INTERNATIONAL CHEMICAL SERIES H. P. TALBOT, PH. D., CONSULTING EDITOR QUANTITATIVE ANALYSIS McGraw-Hill BookComparry Electrical World The Engineering and Mining Journal Engineering Record Engineering News Railway Age Gazette American Machinist Signal ELngin air currents. The case is provided with levelling screws and a spirit level or plumb bob. GRAVIMETRIC ANALYSIS 43 To prevent needless wear on the knife edges the beam and pans should be provided with suitable rests so that the knife edges may not be in contact with their bearings when the balance is not being used. These rests are necessary also for the proper control of the action of the balance, as will be noticed when exercises with the balance are described. The rests for the beam and for the pan supports are usually operated by one piece of mechanism. The exact construction of this varies in different balances but the action of the beam rests belongs to one of two classes, the vertical and the circular. In the first (Fig. 23) the rests move vertically FIG. 23. Beam rests with vertical action. upward. The chief defect of this action is the fact that if the beam is caught at any position except a horizontal one the knife edges are caused to slide upon their bearings, causing unnecessary wear. In the circular action (Fig. 24) the arms of the arrests have the same length as the arms of the beam and they move about the same axis. No matter in what position the beam is caught or how sudden the motion of arrest, no damage results to the knife edges. All designs of mechanism for this purpose include devices for automatically placing beam and pan bearings in their proper position in case they have been accidentally twisted out of posi- tion. These are shown at (a), Fig. 24. The rests under the pans are not for the purpose of lifting knife edges from their bearings, but merely for steadying the pans and for controlling the move- ments of the balance when it is in use. In some balances these 44 QUANTITATIVE ANALYSIS are operated by the same mechanism that operates the other rests, while in others a separate knob is provided. Sensibility. The sensibility of the balance is stated in terms of the displacement of the/beam by a given excess oj load in one pan. More specifically, i^is the number of scale divisions of displace- ment of zero point by an excess of 1 mg in one pan. The sensibility is affected by a number of factors. The bal- ance, in order to possess stability, must have the center of gravity of the moving parts slightly below the point of the middle knife FIG. 24. Beam rests with circular action. edge. This distance is one of the determining factors of the sen- sibility. If large, the sensibility must be relatively small, since a given displacement of the zero point will involve a relatively large displacement of the center of gravity and will, in conse- quence, require a greater difference in load. Every balance has some provision for arbitrarily altering t'he sensibility by altering the location of the center of gravity. The most common device is a weight that can be moved up and down the pointer. The three knife-edge bearings must lie in the same plane, as well as parallel to each other. Since the pans swing freely upon their own bearings, the whole load of the pans is applied at these points. If the plane of these bearings were below that of the middle bearing, an increase in load would lower the center of gravity with reference to the central bearing and thus decrease the sensibility. If their plane were above that of the middle bearing an increase in load would raise the center of gravity to a GRAVIMETRIC ANALYSIS 45 point above the middle point of support and give instability to the balance so that if displaced from its normal horizontal posi- tion the beam would not return. When the three bearings lie in the same plane an increase in load will raise the center of grav- ity but can never raise it to the level of the middle knife edge. The above method of reasoning supposes that the balance beam is perfectly rigid, a property that is never attained in practice. Increase in load, therefore, does actually cause decrease in sensi- bility because the beam is somewhat distorted, causing the center of gravity to be lowered. In order to combine great strength with lightness of weight and so minimize the distortion of the beam, makers have tried many designs and many alloys in the manufac- ture of balance beams. FIG. 25. Illustrating the principle of moments. Another property that has a large influence upon the sensibility is the length of the arms of the beam. It is a well known princi- ple of physics that when a balance is in equilibrium the product of the weight o_ojie_^ideJniQ_jthe _ lengthjof . the corresponding arm must equal the product of the other weight into the length of its arm. This is the " principle of moments." In Fig. 25, aw = a'w r . The greater the inequality of the statical moments (aw and a V) when an excess of weight lies on one side the greater will be the displacement of zero point and this inequality will be greater when a and a' (the lengths of the arms) are large. Lengthening the arms also causes slower swinging so that sensi- bility gained by this means results in loss of time in weighing. This fact sets a practical limit to the length of the beam. 46 QUANTITATIVE ANALYSIS The following properties have been discussed as having an influence upon the sensibility of the balance: 1. Distance between the center of gravity and point of support. 2. Coinci- dence of the planes of the three bearings. 3. Length of the arms of the beam. 4. Reduction of friction to a minimum by finely ground knife edges. In addition to these should be mentioned the weight of the beam. A heavy beam makes a balance that is less sensitive than a balance having a light one. In addition to those features which affect the sensibility, certain others are essential if the balance is to weigh accurately. It is extremely difficult to construct a beam having absolutely the same distance between the central knife edge and the two end ones. Obviously any difference involves a slight difference in the two weights required to bring the balance to equilibrium. From the equation aw = a'w', if a=f=a' then w=w f and the weight of the substance which is being weighed is not the same as that of the weights which counterbalance it. The relative lengths of the arms must be determined for each balance and the observed weights corrected if these lengths are appreciably different. Even if the discrepancy in lengths is sufficiently small to be negligible it may be magnified by a change in temperature or by a change in load, if the beam is not absolutely uniform in material and structure. This last condition is impossible of attainment in practice. It will be made clear in the course of the work in gravimetric analysis that it is not the absolute weight that is* important in most cases but only relative weights because the object of quantitative analysis is to determine the proportionate parts of the constituents of a compound or mixture. From the equation aw = a r w' } if a and a' are not equal the error in w' bears in all cases a definite ratio to w' . While the balance gives the true mass of the object and is in- dependent of the magnitude of the force of gravity, this expres- sion is true only if the buoyancy of the air acts upon weights and objects alike. This can be the case only when the density of weights is the same as the density of objects, a condition that is not fulfilled in the majority of cases. A correction must there- fore be introduced in such cases in order to find the true weight of the object. The amount of correction is negligible in gravi- metric analysis and becomes serious only when the total weight is considerable. The method for applying this correction will GRAVIMETRIC ANALYSIS 47 be explained in the section dealing with volumetric analysis. (See p. 144.) Weights. Sets of analytical weights as purchased fre- quently include weights as small as 1 mg. These are rarely used because the balance provides a more convenient method for making the final adjustment, in the form of a "rider" or small weight of fine platinum or aluminium wire which may be shifted to various positions on the beam. The manner in which the beam is divided varies with the balances of different manufacturers. The lowest subdivision should be at most 0.1 mg. The weight of the rider will depend upon the manner of numbering the milli- gram divisions and the weight will be represented by the number FIG. 26. Balance beam. which is directly over the terminal knife edge. This is because when the rider is placed on this division it is essentially the same as though it were in the pan below. To correctly indicate weights it must then weigh the number of milligrams indicated by this division, which may be 5, 6, 10, 12, or any other number. A mechanism is provided for lifting and adjusting the rider with- out opening the balance case. This may be a simple sliding hook, or an elaborate carrier, such as are found in more expensive balances. It is essential that the carrier be capable of quickly and easily shifting the rider without danger of throwing it from the beam. Every balance is rated for a certain maximum load, it being understood that this is the load for each pan and not the total load. The normal load is fixed by the strength of the knife edges and by the capacity of the beam to resist deformation under stress. 48 QUANTITATIVE ANALYSIS If the knife edges are short and ground to exceeding fineness they are injured more readily by a load than if they are slightly more blunt. If the beam is overloaded it is temporarily deformed to such an extent that there is an unusual loss of sensibility, due to the excessive lowering of the center of gravity. It is thus evi- dent that the weighing of heavy objects requires correspondingly more sturdy balances and these will, of course, be less sensitive. The usual form of a set of metric weights is shown in Fig. 27. The largest weight should not be heavier than the maximum load for which the balance is rated and the least weight should 3 FIG. 27. Usual form of a set of analytical weights. c be such that, used in conjunction with the rider, 10 mg may be made up.V The larger weights are constructed of brass or bronze, plated with platinum or gold to prevent corrosion. The fractional pieces are of platinum in the better sets or of aluminium in the cheaper ones. Even with weights plated with platinum or gold it is comparatively easy to damage the surface by careless handling or by allowing chemicals to touch the weights. Exercises and Rules for Manipulation of the Balance. Printed directions for setting up always accompany a new balance. The following rules deal only with the balance set up and ready for use. GRAVIMETRIC ANALYSIS 49 1. Cleanliness. The pans, beam, bearings and all other parts inside the glass case must at all times be kept free from dust and chemicals. A camel's hair brush 1 inch wide should be provided for this purpose. It is not permissible to weigh any soluble mate- rial in direct contact with the pans because some of this will in- variably stick to the pan and eventually it may cause corrosion. Volatile acids must never be brought inside the balance case un- less securely stoppered in an air-tight container. 2. Adjustment. The balance is levelled by means of the screws provided for that purpose. Examination is made to determine whether the knife edges are in the proper position with respect to their bearings. The pan rests are released to determine whether the pans hang vertically from the stirrups, or whether they swing horizontally when released. If so this swinging is stopped by momentarily touching the pans by the pan rests, repeating the operation until the pans hang quietly upon release. 3. To Set the Beam in Motion. Various methods are used for starting the oscillation of the balance about the central bearing. One pan may be lightly touched with a small camePs hair brush. This is not an easy process to carry out properly because it is difficult to control the impulse given to the pan. Another method is to raise the door and fan one of the pans slightly with the hand. This is open to the same objection as is the first method and, in addition, it defeats the primary aim of the glass case which is to prevent the interference of air currents. Even the slight current started by the hand does not at once die and it must be a disturbing influence for some time after the balance case is closed. A better method than either of the above mentioned is to lower the rider to the beam just before releasing the latter, then to catch up the rider with the carrier after releasing and allowing the proper start. A short practice will enable the operator to give just the desired impulse to the beam to make the pointer swing over from five to ten divisions on either side of the zero of the scale. The rests should be so adjusted that the three knife edges are lifted from their bearings when the rests are raised, but the distance between the edge and bearing should be barely perceptible. If this distance is unduly large the shock to the delicate knife edge is so great that this edge is soon dulled or chipped with a consequent loss of sensibility. 4. To Determine the Zero Point. The zero point may be de- 50 QUANTITATIVE ANALYSIS fined as that point on the scale at which the pointer would eventually come to rest from swinging over the scale. It is never observed by allowing the pointer to actually come to rest because such in- fluences as minute air currents would either prevent this con- summation entirely or would cause the observation of a fictitious zero point. The effect of these influences is counteracted by allowing the pointer to swing a number of times to the right and to the left, taking the average of the indications. : Proceed as follows : If the balance operates all of its. rests by one mechanism care- fully lower these rests and set the beam in motion as directed above. If the pan supports are controlled by a separate button lower the beam and stirrup rests first, then the pan rests. Allow the pointer to swing three or four times in each direction and record the num- ber of scale divisions over which it swings, taking the last reading on the same side as the first. Record in two columns and take the average of each column. Subtract the less average from the greater and divide the remainder by 2. This gives the zero point if the proper direction is assigned to it. Example: Left Right 8.25 7.75 7.25 7.00 6.75 6 50 6.00 5.50 5.00 4.75 4.50 Average 7 . 25 Average 5.15 7.25-5.15 = 1.05. Therefore the zero point is -1, or 1 division to the left of the zero of the scale. Two methods of procedure are now open to the operator. He may either make his weighings with reference to this observed zero point or he may adjust the balance so that the observed zero point is the actual zero of the scale, using for this purpose the small screws provided on the ends of the beam. The first method is preferable for the beginner because any attempt to change the adjustment will probably result in more serious derangement. After skill has been gained by practice time will be gained and .much calculation will be saved if the observed zero point is adjusted to coincide with the GRAVIMETRIC ANALYSIS 51 ideal zero of the scale. A method for making a close approxi- mation of the zero point without resorting to calculations on paper will be explained in a later paragraph. The zero point changes and it must be determined each day, or more often if necessary. 5. To Determine the Sensibility. ^he sensibility in the case of the analytical balance has already been defined as the number of scale divisions that the zero point is displaced by an excess in weight of 1 mg on one side. That the sensibility varies with the total load has already been explained. To determine the sensibility with zero load, first determine the zero point of the balance. Place the rider on the beam at the division marked I and redetermine the zero point. The difference is the sensi- bility. Determine the sensibility when both pans are loaded with 5, 10, 25 and 50 gm, respectively. Record the results on a card and place this in the^balance case for future reference. 6. To Determine the Relative Lengths of the Arms. Weigh a small object, such as a crucible, placing it on the left pan and the weights on the right, then place the object on the right pan and the weights on the left. If the arms of the balance are of unequal length these weights will not be the same. Let W = the true weight of the object, TP' = the sum of the weights when the object is on the left pan, a = the weight added to W when the object is placed on the right pan, r = the length of the right arm and I = that of the left arm. From the principle of moments W'r = Wl Wr= (W'+a)l WW'r 2 =W(W'+a)l 2 W f r*=(W'+a)l* r 2 W' + a 1 2 ~ W This is the proper method for testing the equality of arms. T If the arms are equal then a = and ^ = 1. As has already been explained, the question of equality has little or no importance for purely analytical work. Inasmuch as the chemist has uses for his balance in other lines of work it should be tested. 52 QUANTITATIVE ANALYSIS 7. To Weigh. With all of the rests raised the object to be weighed is placed on the left pan by means of the crucible tongs or by some other method that avoids contact with the fingers. The pan rests are lowered and raised momentarily until the pans stop swinging on their bearings, then these rests are lowered and fastened down unless all of the rests are governed by one mechanism. By means of the weight forceps place one of the weights, judged to be somewhat heavier than the object, on the right pan. Lower the beam rest slowly until the pointer just starts to move to the right or left. If to the right the weight is too light, if to the left it is too heavy. If it is too light raise the beam rest and exchange the weight for the next heavier one and repeat the trial until the first weight is found that is too heavy. Remove this and replace by the next lighter one. Continue the addition of weights, trying after each addition and adding always the next consecutive weight lighter than the one used in the preceding trial. j~'When the weights below 1 gm (milligram pieces) are reached the difference between the loads on the two pans is so small that the pan rests will now readily control the movements of the balance. If these rests are operated by a separate knob they are then raised and the beam and stirrup rests lowered, and the process of trial is continued until the range covered by the rider is reached. v The balance case is now closed, and the pans again steadied if they have shown a tendency toward lateral swinging. Make the preliminary rider trials by placing the rider on the division estimated to be nearest the proper one, slightly releasing the pan rests until the pointer starts to move. Arrest the motion and move the rider by whole milli- gram divisions to the right or left, as may be required, until the milligram nearest to the correct weight is reached. Estimate the proper fraction of a milligram, set the beam in motion as directed in (3) and drop the rider on the estimated division. By observing the swinging determine the zero point and calcu- late from this and the sensibility with this load what change should be made in the position of the rider to bring the balance to equilibrium on the zero as determined with no load. Repeat the trial with the rider on this calculated position and shift if necessary to obtain the exact weight. Raise all of the rests and read the weights as follows: Observe the empty places in the box. If the weights have been systematically placed in the box, GRAVIMETRIC ANALYSIS 53 none being removed except those on the pan, the empty places will give the correct weight. Record this weight in the data book. Confirm by counting the weights as they lie on the pan. Reconfirm by counting as they are removed and replaced in the box. This gives three readings and if these are carefully made a mistake is practically impossible. In recording the weights mental addition may be made if they are taken in order, pro- ceeding from the larger ones to the smaller ones. This is because, as the sets of weights are made, no one order of digits can total more than 9. Each order can be mentally added and recorded . with the certainty that no other order will change the one read. Thus, if there are on the pan the following pieces : 10 gm, 5 gm, 1 gm, 200 mg, 100 mg, 50 mg, 20 mg, 10 mg, ar d 5 mg, and on the rider 3.2 mg, we should read and record thus: Of whole grams 16, of tenths (100 mg) 3, of hundredths 8, of thousandths 8, of ten-thousandths 2. Writing in the same order we should have 16.3882 gm. Several points in the preceding paragraph need additional comment. The beam and stirrup rests must be used when changing weights heavier than 1 gm for two reasons: first, the shock of the weight against the pan must not be allowed to communicate itself to the knife edges when on their bearings and second, the pan rests are held up by a spring that will not support an excess load of more than 1 gm in one pan. In other words, these rests would not control the balance at this point in the experiment. When the milligram pieces are being exchanged the shock of impact with the pan is so small that the knife edges are not damaged and the pan rests offer an easier method of control. In making a trial of a weight the pointer should be allowed to move only far enough to indicate the direction of motion. This indicates the proper change to be made in weights as well as if it were allowed to move half way across the scale and it does not derange the balance. It was stated in (4) that in later work extended calculations of zero point would not be made. On account of the resistance due to friction with the air and in the bearings, any balance decreases the amplitude of vibration with each successive journey. The amount of such decrease varies with different balances but a close approximation can be made by simple observation. 54 QUANTITATIVE ANALYSIS If the zero point of the unloaded balance has been adjusted to coincide with that of the scale, in the final adjustment of weights the loaded balance can also be brought to this ideal zero point without the necessity of extended calculations, by simply noting that the distance to which the pointer swings in one direction is a certain (approximate) fraction of a divi- sion less than the distance in the opposite direction on the next preceding journey. In many cases of quantitative analysis, if the longer (even though slightly more exact) method involving calculations of zero point were followed, the time consumed in weighing would be so great that the weight of the object would change appreciably while on the balance, owing to absorption or evolution of water, carbon dioxide, etc. It is often desirable, in order to simplify calculations, to use a predetermined weight of sample, as 1 gm, 0.5 gm, 5 gm, etc. This is a difficult operation if the ordinary method of weighing by difference is used, because the sample that is to be used is poured from a weighing bottle and if too much is inadvertently poured out it is not easy to return the excess without loss. If the sample is dry and is unaffected by free contact with air one can dispense with the weighing bottle and weigh directly on a watch glass or scoop. For this purpose one may obtain " counterpoised watch glasses," which are pairs of glasses the members of which possess so nearly the same weight that they can easily be balanced by means of the rider so that their weight does not enter into the calculation. The method of obtaining a predetermined weight of a sample is as follows: The glasses are placed on the pans and exactly balanced, if necessary, by using the rider. Weights totaling the desired quantity are placed on the right glass and the pan rests lowered, steadying the pans at the time if necessary. The beam rests are now lowered just enough to allow the central knife edge to come to its bearing and the pointer to perceptibly move to the left. The balance door is lowered about half way and then the sample, in a fine state of division, is carefully poured on the left glass until but a slight excess is obtained, as evidenced by the swing of the pointer to the right. (The total length of swing as allowed by the beam rests should not exceed two scale divisions as otherwise the adjustment of the balance may be disturbed.) Using the spatula, sufficient sample is now removed GRAVIMETRIC ANALYSIS 55 from the glass to make this side slightly too light. This is held over the glass and, by gently tapping the spatula, the sample thus held is gradually dropped to the pan until equilibrium of the balance is nearly or quite attained. The excess is dis- carded. By repeating this process once or twice apparent equilibrium is quickly attained and this is confirmed by closing the case and determining the zero point by the usual method. In this way any desired weight of sample can be obtained in a comparatively short time. The degree of accuracy with which it is finally weighed will depend, as in all other cases, upon the nature of the sample, the total weight being taken and the degree of accuracy possible at other points in the analysis. 8. To Correct the Observed Weight for Inequality of Arms. Certain investigations in physics and physical chemistry require that the weight found shall be the absolute weight, correcting for the inequality of arms and for buoyancy of the air. The former correction need not be made for analytical work but where necessary the method of Gauss or that of Borda may be used. Method of Gauss. Weigh the object first on the left pan and then on the right. Let W be the true weight, a the weights required to counterbalance when the object is on the left pan and b the weights when the object is on the right. By the principle of moments: Wl = ar bl = Wr WHr = ablr W 2 = ab_ W = Vab Therefore the true weight is the square root of the product of the two observed weights. Where the inequality of arms is very slight (the usual case) the arithmetical mean of the two weights is a sufficiently close approximation to the square root of the product. Method of Borda. This is also known as the method of substitution. Place the object to be weighed on the left pan and counterbalance with any other material, such as weights or small shot. Remove the object and substitute accurate weights until the balance is again in equilibrium. These 56 QUANTITATIVE ANALYSIS weights are necessarily the same in value as the object for which they substitute, irrespective of the relative length of the arms. To Calibrate Weights. For analytical purposes it is not necessary that the various pieces in a set of weights shall weigh exactly as indicated by the stamp, although this is desirable. The only requirement is that the pieces shall have the correct ratio to each other. That is, the weight marked " 1 gm " need not weigh exactly 1 gm. Indeed it might conceivably have any other value which is reasonably near to 1 gm, but it is necessary that its weight shall be one-tenth of that of the piece marked "10 gm," 10 times that of the piece marked " 100 mg," etc., or that the deviation from these ratios be known and corrected in calculations of the weights of objects. Commercial weights are seldom accurate unless the cost is high. A calibration, there- fore, must always be made and correction applied in such cases as are made necessary by excessive errors in manufacture. If the calibration is to include correction to absolute weight a standard piece is necessary. Otherwise ony piece of the set may be taken as the standard. Determination. Before beginning the calibration see that in all cases where there is more than one weight of the same denomination the separate pieces bear some distinctive mark. A good method is to make small dots by means of a prick punch. This marks without damaging the plate or changing the weight of the pieces. One of the two 10-gm pieces may be marked (') two of the three 1-gm pieces (') and ("), etc. The method to be followed in the calibration will depend upon whether the arms of the balance have been found to be of equal length. If they are not equal either the method of Gauss or that of Borda may be used for comparing the weights. If the method of Borda is used a second set of weights is convenient for the substitution. This method involves less work of calculating than does that of Gauss. If the arms are equal the simple method of weighing without correction is used. In any case the following comparisons are made using the rider to obtain equilibrium. I II Gram pieces Milligram pieces 1 against 1' 500 against 200+100+100'+50 1 against I" +20+10+10'+rider a t 10 2 against 1 + 1' 200 against 100+100' GRAVIMETRIC ANALYSIS 57 5 against 2+l + l'+l" 100 against 100' 10 against 5+2+1 + 1'+!" 100 against 50+20+10+10'+ 10 against 10' rider at 10 20 against 10+10' 50 against 20+10+10'+rider 50 against 20+10+10'+5+2+l 20 against 10+10' + 1 / +1 // 10 against 10' 10 against rider at 10 Also unmarked 1-gm piece against all milligram pieces +rider. The calculation of correction for pieces of 1 gm and upward is made upon the arbitrary assumption that the unmarked 1-gm piece is correct and this will readily be understood. In calculating the correction for the milligram pieces first assume the 10-mg piece as standard and cal- culate provisional corrections for the other milligram pieces upon this basis. Add these provisionally corrected weights and determine, by comparing their sum with their collective weight as found by the com- parison with the unmarked 1-gm piece, how much each weight must be further corrected. Example: In a certain calibration the sum of the milligram pieces on the basis of 10 as standard was 10.1 (rider) + 10+ 10'+ 19.9+48.6+ 100' + 100.2+200+498.2 = 996.1 mg, or 0.9961 gm. The comparison of the unmarked 1-gm piece against the fractional pieces gave as their sum 1.0023 gm. Therefore, on the new (and final) basis of the 1-gm piece as 1 0023 standard, each of the provisional values should be multiplied by Q Reagents. One of the most vexatious problems with which the analyst has to deal is that of obtaining reagents that are sufficiently pure to suit his purpose. Methods of manufacture are constantly being improved and better chemicals are now available than in the past, but even at this time the reagent that is assumed to be pure often contains small quantities of impurities which interfere with the accuracy of analytical proc- esses. Attempts have been made by manufacturers to indicate on the label the degree of purity. Thus "c.p." for " chemically pure," signifies a reagent containing no impurity in a quantity that could be detected by chemical tests. "Com." for "com- mercial" means a crude unpurified chemical, "medicinal" sufficiently pure for medicinal purposes, "U. S. P." purity as specified by the United States Pharmacopoeia, etc. Zinc might be labeled " arsenic free" to indicate that it could be used without a blank test for a determination of arsenic by Marsh's method, or "iron free" so that it could be used for reducing solutions 58 QUANTITATIVE ANALYSIS in iron analysis without a blank test. If these labels ever did have any real value they early lost it. "c. p." has, like charity, been made a mantle to cover a multitude of shortcomings in packages of grossly impure reagents. " Medicinal" has meant little more than that the manufacturer hoped that the substance so marked might be sold to the unsuspecting for medicinal pur- poses. " Silver free" lead (for assaying silver ores) is often lead from which the manufacturer has removed a certain fraction (or none at all) of the silver originally contained in it. On account of the carelessness evident in preparing and label- ing reagents chemists have come to practically disregard all such indications of purported purity and to rely upon one or both of two sources of information regarding the purity of reagents. These sources are the reputation that the manufac- turer bears for producing reliable chemicals and the chemist's own personal test of the chemicals themselves. Many manu- facturers have now entirely discarded the abbreviation "c. p." and publish on the label a supposed analysis of the substance contained in the package. " Analyzed chemicals" have thus become popular, but the inexperienced chemist will make a great mistake if he forms the too hasty conclusion that the analysis is always correct. It is often very far from being correct. The passage of pure food and drugs acts in this and many foreign countries has resulted in great improvement in the matter of labeling chemicals that are to be used for medicinal purposes, since the label constitutes a legal guarantee as to the contents of the package. When these acts are extended to include the reagents used for scientific purposes the chemist will have a better opportunity for purchasing chemicals of the degree of purity of which he can feel reasonably assured. At present the only safe plan is to make blank tests for such impurities as will interfere in the analysis to be performed. While it will be readily conceded that no substance can be made absolutely pure yet certain reagents can only with difficulty be made even approximately pure. Examples are the strong bases, such as sodium hydroxide, potassium hydroxide, ammo- nium hydroxide, barium hydroxide, etc., which readily attack and dissolve glass so that they are always contaminated with silica. On this account their solutions are seldom kept as stock reagents in the laboratory but are made from the solids as re- GRAVIMETRIC ANALYSIS 59 quired excepting, of course, ammonium hydroxide which is a solution of a gas. In such cases the chemist will simply require that interfering substances shall be absent. Basic solutions often contain precipitated matter. The glass bottle is first attacked, the solution accumulating alkali silicates. These are later hydrolyzed, causing the precipitation of hydrated silica. The rule must never be forgotten that solutions are to be filtered just before using for analytical purposes unless they are already quite clear and free from sediment. Distilled Water. Natural waters always contain dissolved matter which unfits them for use in analytical work. Besides such natural mineral matter and dissolved gases, water will always dissolve certain quantities of the container when allowed to stand. In order to remove dissolved solids the water is distilled and recondensed. Various forms of stills are in use. In any form of such apparatus the vessel in which the water is boiled should be so far separated from the condensing worm that it is impossible for any spray to enter the latter. The boiler itself may be of any material, but the condensing worm should be of pure tin, silver, or platinum because hot water dissolves most other metals and also glass. The cost of platinum of course precludes its use in any but small stills that are to be used for preparing water for exact investigations, such as are carried out in physical chemical work. Pure tin is the metal generally used for the purpose. Distillation does not free water from dissolved gases and for work in which carbon dioxide, oxygen, nitrogen, or ammonia will interfere it is necessary to boil the water immediately before using. Boiling should not be unduly prolonged, since the water thus becomes recontaminated with the material of the contain- ing vessel. Transfer of Liquids. The operations involving pouring rea- gents from bottles, pouring liquids into a filter or pouring from one vessel to another are often so clumsily performed as to cause a loss of part of the liquid through splashing or running down the outside of the pouring vessel, thus vitiating the results of the analysis or at least producing a very disagreeable sort of uncleanness of the apparatus. When pouring from a bottle the stopper should never be laid on the desk but is held between the fingers of the right hand. The bottle is then grasped in 60 QUANTITATIVE ANALYSIS such a way as to bring the label under the hand and then a glass rod is held in a vertical position against the mouth of the bottle. The latter is tilted to pour out the required amount of liquid, the glass rod being kept in position until the bottle is returned to an upright position. No liquid should run down the outside of the bottle in this case. If a drop should escape the label will not be marred because the method of holding the bottle brings the label on the upper side while pouring. When pouring from a beaker the stirring rod is used in the same way for preventing splashing. Records. In no other part of the analysis is system more important than in that of records. Results are many times rendered worthless by uncertainty regarding the meaning of the FIG. 28. Form of label for laboratory reagent bottle. experimental figures or regarding the pieces to which recorded weights belong. All analyses are run at least in duplicate, some in triplicate. If at any stage in the work the beakers, crucibles, burettes, or other pieces become interchanged or if the recorded weights, volumes, temperatures, or other data are not properly labeled and are applied to the wrong pieces, then the calculated results of the analysis are, of course, incorrect. At the very outset the duplicate pieces should be numbered (I and II unless other marks are preferred) in every case where the mark is not objectionable. It is not advisable to mark by labels or pencil any article that is to be weighed because the mark itself often causes changes in weight through rubbing off or absorption of moisture. For articles that are not to be exactly weighed a small label or pencil such as is used for marking glass and porce- lain may be used. The latter has a soft core composed of pig- ment and grease or paraffin so that it will stick to glass. The better grades of glassware are now furnished with a spot rough- ened by sand blasting, this making it possible to use a common GRAVIMETRIC ANALYSIS 61 graphite pencil or a pen for marking without a label. Even in the case of articles that are not to be marked they can be easily identified if the analyst follows the plan of always keeping No. I on the left and No. II on the right when precipitating, filtering, igniting or allowing crucibles to stand in desiccators. SAMPLE OF_ -MARKED GRAVIMETRIC ANALYSIS AMOUNT OF SAMPLE TAKEN WEIGHT OF PEECKNT OF AMOUNT OF SAMPLE TAKEN WEIGHT OF PERCENT OF 'FOUND FOUND I I II II AMOUNT OF SAMPLE WEIGHT OF PEECENT AMOUNT OF SAMPLE WEIGHT OF PEECENT FOUND FOUND 1 I II II AVERAGES 8DB8TANOE DETERMINED PERCENT SUBSTANCE DETERMINED PERCENT FIG. 29. A convenient blank for reporting the results of a gravimetric analysis. Reagent bottles should be marked by a label bearing the name of the student, that of the reagent, the concentration of the solu- tion and the desk number, as shown in figure 28. Many systems of note-book records have been used with greater 62 QUANTITATIVE ANALYSIS or less success. If an ordinary blank note book is used it is nece'ssary to so indicate each measured weight or volume that there is no possibility of uncertainty regarding the meaning of the figures. Loose sheets of paper must not, under any circumstances, be used for records of a permanent character. Loss of such a sheet has frequently caused the loss of days or even weeks of laboratory work. A better device than that of the blank paged note book is found in a systematic record book, with spaces so provided and lettered that mere recording of figures is all that is necessary. Such a page for gravimetric analysis is shown in Fig. 29. No indication of the identity of weights or percents is necessay be- yond the filling of the blanks as shown. CHAPTER III EXPERIMENTAL GRAVIMETRIC ANALYSIS One of the most apparent differences between the methods of study as applied to qualitative and to quantitative analysis lies in the fact that while in qualitative analysis the metals and acids are grouped according to their susceptibility to the action of cer- tain " group reagents," in quantitative analysis widely different reagents and methods are often used for elements or acids that are closely allied in most respects. A general review of the conditions that must be fulfilled for accurate gravimetric analysis will serve to show that no such systematic classification as is found in qualitative work is desirable in quantitative pro- cedure, since the reagent and method must always be selected which will give the precipitate which is the most insoluble and the most easily separated and purified and which assumes the most definite form upon the application of heat. For example, calcium, barium and strontium fall in the same periodic as well as the same qualitative group and yet there is no logical reason for studying these metals together in quantitative analysis because barium is most conveniently precipitated as sulphate and calcium as oxalate from water solutions, while strontium is precipitated as sulphate from solutions containing alcohol. Barium sulphate is stable when ignited, and is weighed as such. Calcium oxalate decomposes and is weighed as carbonate or oxide while strontium sulphate is stable and is weighed in this form. Any one of these metals could be precipitated by ammonium car- bonate and the carbonate changed to the oxide by ignition but this would not, in any case, prove to be the most convenient or the most accurate method. In view of these facts it becomes desirable to select, for each element or radical, that method which is, under the circumstances, the most easily executed and the most accurate and to study these, not in order of qualitative or periodic groups, but in that 63 64 QUANTITATIVE ANALYSIS order which will best develop manipulative skill and accuracy in experimental work. Many methods that were formerly used have become almost obsolete because of the development of better apparatus or more rapid methods. In such cases the older methods will generally receive mention at the proper place and the more modern method will be described more fully. CALCIUM Calcium may be precipitated from ammoniacal solution as carbonate by alkali carbonates or as oxalate by alkali oxalates. Since ammonium carbonate or oxalate yields volatile ammonium salts as byproducts in the reaction and since traces of these will be expelled upon ignition if not completely washed Qut of the precipitate, the ammonium salts are always used in preference to those of sodium or potassium. The solubility of calcium carbonate in water was determined by Kohlrausch and Rose 1 by conductivity experiments and was found to be 0.013 gm (corresponding to 0.0052 gm of calcium) per liter at 18, but the solubility is considerably increased by ammonium salts, such as must be present when ammonium carbonate reacts with a calcium salt in solution. The solubility of calcium oxalate in water was found by Kohlrausch and Rose to be 0.0056 gm of the crystallized salt, CaC 2 O 4 .H 2 O (corresponding to 0.0015 gm of calcium) per liter. The solubility is considerably reduced by an excess of ammonium oxalate. On account of this difference in solubility the oxalate method is preferable. Ammonium chloride should be present in either case to prevent precipitation of traces of magnesium. The reaction involved in the precipita- tion may be expressed as follows: CaCl 2 +(NH 4 ) 2 C 2 04 CaC 2 4 +2NH 4 Cl. Ammonium oxalate does not readily dissolve in water, the saturated solution at containing 2.2 percent of the salt. As the temperature is raised the solubility is increased so that a saturated solution at 20 is easily made by heating in contact with an excess of the salt and allowing to cool. It is not best to keep a stock solution of ammonium oxalate because it readily undergoes hydrolysis, yielding ammonium hydroxide which 1 Z. physik. Chem., 12, 234 (1893); 44, 197 (1903). EXPERIMENTAL GRAVIMETRIC ANALYSIS 65 attacks the glass, and because there occurs a decomposition in solution as follows: (NH 4 ) 2 C 2 O 4 -> (NH 4 ) 2 CO 3 +CO. It is preferable to make a small amount of the solution as needed for the determination. A difficulty that is often encountered by the inexperienced analyst is the formation of a precipitate of calcium oxalate which is so finely crystalline that it passes through the pores of the filter, making its complete separation impossible. Refiltration of the portion that has passed through will partially remedy this trouble but when a precipitate is found to be too finely divided for filtration the only satisfactory cure is found in digestion. Certain grades of filter paper are made for filtering fine precipi- tates, their structure being very dense. This renders filtration less rapid than would be the case with papers of ordinary density. The difficulty nearly or quite disappears if the proper conditions are observed during the precipitation. It has been explained (page 18) that too rapid precipitation causes the formation of a large number of small particles rather than a small number of large particles. Two conditions are found to be suitable for the formation of large crystals of calcium oxalate: (1) boiling tem- perature for solutions of calcium salt and ammonium oxalate and (2) slow addition of reagent. These conditions will be elaborated in the directions for the determination. Calcium may also be determined by precipitating as sulphate from a solution containing alcohol 1 or volumetrically by pre- cipitating as oxalate and titrating with a standard solution of potassium permanganate. The latter method will be described in the section dealing with volumetric analysis. Calcium precipitated as either carbonate or oxalate may be weighed as carbonate, oxide or sulphate. Calcium oxalate decomposes as follows, on the application of heat: CaC 2 4 CaCOs+CO, (1) CaCOs CaO+C0 2 . (2) Reaction (1) begins at quite low temperatures. Reaction (2) begins as low as 500 but requires long heating at this tempera- ture to complete the decomposition. In practice the highest 1 Stolberg: Z. angew. Chem., 17, 269 (1904). ' 5 66 QUANTITATIVE ANALYSIS temperature attainable by the blast lamp is applied and heating is continued until no further loss in weight occurs. Calcium oxide is not reduced by hot carbon and the precipitate may be heated without removing from the paper. Many chemists prefer to ignite the oxalate at a low temperature and to weigh as carbonate but this procedure is of doubtful utility, even for an experienced analyst, on account of the difficulty in stopping the decomposition at a point where all the oxalate has disappeared and no oxide has been formed. \i is also possible to add a few drops of sulphuric acid to the crucible containing the calcium oxalate and to weigh the resulting calcium sulphate, after gentle ignition to expel the oxalic acid and the excess of sulphuric acid. The most important source of error in this procedure comes from the loss by spattering, a loss which even the greatest care can scarcely prevent. There is also danger of decomposing cal- cium sulphate by strong heating: CaS0 4 -CaO+S0 3 . It is much preferable to strongly heat the precipitate converting it quantitatively into calcium oxide. The completion of the conversion can be judged by the constancy in the weight of the substance upon further heating. [ Calcium oxide readily absorbs moisture and carbon dioxide when exposed to the air and the resulting change of weight will become appreciable if the process of weighing is unduly prolonged. If the weight of the crucible and oxide is approximately known most of the weights may be placed on the balance pan before the crucible is removed from the desiccator and the remainder of the process completed in a short time. It is a good plan to keep a small piece of potassium hydroxide in the desiccator in which calcium oxide is to be preserved. This lessens the absorption of carbon dioxide by keeping the atmosphere free from that gas. The converse of this method may be used for the determination of the oxalate radical, precipitation being made by a soluble calcium salt in basic solution. Volumetric methods are, however, preferable. Determination. Fill a clean dry weighing bottle with the calcium compound to be analyzed. Provide two clean Jena beakers having a capacity of 250 cc and mark them I and II. If the substance is of such a nature that it is altered in any way by free exposure to air the EXPERIMENTAL GRAVIMETRIC ANALYSIS 67 sample to be used must be weighed by difference as follows: Touching the bottle with the fingers as little as possible, place it on the balance pan and carefully weigh. Record this weight in the data book at the top of the space marked for sample I, reading the weights as directed on page 52. Carefully remove the stopper, holding over beaker marked I, and pour what is judged to be between 0.2 gm and 0.5 gm into the beaker. Replace the stopper, taking great care that any falling particles drop into the beaker and are not lost, then reweigh the bottle and contents. Record this weight under the first, also at the top of the space for sample II. Remove a second portion to beaker II and reweigh the bottle, recording under the preceding weight. Subtracting the less weights from the greater gives the weights of sample used. If the substance is known to be unaffected by contact with air it may be poured into one of the counterpoised glasses and weighed directly, being then brushed into the beaker by the small pencil brush of camel's hair. After having weighed the, two samples for analysis determine, by qualitative tests on another portion of the substance, whether it is solu- ble in water and, if not, w r hether in dilute hydrochloric acid. If soluble in water dissolve in about 100 cc of distilled water, add 5 cc of 10 percent ammonium chloride solution and treat each sample as directed below. If insoluble in water but soluble in hydrochloric acid first moisten each sample with water then cover the beakers with glasses and carefully add about 20 cc of dilute acid. After effervescence has ceased rinse down the cover glass and the sides of the beaker with water from the wash bottle and gently boil for one minute to expel dissolved carbon dioxide. From this point the procedure is the same as for water soluble salts. Prepare ammonium oxalate solution by heating to boiling 5 gm of powdered ammonium oxalate and 100 cc of water in a Jena beaker. Part of the salt will crystallize when cooled, leaving a saturated solution. Add dilute ammonium hydroxide (filtered unless already quite clear) to the solution of calcium salt untilthe liquid smells very distinctly of ammonia'. In determining this point the ammonia that is already in the air above the liquid must be blown away before testing the odor. x lleat to boiling and add, drop by drop from a pipette, 20 cc of the recently prepared ammonium oxalate solution, stirring vigorously during the addition. If this is carefully done the precipitate should settle readily, leaving a clear liquid above. When this is the case add a few drops more of ammonium oxalate solution, observing whether any precipitate forms. If so, more reagent must be added in the same manner as at first. ^jWhen precipitation has been shown to be complete the liquid is digested at a temperature somewhat below the boiling-point for one-half hour or until the supernatant liquid is quite clear, when it is ready for filtration, y 68 QUANTITATIVE ANALYSIS Prepare two filters of extracted paper, marking the funnels I and II. Carefully decant each solution, while hot, upon the proper filter, allowing to run through into clean beakers. Observe the directions given on page 59 for proper method of pouring from beakers. Before completing the filtration a few drops of ammonium oxalate solution should be added to the clean filtrate that has already run through. If a precipitate forms, the filter must be well washed, the filtrate and washings returned to the beaker in which precipitation was made and more reagent added in the same manner as before, until precipitation is complete. When no pre- cipitate is produced in the filtrate, complete the filtration, washing the precipitate into the filter by a stream from the hot water bottle, rubbing the beakers with a glass rod tipped with rubber tubing. \ Wash the pre- cipitate on the filter until a small amount of the last washings fails to give more than a faint precipitate with silver nitrate, showing that chlorides have been removed. Allow the paper to drain then remove from the funnel, fold as directed on page 31 and place in a previously ignited and weighed porcelain or platinum crucible. The cover should have been weighed with the crucible because the closed crucible will hinder absorp- tion of moisture and carbon dioxide while weighing. The crucible is care- fully heated by the burner until the moisture has been expelled and smok- ing begins. The cover may now be removed and placed on a clean tile while the crucible is heated more strongly until the paper is completely charred. The crucible is now placed on its side, the cover adjusted and complete oxidation of the carbon of the paper is accomplished as described on page 33 . The crucible is then placed in an upright position, is covered and subjected to the hottest flame available from the blast lamp. This ignition is continued for 30 minutes, when the crucible is placed in the proper desiccator, allowed to cool to the temperature of the room and quickly weighed. It is ignited for 10 minutes longer, cooled and re weighed. If there is a decrease in weight of more than 0.0003 gm the crucible is reheated for 10 minutes and weighed, the process of heating and weighing being continued until the weight remains constant within the limit given above. The preliminary weighings should be recorded upon the back of the sheet preceding the one used for the final record, the final weight being recorded in the proper blank space. Calculate the percent of calcium in the sample, using the factor already calculated and recorded on page 8, and using a table of loga- rithms for the arithmetical work. Do not discard the ignited product until after the report has been accepted. Errors may have been made which can be corrected if this has been preserved. SILVER AND THE HALOGENS Silver might be gravimetrically determined as chloride, bromide or iodide. The solubilities of these salts in water, EXPERIMENTAL GRAVIMETRIC ANALYSIS 69 shown in the following table, were determined by Kohlrausch and Rose. 1 Milligrams per liter, soluble at 18 Salt Silver equivalent to salt Silver chloride Silver bromide 0.0017 0.0004 0.0013 0.00023 Silver iodide. . 0.0001 0.00005 Bottger 2 has obtained somewhat different values for the solubility of silver chloride and silver iodide. From the comparative solubilities one might conclude that silver chloride is the least desirable form in which to weigh silver. The bromide and iodide, however, are much more sensitive to the action of light, being more readily decomposed into sub- halides with liberation of free halogen. The stabilities of these salts are in the same relation to each other as are those of halides of other metals and of hydrogen. On this account the gravimetric determination of silver is invariably made by weighing silver chloride, using hydrochloric acid as the precipi- tating reagent. Conversely the determination of chloranion is 1 made by using a soluble silver salt as the reagent. A small excess of either a soluble silver salt or a soluble chloride greatly diminishes the solubility of silver chloride as explained in the section dealing with the principles of precipitation. If more than a very slight excess of a metal chloride is present the solu- bility of silver chloride is increased, because of the formation of soluble double salts. On this account hydrochloric acid is used as the precipitant for silver. Silver chloride shows a well defined tendency toward the formation of a hydrosol in cold water and when this is formed the solubility follows no definite rule. The sol can be flocculated by boiling with dilute acids or other electrolytes. The precipitate of silver chloride is affected by light as are the other silver halides, it being reduced to a subchloride, Ag 2 Cl, with liberation of free chlorine. The darkening of silver chloride under the influence of strong light is due to the appearance of the subchloride which is bluish black in color. 1 Z. physik. Chem., 12, 234 (1893). 2 Ibid., 46, 521 (1903). 70 QUANTITATIVE ANALYSIS While some decomposition undoubtedly occurs in daylight of any intensity, if the precipitation is performed in the darker parts of the room ordinary diffused light will not appreciably affect the weight of the precipitate in a short time. The precipitate cannot be ignited in contact with filter paper on account of the ease with which it is reduced to metallic silver. Use can be made of any of the devices for dealing with such pre- cipitates, as mentioned in the general discussion of the ignition of precipitates; we shall here follow the method of removing the precipitate from the filter paper and, when the determination of chlorine is considered, with the use of the Gooch crucible. Either method may be used in either case. Whenever silver chloride is heated, care must be taken to prevent a rise of tem- perature above the point of fusion, which is 151, since it is sen- sibly volatile at high temperatures. By making the proper changes in procedure the halogens may be determined, silver nitrate being used as the precipitating reagent. These determinations are discussed later. (Page 93.) Determination. Fill a stoppered weighing bottle with the powdered silver salt and weigh out two samples of about 0.3 gm each for analy- sis, placing in 250-cc beakers, which may be of ordinary hard glass. The weighing may be done upon counterpoised glasses or from the weigh- ing bottle, by following the directions given for the determination of calcium. In this, as in all other cases, care must be exercised to avoid spilling any of the substance upon the balance pan, as this invariably results in corrosion of the pan. Dissolve the weighed sample in about 100 cc of water, and heat nearly to boiling. Add, with stirring, 5 cc of dilute hydrochloric acid and digest at a temperature near the boiling-point until the precipitate is completely flocculated and the supernatant liquid is quite clear. Test the clear liquid with more hydrochloric acid as soon as is practicable, to determine whether precipitation is complete. When the precipitate settles completely filter on extracted paper, with or without slight suction, and wash with hot water containing 1 cc of dilute nitric acid in each 100 cc of water, the acid being used in order to prevent the silver chloride from returning to the condition of a hydrosol and thus passing through the filter. Wash until free from chlorides, testing the washings with silver nitrate. Allow the precipitate to drain, then remove the paper, fold over the top and sides (see page 31), place on a watch glass and dry in an oven at 100. When the precipitate and paper are completely dried, place a piece of black glazed paper on the desk, unfold the filter paper and carefully EXPERIMENTAL GRAVIMETRIC ANALYSIS 71 detach as much as possible of the precipitate, using a spatula for this purpose and allowing the precipitate to fall upon the central portion of the glazed paper. While it is desirable to leave as little as possible of the precipitate upon the filter paper it is also essential that no paper fiber be removed with the main portion of the precipitate since this portion is not to be treated to reconvert reduced silver into silver chloride. With the small camel's hair brush the precipitate is now brushed into a pile, loosening any particles that may have been caught by the brush, and is covered with a watch glass. An ignited and weighed crucible is placed upon one corner of the glazed paper. The filter paper is refolded in the same manner as before, is rolled into a tight roll and a stiff plati- num wire is coiled around it in a manner such that the roll can be held over the crucible by means of the wire. Being held in this position it is touched with the oxidizing flame of the gas burner until the paper ignites. The gas flame is to be used only often enough to keep the paper ignited and the outer oxidizing portion of the flame is always to be used for this purpose. The paper is thus burned, the ash falling into the crucible, where the combustion is completed at a low temperature. Some silver has been reduced even with these precautions. To change this again into silver chloride the ash is moistened with a drop or two of concentrated nitric acid and, after a few minutes, a drop of concentrated hydrochloric acid is added. The reduced silver is first [dissolved by the nitric acid, forming silver nitrate, and this is changed to silver chloride by the hydrochloric acid. Evaporate the acids by placing the crucible on a water bath, then carefully brush into the crucible the main portion of the precipitate. Heat gently over the burner until the precipitate shows the first appear- ance of fusion where it is in contact with the sides of the crucible. In case the precipitate has been unduly reduced by light or if it becomes reduced when heated, on account of cellulose derived from the paper, it should be moistened by nitric acid and hydrochloric acid, as directed above, and warmed, when it will usually become white, after which the stronger heating to the fusion point may be performed. Place the crucible in the desiccator and weigh as soon as cool. Calculate the per- cent of silver in the salt, using the factor for silver in silver chloride as already calculated in the table of factors on page 8. ALUMINIUM The gravimetric determination of aluminium, as well as of iron, chromium, nickel, cobalt and copper, may be accomplished by precipitating them as hydroxides, igniting and weighing these as oxides. For reasons that will presently be discussed all of these metals excepting aluminium are now usually determined by volumetric or electrolytic processes. 72 QUANTITATIVE ANALYSIS Aluminium is precipitated as hydroxide by any basic solution, whether it be that of a pure base or of a hydrolyzed alkali salt of a weak acid, such as sodium carbonate or ammonium sul- phide. Aluminium hydroxide readily forms hydrosols which are flocculated by the addition of electrolytes, which must for this purpose be inorganic salts. Since the flocculated hydroxide is also of a colloidal nature (hydrogel) it manifests the phenome- non of adsorption to a marked degree and the inorganic salts are consequently washed out with considerable difficulty. For this reason, as well as for other and more important ones, am- monium hydroxide is chosen as the precipitant because the by- products of the reaction will thereby be ammonium salts and re- maining traces will be volatilized when the precipitate is heated. Aluminium hydroxide, besides being soluble as a hydrosol, also dissolves in solutions of bases. It thus happens that if an excess of the basic precipitant is inadvertently added, part or all of the precipitate returns to the solution, the amount dis- solved depending upon the excess and ionization of precipitant. The strong bases, as sodium or potassium hydroxide, dissolve aluminium hydroxide more readily than the weaker ones and this furnishes a second reason for the use of the weaker base, ammonium hydroxide, as the precipitating reagent. In case an excess of this has been added it is possible to remove it by boiling. The solvent action of bases has been explained upon the sup- posed ability of aluminium hydroxide to exist in both acid and basic form. When precipitation is taking place, the solution, besides holding more or less of the hydrosol, is saturated also with the substance in the condition of molecular aluminium hydroxide in equilibrium with two sets of ions. This equilib- rium within the solution might be expressed simply thus: A1 + +3 OH H + H 2 Afo 3 . , (If the substance is an acid it must ionize in three stages: A1(OH) 3 H+H 2 A1O 3 , H 2 A10 3 -*H+HA1O 3 , HA1O 3 H+A1O 3 . EXPERIMENTAL GRAVIMETRIC ANALYSIS 73 Since it must necessarily be very weakly ionized the ion H 2 A1O 3 must predominate, but the equilibrium represented above can be but an approximate representation of the real conditions.) The addition of either a strong base or a strong acid to such a system would cause aluminium hydroxide to dissolve if the resulting salt were soluble. The effect of the strong acid upon the acid form would be that of suppression of the (already small) ionization. Its effect upon the basic form would be interaction to form a salt: A1(OH) 3 +3HC1-A1C1 3 +3H 2 O. The disturbance of equilibrium resulting from the disappearance of hydroxyl ions (due to the formation of water) would cause more aluminium hydroxide to ionize in this manner and, since unionized aluminium hydroxide is also in equilibrium with the undissolved portion, more would go into solution. The effect of a base would be quite similar to that of an acid, although by a different process and through the formation of a different salt. The basic ionization of the aluminium hydroxide would be suppressed while the added base would react with the acid form: H 3 AlO 3 +NaOH NaH 2 A10 3 +H 2 0. Here again a salt is formed, although the aluminium appears in the anion. The disturbance of equilibrium has the same ultimate result as before, namely, that the solid substance passes into solution. The foregoing explanation of the solvent action of both bases and acids upon aluminium hydroxide is valid if the hypothesis that the latter is an amphoter is correct. It should be noted that there are still some objections to this hypothesis which remain to be satisfied. 1 It has already been stated that the washing of the gelatinous precipitate is more or less difficult on account of the adsorption of dissolved salts. It is impossible to avoid the presence of such salts when making separations or when precipitating aluminium from such compounds as the alums. (Ammonium salts are always present.) There is also danger that, in the case of prolonged washing by water to remove alkali salts or other salts, 1 J. Am. Chem. Soc., 36, 30 (1913). 74 QUANTITATIVE ANALYSIS some of the hydrogel may return to the condition of the hydrosol. In order to prevent this the customary device of having present an ammonium salt in the wash water is used. Some of this necessarily remains with the precipitate at the last. If this salt is ammonium chloride some of the aluminium will be lost by volatilization, the chloride being, as with most other metals, more volatile than the salts of other acids. The chloride will be formed during ignition by interaction of the aluminium oxide or hydroxide and the ammonium chloride: A1(OH) 3 +3NH 4 C1-* A1C1 3 +3NH 3 +3H 2 O. Ammonium nitrate should therefore be used in the wash water. Aluminium nitrate, if formed, is decomposed at high temperatures into aluminium oxide and oxides of nitrogen. If the precipitate of aluminium hydroxide is filtered and washed under diminished pressure, care should be exercised that the liquid is not, at any time before the completion of the washing process, drawn out so nearly completely as that the precipitate should harden and crack. In such a case the wash water that is subsequently used will run through the cracks instead of through the body of the precipitate and complete washing will therefore be accomplished only after the use of much water. If it becomes necessary to allow the precipitate to remain in the funnel from one day to the next and before the washing is completed, the precipitate may be kept moist by plugging the stem of the funnel, covering the precipitate with water, and placing a watch glass over the top. By making suitable changes in the procedure, aluminium chloride might be made a reagent for the quantitative determina- tion of hydroxyl. Volumetric methods are always used instead. Determination. Fill a weighing bottle with the powdered sample of an aluminium salt. Choose the method to be used in weighing according to the nature of the substance and weigh two samples of about 1 gm each into Jena beakers. Dissolve in 100 cc of water and add dilute, recently filtered ammonium hydroxide, stirring until the liquid is distinctly basic, as shown by a bit of litmus paper thrown into the beaker. Boil until the precipitate is coagulated and until the odor of ammonia above the solution is but faint. Boiling after the odor has nearly disappeared may cause some of the precipitate to return to the solution: A1(OH)3+3NH 4 C1-A1C13+3NH3+3H 2 0. EXPERIMENTAL GRAVIMETRIC ANALYSIS 75 Allow the precipitate to settle and then filter through paper, using a filter pump attached to a bell jar or filter flask and placing a supporting cone of hardened paper or platinum in the funnel (page 21). Wash with hot distilled water containing 1 percent of ammonium nitrate until the washings are free from chlorides. Suck the precipitate as nearly dry as possible and transfer the paper and precipitate to a porcelain or plati- num crucible which has been ignited and weighed, folding the paper in the manner already learned. Heat very gently in the covered crucible until the moisture is volatil- ized, then raise the temperature and burn the paper, inclining the cru- cible and placing the cover as in the case of the ignition of the paper containing calcium oxalate. When all of the carbon has been burned, cover the crucible and heat over the blast lamp for 30 minutes. Cool in the desiccator and weigh. Heat again for 10 minutes, cool and weigh. If necessary repeat this process until the weight is constant. Calculate the percent of aluminium in the salt. Aluminium oxide absorbs water from the air, reforming the hydroxide with a corresponding gain in weight. On this account the crucible and oxide should be weighed rapidly. Copper, cobalt and nickel cannot be quantitatively precipi- tated by ammonium hydroxide because of the formation of soluble complex ammonium salts. Sodium hydroxide or potas- sium hydroxide is used as the reagent. Adsorption of the reagent by the precipitate causes a large error and volumetric or electrolytic methods are preferable. BARIUM AND SULPHURIC ACID Barium may be precipitated as sulphate, carbonate or chro- mate. The sulphate and chromate are weighed as such, while the carbonate is ignited to form the oxide, in which form it is weighed. The solubilities are as follows (determined by Kohlrausch and Rose 1 ). Milligrams per liter soluble at 18 Salt Barium equivalent to salt BaCO 3 . . . 22. 15.3 BaSO 4 2.6 1.53 BaCrO 4 3.8 2.06 The sulphate is seen to be the most suitable compound for the separation of barium from solution. The precipitating rea- 1 Z. physik. Chem., 12, 234 (1893). 76 QUANTITATIVE ANALYSIS gent may be sulphuric acid or a soluble alkali sulphate. Since the latter produces by the reaction alkali salts that must be washed from the precipitate, while the former produces volatile acids, sulphuric acid is generally used for the purpose: BaSO 4 +2NaNO 3 . 4 -> BaS0 4 +2HNO 3 . Barium sulphate is appreciably soluble in hydrochloric acid as is shown in the following table: 1 Solubility of barium sulphate, milligrams per liter Hydrochloric acid Barium sulphate Barium equivalent to barium sulphate 1.82 3.65 7.27 18.23 6.7 8.9 10.9 8.6 3.9 5.2 6.4 5.1 If hydrochloric acid is present in the solution it must be nearly neutralized before precipitating the barium sulphate, not only because of its solvent action above shown but also because of its tendency to increase the occlusion of other salts by barium sulphate. 2 Barium sulphate easily precipitates in the form of fine crystals. If precipitation takes place rapidly and from a somewhat con- centrated solution the crystals may be so small as to pass through the filter. Remedies similar to those applied to calcium oxalate may be used also with barium sulphate. These are use of a dense paper for the filter, precipitating slowly from a hot solu- tion and digestion of the precipitate in the mother liquor at a temperature near the boiling point. Sometimes the filter paper is treated before using with a hot, concentrated solution of ammonium chloride which softens and swells the cellulose fibers, making a less permeable filter. Such treatment is of doubtful utility since the ammonium chloride must later be washed out of the paper or cause some volatilization of barium chloride when the precipitate is ignited. No trouble will be experienced if the precipitation is accomplished under proper conditions. The converse of this method may be used for the determina- 1 Banthisch: J. pr. Chem., 29, 54 (1884). 2 Richards: Z. anorg. Chem., 8, 413 (1895). EXPERIMENTAL GRAVIMETRIC ANALYSIS 77 tion of the sulphate radical and for sulphur of any compound that may be changed ,to a sulphate. Barium chloride is then the precipitating reagent and the solution is made slightly acid by adding hydrochloric acid. The latter is necessary to prevent the precipitation of barium salts of certain other acids whose salts might be present. Examples of such salts are carbonates, oxalates, phosphates and borates, the barium salts of all of these being insoluble in water but soluble in hydrochloric acid. Determination. Weigh about 0.2 gm of a barium salt into each of two beakers, dissolve as in the case of calcium salts and heat to boiling. Add, drop by drop, with vigorous stirring, 2 cc of 25 percent sulphuric acid. Allow the precipitate to settle somewhat and test the supernatant liquid, as usual, to determine whether precipitation is complete. Digest at a temperature near the boiling point for 15 minutes or longer, until the precipitate settles readily. Filter without the use of a pump, on a paper of the general grade of the Schleicher and Schull No. 589 " white ribbon" paper (the "blue ribbon" paper may be used if the precipitate is extremely fine) and wash several times with hot, distilled water. Remove the paper from the funnel, fold and place in a weighed cru- cible. Incline the crucible as usual for burning the paper and heat until white. Cover the crucible and heat. over the blast lamp for 15 minutes, cool and weigh. Since no decomposition of the precipitate takes place when it is heated there should be no change in weight after the first few minutes of heating unless washing has not been thorough, leaving salts that slowly volatilize. It is well to test this point by further heating and weighing. Calculate the percent of barium in the barium salt, using the factor for barium in barium sulphate. STRONTIUM Strontium is best determined as sulphate, precipitating from a solution containing alcohol and an excess of dilute sulphuric acid. Its solubility in water at 18 is 114 mg per liter. 1 The solubility is considerably diminished by a small excess of sul- phuric acid and in 50 percent aqueous alcohol the solubility is very slight, although no definite figures are now available. Determination. Weigh the proper quantity of strontium salt to produce 0.2 to 0.3 gm of strontium sulphate. If soluble in water dis- solve in 50 cc of water and add to the solution 60 cc of alcohol. If insoluble in water dissolve in hydrochloric acid, evaporate in a cas- 1 Kohlrausch: Z. physik. Chem., 60, 355 (1905). 78 QUANTITATIVE ANALYSIS serole to expel excess of acid and dilute the solution to 50 cc, then add 60 cc of alcohol. Add, slowly and with stirring, dilute sulphuric acid until precipitation is complete. An excess of about 5 cc is desirable. Stir for some time then allow to stand for 12 hours. Filter and wash twice with 50 percent alcohol containing a few drops of dilute sulphuric acid, then with 50 percent alcohol until the washings fail to give a test for sulphates. Ignite in a weighed crucible at as low a temperature as possible until white. Weigh the strontium sulphate and calculate the percent of strontium in the sample. The separation of barium, strontium and calcium is accom- plished by converting all of the metals into nitrates, evaporating to dryness and taking up with a mixture of alcohol and ether. Calcium nitrate dissolves and the calcium is precipitated as oxalate after evaporating the alcohol and ether and dissolving the residue in water. The barium and strontium nitrates are dissolved in water, barium is precipitated as chromate and stron- tium as sulphate as described above. POTASSIUM AND SODIUM Potassium may be separated from sodium and determined as perchlorate or as chlorplatinate. It may also be precipitated as potassium sodium cobaltinitrite and this determined by a volumetric process or dissolved and the potassium later deter- mined by the other gravimetric methods. It is often stated that it can also be determined by weighing as sulphate or as chloride. Inasmuch as the latter two methods involve no separa- tion by precipitation and filtration but simply conversion of the potassium into a form other than the one in which it formerly existed and since any other metals that might be present would also be converted into sulphates or chlorides and weighed with the potassium, the value of these methods is not apparent. Of the first two methods the perchlorate method has the advan- tage of cheapness, while the chlorplatinate method is more con- venient and probably more accurate. The perchlorate method is based upon the fact that potassium perchlorate is almost insoluble in 97 percent ethyl alcohol, while sodium perchlorate dissolves with greater ease. It involves the use of an aqueous solution of perchloric acid, the preparation of which is somewhat troublesome and dangerous. EXPERIMENTAL GRAVIMETRIC ANALYSIS 79 The solubility of potassium perchlorate in alcohol of various concentrations is as follows: 1 Concentration of alcohol, percent by weight Grams, potassium perchlorate in one liter of solvent Grams, potassium equivalent to potassium perchlorate 97.2 95.8 90.0 0.156 0.20 0.36 0.044 0.06 0.10 The solubility is considerably diminished by an excess of per- chloric acid. Sodium perchlorate dissolves easily in alcohol although no definite data are on record. The chlorplatinate method has been varied in matters of detail but essentially it consists in precipitating potassium chlorplati- nate from an alcoholic solution by adding chlorplatinic acid. K 2 S0 4 +H 2 PtCl 6 K 2 PtCl 6 +H 2 S0 4 . Compounds of sodium and magnesium are not so precipitated and potassium is separated from these by filtration but other metals must be absent because of the small solubility of most chlorplatinates. Potassium chlorplatinate may be either weighed as such or ignited in a current of hydrogen: K 2 PtCl 6 +2H 2 ->2KCl+Pt+4HCl. The potassium chloride is washed out of the reduced platinum which is then weighed. In practice potassium chlorplatinate is usually weighed without ignition. The separation from alcoholic solution can be accomplished only under certain conditions, owing to the small solubilities of certain other compounds that may be present. Sodium chlor- platinate is easily soluble in alcohol but ammonium chlorplatinate is soluble to a very slight extent. Ammonium compounds must therefore be volatilized by heating, before the addition of chlor- platinic acid. Sodium chloride and sulphate are nearly insoluble in alcohol, and must be changed into more soluble substances before a separation of sodium and potassium can be accomplished. The following table will serve to show the nature of the questions that must here be met. There is considerable disagreement between the results as obtained by different investigators but these figures may be regarded as at least approximately correct. 1 Wenze: Z. angew. Chem., 6, 691 (1891). 80 QUANTITATIVE ANALYSIS Percent alcohol in water Solubility, grams salt per liter of solvent Sodium sulphate 1 0.7 127.0 19.4 26.0 72.0 <0.001 Sodium chloride 80 about 5 Ammonium chlorplatinate 2 55 0.150 76 0.067 95 0.0037 Potassium chlorplatinate 3 7.742 10 3.72 20 2.18 30 1.34 40 0.76 50 0.491 60 0.265 70 0.128 80 0.085 90 0.025 100 0.009 The method of Lindo 4 consisted in obtaining a solution con- taining chlorides of no other metals than sodium and potassium, adding sufficient chlorplatinic acid to convert all of the chlorides into chlorplatinates, evaporating nearly to dryness and adding strong alcohol. Sodium chlorplatinate dissolves and is separated from the potassium chlorplatinate by filtration. The potassium salt is washed with alcohol and weighed or treated as already de- scribed. Lindo also showed that the solubility of very fine crystals of potassium chlorplatinate is greater, than that of larger crystals (cf. p. 19). Using this method, it was necessary that no sulphate should be present because of the very slight solubility of sodium sul- phate in alcohol. The discussion of the laws of precipitation (pages 13 et seq.) will make it clear that even if enough chlor- platinic acid were present to combine with all of the sodium present to form sodium chlorplatinate, the less soluble sodium sulphate would still be precipitated. Even when all of the metals are present as chlorides there is without doubt some contamination 1 de Bruyn: Z. physik. Chem., 32, 63 (1900). 2 Fresenius: Z. anal. Chem., 36, 322 (1897). 3 Archibald, Wilcox and Buckley: J. Am. Chem. Soc., 30, 747 (1908). 4 Chem. News, 44, 77, 86, 97 and 129 (1881). EXPERIMENTAL GRAVIMETRIC ANALYSIS 81 of the precipitated potassium chlorplatinate by sodium chloride. It was early observed 1 that high results would be obtained by this method for determining potassium if the factor for potassium in potassium chlorplatinate were calculated by the use of the atomic weight of platinum as determined by Seubert 2 (194.8) or even 195.2, which is that given in the table of atomic weights for 1913, while a factor calculated from the atomic weight 197.2, which had previously been accepted, gave correct results. This may be partly due 3 to the fact that the precipitated potassium chlorplatinate contains also some compounds with composition as represented by such a formula as H 2 PtCl 5 OH or H 2 PtCl 4 O. It is undoubtedly also partly due to the presence of some sodium chloride in the precipitate. On this account the method has been modified by using 80 percent alcohol instead of absolute alcohol. Reference to the solubility table above will show that alcohol of this concentration will dissolve potassium chlorplatinate to a greater extent than does absolute alcohol and this negative error seems to practically balance the positive error discussed. The fact that volatile acids, organic compounds, ammonium salts, etc., can be easily volatilized by heating makes it desirable to obtain the sodium and potassium in a form in which they can be heated to redness without danger of loss. Both chlorides are sensibly volatile at such temperatures while the sulphates are not but, as already stated, sodium sulphate must not be al- lowed to form because of its small solubility in alcohol. To meet this difficulty Gladding suggested 4 a further modification of the Lindo method in which the sodium sulphate was to be washed out, after {he removal of the excess of chlorplatinic acid, by a water solution of ammonium chloride. The solubility of sodium sulphate in water is considerably increased by the pres- ence of ammonium chloride. This follows the general law that *the addition of an electrolyte which does not contain an ion in common with the first electrolyte increases its solubility. To avoid loss of potassium chlorplatinate the washing liquid is previously saturated with the pure salt. In using such a solution it is important that no great change in temperature shall occur in 1 Dittmar and McArthur: J. Soc. Chem. Ind., 6, 799 (1887). 2 Ann., 207, 1 (1881). 3 Dittmar and McArthur: loc. cit. 4 U. S. Dept. of Agr., Chem. Bull., 7, 38. 6 82 QUANTITATIVE ANALYSIS the solution after it is withdrawn from the bottle and before it is used for washing the precipitate. The solution is kept saturated by an excess of potassium chlorplatinate in the bottle. If the temperature should rise the solution which was saturated in the bottle now becomes undersaturated and will dissolve some of the precipitate on the filter. On the other hand, if the funnel is at a much lower temperature than the reagent bottle (due to working in a colder part of the room) it will cool the solution and cause a deposition of potassium chlorplatinate upon the precipitate already present. This modified method, known as the Lindo-Gladding method, is now used quite generally, particu- larly for the determination of potassium (and indirectly of sodium) in industrial products, minerals, etc. If it is desired to accomplish the removal of ammonium salts and organic matter by ignition but to avoid the use of the am- monium chloride washing solution the sulphates, first obtained by evaporating with sulphuric acid and igniting, are dissolved and precipitated by barium chloride which precipitates barium sulphate and leaves sodium chloride and potassium chloride in solution. The excess of barium is then precipitated by sulphuric acid. There is no method known for the direct determination of sodium if we exclude the weighing as sulphate or chloride, methods of very limited usefulness. This is because no sodium compound has sufficiently small solubility to make possible its separation from the corresponding salt of potassium. Sodium is usually determined by weighing it with potassium in the form of sulphate or chloride, determining potassium and calculating sodium by difference. It should be noted that such a method throws all of the errors of the potassium determination upon that of sodium, in addition to any errors that may have occurred in the weighing of the combined chlorides or sulphates. Chlorplatinic acid is also used as a reagent for the quantita- tive determination of the ammonium radical but potassium must be absent. On account of the difficulty experienced in the removal of potassium from ammonium the latter is more conveniently determined by volumetric methods. The converse of the Lindo method for potassium is used for the determination of platinum. Either ammonium chloride or potassium chloride may be used as the reagent but the former is EXPERIMENTAL GRAVIMETRIC ANALYSIS 83 generally used because of its greater solubility in alcohol, which makes the removal of the excess of reagent more easy. Determination by the Lindo-Gladding Method. (To be performed in an atmosphere which is free from ammonia) . Use portions of about 0.3 gm of a sample containing salts of potassium and sodium and weigh into small weighed evaporating dishes. Dissolve in a small amount of hot water, add 0.5 cc of concentrated sulphuric acid, evaporate to dryness under the hood, using care to avoid spattering, and ignite at bright red- ness until no more white fumes are evolved and the residue is white. Cool, weigh and ignite again to constant weight. Record the weight of sulphates of sodium and potassium. Dissolve the resulting potassium sulphate (mixed with sodium sulphate) in 50 cc of hot water, add 5 drops of concentrated hydrochloric acid and then add chlorplatinic acid, using about 1 cc more than the theoretical amount, calculated upon the assump- tion that the original salt was potassium chloride. Evaporate on the water bath to a thick paste but not to dryness, cool and add 50 cc of 80 percent alcohol, stir up the solid matter and allow to stand, covered, for 30 minutes. If the liquid is not visibly colored too little reagent has been used. In this case new samples should be taken and the quantity of chlor- platinic acid increased. Filter and wash the precipitate thoroughly with 80 percent alcohol, washing several times after the washings pass through colorless. The wash bottle should be provided with ground- glass joints so that no rubber will come into contact with the alcohol. Remove the filtrate and washings, pouring these into the bottle provided for platinum waste residues, and wash the precipitate with five or six portions of 10 cc each of 10 percent ammonium chloride solution which is saturated with potassium chlorplatinate. Wash again, thor- oughly, with 80 percent alcohol, using particular care in washing the upper part of the paper free from ammonium chloride. Wash until only a faint turbidity is produced by the addition of a drop of silver nitrate to the last washings. Drain most of the alcohol from the paper, slip the latter out of the funnel and dry in the oven. Place a weighed porcelain crucible upon a piece of glazed paper, remove most of the precipitate to the crucible, brushing up any particles that may have fallen upon the glazed paper, and then replace the paper in the funnel. Place the crucible under the funnel and dissolve the remainder of the precipitate in the smallest amount of nearly boiling water, allowing the solution to run into the crucible. Evaporate to dryness on the water bath, carefully wipe the outside of the crucible with a clean towel and dry for 30 minutes at a temperature between 100 and 105. Weigh and calculate the percent of potassium in the salt analyzed. Calculate also the weight of po- 84 QUANTITATIVE ANALYSIS tassium sulphate, subtract this weight from that of the mixed sulphates, and from the remainder calculate the percent of sodium. Optional Method, Using a Gooch Crucible. Proceed as above until ready to filter out the potassium chlorplatinate. Prepare two Gooch filters as directed on page 21, paying attention to the precautions sug- gested, and using strong suction in forming the asbestos felt. Rinse the crucible with alcohol, remove, wipe the outside and dry at 100 to 105 for 30 minutes or until the weight is constant. Weigh and replace in the holder. Before the suction pump is again turned on moisten the asbestos 'with one or two drops of water. Start the pump and filter and wash the precipitate exactly as above directed. Remove the crucible, dry in the oven and weigh. Calculate potassium and sodium as before. In the foregoing exercise the procedure is based upon the as- sumption that sodium or ammonium salts or both may be pres- ent. The latter are volatilized by heating with sulphuric acid. The former are removed by the ammonium chloride solution. Recovery of Platinum from Waste and the Preparation of the Reagent, Chlorplatinic Acid. The following is essentially the method recommended by Wiley. 1 Waste Solutions. Add alcohol if not already present. Make basic with sodium carbonate and boil until reduced. If any solid potassium chlorplatinate is to be added this must be done gradually while boiling. Decant from the reduced platinum, add hydrochloric acid (1:1), boil, decant and repeat the treatment with acid. Wash by decantation with hot water and ignite to destroy organic matter. Heat nearly to boiling with concentrated nitric acid and decant. Dissolve the platinum by adding four times its weight of concentrated nitric acid, heating on the water bath and adding concentrated hydrochloric acid gradually. Evaporate until a drop removed on a stirring rod crystallizes on cooling. Add water, filter and dilute so that 1 cc shall contain 0.1 gm of plati- num. The specific gravity of the solution should be 1.18. Scrap. Dissolve in nitric acid and hydrochloric acid as above, evaporate, dilute and precipitate platinum as ammonium chlorplatinate to separate from iridium. Reduce the precipitate by alcohol in basic solution and proceed as above. A method for the recovery of platinum from scrap by electrol- ysis is described by Weber. 2 Chlorplatinic acid prepared by this method is quite free from traces of nitric acid. 1 Principles and Practice of Agricultural Analysis, Vol. II, 287. 2 J. Am. Chem. Soc., 30, 29 (1908). EXPERIMENTAL GRAVIMETRIC ANALYSIS 85 MAGNESIUM AND PHOSPHORIC ACID The determination of magnesium is usually made by pre- cipitating from a basic solution as magnesium ammonium orthophosphate. This is ignited and weighed as magnesium pyrophosphate. The reactions may be expressed thus: MgCl 2 +NH 4 OH+Na 2 HP0 4 MgNH 4 PO 4 +2NaCl+H 2 O, 2MgNH 4 P0 4 Mg 2 P 2 7 +2NH 3 +H 2 0. No other metals than those of the alkali group may be present, as the phosphates of practically all others are insoluble in am- monium hydroxide. Any soluble phosphate may be used as the precipitating reagent but the ones most used in practice are disodium orthophosphate and sodium ammonium acid ortho- phosphate (microcosmic salt). The following tabular statement from the work of Ebermayer 1 shows the solubility of crystallized magnesium ammonium ortho- phosphate in mixtures of ammonium hydroxide and water. Percent by volume of Grams of . 11 i TVT ATTT r>r\ ^TT ^ Equivalent grams of ammonium hydroxide of | MgNH 4 PO 4 .6H 2 O _ M * -i .... f , ,_ Mg per liter of solvent sp. gr. 0.96 per liter of solvent at 15 0.074 0.0072 25 0.027 0.0026 50 0.023 0.0022 75 0.019 0.0018 This statement of the solubility of the precipitate in solutions containing various concentrations of ammonium hydroxide would lead to the conclusion that precipitation from the more concentrated solutions of ammonium hydroxide would result in greater accuracy because of the small solubility of magnesium ammonium orthophosphate. From this standpoint alone the conclusion would be correct. It happens, however, that the basicity of the solution, as well as the presence of other salts, has an important influence upon the composition of the precipitate. The decrease of solubility with increasing concentrations of ammonium hydroxide and also of ammonium salts is to be expected as a consequence of the mass law since these substances increase the concentration of the ion NH 4 , a constituent of the precipitate. Substances other than monammonium magne- 1 J. prakt. Chem., 60, 41 (1853). 86 QUANTITATIVE ANALYSIS slum orthophosphate ar^ precipitated to some extent under the following conditions: If the solution is strongly basic when the reagent is added there is formed some normal magnesium orthophosphate, Mg 3 (P0 4 ) 2 , and the quantity of this substance is increased by slow addition of the reagent. This is not decomposed upon heating and the ignited precipitate is not all magnesium pyrophosphate. This fact makes it undesirable that too much ammonium hy- droxide should be present, even though the solubility of the precipitate is lessened thereby. The solubility of magnesium monammonium orthophosphate is much less than that given by Ebermayer, according to the work of Bube 1 who states that the saturated solution in pure water contains about 0.00014 gm in 1000 cc. Bube also states that in such a solution the solubility product of normal magnesium orthophosphate is far exceeded and that the solubility of magnesium ammonium orthophosphate is increased by large concentrations of ammonium ions. This would probably account for the increased precipitation of magnesium ortho- phosphate in solutions made strongly basic by ammonium hydrox- ide, the magnesium ammonium salt changing into magnesium phosphate and ammonium phosphate, the magnesium salt precipitating : 3MgNH 4 P0 4 -Mg 3 (P0 4 )2+(NH 4 ) 3 P0 4 . If the solution contains excessive quantities of ammonium salts, whether the precipitation takes place from a strongly basic or weakly basic solution the magnesium monammonium ortho- phosphate will contain certain quantities of magnesium tetram- monium orthophosphate, Mg(NH 4 ) 4 (P0 4 ) 2 . This substance, when strongly heated, passes into magnesium metaphosphate, Mg(P03)2, a substance which can be converted into magnesium pyrophosphate only after prolonged heating at high temperatures (2Mg(P0 3 ) 2 'Mg2P 2 O7+P2O 5 ). Ammonium salts should thus be nearly or entirely absent. If they have accumulated in the solution as a result of the use of ammonium hydroxide in the separation of other metals, they should be removed before precipitation, by (a) evaporating to dryness and heating strongly or (b) evaporating to small volume and heating with concen- 1 Z. anal. Chem., 49, 525 (1910). EXPERIMENTAL GRAVIMETRIC ANALYSIS 87 trated nitric acid or (c) performing a double precipitation, dissolv- ing the first impure precipitate in hydrochloric acid and re- precipitating. Method (a) or (c) is to be preferred. Most chemists prefer to precipitate magnesium from a cold solution although Gibbs 1 recommends a boiling solution. Whether the cold or hot solution is used one of two procedures may be followed in order to conform to the principles outlined above. The entire amount of disodium phosphate solution may be added at once to an acid solution and then dilute ammo- nium hydroxide slowly added until the solution is basic. After standing a short time most of the precipitate will form and the remaining magnesium can be precipitated by the addition of concentrated ammonium hydroxide. Instead of following this method the solution may be made neutral or faintly basic and disodium phosphate added slowly, thus precipitating nearly all of the substance, when concentrated ammonium hydroxide may be added as before. The second method is recommended. The effect of excessive concentrations of ammonium salts upon the composition of the double phosphate of magnesium and ammonium is also to be expected as a consequence of the mass law. The reason for the difference in the mode of decom- position of the precipitate containing more of the ammonium radical from that of the monammonium magnesium salt is apparent when the properties of the three phosphoric acids and of the salts are examined. Phosphorus pentoxide, by combining with different propor- tions of water, gives rise to three different acids: P 2 05+ H 2 ^2HPOs, metaphosphoric acid, P 2 O5+2H 2 'H 4 P 2 07, pyrophosphoric acid, P 2 O 5 +3H 2 2H 3 P0 4 , orthophosphoric acid. Either metaphosphoric or pyrophosphoric acid will be trans- formed into the one containing more water if allowed to stand in solution. Also the acids may be changed in the opposite sense by heating. At about 213 orthophosphoric acid loses water and yields pyrophosphoric acid. 2H 3 P0 4 ->H 4 P 2 O 7 +H 2 O. iAm. J. Sci., [3] 6, 114. 88 QUANTITATIVE ANALYSIS At about 400 pyrophosphoric acid loses one molecule of water arid yields metaphosphoric acid. H 4 P 2 7 2HP0 3 +H 2 0. When heated to higher temperatures the remaining molecule of water is lost and phosphorus pentoxide remains. 2HP0 3 P 2 5 +H 2 0. Phosphorus pentoxide is thus seen to be the final product of any of the three acids when the acid is heated to a high temperature and this is because a volatile substance (water) is produced by heating. Just as the acids are compounds of phosphorus pent- oxide and water, so the salts may be regarded as compounds of phosphorus pentoxide and metallic oxide (which is analogous to hydrogen oxide). Consequently the extent to which the salts may be decomposed by heating will be conditioned by the nature of the metallic oxide or, in other words, by its degree of volatility. Thus the normal phosphates of sodium, potassium, magnesium, calcium, etc., are not decomposable at all, except at extremely high temperatures where phosphorus pentoxide begins to be volatile, while the acid phosphates of these metals are decomposable to whatever extent is denoted by the propor- tion of water that may be formed. Ammonium salts are con- verted completely into phosphorus pentoxide because, instead of the hypothetical metallic oxide ; (NH 4 ) 2 0, there are formed ammonia and water and both of these substances are volatile. Orthophosphoric and pyrophosphoric acids are polybasic and a considerable variety of salts may be prepared, containing varying amounts of metals, ammonium and hydrogen, so that they may be regarded as containing varying amounts of metallic oxide, ammonia, water and phosphorus pentoxide. The composition and decomposition of the three phosphoric acids and typical examples of their salts are shown in the follow- ing statement: Composition Decomposition by Heat Acids 2HPOs =D=H 2 O.P 2 O 6 H 4 P 2 7 =0= 2H 2 O.P 2 O 6 EXPERIMENTAL GRAVIMETRIC ANALYSIS ' 89 Composition | Decomposition by Heat Normal Potassium Salts 2KPOs K 4 P 2 O 7 =0= 2K 2 O.P 2 S 2K 3 P0 4 <^3K 2 O.P 2 O 6 Not decomposed except by slow loss of P 2 O6 at high temperatures Potassium Acid Salts No acid metaphosphate possible K 2 H 2 P 2 O 7 =C: K 2 O.H 2 O.P 2 O6 2K 2 HP0 4 =0= 2K 2 O.H 2 O.P 2 6 2KH 2 P0 4 =0= K 2 O.2H 2 O.P 2 6 K 2 H 2 P2O 7 -2KP0 3 + H 2 O 2K 2 HP0 4 ^K 4 P 2 7 + H 2 Potassium Ammonium Salts No double metaphosphate possible K 2 (NH 4 ) 2 P 2 7 K 2 O.(NH 4 ) 2 O.P 2 5 2K 2 NH 4 P0 4 2K 2 O.(NH 4 )2O.P2O 8 2K(NH 4 ) 2 P0 4 =C=K 2 O.2(NH 4 ) 2 O.P 2 O 6 K 2 (NH 4 ) 2 P2O 7 ->2KPO 3 + 2NH 3 -t-H2O 2K 2 NH 4 PO 4 ->K 4 P 2 O 7 + 2NH 3 + H 2 O Magnesium tetrammonium orthophosphate, Mg(NH 4 ) 4 (PO 4 ) 2 , is analogous to monopotassium diammonium orthophosphate, K(NH 4 ) 2 PO 4 , as may be seen from the structural formulae : K\ NH 4 NHAP0 4 and NHAPO 4 NH 4 / Mg / NHAP0 4 / Its decomposition can therefore proceed as far as magnesium metaphosphate : Mg(NH 4 ) 4 (P0 4 ) 2 Mg(P0 3 )2+4NH 3 +2H 2 0. while monammonium magnesium orthophosphate can decom- pose only as far as the pyrophosphate : 2MgNH 4 P0 4 Mg 2 P 2 7 + 2NH 3 + H 2 O . This makes it necessary that such conditions shall be maintained as will make possible the formation of but one double salt, in order that the composition of the ignited precipitate may be definite and constant. Bube 1 does not believe that magnesium tetrammonium ortho- phosphate is precipitated at all or that magnesium metaphos- phate is found in the ignited precipitate but that the partial precipitation of ammonium phosphate, which leaves a residue of phosphorus pentoxide when ignited, has been mistaken for 1 LOG. cit. 90 QUANTITATIVE ANALYSIS the formation of magnesium tetrammonium orthophosphate and, later, of magnesium metaphosphate. The complete precipitation of magnesium monammonium orthophosphate takes place only after standing for some time. Formerly it was considered necessary to allow 24 hours for the action to proceed. It is now generally thought that from 2 to 3 hours is sufficient for the precipitation of all but a minute amount, negligible under ordinary circumstances. The usual method of testing with excess of reagent, to determine whether precipitation is complete, is rendered useless because of the slow crystallization of the precipitate unless several hours are allowed for the possible precipitation of small amounts. The crystalline precipitate may be readily filtered and washed by a dilute solu- tion of ammonium hydroxide or ammonium nitrate. The pre- cipitate is appreciably soluble in distilled water. An application of the laws of solubility, discussed under the head of " Precipi- tation" (page 13) would lead to the conclusion that any one of the three classes of soluble compounds: phosphates, ammonium salts or magnesium salts, will lessen the solubility of ammonium magnesium phosphate since the latter dissociates into the three ions NH4, Mg and PC>4. The addition of magnesium salts to the washing fluid is clearly out of the question if magnesium is to be determined. Phosphates must themselves be removed by washing because only the ammonium phosphates are entirely volatile and these only with some difficulty. Either ammon- ium hydroxide or ammonium nitrate is suitable for the purpose, excess of either being driven off during drying and ignition of the precipitate. Considerable difficulty is often experienced in obtaining pure, white magnesium pyrophosphate by igniting the ammonium magnesium orthophosphate. This is usually due to imperfect washing, sodium phosphate being left in the precipitate. Upon heating, traces of the salt cause partial fusion, particles of carbon being enclosed and oxidation made difficult. Thorough wash- ing followed by long heating at high temperatures is the only remedy^ This method, suitably modified, will be used later (see Analysis of Iron and Steel, also of Fertilizers) for the determination of phosphorus and of the orthophosphoric acid radical. In this case the reagent will be a soluble magnesium salt in the form of EXPERIMENTAL GRAVIMETRIC ANALYSIS 91 " magnesia mixture " which is a solution containing magnesium chloride to which has been added ammonium hydroxide and ammonium chloride, the latter to prevent precipitation of mag- nesium hydroxide. The method may also be adapted to the determination of arsenic acid, the precipitate of magnesium ammonium arsenate being heated at a moderate temperature until it forms magnesium pyroarsenate : 2MgNH 4 AsO4-Mg 2 As 2 O7+2NH 3 +H 2 O. Determination. Weigh into Jena beakers portions of about 0.3 gm of a magnesium salt. If the salt is soluble in water dissolve in about 100 cc of distilled water and add a drop of dilute hydrochloric acid. If not soluble in water dissolve in hydrochloric acid (1 part of concen- trated acid to 1 part of water), warming if necessary. Cool and drop in a very small piece of litmus paper and then add, slowly and with stirring, dilute ammonium hydroxide until the solution is faintly basic. Now add from a pipette, slowly and with continual stirring, 10 cc of a 10 percent solution of disodium orthophosphate. Allow to stand for 15 minutes until a considerable part of the precipitate has appeared, then add concentrated ammonium hydroxide solution (sp. gr. 0.90), in such quantity that the solution shall finally contain ammonium hydrox- ide equivalent to one-ninth of its total volume. Cover and allow to stand for three hours or stir continually for 30 minutes. Filter the precipitate on a filter of extracted paper or in a platinum Gooch crucible and wash until free from chlorides with a solution con- taining 2 percent of ammonia or 5 percent of ammonium nitrate, test- ing the washings with silver nitrate after acidifying with nitric acid. If a Gooch crucible has been used place the cap on the bottom and heat over the burner until dry, then over the blast lamp for 20 minutes. If a paper filter was used remove the paper from the funnel and, if sufficient precipitate is present to make its removal from the paper feasible, dry and remove most of the precipitate to a sheet of glazed paper, refold the paper and place in a weighed porcelain or platinum crucible. Incline the crucible with the cover leaned against it and heat gently over the burner until the paper is completely burned and the precipitate is nearly white. This is usually a rather slow process but no time will be saved by attempting to burn carbon over the blast lamp. After the precipi- tate is white or gray the crucible is heated for 20 minutes over the blast lamp, cooled in the desiccator and weighed. From the weight of magnesium pyrophosphate calculate that of magnesium and the percent of magnesium in the original sample. 92 QUANTITATIVE ANALYSIS MANGANESE The method of Gibbs for manganese uses the same chemical principles as are involved in the determination of magnesium. A soluble orthophosphate is added to the solution of the man- ganese salt and the solution is then made basic with ammonium hydroxide. Manganese ammonium orthophosphate is precipi- tated and this, when ignited, gives manganese pyrophosphate. 2MnNH 4 PO 4 Mn 2 P 2 O 7 +2NH 3 +H 2 O. If the manganese is already in its lowest state of oxidation, precipitation is accomplished without further change. If it is in the form of a permanganate or of manganese dioxide it is first reduced by sulphurous acid: 2KMn0 4 +5H 2 SO 3 2MnS0 4 +2H 2 SO 4 +K 2 S0 4 +3H 2 O; Mn0 2 +H 2 S0 3 MnS0 4 +H 2 O. Determination. Weigh enough of the sample to contain about 0.1 gm of manganese. If the sample is a permanganate or manganese di- oxide dissolve in 50 cc of a saturated solution of sulphurous acid, con- taining also 1 percent of hydrochloric acid, filter if necessary and boil to expel the excess of sulphur dioxide. If the sample is a soluble manganese salt omit the treatment with sul- phurous acid. In either case proceed as follows: Add to the solution in a Jena beaker, 3 percent more than the quan- tity of 10 percent disodium phosphate solution calculated to be neces- sary for complete precipitation of the manganese. Heat to boiling and add dilute ammonium hydroxide solution, drop by drop with constant stirring, until a precipitate begins to form. Boil and stir until this pre- cipitate becomes crystalline, then add another drop of ammonium hy- droxide and stir and boil until the additional amorphous precipitate becomes crystalline. Continue this process until further addition of ammonium hydroxide produces no precipitate. All of the precipitate should now be in the crystalline condition. Add 0.5 cc excess of ammo- nium hydroxide, then cool the solution by placing the beaker in ice- water. Filter and wash with a clear, slightly basic, 10 percent solution of am- monium nitrate or a 2 percent solution of ammonium hydroxide until free from chlorides, then ignite with the same precautions as were ob- served in the ignition of magnesium ammonium phosphate. From the weight of manganese hyrophosphate obtained calculate the percent of manganese in the sample. EXPERIMENTAL GRAVIMETRIC ANALYSIS 93 CHLORINE, BROMINE AND IODINE The members of the halogen group may occur in different forms, requiring different methods of procedure. This occur- rence may be as free halogens, as oxyacids or salts, as hydracids or salts, or as organic compounds. The gravimetric determina- tion of the negative radical of the halogen hydracids is invariably made by precipitating and weighing the silver salt. The solubilities of the latter, as well as the principles involved in the precipitation, washing and ignition, were discussed under the description of the determination of silver. The procedure is also similar to that involved in the determination of silver, the silver salt (silver nitrate) being, in this case, the precipitant while the chloride, bromide or iodide is the substance being investigated. Separation of Chlorine and Iodine. If chlorides and iodides occur together the iodine may be precipitated as palladious iodide by a solution of palladious chloride, PdCl 2 . In another portion the total halogen may be precipitated by silver nitrate and weighed as a mixture of silver iodide and chloride. The proper weight of silver iodide, as calculated from the weight of pal- ladious iodide found, is subtracted and chlorine calculated from the remainder. The chlorine and iodine may be determined indirectly by precipitating by excess of silver nitrate, weighing the mixed chloride and iodide, then converting the silver iodide into chloride by heating in an atmosphere of chlorine and reweigh- ing. If x = weight of chlorine, a = weight of silver chloride and iodide, b = weight after conversion of silver iodide into chloride 143 34 and y = weight of iodine, then ' ^ x = weight of silver chloride and vs/nvo y = weight of silver iodide, therefore : iZv.yz 143.34 , 234.80 35^ X+ 126792 y = a > 143.34 ' 143. 34 y=b - Subtracting (2) from (1), -^5 92 94 QUANTITATIVE ANALYSIS Since a and b are both known quantities the values of x and y may be easily obtained. Chlorine and iodine may also be separated by the method of graded oxidation, to be described in connection with the separa- tion of the three halogens. Separation of Bromine and Iodine. Mixtures of bromides and iodides may be analyzed by methods similar to those de- scribed above. Palladious bromide is sufficiently soluble to make possible the precipitation of palladious iodide in the pres- ence of the bromide. Also either the indirect analysis or the method of graded oxidation may be used. Separation of Chlorine and Bromine. For mixtures of chlorides and bromides either the method of graded oxidation or that of indirect analysis may be used. For the latter silver chloride and bromide are weighed together and the bromide is then converted into silver chloride and reweighed. Separation of Chlorine, Bromine, and Iodine, by Graded Oxi- dation. In the discussion of the decomposition voltages of electro- lytes (page 115) it is shown that any electrically neutral element that is capable of ion formation will, when placed in contact with a solution containing its ions, generate a definite potential difference whose magnitude depends upon the solution tension of the element and the concentration of its ions already in solu- tion. Two such systems will generate a definite electromotive force if external connection is made between the non-ionized elements and if the two solutions are brought into contact. This electromotive force is always in the direction that would cause a current to flow externally from the element having the less solution tension (if its ions are positive, or the greater if its ions are negative) to the one having the greater solution tension. When a metal passes into solution and forms positive ions it is thereby oxidized. When an element capable of negative ion formation passes into solution and forms ions it is thereby reduced. Conversely, when metallic ions are converted into massive, uncharged metal they are reduced and when non- metallic or negative ions are discharged they are thereby oxi- dized. According to this view oxidation consists in the addition of positive charges or the removal of negative ones, while reduc- tion is the addition of negative charges or the removal of positive + + + + + ones. Thus the change: Fe *Fe (change of the ferrous ion to EXPERIMENTAL GRAVIMETRIC ANALYSIS 95 the ferric ion) is oxidation, while the reverse is reduction and MnC>4 ^MnO 4 (change of the permanganate ion to the manga- nate ion) is reduction, while the reverse is oxidation. It fol- lows from these statements that the metal having the greater solution tension is the stronger reducing agent while the non- metallic element having the greater solution tension is the better oxidizing agent. It thus becomes possible to compare the activities of two oxidizing or reducing agents by measuring the magnitude and direction of the electromotive force pro- duced by combining two systems made up of these agents in contact with solutions containing the respective products of reduction or oxidation. If the ' oxidizing or reducing agents are solids the electrodes are composed of these solids. If gases the electrode is of some material that will superficially dissolve the gases, while if the agents are solutions they are merely brought into contact with electrodes made of indifferent metals, such as platinum. Thus silver as a reducing agent would itself be made to form the electrode material, in contact with an ionized silver salt. Oxygen as an oxidizing agent would be caused to bubble into the solution in contact with an electrode of platinum which is coated with platinum black, in which oxygen dissolves, and this would be immersed in a solution containing hydroxyl ions. Potassium permanganate would be used in simple contact with a platinum electrode and the solution would also contain the * ** positive manganese ion, Mn, the product of reduction of the ion MnC>4. In the last case the force, solution tension, is re- placed by the tendency of the permanganate anion, already in solution, to become reduced. All of the elementary halogens are oxidizing agents because they exhibit a tendency toward negative ion formation: C1C1, BrBr, etc. Conversely the change of halogen ions into neutral elements is an oxidation of these ions. In order to bring about this oxidation it is necessary to apply another oxidizing agent whose " oxida- tion potential" (F electrode-electrolyte) is greater (in the positive sense). This is analogous to the decomposition of a halide by means of the current, which is an oxidizing agent for the negative 96 QUANTITATIVE ANALYSIS ion and a reducing agent for the positive ion. If an oxidizing agent can be found, having an oxidation potential greater than that of one of the halogens and less than that of another this agent may be used for separating the two halogens by oxidation of the salt (or acid) of one, removing the liberated element by distillation and leaving the other, which was incapable of being oxidized. This is analogous to the electrolytic separation of metals by grading the electromotive force which is applied to two electrodes in the solution. The measurement of oxidation potentials should therefore furnish valuable information for as- sisting in the selection of oxidizing agents suitable for graded oxidation of the anions of the halogen hydracids. Bancroft has shown 1 that the following differences exist between the oxi- dation potentials of chlorine, bromine and iodine in solutions of salts of their respective hydracids: Chlorine in potassium chloride bromine in potassium bro- mide =0.241 volt. Bromine in potassium bromide iodine in potassium iodide = 0.535 volt. These differences vary somewhat if the concentrations are altered. As examples of oxidizing agents which will serve for the iodine anion without thereby oxidizing bromine or chlorine anions, may be mentioned monopotassium arsenate and nitrous acid. These were suggested by Gooch. 2 The oxidation potential of nitrous acid (or potassium nitrite and sulphuric acid), according to the measurements of Bancroft, is 0.249 volt higher than that of iodine in potassium iodide, and 0.285 volt lower than that of bromine in potassium bromide. For the selective oxidation of the bromine anion in presence of the chlorine anion the following substances have been used: potassium permanganate in acid solution, lead peroxide in acid solution, potassium dichromate in acid solution, ammonium persulphate in neutral solution and potassium iodate in acid solution. With the exception of the last named oxidizing agent all of these substances possess oxidation potentials higher than that of chlorine in potassium chloride and could therefore be made to serve for a quantitative separation of bromine and 1 Z. physik. Chem., 10, 387 (1892). 2 Chem. News, 61, 235 (1890). EXPERIMENTAL GRAVIMETRIC ANALYSIS 97 chlorine only by carefully regulating the concentrations of oxi- dizing and reducing agents and by stopping the distillation at exactly the correct time. Attempts have been made to regulate the speed of oxidation by acidifying with substances that can furnish only a small concentration of hydrogen ions, since it is only in presence of hydrogen ions that the reactions can proceed. Weakly acid substances that have been used for this' purpose are acetic acid, potassium acid sulphate, ferrous sulphate and aluminium sulphate. The last two are weakly acid through hydrolysis. Potassium permanganate is the oxidizing agent used by Jan- nasch and Aschoff 1 and the oxidation of hydrochloric acid is prevented by acidifying with an acid no stronger than acetic acid and by employing a large dilution. The oxidation potential of potassium permanganate with sulphuric acid is 0.097 volt higher than that of chlorine with potassium chloride. The reduction of potassium permanganate is really a reduction of manganese itself, being a change of heptavalent into bivalent manganese. The complete equation is KMnO4+5HBr+3HC 2 H 3 O 2 KC 2 H 3 02+Mn(C 2 H 3 O 2 )2 + 5Br+4H 2 O. The ionic change involving manganese is 4 Mn+20 2 . The electrical change of manganese itself is not oxidation, as would appear from the last equation, but reduction, because the univalent anion, MnO 4 , is composed of one atom of heptavalent positive manganese and four atoms of bivalent negative oxygen, so that the change of manganese is really Mn * If the substance being analyzed is known to be a pure mixture of only two of the halides, one of the halogens may be liberated and removed by distillation without subsequent absorption, the other being determined in the residual solution. If it is not a pure mixture or if it contains salts of three halogens it is necessary to absorb at least one of these and make a direct determination of it in the absorbing solutions. 1 Z. anorg. Chem., 1, 144 and 245 (1892); 6, 8 (1894). 7 98 QUANTITATIVE ANALYSIS Bugarszky 1 used potassium iodate and sulphuric acid for separating bromine and chlorine, distilling the bromine without absorption. The oxidation potential of acidified potassium iodate was found by Bancroft to be 0.064 volt higher than that of bromine and 0.178 volt lower than that of chlorine. The reaction is as follows: KI0 3 +5KBr+3H 2 SO 4 3K 2 SO 4 +5Br+I+3H 2 0. Both bromine and iodine are liberated and distilled and account of this must be taken if the free halogens are absorbed and subse- quently determined. Chlorine is determined in the residual solution, after reducing the excess of iodic acid to hydriodic acid by means of sulphurous acid, then oxidizing by nitrous acid and distilling. Andrews 2 modified the method of Bugarszky by substituting nitric acid for sulphuric acid and by reducing the excess of iodic acid by means of phosphorous acid. His method was not tested, however, except for the determination of chlorides in crude bromides and of chlorine in crude bromine. In both cases the chlorine was present in relatively small quantities (less than 10 percent) and it was not adapted to the determination of both bromine and chlorine. For the determination of chlorine, bromine and iodine by direct means, the method of Jannasch and Aschoff is probably the best of all methods yet proposed, even though permanganic acid is not an ideal oxidizing agent for the separation of chlorine and bromine. In this method the solution of mixed chlorides, bromides and iodides is first acidified with sulphuric acid and potassium nitrite is added: KI+KNO 2 +2H 2 SO 4 2KHSO 4 +NO+H 2 0+I. The liberated iodine is distilled, absorbed, and subsequently determined. The sulphuric acid is then neutralized by sodium hydroxide, acetic acid and potassium permanganate are added and the bromine is distilled, absorbed and determined. In the residual solution the excess of permanganate is reduced and the chlorine is determined gravimetrically. The absorbent which best serves for iodine and bromine is a solution containing sodium hydroxide and hydrogen peroxide. 1 Z. anorg. Chem., 10, 387 (1895). 2 J. Am. Chem. Soc., 29, 275 (1907). EXPERIMENTAL GRAVIMETRIC ANALYSIS 99 Bromine and iodine react with sodium hydroxide to form sodium bromide and sodium hypobromite in the one case and sodium iodide and sodium hypoiodite in the other: 2NaOH + 2BrNaBr + NaBr + H 2 O, 2NaOH+2I NalO +NaI +H 2 O. If the solutions are allowed to stand for some time bromates and iodates are formed: 3NaBrO~>NaBrO 3 +2NaBr, 3NaIO ->NaIO 3 +2NaI. FIG. 30. Apparatus for the separation of the halogens. The last change does not take place if hydrogen peroxide is present and the solution is kept cold, the hypobromite and hypoiodite being reduced as fast as formed: NaBrO+H 2 O 2 NaBr+H 2 0+O 2 , NalO +H 2 2 -NaI +H 2 O+0 2 . In the resulting solutions bromine and iodine may be determined as the silver salts in the usual manner after acidifying with sulphuric acid. If the absorbing solution has been allowed to become warm some iodate or bromate will be formed. In this case acidification will cause the liberation of free halogen which will escape precipi- tation. If silver nitrate is added before acidifying, any iodate or bromate will remain in solution as the silver salt. This inter- ference of oxy salts may also be prevented by the addition of a 100 QUANTITATIVE ANALYSIS sulphite before the addition of acid. The resulting sulphurous acid then reduces the iodate or bromate to iodide or bromide. During the distillation of bromine and iodine it is essential that contact with cork or rubber be avoided, since the halogens are thereby reduced and absorbed. Ground-glass stoppers are necessary in all parts of the apparatus where such contact would occur and, where rubber connections are used, the glass tubes inside must be pushed together so as to expose as little of the rubber tubing as is possible. All reagents must be tested and found free from the halogens. Determination. Weigh about 1 gm of the mixture of halides, placing the sample in a round bottomed, glass stoppered distilling flask, having a capacity of 1000 cc, and having an inlet tube sealed into the side of the neck and reaching to the bottom of the flask. Connect the appa- ratus as shown in Fig. 30. A is a vessel in which steam may be generated, B is the distilling flask, C and D are bubble tubes having a capacity of 150 cc. The tube a should reach to the bottom ot the steam generator and should extend about 18 inches above. This tube provides an inlet for air, in case there is any tendency toward drawing liquid back from B. Each of the absorption tubes C and D contains 50 cc of 5 percent sodium hydroxide and 50 cc of hydrogen peroxide. The union between B and C and between C and D should be made by bringing the glass tubes quite together inside the rubber connections. Dissolve the weighed sample in about 600 cc of water, add 5 cc of 25 percent sulphuric acid and 2 gm of sodium nitrite. Heat the solution nearly to boiling and pass steam through the flask for twenty minutes after the solution is colorless. During this time the tubes C and D must be kept cool by immersion in ice water. When all of the iodine has been distilled the boiling is interrupted, the absorption tubes are disconnected and their contents washed into a 300 cc beaker. The tubes are then returned to the apparatus and are refilled with sodium hydroxide and hydrogen peroxide as before. The solution in the flask is barely neutralized with sodium hydroxide solution and evaporated to a volume of about 500 cc. 1.5 gm of potassium permanganate and 60 cc of 33 percent acetic acid are added and the bromine thus liberated is distilled and absorbed in hydrogen peroxide and sodium hydroxide in the cooled tubes. When steam has been passed through the solution for some time after the latter has become colorless the distillation is again stopped and the contents of the tubes washed into another beaker. The solutions now containing the iodine and bromine are boiled until the excess of hydrogen peroxide is completely decomposed. 0.5 gm EXPERIMENTAL GRAVIMETRIC. ANALYSIS ; , . 101 of sodium sulphite is added and then dilute sulphuric acid until the solu- tion is slightly acid in character. If any color appears at this point it is due to the presence of iodine or bromine produced by iodate or bromate, showing that insufficient sodium sulphite has been added. In this case 0.5 gm more is at once added to reduce the free halogen. When the solutions are acid and colorless a 5 percent solution of silver nitrate is added, drop by drop from a pipette, stirring vigorously until no further precipitation occurs. The liquid is digested at near the boil- ing temperature until the precipitate settles readily, after which it is filtered on a Gooch crucible, as directed on page 21, and the precipitates are washed free from silver nitrate, testing the washings with dilute hydrochloric acid. The crucibles are finally washed once with alcohol to promote rapid drying and are then dried at 110 for one-half hour or until the weight is constant. The percent of iodine and of bromine is calculated. The solution in the large distilling flask is boiled with alcohol to reduce the excess of potassium permanganate and is then poured into a beaker or evaporating dish and evaporated to a volume of not more than 150 cc. 5 cc of dilute nitric acid is added and the chlorine is precipitated and weighed exactly as directed in the case of iodine and bromine. Halogen Oxyacids. The oxy acids of the halogens (or their salts) may be reduced to the hydracids by warming with hydrogen peroxide, after which the separation and determination may be accomplished as above directed. Free Halogens existing in solution may be converted into oxysalts by treatment with alkali bases, after which their separa- tion and determination may be carried out by methods already discussed. Their determination is more conveniently made by volumetric methods which will be discussed later. Chlorine in gas- eous mixtures is also determined by absorption followed by a volumetric process. Organic Halogen Compounds. Compounds of the halogens with organic residues cannot be analyzed by the usual methods because such compounds do not, as a rule, ionize to form the anions of the halogen acids. The compound must be decomposed in such a manner as to leave the halogen in the form of an inorganic compound of one of the well-defined acids. In such cases either the lime method or the method of Carius may be used. In the lime method the material is mixed with pulverized lime, free from halogens, and is placed in a hard glass tube, closed at one end. Lime is placed in the open end of the tube, which is then 102 . _ . [^QUANTITATIVE ANALYSIS heated in a combustion furnace. The organic compound is decomposed and the halogen unites with the calcium oxide to form calcium halide, from the acid solution of which the halogen may be precipitated by silver nitrate. If more than one halogen is present the separation may be made, after the heating is finished, by methods already outlined. , In the Carius 1 method the material is heated in a closed tube in contact with fuming nitric acid and silver nitrate. The organic compound is oxidized and the free halogen thus produced is converted into the hydracid. The halogen hydracid at once reacts with silver nitrate and the silver halide is later weighed. The method is not well adapted to separation of the halogens, since a mixture of silver salts is obtained in the tube. Determination. Carius tubes of Jena glass may now be obtained with one end already closed. The tube should be approximately 50 cm long and 2 cm in diameter. If such a tube is not at hand a good grade of combustion tubing may be used. One end is closed as follows: The tube is carefully heated at a point about 10 cm from one end by rotating in the flame of the blast lamp. When the glass has softened the tube is quickly drawn out, until nearly closed. It is allowed to cool and is then cut at the narrow part. The nearly closed end is then fused together until a well-rounded end is produced. This must be annealed with great care or disastrous breaks will occur later. Having prepared a tube that is clean and dry, another small tube about 4 cm long is closed at one end to serve as a weighing tube. About 0.2 gm of the organic material is weighed into the latter. Into the Carius tube is carefully placed about 0.5 gm of powdered silver nitrate and 2 cc of fuming nitric acid, free from halogens. The acid is intro- duced through a funnel with a long stem which reaches at least half way to the bottom of the tube, thus keeping the upper half dry. The weighing tube containing the substance to be analyzed is inserted into the end of the Carius tube, the latter being placed in a slanting position. Mixing of the contents of the weighing tube with the acid should not occur until after the Carius tube is sealed. The latter is now heated about 10 cm from the open end, the tube is drawn out while in the flame and the walls are sealed together. A more or less blunt point should be left here as shown in Fig. 31. Since a high pressure will be generated within the tube when heating begins it is necessary to place the tube inside an iron tube having caps screwed over the ends. The glass tubes frequently break on account of high pressure. The iron tube is now placed in a suitable furnace 1 Z. anal. Chem., 1, 240 (1861); 4, 451 (1864); 10, 103 (1871).. EXPERIMENTAL GRAVIMETRIC ANALYSIS 103 in which it may be gradually and uniformly heated. The temperature and time necessary for heating will vary with the nature of the substance under examination. Most organic compounds will be completely decomposed by heating for three hours at 300, while many aliphatic compounds will require a temperature no higher than 150. After the decomposition is completed the furnace is allowed to cool, the iron pipe containing the tube is carefully removed, the cap unscrewed and the glass tube taken out. The latter is wrapped in a towel, to minimize the danger due to possible explosions, and the point of the tube, where it was last sealed off, is held in a flame until softened. The internal pressure causes the glass to blow out and the gas escapes, after which the tube may be handled without risk of injury. A scratch is FIG. 31. Sealed end of carius tube. made near the blown-out end, but on the wide part, and this end is broken off by touching the scratch with a hot glass rod. The contents of the tube are rinsed into a beaker, diluted with water and filtered; the pre- cipitate is washed and weighed by the ordinary process, using either a paper filter or a Gooch crucible. From the weight of silver halide found the percent of halogen in the organic compound is calculated. CARBONIC ACID AND CARBON DIOXIDE The following cases are to be considered: Carbon dioxide in gaseous mixtures, solutions of carbonic acid and salts of car- bonic acid. Carbon Dioxide in Gaseous Mixtures (air, chimney gases, etc.) This determination is best made by gasometric methods which will be considered in a later section (pages 282 and 290). Carbonic Acid in Solution. The. most frequently occurring case is that of underground waters. Such waters, coming from regions of low temperature and high pressure, often contain con- siderable quantities of carbonic acid. When the water reaches the surface, diminished pressure and rise in temperature cause the release of more or less carbon dioxide, so that a determina- tion is always subject to some uncertainty regarding the rela- tion of the original concentration of carbonic acid to that in the water as the analyst receives it. Determinations are also re- 104 QUANTITATIVE ANALYSIS quired of carbonic acid in carbonated drinks. In such cases provision must be made for transferring the solution from the pressure bottle to the apparatus in which the determination is to be made without loss of carbon dioxide. The procedure for the determination of carbonic acid in water is given on page 342. Carbon Dioxide in Carbonates. Determinations of this class are by far the most common in general analytical practice. The FIG. 32. Rohrbeck's appara- tus for determination of carbon dioxide by loss. FIG. 33. Mohr's apparatus for determination of carbon dioxide by loss. carbonate is decomposed by means of a stronger acid than car- bonic acid and the carbon dioxide determined in one of three ways: (1) by a determination of loss in weight, (2) by measuring the gas disengaged, or (3) by weighing this gas after absorbing by reagents in a suitable apparatus. Determination by Loss. Many forms of apparatus may be purchased for the determination of carbonic acid by loss. Three of these are shown in Figs. 32, 33, and 34. Any such apparatus EXPERIMENTAL GRAVIMETRIC ANALYSIS 105 must include means for drying incoming air and outgoing gas. It must also be compact and not too heavy to be weighed on the analytical balance. In using such apparatus the sample is weighed and brushed into the lower generating vessel. Hydro- chloric or sulphuric acid is placed in the upper bulb and the bubble tubes are partly filled with concentrated sulphuric acid. The whole apparatus is accurately weighed, after which the cock is carefully opened so that acid drops upon the car- bonate, evolving carbon dioxide at a moderate rate. This car- bon dioxide passes out through the sulphuric acid in the bubble tube, being freed from moisture by so doing. The apparatus is finally heated and air is drawn through to displace the re- maining carbon dioxide. The loss in weight is taken to represent carbon dioxide. This is not accurately the case unless the air that is drawn through the apparatus is first dried. The determination by means of such apparatus is quickly made but is sub- ject to a rather large error on account of the large weight of the apparatus, because of the large surface and largely because of the difficulty en- countered in the drying and purifica- tion of the out-going gases unless un- duly large quantities of sulphuric acid are used, as well as an absorbent for acid vapors. Determination by Absorption. -The FlG ' f.-Schrotter's p- , . . , . . paratus for determination of direct determination by a somewhat carbon dioxide by logg more elaborate apparatus is to be preferred if accuracy is an object. In such a method the purifi- cation of the carbon dioxide is rendered complete by elaborating that part in which the purification is accomplished, providing better contact of the gases with drying agents and acid absorb- ents. Instead of weighing the entire apparatus before and after expulsion of carbon dioxide from the carbonate the carbon dioxide is absorbed in a weighed amount of potassium hydroxide, 106 QUANTITATIVE ANALYSIS which is again weighed after the absorption. Many variations in the apparatus have been employed but the apparatus here described embodies the essential features of most of these. In Fig. 35, A is a generating flask into which the weighed sample of carbonate is placed. B is a dropping funnel having a capacity of 50 cc, and having the lower end drawn out to a point and turned upward. This part should extend to the bottom of the flask. At the top of the dropping funnel a drying tube C is connected by means of a rubber stopper and a bent glass tube. FIG. 35. Assembled apparatus for determination of carbon dioxide by absorption. The drying tube is filled with soda lime for the absorption of carbon dioxide from the air that is later to be drawn through. Following the generating flask is a short condenser D and then U-tubes E, F and G. The first U-tube is omitted if sulphuric acid is to be used for decomposing the carbonate, or is filled with an ab- sorbent for hydrochloric acid vapors if this acid is used. The U-tubes F and G are filled with granular calcium chloride which absorbs moisture from the gas mixture. Following these is the apparatus H in which potassium hydroxide is placed for the ab- sorption of carbon dioxide. This apparatus also carries a small tube filled with calcium chloride to prevent the removal of mois- ture from the apparatus, which would occur if the dry entering EXPERIMENTAL GRAVIMETRIC ANALYSIS 107 gases were allowed to leave the apparatus saturated with mois- ture. To provide a means for drawing air through the whole apparatus the aspirator / is placed at the end of the series, while to prevent moisture from diffusing backward into the absorption apparatus the calcium chloride tube I is interposed. Choice of Acid. The choice of acid to be used in decomposing the carbonate will depend upon the nature of the latter. Sul- phuric acid is to be preferred where it can be used, because it is non- volatile and thus needs no absorbent in the purifying apparatus. If, however, the carbonate is one of a metal which forms a sul- phate of small solubility, (e.g., calcium carbonate or barium car- bonate) sulphuric acid soon coats the particles with insoluble sulphate which hinders the decomposition of the interior of the particles. Decomposition is slow and uncertain and for this reason hydrochloric acid is used instead* of sulphuric acid. A preliminary test should be made to ascertain whether sulphuric acid forms a complete solution of the carbonate to be analyzed. Absorbent for Hydrochloric Acid. If hydrochloric acid must be used a suitable absorbent is placed in the U-tube E } following the generating flask. Absorbents which serve best for this purpose are silver sulphate and anhydrous copper sulphate. For such a purpose the copper sulphate is prepared by first dropping red hot pieces of pumice stone into a concentrated solution of copper sulphate, removing the pumice stone, allowing to drain and then drying at 200. A supply of the dry pieces is kept in a desiccator and fresh pieces placed in the U-tubes for each determination. A more satisfactory absorbent is a saturated solution of silver sulphate in concentrated sulphuric acid. . This may be absorbed by pieces of pumice and used in the same manner as copper sul- phate. When sulphuric acid is used in this manner it must not be allowed to come into contact with corks, cotton or any other organic matter. The evidence of such contact is blackening. The result is the formation of both carbon dioxide and sulphur dioxide. Both oxides are absorbed in the potassium hydroxide and give rise to errors in the determination. A U-tube with glass stoppers should be used. Silver nitrate cannot be used because its reaction with hydrochloric acid produces nitric acid, which is nearly as volatile as hydrochloric acid, and also chlorine. Soda Lime. The soda lime which is used for the removal of carbon dioxide from the entering air should be fresh and in the 108 QUANTITATIVE ANALYSIS form of lumps. A powdered condition is evidence of having been air-slaked, in which case it is unfit for use since it is already saturated with carbon dioxide. ^ Calcium Chloride. The calcium chloride used for the absorp- tion of moisture should be the granular form which has been fused. Fusion is necessary in order to produce an anhydrous material. This fusion always produces a certain amount of calcium oxide which, if allowed to remain as such, will absorb a certain quantity of carbon dioxide as well as of water. It is best to treat the material directly in the bottle by passing dried carbon dioxide through for several hours, then displacing the carbon dioxide by drawing through dried air. In filling U-tubes only lumps should be used. The tube is filled to just below the side branches and then a loose plug of cotton or glass wool is placed on top in each side to prevent drawing out of any grains of powder that may subsequently be produced. For sealing the tubes a cork is pressed in until it begins to fit closely. It is then cut off even with the top and the smaller part is pressed into the tube about one-eighth inch farther. The shallow cup thus formed is poured full of melted paraffin or sealing wax. When this solidifies an air-tight seal should result, unless bubbles have formed in the sealing material. In the latter case a flame may be lightly touched to the surface of the solid paraffin or wax, which will cause the bubbles to break. Absorbent for Carbon Dioxide. Potassium hydroxide is generally used for the absorption of carbon dioxide, a solution 33 percent by weight being commonly employed. In practice it is found that absorption becomes so slow as to be uncertain before the point of complete saturation is reached. The prac- tical limit is reached when 0.10 gm of carbon dioxide has been absorbed by each cubic -centimeter of potassium hydroxide solution. To determine the amount of gas that can be absorbed by the solution in the apparatus tile latter is first filled with water to the height at which the liquid is to stand. This is emptied out and measured. The number of cubic centimeters times 0.1 gm is the weight of carbon dioxide which can be absorbed before the solution becomes inefficient. By adding together the weights of gas absorbed in successive experiments it is easy to determine when the bulbs need refilling. The bulbs in which the absorption is to take place furnish the greatest EXPERIMENTAL GRAVIMETRIC ANALYSIS 109 source of error to be encountered in this method. Inaccuracies are due to the large weight, the large surface and the possibility of moisture being carried out by the outgoing air. The effect of the comparatively large weight of the bulbs and their contents is to decrease the sensibility of the balance. The surface gives rise to a possible error because of the variable amount of moisture which is always dissolved in the surface of the glass. This error may be considerable if the two weights (before and after the absorption) are observed under different atmospheric conditions of humidity. For this reason it is necessary that the two readings of weights shall be made on the same day and as near to each other in point of time as possible. The danger of loss of moisture from the potassium hydroxide solution to the dry air which enters is magnified by the necessary limit which the already large weight of the bulbs places upon the tube which carries the calcium chloride for drying the outgoing air. For this reason many analysts prefer to separate this drying tube from the bulbs, using a small U-tube 1 or even two such tubes, and weighing the apparatus in the two or three parts. Sometimes there is placed in the first half of the U-tube so used, or in the first U-tube if two are used, solid potassium hydroxide to insure complete absorption of carbon dioxide. While this procedure may make more certain the complete drying of the air and thus prevent a loss of weight from this cause, an added uncertainty is introduced due to the accumulation of the errors of four weighings. It is possible to insure complete detention of the moisture by passing the gas at a regular, specified rate, not exceeding a maximum found by experience. Determination. Procure the following parts for assembling: 1 dropping funnel, 50 cc, with 1-hole rubber stopper, 1 short, wide flask, 75 cc, such as is used for fat extractions, with 2-hole rubber stopper, 1 condenser with body not more than 6 inches long, ' 3 U-tubes with corks to fit, 1 U-tube with glass stoppers, 1 straight drying tube with 1-hole rubber stopper, 1 set Geissler "potash bulbs" or bulbs of some other approved form, 1 aspirator bottle, tubulated near bottom, with 1-hole rubber stoppers to fit, 1 piece glass tubing, about 2 feetX^ inch, for supporting apparatus, 2 clamps, 110 QUANTITATIVE ANALYSIS 2 pinch cocks, 1 small screw clamp (Hoffman screw), 2 retort stands, Glass and rubber tubing for connections. Fill and connect the apparatus in the manner previously described. Measure the capacity of the absorption bulbs by drawing in distilled water, then blowing out and measuring the water. This volume will be used as a basis for the calculation of absorbing power as already directed. When filling the absorption bulbs with potassium hydroxide solution the latter should not be warmer than the air of the room. The bulbs are detached from the apparatus and the solution is drawn in through a tube attached at a, suction being applied at 6. The solution should about half fill the bulb c when air is bubbling through. The ground- glass joint between the drying tube b and the bulbs should be lightly coated with vaseline and the tube then twisted on until it fits closely enough that there will be no danger of loosening during the course of an experiment. Any surplus vaseline is removed from the outside of the joint. Place the bulbs in position, close the cock of the dropping funnel and open the pinch cock at e to allow water to flow from the aspirator. Bubbles of air will at first pass through the bulbs but this action will finally cease unless there is a leak in the apparatus, in which case it must be found and closed. It is important that all glass tubes be brought entirely together inside the rubber connections since rubber is slightly permeable to gases. After the apparatus has been shown to be free from leaks the pinch cock at / is closed, the cock of the separatory funnel slowly opened and, after equilibrium is established, the clamp k is so adjusted that when clamp / is opened air will pass through the bulbs at a rate not greater than 3 bubbles per second. Clamp k is not thereafter changed. This provides against too rapid flow of gas under any conditions. Clamp / is now closed, the bulbs are removed, the inlet and outlet tubes are closed by short rubber tubes containing glass plugs and the bulbs are wiped clean and placed in the balance case . The bulbs should be allowed to stand for 15 minutes before weighing. In the meantime about 1 gm of the carbonate is weighed and brushed into the generating flask and a small amount of water is added to moisten the sample. After the absorption bulbs have stood for 15 minutes the tubes carrying the plugs are removed and the bulbs are weighed. The plugs are then replaced and left so until the bulbs can be connected in the apparatus. 50 cc of dilute sulphuric acid or hydrochloric acid is placed in the dropping funnel, a test having previously been made to determine whether sul- phuric acid will form a clear solution with the carbonate. If such a solution is not produced, of course hydrochloric acid must be used and EXPERIMENTAL GRAVIMETRIC ANALYSIS 111 silver sulphate and pumice must be placed in tube E. Reconnect the apparatus and open all cocks except the stop-cock in the dropping fun- nel, leaving the clamp k set for the proper rate of gas flow, as previously determined. Slowly open the cock of the dropping funnel, allowing acid to drop just fast enough to evolve carbon dioxide at the prescribed rate. The constant attention of the operator is necessary at this point, for by causing too rapid evolution of gas some moisture may escape absorption in the small tube of the absorption bulbs and the experiment be rendered worthless. The acid should be allowed to run in until about 1 cc is left above the stopcock, this acting as a seal during the subsequent boiling. After the decomposition of the carbonate is complete the solution in the flask is slowly heated until it boils, always with due regard to the rate at which the gas is made to flow through the absorption bulbs. The boiling is continued for one minute, when the flame is withdrawn, the cock of the dropping funnel being opened at the same time to allow air to enter so that no back suction occurs, due to the cooling effect. Air is now drawn through the apparatus until 1000 cc of water has flown from the aspira- tor. This amount of air should be sufficient to sweep all of the carbon dioxide into the absorption bulbs. The clamp / is now closed, and the absorption bulbs are removed, plugged and placed in the balance case. After 15 minutes they are weighed, the increase in weight being the weight of carbon dioxide. From this and the weight of sample the percent of carbonic anhydride (combined carbon dioxide) is calculated. For the duplicate or any subsequent determination the generating flask and the dropping funnel are washed absolutely free from acid, so that no decomposition of the next carbonate sample may occur before the bulbs are in place. The first U-tube should also be emptied and recharged with absorbent, if such is to be used for the next determination. If a large number of determinations is to be made with the same apparatus much time will be saved by providing two decomposition flasks and two absorption bulbs. While one determination is being made another sample may be weighed into the duplicate flask and the second absorption bulb may be weighed. The next determination may then be started while the first bulbs are standing in the balance case, pre- liminary to the final weighing. It is also necessary to determine when the various absorbents have become so saturated as to be inefficient for further work. Soda lime in the tube C is good until the lumps have fallen into a powder. Silver sulphate in the pumice of tube E may become inefficient through absorption of hydrochloric acid or through the accumulation of water in the tube. The solubility of silver sulphate in water is much less than in concentrated sulphuric acid. If the acid solution becomes diluted the silver salt crystallizes and will not there- 112 , QUANTITATIVE ANALYSIS after readily absorb hydrochloric acid. As the silver sulphate becomes saturated with hydrochloric acid it darkens, on account of the action of light. When the darkening effect has proceeded as far as the middle of the tube the material should be replaced. Calcium chloride must be replaced when it becomes visibly moist for the first third of any absorbing tube. A method has already been given for the determination of the amount of carbon dioxide which can be absorbed by the solution in the bulbs. CHAPTER IV ELECTRO -ANALYSIS We have here to deal with a class of work that, while also gravimetric in most cases, is sufficiently different from what has already been considered to be treated as a separate division. In all of the preceding exercises the element or radical to be deter- mined was precipitated from a solution by chemical reactions produced by other substances which were added for the purpose. In the cases now to be considered the precipitation will be brought about by electrical action, the passage of a current through the solution causing the deposition of a metal upon a cathode in such a form that it can be weighed, or the accomplishment of some change which makes possible the determination of a sub- stance not a metal. The electrolysis of silver sulphate will serve as an example. When a solution of this salt is electrolyzed at platinum electrodes the metal is plated on the cathode and sulphuric acid is produced at the anode, thus: 2Ag+S0 4 -> 2Ag+S0 4 , 2SO 4 +2H 2 2H 2 SO 4 +O 2 . The silver can then be weighed and the sulphuric acid deter- mined volumetrically. While the electrolysis of a simple salt is frequently a tolerably simple and well understood process, the practical accomplish- ment of such a process for the purpose of a quantitative analysis is usually possible only when a certain set of conditions is main- tained. The principal reasons for failure to attain accuracy are three: (l) Deposition may not occur upon passage of a current. (2) The deposit may be contaminated by other products of electrolysis. (3) The deposit may not have the proper physical character, so that it will not adhere to the electrode but crumbles off during the electrolysis or during the process of washing. We have thus to consider the nature of salt to be used, solvents, tem- perature, electrolytic pressure (voltage), current density and nature and kind of electrode. 8 113 114 QUANTITATIVE ANALYSIS Nature of Electrolyte. Electrolytic methods are more fre- quently applied to the determination of metals than of non- metals, although methods have lately been perfected for the determination of the latter. If the metal alone is to be deter- mined it will usually be possible to obtain it in the form of what- ever salt gives the best results. Certain anions must be excluded in specific cases, either because they yield substances that at- tack the anode or because the acids that are produced by their electrical discharge cause the metal to deposit in an undesirable physical form. As an example of corrosive action upon the anode it is sufficient to here mention the formation of nascent chlorine at a platinum anode when a chloride is electrolyzed. With regard to the effect of the acid that accumulates in the solution as electrolysis proceeds it may be stated that there is little known, at present, of the reasons for the effect of acids, bases and other substances that may be in the solution, upon the nature of the deposit. Experiment shows, however, that such substances often exert a very important influence upon the physical character of a deposited metal and they are often added for this reason, although they may be objectionable for other reasons. A solu- tion of copper sulphate, if electrolyzed without the addition of another substance, usually gives a dark red or brown deposit of finely divided copper which is liable to powder and be lost during washing. If a small amount of sulphuric acid is first added the deposit is improved, while nitric acid causes a still better deposit of bright red, firm and adherent metal. For this reason nitric acid is usually added although it gives rise to more or less danger of resolution when the cathode copper is being washed. On the other hand, a silver salt is best electrolyzed in the absence of nitric acid. If silver nitrate is electrolyzed from water solution with or without the addition of nitric acid (the latter is formed by the electrolysis) the plate of silver on the cathode is so decidedly crystalline that it is very easily detached. The addition of potassium cyanide in quantity sufficient to redissolve the precipitate of silver cyanide first formed gives a solution from which silver will deposit as a white firm plate. The solution in potassium cyanide has a comparatively high electrical resistance so that more energy is consumed in the ac- complishment of its decomposition, nevertheless potassium cyanide is generally added. ELECTRO-ANAL YSIS 115 Other examples of similar effects will appear in the exercises. It is desirable to note that little is known of the cause of such effects, also to guard against a very common misconception regarding the purpose of adding other electrolytes to solutions that are to be electrolyzed. It is frequently stated that such substances are added in order to increase the conductivity of the solution. If such substances could increase the ionization .of the salt that is to be electrolyzed, or in any manner diminish the fric- tional resistance to the passage of the ions, such an effect would be desirable. It is evident, however, that the addition of a foreign electrolyte can usually increase the conductivity only by itself acting as a carrier of current, in which case it has accomplished no desirable effect since the prime object is not to use a large current but to make the minimum current do the maximum work in discharging an ion already in solution. Solvent. Very little work has been done in any solvents other than water. The use of organic solvents may, in some cases, prove advantageous in producing good deposits where other con- ditions fail to do so. Temperature. Conductivity of solutions usually increases with rise in temperature. This is not due to increased ionization (ionization usually decreases with rise in temperature) but to re- duced viscosity and consequent reduction in frictional resistance to ionic migration. If the complete electro-decomposition of a substance requires considerable time it is not convenient to heat to any definite elevated temperature. In most cases, therefore, the temperature of the solution is not raised above that of the laboratory except at the beginning. Electrolytic Pressure. For every electrolyte in solution there is a definite minimum voltage, below which no decomposition will take place. If but one electrolyte is present and the voltage lies below this minimum, a continuous current cannot flow. The minimum voltage necessary to produce a continuous flow of current is called the " decomposition voltage" for the sub- stance in question. If salts of more than one metal are present in solution, the deposit on the cathode will consist of any metals, the decomposition voltage of whose salts has been exceeded. If there is sufficient difference in the values of decomposition voltage for the different salts, separation may be made. It is only necessary to so adjust the voltage that it shall exceed the 116 QUANTITATIVE ANALYSIS decomposition voltage of the metal that is most easily discharged. After this metal has been completely removed from solution and weighed the voltage is raised until it exceeds the decomposition voltage of the metal next in order and so on. While this. consti- tutes the general procedure for electrolytic separations it is necessary to make certain changes in the nature of the solution after each metal is removed in turn as will be understood from the discussion of the solutions to be employed. In order to understand the origin of the decomposition voltage it will be necessary to briefly consider the underlying principles of electrolysis. If a metal is placed in contact with a solution of one of its salts it will be found that a difference in potential exists between the metal and solution. This difference may be either positive or negative, i.e., the metal may be at a higher or lower potential than that of the solution. The difference may be zero in certain cases but such cases are special. The conception of Helmholtz 1 regarding the cause of this potential difference may be thus stated: Whenever two dissimilar substances are in contact a potential difference is established because of the passage of one into the other. In the case of metals and their salt solutions, one of two things may happen: either some metal atoms pass into solution and become charged ions or some ions are discharged by the mass of metal and themselves become elementary. In the first case the solution assumes a higher potential than the metal because positive charges have been transferred from metal to solution. In the second case the solution is at a potential lower than that of the metal because positive electricity has passed from solution to metal. The direction of the change is determined by the relative magnitude of two opposing forces. The metal shows a tendency to pass into solution in the ionic condition in obedience to a force called by Nernst 2 " electrolytic solution tension." This force varies with different elements, but is constant for a given element and may be relatively large or small. When positive ions have been thrown into the solution the potential difference thus established gives rise to an attraction of an electrostatic nature between the positively charged solution and the negatively charged metal. This may be represented diagrammatically as in Fig. 36. 1 Wied. Ann., 7, 337 (1879). 2 Z. physik. Chem., 4, 129 (1889). ELECTRO-ANALYSIS 117 Helmholtz considered that a " double layer" was thus formed, composed of positive and negative charges, and that the compo- nents of this layer were very close together. The attraction existing between the components of the double layer increases as more ions are formed and finally reaches equilibrium with the solution pressure. If ions of the metal in question were already in the solution, then osmotic pressure would oppose the entrance of more ions into the solution and would thus act in conjunction with the electrostatic attraction, so that equilibrium would be FIG. 36. Diagram illustrating the "double layer." reached with a smaller potential difference. Evidently, then, the potential difference, F e i ectrode . e i ectro i yte in the case of a given metal and its salt solution will be greatest (in a negative sense) when the initial ion concentration is least, and least when the latter is greatest. If the solution pressure is small and the ion concentration large, equilibrium may be reached only by the actual deposition of ions upon the metal. In this case F 'electrode-electrolyte will be positive. If the different elements are compared, with regard to the potential difference established between them and their ion solu- tions, the ion concentration being the same in all cases, a series of different values is obtained. It is to be noted that if an element takes a negative charge upon becoming ionized the potential difference is reversed in sign. In the following table 1 several of the elements are given with values for the potential 1 Wilsmore: Z. physik. Chem., 35, 291 (1900). 118 QUANTITATIVE ANALYSIS difference. These differences are measured by a method that need not be here discussed, involving the use of an arbitrary standard, so that only the relative values are important. These values are for solutions which contain 1 gm-ion per liter. Element | F electrode-electrolyte Element F electrode-electrolyte Potassium Sodium -3.20 -2 82 Hydrogen Arsenic. ... 0.000 <+0 293 Barium Strontium Calcium . -2.82 -2.77 2 56 Copper Bismuth Antimony + 0.329 <+0.391 <+0 466 Magnesium 2 54 Mercury + 750 Aluminium Manganese Zinc -1.276 -1.075 -0 770 Silver Palladium Platinum . . . + 0.771 <+0.789 <+0 863 Cadmium Iron -0.420 -0.340 Gold Fluorine < + 1.079 + 1.96 Thallium -0.322 Chlorine + 1.417 Cobalt 232 Bromine + 993 Nickel 228 Iodine . . + 520 Tin. < 192 Oxygen + 1 119 Lead -0.148 In the electrolysis of a salt solution both positive and negative ions are discharged at the electrodes, both electrodes become coated with the products of decomposition (polarized), and both then become essentially electrodes of the respective elements, no matter what the original substance might have been. Hence such a system as that just discussed may be considered to exist at each electrode. Since, after electrolytic decomposition has begun, electrolysis consists of electrical discharge of ions it is evident that it must act in opposition to solution pressure and in conjunction with osmotic pressure. That is, in order to produce continuous electrolysis a voltage must be applied to the electrodes that is at least as great as the algebraic difference between the single potential differences normally established at the cathode and anode. This difference constitutes the theoretical "decom- position voltage" of a given salt solution, and no appreciable electrolysis can take place as a result of the application of a lower voltage. The attempt to calculate the decomposition voltage of a solution from the single potential differences, ex- perimentally determined, does not always give results that agree with those found by experiment. This is because the ion con- centration is not necessarily equal to 1 gm-ion per liter and it changes as electrolysis proceeds. Also where gases are evolved at the anode (a condition generally noticed) the phenomenon ELECTRO-ANALYSIS 119 of " overvoltage " exerts a very important influence upon the practical decomposition voltage. The following examples are taken from the work of LeBlanc 1 and others, showing practical agreement between calculated and experimental values. Salt Decomposition voltage Calculated | Experimental ZnBr 2 . 1.76 1.65 1.84 1.65 1.80 1.85 1.88 1.78 NiCl 2 CdCl 2 CoCL., Agreement is not at all satisfactory in the case of salts of oxy acids. This is partly because oxy salts are not normal with respect to the oxygen or hydroxyl ion, also because^ oxy- gen shows overvoltage to a marked degree, giving a much larger decomposition voltage than would be calculated. The matter that is here important is not the possibility of calculating decomposition voltages from known single potential differences but the recognition of the reasons for the fact that a minimum decom- position voltage must exist for every compound, under definite con- ditions, and that if this value can be determined a method for elec- trolytic separations is available. We shall understand that " de- composition voltage" refers to the fall in potential, measured across the electrodes, below which electrolysis cannot take place continuously. So far as we now know there is no upper limit to the voltage that should be used, excepting that set by the current density that should be employed, unless separations are to be made. Because the variable factors which influence the practical de- composition voltage (such as overvoltage of the anion, varying temperature and added electrolytes) and the consequent diffi- culty that is experienced in its calculation, Sand 2 proposed to measure merely the cathode potential difference, Feiectrode- eiectroiyte and to make metal separations by properly grading this potential. This is, no doubt) the ideal method. For making such measurements, however, more elaborate apparatus is re- quired and much care must be exercised. For most purposes such refinement is entirely unnecessary because the practical X Z. physik. Chem., 8, 299 (1891). 2 J. Chem. Soc., 91, 374 (1906). 120 QUANTITATIVE ANALYSIS decomposition voltage is usually fairly accurately known as the result of experiments with a given salt, electrolyzed under speci- fied conditions. Current Density. The relation between the amount of current flowing through a solution of an electrolyte and the amount of substance decomposed is stated in the law of Faraday 1 : (a) For any given electrolyte the amount of decomposition is directly proportional to the amount of current; (b) the amounts of different substances decomposed by the same current are proportional to the combining weights of the substances. According to the first part of this law the rate of decomposition and electro-deposition in any experiment will depend upon the current strength, except- ing the part played by other electrolytes that may be present. Being thus able to limit and control the rate of deposition the question arises as to whether there is any suitable current strength, above or below which good results will not be attained. If it were not possible for any current to pass through a solution ex- cept that carried by the ions of the salt which we desire to de- compose there would probably be no definite limit to the prac- tical current strength to be employed. The ions encounter a large frictional resistance in their migration toward the elec- trodes. In order to overcome this resistance the pressure (volt- age) is raised, often considerably above the decomposition voltage of the metal salt being analyzed, in order to hasten the action. If no other cation is present there is no upper limit to the prac- tical voltage. There is, however, another positive ion that is present in all aqueous solutions and particularly when acids are present. The ion referred to is that of hydrogen. If the pres- sure is raised above its discharge potential it can discharge at the cathode and will do so unless the current strength, concen- tration of hydrogen ion and concentration of metal ion are so related that the current can easily be carried from solution to cathode by the metal ion without the necessity for discharge of the hydrogen ion. This relation will evidently be such that there is a relatively small current, large metal ion concentration and small hydrogen ion concentration. The objection to the deposition of hydrogen on the cathode is based upon the fact that minute bubbles of gas prevent the proper coherence of the deposited metal. Apparently, then, the upper limit of current 1 Pogg. Ann., 33, 301 and 481 (1834). ELECTRO- A NAL YSIS 121 will be fixed by the point at which noticeable evolution of hy- drogen (or other gas) occurs. This limit cannot well be calcu- lated but is determined by experiment. It should be noted that the value to be measured is not that of total current flowing across the solution but is that of the current flowing into unit area of the electrode where the desired deposition is taking place, usually the cathode. This gives rise to the term " cur- rent density," abbreviated to CD. In stating the conditions to be observed in electro- depositions the current density may be more specifically defined as the current in amperes flowing into each 100 sq cm of cathode surface. This is denoted by CDioo. _ . , ,. total amperes Evidently CDioo = 5 . u , - e . The square decimeters cathode surface proper current density to be employed in a given case is called the " normal density" for that experiment. This is indicated by NDioo. The normal density is fixed by the conditions already * discussed. There is considerable variation, however, based also upon the form of the electrodes. This will be taken up in the next paragraph. Nature of Electrodes. Electrodes must possess certain proper- ties in order to be suitable for use in quantitative analysis. The electrode material must be insoluble in the solution of electro- lyte, with or without current action. In the process of metal plating as used in the arts it is customary to make the anode of the metal being plated, so that the rate of deposition at the cath- ode is equal to the rate of solution at the anode, the mean concentration of the metal in the solution remaining constant. This is obviously out of the question in quantitative analysis, where the total metal in solution, and no more, is to be deposited. The material most used for electrodes is platinum. The con- tinued advance in the cost of platinum has led to a search for less expensive materials. However, the combination of high electrical conductivity and low solubility is a rare one. Other metals could be used for cathodes because the current action prevents their resolution, but when the deposited metal is to be removed after the process is finished the solvent used will gen- erally dissolve some of the electrode also. So long as the cost of platinum is sufficiently low to make it possible to provide an adequate supply of platinum electrodes it is doubtful whether any other material will supplant it to any great extent. 122 QUANTITATIVE ANALYSIS Gooch and Burdick 1 have recently perfected a method for making electrodes, by which a very small amount of platinum is spread over a relatively large surface of glass. A mixture of glycerine and chlorplatinic acid is spread over the glass surface, which is then heated. The glycerine is evaporated and the chlorplatinic acid is decomposed, elementary platinum fusing into the glass surface. Carbon is a good conductor of electricity but it has not been successfully used for electrodes. Turrentine 2 has used a process for making a graphite electrode of exceptional density but any known process fails to produce an electrode of this material FIG. 37. Turpentine's graphite FIG. 38. Turpentine's graphite anode. cathode. that is not sufficiently porous to permit absorption of the solu- tion and that does not lose weight by rubbing off particles of graphite. From such an electrode it is not possible, in practice, to remove all absorbed matter, or to completely dry it except by long heating. Mercury is used as a cathode for a certain class of work and this will be discussed in a later paragraph. The electrodes must also be chemically unaltered by the passage of a current or else altered in a definite manner. The first condition is more often realized but there are cases where one electrode is altered in a definite manner, as a silver anode, used for the determination of chlorine, bromine or iodine anion becomes coated with chloride, bromide or iodide of silver. 1 Z. anorg. Chem., 78, 213 (1912). 2 J. Phys. Chem., 13, 438 (1909); Chem. News, 100, 43 (1909). ELECTRO-ANALYSIS 123 The electrode that is to be weighed and is to receive the deposit (usually the cathode) should present the maximum surface for the minimum weight of electrode material. Since a practical limit is placed upon the current density the duration of the process of deposition will be inversely proportional to the total electrode surface exposed. The weight of the electrode must not be too large for accurate work and these considerations naturally lead us to consider the form of material where the ratio of surface to weight will be as large as possible. In general, any piece of platinum, small enough to be weighed, may be used as a cathode for metal deposition. Any chemist who has a dish or crucible may make at least an occasional analysis by the use of such an article as cathode. The dish is simpler because the solution may be placed directly in it and a coil of platinum wire used as an anode. The ratio of surface to weight is not large in this case, especially as only one surface is effective. Moreover if there is any sedi- ment in the solution this may be partly caught by the depositing metal and weighed along with the latter. The cathode dish designed by Classen is quite thin and presents a larger surface. A dish of this kind weighing about 40 gm has a capacity of 250 cc and presents an inner surface of about 150 sq cm to the solu- tion. A crucible may be used as a cathode by connecting as in Fig. 39. A rubber stopper is used to help support the crucible, a metal rod passing through and connecting with the negative of the current source. A small platinum wire serves to complete the connection of crucible with rod. This form of cathode also possesses a small relative surface. Other forms of cathodes are open cylinders and cones of foil,, and gauze cylinders and plates, made from gauze of small mesh and fine wire. These forms are shown in the illustrations (Figs. 40 and 41). Of all of these forms the gauze electrode is most effi- cient, not only because the relative surface is greatly increased by constructing of fine wire but also because practically all parts of the surface are equally effective. The latter condition does FIG. 39. Crucible cathode. 124 QUANTITATIVE ANALYSIS not obtain for foil electrodes of any form, the surface farthest from the anode being in a relatively weak electrolytic field. Gauze electrodes also permit better mixing of the solution and very much higher current densities may be used. Special forms of electrodes for rapid rotation will be discussed later. FIG. 40. Platinum foil cathodes. Other Apparatus. The necessary apparatus for electro-analysis will include, besides the electrodes, a generator of direct current, variable resistance, voltmeter and ammeter. The source of current may be a dynamo, any of the forms of primary cells, secondary or storage cells or thermoelements. The thermoele- ment is not a practical source of current, being both inefficient and unreliable. The direct current from a dynamo may be used and is better than primary cells, the latter being trouble- some in the matter of maintenance. The chief objection to ELECTRO-ANALYSIS 125 the dynamo current lies in the fluctuations usually resulting from a variable load on the line from the generator. The best and most satisfactory current producer for this class of work is the secondary or storage element. Any of the various forms of accumulators will prove satisfactory, the lead-lead peroxide cell being the best known. The great merit of the secondary cell is its constancy and reliability. The E. M. F. of the lead cell is about 2 volts and the necessary voltage for the work may be obtained by connecting several cells in series. Any rheostat will do for this work, provided that the range in resistance is properly related to the other factors entering into the determination of cur- rent strength. In the absence of such a rheostat carbon lamps may be used for the current control, if the line volt- age is high. The resistance of a 16 c.p. carbon lamp is about 220 ohms and by arranging several in parallel a fairly satisfactory regulation of current may be provided. Voltmeters and ammeters should have the scales graduated with a range as limited as is consistent with the current conditions to be employed, so that each subdivision may represent a small frac- tion of a unit. A satisfactory plan is to have double scale instruments, the range of one scale being ten times that FlG - 41. Platinum gauze of the other. cathode and spiral anode. The following is a description of the apparatus now in use in the Purdue laboratory. Purdue University Laboratory for Electro -Analysis. Current is furnished by storage cells of the lead type, each having a capacity of 48 amp-hours and a maximum charge and discharge rate of 6 amp. They are placed in a closed battery case which is outside the room for electro-analysis. The cells are provided with sand trays and insulators, and also with glass covers which 126 QUANTITATIVE ANALYSIS almost entirely prevent the annoyance due to acid spray during charging, and the case ventilator makes charging absolutely inoffensive. The interior of the case is protected against the attack of acid by asphaltum paint. Wires from the cells run into the special laboratory where they are connected with the distributing switchboard. This is FIG. 42. Distributing switch board of the Purdue University Laboratory for electro-analysis. a 28-in.X72-in. board of black oiled slate, providing switches and plug receptacles for the control of all current which is used for any purpose within this room. The cells are con- nected in series groups of three each, the outside terminals of each group having double receptacles. This arrangement en- ELECTRO-ANALYSIS 127 ables the operator to connect his cells with any number of groups in multiple, thus giving greater latitude in the selection of voltage and current strength than is possible with the usual series connections. The 110-volt charging current enters the board through a switch which can be connected by plug connectors with any cell or combination of cells, and a slide-wire rheostat on the back of the board makes it possible to charge any number of cells at a time. The cells are protected during charging by an under-load circuit-breaker, and also during both charge and discharge by a fuse panel which is placed on the back of the board. An ammeter with a range of 20 amp. and a double scale voltmeter, with ranges of 150 volts and 15 volts, are provided for proper control of the charging process. The 15-volt scale is used for testing the voltage of the cells when they are not in use. A switch on the distributing board controls the 110-volt alternating current which is used for the lights and motors. Finally, on this board are the terminals for all of the desks, so that any operator may connect with his desk any of the cells not then in use, and in almost any combination. It will be seen that the connections on this board make the different cells and desks practically independent of each other; for instance, a part of the cells may be charging while the remainder may be dis- tributed to the various desks as wanted. At each working desk is a 24-in.X36-in. slate panel which carries all of the apparatus that will be needed by the analyst, making each board an independent working unit. The volt- meter and ammeter on each board are double-scale instruments with ranges of 2 volts and 20 volts and amperes, respectively. The multiplier for the voltmeter is controlled by a small knife switch, and the shunts for the^ammeter are joined to plug receptacles. The current to each desk panel is controlled by a slide-wire taper rheostat. These rheostats are wound to give a total re- sistance of 130 ohms, in 254 steps. The carrying capacity of the first step is 1.3 amp. and that of the last step 25 amp., on continuous work. For working with rotating electrodes, 1/30 h.p. alternating- current, series motors of the commutator type are mounted on 128 QUANTITATIVE ANALYSIS the board and are controlled by a switch and 5-step rheostat. Induction motors are not used, because it is desirable to make variation in speed possible. These motors have a maximum speed of 2200 r.p.m. on 110 volts and are provided with three pulleys of different sizes. FIG. 43. A single desk panel and rotator, Purdue University Laboratory. In many laboratories it is the practice to mount the motor directly on the electrolyzing stand, thus avoiding all belting and making possible direct connection with the rotating electrode. On the other hand, the application of a small belt is a simple operation and not only very much decreases the vibration of ELECTRO- ANAL YSIS 129 the stand, and consequently the danger of dust particles fall- ing into the bath, but also removes the motor from the region of the bath, which in many cases contains acids and which is frequently hot. Corrosion of the motor parts is in this way largely prevented. The stand for holding the electrodes is of iron, is quite heavy, to prevent vibration, and is fitted with rubber feet. Every portion of the base and vertical rod is heavily enameled and the electrode supports are clamped to the rod by means of heavy thumb screws, but in such a manner that the screw does not come FIG. 44. Group of five desk panels, Purdue University Laboratory. into contact with the rod, so that the enamel is not injured by the grip. Each clamp is insulated from the rod by a fiber bushing and each carries a binding post. There is no glass about the stand, perfect insulation of the electrode clamp being secured by the fiber bushings. The supporting ring for a dish cathode has three brass screw contacts which are ad- justable for dishes of different sizes. Finally, for stationary electrodes, simpler clamps are provided to take the place of 'the rotator. The rotator, which carries three pulleys of different 130 QUANTITATIVE ANALYSIS sizes, is a vertical shaft, the lower end of which carries a universal chuck, electrical contact being insured by a brass brush. COPPER If a copper salt is to be used a nitrate or sulphate is best suited. Other salts of volatile acids may be converted into the sulphate by evaporating with sulphuric acid, stopping the evaporation before any decomposition into copper oxide occurs. Copper deposits in a coherent form from solutions containing sulphuric acid, nitric acid, oxalic acid and ammonium oxalate, potassium cyanide, phosphoric acid, formic acid or ammonium hydroxide. ! Of all of these, nitric acid produces the best results and probably sulphuric acid is next best. Chlorides should not be present. If metallic copper is to be analyzed it may be dissolved in nitric acid and the undesired excess of acid removed by evaporation. Determination. Use enough sample to yield 0.25 to 0.50 gm of I copper. Dissolve in such a manner that 200 cc of solution will contain 1 about 2 cc concentrated nitric acid. It is sometimes desirable to make IB, larger quantity of solution, as 250 cc, and to use an aliquot part for each determination. In this case the acid may be added to the solution as used. The electrolysis may be begun and finished at the temperature of the room, but it will be hastened by warming the solution to about 70 at the beginning. Connect the weighed electrodes, add enough water to i cover the cathode, place split cover glasses on the beaker and electrolyze with a pressure not below 1.7 volts and not above 2.0 volts unless other metals are known to be absent. The current density that may be used will depend upon the kind of electrodes. For foil cones or cylinders or for dishes, NDiw= about 0.1 amp. For gauze electrodes NDi 00 may some- times be as high as 5 amp. In any case the analyst must use his judg- ment, watching the deposited metal to discover its character. The cop- per should appear as a bright red metal with no spots of brown and no tendency to crumble off the cathode. When the disappearance of color indicates that the metal is deposited remove a few drops to a test tube by means of a pipette and test for traces of copper by adding a slight excess of concentrated ammonium hydroxide or of potassium ferrocyanide. The former is preferable because if copper is found the solution can be neutralized with nitric acid and returned to the beaker. When all of the metal is found to be deposited arrange a small siphon tube in such a manner that the solution may be drawn from the bottom of the beaker without interrupting the current. As the solution is ELECTRO-ANALYSIS 131 removed wash down the exposed portion of the cathode removing every trace of acid before the E. M. F. is removed from the system. It is best to add water fast enough to keep the bottoms of both electrodes covered until the acid has become so diluted that no further action is to be feared. Siphon out the remaining liquid and perform the next operations as quickly as possible. Lower the beaker and remove the cathode, taking care to .avoid touching cathode to anode and thus making a short circuit, and quickly wash with much water. Set aside until the duplicate cathode has been treated in the same way, then wash both cathodes with redis- tilled alcohol and dry at 100. The alcohol washing may be omitted, but drying is hastened by this means. Weigh and calculate the per- cent of copper in the sample. SILVER The deposition of silver from solutions containing free acids may be accomplished, but the deposit is usually either spongy or crystalline so that it cannot be washed without loss. When nitric acid is present there is also a deposit of silver peroxide at the anode if high voltage is applied. The addition of potassium cyanide, although materially lowering the concentration of silver cations, is desirable because it entirely prevents the formation of silver peroxide and also yields a coherent deposit of silver at the cathode. The deposit is without lustre and should be white. The deposition of silver peroxide upon the anode, when high voltage is applied to solutions containing nitric acid, is probably due to the existence of the anion of a silver oxyacid, H 2 AgO 2 * + 2H-|-Ag0 2 . This is a theoretical derivative of the dihydroxide of silver, Ag(OH) 2 . The decomposition potential of this anion is higher than that of the univalent silver cation and its concentra- tion is always small, yet it will discharge to some extent at the same time that silver is depositing upon the cathode, if high vol- tage and current density are used:. AgO 2 *AgO + 0. Such a deposit having formed upon the anode it will finally redissolve as the silver becomes more dilute in the solution because the equilibrium is disturbed by the removal of silver cations, Ag, which are dis- charging at a much greater rate on account of their lower decom- 132 QUANTITATIVE ANALYSIS position potential. The discharge and deposition of silver peroxide is entirely prevented by potassium cyanide. When this is added to a solution of silver nitrate a precipitate of silver cyanide is first formed: AgN0 3 +KCN-AgCN-t-KN0 3 . An excess of potassium cyanide redissolves this precipitate, forming a salt of a complex anion: AgCN+KCN-*KAg(CN) 2 . In this way the equilibrium, is also disturbed by the conversion of silver into the new anion Ag(CN) 2 , which, presumably, has a high decomposition potential. The double cyanide of potassium and silver cannot all be in the form represented by the formula KAg(CN) 2 *as in this case no silver could be discharged at the cathode, but only hydrogen. There must, therefore, be equilibrium between the salt having the composition represented above and the single cyanides: K Ag (CN) 2 ^KCN + AgCN, or the ionic equilibrium Ag(CN) 2 ^Ag+2CN. It is to be supposed that the relatively high decomposition potential of the anion Ag(CN) 2 prevents its discharge under ordinary conditions. Determination. Use sufficient sample to give about 0.3 gm of silver. Dissolve this in a small amount of water and add 3 gm of potas- sium cyanide, stirring the solution until the precipitated silver cyanide is all redissolved, avoiding unnecessary excess. Fasten the weighed electrodes in place and dilute with water until the cathode is covered. The E.M.F. required will be about 2 volts and NDioo = 0.04 to 0.10 amp. for electrodes not of gauze or as high as 2 amp. if gauze electrodes are used. If the current density is too large the potassium cyanide will be decomposed around the anode, giving a dark solution of organic matter that will eventually reach the cathode and darken the silver. Such ELECTRO-ANALYSIS 133 darkening will occur also if the potassium cyanide is impure. A cyanide of high grade is required for this purpose. When the deposition is thought to be complete a few drops of the solu- tion may be tested for silver by adding concentrated hydrochloric acid in sufficient quantity to decompose the potassium cyanide and to precip- itate any remaining silver as chloride. When the deposition has been shown to be complete the solution is removed and the cathode washed, dried and weighed as in the preceding exercise. Remove the silver from the electrode by dipping into dilute nitric acid. Caution. Avoid inhaling the vapors that arise during the progress of electrolysis. Do not use pipettes and do not fill the siphon by suction. IRON Iron does not deposit well from solutions containing nitrates, chlorides or strong inorganic acids. If the iron salt is derived from such an acid this acid will be formed as electrolysis proceeds. To prevent its formation a salt of a weak inorganic or organic acid may be added. Examples of salts that are used for this purpose are ammonium oxalate, ammonium tartrate and sodium citrate. There is always a possibility of depositing some carbon from such solutions and this is least likely to occur when the oxalate is used. The iron or iron salt should be converted into the sulphate before electrolyzing. Determination. Calculate the weight of sample that will be required to give 0.25 to 0.5 gm of iron. If the sample is an iron salt, soluble in water, dissolve in water, using no more than is necessary. If the sample is iron or steel, dissolve in the proper quantity of dilute sulphuric acid, avoiding an excess. In either case, add, stirring, a saturated solu- tion of ammonium oxalate until the precipitate of iron oxalate is redis- solved. Dilute, after the electrodes are in place, until the solution covers the cathode. The decomposition voltage for iron in such a solu- tion is about 2 and 2VD 100 = 0.1 to 0.5 amp. The deposited iron should be bright. It does not easily redissolve in the solution when the current is interrupted and may be readily washed. The end of the process is tested by the use of potassium ferricyanide or potassium thiocyanate. If the latter is used the solution should be previously warmed with a drop of nitric acid, since the iron (if any is present) has been reduced to the ferrous condition by the current action. After weighing the iron it should be removed from the cathode by dissolving in dilute sulphuric acid. 134 QUANTITATIVE ANALYSIS LEAD , When salts of lead in solution are subjected to the action of a current the lead may deposit upon both cathode and anode upon the former as elementary lead and upon the latter as lead peroxide. Upon the cathode the lead is so spongy that it becomes impossible to wash and dry it properly. This is the familiar action of the lead storage cell during charging, where lead sulphate is electrolyzed, producing spongy lead at the " negative" and lead peroxide at the " positive." If the lead salt solution contains a considerable excess of nitric acid the entire quantity of lead deposits upon the anode as peroxide and this is the only practicable quantitative method for the electroly- sis of lead salts except where the mercury cathode is used. Lead dioxide cannot be completely dehydrated unless heated to a temperature above 200. The deposition of lead peroxide upon the anode when nitric acid is present is to be explained exactly as in the case of silver. The solution contains a small concentration of the amphoteric lead per hydroxide, Pb(OH) 4 , which furnishes anions of an oxy- acid of lead, H 4 Pb04. The electrolysis of this acid and dis- charge of its anion produces lead peroxide and oxygen : Fb0 4 -+Pb0 2 -{-0 2 . As in the case of silver the formation of the peroxide may be prevented by the addition of some substance which will diminish the concentration of lead cations, such as ammonium oxalate. Since the cathodic deposit of lead cannot well be washed and dried without loss or oxidation the anodic deposition of peroxide is assisted by addition of nitric acid. Determination. Weigh enough lead salt (preferably nitrate) to produce about 0.1 gm of lead, dissolve in the proper quantity of water to cover the electrodes and add 20 cc of concentrated nitric acid for each 100 cc of solution. Connect the electrodes so that the one with the largest surface will be the anode, instead of the cathode as is the case in the electrolysis of most other metals. Warm to about 50 and keep at this temperature until the electrolysis is finished. Use 2.4 volts. NDiQQ = 1.5 amp. At the end of the operation test the solution for lead by adding a few drops of hydrogen sulphide solution to a small amount of the electrolyte. Carefully remove the anode, having previously washed it in the usual way. Dry at 200 to 230 until the weight is constant. ELECTRO-ANALYSIS 135 NICKEL The best solution from which to deposit nickel is that of the sulphate, containing ammonium sulphate and ammonium hy- droxide. If nitric acid is present there is usually some trouble, due to the oxidation of the deposited nickel. Nickel may also be precipitated from solutions containing ammonium oxalate, tartrate or citrate or from solutions containing an excess of potas- sium cyanide. Determination. Separation of Copper, Nickel and Iron. Thoroughly clean and dry a nickel coin, weigh it accurately, place in a casserole and dissolve in nitric acid (sp. gr. 1.2) the casserole being covered while the coin is dissolving. Carefully add 10 cc of concentrated sulphuric acid and evaporate over a flame until the characteristic dense, white fumes of sulphuric acid appear. The evaporation should be accomplished while holding the casserole in the hand, giving it a continuous rotary motion to hasten evaporation and prevent spattering. Allow the mate- rial to cool then wash into a 250 cc graduated flask and dilute to the mark. Measure 50 cc of the solution into the vessel in which electrolysis is to be accomplished and deposit the copper in the manner already described. The voltage should not exceed 2.7, which is nearly the decomposition voltage of nickel. Unusual care should be exercised in washing and saving the washings because other metals are to be deter- mined. Evaporate the solution from which the copper has been removed, until the volume is about 100 cc. Neutralize with ammonium hydrox- ide, boil to flocculate the colloidal ferric hydroxide which is always present and filter off the precipitate, washing the paper and precipitate with hot water. Add to the filtrate 7 gm of ammonium sulphate and 20 cc of ammonium hydroxide (sp. gr. 0.90), and electrolyze. Decomposition voltage is about 2.8 and any voltage above this value may be used, the upper limit being fixed by the nature of the deposit obtained. Record the current density. Dissolve the ferric hydroxide in the filter paper with 1 cc of oxalic acid solution, saturated at about 20. Wash the solution out of the paper with hot water and into a solution of 5 gm of ammonium oxalate in 100 cc of water. Electrolyze as previously directed. Moving Electrodes. In the discussion of decomposition voltage it was noted that if the voltage is unduly increased in order to hasten the decomposition, gas evolution prevents the formation of a dense deposit of metal. Migration of the ions 136 QUANTITATIVE ANALSYIS is comparatively slow and current is transferred, not only across the solution, but also from the solution to the cathode, by hydro- gen ions. If the migration of the metal ions is aided by stirring the solution large currents may sometimes be carried without the deposition of enough hydrogen on the cathode to injure the metal deposit. Stirring may be accomplished by any one of five different methods: (1) Heat- ing to produce convection currents, (2) use of stirring apparatus not connected with the elec- trodes, (3) rotation of the anode, (4) rotation of the cathode, (5) electromagnetic action. Convection currents are of limited usefulness because they are not sufficiently rapid. They will, however, materially shorten the time of elec- trolysis. Mechanical stirring, whether by the second, third or fourth method, has practically the same use. Which method of these three is to be chosen will be decided chiefly by matters of convenience. If a stirrer of glass or other non-conducting material is to be used it will require room for its movement and, since it is as easy to rotate one of the electrodes, the stirrer is generally made one of these. Either elec- trode may be rotated with success. The anode is generally the one chosen for this purpose be- cause it has not the large surface of the cathode and is therefore more easily manipulated. Fig. 45 shows one of the forms of anodes that may be rotated rapidly without becoming bent or distorted. The cathode is most frequently a dish. In order to provide a dish of large sur- face and capacity Classen devised a very thin platinum dish, shown in Fig. 46. The speed of rotation of the anode should be as high as may be attained without danger of throwing the solution out of the vessel. 500 to 1000 r.p.m. may be used. Recently a comparatively slow rota- tion (150 r.p.m.) of the ordinary spiral anode inside a gauze cathode has been used, with a considerable degree of success. Indeed, it is questionable whether this is not more practicable than the rapid rotation because the gauze electrodes may be used with- FIG. 45. Anode suitable for rotating. ELECTRO-ANAL YSIS 137 out danger and the apparatus needs no watching after starting. The time necessary for complete deposition of the metal may be made about one-fifth of that required when stationary elec- trodes are used. In the electromagnetic stirring apparatus devised by Frary 1 the solution is placed within an electromagnetic field, generated by a current passing through a solenoid surrounding the elec- trolyte. The moving ions constitute a conductor in which the current moves radially while the electromagnetic field is vertical. FIG. 46. Classen's dish cathode. In another form the apparatus is changed using a vertical field and vertical current lines. In either case the mutual action of the fields causes rotation of the solution within. The Mercury Cathode. By making the cathode of mercury instead of platinum two important gains are made. The large expenditure for electrodes is largely eliminated because of the relative cheapness of mercury, also there is no longer any ques- tion as to the satisfactory nature of the deposit of metal, since the latter amalgamates with the mercury instead of forming a surface deposit. The one obstacle to a nearly universal use of this form of electrode lies in the small surface that may be used, the large specific gravity of mercury prohibiting the use of more than a few cubic centimeters. The most satisfactory form of apparatus for this purpose is a glass cup having a small platinum 1 J. Am. Chem. Soc., 29, 1592 (1907). 138 QUANTITATIVE ANALYSIS wire fused into the bottom for the cathode connection (Fig. 47). The upper limit to 'the normal density is approximately fixed by the undesirable heating effect of large current densities. The anode should be rotated. In the following exercises are given the changes necessary to adapt the foregoing exercises to the use of rotating electrodes and the mercury cathode. FIG. 47. (a) Mercury cathode cell with (b) drying apparatus. Determination of Copper by Use of the Rotating Anode. Set up the apparatus and add to the solution 1 cc of dilute sulphuric acid instead of nitric acid. Use rapid rotation and note the current density that can be employed. For slow rotation use the gauze cathode and spiral anode. Use the same solution as with stationary electrodes but increase the current density as much as possible, noting the character of the deposit in deter- mining the practicable maximum. Determination of Copper by Use of the Mercury Cathode and Rotating Anode. Use the small amount of solution made necessary by the size of the cup. Thoroughly clean the cup and place in it pure mercury until the whole apparatus weighs not more than 60 gm. Wash with distilled water, then with redistilled alcohol and finally dry by passing dried air through the cup. The drying tube on the inlet of the drying apparatus must be so arranged that the air is well filtered by cotton and no calcium chloride can enter the cell. The drying will cool the mercury and this must be allowed to warm to the temperature of the ELECTRO-ANALYSIS 139 laboratory before weighing. Connect the platinum wire with the nega- tive binding post and place the anode in position. Pour in the solution but do not add acid of any kind. Rotate the anode as rapidly as may be done without loss of solution by spattering. Limit the current density only by the tendency of the solution to boil. After the electroly- sis is completed stop the rotation of the anode, reduce the voltage (but not below 2 volts), lower the anode until it almost touches the mercury and siphon and wash until no acid remains. Finally rinse the cell with redistilled alcohol and dry at the temperature of the laboratory. The copper amalgam may be used as a cathode in more experiments but it should be replaced by pure mercury as soon as it shows any ten- dency to form a scale of undissolved copper. Determination of Silver by Use of the Rotating Anode. Use the same solution as with stationary electrodes. The apparatus and general manipulation are the same as with copper. Either rapid or slow rotation of the anode will prove to be satisfactory. Determination of Silver by Use of the Mercury Cathode and Rotating Anode. Dissolve the silver salt in the proper amount of water and do not add potassium cyanide. Proceed as with copper. The deposit of silver peroxide that usually appears upon the anode at first should later redissolve so that all of the silver will finally amalgamate at the. cathode. Determination of Iron by Use of the Rotating Anode. The iron should be in the form of sulphate. Note the current conditions required to produce a good deposit. Determination of Iron by Use of the Mercury Cathode and Rotating Anode. Use the same solution as in the preceding exercise and same current conditions. CHAPTER V VOLUMETRIC ANALYSIS The gravimetric process involves the conversion of a given constituent of a substance into a compound of known composi- tion by the addition of an excess of a precipitating reagent. The volumetric process consists in the addition of a reagent of accurately known concentration (a " standard solution") until a definite reaction with the substance is exactly completed. In the first case the compound produced is weighed while in the second case a solution of one of the substances reacting is measured by volume, the weight of the reacting substance being thus obtained indirectly. In the gravimetric process the constituent to be determined is actually weighed in a new compound and its weight calculated from the known composition of that compound. In the volumetric process the constituent to be determined is cal- culated from its known reacting ratio to the indirectly observed weight of reagent. Since a measurement of volume is much more quickly and easily made than is the case with the necessary filtration, washing, drying, ignition, cooling and weighing of a gravimetric determination, it follows that the volumetric method frequently results in a great saving of time. This is, however, not always the case and the method to be chosen will be that which, all circumstances considered, can be carried out most easily, quickly and accurately. No generalization can, at pres- ent, be made regarding this choice. The choice itself will not be difficult or uncertain after some experience is gained in general quantitative analysis. Apparatus. It may be observed at the beginning that the balance is practically always concerned even in the volumetric process. The concentration of the standard solution must be determined and this determination is generally gravimetric. This fact might seem to remove the time-saving element from 140 VOLUMETRIC ANALYSIS 141 the new class of methods. This is not so because one gravi- metric determination suffices for a large number of volumetric determinations if a sufficient quantity of standard solution is made. The volumetric process will involve many very accur- ate measurements of volume and consequently several forms of graduated apparatus. These will be described with some detail. Flasks. For measuring relatively large quantities (50 cc to 2000 cc) of liquids in one portion the graduated flask may be employed. This is the form of apparatus that possesses the least relative surface and consequently causes the least trouble in washing, draining, etc. The one reading is made at a mark on the neck. The neck must be sufficiently small to permit a read- ing with a slight percentage error but must be large enough to permit filling and empty- ing without trouble. These are practical limits, fixed as the result of experience. The requirements of practice also demand a neck of uniform bore and of some margin above and below the mark. Volumetric flasks may be graduated to contain a stated amount or to deliver this amount when emptied. No container can be made to deliver all of the liquid con- tained in it if the liquid is one that wets glass, a condition that obtains with water FIG. 48. Volumetric and solutions in water. If the flask is to flask, be graduated to deliver a stated amount the mark must be placed higher than if it indicates the same amount contained in the flask. If a flask, upon empty- ing, could be made to drain uniformly it could be accurately calibrated for deliverance. On account of the necessary form of the flask this is impossible and for accurate work the flask is always calibrated for containing the amount indicated by the inscription upon it. Pipettes. Pipettes are filled by suction and allowed to de- liver the liquid by the action of gravity. Common forms of pi- 142 QUANTITATIVE ANALYSIS pettes are shown in the illustrations. The pipette which is grad- uated to deliver one fixed and stated amount is known as a " transfer pipette." On account of the smallness of the bore at the point where the mark is placed the pipette may be made to measure liquids with a relatively high degree of accuracy. Certain errors in their use often render the readings highly inaccurate. These are chiefly due to inconstancy in the length of the period of draining. The pipette must be quite clean, espe- cially with regard to the slight film of oily matter that so easily deposits within it. Also the same period of time must be allowed for draining in all cases where a given instrument is used and, in the case of pipettes, a uniform procedure must be followed regarding the removal of the last drop which is retained in the point of the pipette. The approved practice is to touch the point of the instrument against the vessel into which the liquid flows but not to blow out the drop that is still retained. Burettes. The burette finds a more extensive use than any other form of volumetric apparatus. The flask is used in making the standard solu- tion, often in making solutions of the substance being analyzed and sometimes in subdividing these solutions. The burette is used in each determina- tion and it must be so graduated as to measure any quantity of liquid between the extreme limits of its graduations. Its construction, calibration and use therefore require exceptional care. Its bore must be uniform, its graduations sharp, dis- tinct and correctly placed. It must drain freely v FIG. 49. (a) Transfer pipette and (b) measur- an d uniformly and it should be provided with a ing pipette, cock of proper construction to permit easy control of the outflow. In reading burettes considerable errors sometimes result from parallax. This is because the part of the surface of the liquid which is being observed is the lowest point of the meniscus and this point is in the center of the bore. If, therefore, the eye is not in the same horizontal VOLUMETRIC ANALYSIS 143 plane with this point or if the burette is not in a vertical position the line upon the exterior of the burette, apparently marking the position of the meniscus, does not represent the correct reading. This is made evident from Fig. 50 in which the errors are pur- posely exaggerated. In order to increase the accuracy of reading and to prevent parallax, various devices are used. In Schell- bach's burette, Fig. 51, a background of white glass bears a stripe of blue. The meniscus appears against this as a point. This im- provement is of doubtful value except in rooms where the light is not good, because it does not prevent parallax. The use of FIG. 50. Effect of Parallax. FIG. 51. Meniscus as seen in Schellbach's burette. floats, Fig. 52, sometimes renders readings more easily made, especially when the liquid is so dark in color as to be nearly opaque. The mark on the float is brought so near to the side of the burette that parallax is also largely prevented. Trouble due to sticking of the float is sufficient cause for dispensing with its use whenever possible. The best construction of burettes yet devised is that specified by the U. S. Bureau of Standards. 1 Upon burettes made according to these specifications the marks for whole cubic centimeters extend entirely around the burette while those for subdivisions extend half way around. This ar- 1 Bull. Bur. Standards, Vol. 4, No. 4, pages 556 to 563. This discussion is also to be obtained in Bull. Bur. Standards, Reprint No. 92. 144 QUANTITATIVE ANALYSIS rangement absolutely obviates the troubles of parallax and makes quite sharp readings possible. Certain devices are sometimes used for promoting rapid fill- ing of the burette. Fig. 54 illustrates some of these. To set up such a burette and keep it in working order requires a certain amount of attention and such devices are of value chiefly in works laboratories where large numbers of routine determinations are to be made by means of the same standard solution. The burette with a plain glass cock is most serviceable for ordinary use. All bu- rettes should be covered by a cap when in use. This excludes dust and lessens evaporation of the solution. Units of Volume. For the requirements of volumetric analysis the same accuracy may be obtained without regard to the particular unit of volume adopted, provided that all of the different pieces are calibrated upon the basis of the same unit. The liter is defined by the International Bureau of Weights and Measures to be the volume occupied by water having a mass of 1 kg at 4 C. This is almost exactly 1000 cc and for all practical purposes may be regarded as such. (The milli- liter is 1.000029 cc). It is now customary to use this true liter as the standard, calibrating apparatus upon this basis, the apparatus to be used at the average room temperature. In America the working temperature is usually taken to be 20. Other temperatures are used as standard working tem- peratures, particularly abroad where 17.5, 15.5 or 15 is used as a calibration and working temperature. When the true liter is made the basis of calibration and higher temperatures than 4 are used for the experimental part of the calibration corrections must be made for the difference in density of water used for calibrating and, if the water is weighed, also for the buoyant effect of air. In order to avoid making such corrections Mohr 1 suggested a different unit, the "Mohr 1 Lehrbuch, Chemisch-analytischen Titriermethoden, 6th ed. (Rev. by A. Classen), 42. FIG. 52. Burette floats. VOLUMETRIC ANALYSIS 145 liter/' which is defined to be the volume of 1000 gm of water weighed in air at a standard pressure of 760 mm of mercury and at a temperature of 17.5. Tolerance. Certain experimental errors in the graduation of volumetric apparatus may be regarded as reasonable errors, on , FIG. 53. Form of bur- ette approved by the Bureau of Standards. FIG. 54. Burette with automatic filling and overflow devices. account of which the apparatus should not be rejected. This does not mean that corrections should not be made after the results of calibration are known. Maximum permissible errors in graduation are known as " tolerances" and if the tolerance is exceeded in a given case the piece should not be used without regraduation. 10 146 QUANTITATIVE ANALYSIS The following quotation from the bulletin of the U. S. Bureau of Standards already referred to gives the requirements of that bureau for volumetric flasks, burettes and pipettes to be accepted for testing. These requirements are recognized as, at once, rigid and scientific. Apparatus used, even by the student beginning the study of volumetric analysis, should, whenever possible, conform to these requirements. GENERAL SPECIFICATIONS. "(a) Units of Capacity. The liter, defined as the volume occupied by a quantity of pure water at 4 C. having a mass of 1 kg, and the one- thousandth part of the liter, called the milliliter or cubic centimeter, are employed as units of capacity. "(6) Standard Temperature. 20 C. is regarded by the bureau as the standard temperature for glass volumetric apparatus. "(c) Material and Annealing. The material should be of best quality of glass, transparent and free from striae, which adequately resists chem- ical action, and has small thermal hysteresis. All apparatus should be thoroughly annealed at 400 C. for 24 hours and allowed to cool slowly before being graduated. " (d) Design and Workmanship. The cross section must be circular and the shape must permit of complete emptying and drainage. "instruments having a base or foot must stand solidly on a level sur- face, and the base must be of such size that the instruments will stand on a plane inclined at 15. Stoppers and stopcocks must be so ground as to work easily and prevent leakage. "The parts on which graduations are placed must be cylindrical for at least 1 cm on each side of every mark, but elsewhere may be enlarged to secure the desired capacities in convenient lengths. "The graduations should be of uniform width, continuous and finely but distinctly etched, and must be perpendicular to the axis of the apparatus. All graduations must extend at least halfway around, and on subdivided apparatus every tenth mark, and on undivided apparatus all marks must extend completely around the circumference. "The space between two adjacent marks must be not less than 1 mm. The spacing of marks on completely subdivided apparatus must show no evident irregularities, and sufficient divisions must be numbered to readily indicate the intended capacity of any interval. Apparatus which is manifestly fragile or otherwise defective in construction will not be accepted. VOLUMETRIC ANALYSIS 147 "(e) Inscriptions. Every instrument must bear in permanent legible characters the capacity in liters or cubic centimeters, the temperature in Centigrade degrees at which it is to be used, the method of use, i.e., whether to contain or to deliver, and on instruments which deliver through an outflow nozzle the time required to empty the total nominal capacity with unrestricted outflow must be likewise indicated. "Every instrument should bear the name or trade-mark of the maker. Every instrument must bear a permanent identification number, and detachable parts, such as stoppers, stopcocks, etc., belonging thereto, must bear the same number. SPECIAL REQUIREMENTS. "(a) Flasks. -^A.t the capacity mark or marks on a flask the inside diameter should be within the following limits: Capacity of flask (in cc) up to and including. Maximum diameter (in mm) 2,000 25 1,000 20 500 18 250 15 200 13 100 12 50 10 25 s Minimum diameter (in mm) 18 14 12 10 9 8 6 6 "The neck of a flask must not be contracted above the graduation mark. "The capacity mark on any flask must not be nearer the end of the cylindrical portion of the neck than specified below: Capacity Distance from upper end Distance from lower end 100 cc or less More than 100 cc 3 cm 6 cm 1 cm 2 cm "Flasks of 1 liter or more but not less may be graduated both to contain and to deliver, provided the intention of the different marks is clearly indicated. "(6) Transfer Pipettes. Pipettes for delivering a single volume are designated "transfer" pipettes. "The suction tube of each transfer pipette must be at least 16 cm long, and the delivery tube must not be less than 3 cm nor more than 25 cm long. "The inside diameter of any transfer pipette at the capacity mark must not be less than 2 mm and must not exceed the following limits : Capacity of pipettes (in cc) up to and including . . . Diameter (in mm) 25 4 50 5 200 6 "The outside diameter of the suction and delivery tubes of transfer pipettes exclusive of the tip must not be less than 5 mm. 148 QUANTITATIVE ANALYSIS "The capacity mark on transfer pipettes must not be more than 6 cm from the bulb. "The outlet of any transfer pipette must be of such size that the free outflow shall last not more than one minute and not less than the following for the respective sizes : Capacity (cc) in up to and. including 5 10 50 100 200 Outflow time (in seconds) .. 15 20 30 40 50 " (c) Burettes and Measuring Pipettes. Only those emptying through a nozzle permanently attached at the bottom are accepted for test. "The distance between the extreme graduations must not exceed 65 cm on burettes nor 35 cm on measuring pipettes. "The rate of outflow of burettes and measuring pipettes must be restricted by the size of the tip and for any graduated interval the time of free outflow must not be more than three minutes nor less than the following for the respective lengths : Length graduated | Time of outflow | Length graduated | Time of outflow 65 cm 140 sec 35 cm 60 sec 60 cm 120 sec 30 cm 50 sec 55 cm 105 sec 25 cm 40 sec 50 cm 90 sec 20 cm 35 sec 45 cm 80 sec 15 cm 30 sec 40 cm 70 sec " The upper end of any measuring pipette must be not less than 10 cm from the uppermost mark and the lower end not less than 4 cm from the lowest mark. "(d) Burette and Pipette Tips. Burette and pipette tips should be made with a gradual taper of from 2 cm to 3 cm, the taper at the extreme end being slight. "A sudden contraction at the orifice is not permitted and the tip must be well finished. "In order to facilitate the removal of drops and to avoid splashing when the instrument is vertical, the tip should be bent slightly. "The approved form of tips for burettes, measuring pipettes, and transfer pipettes is shown in Fig. 55. " Special Rules for Manipulation. These rules indicate the essential points in the manipulation of volumetric apparatus which must be observed in order that the conditions necessary to obtain accurate measurements may be reproduced. "(a) Test Liquid. Apparatus will be tested with water and the capacity determined will, therefore, be the volume of water contained or delivered by an instrument at its standard temperature. VOLUMETRIC ANALYSIS 149 "(b) Method of Reading. In all apparatus where the volume is limited by a meniscus the reading or setting is made on the lowest point of the meniscus. In order that the lowest point may be observed it is necessary to place a shade of some dark material immediately below the meniscus, which renders the profile of the meniscus dark and clearly visible against a light background. A convenient device for this purpose is a collar-shaped section of thick black rubber tubing, cut open at one side and of such size as to clasp the tube firmly. " (c) Cleanliness of Apparatus. Apparatus must be sufficiently clean to permit uniform wetting of the surface. FIG. 55. Tips of burette and pipettes, approved by the Bureau of Standards. " (d) Flasks and Cylinders. In filling flasks and cylinders the entire interior of the vessel will be wetted, but allowed a sufficient time to drain before reading. Before completely filling to the capacity mark flasks should be well shaken to completely mix the contents. " Flasks and cylinders when used to deliver should be emptied by gradually inclining them until when the continuous stream has ceased they are nearly vertical. After half a minute in this position the mouth is brought in contact with the wet surface of the receiving vessel to remove the adhering drop. "(e) Pipettes and Burettes. In filling pipettes and burettes excess liquid adhering to the tip should be removed when completing the filling. "In emptying pipettes and burettes they should be held in a vertical position, and after the continuous unrestricted outflow ceases the tip 150 QUANTITATIVE ANALYSIS should be touched with the wet surface of the receiving vessel to com- plete the emptying. "Stopcocks, when used, should be completely open during emptying. "Burettes should be filled nearly to the top, and the setting to the zero mark made by slowly emptying. "While under normal usage the measurements ordinarily are from the zero mark, other initial points may be used on burettes of standard form without serious error. "Tolerances. (a) Flasks Capacity less than and including Limit of error If to contain | If to deliver 25 cc 0.03 cc 0.05 cc 50 cc 0.05 cc 0.10 cc 100 cc 0.08 cc 0.15 cc 200 cc 0.10 cc 0.20 cc 300 cc 0.12 cc 0.25 cc 500 cc 0.15 cc 0.30 cc 1,000 cc 0.30 cc 0.50 cc 2,000 cc 0.50 cc l.OOcc (b) Transfer pipettes Capacity less than and including Limit of error 2 cc 0.006cc 5 cc 0.01 cc*y 10 cc 0.02 cc 30 cc 0.03 cc 50 cc 0.05 cc 100 cc 0.08 cc 200 cc 0.12 cc (c) Burettes and measuring pipettes Capacity of total gradu- ated portion less than and including Burettes Measuring pipettes 2 cc 01 cc 5 cc 0.01 cc 0.02 cc 10 cc. 0.02 cc 0.03 cc 30 cc 0.03 cc 0.05 cc 50 cc 0.05 cc 0.08 cc 100 cc 0.10 cc O.lScc "Further, the error of the indicated capacity of any ten consecutive subdivisions must not exceed one-fourth the capacity of the smallest subdivision." VOLUMETRIC ANALYSIS 151 Calibration. For accurate work the apparatus as supplied by the makers should never be regarded as correctly graduated until it has been tested (calibrated) by the user. Some manu- facturers use great care in graduating, especially with such appara- tus as must pass inspection by one of the national standardizing bureaus. Others are less careful and pieces are often found to have large errors in their graduation. Two general methods are in use for calibrating instruments for capacity. In the first the quantity of pure water or other liquid of known density which would exactly occupy the desired volume at the stated temperature is measured by weighing. The position of the meniscus is then compared with the mark upon the apparatus. If the latter was previously unmarked the position of the menis- cus is then marked. In the second method the capacity of the instrument is determined by allowing water or another liquid to flow into it from a previously standardized piece and the capacities are compared. In the first method temperatures must be accurately noted and corrections made for any departure from the standard temperature, also for air displacement. In the second method such corrections have already been made when the standard piece was calibrated and we have merely a comparison to make of two instruments of capacity. The second method is, therefore, shorter in point of time. Errors may occur if proper attention is not given to certain details of manipulation. Calibration by Weighing. Water is the most conveniently used liquid for this purpose. Since water solutions are generally to be used in volumetric analysis water possesses a second advantage in that the form of meniscus is most nearly that of the solutions later to be measured. The problem in calibrating, in the case of flasks or other pieces having but one or two marks and therefore easily remarked, is to determine the correct posi- tion of the mark when the piece contains the rated quantity of liquid. For this reason, in laboratories where all of the appara- tus purchased is regularly calibrated it is best and cheapest to purchase flasks unmarked, though conforming to certain specifi- cations as to dimensions and shape. If the true liter is to be taken as a basis it will first be necessary to determine the apparent weight of this volume of water in air, correcting also for the expansion due to the difference in tempera- ture between that at which the apparatus is calibrated and used 152 QUANTITATIVE ANALYSIS and the temperature upon the basis of which the liter is defined (4) . From the following table it is seen that the density of water at 20 working temperature is 0.99823. EXPANSION OF WATER ACCORDING TO P. CHAPPUIS 1 Density of pure water free from air, by tenths of degrees from to 40 and under standard pressure Standard degrees Tenths of degrees Mean differ- ences 1 2|3 4 5|6|7 8 9 0.999 8681 8747 8812 8875 8936 8996 9053 9109 9163 9216 + 59 1 9267 9315 9363 9408 9452 9494 9534 9573 9610 9645 + 41 2 9679 9711 9741 9769 9796 9821 9844 9866 9887 9905 + 24 3 9922 9937 9951 9962 9973 9981 9988 9994 9998 *0000 + 8 4 1.000 0000 *9999 *9996 *9992 *9986 *9979 *9970 *9960 *9947 *9934 - 8 5 0.999 9919 9902 9884 9864 9842 9819 9795 9769 9742 9713 - 24 6 9682 9650 9617 9582 9545 9507 9468 9427 9385 9341 - 39 7 9296 9249 9201 9151 9100 9048 8994 8938 8881 8823 - 53 8 8764 8703 8641 8577 8512 8445 8377 8308 8237 8165 - 67 9 8091 8017 7940 7863 7784 7704 7622 7539 7455 7369 - 81 10 7282 7194 7105 7014 6921 6826 6729 6632 6533 6432 - 95 11 6331 6228 6124 6020 5913 5805 5696 5586 5474 5362 -108 12 5248 5132 5016 4898 4780 4660 4538 4415 4291 4166 -121 13 4040 3912 3784 3654 3523 3391 3257 3122 2986 2850 -133 14 2712 2572 2431 2289 2147 2003 1858 1711 1564 1416 -145 15 1266 1114 0962 0809 0655 0499 0343 0185 0026 *9865 -156 16 0.998 9705 9542 9378 9214 9048 8881 8713 8544 8373 8202 -168 17 8C29 7856 7681 7505 7328 7150 6971 6791 6610 6427 -178 18 6244 6058 5873 5686 5498 5309 5119 4927 4735 4541 -190 19 4347 4152 3955 3757 3558 3358 3158 2955 2752 2549 -200 20 2343 2137 1930 1722 1511 1301 1090 0878 0663 0449 -211 21 0233 0016 *9799 *9580 *9359 *9139 *8917 *8694 *8470 *8245 -221 22 0.997 8019 7792 7564 7335 7104 6873 6641 64C8 6173 5938 -232 23 5702 5466 5227 4988 4747 45C6 4264 4021 3777 3531 -242 24 3286 3039 2790 2541 2291 2040 1788 1535 1280 1026 -252 25 077G 0513 0255 *9997 *9736 *9476 *9214 *8951 *8688 *8423 -261 26 0.996 8158. 7892 7624 7356 7087 6817 6545 6273 6000 5726 -271 27 5451 5176 4898 462C 4342 4062 3782 3500 3218 2935 -280 28 2652 2366 2080 1793 1505 1217 0928 0637 0346 0053 -289 29 0.995 9761 9466 9171 8876 8579 8282 7983 7684 7383 7083 -298 30 6780 6478 6174 5869 5564 5258 4950 4642 4334 4024 -307 31 3714 3401 3089 2776 2462 2147 1832 1515 1198 0880 -315 32 0561 0241 *9920 *9599 *9276 8954 8630 *8304 *7979 *7653 -324 33 0.994 7325 6997 6668 6338 6007 5676 5345 5011 4678 4343 -332 34 4007 3671 3335 2997 2659 2318 1978 1638 1296 0953 -340 35 0610 0267 *9922 *9576 *9230 *8883 *8534 *8186 *7837 *7486 -347 36 0.993 7136 6784 6432 6078 5725 5369 5014 4658 4301 3943 -355 37 3585 3226 2866 2505 2144 1782 1419 1055 0691 0326 -362 38 0.992 9960 9593 9227 8859 8490 8120 7751 7380 7008 6636 -370 39 6263 5890 5516 5140 4765 4389 4011 3634 3255 2876 -377 40 2497 2116 1734 1352 0971 0587 0203 *9818 *9433 '9047 -384 41 0.9-91 8661 Travaux et Memoires, Bur. Intern. Poids Mesures, 13 (1907). VOLUMETRIC ANALYSIS 153 One liter of water would therefore in a vacuum weigh 998.23 gm. In air both water and weights are apparently lighter than in a vacuum. If the density of these were equal the effect of air upon them would be equal and no error would be introduced. Since the density of the weights is greater and their volume less, the buoyant effect is greater upon the water and the apparent weight of the water in air is less than the true weight. One liter of air, at 760 mm pressure and at 20, weighs 1.2 gm. This is the buoyant effect upon the water. Analytical weights, whether plated or not, are usually constructed of brass. The density of brass may be taken as 8.4. The weight of air displaced by 998.23 gm of brass weights is then 0.99823X1.2 = 0.14gm. This is the buoyant effect upon the weights. The difference 1.2 0.14 = 1.06 gm is the apparent loss in weight of the liter of water when weighed in air. 998.23-1.06=997.17. There- fore one liter of water at 20 and in air apparently weighs 997.17 gm. The weight of fractions of a liter will be calculated from this. Calibration by Standardized Bulbs. Any bulb or tube that is to be used as a comparison standard for calibrating by the second method must be of such a form that it will drain well upon emptying. It must also have the graduated portion small enough to make possible accurate readings at this part. The apparatus must be quite rigid so that varying pressure may have only an inappreciable effect upon the volume. The apparatus shown in Fig. 56 is that used by the Bureau of Standards. Its use is described as follows: "The arrangement used in testing 100 cc flasks permits the testing of long-necked flasks and graduated cylinders without placing the standard pipette at an inconvenient height. The standard pipette is inserted into a heavy rubber connection C. The delivery nozzle meets the con- necting tube B in a ground joint. "The pipette is filled through a glass nozzle, which is connected to a water faucet as shown in the illustration and is directed into the top of the pipette. The pipette is filled to the top, the nozzle is swung aside, and by opening the stopcock the meniscus is slowly lowered to the zero mark. Excess of water is then removed from the outflow nozzle, and the flask to be tested is placed on the platform E immediately under the 154 QUANTITATIVE ANALYSIS FIG. 56. Calibrating device of the Bureau of Standards. VOLUMETRIC ANALYSIS 155 outflow nozzle. The platform is raised by turning the wheel F until the outflow nozzle is just inside the neck of the flask. The stopcock is opened wide and the flask rotated to wet the entire neck, and the flask is then raised until the nozzle is from 1 to 2 cm above the mark. Before completing the filling of the flask it is removed and the contents shaken as directed in the rules for manipulation. The filling is completed with the tip in contact with the wetted wall 1 to 2 cm above the mark. The meniscus in the flask is finally brought to the mark by breaking contact of the tip with the wetted surface. "The standard pipette is read at the end of its normal outflow time, plus 15 seconds. The pipette reading plus the instrumental correction is the capacity of the flask at the standard temperature of the pipette that is, 20 C. The object of shaking the water is to disperse the con- taminations and thus produce a meniscus of normal volume. This manipulation reproduces the conditions of ordinary use. If the test is to merely ascertain whether the capacity is within the allowed limits of error, this detail is omitted unless the error is too near the limit to allow discrimination, in which case a retest is made. The magnitude of the possible error due to contamination has already been indicated in the previous discussion of variation in capacity of flasks. "This method of testing flasks obviates observations of temperature and weight and the calculations required in their reduction to equiva- lent volume, while it affords equal accuracy if care is observed. The method assumes that the temperature of the water is the same in the pipette and in the flask and that the coefficients of expansion of the two vessels are nearly equal. No large systematic error is peculiar to the volumetric method except, under very unfavorable conditions, this error of temperature. "The pipettes used for this purpose at this bureau are interchangeable, it being only necessary to employ a nozzle of the proper size in order to use any pipette with the holder and outflow tube. "The use of standard graduated pipettes for determining the capacity of flasks has been described by Morse and Blalock, and the method described above merely introduces certain details of refinement adapting it to the immediate requirements of the testing laboratory." In Fig. 57 are shown the standard bulbs devised by Morse and Blalock. 1 The three pieces shown provide a means for calibrating vessels of all of the various capacities common to the analytical laboratory, if proper combinations are made. In calibrating these bulbs it is necessary to determine the capacity from the single mark to the first stem division, also 1 Am. Chem. J., 16, 479 (1894). 156 QUANTITATIVE ANALYSIS the capacity of the stem for the smallest subdivision. If the water used is kept at the standard working temperature no correction for this factor need be introduced and the value 997.17 gm, previously deduced as the apparent weight of a liter of water, may be used without change. The bulbs are supported in such a manner that they may be readily filled from u FIG. 57. Morse-Blalock calibrating bulbs. a reservoir of distilled water at 20. The water from the bulb is carefully weighed and its volume calculated. That from the entire graduated portion of the stem is then weighed in a smaller vessel and the calculated volume is divided by the total number of stem divisions and the result recorded as the value of one division. In order to use the bulbs for the calibration of" VOLUMETRIC ANALYSIS 157 vessels that are to contain a specified volume of liquid, the vessel to be calibrated must first be cleaned as hereafter directed. The proper bulb is then placed in such a position that it may drain directly into the instrument being calibrated and the latter is marked at the meniscus. Instruments to be calibrated to deliver, such as burettes, are better calibrated by weighing the water delivered. If, however, it is desired to use the standard bulbs for this purpose the burette is so connected that it may empty into the bulb from below. The details of manipulation will be made clear in the exercises that follow. FIG. 58. Morse-Blalock bulb arranged for calibrating flasks. Cleaning Solution. Prepare a cleaning solution by dissolving 5 gm of powdered commercial sodium dichromate or potassium dichromate in 500 cc of commercial sulphuric acid. The solution may be kept in a bottle having a wide mouth, such as those in which dry chemicals are purchased. Burettes may be inverted and left standing in the bottle the solution then being drawn up by suction and held in the burette by closing the cock. For cleaning flasks the solution may be allowed to remain in the flask for some time or a small amount may be warmed and 158 QUANTITATIVE ANALYSIS the flask rinsed with it. The chromic acid produced by the interaction of sulphuric acid and sodium dichromate oxidizes all organic matter and leaves the glass thoroughly free from it. Exercise: Calibration of the Standard Bulbs. Clean the bulb and set it up in a manner similar to that shown in Fig. 56. Distilled water must be used. Place the bulb so that the graduated stem extends down- ward. A glass stopcock must be used for controlling the outflow, since pinchcocks and rubber connections would involve an uncertain change in volume of the apparatus. Fill the bulb to the upper mark. With the stopcock wide open allow the water to flow into a previously dried and weighed flask until the first division (zero) on the lower stem is reached. After 15 seconds adjust the water level to coincide with the mark, put the stopper in the flask (a glass stopper is desirable) and weigh. Place a weighing bottle under the delivery tube and allow the stem to drain to the last mark, stopper the bottle and weigh. If the water was at 20 divide its weight in grams by 0.99717 and record the result as cubic centimeters capacity of bulb and stem. Record also the value of each stem division and the division to be used as the mark for the rated capacity of the bulb. If the temperature was not 20 determine from the table on page 152 the weight of a cubic centimeter of water at the observed temperature, making the correction for air displacement as already explained. In the preceding exercise as in those that follow the bulb and reservoir may also be set up as in Fig. 58. The chief objection to this method of assembling lies in the fact that the water entering the bulbs must, for reasons already explained, pass through a glass stopcock, which is necessarily lubricated with some kind of grease. The result is that no matter how well the bulb may have been previously cleaned it acquires, at the first filling, a film of oil that absolutely prevents the proper draining at the next stage in the experiment. If the first arrange- ment is used and the bulb is filled from above a rubber connec- tion and pinchcock may be used and this annoyance avoided. Exercise : Calibration of Flasks by the Standard Bulbs. With the proper bulb in the position used in the preceding exercise, and with the dried flask to be calibrated placed under the delivery tube, allow water to flow into the flask until the proper mark on the lower stem is reached, exactly following the directions as to time of outflow as given in the pre- ceding exercise. Avoid handling either bulb or flask at the parts con- taining the water as the temperature is thereby raised. It has already been explained that after the bulbs have been standardized they may be used for further calibrations without regard to the temperature of the VOLUMETRIC ANALYSIS 159 water provided only that the temperature does not change during the progress of the experiment. To mark the flask cut a strip of gummed label, long enough to reach around the neck and about 1/4 inch wide. Carefully paste this with the original straight edge at the level of the meniscus, where the mark is to be made. Melt a small quantity of paraffin and brush a thin layer over the label and over a space of about 1 inch on either side of it. Using the point of a knife or of a sharpened piece of wood trace the straight edge of the label around the neck of the flask, making a mark sufficiently wide to be easily visible. The label here merely serves as a guide, making a regular line possible. Using a small feather as a brush apply a few drops of hydrofluoric acid and allow this to remain on the flask for two or three minutes, after which the acid may be washed off and the paraffin removed by warming. In case the flask already has a graduation and the calibration shows this mark to be in- correctly placed it is desirable to indicate the new mark by making a small, well-defined arrow with the point resting exactly upon the new mark. The operator's initials may be placed beside the arrow and if this is done carefully, no interference will result (Fig. 59) . If the flask contains no inscription etch the side like that shown in Fig. 48, page 141. Exercise : Calibration of Flasks by Weigh- ing. This is, in many respects, the most satisfactory method of calibration although more time is required. Have the flask clean and quite dry. Place on a balance of capac- ity sufficiently great to carry the filled flask. Counterpoise, then add weights to the right pan at the rate of 997.17 gm for each liter. Remove the flask from the balance and fill with distilled water at 20, nearly to the point where it is thought that the mark will be placed. Replace the flask upon the balance pan, then carefully drop in water from a pipette until the balance is in equilibrium. Mark as directed in the preceding exercise. In both of these exercises the flasks have been calibrated to contain the rated quantity and this is the only way in which flasks wjirbe used in this work. FIG. 59. Marking on the neck of a calibrated volumetric flask. 160 QUANTITATIVE ANALYSIS Exercise : Calibration of Burettes by Weighing. The marking of a burette is too complex to be easily changed and the calibration will therefore consist of finding what, if any, corrections must be applied to the existing graduations. Clean the burette with cleaning solution and distilled water. Fill with distilled water at 20. Weigh accurately a 25 cc weighing bottle to the third decimal then measure 5 cc of water into it from the burette, and reweigh. Add another 5 cc and weigh, continuing until the bottle is full. Empty the bottle, reweigh and continue the process until the water from the entire graduated portion of the burette has been weighed. Repeat the process in order to have a check upon the work. Calculate the true capacity of each of the ten portions, using the weight 0.99717 gm for 1 cc of water. Record as follows, the capacities in the last two columns being recorded only as far as the second decimal place. Weight of water, True capacity, each True total capacity, zero to end each interval. interval. of interval. Mark. Construct a curve showing the true reading at all points, using the plan shown in Fig. 60. In case any marked irregularity is observed at any part of the burette so that corrections taken from the curve would be inaccurate, recalibrate this portion, using 1 cc at a time. Exercise: Calibration of Burettes by the Standard Bulbs. Set up the apparatus as in Fig. 61. The reservoir must be higher than the Burette Reading. FIG. 60. Curve expressing the results of a burette calibration. top of the burette and this, in turn, must be placed so that the lowest graduation is higher than the bulbs. The tubing leading from the reser- voir to the burette may be of well-cleaned rubber. That between the stop-cock a and the bure'tte and bulbs respectively must be of glass, the VOLUMETRIC ANALYSIS 161 necessary connections being of heavy rubber tubing with the glass tubes pushed together until they touch inside the rubber. With the three-way cock b closed, open the cock a and fill the burette with water. Close a and open b so that the 2 cc and 3 cc bulbs may fill, then drain the burette to the zero mark and the bulbs to that mark on the stem of the 2 cc bulb which represents exactly 2 cc. This leaves the bulbs moistened as they will be throughout the experiment. Leave the burette cock open. Turn the cock b and measure 5 cc of water from the burette into the small bulbs. Observe the position of the meniscus upon the stem of the 3 cc bulb and calculate the true capacity of the first portion of the burette, using the values for the stem divisions as determined in the calibration of the bulbs. Repeat this process for the other nine portions of the burette and record as follows : Mark. Bulb reading. True capacity, each True total capacity, zero to end interval. of interval. For a more nearly complete calibration the burette may be calibrated 2 cc at a time, using the 2 cc bulb alone or 1 cc or any fraction at a time, using the standard tube, Fig. 62. Such calibration is necessary if the bore of the burette is found to be very irregular. Calibration of Pipettes. Pipettes which are graduated . in small subdivisions from zero to full capacity ("measuring pi- pettes") are calibrated in the. same manner as are burettes. Transfer pipettes are best calibrated by the method of weighing. Exercise: Calibration of Transfer Pipettes. Provide a weighing bottle having a capacity of 10 cc, also a larger one having a capacity equal to that of the pipette. Cut a strip of paper about 2 mm wide and 5 cm long and carefully rule this in divisions of centimeters, mark- ing from to 5, and subdivisions of millimeters, using fine lines. Deter- mine the approximate location of the capacity mark on the pipette by a rough experiment, unless the pipette is already marked. Paste the paper strip on the stem of the pipette with the division 2.5 at the supposed place for the capacity mark and with the zero toward the point of the pipette. Having cleaned the pipette with chromic acid solution it is drawn full of distilled water which is at a temperature of 20, and the water is allowed to flow out until the zero mark is exactly reached. The pipette must be held in a vertical position and the eye must be in the same horizontal plane as is the meniscus. The finger is now removed from the top of the pipette and the water is allowed to flow, at full speed, into the larger weighing bottle, which has already been weighed. The tip is immediately touched to the side of the weighing bottle to remove the hanging drop. The weighing bottle is then stoppered and weighed. 11 162 QUANTITATIVE ANALYSIS Calculate the volume of the water from the observed weight and record this as the capacity of the pipette to the zero mark. Using the small weighing bottle determine in a similar manner the capacity of the pipette stem between and 5. Divide this capacity by 50 in order to obtain the value of the smaller subdivisions. FIG. 61. Morse-Blalock bulb set up for calibrating burettes. From the capacities so determined calculate the number of stem divisions to be added to the zero in order to obtain the rated capacity of the pipette. Mark the point so determined, using the method direct- ed for marking flasks. VOLUMETRIC ANALYSIS 163 CALCULATION OF THE RESULTS OF VOLUMETRIC ANALYSIS Although the first exercises in volumetric analysis will necessar- ily have to do with the making of solutions and with their standardization and adjustment to desired concentration, it will be simpler to deal first with the calculation of the results of the analysis. When making the determination of silver by the gravimetric method, a definite amount of the silver compound was weighed, dissolved in water and a slight but somewhat indefinite ex- cess of hydrochloric acid was added, thus precip- itating all of the silver as silver chloride, the following reaction taking place : AgN0 3 +HCl-+AgCl+HN0 3 . The silver chloride, representing the entire amount of silver present in the compound of un- known composition, was then filtered, washed, dried, and weighed and from this observed weight and the known weight relations between silver chloride and silver the percent of the latter was calculated. The formula AgCl expresses the fact that for each 143.34 parts, by weight, of silver chloride, there was involved 35.46 parts of chlor- ine and 107.88 parts of silver. In other words, in any given weight of silver chloride, .. * . of , 107.88 . this weight is chlorine and .,. ~ . is silver. If the weight of silver chloride found in the analysis 1 07 88 is multiplied by the fraction 14 o 04 and the re- sult divided by the weight of sample taken, the quotient will be, when multiplied by 100, the percent of silver in the sample. If TF=the FlG 52 Morse- weight of silver chloride found, and = the Blalock tube, weight of sample taken, this would be expressed shortly as follows : 107.88TFX100 = percent of silver in sample. (I) 164 QUANTITATIVE ANALYSIS Instead of adding the hydrochloric acid in slight but indefinite excess to the solution of the silver salt, one might add exactly the amount required to complete the reaction, but no more. This would involve the use of some method for determining when the reaction is exactly completed (the "end point"), guch as noting when another drop of hydrochloric acid solution fails to produce any further precipitation of silver chloride. Suppose, also, that the concentration of the hydrochloric acid solution, in grams per cubic centimeter, were very accurately known. We should then have the following data: (a) Weight of silver salt taken, (6) Volume of hydrochloric acid required to react with silver, (c) Concentration of hydrochloric acid. Just as the formula for silver chloride expresses the weight relations between silver, chlorine and silver chloride, so the equation for the reaction between silver salt and hydrochloric acid expresses the weight relations between all of the elements and compounds involved. We are here particularly concerned with the relations between silver and hydrochloric acid, and we note that for every 107.88 parts, by weight, of silver, we require 36.468 parts of hydrochloric acid for complete precipitation of the silver as silver chloride. Conversely, if the reaction has been exactly completed, for every 36.468 parts of hydrochloric acid used there will have been present 107.88 parts of silver. The weight of pure hydrochloric acid used is found by multiplying the number of cubic centimeters by the concentration in grams per cubic centimeter, i.e., VC = Wt. HClused, where V = cc of acid solution used and C = gm hydrochloric 107 88 acid in 1 cc. If this weight is multiplied by the fraction ~oc~r^~ the result will be the weight of silver in the sample. Expressed briefly : 107.88FCX100 36.46S' percent silver. (II) This is the most general expression for the calculation of the results of a volumetric analysis. A comparison of expressions (I) and (II) will show that the volumetric calculation differs VOLUMETRIC ANALYSIS 165 from the gravimetric calculation in two respects only: (1) A weight of a substance is obtained indirectly by measuring the volume of a solution of known concentration, instead of directly by weighing the substance. (2) The substance whose weight is desired is one which reacts with the substance being determined instead of one which is produced by this substance. The gravi- metric factor for the ratio of the weight of one substance to the weight of another substance which contains the first becomes the volumetric factor for the ratio of the weight of one substance to the chemically equivalent weight of another substance which does not contain the first. The substance which is a visible indication of the end point of the reaction is called the " indicator." Indicators will be discussed at length in a later section. The solution whose con- centration is accurately known and of which we measure the volume required, is called a " standard solution" because it is actually a standard by which the quantity of the substance under investigation is measured. The process of running in the stand- ard solution until the end point is reached is called "titration." The examples given below will serve to illustrate the principles above outlined: 1. 0.5436 gm of a silver salt was dissolved and titrated by a standard solution of hydrochloric acid, 1 cc of 'which contained 0.00304 gm of the pure acid. 27.2 cc of the standard was used. Required, the percent of silver in the salt. 27.2 X 0.00304 = gm of pure acid used; 107 88 -ofi 46 Xgm of hydrochloric acid = gm of silver present. 107.88X27.2X0.00304X100 ~~36. 46X0.5436 -45. 00 = percent sil- ver in the original sample of silver salt. Use of Aliquot Parts. It often happens that, in order to elimi- nate the error due to the lack of uniformity of a sample being analyzed, or for reasons of convenience, a larger quantity than is necessary for the titration is weighed and this dissolved in a definite quantity of solvent and an aliquot part taken for the titration. In such a case the final calculation must include an expression of this fact. Thus in Example 1 3 instead of weighing 0.5436 gm of the silver salt, suppose that 10.8720 gm were 166 QUANTITATIVE ANALYSIS weighed, dissolved, and the solution diluted to 1000 cc, 50 cc being then titrated. The statement would then be 27.2X0.00304X107.88X20X100 10.8720X36.46 = percent Sllver m the Sample ' 2. In a sample of undried, but otherwise pure, sodium hydrox- ide, the percent of the base was to be determined. For this purpose 5.5310 gm of the sample was weighed and dissolved and diluted to 250 cc. Portions of 25 cc each were measured and titrated by a standard solution of hydrochloric acid, the average amount of acid solution required for complete neutrali- zation being 45.1 cc; 1 cc of the standard acid contained 0.00960 gm of the pure acid. Required, the percent of sodium hydroxide. The solution of the problem is as follows:^ 45. 1X0. 0096 = gm of hydrochloric acid used; 40 008 ^ hydrochloric acid = gm of sodium hydroxide 36 4fi in 25 cc of solution. 250 of sodium hydroxide = gm in 5.5310 gm of sample. The condensed expression is 4 . 51 X . 0096 X 40 . 008 X 250 X 1000 36.46X25X5.5310 = percent Sodmm hydroxide. The " equivalent weight" of a substance is the number oj weight-units chemically equivalent to eight weight-units of oxygen. This definition is sufficiently broad to apply to any system of weights, '.although there are very few cases in scientific work where " grams" might not be substituted for "weight-units," since the metric system is quite universally accepted and used in scientific work. The result of this substitution is the " gram- equivalent." In the effort to determine what is the equivalent weight of a substance it is always necessary to inspect the equation for the reaction that occurs in that particular case. In the reaction HCl-fNaOH NaCl+H 2 0, it is easily seen that the equivalent weights for all of the elements, radicals, ions or compounds are the atomic, radical, ionic or molecular weights, respectively. In the reaction: H 2 SO 4 +BaCl 2 BaS0 4 +2HCl, VOLUMETRIC ANALYSIS 167 the equivalent weights of the compounds are seen to be one- half of their molecular weights, with the exception of hydro- chloric acid, whose equivalent weight is its molecular weight. This will be more easily understood if we first determine the " hydrogen equivalent" of each substance, this being the number of atoms of hydrogen chemically equivalent to one molecule, atom, radical, etc., of the substance under consideration, as denoted by the equation for the reaction that has taken place. The hydrogen equivalent of sulphuric acid is 2 because it reacts by substituting another element for two hydrogen atoms. That of barium chloride is 2, because one atom of a bivalent element gives place to hydrogen or because 2 atoms of a univalent element give place to one radical which is bivalent. That of barium sulphate is 2 and that of hydrochloric acid is 1, for similar reasons. Since one atom of oxygen is chemically equivalent to two atoms of hydrogen, it follows that any body that is equivalent to 1.008 weight units of hydrogen will be equivalent to 8 weight units of oxygen and therefore the equivalent weight is the molecular (atomic, etc.) weight divided by the hydrogen equivalent. PROBLEMS Find the equivalent weights of the substances whose formulas are in bold face in the following equations: 1. 2HCl+Na 2 C0 3 ->2NaCl+H 2 C0 3 . 2. HCl+NaHC0 3 -+NaCl+H 2 CO 3 . 3. HCl+Na 2 CO 3 ^NaHCO 3 +NaCl. 4. (NH 4 ) 2 C 2 O 4 +CaCl 2 -+CaC 2 O4+2NH 4 Cl. 5. (NH4) 2 C 2 O4+HC1^NH 4 C1+NH 4 HC 2 O4. 6. (NH 4 ) 2 C 2 4 +2HC1-2NH 4 C1 + H 2 C 2 O 4 . 7. FeCl 2 +2AgNO 3 -Fe(NO 3 ) 2 +2AgCl. 8. 2FeCl 2 +Cl 2 -+2FeCl 3 . 9. H 2 S0 4 +Zn-+ZnSO 4 +H 2 . 10. H 2 SO 4 +2H^SO 2 +2H 2 O. 11. CuCl,+2AgNO,->Cu(NO,)+2AgCl. 12. 2CuCl 2 +Fe-^2CuCl+FeCl 2 . Calculation of the Weight of One Substance, Chemically Equiv- alent to a Stated Weight of Another. In the solution of nearly all problems of quantitative chemistry there is involved a calcu- lation of the weight of one substance chemically equivalent to a given weight of another. In the examples just considered this is a calculation of the weight of silver or of sodium hydroxide 168 QUANTITATIVE ANALYSIS equivalent to the weight of hydrochloric acid that is contained in the quantity of standard solution used. In example (1) this 1 07 88 was expressed as Q ~ V^-FX 0.00304 and in example (2) as 3 ' 6 7X0.0096. It is easily seen that both of these cal- culations involve the multiplication of the weight of hydro- chloric acid by a ratio of equivalent weights. From this fol- lows the rule that to find the weight of one substance chemically equivalent to a stated weight of another, multiply the stated weight by the fraction: equivalent weight of substance calculated equivalent weight of substance given This is a simple and very useful rule and its application will obviate the use of the more cumbersome rule of proportions. Applied to gravimetric analysis the fraction given above is the factor. PROBLEMS 13. What weight of oxygen is equivalent to 0.3460 gm of hydrogen, direct oxidation to water being understood? 14. What weight of carbon dioxide is equivalent to 0.5693 gm of carbon, direct oxidation being understood? 15. Calculate the weight of ferric chloride and of iron equivalent to 0.5243 gm of chlorine, the following reaction taking place: 2Fe+3Cl 2 ->2FeCl 3 . 16. Calculate the weight of potassium hydroxide equivalent to 1.7521 gm acetic acid, assuming complete neutralization. 17. In problems (1) to (12) calculate the weights of the substances whose formulas are in bold face, equivalent to 3 gm of the substances reacting with them. 18. A solution of hydrochloric acid of specific gravity 1.05 contains 10 percent by weight of the pure acid. What volume of the solution is required to precipitate the silver from 0.75 gm of silver sulphate? 19. 0.4321 gm of impure potassium sulphide was oxidized to potassium sulphate and precipitated by barium chloride. 0.8035 gm of barium sulphate was produced. What was the percent of sulphur in the sample? 20. What weight of tartaric acid is equivalent to 3.52 gm of sodium hydroxide, the following reaction taking place? VOLUMETRIC ANALYSIS 169 Use of a Standard Solution for the Titration of but One Sub- stance. When a standard solution is to be used for the titra- tion of but one substance the calculations will all involve the constants representing (1) equivalent weight of the active sub- stance in the standard solution, .(2) equivalent weight of the substance to be determined and (3) concentration of the standard solution. If the standard solution of example (1) is to be used for 107.88X0.00304 the determination of silver, the expression: ^ ^ contains quantities that are constants for all such determinations. These constants should then be combined in a single constant: 0.00899. From what was said in the preceding paragraph this is seen to be the weight of silver equivalent to 1 cc of this par- ticular standard solution of hydrochloric acid. All such calcu- lations would therefore be made by the expression VX 0.00899X100 : = percent silver (III) This is obviously a very simple calculation and such simplifica- tion is possible and should be made whenever a given standard solution is to be used for a considerable number of determinations of a single substance. Burette Reading a Direct Percentage Reading. If some care were exercised in adjusting the weight of silver salt used for analysis where statement (III) is to enter the calculation, so that exactly 0.8990 gm of sample were used, the expression would become FX0.00899X100 0.8990 = percent Sllver whence, V= percent silver. In this case the volume of standard solution used is the percent of silver in the sample. From this follows the rule: To make the burette reading a percentage reading first calculate the weight of the titrated substance that is equivalent to 1 cc of the standard solution, then use 100 times this weight of sample. PROBLEMS 21. What weight of soda ash must be used for analysis in order that 1 cc of the hydrochloric acid solution containing 0.0031 gm shall be equivalent to 1 percent of sodium carbonate, assuming complete decomposition? 170 QUANTITATIVE ANALYSIS 22. A standard solution of sulphuric acid contains 40.2 gm in 1000 cc What weight of potassium hydroxide must be taken so that each cubic centimeter of the standard acid required shall indicate 0.1 percent of potassium hydroxide? 23. A standard solution of barium hydroxide contains 20.35 gm in 1000 cc. What weight of vinegar is necessary in order that 1 cc of barium hydroxide solution shall indicate 0.1 percent of acetic acid in the vinegar? No System. In the examples given above there was no definite basis for the choice of the concentration of the standard solution, all that was required being an accurate knowledge of the existing concentration. Thus, in the first example the standard hydro- chloric acid contained 0.00304 gm in 1 cc, while in the second it contained 0.00960 gm in 1 cc. There is no connection, apparent or real, between these concentrations; they were chosen, at least to a certain extent, at random, upon the assump- tion that the determination of the concentration (standardiza- tion) was carried out with the greatest possible accuracy but that in making the solution no particular care was exercised. The method of calculating analyses made by means of such standard solutions would in all cases be analogous to these examples and any substance can be determined by means of such solutions, provided that the reaction involved is definite, complete and well understood, and that the end point can be determined accurately. Normal System. The calculations of volumetric analysis may be considerably shortened by the proper adjustment of the concentration of the standard solution. The reaction between hydrochloric acid and silver nitrate is expressed by the equation : HCl+AgNOs HNOa + AgCl, and expression (II) was deduced for the calculation of the per- cent of silver in a salt that had been titrated by a standard solution of hydrochloric acid. This expression was 107.88FCX100 36.463"" = Percent silver. By using the proper indicator in each case we might have completed such reactions as the following for the titration of the substances indicated: VOLUMETRIC ANALYSIS 171 HCl+NaOH->NaCl+H 2 O, (a) HC1 + KOH->KC1+H 2 O, (b) HC1+NH 4 OH-NH 4 C1+H 2 O, (c) 2HCl+Na 2 CO 3 2NaCl+H 2 O+CO 2 , (d) HCl+NaHC0 3 NaCl+H 2 0+C0 2 , (e) 2HCl+Ba(OH) 2 BaCl 2 +2H 2 O, (f) 2HCl+CaCO 3 CaCl 2 +H 2 O+CO 2 . (g) Many other substances may also be titrated by this same standard solution and in each case the expression for the per- cent of the substance to be calculated would be the same as (II) with the exception that, for the equivalent weight (combining weight) of silver (107.88) we should substitute the equivalent weight of the substance to be calculated; we should then have: 40.008FCX100 36 46 S = P ercent NaOH (a') 56.108FCX100 ~36 46 S -- = P ercent KOH ( b ) 35.05 FCX100 = percent NH 4 OH (c') 17.03 FCX100 . . 36 46 ff = percent NH 3 (c") 53 FCX100 = percent Na 2 CO 3 84. 008 FOX 100 ^ AT _ , ,, 36 46 g = percent NaHC0 3 (e') 85.69370X100 , m 36 46 ~ = percent Ba(OH) 2 (f ') 50. 045 FOX 100 36 46 g = percent CaCO 3 (g ) In all of these expressions for the percent of the various substances as titrated by a single standard solution, the only difference lies in the equivalent weight of the substance. The volume of standard required will depend, among other things, upon the purity of the sample and, since this is unknown, the volume required cannot be predicted. The concentration of the standard is under control and may be arbitrarily fixed at any desired figure. The equivalent weights concerned are constants, in any given case, and the weight of sample may be made whatever is desired. 172 QUANTITATIVE ANALYSIS If the standard solution is made of such strength that the number of grams contained in 1000 cc will be represented by the equivalent weight (in the case of hydrochloric acid 36.46 gm) the concentration in grams per cubic centimeter will then be C 0.03646, and the fraction ^ ^ which is involved in all of the expressions, will become -~T-T^- = 0.001, so that we shall then have 0.040087X100 , , = percent NaOH (aO 0.056108FX100 ,, , = percent KOH (bi) 0.035057X100 , , a = percent NH 4 OH (ci) and so on. The standard solution of hydrochloric acid thus made, con- taining 1 gram-equivalent of the active substance in 1000 cc of solution, is a solution of general application and the calcula- tion of the results of analyses of various substances is simplified by this choice of concentration. Such a solution is called a "normal solution" which will be defined as a solution containing 1 gram-equivalent of the active substance in 1000 cc. Because of the fact that each cubic centimeter of the normal solution contains one-thousandth of a gram-equivalent of the active substance, it follows that 1 cc of any normal solution is equivalent to 1 cc of any other normal solution. It is therefore true that 1 cc of any normal solution is equivalent to 0.001 gram- l equivalent of any other substance. The last relation is one of great importance and should be well understood. It follows from this that the number of cubic centimeters of normal solution used, multiplied by the milligram-equivalent of the substance titrated, gives the weight of the latter in the sample. This, divided by the weight of sample and multiplied by 100, gives the per- cent of substance in the sample. It frequently happens that the normal solution is too concen- trated or too dilute for convenient use in a given analysis. In this case the advantage of the normal solution may be retained by making the concentration of the solution to be some simple multiple of the concentration of the normal solution, such as VOLUMETRIC ANALYSIS 173 2> 3, , 77;, ^70 77.7., etc. This factor must then be introduced O 1U DU 1UU into the calculations involving the solution. Solutions made of normal or a simple multiple of normal strength are said to be made in the " normal system" and are, for the sake of brevity, N N N designated as N, 2N, ., -, , etc. PROBLEMS 24. 1 cc of normal acid is equivalent to what weight of ammonium car- bonate, assuming complete decomposition? N 25. 1.1256 gm of a silver alloy is dissolved and titrated by ^. potassium thiocyanate solution according to the following equation: KCNS + AgNOa^KNOs + AgCNS. 35.2 cc of standard solution is required. What is the percent of silver in the alloy? N 26. 0.5 gm of limestone was dissolved in 50 cc of -^ acid. The unused o N excess of acid was titrated by 16.2 cc of VQ base. What was the percent of calcium carbonate in the limestone? What percent of calcium? N 27. 0.4 gm of soda ash was titrated by 20.9 cc of -^ acid. What was o the percent of sodium carbonate in the sample? 28. 0.5 gm of an ammonium salt was decomposed by sodium hydroxide N and the resulting ammonia distilled into 50 cc of -=- acid solution. The o N unused excess of acid was titrated by 29.3 cc of ^ base. What percent of ammonia in the salt? Decimal System. Instead of using the normal system, a further simplification may be made by adjusting the standard until each cubic centimeter shall be equivalent, not, as in the normal system, to a decimal fraction of a gram-equivalent of the substance to be titrated but to a decimal or simple fraction of a gram of the substance. For example, a solution of hydro- chloric acid would be made with each cubic centimeter equiva- lent to 0.0100 gm, 0.0010 gm, 0.0050 gm, etc., of silver. This results in a very much simplified calculation and still more time is saved if the weight of sample used bears a definite and simple relation to the equivalence of the standard. 174 QUANTITATIVE ANALYSIS Such solutions as these are frequently made for technical work in industrial laboratories, where large quantities of standard solutions are often required for the titration of a single con- stituent of a large number of samples. Mention may be made of the use of potassium permanganate or potassium dichromate solutions for the titration of iron in ores, sodium thiosulphate solutions for the determination of the available chlorine in bleach- ing powder, potassium ferrocyanide solutions for the determina- tion of zinc and hydrochloric acid solutions for the determina- tion of hardness of water. The method of calculation of the necessary concentration of a solution to be made in the decimal system is the reverse of the method for calculating the equivalence of a solution of given concentration. Example: What must be the concentration of a solution of potassium hydroxide in order that each cubic centimeter shall be equivalent to 0.001 gm of sulphuric acid? The reaction involved is: 2KOH + H 2 S0 4 K 2 S0 4 +2H 2 O. The equivalent weight of potassium hydroxide is 56.108 and that of sulphuric acid is 49.02. Each cubic centimeter must con- tain . ' 2 X 0.001 gm of potassium hydroxide. This is 0.00114 gm. PROBLEMS 29. Calculate the concentration ( j of a standard solution of hydro- chloric acid such that 1 ceo the following weights of other substances: 0.002 gm of silver; 0.005 gm of silver chloride; 0.010 gm of potassium hydroxide; 0.005 gm of sodium hydroxide; 0.002 gm of sodium; 0.002 gm of ammonia. 30. Calculate the concentration of a nitric acid solution such that 1 cc=c= the following weights of substances: 0.040 gm of potassium hydroxide; 0.005 gm of calcium carbonate; 0.001 gm of nitrogen as ammonia. 31. What is the concentration of a potassium hydroxide solution of which 1 cc =0=0.010 gm of potassium acid tartrate? Choice of System. Summarizing, it has been shown that volumetric analysis may be carried out by the use of standard VOLUMETRIC ANALYSIS 175 solutions made in "no system/' in the " normal system" or in the " decimal system, " and that for any of these systems a defin- ite, precalculated weight of sample may be taken so that the burette reading in cubic centimeters will indicate directly the percent or simple fractions of percent of the constituent being determined. Which 1 of these systems shall be selected in prac- tical work will be determined by the circumstances. If but a few titrations are to be made with a given standard solution the time saved in simplified calculations will not justify the expendi- ture of time required for adjusting the concentration to the nor- mal or decimal system. If many titrations are to be made, one of the latter two systems will be used. The normal system is most useful for standard acids and bases because their application is more general and a solution so made will give simplified cal- culations for the titration of many other substances. There are many standard solutions which cannot be used so generally but which are made for the titration of but one substance. In such instances the decimal system will always be used. The use of standard acids and bases provides a means for the quantitative determination of practically any acid or base and of many salts. This is an extremely useful department of work, in view of the fact that no gravimetric method will serve to de- termine the essential constituent of acids and bases, the ionizable hydrogen and hydroxyl. For example, from potassium hy- droxide potassium may be determined as chlorplatinate or perchlorate, but this gives no information concerning ^he per- cent of potassium hydroxide since potassium from any salt present is also precipitated and weighed. By using the proper indicate^ salts of strong bases with weak acids or of strong acids with weak bases may also be titrated. Thus sodium carbonate may be titrated by standard hydrochloric or sulphuric acid if methyl orange is used as indicator. CHAPTER VI COLOR CHANGE OF INDICATORS In a broad sense the word " indicator" applies to all substances which, by undergoing any visible change, indicate the end point of reactions. When the indicators are inorganic the reactions are usually definite and well understood. The indicators used in acidimetry and alkalimetry are organic and the direct cause of color change is, even now, not thoroughly understood. Many of the organic dyes show, in acids, a color different from that in bases. The color change is generally reversible an indefinite number of times. The molecular structure of the dye is often very complex and it is not easy to follow the 'changes in structure. Simple lonization Theory. Most or all of the indicators of this class are known to possess, in certain conditions at least, the prop- erties of acids or bases. The acid or basic nature is usually weakly emphasized. From this Ostwald deduced a theory as to the cause of color change. 1 According to Ostwald these dyes are, when un- combined with other acids or bases, weak electrolytes and largely in the molecular state. If a base is added to a weakly acid indi- cator, the salt is formed and this is highly ionized, according to the general rule. The molecule possesses one color (or is colorless), while the anion shows a different color. The result of the addi- tion of a base is therefore a color change. If another acid is now added to the ionized salt the weak acid is reformed, the molecule reappears and the color change is reversed. The added acid has a further effect upon the indicator acid in suppressing the already small ionization. Similar reasoning would apply to basic indica- tors. Phenolphthalein is in the presence of acids, a derivative of phthalic anhydride and phenol having the following constitu- tion: CO .= (C 6 H 4 OH) 2 . 1 Scientific Foundations of Analytical Chemistry, 118. 176 COLOR CHANGE OF INDICATORS 177 According to the theory of Ostwald this is a very weak acid, giving a small concentration of ions thus: CO CO C 6 H 4 <^ ^>0 ^> C 6 C = (C 6 H 4 OH) 2 \C 6 H 4 OH The ionization constant is very small and equilibrium occurs with an inappreciable concentration of the anion. Upon the addition of a base the ionized acid is neutralized, equilibrium is disturbed and the ionized salt is produced, hence the color of the anion (red) appears. Methyl orange is known to have, under certain conditions, the structure (CH 3 ) 2 N-C 6 H 4 -N = N-C 6 H 4 S0 3 H. This is an acid, the red molecular form predominating in acid solutions, and the yellow anion appearing in basic solutions. (CH 3 ) 2 N-C 6 H 4 -N = N- Theory of Chromophors. This explanation is not sufficient in itself for several reasons. The silver salt of phenolphthalein is intensely purple, even when dry, and the dry salt cannot be highly ionized. Ethers of tetrabromphenolphthalein have been prepared; 1 these are non-ionizable but colored. The monoethyl ether is xC0 2 C 2 H 5 C 6 H 4 < Litmus is known as both blue and red in the dry state, when it must be chiefly molecular, no matter what the color may be. Also the studies of recent years upon the constitution of organic dyes have shown that in many cases a change of molecular structure takes place upon the addition of an acid or a base. Phenolphthalein is known to have the structure shown above but in basic solutions there is a salt of a carboxyl acid which is a quinone derivative. The phenol derivative is then in equilibrium 1 Nietzki and Burckhardt: Ber., 30, 175 (1897). 12 178 QUANTITATIVE ANALYSIS with the quinone derivative and this equilibrium is disturbed in one direction or the other by the addition of an acid or a base. CO COO / \ / + (~*\ TT / \/"^ ^ f^ TT / i TT Oerl 4 x >U ?=z v^ 6 i 4Ns ^ C = (C 6 H 4 OH) 2 If an acid is added the first (colorless) molecular form is produced because suppression of ionization results from the increase in concentration of hydrogen ions. If a base is added the ionized form is neutralized, forming ionized salt and water, and thus the new structure predominates. When methyl orange changes from the sulphonic acid to one of its salts a change of structure also takes place. The structure peculiar to the non-ionized body (present when an acid is added) is not that of an azo compound but one containing the quinone ring. There is then equilibrium between the two forms : Yellow, predominates in basic solution. 2 N = C 6 H 4 = N-NH-C 6 H 4 S0 3 Red, predominates in acid solution. The acid, by suppressing the ionization of the first form, causes the second, a lactonic form, to predominate. A base, by forming salt and water from the sulphonic acid, causes the first form to predominate. With the acid the quinone ring gives a red color. With the base the azo group gives a yellow color. The quinone ring, =C 6 H 4 = , is one of a class of groups known as "chromo- phors" because, wherever they appear in any compound they give rise to color. Other well characterized chromophors are the azo group N = N , the nitro group, N0 2 , and the dicarbonyl group, CO CO .! Theory of Absorption by Electronic Vibration. While the appearance of chromophors and of color are coincident there is still to be explained the direct cause of color. Visible color is but a special case of absorption of light waves. White light, being composite, may have waves of certain frequencies removed by passing through a transparent body or by being reflected from 1 Vide Hantzsch: Ber.,32, 575 (1899), and Stieglitz: J. Am. Chem. Soc., 25,1112(1903). COLOR CHANGE OF INDICATORS 179 the surface layers of an opaque body. If the frequencies of the absorbed waves correspond with those of waves in the visible spectrum the resultant beam of light is no longer " white" but colored. Selective absorption is due to the presence of some body that can vibrate synchronously with the light waves of certain frequencies, thus absorbing their energy and converting it into heat or chemical energy. The absorbing body for rays in the infrared is probably, in most cases, the atom or an atomic group. That for the rays in the visible and ultraviolet is thought to be the electron. Probably all colored organic compounds are tautomeric. The view is gaining ground that the absorption is due to the vibration of "valency" electrons as the tautomeric forms in equilibrium pass reversibly from one condition to the other. If this be true the groups known as chromophors possess the property of giving color because they always exist in dynamic equilibrium with groups of tautomeric forms and not because they can directly, as units, absorb any of the waves of white light. In the case of phenolphthalein it is the reaction: I CO COOH C = (C 6 H 4 OH) 2 C = C 6 H 4 = O \C 6 H 4 OH and not the mere presence of a given group, that gives the color. The changes from phenol to quinone and from quinone to phenol are continually taking place. The benzene ring continually changes reversibly to the quinone ring : C X / C \ ^C -C/ ^C- C II C The change of "bonding" is to be regarded as a change in posi- tion of those electrons that give rise to valence and it is this vibration of electrons from one position to another which makes possible selective absorption. It was shown above that the addition of acid or base to the solu- tion of indicator causes one or the other of the tautomers to 180 QUANTITATIVE ANALYSIS predominate. This, in itself, could not account for the change of color when it is known that a body as small as the electron must be responsible for the property. Acids and bases act as catalyzers for such reversible reactions and in such a manner that the velocity of both reactions may be changed with or without affecting the final condition of equilibrium. In other words the fact that the acid or base causes a predominance of one or the other form is not of as great importance as is the fact that it causes a change in the velocity with which the change from one to the other takes place and therefore causes a change in the velocity with which the valency electrons vibrate. Here we have the final explanation of the cause of color change when the nature of the solution is changed. The third theory seems to be the most comprehensive and reasonable. Further investigation will no doubt add much that is interesting. One point is common to all of the theories. No matter what may be the direct cause of color change, such change is conditioned upon a chemical reaction of the indicator with the excess of acid or base. The ideally perfect indicator is the one that gives a visible indication when the least excess of standard is added after the point of neutrality is reached. If the indicator must react with this excess in order to change color it is obviously necessary to work with the least quantity of indicator that can be made to give a perceptible color. The usual inclination of the beginner is to use too much indicator, having in mind the idea that intensity of color will promote sensitiveness. This is easily seen to be a mistake. The sensi- tiveness of eyes with respect to color perception varies and no general rule regarding the quantity of indicator to be used can be stated, but the operator should accustom himself to the use of the least possible quantity in all cases. With the indicator solutions as made in the laboratory one drop is generally sufficient. Classification of Organic Indicators. Ostwald classified the indicators according to their supposed dissociation constants into three groups: (a) Very weak bases and relatively strong acids. (b) Moderately strong acids and bases. (c) Very weak acids and relatively strong bases. Since he explained the color change upon the basis of salt formation it would necessarily follow that the relative sensitive- COLOR CHANGE OF INDICATORS 181 ness would vary in the three classes. In class (a) the indicators would be highly sensitive to bases but not easily affected by acids, except by very strong ones. The indicators of class (b) would be moderately sensitive to both acids and bases, while in class (c) they would be highly sensitive to acids and to none but strong bases. While some of these indicators are here called " relatively strong" acids or bases, it must be remembered that, compared with the strongest electrolytes, all are weakly ionized and all lie in the class of weak electrolytes. While the theory of color change by ionization must be regarded as based upon incorrect assumptions, the above classi- fication is still a convenient one since the same relative sensi- tiveness would follow from the application of any of our theories. Phenolphthalein may be taken as an illustration. According to the simple ionization theory there is to be considered merely the following system in equilibrium: where Ph is understood to mean the negative radical C 6 H 4 (CO)2C 6 H40H.C 6 H 4 0. Phenolphthalein falls in the class of very weak acids and it is consequently chiefly molecular unless a strong base be present, a weak base forming an easily hydrolyzed salt. Even weak acids can decompose the salt, therefore phenolphthalein will be easily affected- by acids but will not be highly sensitive to bases. According to the view that color is due to the existence of chromophors the equation CO COO C e H 4 / No <= C 6 H 4 NaH 2 P0 4 +H 2 0. In the presence of phenolphthalein the end point is somewhat in- definite but occurs at the neutralization of two-thirds of the acid : 2NaOH+H 3 PO 4 Na 2 HPO 4 +2H 2 O. DESCRIPTION OF INDICATORS Following is a brief discussion of the preparation and properties of the indicators named in the table on page 182. 184 QUANTITATIVE ANALYSIS Phenolphthalein. The chemical nature and changes of phenol- phthalein have already been discussed. The compound is pre- pared by heating together 5 parts of phthalic anhydride, 10 parts of phenol and 4 parts of concentrated sulphuric acid for several hours at a temperature between 120 and 130. The mass is then boiled with water, filtered and the residue dissolved in dilute sodium hydroxide solution. The solution is filtered and neutralized with hydrochloric acid. Phenolphthalein precipi- tates and is purified by recrystallization from alcohol. The pure substance is a yellowish-white crystalline powder, practically insoluble in water but soluble in alcohol. For use in volumetric analysis a 1 percent solution in 50 percent alcohol is used. Rosolic Acid (Corallin). Commercial rosolic acid is a mix- ture of aurine, Ci 9 H 4 4O 3 , oxyaurine, CigHieOe, methylaurine, C 2 oH] 6 O3 and pseudorosolic acid, C 2 oHi 6 O4. Each of these sub- stances contains the quinone ring. It is prepared by heating together phenol, sulphuric acid and oxalic acid. The changes occurring as acids or bases are added are not thoroughly under- stood. The solution as used in the laboratory is a 1 percent solutiori in 60 percent alcohol. The indicator is red with bases and yellow with acids. It is highly sensitive to acids and can be used for the titration of weak acids. Litmjus. Litmus is obtained by the action of ammonium hydroxide and potassium hydroxide upon certain species of plants, followed by fermentation. The essential constituent of the indicator is azolitmin, OrB^NC)^ of unknown constitution. It is colored red by acids and blue by bases. Its sensitiveness toward both acids and bases is only moderate and this fact makes it a very valuable indicator for general qualitative purposes but of little use for quantitative analysis. A 10 percent solution in water is used in the laboratory. p-Nitrophenol. This indicator is prepared by the action of /N0 2 (4) nitric acid upon phenol. Its formula, C 6 H 4 <(^ X)H (1), is indicated by its name. It is yellow with bases and colorless with acids. It is applicable to the same uses as is litmus. The indicator solution is a 0.02 percent solution in water. It should not be kept in a closely stoppered bottle. Methyl Orange. The constitution and chemical properties COLOR CHANGE OF INDICATORS 185 have already been discussed. The substance as obtained in commerce is usually the sodium salt. This is a yellow powder, soluble in water. A 0.1 percent solution is used. In presence of ammonium salts the indicator is not very sensitive. Ethyl Orange. This substance is analogous in constitution to methyl orange, it being the diethyl ester of amidoazobenzene- sulphonic acid. Its properties and uses are the same as those of methyl orange. Cochineal. Cochineal is the dried female insect Coccus cacti Linne". The essential coloring matter is carmiiiic acid, CnH^O? whose constitution is not definitely established. The solution for use as an indicator is made by digesting 3 gm of the dried, unpowdered insects with 250 cc of 75 percent alcohol until the coloring matter is extracted, then decanting. The bottles containing the solution should not be closely stoppered. The indicator is violet with bases and red with acids. Its sensitive- ness is not diminished by ammonium salts. Lacmoid. Lacmoid is prepared by heating together resorcin, sodium nitrite and water. It is a deep blue dye of unknown constitution; the molecular composition is probably represented by the formula Ci 2 H 9 NO4. It is soluble in alcohol and less so in water. In solution it is colored blue by bases and red by acids. Its sensitiveness toward both acids and bases is moderate but it finds an application in the titration of carbonates in boiling solution, carbonic acid being decomposed as fast as it is formed. Litmus may be used in the same way but is not so sensitive in hot solutions. The indicator solution is a 0.2 percent solution of purified lacmoid in alcohol. Erythrosine (lodeosine). This dye is tetrabromfluorescein, a synthetic derivative of fluorescein, and it has the constitution represented by the formula CO C 6 HBr 2 OH C 6 HBr 2 OH. The substance is therefore a phthalein. It is almost insoluble in water but is soluble in hot alcohol and ether. A very dilute solution (0.002 gm per liter) in ether is used as indicator. It is colored red by bases and light yellow by acids. It i%ighly 186 QUANTITATIVE ANALYSIS sensitive to bases and is therefore applicable to the titration of the alkaloids. Its sensitiveness to acids is correspondingly small so that it may be used for the titration of carbonates of the alkali and alkaline earth metals without boiling to expel carbonic acid. On account of its limited solubility titrations are made by adding 10 to 20 cc of the solution in ether to the titrating solu- tion, shaking after each addition of acid. CHAPTER VII STANDARDIZATION Thus far we have dealt with only the calculation of the results of analyses, assuming that the standard solution was ready for use in the experiment. The determination of the concentration of the standard solution is called "standardization." The details of the experimental work will be treated later and will be mentioned here only so far as they may serve to illustrate the methods of calculation. Standardization may be accomplished by one or more of four methods : (1) Direct Weighing. The active substance of the solution is accurately weighed and dissolved to make a definite volume of solution. This method is applicable to only those substances that can be obtained in a pure state or in a state of uniform and accurately known composition. Most of such substances are crystallized salts or acids, or soluble gases. (2) Weighing a Substance Produced by a Measured Volume of the Solution. Sulphuric acid solution may be standardized by precipitating a measured volume by adding an excess of barium chloride. From the weight of barium sulphate found the weight of sulphuric acid may be calculated by the method given on page 167. Similarly hydrochloric acid solution may be standardized by precipitating as silver chloride. (3) Measuring the Volume of Solution Required to React with a Known Weight of a Substance of Known Purity. An acid may be allowed to react with a pure carbonate and the required volume noted. Sodium thiosulphate solution may likewise be titrated against a weighed quantity of iodine or (indirectly) against a weighed quantity of arsenic trioxide. (4) Titration Against Another Solution Which has Already Been Standardized. This is a very common expedient. 187 188 QUANTITATIVE ANALYSIS The following examples will illustrate the methods of calcula- tion in each of the cases discussed. (1) The method of calculation for the first method of stand- ardization is self-evident. The normality is equal to the ratio of the number of grams dissolved in 1000 cc to the number of grams in 1000 cc of a normal solution. That is, grams per 1000 cc normality 5 : f- ^uT' equivalent weight (2) A solution of hydrochloric acid was standardized by precipitating the chlorine from 40 .cc, as silver chloride. The weight of silver chloride found was 0.6327 gm. Required, the normality of the solution. 0.6327 1 cc acid solution^ j~ m S1 lver chloride. 1 cc normal acid solution =0=0. 1433 gm silver chloride. fi^97 Therefore normality = * -s-0.1433 = 40 xO 1433 = (U107N - To make the solution decinormal 1000 cc would be diluted to 1107cc. (3) A similar solution was standardized by titration of pure sodium carbonate in presence of methyl orange, the following reaction being completed: Na 2 C0 3 +2HCl 2NaCl+H 2 C0 3 . It was found that 32.2 cc acido 0.1638 gm sodium carbonate. Required the normality. 0.1638 1 cc acid 00 gm sodium carbonate and oZ . Z 1 cc normal acid 0.053 gm sodium carbonate. n Therefore normality = 00 ' '-^=0.9598 N. . Uoo (4) Another acid solution was standardized by titration against a measured volume of standard potassium hydroxide solution in presence of methyl orange according to the equation : HCl+KOH KC1+H 2 0. 1 cc of the standard solution contained 0.00468 gm of potas- sium hydroxide. It was found from the titration that 50 cc STANDARDIZATION 189 of potassium hydroxide solution =s= 43. 5 cc of hydrochloric acid solution. The weight of potassium hydroxide in 50 cc of solution = 50X0.00468 gm. Since this weight was equivalent to 43.5 cc of acid, the potassium hydroxide equivalent to 1 cc acid = 50X0.00468 txfc'Vji ui AO~X -- S m - The normality of the hydrochloric acid solu- 50X0.00468 Primary Standards. Any substance which is used for stand- ardizing a solution and whose purity is known or assumed is a primary standard. The primary standard is the starting-point for all of the calculations of the results of analyses made by volumetric processes and no assumption as to its purity should be made without a careful analysis. PROBLEMS 32. 30.0 cc of sulphuric acid solution yields 0.3625 gm of barium sul- phate. Calculate the normality and the dilution necessary to make the solution decinormal. 33. 44.6 cc of silver nitrate solution yields 1.2870 gm of silver chloride. Calculate the normality and the dilution necessary to make the solution fifth-normal. 34. 39.7 cc of barium chloride solution yields 2.5346 gm of barium chromate. Calculate the normality and dilute to make the solution half-normal. 35. An acid solution is standardized by titrating pure sodium carbonate, using methyl orange as indicator. 45.1 ceo 2.4065 gm of sodium car- bonate. Calculate the normality and dilution necessary to make the solu- tion exactly normal. 36. 50 cc of nitric acid solution is added to 0.4530 gm of pure calcium carbonate. The unused excess of acid is titrated by a solution of a base and 6.15 cc of the latter is required. The base is then titrated against the acid in order to compare their concentrations and 21.3 cc of acid is found to be equivalent to 19.2 cc of base. Calculate the dilution necessary to make each solution fifth-normal. 37. A sulphuric acid solution is standardized by titrating a sample of potassium bicarbonate which contains 98.45 percent of the pure compound. 35 cc acid =0=0.0391 gm of the sample. Calculate the normality and the dilution necessary to make the solution centinormal. 38. 32.9 cc of potassium hydroxide solution exactly neutralizes 0.3118 gm of pure potassium acid tartrate, KIK^EUOe. Calculate the dilution necessary to make the solution twentieth-normal. 39. 45.9 cc of sodium hydroxide solution was added to a solution coil- 190 QUANTITATIVE ANALYSIS taining 0.25 gm of crystallized oxalic acid, H 2 C 2 O4.2H 2 O, the indicator being phenolphthalein. The excess of base was titrated by 1.3 cc of acid. 4.9 cc of the acid is found to be equivalent to 50 cc of base. Calculate the normality of each solution. 40. A solution of barium hydroxide was standardized by titration against succinic acid, H 2 C 4 H 4 O4, in presence of phenolphthalein. 20.9 cc of barium hydroxide solution neutralized a solution of 1.22 gm of succinic acid. Calculate the normality. 41. 38.1 cc of a sodium bicarbonate solution exactly neutralizes 36.7 cc of a decinormal solution of hydrochloric acid. What is the normality of the first solution? CHAPTER VIII EXPERIMENTAL VOLUMETRIC ANALYSIS Standard Acids. It has already been shown that the most serviceable acids or bases for standard solutions are those that belong to the class of strong electrolytes. For standard acids this practically limits one to the use of hydrochloric, sulphuric or nitric acid. On account of the ease with which it may be re- duced, nitric acid is not to be recommended for standard solu- tions of general application. Of the first two acids named, hy- drochloric acid is usually to be preferred because it is monobasic and cannot form acid salts. Materials for Standardization. For standardizing an acid use may be made of any method which involves a definite re- action with a pure substance or which produces a precipitate or gas that may be weighed or measured. This makes either volumetric or gravimetric methods available. Since most strong bases are hygroscopic and also combine readily with carbon di- oxide, purification and weighing are difficult and these substances are unsuitable for use as primary standards for standardizing acid solutions. We have left the alkali and alkaline earth car- bonates for this purpose. Calcium carbonate and sodium car- bonate are suitable and both substances may be obtained nearly pure. Precipitated calcium carbonate may be used but it is seldom free from other salts and an analysis must be made be- fore it is used for standardizing. The form most often used is the natural crystallized calcite known as Iceland spar, which is often nearly 100 percent calcium carbonate, although its purity should not be assumed without analysis. The chief disadvan- tage in the use of any form of calcium carbonate for standardizing acids lies in the fact that it is insoluble in water and an excess of acid must be used in order to hasten the process of solution. In^such*a case a direct titration cannot be made but the excess 191 192 QUANTITATIVE ANALYSIS of acid must be determined by titration with a solution of a base, whose concentration as compared with the acid must be known. Several of these difficulties may be obviated by the use of sodium carbonate, which is soluble in water. As this substance is obtained in commerce it contains variable quantities of water and salts incident to the process of manufacture, such as sodium sulphate and sodium chloride. This is largely due to the com- paratively large solubility of the salt and the consequent diffi- culty in purifying it by crystallization. At 20 a saturated solution contains sodium carbonate to the extent of 17.7 per- cent of its weight. The pure salt is best obtained from the bicarbonate by heating. Sodium bicarbonate dissolves to the extent of 8.8 percent of the weight of solution at 20. It may therefore be more readily purified by fractional crystallization, especially if the purest obtainable commercial 'salt is used for the purpose. When heated sodium bicarbonate decomposes according to the equation 2NaHCO 3 Na 2 CO 3 +H 2 O+C0 2 . The dissociation tension of carbon dioxide from sodium bicar- bonate is as follows: 1 Temperature. 55 60 70 80 90 100 Tension, mm of mercury 19 25 43 70 125 310 The tension of carbon dioxide in the atmosphere is less than 1 mm and, in consequence, sodium bicarbonate will slowly de- compose at temperatures below 55. At 100 the decomposition is fairly rapid and the bicarbonate is completely changed into normal carbonate by heating for a short time at a temperature between 270 and ,300. Gravimetric Standardization. Acids may also be standardized gravimetrically in case insoluble salts can be produced. Such a method will apply to hydrochloric or sulphuric acid but not to nitric acid, since no insoluble nitrate is known. A point fre- quently overlooked is that this method is really a standardiza- tion with respect to the negative radical and is an acid standardi- *Lescoeur; Ann. chim. phys., [6] 28, 423 (1892), EXPERIMENTAL VOLUMETRIC ANALYSIS 193 zation only in case no salts of the acid are present. Even in the purest commercial acids ammonium salts are often present and the weighing of silver chloride or of barium sulphate will thus not give a basis for the correct calculation of acid strength. Standardization by Direct Weighing. It is possible to weigh the active substances directly in the exact amount necessary for a solution of desired strength only in case the substance is available in pure form. This is not the case with most of the inorganic acids 1 and with comparatively few salts. It then becomes necessary to calculate the approximate quantity re- quired to make a solution somewhat stronger than that desired, to standardize the solution so made and dilute exactly to the required strength. Such dilution may be accomplished wi.th accuracy in case the water to be added may be measured in a burette, i.e., if less than about 25 cc is required. The final volume obtained by dilution is the sum of the initial volume and that of the added water only in case no expansion or contrac- tion occurs upon mixing. This is practically the case if the solu- tion is already dilute and the relative amount of water to be added is small. Dilution may then be accomplished by measuring a specified amount of solution in a flask, running in the calculated amount of water from a burette and mixing directly in the gradu- ated flask. The neck of the flask must be capable of easily holding the required water above the graduation. This fact, together with considerations of volume changes already men- tioned, places a practical limit upon the amount of dilution that may be accurately made in one process. If more than 25 cc of water must be added to 1000 cc of solution it is necessary to dilute to nearly the required amount, restandardize and redilute to the exact value required. For example, if it is found N that a solution is 1.3462X TTJ and it is necessary -to dilute the solution to make it exactly decinormal, each 1000 cc must be diluted to a volume of 1346.2 cc The addition of 346.2 cc of water could not be accurately made because there is no gradu- ated vessel capable of accurately measuring this quantity. Such an addition might also cause an appreciable volume change. 1 Vide, Moody: J. Chem. Soc., 73, 658 (1898), and Acree: Am. Chem. J., 36, 117 (1906). 13 194 QUANTITATIVE ANALYSIS The correct procedure would be to add first to each 1000 cc of solution about 335 cc of water, measured in a graduated cylinder, restandardize and then carefully complete the dilution in the manner already explained. Exercise : Preparation of Pure Sodium Carbonate. Make a saturated solution of sodium bicarbonate by heating the purest obtainable salt with distilled water. Decant from any undissolved matter remaining and evaporate the solution until crystals begin to separate. Allow to cool and stand until about 25 gm of crystals have formed, pour off the solu- tion, wash the crystals once with cold water and press between filter paper. Dry at 100, powder and heat in a platinum crucible at a tem- perature between 270 and 300 until the weight is constant. The product should be pure sodium carbonate but a test for sulphates and chlorides should be made. Preserve in a closely stoppered weighing bottle. Exercise : Preparation of Decinormal Hydrochloric Acid. Determine the specific gravity (if not already known) and from this the percent of hydrochloric acid in the concentrated solution found in the laboratory. From the data so obtained calculate the weight or volume necessary to make 2500 cc of decinormal solution. Measure 3 percent more than this amount into a 1000 cc graduated flask and fill to the mark with water. Empty into a bottle having a capacity of about 2500 cc and add 1500 cc more of water. Stopper and mix thoroughly by shaking. Since the solution has been warmed by the reaction between acid and water it should be allowed to stand until the temperature of the room is attained before standardizing. Calculate the weight of sodium carbonate necessary to make 200 cc of a decinormal solution. Weigh this quantity of the prepared pure material from a weighing bottle, emptying into a beaker. Dissolve the weighed carbonate in distilled water and carefully rinse into a 200 cc graduated flask. Fill to the mark and mix thoroughly. Imperfect mixing is often found to be the source of discrepancies in titrations with the acid solution. The solution will not remain constant in concentra- tion and should not be kept for more than one day. Fill a burette with the solution and another with the acid solution. Before making the titrations practice reading the color change as follows: Place 100 cc of distilled water in a beaker and add a drop of methyl orange and 0.5 cc of sodium carbonate solution. Drop in the acid solution until the last drop changes the tint from yellow to pink. Now drop in sodium carbonate solution until the yellow color reappears. Repeat the alter- nate additions of carbonate and acid until the color change can be observed when but one drop of either solution is added. It will aid in the next process if this solution is preserved and another prepared, the two EXPERIMENTAL VOLUMETRIC ANALYSIS 195 showing the two colors of methyl orange. These may be set aside for comparison. Measure 40 cc of the carbonate solution into another beaker or Erlenmeyer flask, dilute to about 100 cc and titrate with the acid solution in presence of a drop of methyl orange. Calculate the normality of the solution, also the volume of water to be added to each 1000 cc to make exactly decinormal. If water to be added is more than 25 cc add nearly the required amount to each liter of the acid, mix, and restandardize. If the quantity to be added is less than 25 cc the acid is diluted as follows: Fill a dry 1000 cc graduated flask to the mark with the acid solution. This flask should be capable of holding the required amount of water above the mark. From a burette add the calculated quantity of water directly to the solution in the flask and mix thoroughly. Pour into a dry bottle and make a second liter of diluted solution in the same manner, having first rinsed and dried the graduated flask. Check the accuracy of the dilution by restandardization. Record upon the label of the bottle your name, the name of the standard solution, its normality and the date of standardization, thus: Hydrochloric Acid N/10 FIG. 63. Form of label for standard solution. SODA ASH The commercial grade of sodium carbonate known as "soda ash" contains, in addition to sodium carbonate, considerable water, some potassium carbonate and varying quantities of other sodium and potassium salts, such as sulphates and chlorides. A complete analysis would include the determination of all radicals but on account of the fact that soda ash is used in many 'indus- tries because of its basic properties its valuation generally in- cludes a determination of basicity and of water with other impurities. The first determination may be made by direct titration by a standard acid in the presence of methyl orange, while the last may be determined directly or taken by difference, in which case the percent of water includes all other impurities. 196 QUANTITATIVE ANALYSIS In view of the fact that the basicity of the alkali carbonates toward methyl orange is due to the hydrolysis of a salt of a strong base and a weak acid, it is obvious that any salt derived similarly will likewise be basic and that therefore the titration by acid is really a method for determining the radical of salts of weak acids and is not a basis for the calculation of any particular salt, such as sodium carbonate. Thus, for example, if a mixture of sodium carbonate, potassium carbonate, sodium bicarbonate, sodium silicate, and sodium borate were being titrated these salts would all be decomposed by the standard acid before an end point with methyl orange would be reached. In the absence of a more extended analysis it would be impossible to calculate the percent of any one of these radicals or compounds. Since all are basic in solution and since all would serve for most purposes where sodium carbonate is used industrially it is customary to report the percent, arbitrarily calculated, of either sodium carbonate or of sodium oxide (regarded as being combined), assuming that all basicity of soda ash is due to sodium carbonate. It should also be noted that sodium carbonate could contain either alkali hydroxides or bicarbonates but not both in the same sample. Ordinarily no attention is given to either. On account of the lack of uniformity of most commercial soda ash it is necessary to select a rather large sample, dissolving in water and measuring an aliquot part for the titration. The directions for sampling on pages 8 to 13 should be carefully followed but exposure to air should not be unduly prolonged. Determination. Fill a 20 cc weighing bottle with soda ash, properly sampled. Assuming that the sample is pure sodium carbonate, calcu- late the approximate wieght that should be contained in 500 cc of solution so that 25 cc shall require about 35 cc of fifth-normal acid for its titration. Weigh this quantity of soda ash into a graduated 500 cc flask, dissolve and dilute to the mark with water. Mix thor- oughly by inverting the flask and shaking. Fill a burette with the solution and measure out 25 cc, allowing this portion to run into 200 cc Jena beakers or Jena Erlenmeyer flasks. Add just enough methyl orange to tint the solution and titrate with the fifth-normal acid. Make at least two more titrations and calculate the percent of sodium car- bonate, also of sodium oxide, assuming that the basicity of the substance is entirely due to the sodium oxide combined as carbonate. "Pearl ash" (crude potassium carbonate) may be evaluated in a similar manner. EXPERIMENTAL VOLUMETRIC ANALYSIS 197 MIXTURES OF CARBONATES AND BASES The use of the two indicators, methyl orange and phenol- phthalein, provides a means for the determination of carbonates and bicarbonates when in mixture, and also of carbonates and soluble bases. Bases and bicarbonates (acid salts) cannot occur in the same mixture. If phenolphthalein is added to a solution con- taining sodium carbonate and sodium hydroxide and the solution is titrated by a standard acid, the end point is reached when the sodium hydroxide is neutralized and the sodium carbonate is changed to bicarbonate: NaOH+HCl NaCl+H 2 0, (1) Na 2 CO 8 +HCl-*NaHC0 8 +NaCl. (2) Since the solution has now become colorless, methyl orange may be added and the titration continued until the red tint appears, when the sodium bicarbonate has been completely decomposed: NaHCO 3 +HCl NaCl+H 2 CO 3 . (3) Represent by A the cubic centimeters of acid used in completing the first titration and by B that used in the second. SXnormalityX0.106 = gm of sodium carbonate, (A B) X normality XO. 040 = gm of sodium hydroxide. It must be noted that here the equivalent weight of sodium carbon- ate is 106 instead of 53, since by equation (3) lHCl~lNaHC0 3 and by equation (2) lNaHCO 3 olNa 2 C0 3 . The quantitative conversion of sodium carbonate into sodium bicarbonate by means of an acid can take place only when care is taken to prevent the escape of carbon dioxide. At the point where the acid enters the solution there is at first complete con- version of a part of .the carbonate into the normal sodium salt of the added acid. If carbon dioxide escapes from the solution at this point more acid will be required to produce the first end point than would otherwise be the case. The escape of gas may be prevented by keeping the solution at a temperature near and by adding the standard acid very slowly and while stirring vigorously, 1 1 Kuster: Z. anorg. Chem., 13, 127 (1897). 198 QUANTITATIVE ANALYSIS Determination of Sodium Hydroxide and Sodium Carbonate in Com- mercial "Caustic Soda." Arbitrarily assuming that the sample is pure sodium hydroxide, calculate the approximate quantity necessary to dissolve and dilute to 1000 cc so that 50 cc shall require about 40 cc of decinormal acid. From a stoppered weighing bottle weigh this amount of well-mixed " caustic soda" into a 1000 cc graduated flask. Dissolve in 500 cc of water and cool to 20. Dilute to the mark, measure out 50 cc, add a drop of phenolphthalein, cool in ice water and titrate to the disappearance of the pink color. Add a drop of methyl orange and continue the titration to the next color change. Calculate the percent of sodium carbonate and of sodium hydroxide. MIXTURES OP CARBONATES AND BICARBONATES If a mixture of a carbonate and bicarbonate is to be investi- gated the procedure is the same as in the preceding exercise. In this case the phenolphthalein changes color at the completion of the reaction Na 2 C0 3 + HC1-* NaHC0 3 + NaCl. When the color change of methyl orange occurs the sodium bi- carbonate so produced, as well as that originally present, has been decomposed. If A is the acid used in the first titration and B that used in the second titration A X normality X0.106 = gm of sodium carbonate, and (B A) X normality X 0.084 = gm of sodium bicarbonate. These calculations are based upon the same arbitrary assumption regarding the presence of other salts as is noted in the discussion of the valuation of soda ash. Determination of Sodium Carbonate and Bicarbonate in a Mixture. Proceed as in the preceding exercise, except that the calculation is to be made as above indicated. HARDNESS OF WATER The difference between "hard" and "soft" water lies in the fact that the former contains various inorganic salts which form insoluble soaps when used for washing. The salts of calcium and magnesium are chiefly responsible for this action. When soap is added to such a water calcium and magnesium salts of the fatty acids are precipitated. Bicarbonates of these metals are EXPERIMENTAL VOLUMETRIC ANALYSIS 199 more common than other salts and when the water is boiled carbonates are precipitated: Ca(HC0 3 ) 2 -CaC0 3 +H 2 0+C0 2 ; Mg(HC0 3 ) 2 -> MgC0 3 +H 2 0+C0 2 . This results in a " softened" water and on account of this fact it is customary to speak of such hardness as is caused by bicar- bonates as "temporary hardness." Calcium and. magnesium salts of strong acids, as well as other salts of the heavy metals and strong acids, also cause hardness. Since these salts are not precipitated by simple boiling, such hardness is known as "per- manent" hardness. Examples of salts that cause permanent hardness are chlorides, sulphates and nitrates of calcium, mag- nesium and iron. Calcium sulphate is one of the most abun- dant of these and permanent hardness is usually arbitrarily expressed in terms of this salt alone just as temporary hardness is expressed in terms of calcium carbonate which is precipitated from calcium bicarbonate. The methods used in the titration of carbonates and bicarbon- ates of sodium and potassium will serve for the determination of temporary hardness. Applied to this purpose this is known as Hehner's method. 1 . Most natural waters contain relatively small amounts of dissolved salts and it is necessary to use a solution of acid more dilute than tenth-normal. The latter may be diluted for this purpose to fiftieth-normal. For the determination of permanent hardness by Hehner's method a standard solution of sodium carbonate is necessary. The method depends upon the small solubility of the car- bonates of the alkaline earth and earth metals. When sodium carbonate is added to the water a reaction occurs, removing sodium carbonate in a quantity equivalent to the soluble salts of the metals which cause permanent hardness. After evapora- tion and filtration to remove the insoluble carbonates the sodium carbonate remaining in solution is titrated. The chief reac- tions are: CaCl 2 +Na 2 C0 3 CaC0 3 +2NaCl, (1) CaS0 4 +Na 2 C0 3 CaCO 3 +Na 2 SO 4 , (2) MgCl 2 + Na 2 CO 3 MgC0 3 + 2NaCl, (3) MgSO 4 +Na 2 CO 3 MgC0 3 + Na 2 S0 4 . (4) 1 Analyst, 8, 77 (1883). 200 QUANTITATIVE ANALYSIS Sodium carbonate also reacts with the bicarbonates present: Ca(HCO 3 ) 2 +Na 2 CO 3 CaC0 3 +2NaHCO 3 , (5) Mg(HC0 3 ) 2 +Na 2 C0 3 MgCO 3 +2NaHC0 3 . (6) That these reactions do not affect the determination of permanent hardness is due to the fact that there is formed in solution an amount of sodium bicarbonate equivalent to the sodium carbon- ate added. This method gives low results, however, because in reactions (5) and (6), above, equilibrium occurs with appreciable quantities of calcium bicarbonate and magnesium bicarbonate in solution. In case the permanent hardness is quite low the method may even give negative results, i.e. } more standard acid will be required than is equivalent to the standard sodium carbonate used. The solubility of carbonates of calcium and magnesium is diminished by the substitution of "soda reagent" for standard sodium carbonate. This is a standard solution containing equal parts of sodium carbonate and sodium hydroxide. The cause of the diminished solubility is the fact that a smaller quantity of sodium bicarbonate is produced by the reactions, nor- mal sodium carbonate being the chief product : Ca(HC0 3 ) 2 +2NaOH- CaC0 3 +Na 2 C0 3 +2H 2 0. Clark's method for the determination of total hardness is based upon the use of a standard soap solution, added until a permanent lather is produced. It was formerly extensively used but it is inaccurate and is now little used. The small percent of dissolved salts usually found makes de- sirable a method of expressing results different from that based upon percent. Instead of percent it is customary to report parts per hundred thousand of water, parts per million or grains per gallon. In England the Imperial gallon of water weighs 70,000 grains while the United States gallon weighs in air at 15.5 and 760 mm pressure, 58335+ grains. This gives at least four different systems which have been at various times and in various countries commonly used for the expression of the results of water analysis. Hardness itself has also been expressed in Clark's degrees (grains of calcium carbonate per Imperial gallon), German degrees (parts of calcium oxide per 100,000 parts of water) and French degrees (parts of calcium carbonate per 100- 000 parts of water). This has resulted in much confusion but EXPERIMENTAL VOLUMETRIC ANALYSIS 201 there is now a general tendency toward the practice of expression in parts per million or, more exactly, milligrams per liter, although in industrial operations the report is often made as grains per United States gallon. Upon the assumption that one liter of water, at the usual working temperature, weighs 1000 gm, every milligram of dissolved matter will represent one part per million of water. This assumption is not absolutely correct and " milligrams per liter" is a better expression than " parts per million." The following exercises may profitably be omitted at this time by students who will carry out a more extensive analysis of water later. (See page 339.) Determination of Temporary Hardness. Dilute 50 cc of tenth- normal hydrochloric acid to 250 cc to make the solution fiftieth-normal. Allow the water to run from the laboratory faucet for 5 minutes in order to cleanse the pipes of standing water. Thoroughly rinse a 2500 cc bottle with the fresh water, fill and use from this as the sample. Fill a 100 cc graduated flask with this water and empty into a 200 cc beaker, rinsing the flask with distilled water. Titrate with fiftieth-normal acid, using methyl orange or erythrosine as indicator. Calculate the tem- porary hardness of the water, expressing the result as milligrams of calcium carbonate per liter. Determination of Permanent Hardness. Make 500 cc of a fiftieth- normal solution of sodium carbonate and check by titration against the fiftieth-normal acid. Measure 100 cc of the water into a graduated flask and evaporate from a platinum dish placed on the water bath, adding 50 cc of fiftieth-normal sodium carbonate solution to the dish from a burette. It is not necessary that the platinum dish should hold the entire amount of water as this may be poured in as evaporation pro- ceeds. Evaporate to dryness, then add distilled water and make up to 100 cc in a graduated flask. Allow to settle, then fill a burette with the clear solution and titrate 25 cc with fiftieth-normal acid, using methyl orange or erythrosine. Calculate the permanent hardness as milligrams of calcium sulphate per liter of water. Determination of Permanent Hardness by Use of "Soda Reagent." Make a standard solution of equal parts of sodium carbonate and sodium hydroxide, standardized and diluted to make twenty-fifth nor- mal when titrated by acid in presence of erythrosine. Measure 200 cc of water into an Erlenmeyer flask of Jena glass. Boil ten minutes to expel free carbonic acid and add 25 cc of standard " soda reagent." Boil to a volume of 100 cc, cool, rinse into a 200 cc volumetric flask and dilute to the mark with recently boiled distilled water. Filter 202 QUANTITATIVE ANALYSIS through a dry filter, rejecting the first 50 cc of the filtrate, and titrate 100 cc with fiftieth-normal acid, using erythrosine. Standard Bases. Standard solutions of bases are subject to change in basic concentration and must be frequently restand- ardized. This is because glass is appreciably soluble in bases and the accumulation of alkali silicates in the solution gives an increase in .basicity toward all indicators. Bases also absorb carbon dioxide when exposed to the air and this results in a decreased basicity toward indicators of the class of phenolphthalein. For this reason it is desirable to avoid unnecessary contact with the air after standard- ization. For the preparation of standard basic solutions free from carbonates one may either use a substance whose carbonate is insoluble or one which contains little carbo- nate and whose carbonate may be precipitated by the addition of another sub- stance. For the first method barium hydroxide is gener- ally used. This base always contains some barium car- bonate but this remains un- dissolved when the solution is made. If, thereafter, carbon dioxide is absorbed by the solution, an equiva- ^ amount Qf bar j um . . bonate is precipitated and the solution remains free from carbonate, although it must be restandardized. Sodium hydroxide or potassium hydroxide may be obtained nearly free from carbonates by dissolving in alcohol, decanting or filtering from undissolved carbonate and evaporat- FIG. 64. Burette with three-way stop- cock, connected with reagent bottle. EXPERIMENTAL VOLUMETRIC ANALYSIS 203 ing the alcohol in an atmosphere that is free from carbon dioxide. Bases so prepared may now be obtained from the manufacturers and should be used for the preparation of standard solutions whenever possible. For the second method potassium hydroxide or sodium hy- droxide may be used and a slight excess of barium chloride added to the solution, barium carbonate being thereby precipitated. In either case reabsorption of carbon dioxide is prevented by passing entering air through a soda lime tube, removing the solu- tion through a siphon directly to the burette, as in Fig. 64. Covering the solution with a layer of toluene or of any oily sub- stance is not to be recommended because this results in fouling the burette. It should be understood that these precautions are unneces- sary in most titrations. While it is true that alkali carbonates have not the same basicity toward some of the indicators as have the alkali hydroxides, a proper correction is made by standardiz- ing in presence of the indicator that is to be used in the deter- minations. This is a principle that must always be observed. One must not, for instance, use the standardization in presence of methyl orange as a basis for the calculation of determinations made in presence of phenolphthalein. Standardization. The standardization of solutions of bases may be accomplished by indirect methods, titrating against a previously standardized acid solution, or by direct methods, titrating against a weighed substance of known purity. In general the first method is to be recommended because the avail- able solid acids of uniform purity are limited to the organic acids. This makes necessary the use of phenolphthalein in the standardization, even if other indicators are necessary for later determinations. Such substances are oxalic acid, H 2 C204.2H 2 O, potassium tetroxalate KHC2O4.H2C2O4.2H 2 and succinic acid, H 2 C4H 4 04. No direct gravimetric method can be used because there is no precipitating reagent for the hydroxyl radical and a determination of the metal would not give the basic strength because of the invariable presence of salts of the same metal. Exercise : Preparation of Decinormal Potassium Hydroxide Solution. Select the best grade of potassium hydroxide obtainable, that purified by alcohol being preferable. Calculate the weight necessary for 2500 cc of decinormal solution and weigh out the quantity with 2 percent added 204 QUANTITATIVE ANALYSIS to compensate for carbonates, water, and other impurities. Dissolve in a 2500 cc bottle having a solid rubber stopper. Fill the bottle with distilled water, mix thoroughly and allow to cool to the temperature of the room. Titrate portions of about 30 cc each against the fifth- normal acid, using, in turn, methyl orange, cochineal, and phenolph- thalein. Continue these practice titrations until the results with a given indicator agree closely among themselves. The results with phenolphthalein will indicate a weaker solution than with the other indicators because of the presence of carbonates in the solution of base. Dilute to make exactly tenth-normal, using the figure obtained with methyl orange as indicator as the basis for the calculation. Record upon the label, besides your own name, the name of the solution and the normality as calculated from the titration by each indicator, also the date of standardization. Determination of the Concentration of the Laboratory Acids. Use the " dilute *' acids, either sulphuric, hydrochloric, nitric or acetic. The sample should not be weighed in the analytical balance and it is better to determine the specific gravity and then take a measured volume. Determine the specific gravity with an accurate hydrometer. From the approximately known percent of acid in the solution calculate the dilu- tion required to make a solution approximately equivalent in concentra- tion to the standard base. Make 500 cc of solution, measuring accur- ately the volumes used. Titrate the finally diluted solution against the standard base, using methyl orange for any of the acids above named except acetic acid. For this use phenolphthalein. Calculate the per- cent of acid, by weight and by volume, in the original solution. If tables are at hand compare the results of the experiment with the per- cent corresponding to the specific gravity found. Determination of the Purity of Citric Acid. Weigh about 3 gm of commercial citric acid, dissolve and dilute to 500 cc. Titrate portions of 30 cc to 40 cc by the standard base, using phenolphthalein. Calculate the percent of the tribasic acid H 3 C4H 5 07.H 2 assuming that no other acid is present. VINEGAR Vinegar contains from 3 to 6 percent of acetic acid, in addi- tion to coloring matter, dissolved solids and sometimes unfer- mented sugar or alcohol. Cider vinegar contains also from 0.08 to 0.16 percent of malic acid. Vinegar is sometimes adulterated with other added acids, notably sulphuric acid. The complete analysis will include the determination of the sub- stances just mentioned and others that serve to characterize the EXPERIMENTAL VOLUMETRIC ANALYSIS 205 vinegar with respect to its origin or quality. The determination of total acidity alone is of value in determining the " strength" of the vinegar. This is practically due to acetic acid alone in pure t vinegars other than those made from cider. In cider vinegar the determination of malic acid is also of importance as indicating its origin. Determination of Total Acidity. Determine the specific gravity of the vinegar by means of a hydrometer and measure about 6 cc for analysis, or weigh about 6 gm in a weighing bottle. Dilute until the color is faint, add a drop of phenolphthalein and titrate with decinormal base. Calculate total acidity in terms of acetic acid. BORIC ACID Boric acid is one of the weakest of all inorganic acids, having a percentage ionization not far above that of hydrocyanic acid. It is therefore not possible to titrate it by standard bases in ordinary solution because no indicator will give a sudden color change at any point in its neutralization. It can be determined by titrating in presence of glycerine, phenolphthalein being used as indicator. 1 Hydroxylated organic compounds form ester- like compounds with the various boric acids, the result being substances more strongly ionized than boric acid. Tartaric acid forms such substances, with a resultant change of its optical rotatory power. With glycerine and orthoboric acid the com- pound C3H 5 (OH).HBO 3 is probably formed because the substance reacts as a monobasic acid. Boric acid combined as salts may be determined by first adding, in presence of methyl orange, enough hydrochloric or sulphuric acid to completely decompose the borate. In case carbonates are present carbonic acid is also produced and this must be expelled by boiling because phenol- phthalein is to be used in the last titration. If the more common biborates are thus decomposed they yield orthoboric acid, methyl orange indicating the end-point at the completion of the reaction : Na2B 4 07+5H 2 + 2HCl 2NaCl+4H 3 B0 3 . The addition of glycerine produces the reaction: C 3 H5(OH)3+H 3 B03 C3H 5 OH.HBO3H-2H 2 O. The monobasic acid is then neutralized in the titration: C 3 H 5 OH.HBO 3 +KOH C 3 H 5 OH.KBO 3 +H 2 0. 1 Thomson: J. Soc. Chem. Ind., 12, 432 (1893). 206 QUANTITATIVE ANALYSIS Commercial borax is essentially crystallized sodium biborate, Na 2 B 4 O7.10H 2 O. It loses water rapidly and seldom corresponds to this formula. Determination. Crush and mix a sample of borax quickly. Weigh about 10 gm, dissolve and dilute to 500 cc. Titrate measured portions of 20 cc with decinormal acid in presence of methyl orange, first diluting the solution to 50 cc. Now add to other diluted portions of the borate solution the exact amount of acid indicated by the titration but no indi- cator. Boil for a few minutes, then cool. Add a drop of phenolphthal- ein and 30 cc of glycerine. The glycerine must be neutral. Titrate with standard base to the appearance of a pink color. Add 10 cc more of glycerine and if the pink disappears add more base. Continue the addition of glycerine and standard base until a permanent end-point is attained. Calculate the percent of boron trioxide, B 2 03, in the original borax. Reference to the formula for the complex of boric acid and glycerine will show that the equivalent weight of boron trioxide must be one-half its molecular weight. CHAPTER IX OXIDATION AND REDUCTION Reactions of neutralization constitute a very important group in quantitative chemistry. Of no less importance is that group which is composed of reactions of oxidation and reduction. These do not involve the use of the organic indicator but of some inorganic substance, this being, in some cases, the standard reagent itself. While the quantitative relations between the reacting substances are here different from those of neutraliza- tion, the same principles will serve for the calculation of the results of the titration. The equivalent weights will be deter- mined by dividing the molecular weights by the hydrogen equivalents but the latter will be determined, not by the valence of the reacting parts but by the change of valence in the reaction, because oxidation always involves a change of valence of the oxidizing and reducing agents. In the reaction: BaCl 2 +H 2 S0 4 BaS0 4 +2HCl the hydrogen equivalent of each radical is plainly represented by its valence, for it is in this measure that it may enter into the exchange of double decomposition. In the reaction: 2KMn0 4 +10FeSO 4 +8H 2 SO 4 K 2 S0 4 +2MnSO 4 +5Fe 2 (S0 4 ) 3 -f- 8H 2 O the hydrogen equivalents of potassium permanganate and ferrous sulphate cannot be so represented because something that is different from ordinary double decomposition has taken place. Manganese has left a negative radical and entered a positive one and has thereby changed its valence and has been reduced. The change of valence will represent its power as an oxidizing agent. Iron has taken on more of the radical with which it was already combined, has increased its valence and has been oxidized. It is not difficult to determine the change of valence of manganese if its state of oxidation in the two compounds is first determined. 207 208 QUANTITATIVE ANALYSIS For the purposes of this inspection the question of the actual valence need not be considered. There has been much difference of opinion regarding the structural formulas that should represent inorganic compounds and the real valence of an element in an oxide is indicated by the structural formula of that oxide. For example, the dioxide of manganese might be represented by the formula Mn , in which the valence of manganese is 4, or by ,0 Mn^ | in which the valence is 2. If the element is regarded as X always being in direct combination with the oxygen the latter may be taken as a measure of the apparent valence and the loss of oxygen or its equivalent in other elements, as the result of a reaction, will be measured by the change of apparent valence. This change will therefore be the hydrogen equivalent of either oxidizing or reducing agent. Potassium permanganate may be represented by the older oxide formula, K 2 O.Mn 2 07, which shows that it is derived from the oxide Mn 2 07. Manganous sulphate may also be written MnO.SO 3 . Manganese has thus changed its apparent valence from 7 to 2 and its hydrogen equivalent is therefore 5. The equivalent weight of potassium permanganate, containing one atom of manganese in each molecule, is one-fifth of its molecular weight or 31.606. Iron changes in the reaction from FeO.S0 3 to Fe 2 3 .3S0 3 . Its change of apparent valence is from 2 to 3, its hydrogen equivalent is therefore 1 and its equivalent weight is its atomic weight. In the reaction: KiCrj0 7 -HFeCl2+ 14HC1 2KC1 + 2CrCl 3 + 6FeCl 3 +7H 2 0, potassium dichromate is the oxidizing agent. Closer examina- tion shows that the element chromium is the real cause of the oxidizing action, since it leaves the negative radical by changing to a base-forming lower oxide. Potassium dichromate may be written K 2 O.2CrO 3 . Chromium chloride is seen at once to contain trivalent chromium. The change of valence (reduction) of chro- mium is from 6 to 3, its hydrogen equivalent is 3, and that of potassium dichromate, containing two atoms of chromium in the molecule is 6. The equivalent weight of potassium dichro- mate is then one-sixth of its molecular weight. OXIDATION AND REDUCTION 209 The assignment of apparent valence to the oxidizing and reduc- ing elements is a valuable aid in balancing oxidation and reduction equations. For example the equation last given is to be balanced. The empirical equation is K 2 Cr 2 O 7 +FeCl2+HCl-*KCl+CrCl3+FeCl3+H 2 0. First inspect the equation to determine the oxidizing and reducing elements, which are seen to be chromium and iron. Determine the changes in apparent valence and from these their hydrogen equivalents and those of the compounds containing them. Write these above the respective compounds thus: VI I K 2 Cr 2 7 +FeCl2+HCl-*KCl+CrCl3+FeCl3+H 2 O. The hydrogen equivalent of the reducing agent is to be the coefficient of the oxidizing agent, and vice versa. These coeffi- cients will not thereafter be changed. From these will follow the coefficients of potassium chloride, chromium chloride and ferric chloride and from all of these will be calculated the coefficient of hydrochloric acid. From the latter will follow the coefficient of water. The balanced equation is then: K 2 Cr 2 O 7 +6FeCl 2 +14HCl 2KC1 + 2OC1 3 + 6FeCl 3 + 7H 2 O. PROBLEMS 42. Determine the equivalent weights of the oxidizing and reducing agents in the following equations and balance the latter: (a) KMnO 4 +MnSO 4 +KOH K 2 SO 4 +MnO 2 +H 2 O. (b) K 2 Cr 2 O 7 +SnCl 2 +HCl KCl+SnCl4+CrCl 3 +H 2 O. (c) H 3 AsO 3 +I 2 +H 2 O H 3 AsO 4 +HI. (d) HgCl 2 +SnCl 2 HgCl+SnCl 4 . (e) HgCl 2 +SnCl 2 Hg+SnCl 4 . 43. How much potassium permanganate must be contained in 1000 cc of solution so that 1 cc shall be equivalent to 0.002 gm of iron? To 0.002 gm of manganese? 44. How much potassium dichromate must be contained in 1000 cc of solution so that 1 cc shall be equivalent to 0.005 gm of iron? 45. A solution of potassium permanganate contains 2.83 gm in 1000 cc. What weight of pure ferrous ammonium sulphate, FeSO 4 (NH 4 ) 2 SO 4 .6H 2 O, should be taken for standardization so as to require 30 cc of the permanga- nate solution? 46. A solution of potassium dichromate is of such concentration that 1 cc =c= 0.005 gm iron. A solution of potassium permanganate contains 3.26 gm of 14 210 QUANTITATIVE ANALYSIS the salt in 1000 cc of solution. To what volume should 1000 cc of the stronger solution be diluted to make the two equivalent to the same weight of iron? 47. What weight of iodine is required for 1000 cc of a tenth-normal oxidiz- ing solution? 48. What weight of arsenic trioxide is equivalent to 1 cc of tenth-normal iodine solution? Potassium Permanganate. This substance is a very conveni- ent one for use as a standard oxidizing agent, not only because it is readily and quantitatively reduced by many reducing agents, but because it serves as its own indicator. The color of even a dilute solution is quite intense, while the reduced salt, man- ganous sulphate, forms colorless solutions. The least excess, after the oxidation is finished, is made evident by the color of the unchanged permanganate. Practically, standard potassium permanganate is most often used for the determination of iron, less frequently for the determination of manganese, and occa- sionally for the determination of tin or antimony and indirectly for the determination of phosphorus and many other elements. Theoretically it could be used for the direct or indirect deter- mination of any reducing agent or oxidizing agent but other methods are frequently better for substances other than those named. A solution of potassium permanganate, if made by simply dissolving the salt in water, decomposes slowly and must be fre- quently restandardized. Morse 1 has shown that this is due to the presence of manganese dioxide, traces of which may have been contained in the original salt or it may have been formed by reduction of potassium permanganate by dust or organic impuri- ties. The work of Morse shows that if the solution is filtered through asbestos, and is kept in clean, glass-stoppered bottles its concentration will remain constant almost indefinitely. IRON Iron may be readily and quickly titrated by a solution of po- tassium permanganate. For this purpose the standard solution should be made in the decimal system. The normal system is not adapted to this determination because the potassium per- manganate will not often be used for the determination of sub- Am. Chem. J , 18, 401 (1896); 20, 521 (1898). OXIDATION AND REDUCTION 211 stances other than iron and the decimal system provides more sim- ple calculations. If the iron compound, as an ore, is taken in such quantity that the weight of sample is a simple multiple of the iron equivalent of the standard, the burette reading is a direct percentage reading. Thus, if 1 cc of standard is equivalent to 0.002 gm of iron and if 2 gm of iron ore is taken for analysis each cubic centimeter of standard represents 0.1 percent of iron. One serious disadvantage in the use of potassium permanganate for the titration of iron lies in the fact that if hydrochloric acid is present, it also is oxidized by the standard, the result being a fictitious value for the percent of iron. The red color of ferric chloride also obscures the end-point with potassium permanga- nate. Most iron ores dissolve best in hydrochloric acid and the solubility in sulphuric acid is very slight. After solution of the ore is accomplished it is necessary to remove the hydrochloric acid or to find some method for avoiding its reducing action upon the permanganate. To remove the acid, the solution of the ore may be evaporated with sulphuric acid. Instead of removing the hydrochloric acid it is possible to minimize its reducing action by the addition of manganous sulphate to the solution. The exact cause of this action of manganous sulphate is not known. No explanation has been offered, that has an undoubted experi- mental foundation. 1 The red color of ferric chloride may be avoided by the addition of phosphoric acid 2 to the solution. This probably produces a weakly ionized ferric phosphate, which is colorless. Primary Standards. Potassium permanganate cannot be obtained in a sufficiently high state of purity to make possible direct weighing of the substance for standardization. Primary standard reducing agents of known and uniform purity must be used. Some reducing agents that may be used for this purpose are iron, ferrous sulphate, ferrous ammonium sulphate, oxalic acid and ammonium oxalate. Iron. Pure iron is not obtainable commercially and can be prepared only with great difficulty. Iron wire may be obtained having a fairly high degree of purity. A common fallacy has found acceptance by many chemists, to the effect that the so- called " piano wire, " purchased for standardizing, is uniformly 99.6 i Vide Manchot: Ann., 325, 105 (1902). 5 Reinhardt: Z. physik. Cbem., 28, 33 (1899). 212 QUANTITATIVE ANALYSIS percent or some other stated percent of iron. The purity of such wire varies widely and this material cannot be used as a primary standard for accurate work unless it has been carefully analyzed. Such analysis is best accomplished by electrolysis. The gravi- metric determination of iron by precipitation as ferric hydrox- ide and weighing as ferric oxide is not accurate unless made with extreme care, on account of the difficulty experienced in puri- fication of the precipitate. If iron is to be used for standardizing solutions, weighed portions of the previously analyzed material are dissolved in dilute sulphuric acid. The necessary li :U- tions as to accuracy render the use of iron as a primary standard decidedly unsatisfactory, especially since it has become pos- sible to obtain iron salts in a high state of purity. Ferrous Sulphate. Ferrous sulphate may be obtained in a satis- factory condition for standardizing purposes and it may be read- ily analyzed by electrolysis. The salt easily oxidizes, even in the crystalline condition, and it also hydrolyzes by contact with the moisture of the air. These properties render it difficult to pre- serve the salt unchanged. Ferrous Ammonium Sulphate. Ferrous ammonium sulphate ("Mohr's salt") is free from the objections above noted. Many analyses conducted upon high-grade samples have shown that the salt may be obtained in a state of practical purity, the per- cent of iron being almost exactly that calculated from the formula. Oxidation in air does not readily occur, so that the standard sample may be preserved almost indefinitely if kept in a stop- pered bottle. Oxalic Acid. Oxalic acid and ammonium oxalate may be puri- fied by recrystallization to the condition required for primary standards. If previously analyzed they, are equally as satisfac- tory as ferrous ammonium sulphate. The student is cautioned against the practice of assuming the purity of any of his primary standards without an analysis as the basis for the assumption. Reduction of the Iron Solution. Iron exists in the ferric con- dition in most ores or other minerals. In order to reduce the solution of ferric salt either stannous chloride, zinc, sulphurous acid or hydrogen sulphide may be employed. The first two are the only ones now commonly used. Stannous chloride, in solu- tion, possesses the advantage of instantaneous action if added to the hot solution of ferric chloride. If the iron is to be reduced OXIDATION AND REDUCTION 213 by stannous chloride an addition of this salt to the ore during the process of solution will materially hasten the action. For the final reduction the stannous chloride solution may be added from a burette, the disappearance of the red color of ferric chloride providing an approximate indication of the end-point. After a slight excess has been added the solution is cooled and a considerable excess of mercuric chloride is added, the unused stannous chloride being thereby oxidized: 2HgCl 2 +SnCl 2 SnCl 4 +2HgCl: Mercuric chloride will not oxidize ferrous chloride and hence may be left in the solution. If an insufficient excess of mercuric chloride is used, or if it is added too slowly, free mercury may be produced: HgCl 2 +SnCl 2 SnCl 4 +Hg. The indication of such action is the appearance of a gray precipi- tate of mercury instead of the characteristic white silky crystals of mercurous chloride. If mercury is so produced the determi- nation is ruined because this mercury will itself reduce some of standard oxidizing solution during the process of titration of the iron. Stannous chloride cannot be used to reduce ferric solutions previous to titration by potassium permanganate unless the interference of chlorides is to be prevented by the addition of manganous sulphate. Instead, pure zinc or zinc of predetermined iron content may be used to reduce the iron. For this purpose an approximately weighed quantity of granular zinc or zinc dust may be added directly to the solution and dissolved in it, or the ferric solution may be passed through a reductor. The latter is a tube having the dimensions of a burette, filled with amalga- mated zinc and having a dropping funnel fixed in the top. The acidified solution is passed through the tube once or twice and the iron is thereby reduced. Most zinc contains iron or other reducing matter and if it is to be dissolved in the iron solution, as above described, a blank determination should be made to determine the amount of standard that will be required to oxidize the reducing matter of the zinc. On account of the slow reducing action of zinc, stannous chloride is much to be preferred, where conditions will permit its use. 214 QUANTITATIVE ANALYSIS Sulphurous acid and hydrogen sulphide reduce iron solutions quickly but the disadvantages involved in their preparation prevent their extensive use for this purpose. Exercise : Preparation of Standard Potassium Permanganate Solu- tion. Calculate the weight of potassium permanganate required for 2500 cc of solution, each cubic centimeter to be equivalent to 0.005 gm of iron, using for this purpose the method shown on page 167. Weigh 2 percent more than the calculated weight and dissolve in about 1000 cc of distilled water. Allow to stand at least an hour, then filter through a double asbestos filter arranged as in Fig. 65, all vessels having been previously cleaned by chromic acid mixture. Dilute the solu- tion to 2500 cc and standardize as follows : Calculate the weight of ferrous ammonium sulphate that will be approximately equiva- lent to 35 cc of the permanganate solution. Weigh three such portions into Erlenmeyer flasks of 250 cc capacity. Dissolve each por- tion, immediately before titrating, in 50 cc of distilled water and add 10 cc of dilute sul- phuric acid. Titrate at once with the potas- sium permanganate solution, the first ap- pearance of a permanent pink tint being taken as the end-point. Calculate the value of the permanganate solution in terms of iron and dilute two liters of the solution to the exact equivalence of 0.005 gm of iron per cubic centimeter. Determination of Iron in an Ore. Sample the ore and grind the last selection to pass the 100-mesh sieve. Weigh exactly 0.5 gm on the counterpoised glasses, placing this amount of ore in each of three casseroles. Add to each 25 cc of concentrated hydro- chloric acid. If method (b) is to be used for reducing the iron" add also at this point 8 cc of 5 percent stannous chloride solution. Cover and warm until solution is complete or until no further action appears to take place. If the residue is not colored, proceed, without filtration, by one of the methods (a) or (b) given below. If the residue is colored it may contain iron. In this case filter on a small paper and wash the paper free from iron solu- FIG. 65. Apparatus for filtering potassium permanganate solution. OXIDATION AND REDUCTION 215 tion with hot water. Set the filtrate and washings aside and burn the paper in a platinum crucible. If the residue is small in amount and apparently contains little silicious matter it may be decomposed by fus- ing with potassium acid sulphate. If the proportion of silicates seems to be large, treat the residue, after burning the paper, with a drop of concentrated sulphuric acid and four or five drops of hydrofluoric acid. Warm until the acids are volatilized then fuse at a red heat with 1 gm of potassium acid sulphate. Cool and dissolve the mass in hot water, adding the solution to the former nitrate. Now proceed by one of the methods given below. (a) Add 4 cc of concentrated sulphuric acid to the iron solution and evaporate by holding the casserole over a free flame, keeping in continual motion to hasten evaporation and prevent bumping. Evaporate until the characteristic white fumes of sulphuric acid appear, this being the point at which all water and hydrochloric acid have been expelled. Cool and dilute to 50 cc, rinsing the solution into a 250 cc Erlenmeyer flask. Add 2 gm of granular zinc, as free as possible from iron, place a funnel with a short stem in the flask and warm until the zinc is dissolved. The iron solution should now be quite colorless or faintly green. Cool and titrate at once with standard potassium permanganate solution. Make a blank determination without the ore, to determine the amount of iron and other reducing matter in the zinc, by dissolving 1 gm of zinc in 25 cc of dilute sulphuric acid, under the same conditions as noted above, and titrating. Express the result of the blank experiment as the number of cubic centimeters of potassium permanganate reduced by 1 gm of zinc. The proper value will then be subtracted from the volume of permanganate used in the iron titration and the percent of iron then calculated. If the zinc is nearly pure it will dissolve very slowly. Solution may be hastened by dropping a coil of platinum wire into the flask, and keep- ing it in contact with the zinc. (b) Concentrate the iron solution, if necessary, to about 50 cc and transfer to a 250 cc Erlenmeyer flask. While the solution is nearly boil- ing add, drop by drop from a burette or pipette, a 5 percent solution of stannous chloride until the ferric chloride has just been reduced, this being made evident by the disappearance of the red color. Add 0.5 cc more of stannous chloride solution then cool quickly by immersing the flask in running water. When cool add, all at once, 25 cc of a 5 percent solution of mercuric chloride and mix well with the solution. The pre- cipitate should be pure white mercurous chloride without a trace of gray mercury. Dilute to 100 cc and add 50 cc of a solution containing 144 gm of phosphorous pentoxide, 235 gm of sulphuric acid and 67 gm of crystallized manganous sulphate in each liter of solution. Titrate at 216 QUANTITATIVE ANALYSIS once with standard potassium permanganate solution and calculate the percent of iron in the ore. CALCIUM Calcium may be precipitated as oxalate, filtered and washed free from ammonium oxalate, then dissolved in hot, dilute sul- phuric acid and the resulting oxalic acid titrated with potassium permanganate. CaC 2 O4+H 2 S04->CaS04+H 2 C 2 O 4 , 2KMnO 4 +5H 2 C 2 04+3H 2 S04 K 2 SO 4 +2MnS0 4 + .'.-' 10C0 2 +8H 2 0. In order to determine the equivalent weight of oxalic acid it is necessary here, as in other cases, to note the change which it undergoes as it becomes oxidized. The formula may be written H 2 O.C0 2 .CO, to show the state of oxidation of the carbon. The products of oxidation are water and carbon dioxide. It is evident that each molecule of the acid is oxidized by one atom of oxygen. Its hydrogen equivalent is therefore 2 and its equivalent weight is one-half its molecular weight. When oxalic acid reacts as an acid its hydrogen equivalent is also 2, although there is no neces- sary connection between the two cases. Since one molecule of oxalic acid is formed by the decompositon of one molecule of calcium oxalate, containing one atom of calcium, the equivalent weight of calcium is also one-half its atomic weight. The cal- cium equivalent of the potassium permanganate solution may be calculated from the iron equivalent by the expression: eq. wt. of calciuni . . f. Xwt. iron = wt. ot calcium, eq. wt. ot iron PROBLEMS 49. 1 cc of a solution of potassium permanganate is equivalent to 0.002 gm of iron. Calculate the weight of calcium, calcium oxalate and oxalic acid equivalent to 1 cc. 50. What weight of potassium dichromate is equivalent to 4.75 gm of potassium permanganate, both being used for the oxidation of iron? 51. What weight of crystallized oxalic acid, H 2 C 2 O4.2H 2 O, is equivalent to 3.56 gm of crystallized ferrous ammonium sulphate, Fe(NH 4 ) 2(864) 2.6H 2 O? OXIDATION AND REDUCTION 217 52. What is the normality of a solution of potassium permanganate, 1 cc of which is equivalent to 0.005 gm of iron? 53. 30 cc of potassium permanganate solution oxidizes 0.1905 gm of crystallized oxalic acid. How dilute 1000 cc of the solution to make exactly decinormal? Determination. Use samples of not more than 0.2 gm of the calcium compound. Dissolve, precipitate, filter and wash the calcium oxa- late according to the method already learned in an earlier exercise. Pierce the point of the filter paper and wash the precipitate into a beaker with the least possible quantity of hot water. Thoroughly wash the paper in the funnel with successive portions of 5 cc of hot dilute sulphuric acid until any remaining precipitate shall have been dissolved. Again wash the paper with hot water, then warm the acid and precipitate in the beaker until the precipitate is dissolved. Titrate while warm (60 to 70) with standard "potassium permanganate/ and calculate the percent of calokim in the sample. MANGANESE In aci(J solutions ^potassium permanganate is always reduced to the form of manganous salts, manganese being thereby reduced to its lowest state of 'oxidation, corresponding to the monoxide. In basic solutions^ the reduction goes only so far as to produce manganese dioxide. If the reducing agent is a manganous salt it is also oxidized to manganese dioxide: 2KMn0 4 +3MnS0 4 +2H 2 K 2 SO 4 +5Mn0 2 +2H 2 SO 4 . (a) This reaction may be made the basis of a determination of manganese by titration with potassium permanganate. 1 The manganese ore is dissolved in hydrochloric acid, manganous chloride being formed. The chloride is converted into sulphate and the hydrochloric acid removed by evaporation with sul- phuric acid. When the titration is made the solution must be only feebly basic. If a strong base is present a reduction of potassium permanganate to potassium manganate occurs, the reducing agent being manganese dioxide or manganese hydroxide: 2KMn0 4 +4KOH+Mn0 2 3K 2 MnO 4 +2H 2 0, (b) 4KMnO 4 +6KOH+Mn(OH) 2 5K 2 MnO 4 +4H 2 0. (c) These undesirable reactions may be prevented by having present a considerable excess of a weak base, such as is produced by shak- iVolhard: Chem. News, 40, 207 (1879). 218 QUANTITATIVE ANALYSIS ing an excess of zinc oxide with water, this giving a suspension of zinc oxide in a saturated solution of zinc hydroxide. The latter is so dilute and so weakly ionized that the formation of potassium manganate does not take place. It does provide, however, suffi- cient base to neutralize the sulphuric acid produced by reaction (a), because the excess of zinc oxide keeps the solution saturated with zinc hydroxide. Manganese dioxide possesses, to a slight extent, acid-forming properties, since it is able to produce a class of salts that are theoretical derivatives of manganous acid, H 2 Mn0 3 ( = H^O.MnC^) . This acid is not known in the free state but certain manganites, as those of calcium and zinc, CaMnO 3 and ZnMnO 3 , are produced when manganese dioxide is formed in presence of soluble calcium or zinc compounds. If no such metal is present at the moment of oxidation of manganous salts to manganese dioxide, manganous manganite, MnMn0 3 , is precipitated, thus removing a certain amount of unoxidized manganese from the solution. An error would thereby be introduced but this is prevented by having zinc hydroxide present. The saturated solution of manganese dioxide can have but a small concentration of manganous acid and this is at once precipitated as zinc manganite. The addition of zinc oxide therefore serves a double purpose. It maintains a feebly basic solution throughout the titration and also prevents the precipitation of manganous manganite. PROBLEMS 54. From the equation for the reaction between potassium permanganate and manganous sulphate, calculate the equivalent weight of potassium permanganate and of manganese. 55. What weight of potassium permanganate must be contained in 1000 cc of a solution, 1 cc of which will oxidize 0.002 gm of manganese? 56. If a solution of potassium permanganate is fifth-normal with respect to iron in acid solution what is its normality with respect to manganese in basic solution? 57. 1 cc of a solution of potassium permanganate is equivalent to 0.010 gm of iron. What weight of manganese will be oxidized by 1 cc? Determination. Calculate the approximate weight of pyrolusite necessary to reduce about 40 cc of the standard potassium permanganate solution already made, arbitrarily assuming that the pyrolusite is pure manganese dioxide and also that one-fifth of the sample is finally to be titrated. Dry the sample to constant weight at 120 and weigh the OXIDATION AND REDUCTION 219 calculated quantity, placing in a casserole. Dissolve by warming with concentrated hydrochloric acid. When solution is complete or when no further action is apparent filter and wash the residue and paper, preserv- ing the nitrate and washings. If the residue contains any dark material, burn the paper in a platinum crucible and fuse with 1 to 2 gm of sodium carbonate. If manganese is present the fusion will be colored green. Dissolve the fused mass in hot water and add to the main solution. Remove hydrochloric acid by evaporating with sulphuric acid (about 2 cc). Redissolve, adding a little nitric acid if necessary, wash into a 250 cc volumetric flask, dilute to the mark and mix. Treat 50 cc por- tions as follows: Measure into an Erlenmeyer flask of 1000 cc capacity and neutralize by the addition of zinc oxide suspended in water, shaking and continuing the addition until the iron is all precipitated as ferric hydroxide and a small excess of zinc oxide is present. Dilute to about 300 cc, heat nearly to boiling and titrate, adding the potassium permanganate solution 5 cc at a time until a permanent color is produced. Treat a second 50 cc portion of the manganese solution in a similar man- ner but adding, all at once, 5 cc less of standard solution than was added to the first portion then adding 1 cc at a time. To a third portion add the entire quantity used in the second, less 1 cc, then complete the titra- tion by adding the standard solution 0.1 cc at a time. Treat the fourth portion in the same manner as the third. From the last two titrations calculate the percent of manganese in the ore. AVAILABLE OXYGEN Manganese dioxide is used not only as an ore of manganese but also as an oxidizing agent in various laboratory processes and in a commercial way, as, for example, in the production of chlorine from hydrochloric acid. In such cases the percent of manganese is not as important as is that of oxygen available for oxidation processes. In most cases the available oxygen may be calculated with sufficient accuracy for commercial requirements from the percent of manganese. This can be accurately done only in case no other manganese compound and no other peroxide is present. Generally no other peroxide is present, although manganese fre- quently occurs in pyrolusite in small quantities as other compounds than the peroxide. If an accurate determination of available oxygen is required it may be made by reducing a weighed sample of the manganese dioxide by a measured amount of a reducing agent, titrating the excess of the latter by standard potassium permanganate. The reducing agent may be any of those already 220 QUANTITATIVE ANALYSIS discussed in connection with standardization of potassium per- manganate. Ferrous ammonium sulphate or oxalic acid is to be preferred. The reaction between manganese dioxide and these reducing agents in presence of sulphuric acid is represented by the following equations, which should be balanced by the student as an exercise in calculation of hydrogen equivalents. Determine also what fraction of the total oxygen of manganese dioxide is " available." Mn0 2 +FeS0 4 +H 2 S0 4 MnS0 4 +Fe 2 (S0 4 ) 3 +HA Mn0 2 +H 2 C 2 O 4 +H 2 S0 4 MnSO 4 +C0 2 +H 2 0. Another method for determining the available oxygen is described on page 227. Determination. Dry the sample of either pyrolusite or commercial manganese dioxide to constant weight at 120. Calculate the weight that would be equivalent to approximately 40 cc of the standard potas- sium permanganate already made, arbitrarily assuming that the sample is pure manganese dioxide. Weigh samples of the calculated weight into 250 cc Erlenmeyer flasks. Calculate the weight of crystallized ferrous ammonium sulphate or oxalic acid that would reduce approxi- mately 50 cc of the standard solution of potassium permanganate and add this quantity to each flask. Calculate the approximate volume of dilute or concentrated sulphuric acid necessary to enable the reactions to proceed and add three times this volume to each flask. Add 50 cc of water, warm to 70 and titrate immediately with standard potassium permanganate solution. Calculate the percent of available oxygen in the sample. Potassium permanganate solution is also useful for the titra- tion of hydroferrocyanic acid and hydroferricyanic acid. Ferro- cyanides are oxidized in acid solution: 10H 4 Fe(CN) 6 +2KMn0 4 -|-3H 2 SO 4 ->10H3Fe(CN)6+K 2 SO 4 + 2MnS0 4 +8H 2 O. Ferrocyanidesmay be reduced in basic solution by ferrous sulphate and then titrated by potassium permanganate. The reaction is as follows: K,Fe(CN)+FeS0 4 +3KOH K 4 Fe(CN) 6 +Fe(OH) 3 +K 2 S0 4 . Potassium Dichromate. The equation for the reaction of potassium dichromate with ferrous salts is given on page 209. This substance possesses several advantages over potassium per- OXIDATION AND REDUCTION 221 manganate as a standard oxidizing agent. It is relatively more stable and therefore may be obtained in a state of uniform purity. This makes it possible to standardize solutions by direct weighing when the degree of purity of the salt has been established by analy- sis. The relative stability is the same with solutions and the standard solution can be kept almost indefinitely without chang- ing its concentration. Potassium dichromate may also be used for the titration of iron and other reducing agents in presence of hydrochloric acid or chlorides, without oxidation of the latter taking place. This is a very decided advantage in the determina- tion of iron since it makes possible the use of stannous chloride as a reducing agent without the addition of manganous sulphate and phosphoric acid. There is no indicator that can be added directly to the solution which is being titrated by potassium dichromate and the color of the standard solution is not suffi- ciently intense to be of any use for this purpose. The indicator that is commonly used is potassium ferricyanide, placed in drops on a white porcelain "spot plate." Drops of the solution are removed from time to time by means of a stirring rod and allowed to touch the drops of ferricyanide. So long as ferrous iron is present the blue of ferrous ferricyanide is apparent on the spot plate. When the last trace' of iron has been oxidized there is produced on the plate only the light brown ferric ferricyanide. There being nothing in the appearance of the solution of the iron salt to indicate the approach to the end-point, the titration is necessarily somewhat tedious unless a system is devised for rapid readings. Such a system has been used in connection with the determination of manganese and is indicated in the next exercise and this removes the last objection to the use of potas- sium dichromate for the titration of iron. PROBLEMS 58. A solution of potassium permanganate contains 25.38 gm in 1000 co. What must be the concentration of a potassium dichromate solution in order that it shall have the same oxidizing power toward iron? 59. Balance the following equation and calculate the equivalent weight of tin. K 2 Cr 2 O 7 -fSnCl 2 +HCl KCl+CrCl 3 +SnCl 4 +H 2 O. Exercise: Preparation of Standard Potassium Dichromate Solu- tion. The solution should be of such concentration that 1 cc is equivalent 222 QUANTITATIVE ANALYSIS 0.005 gm of iron. Calculate the weight of potassium dichromate neces- sary for 2000 cc of such a solution. If the salt is known to be pure, weigh exactly the calculated weight and omit further standardization. If it is not pure but its oxidizing power known from previous determina- tions, calculate the weight of impure sample required and use this weight. If nothing is known of the purity use 1 percent more than the weight of pure salt required for 2500 cc of solution and standardize the solution as directed below. In any case dissolve the weighed salt and dilute to the proper volume. In case titration for standardization is to be omitted and direct weighing is to be made the basis for standardization, 2000 cc of the solution should be accurately made and poured into a dry bottle. Standardization, if this should be necessary, is accomplished by titra- tion against ferrous ammonium sulphate or iron wire, the first method being preferable. Write and balance the equation for the oxidation of ferrous sulphate by potassium dichromate in presence of sulphuric acid referring, if necessary, to the equation for the oxidation of the chloride, page 209. Calculate the weight of ferrous ammonium sulphate nec- essary to reduce 35 cc of the dichromate solution. Weigh five portions of exactly this weight into 250 cc beakers and dissolve each in 50 cc of recently boiled and cooled water just before titrating. Prepare a 1 per- cent solution of potassium ferricyanide and place a drop in each of the depressions of a white porcelain spot plate. Add to the solution of fer- rous ammonium sulphate three times the calculated amount of sulphuric acid necessary, as indicated by the equation, and titrate at once, as follows: To the first solution add the dichromate solution 5 cc at a time, stirring well after each addition, and test by removing a drop by means of the stirring rod and touching to a drop of potassium ferricyanide solu- tion on the spot plate. The end point is reached when a blue color is no longer produced on the plate. Titrate the second solution by adding 5 cc less than the amount of dichromate solution used in the first, then adding 1 cc at a time. Titrate the third solution by adding 1 cc less than the total used in the second, then adding 0.1 cc at a time. Titrate the fourth and fifth solutions in the same manner and take the average of the last three titrations for permanent record. Calculate the value of the solution in terms of iron. Dilute to make 1 cc equivalent to 0.005 gm of iron. Instead of weighing five portions of ferrous ammonium sulphate a standard solution may be made by dissolving ten times the required amount, adding the necessary sulphuric acid and diluting to 500 cc. Portions of 50 cc are then measured ajid titrated. The solution oxidizes upon exposure to air and the method of weighing separate portions and dissolving just before titration is preferable, OXIDATION AND REDUCTION 223 Determination of Iron. Prepare a sample of iron ore by grinding to pass a 100-mesh sieve. Weigh five portions of exactly 0.5 gm each, using the counterpoised glasses and brushing the ore into casseroles. Dissolve in hydrochloric acid, with or without the addition of stannous chloride, and reduce each solution just before titration, following the directions given for dissolving and reducing by method (b) of the per- manganate method. The titration is carried out exactly as directed for standardizing potassium dichromate solution. Calculate the percent of iron in the ore. CHROMIUM The most important ore of chromium is a compound of iron and chromium known as "chromite," having a composition corresponding with the formula FeO.Cr 2 3 . Although chromium is here in its lowest state of oxidation the substance is thought to be a salt of a hypothetical chromous acid, H 2 Cr 2 O 4 . Chromite cannot be dissolved in acids nor is it possible to decompose it easily by fusion with alkali carbonates. Fusion with sodium peroxide decomposes it, oxidizes the iron to ferric oxide and the chromium to chromium trioxide, forming then sodium chromate. Upon dissolving in water and filtering, ferric oxide is removed. The addition of acid produces sodium dichromate. This can then be reduced by adding an excess of a standard reducing agent, such as ferrous ammonium sulphate, titrating the excess by stand- ard potassium dichromate or permanganate. The reactions are expressed by the following equations, which should be balanced by the student. FeCr 2 O 4 +Na 2 2 Fe 2 3 +Na 2 CrO 4 +Na 2 0, Na 2 Cr0 4 +HCl Na 2 Cr 2 7 +NaCl+H 2 O, Na 2 Cr 2 7 +FeS0 4 +HCl NaCl+Fe 2 (S0 4 )3+FeCl 3 +CrCl3 + H 2 0. Iodine and Sodium Thiosulphate. Iodine and thiosulphates react quantitatively, forming sodium iodide and sodiumte tra- thionate : 2Na 2 S 2 3 +l f +-Na 2 S 4 O 6 +2NaI. This is an oxidation of sodium thiosulphate by iodine, which is itself reduced. The solution may be originally neutral or acid, or alkali bicarbonates may be present. Normal carbonates or 224 QUANTITATIVE ANALYSIS hydroxides should not be present since they also combine with iodine : 2NaOH + I 2 ->NaIO + Nal + H 2 O, 2Na 2 CO 3 +I 2 -NaIO+NaI+CO2. The color of dilute solutions of iodine is sufficiently intense to serve as a fairly accurate indicator. Much more accurate re- sults are obtained by the use of starch as an indicator, mere traces of iodine producing a visible blue or rose red color with starch. Because of the fact that iodine is an excellent oxidizing agent for many substances when a bicarbonate is present, and that hydri- odic acid is oxidized by many oxidizing agents when an acid is present, free iodine being liberated, the two standard solutions of iodine and sodium thiosulphate form a most useful pair for volumetric analysis. As an example of their use the reactions of arsenic may be noticed. Arsenious acid or an arsenite is oxi- dized by free iodine thus: H 3 AsO 3 +l2+H 2 0<=H 3 AsO 4 +2HI, This reaction does not take place quantitatively but is reversible. If, however, sodium bicarbonate is present in excess the hydri- odic acid is neutralized as fast as it is formed and the reaction is completed. Standard iodine solution may, in this way, be used for the titration of arsenious acid. On the other hand arsenic acid is reduced by hydriodic acid: H 3 AsO 4 +2HI H 3 As0 3 +H 2 0+I 2 . This is seen to be the reverse of the reaction expressed above and it would follow that it also is incomplete unless one of the products is removed. This may be done by adding sodium thiosulphate to remove the iodine, in which case the standard sodium thiosulphate indirectly titrates the arsenic acid. In prac- tice hydriodic acid is not kept as a reagent because of its insta- bility but potassium iodide and hydrochloric acid are used in- stead, hydriodic acid being thus made available in the solution. Standardization. By properly purifying iodine standard solutions may be made by direct weighing. Commercial iodine is usually not sufficiently pure for this purpose and must be analyzed if it is to be used in this way. Iodine solutions will not remain constant in oxidizing power because of interaction between OXIDATION AND REDUCTION 225 iodine and water, and it is usually not advisable to attempt to dilute solutions to a definite concentration because they must be restandardized after short intervals of time. For this reason standardization by direct weighing is not practicable and the iodine need not be purified before dissolving. The solution may then be standardized by titrating against any standard reducing solution. The best substances for this purpose are sodium thio- sulphate and arsenious oxide. Iodine does not dissolve easily in water but is readily soluble in a solution of potassium iodide or sodium iodide. Such a solu- tion probably contains an iodide having the formula KI 3 . Many organic liquids are good solvents for iodine. Examples are the alcohols and acetic acid. These will be discussed in the section dealing with the analysis of fats and oils. When starch and iodine are brought together a deep, indigo-blue color is produced and this serves as a very delicate test for either starch or iodine. The nature of the blue substance has long been the subject of investigation and discussion. It is probably a solid solution of iodine in starch. Sodium thiosulphate may sometimes be obtained in a suffi- ciently pure condition to allow standardization by direct weigh- ing. It is better to make the solution somewhat more concen- trated than that desired and to standardize and dilute to a definite concentration. For standardization the solution may be directly titrated against standard iodine solution or indirectly against potassium dichromate or a salt of copper. It has already been stated that potassium dichromate may be obtained in a state of uniform purity. If to a standard solution of this salt potassium iodide and hydrochloric acid are added, iodine is liberated as follows : K,Cr 2 7 +6KI+14HCl 8KCl+2CrCl 8 +7H 2 O+6I. - x The liberated iodine may be titrated by sodium thiosulphate and the latter thus standardized. The solution of potassium dichro- mate used for iron determinations may be used also for this purpose. It was standardized in the decimal system, however, and it will be necessary to calculate its value in the normal system because the solution of sodium thiosulphate is to be used for the determination of several different substances. The following ex- ample will illustrate the method of calculation of standardization. 15 226 . QUANTITATIVE ANALYSIS Example. 40 cc of a solution of potassium dichromate liberates iodine equivalent to 22 cc of sodium thiosulphate solution. 1 cc of potassium dichromate solution is equivalent to 0.005 gm of iron. What is the normality of the thiosulphate solution? 40 1 cc of sodium thiosulphate solution is equivalent to ~~ cc of potas- 40 sium dichromate solution and to ^ X 0.005 gm of iron. A normal Cfi solution would be equivalent to 0.05584 gm of iron. Therefore the normality is |*|__o. le28 N . The standardization against a salt of copper is also an excellent method. This is described on page 231 in connection with the determination of copper. Sodium thiosulphate is quite stable in solution and may be kept for months without appreciable change in concentration if the water contains no trace of acid. Even carbonic acid causes decomposition and free sulphur is deposited from the solution, sulphurous acid being formed. This is because thiosulphuric acid is very unstable and rapidly decomposes: Na 2 S 2 O 3 +H 2 CO 3 Na 2 C0 3 +H 2 S 2 3 , H 2 S 2 O 3 H 2 S0 3 +S. Even a very small amount of carbonic acid is sufficient to start the decomposition by liberating some thiosulphuric acid. As sulphurous acid accumulates it aids the decomposition which is thus progressive: Na 2 S 2 3 +H 2 S0 3 Na 2 S0 3 -fH 2 S 2 3 , H 2 S 2 O 3 ->H 2 S0 3 +S. In order to avoid starting this series of reactions the water should be boiled and cooled before making the solution. PROBLEMS 60. A solution of potassium dichromate contains 4.95 gm of the salt in 1000 cc. What weight of sodium thiosulphate is equivalent to 1 cc? 61. A solution of potassium dichromate contains 6.235 gm in 1000 cc. 30 cc of this solution is equivalent to 42.9 cc of sodium thiosulphate solution. What is the normality of the latter? 62. What weight of potassium dichromate must be dissolved in 250 cc to make a solution, 25 cc of which is equivalent to 35 cc of sodium thiosulphate solution containing 13.65 gm of the crystallized salt in 1000 cc? OXIDATION AND REDUCTION 227 63. 1 cc of potassium dichromate solution is equivalent to 0.005 gm of iron. What is the iodine equivalent? 64. 25 cc of iodine solution is equivalent to 0.125 gm of potassium dichro- mate. To what volume should 1000 cc be diluted to make the solution decinormal? 65. 20 cc of potassium dichromate solution oxidizes 0.0240 gm of oxalic acid, H 2 C 2 O4.2H 2 O. 1 cc of the same solution oxidizes the same weight of iron as does 1.2 cc of potassium permanganate solution. What is the nor- mality of the latter solution? Exercise: Preparation of Decinormal Sodium Thiosulphate Solu- tion. Calculate the weight of crystallized sodium thiosulphate, Na2S 2 03.5H 2 0, required for 2500 cc of tenth-normal solution. Crush the salt and dissolve 2 percent more than this weight in cold, re- cently boiled water and dilute to 2500 cc. Keep the bottle well stoppered and out of direct sunlight. Make 200 cc of a solution containing 30 gm of potassium iodide. The starch solution is made as follows: Moisten 1 gm of starch with cold water to make a thick paste. Heat 200 cc of water to boiling and pour it into the starch paste. Boil, wit^i constant stirring, for one minute. The solution does not keep well and should be made each day as required . Standardize the sodium thiosulphate solution against potassium dichromate. If the solution used in iron determinations is at hand, use this, otherwise make 250 cc of exactly tenth-normal solution by weighing the salt, dissolving and diluting to the required volume. Measure 35 cc of either solution into an Erlenmeyer flask, add 40 cc of potassium iodide solution and 10 cc of concentrated hydro- chloric acid. Titrate at once with sodium thiosulphate solution, deferring the addition of starch as long as possible. The solution of chromium chloride, formed by reduction of potassium dichromate, is green. The solution has an amber tint as long as much free iodine is present. Upon the addition of starch the solution acquires a blue- green color and the change to pure green at the end point may be dif- ficult to detect at first trial. With a little experience the difficulty will disappear. Make at least three titrations and calculate the normality of the sodium thiosulphate solution. Make a blank test upon the potassium iodide, omitting the potassium dichromate but adding the hydrochloric acid. If iodine is found, correct the observed volume of sodium thiosulphate before calculating its concentration. OXIDIZING POWER OF PEROXIDES Such peroxides as those of manganese, barium, lead, and' hy- drogen readily oxidize hydriodic acid and liberate iodine. The titration of the latter by standard sodium thiosulphate solution 228 QUANTITATIVE ANALYSIS constitutes an indirect determination of the oxidizing power, or ''available oxygen" of the peroxide. In practice it is sometimes not found convenient to add potassium iodide and hydrochloric acid directly to the peroxide because the solution is usually colored by impurities dissolving as chlorides. In such a case hydrochloric acid is added to the peroxide and the liberated chlorine is distilled into potassium iodide solution. In the case of manganese peroxide the reactions may be represented thus: MnO 2 +4HCl-> MnCl 2 +H 2 O+Cl 2 , C1 2 +2KI->2KC1+2I. PROBLEM 66. Calculate the equivalent weight of manganese dioxide and of available oxygen and find the weight of each that is equivalent to 1 cc of decinormal sodium thiosulphate solution. FIG. 66. Modified Bunsen's apparatus for the determination of available oxygen. These reactions are analogous to those occurring with other peroxides and the determination of available oxygen in manganese dioxide is the most frequently made of all. The apparatus for carrying out the decomposition and distillation should have ground glass joints and should not allow contact of iodine or chlorine with organic matter. The modified apparatus of Buri- sen, Fig. 66, may be used. The receiver must be kept cold in order to avoid loss of iodine. OXIDATION AND REDUCTION 229 Determination. Dry 2 to 4 gm of either commercial manganese dioxide or pyrolusite at 120 until the weight is constant. The sample already used for the determination of manganese may be used for this determination and the results obtained by the two methods compared. Weigh enough sample to be equivalent to about 35 cc of the standard sodium thiosulphate solution and place in the flask of a Bunsen distil- ling apparatus or of some other suitable type. Place in the receiver 2 gm of potassium iodide, dissolve this in water and dilute until the bend is just sealed when the apparatus is in the proper position. Immerse the receiver in ice water, then add to the flask containing the manganese dioxide 30 cc of concentrated hydrochloric acid and quickly insert the stopper carrying the delivery tube. Warm the acid gently, distilling the chlorine into the potassium iodide solution. Raise the temperature gradually until the acid is boiling and boil for five minutes after action is completed. While the burner is still under the flask lower the receiver until the delivery tube is entirely out of it, then remove the burner. Remove the delivery tube from the flask and rinse it inside and outside, the water flowing back to the receiver. Rinse the whole iodine solution into an Erlenmeyer flask and titrate with sodium thio- sulphate solution. Calculate the percent of available oxygen, also the theoretical percent of manganese and of manganese dioxide. If the sample is the same as that used for the direct determination of man- ganese and of available oxygen by potassium permanganate an interest- ing comparison of results of different methods may be made, although the calculation of available oxygen from the percent of manganees may not check with the direct determination, for reasons already discussed. Sodium thiosulphate may be used to titrate the iodine produced by the action of almost any oxidizing agent upon a solution of potassium iodide and hydrochloric acid. Peroxides have already been discussed. Other substances that may be determined are free halogens (chlorine and bromine being allowed to displace iodine from potassium iodide), easily reducible oxyacids and their salts, as the halogen oxyacids, nitrous acid and persulphuric acid, oxysalts of metals that exist in acid radicals, as dichromates, chromates, permanganates and manganates, and salts of metals that possess more than one valence, as iron, copper, mercury and arsenic. While sodium thiosulphate may be used for the determination of almost any oxidizing agent it is not necessarily true that this provides the best method for all such materials. In many cases 230 QUANTITATIVE ANALYSIS other methods will be found to give better results or to be more conveniently applied. PROBLEM 67. Complete the following equations, balance and determine the equiva- lent weights of each of the oxidizing agents. Br 2 +KI KBrO+KI+HCl * KBrOs+KI+HCl-* KClO+KI-fHCl K 2 Cr 2 O 7 +KI+HCl > 2KMn0 4 ^KH4lCl H*CL+ Kcl-f^ FeCls+KI+HCl > CuCl 2 +KI+HCl COPPER The gravimetric determination of copper may be made by precipitating as cupric hydroxide, heating and weighing as cupric oxide, or by precipitating as cupric sulphide, heating with sulphur and hydrogen and weighing as cuprous sulphide. Both methods are difficult of execution and are subject to considerable errors. Electrolytic methods are more accurate and more easy of accomplishment. Copper may be determined volumetrically by several methods, one of the best being Low's "iodide method." 1 This method depends upon the insolubility of cuprous iodide and the instability of cupric iodide. If to a solution of a cupric salt, containing no highly ionized acid and no other oxidizing agent, potassium iodide is added there is an immediate precipita- tion of cuprous iodide with liberation of iodine: Cu(C 2 H 3 O 2 ) 2 +2KI->CuI + 2KC 2 H 3 2 +L The iodine may be titrated by standard sodium thiosulphate solution and copper calculated. PROBLEMS 68. Calculate the equivalent weight of copper and the weight which is equivalent to 1 cc of decinormal sodium thiosulphate solution. 69. What weight of a copper ore should be taken for analysis in order that 1 cc of fifth-normal thiosulphate solution should indicate 1 percent of copper in the ore? 1 J. Am. Chem. Soc., 18, 458 (1896); 24, 1082 (1902). See also a comparison of methods by Fernekes and Koch: Ibid., 27, 1224 (1905). OXIDATION AND REDUCTION 231 If sodium thiosulphate solution is to be standardized against pure copper, the metal is dissolved in nitric acid, most of the nitrogen oxides are expelled by boiling and any remaining trace of nitrous acid is oxidized by bromine. The excess of nitric acid is then neutralized by ammonium hydroxide, acetic acid and potassium iodide are added and the free iodine titrated at once. If a copper ore or crude copper is to be analyzed all metals whose iodides are insoluble or whose salts will oxidize potassium iodide must first be removed. The addition of metallic alumin- ium to the solution containing sulphuric acid will precipitate copper and leave in solution all other metals of higher decomposi- tion potentials as well as those soluble in sulphuric acid, providing that nitric acid be absent. The latter can be removed by evapo- rating with sulphuric acid. This treatment also precipitates lead as sulphate, which may be removed by filtration. After the copper is precipitated by aluminium it may be removed by filtration, washed, dissolved in nitric acid and determined as in the standardization of sodium thiosulphate. Exercise: Standardization of Sodium Thiosulphate Solution. The standard solution already prepared may be used and the copper equiva- lent calculated. In case it is desired to standardize against copper or a copper salt, proceed by one of the following methods : (a) Standardization against Metallic Copper of Known Purity. Weigh sufficient copper to require about 35 cc of sodium thiosulphate solu- tion. Place in a 250 cc flask and dissolve by warming with 5 cc of a mixture of equal volumes of concentrated nitric acid and water. Dilute to 25 cc and boil to expel nitrogen oxides. Add 5 cc of bro- mine water and boil until all excess bromine is removed. Cool and add strong ammonium hydroxide until a clear blue solution is obtained, then boil to remove excess. Acidify with acetic acid and boil, if necessary, to dissolve any precipitated cupric hydroxide. Cool, add 3 gm of potassium iodide and titrate the liberated iodine with sodium thio- sulphate solution. Calculate the copper equivalent of the solution. (6) Standardization against a Copper Salt. Weigh the proper amount of cupric sulphate of known purity, dissolve in 25 cc of water, make slightly basic with ammonium hydroxide and from this point proceed as in (a). Determination. Dissolve 0.5 gm of ore in a covered casserole with 10 cc of hydrochloric acid and 5 cc of nitric acid, boiling if necessary to aid solution. Add 7 cc of concentrated sulphuric acid and evapo- 232 QUANTITATIVE ANALYSIS rate until sulphuric acid fumes appear. Cool, add 25 cc of water and boil to dissolve the sulphates. Filter to remove lead sulphate and gangue, allowing the filtrate to run into a beaker. Wash the residue and paper and dilute the filtrate and washings to 75 cc. Cut a strip of sheet aluminium about 2.5 cm wide and 14 cm long, bend into a triangle and stand on its edge in the solution. Cover and boil until all copper is precipitated and the solution is colorless or green from ferrous sulphate. If this condition cannot be attained it is because nitric acid was not completely removed when evaporating with sul- phuric acid. When all copper is precipitated, wash down the sides of the beaker with a jet of hydrogen sulphide solution, pour the solution into a filter paper and filter quickly. Transfer the copper to the filter, washing the aluminium with hydro- gen sulphide solution while still in the beaker. Wash thoroughly with hydrogen sulphide solution and then place a clean flask under the filter. Add to the beaker containing the aluminium 6 cc of nitric acid, sp. gr. 1.2. Boil shortly to dissolve adhering copper then pour the acid slowly over the filter to dissolve the copper on the paper. When all copper seems to be dissolved pour over the paper 5 cc of bromine water. Wash beaker and paper thoroughly with hot water then open the paper and wash into the flask any particles of copper that have escaped the action of the acid. Boil until all bromine is removed, add strong ammonium hydroxide until a deep blue is obtained, boil to expel excess of ammonia and from this point proceed as in the standardization of sodium thiosulphate solution. Calculate the percent of copper in the ore. BLEACHING POWDER When gaseous chlorine is passed over slaked lime it is absorbed with formatign of an unstable compound that is easily made to yield chlorine under certain conditions and the compound pro- vides a convenient means for storing and transporting chlorine to be used for bleaching, disinfecting, etc. This compound, known as "bleaching powder," is a double salt of calcium with hydro- chloric and hypochlorous acids and may be represented by the /Cl formula Ca\ . When dissolved in water it is probably X C10 ionized in the manner characteristic of both acids. When any acid, even carbonic acid, is added to bleaching powder chlorine is liberated: CaCLClO+HaCOs-* CaCO 3 -f-H 2 O+Cl 2 . OXIDATION AND REDUCTION 233 This is due to the fact that when hydrochloric acid and hypochlor- ous acid come together, even in dilute solutions, they act upon each other with the formation of chlorine: HC1+HC1O H 2 O+C1 2 . Because of the easy decomposition of bleaching powder by car- bonic acid it rapidly deteriorates when exposed to air, chlorine escaping. Loss of efficiency also occurs through loss of oxygen: 2CaCl.ClO-* CaCl 2 +O 2 , and through a decomposition such that calcium chlorate is formed : GCaCLClO-* Ca(ClO 3 ) 2 +5CaCl 2 . The decompositions represented by the last two equations result in the formation of chlorine compounds in which the chlorine is not liberated upon acidification. A determination of total chlor- ine would therefore be of little value as an estimate of the useful- ness of bleaching powder. " Available chlorine " is better determined by a volumetric process. For this purpose the acidi- fied solution may be treated with potassium iodide and the liber- ated iodine titrated with standard sodium thiosulphate solution, or the solution may be titrated directly by a standard solution of sodium arsenite. For the last titration the indicator is a paste of starch and potassium iodide used on a porcelain plate or absorbed by filter paper and dried. This method of reading the end point is inconvenient and the first method of titration is the better one. If calcium chlorate is present in bleaching powder and a strong acid is used for liberating the chlorine, the chlorate will be decomposed, though but slowly. This is because chloric acid is formed by the reaction of chlorate with added acid and chloric acid is slowly reduced by hydrochloric acid, liberating chlorine: HC10 3 +5HC1 3H 2 0+3C1 2 . During the titration the effect of these reactions is seen in an uncertain end point. As sodium thiosulphate is added the blue color of starch iodide disappears and then returns and deepens. As the addition of thiosulphate is continued the blue finally permanently disappears, but this end point does not represent 234 QUANTITATIVE ANALYSIS the titration of chlorine really available in bleaching processes because that which comes from calcium chlorate is evolved too slowly to be of much use. This interference with the titration may be almost entirely averted by using a weak acid instead of a strong one for the decomposition of the chlorohypochlorite. The concentration of chloric acid does not then become sufficiently large to cause more than slight oxidation of hydriodic acid. The most suitable acid for the purpose is acetic acid. Determination. If bleaching powder were pure calcium chloro- hypochlorite, CaCl.CIO, it would contain about 56 percent of chlorine. For reasons already discussed the amount of available chlorine is much less than this and in the average commercial product it is not much more than 25 percent. Upon this basis calculate the weight that should be used when 50/1000 of the weighed sample is to be taken for the final titration. Weigh from a closed weighing bottle into a 1000 cc graduated flask. Fill to the mark with water and agitate until the powder is thor- oughly disintegrated and all soluble matter is in solution. Measure 50 cc portions into flasks, add 5 gm of potassium iodide and 25 cc of 10 percent acetic acid to each and titrate with sodium thiosulphate solution. Calculate the percent of available chlorine in the powder. ARSENICAL INSECTICIDES Two of the most important insecticides containing arsenic are London purple and Paris green. The former is a waste product of certain aniline dye industries and contains much dye in addi- tion to a fairly large quantity of arsenic. Paris green is a definite compound of cupric arsenite and cupric acetate, represented by the formula: Cu 3 (AsO 3 )2.Cu(C2H 3 O 2 )2. This compound is de- composed by boiling with sodium hydroxide, precipitating cuprous oxide and forming sodium arsenate and arsenite in solution. Dur- ing the boiling some oxidation of arsenic also occurs, sodium arse- nate being produced. If the solution is to be titrated for the de- termination of total arsenic this arsenate must first be reduced. For this purpose the solution is concentrated, then hydrochloric acid and potassium iodide are added and the resulting free iodine is removed by sodium thiosulphate. Na 3 AsO 4 +2HCl+2KI^Na 3 AsO 3 +2KCl+H 2Na 2 S 2 3 +l2Na 2 S 4 6 +2NaI. OXIDATION AND REDUCTION 235 The exact removal of iodine must be determined without the aid of starch. In strongly acid solutions starch is partly inverted, dextrine being one of the intermediate products and dextrine forms with iodine a deep red color which is not later removed and which interferes in the titration of iodine solution. The. first equation above represents a reaction that can be quantitatively reversed at will. The complete reduction of pentavalent arsenic has just been accomplished in acid solution, one of the products (iodine) being removed. If the solution is now made basic, thus removing one of the products (hydro- chloric acid or hydriodic acid) of the reverse reaction and if standard iodine solution is added a quantitative oxidation of trivalent arsenic occurs. The addition of a strong base is not permissible because this will combine with iodine: 2KOH+I 2 ->KIO+KI+H 2 O. Neither is it possible to use a normal carbonate, for similar rea- sons. Alkali bicarbonates may be present and are used in prac- tice for neutralizing the acid. Determination of Total Arsenious Oxide in Paris Green. To 2 gm of Paris green in a 250-cc flask add about 100 cc of a 2-percent solution of sodium hydroxide. Boil until all of the green compound has been decomposed and only red cuprous oxide remains. Cool, filter into a 250-cc volumetric flask, washing the paper well, and dilute to the mark. Mix well and filter through a dry paper, rejecting the first 30 to 40 cc. Measure two or three portions of 50 cc of the solution into 250-cc flasks and concentrate by boiling to about half the original volume. Cool to 80, add 10 cc of concentrated hydrochloric acid and 5 gm of potassium iodide. Mix and allow to stand for about ten minutes. From a burette carefully add sodium thiosulphate solu- tion until the iodine is all reduced. Starch should not be added but care should be exercised in reaching the end point. Immediately add, as rapidly as can be done without loss by effervescence, 15 gm of so- dium bicarbonate, free from lumps. Titrate at once with standard iodine solution, deferring the addition of starch until near the end point. Calculate the percent of total arsenious oxide in the Paris green. CHAPTER X TITRATIONS INVOLVING THE FORMATION OF PRECIPITATES The completion of the reactions of neutralization depends upon the small ionization of one of the products, water. The completion of reactions of oxidation and reduction depends upon the relative potentials of oxidizing and reducing agents. Certain other reactions are made the basis of volumetric determinations, completed because of the formation of a precipitate. In some cases an indicator is added while in others the cessation of pre- cipitation with further addition of standard solution is the indicator. SILVER An example of titration without an added indicator is to be found in Gay-LussacV method for silver. This method is one of the oldest of those analytical methods that have survived to the present day and, while it is not now extensively used because it is somewhat troublesome in the matter of execution, it is one of the most exact of all known volumetric processes. It depends upon the titration of the solution of a silver salt by a standard solution of sodium chloride. The very small solubility of silver chloride renders the reaction practically complete. The con- verse of this method maybe used for the determination of chlorine, bromine, or iodine in soluble halides. Exercise : Preparation of Standard Solutions. Calculate the weight of pure sodium chloride that is equivalent to 5 gm of silver, weigh this quantity, dissolve in distilled water and dilute to 1000 cc in a volu- metric flask. Make a second solution by diluting 100 cc of this solu- tion to 1000 cc. Record the silver equivalent of 1 cc of each of these solutions. Determination. Silver may be determined in any alloy that contains no other metal forming insoluble chlorides but the approximate percent Instruction sur Fessai des matieres d' argent par la voie humide. Paris, 1832. 236 THE FORMATION OF PRECIPITATES 237 of silver should be known. A silver coin may be used. United States sil- ver coinage contains approximately 90 percent of silver. Weigh enough of the alloy to give 0.5 gm of silver, place in a 250 cc flask having a ground glass stopper and dissolve in 10 cc of a mixture of equal volumes of water and concentrated nitric acid. Both water and acid must be tested and found free from chlorine. Boil to expel oxides of nitrogen, assisting this action by drawing air through the flask by means of a filter pump. Add to the solution in the flask exactly 99 cc of the more concentrated standard salt solution, stopper and shake until the pre- cipitated silver chloride flocculates and settles readily. Add from a second burette the more dilute standard solution, 0.5 cc at a time, allowing the solution to run down the sides of the flask and observing whether turbidity is produced. Shake the flask if more silver chloride is formed and continue the addition of the dilute standard solution until the last 0.5 cc fails to produce a visible precipitate in the clear, supernatant liquid. Do not use the last 0.5 cc in the calculation. It may sometimes happen that the percent of silver in the alloy is not known with sufficient accuracy and either too much or too little of the more concentrated solution is used. In the first case the first addition of the dilute solution fails to produce a precipitate while in the second case an unduly large quantity of the dilute solution is required to reach the end point. In either case the determination should be begun again, the proper alteration being made in either the weight of sample taken or the volume of concentrated standard solution. From 'the results of the titration calculate the percent of silver in the alloy. In the determination of silver by the method of Volhard 1 an inorganic indicator is added to the solution. The silver should be in the form of nitrate, a solution of a ferric salt, acidified to suppress hydrolysis, is added and the silver is titrated by a standard solution of potassium thiocyanate or ammonium thio- cyanate. Silver is precipitated as silver thiocyanate: AgNOs+KCNS AgCNS+KNO 3 . When all of the silver is removed from the solution an additional drop of the standard solution of thiocyanate produces the red color of soluble ferric thiocyanate: Fe(N0 3 ) 3 +3KCNS Fe(CNS) 3 +3KN0 3 . Mercury thiocyanate is insoluble in dilute nitric acid and mercury must therefore be absent. The color of salts of copper, 1 J. prakt. Chem., [2] 9, 217 (1874), 238 QUANTITATIVE ANALYSIS nickel and cobalt obscures the end point and these metals should be absent although as much as 60 percent of copper may be present. The converse of this method may be used for the determination of the thiocyanate radical. Exercise : Preparation of Solutions. Make a solution of silver nitrate, 1 cc of which contains 0.005 gm of silver. Standardize gravimetric- ally by precipitating and weighing silver chloride, or by Gay-Lussac's volumetric method. Make 1500 cc of a solution of potassium thiocyanate or ammonium thiocyanate by weighing 2 percent more than the calculated quantity of salt required to make 1 cc equivalent to 0.005 gm of silver. Make 100 cc of a solution (saturated without heating) of ferric ammonium sulphate, adding enough nitric acid to remove turbidity and to cause the red color to give place to pale yellow. Standardize the thiocyanate solution as follows: Measure 35 cc of the silver nitrate solution into a beaker or Erlenmeyer flask, dilute to about 75 cc, add 1 cc of ferric ammonium sulphate solution and titrate with the thiocyanate solution until a permanent red tint is obtained. Determination. Weigh not more than 0.25 gm of a silver alloy con- taining no mercury, nickel or cobalt and not more than 60 percent of cop- per and place in a 250 cc flask. Dissolve in 10 cc of a mixture of equal volumes of concentrated nitric acid and water, boiling to expel oxides of nitrogen. Cool, dilute to about 75 cc and titrate exactly as in the standardization of the thiocyanate solution. Calculate the percent of silver in the alloy. HALOGENS AND THE CYANIDE RADICAL Volhard's method also applies to the determination of the halogen hydracids and cyanogen. A measured excess of stand- ard silver nitrate solution is added, precipitating all of the chlorine, bromine, iodine % or cyanogen. The excess of silver nitrate is determined by titration by standard thiocyanate solution by the method already described. In the original method the pre- cipitated silver halide was not removed by filtration before titration of the excess of silver. Rosanoff and Hill have shown 1 that the silver chloride reacts with the red soluble ferric thiocyanate, which is produced at the end point, as follows: 3AgCl+Fe(CNS) 3 FeCl 3 +3AgCNS. 1 J. Am. Chem. Soc., 29, 269 (1907). THE FORMATION OF PRECIPITATES 239 This occurs to an appreciable extent, even though the solu- bility of silver chloride is less than that of silver thiocyanate. Rosanoff and Hill found that as much as 43 percent of ammonium thiocyanate is changed in two minutes by reaction with silver chloride. It is therefore necessary to remove the precipitate by nitration before the final titration. Determination. Use the standard thoicyanate and silver nitrate solutions prepared for the preceding exercise. Weigh enough of a soluble chloride, bromide, iodide or cyanide to be equivalent to about 40 cc of the silver nitrate solution. Dissolve in a small amount of water, acidify with nitric acid and add 50 cc of the standard solution of silver nitrate. Filter and wash thoroughly and titrate the excess of silver nitrate by standard thiocyanate solution. Calculate the percent of halogen or cyanogen in the sample. A method for the direct titration of the halogens by standard silver nitrate solution is described on page 338 in the discussion of water analysis. CHAPTER XI ANALYSIS OF INDUSTRIAL PRODUCTS AND RAW MATERIALS In most of the exercises in the preceding portion of this book determinations have been made of single constituents of various substances and interfering substances have usually been either absent or capable of being removed with comparative ease. Standard methods have been employed and attention has been centered upon the chemical principles underlying the method and the proper manipulation. In the pages that follow the student will become acquainted with the application of these and other determinations to the testing and analysis of some materials which are of importance to our industrial life. Such materials are often quite complicated in composition and most varied procedures are necessary in a determination of their industrial value. The chemist will then find it necessary to have at his command all of the chemical principles and methods of analysis that have already been learned and to apply these to an intelli- gent study of the material under examination. He will also be prepared to take up other methods of testing. Some of the tests are purely physical but they are, in industrial practice, applied by the chemist and not by the physicist because the former is usually engaged in the analysis of the same or similar materials. Other analytical determinations are empirical, rather than exact, in their nature but must be made with the same degree of care and attention as the determinations involving definite elements or compounds. CARBONATE MINERALS The most important and abundant of the carbonate minerals are the calcites and the dolomites. The calcites consist essen- tially of calcium carbonate and the dolomites of double carbonates of calcium and magnesium but these compounds seldom or never occur in a pure state in nature. Iceland spar is one of the best- 240 ANALYSIS OF INDUSTRIAL PRODUCTS 241 known examples of a nearly pure natural variety of calcium car- bonate, yet in many samples of Iceland spar substances other than calcium carbonate occur in appreciable amounts. For pur- poses of geological investigation there is usually required a com- plete analysis with the utmost accuracy that can be attained. For technical purposes this is not the case. The mineral is to be used for a given industrial purpose where the essential constituent is the one of chief importance and where impurities are important only to the extent that they may reduce the percentage of the essential constituent or that they exert an undesirable influence in the industrial operation to which the mineral is to be submitted. The particular application of the mineral to the industrial proc- ess will determine which impurities are of considerable and which are of minor importance. Those of minor importance are fre- quently grouped, with no attempt at separation, into certain arbi- trary classes. For example a limestone may be used as a source of quick lime, as a flux in iron smelting, as a paving material, as a building stone, as a raw material for hydraulic cements, or for any one of a variety of other purposes. All limestone contains more or less of material insoluble in acids, consisting chiefly of various sili- cates and of quartz. For the first purpose named these substances are important only as they act as diluents of the essential calcium carbonate, unless they occur in relatively large quantities. For such a purpose the analysis would be so made as to include all such materials as simply " insoluble" or "silicious matter," no separation of the components being made. If the limestone were to be used as a flux in the smelting of iron ore, the nature of this insoluble material should be more exactly determined, since it not only reduces the actual percent of calcium carbonate but also may contain substances that themselves require a flux or that may even add very objectionable impurities, such as sulphur or phosphorus, to the iron itself. For paving or building material the physical properties of the mineral are of great importance and the chemical analysis might be considerably condensed. As another example of such empiricism in analysis, may be men- tioned the usual report on calcium. This element is usually pre- cipitated as the oxalate. It will readily be understood, however, that if barium or strontium is present and not previously separated it will also precipitate and will be included in the finally weighed oxide. Unless it is known that barium or strontium is present 16 242 QUANTITATIVE ANALYSIS * in more than very small amounts the percent of " calcium" alone is made a part of the report for technical purposes, strontium or barium serving the same purpose as does calcium. This, obviously, involves a slight error, not only in the naming of the element but in the percent as well, because the factors for these three metals in their oxides are all different. For exact scien- tific purposes the separation and determination of all elements or radicals may be necessary while for technical purposes the analysis will be ordered according to the use to which the sub- stance is applied. This is an example of the so-called " proximate' ' analysis, as distinguished from the " ultimate" analysis. It is important to note that the term " proximate" does not imply carelessness in working or neglect of sources of error. It should not even convey the idea of inexact figures, but merely grouping together of more than one substance to be reported by one generic term, as, for example, "insoluble matter" above. The proximate analysis of coal will include the determination of percents of "volatile combustible matter," "fixed carbon," "ash," and "moisture," yet each one of these terms covers many substances which are all determined together with no attempt at a separation into the ultimate constituents, simply because the figures so determined serve a useful purpose in fixing a valuation on the coal. It was formerly the custom to report the analysis of acids, bases and salts, not as radicals but as anhydrides. Calcium carbonate would be reported as calcium oxide and carbon dioxide, sulphuric acid as water and sulphur trioxide, etc. This custom has now largely fallen into disuse in most lines of analytical chemistry but has been retained in the analysis of minerals. The ultimate analysis of carbonate minerals is exhaustively and scientifically treated in a bulletin of the U. S. Geological Survey and only reference to this will be made. 1 The exercise to follow will deal with the analysis made with industrial ends in view. This exercise will be the student's introduction to separations in quantitative analysis. Heretofore the work with the solution has terminated with the filtration and the removal of the precipitate. The filtrate could contain noting but impuri- ties and by-products of the reaction and therefore could be of no further importance to the analyst. In the next and in many 1 TJ. S. Gepl. Surv., Bull. 422, by W. F. HiUebrand. ANALYSIS OF INDUSTRIAL PRODUCTS 243 later exercises the filtrate must be carefully conserved because it contains substances still to be determined. The quantity of wash liquid must be made as small as possible, not merely to minimize its solvent action upon the precipitate but also because the washings must be added to the chief nitrate and the total bulk must not be excessive for subsequent operations. Even with the exercise of great care in this regard an occasional con- centration of the solution by evaporation is necessary in order to reduce its volume to a workable value. Another point that will here appear for the first time is that many of the elements or radicals that must be separated and determined are present in the mineral in relatively small quanti- ties. The student has been accustomed to a rapid appearance of a considerable quantity of precipitate and if this should not appear when the appropriate reagent is added he is likely to conclude that none of the substance is present and to pass to the next determination. None of the constituents ordinarily present in a given mineral or other complex material should be assumed to be absent. The reagent should be added and sufficient time allowed for the precipitation to become completed, remembering that precipitation starts and proceeds slowly from very dilute solutions. Even when no precipitate is finally visible it is the safest plan to filter, wash and ignite the paper in a weighed cruci- ble, when a small amount of precipitate will often be detected, when otherwise it would have been weighed with the next precipitate to be produced. Determination. Read again the discussion of sampling on page 8 and apply this to the preparation of a sample of limestone, dolomite or other carbonate mineral, for analysis. The small sample finally used should weigh about 10 gm and should pass a sieve having 100 meshes in each linear inch. Carbon Dioxide. Determine carbon dioxide exactly as directed on page 109, noting that if dolomite is under investigation solution will proceed rather slowly while the acid is cold. It is obvious that hydro- chloric acid must be used since considerable quantities of calcium are present and the solubility of calcium sulphate is not large. Silica or Insoluble Matter. The residue from the carbon dioxide determination may be used for this determination but it is better to use new samples. Weigh duplicate portions of 0.5 gm each into casseroles. Dissolve in 5 cc of concentrated hydrochloric acid, covering the cas- 244 QUANTITATIVE ANALYSIS serole while the mineral is dissolving. Rinse down the cover glass and the sides of the casserole and evaporate to dryness on the steam bath. Heat the dry material at dull redness until the organic matter has been oxidized, leaving the residue white or reddish brown from iron oxide. Cool, add 5 cc of concentrated hydrochloric acid and warm until all soluble matter has passed into solution. Dilute to about 50 cc with hot water, boil and filter into a Jena beaker, using particular care in removing all of the residue to the filter paper, since the white casserole makes this process somewhat uncertain. Wash the residue free from chlorides with hot water, collecting the washings and filtrate in the same beaker. The total volume, after filtration and washing, should not be greater than 100 cc. If it is greater than this amount it should be con- centrated by evaporation. Place the paper and residue in a weighed platinum crucible, burn the paper and then ignite for 10 minutes over the blast lamp. Report the percent of " insoluble matter." If insoluble matter amounts to more than 0,5 percent it should be separated into its constituents. In this case add to the residue in the crucible 2 gm of sodium carbonate and fuse over the blast lamp until the silicate is com- pletely decomposed, as shown by the cessation of effervescence. Cool, place the crucible in a casserole containing 50 cc of water and warm until the material is completely dissolved or disintegrated. Carefully add to the covered casserole concentrated hydrochloric acid until efferves- cence no longer occurs, then remove the crucible and rinse. Evaporate the solution and heat at about 120 for 10 minutes. Add 5 cc of con- centrated hydrochloric acid and warm until soluble matter is dissolved, then dilute with 5 cc of water and filter on an extracted paper. Wash with hot water until chlorides are completely removed, adding the filtrate and washings to the original solution of the mineral. Ignite the residue and paper in a weighed platinum crucible, weigh and report as silica. Iron and Aluminium. If the solution has a volume greater than 100 cc it should be evaporated to concentrate to about this volume. Drop into the solution a very small bit of litmus paper and then add dilute ammonium hydroxide, stirring, until the solution is distinctly basic, avoiding undue excess of ammonium hydroxide. Boil for 5 minutes or until the odor of ammonia is faint. Filter through an extracted paper and wash until free from chlorides, adding the washings to the filtrate. Remove the paper from the funnel, fold and burn in a weighed crucible, which should be of platinum if a separation of iron and alumin- ium is to be made. Burn the paper at a low temperature in presence of an excess of air, inclining the crucible to facilitate oxidation. Weigh, and if the amount of the oxides is not greater than 0.5 percent report as aluminium oxide and iron oxide. If more than this amount is present a ANALYSIS OF INDUSTRIAL PRODUCTS 245 separation is usually made. In this case add 1 gm of potassium acid sulphate to the crucible containing the oxides of iron and aluminium. Fuse at a relatively low temperature until violent effervescence has ceased then heat to redness until the oxides have been completely dissolved. Cool the crucible and dissolve the mass in hot water. Reduce the iron and titrate with standard potassium dichromate or potassium permanganate according to the methods already learned. Manganese. Add bromine water to the nitrate and washings from iron and aluminium until a yellow color is produced, then boil. If manganese is present it will precipitate as brown manganese dioxide. The quantity is usually small but it must not be disregarded. Filter, wash free from chlorides and ignite. Weigh the oxide Mn 3 6 4 and calculate as Mn0 2 , assuming that the manganese was originally pres- ent in this form. (This is an arbitrary assumption because manganous carbonate is of common occurrence.) Calcium. Acidify the solution with hydrochloric acid and concen- trate to about 100 cc, boiling until all bromine is removed. Add a bit of litmus paper, then ammonium hydroxide until basic. Heat to boiling and add, drop by drop with stirring, 10 cc of a saturated solu- tion of ammonium oxalate, or enough to precipitate all of the calcium. Determine the calcium as directed on page 66 or 217, with the following addition, designed to complete the separation of magnesium: Filter the precipitate of calcium oxalate and wash once with hot water but without making any attempt to transfer completely to the filter. Place the beaker under the paper and add to the precipitate on the filter enough concentrated hydrochloric acid to dissolve all calcium oxalate. 2 cc should be sufficient. Wash the paper thoroughly with hot water, precipitate the calcium once more and determine as already directed. Add the filtrate from the second filtration to that from the first. Calcu- late the percent of calcium oxide in the sample. Magnesium. Determine the magnesium in the filtrate and washings as was done in the case of a magnesium salt. The determination is discussed on page 85. Report the percent of magnesium oxide. Sodium and Potassium. These elements are not often present in more than traces in the carbonate minerals and their determination is not often required for industrial purposes. If such a determination is to be made, a new sample of mineral should be used. Follow the pro- cedure directed under the analysis of silicate minerals, page 250. If the analysis has been made with care and every substance present has been determined the sum of the percents of all of the constituents of the mineral should be 100. This will serve as a check upon the accuracy of the work but the sum will rarely be exactly 100. The omission of the determination of other substances present in small 246 QUANTITATIVE ANALYSIS quantity will give rise to a negative error, while imperfect washing and other experimental errors will summate as a positive error, so that the sum of all percents may be either greater or less than 100. The student should be able to work so that the sum of all the errors should not be greater than 1 percent. SILICATE MINERALS Silica, as a constituent of various simple and complex silicates, is distributed widely in the earth's crust. Associated with other minerals or in a nearly pure form silica itself is also to be found. These minerals are only slightly soluble in acids or bases and their analysis requires a preliminary decomposition by some agent which will react at elevated temperatures. When silicon dioxide or a silicate is heated with an alkali carbonate to the point of fusion the corresponding alkali silicate is produced, carbon diox- ide is evolved and whatever heavy metals may have been origi- nally present as silicates are left in the form of oxides. The alkali silicates are soluble in water (as colloids) and most of the metallic oxides so produced are soluble in hydrochloric acid. The pre- viously insoluble mineral is, by this means, obtained in solution and the ordinary analytical processes will henceforth apply. All of the natural silicates may be regarded as being derived from silicon dioxide, the anhydride of the various silicic acids. These acids are not known in the free state but their existence may be supposed from the composition of the salts. ThusH 2 Si0 3 = H 2 O.Si0 2 , H 4 Si0 4 = 2H 2 O.Si0 2 , H 6 Si 2 7 = 3H 2 0.2Si0 2 , H 4 Si 2 6 = 2H 2 0.2Si0 2 , H 4 Si 3 O 8 = 2H 2 0.3Si0 2 , H 2 Si 2 0; = H 2 0.2Si0 2 . These may be taken as the acids from which various natural silicates are derived. Kaolin, the essential constituent of the various impure clays, is Al 2 Si 2 07.2H 2 0, a salt of H 6 Si 2 C>7. The felspars are double silicates derived from the acid EUSisOg. As examples of the felspars may be mentioned orthoclase, KAlSi 3 Os, and albite, NaAlSi 3 8 . Fusion of kaolinite (china clay) with sodium carbonate causes reactions which may be simply represented thus: Al 2 Si 2 7 +2Na 2 C0 3 2Na 2 SiOs+Al 2 (CO 8 ) 3> A1 2 (CO 3 ) 3 A1 2 3 +3CO 2 . Aluminium oxide may then form, to some extent, sodium alu- minates by reaction with sodium carbonate. The completion of ANALYSIS OF INDUSTRIAL PRODUCTS 247 the first reaction is assured by the presence of a considerable excess of sodium carbonate and by the decomposition of alumin- ium carbonate, which may be regarded as being momentarily present in small concentration. According to the terms of the mass law the reaction should be completed by simply heating at a sufficiently high temperature to decompose completely any car- bonates that may be formed by the decomposition. The total reaction of orthoclase with sodium carbonate is approximately expressed as follows: t The mass of oxides and silicates resulting from the fusion may be decomposed by hydrochloric acid but if this is not preceded by disintegration and solution of the water soluble parts by hot water the result of such treatment will be to form a protective coating upon lumps of the fusion, thus retarding the action of the acid. Upon addition of hydrochloric acid to the mixture of substances after treatment with water, oxides of earth and alka- line earth metals form soluble chlorides, while the alkali silicates are decomposed with formation of alkali chlorides and silicic acid. The first separation occurs in the removal of the silicic acid which must first be converted into the less soluble silicon dioxide. This conversion is partly, but imperfectly, accomplished by evapora- tion to dry ness and heating to about 120. Rehydration readily occurs and the silica partly redissolves because of its marked tend- ency toward the formation of hydrosols. This tendency is diminished by long heating at high temperatures since such treat- ment results in incipient fusion and change into the irreversible colloid, silicic acid, the production of the latter being promoted by the presence of strong acids. It is practically impossible to com- pletely separate silica by one evaporation and filtration, a small proportion invariably returning to the solution. By evaporating the filtrate, heating, and again filtering, all but a trace of silica may be removed. The residue of silica is never pure but con- tains small amounts of oxides of iron, aluminium and calcium. In order to correct the error arising from this cause the precipitate is treated with hydrofluoric acid, which concerts silica into the gaseous tetrafluoride. After volatilization of this and of the hydrofluoric acid the residue is weighed and its loss reported as silica. 248 QUANTITATIVE ANALYSIS After the separation of silica the metals will be determined by the usual methods, such as those used in the analysis of carbonate minerals. It will readily be seen that, after the fusion of the sili- cate with sodium carbon ete or potassium carbonate, a determina- tion of the alkali metals in this portion of the sample will be with- out significance. Other methods must be employed for decom- posing the silicate. Two such methods are in general use. J. L. Smith Method. The method of J. Lawrence Smith 1 depends upon the action of calcium chloride upon silicates at about 800, resulting in the formation of alkali chlorides and sili- cates of calcium and other metals. The sample is intimately mixed with ammonium chloride and precipitated calcium carbon- ate, and gently heated. The reaction that occurs might be repre- sented as follows, assuming orthoclase to be the silicate. 2KAlSi 3 O8+6CaCO3+2NH 4 Cl 2KCl+Al 2 O3+6CaSiO 3 +H 2 O + 2NH 3 +6C0 2 . This can be only an approximate representation of what has really happened during the heating. Ammonium chloride dis- sociates at about 450 into ammonia and hydrochloric acid: NH 4 C1 NHs+HCl. The ammonia escapes while the hydrochloric acid combines with calcium carbonate: CaCO 3 +2HCl CaCl 2 +H 2 + CO 2 . That the decomposition of silicates is, in a large measure, due to calcium chloride is undoubtedly true. That calcium carbonate, as such, also plays an important part in the reactions would be inferred from the above interpretation of the reaction, since in this reaction only one-third of the calcium carbonate could form the chloride. The significance of this equation is lessened by the fact that it is a representation of a series of reactions that cannot well be tested. The silicate resulting from the decomposi- tion is probably not calcium metasilicate alone but is much more complex than this. After the decomposition is complete the mass is treated with hot water which dissolves chlorides of sodium, potassium and calcium, as well as those of other metals present. After filtra- 1 Am. J. Sci., [3] 1,269 (1871). ANALYSIS OF INDUSTRIAL PRODUCTS 249 tion the most of the calcium, iron and aluminium is precipitated by ammonium carbonate and ammonium oxlaate; the solution is then evaporated in a platinum dish and heated to expel ammonium salts. If desired, sulphuric acid may be added to convert sodium, potassium and ammonium salts into sulphates, thus providing less liability of loss of sodium and potassium during the heating. In this case the Gladding modification of the Lindo method must be used or else the sulphates must be converted into chlorides by precipitating with barium chloride. If sulphuric acid is not added the volatilization of ammonium salts must be conducted with greater care, since the chlorides of sodium and potassium are appreciably volatile at a temperature of bright redness. Read again the discussion of the determination of sodium and potassium on page 78. Hydrofluoric Acid Method. Another method for treating silicates without the addition of sodium or potassium carbonate is that of decomposing by means of hydrofluoric acid. The finely powdered silicate is moistened with concentrated sulphuric acid and then hydrofluoric acid is added. The silica is volatilized, upon warming, as silicon tetrafluoride. After evaporation of the excess of hydrofluoric acid and of sulphuric acid the residue is dissolved in water and the solution analyzed by practically the same procedure as is followed in the Smith method. Determination. To insure complete decomposition of the silicate it must be ground much more finely than is necessary for most other minerals. After it is pulverized to pass a. 100-mesh sieve about 3 gm of the sample is ground in an agate mortar until it will pass a 200-mesh sieve. Moisture. Weigh about 0.5 gm of the silicate into a platinum cru- cible, and dry for one hour at 100 to 105, the loss being calculated as hygroscopic moisture. If combined water is also to be determined this can be done by heating to a temperature above 200 in a combustion tube in an atmosphere of dried carbon dioxide, absorbing the moisture in weighed tubes filled with calcium chloride. Silica. Mix with the sample about 0.5 gm of sodium carbonate, approximately weighed. Place the cover on the crucible but slightly toward one side so the contents may be observed. Heat gently at first in order to avoid violent effervescence and consequent loss by spatter- ing, gradually raising the temperature until the full heat of the burner fails to cause more than slight action. Place the crucible over the blast lamp and heat for 15 minutes after carbon dioxide has ceased to be 250 QUANTITATIVE ANALYSIS evolved. Remove the lamp, lift the crucible by means of the tongs, using care to avoid contact of the latter with the fusion, and slowly rotate the crucible in such a manner that the fused mass will be spread over the sides of the crucible as it solidifies. After cooling place the crucible and its contents in a Jena beaker, a platinum dish or a porcelain casserole and cover with hot distilled water. Digest until the entire mass has become disintegrated and re- moved from the crucible. Cover the beaker and add, gradually, from a pipette, concentrated hydrochloric acid until the carbonates are com- pletely decomposed. Remove the crucible and cover by means of a stirring rod, rinsing well. Crucible tongs should not be used for this purpose unless they are tipped with platinum. The solution is now evaporated to dryness. If speed is essential and if a casserole has been used the greater part of the liquid may be evaporated by holding over a free flame, giving a rotary motion to the casserole to prevent spatter- ing or bumping. If other work may be carried on at the same time the solution should be evaporated over the steam^bath. When completely dry heat for 15 minutes at 120, cool and add 5 cc of concentrated hydro- chloric acid and 50 cc of water, warm until soluble salts are dissolved and filter the residue of silica at once, washing until free from chlorides, adding the washings to the filtrate. Evaporate the solution again to dryness, heat at 120 and repeat the treatment with hydrochloric acid and water, filtering in a different paper. Place both papers with their residues in a platinum crucible, dry and burn the papers in the usual way, then ignite over the blast lamp for 20 minutes, cool and weigh. Small quantities of metal salts will have been retained in the residue. In order to correct for their presence the residue is moistened with one or two drops of sulphuric acid and about 5 cc of hydrofluoric acid is added. The silicon tetrafluoride being volatilized, the acids are evaporated and the residue, which should be small, is heated over the burner. The loss in weight is taken to repre- sent silica. The residue of oxides is dissolved in a few drops of hydro- chloric acid and added to the filtrate from the silica. If hydrochloric acid fails to dissolve the residue evaporate the acid, add 1 gm of potas- sium acid sulphate, fuse and finally heat to redness until a clear solu- tion is obtained. Cool, dissolve in hot water and add to the main solution. Iron, Aluminium, Manganese, Calcium, and Magnesium. Concen- trate the filtrate, if necessary, to about 100 cc then determine iron, aluminium, manganese, calcium and magnesium exactly as directed for the analysis of carbonate minerals, calculating each as the oxide. Sodium and Potassium. For the determination of sodium and potas- sium weigh about 0.5 gm of the finely ground sample (passing a 200- ANALYSIS OF INDUSTRIAL PRODUCTS 251 mesh sieve) and mix thoroughly in a platinum crucible with 0.5 gm of dry ammonium chloride and 4 gm of precipitated calcium carbonate. A 20 cc crucible of the usual form will do for the purpose but the form suggested by Smith is better. This is a crucible 8 cm long and 2 cm wide, with a cap instead of the usual loose cover. The mixture should not entirely fill the crucible. It is well to insert the latter in a piece of asbestos board so that the extreme upper portion is protected from the action of the flame. This part of the crucible will then condense any vapors of alkali chlorides that may form in the lower part. Heat the lower part of the crucible gradually until the evolution of am- monia becomes slow, then more strongly with the full flame of the burner. This heating should be finally applied to all but the upper end. After 45 minutes of such heating the crucible is allowed to cool and the charge emptied into a platinum dish. The materials will usually be sintered to- gether in a mass that has shrunken away from the crucible. Actual fusion into a slag that is not decomposed by water is an indication that the tem- perature was too high and that there may have been a consequent loss of alkali chlorides by volatilization. Add 75 cc of water to the materials in the beaker and warm until the mass is thoroughly disintegrated. Boil for a minute, allow to settle and filter. Add 50 cc more of water to the residue, FIG. 67. Smith's crucible for boil, allow to settle and filter through the determination of alkali metals the same paper, transferring all of the in silicates, residue to the paper. Wash with hot water until a few drops of the washings show only a faint test for chlorides. To determine whether decomposition was complete in the crucible, dissolve the washed residue in hydrochloric acid. Any gritty residue must be retreated as before, as this indicates insufficient heating. Evaporate the combined filtrates and washings to a volume of 75 cc or less, then add 5 cc of dilute ammonium hydroxide- and sufficient ammonium carbonate solution to precipitate all iron, aluminium and calcium present. Digest at a temperature just below the boiling point 252 QUANTITATIVE ANALYSIS until the precipitate settles readily, then filter and wash. Evaporate the nitrate and washings in a weighed platinum dish. When dry, care- fully heat over the free flame of the burner until all ammonium salts are volatilized, but without, at any time, allowing the dish to become red. Cool, dissolve in not more than 10 cc of hot water, add a few drops of dilute ammonium hydroxide solution and 1 cc of saturated, recently prepared ammonium oxalate solution. Digest until the small amount of calcium oxalate settles, then filter and wash the paper, collecting filtrate and washings in the platinum dish. Add 1 cc of concentrated hydrochloric acid and evaporate to dryness. Heat care- fully to volatilize ammonium chloride. Cool in a desiccator and weigh as sodium chloride and potassium chloride. Dissolve in a few cubic centimeters of hot water and transfer to another dish. Dry, ignite and weigh the first dish and subtract this weight from the one already observed. The difference is sodium chloride and potassium chloride. Determine the potassium in this mixture as directed on page 83, omitting the washing with ammonium chloride solution, since we are here dealing with chlorides instead of sulphates of sodium and potassium. From the weight of potassium chlorplatinate found, calculate (a) the percent of potassium oxide in the silicate and (b) the weight of potassium chloride in the mixed chlorides. Subtract the latter weight from the total sodium and potassium chlorides and from the remain- ing sodium chloride calculate the percent of sodium oxide in the silicate. The sum of all oxides determined should approximate 100 percent. Since experimental errors accumulate and since small quantities of other substances than those named will generally remain undetermined, the sum of all will usually be less, rather than more than 100 percent. It is customary with many chemists to report this discrepancy as "undeter- mined" but it will be remembered that such a percent is not merely that of undetermined matter but that it also includes the accumulated errors of all of the other determinations. COAL AND COKE As might be expected from a knowledge of its origin, coal is made up of a large number of organic and some inorganic com- pounds. The attempt to separate and identify these compounds has long engaged the attention of chemists and geologists but comparatively little progress has been made in this direction. The reason for the lack of success in this attempt is that most chemical methods of analysis involve the breaking up of organic ANALYSIS OF INDUSTRIAL PRODUCTS 253 substances and the disappearance of the original forms. Such an analysis would prove extremely useful from the stand- point of geology and would, no doubt, be of service in indus- trial applications. In the absence of adequate methods for this purpose, the examination of coal is made with one or more of three ends in view: (1) To determine the geological origin of the coal; (2) to determine its adaptability to various industrial uses, such as steaming, heating, manufacture of pro- ducer gas or illuminating gas, coke, tars, etc. ; or (3) to determine its fuel value in "heat units" per unit weight of coal. The methods used are classified under the head of "proximate analy- sis" or of "ultimate analysis." The proximate analysis of materials may be defined as "the determination, not of elements or radicals but of groups of compounds falling within approxi- mate limits of composition and having similar properties." The ultimate analysis is, as the word indicates, a determination of the elementary composition. It is made with greater difficulty, involves more expensive apparatus and requires longer time than the proximate analysis, often gives no more useful informa- tion than is given by the proximate analysis and is, for this reason, less frequently made. Methods for the analysis of coal were standardized by a com- mittee of the American Chemical Society in 1899. 1 A joint com- mittee of the American Chemical Society and the American So- ciety for Testing Materials has recently made a preliminary report, 2 revising, in many respects, the methods of the original committee. Proximate Analysis. The proximate analysis of coal as usually carried out includes the determination of moisture, volatile combustible matter, coke, "fixed carbon" and ash. The figures thus obtained give considerable information as to the geological age of the coal, determine its fitness for industrial uses and provide a basis for an approximate calculation of fuel value. Moisture. The accurate determination of moisture in coal is a difficult operation. If the coal is heated to 100 or above this temperature there is danger of loss of volatile constituents other than moisture, whether these were originally present or were formed upon heating. Oxidation also takes place during heating. 1 J. Am. Chem. Soc., 20, 281 (1898); 21, 1116 (1899). 2 J. Ind. Eng. Chem., 5, 517 (1913). 254 QUANTITATIVE ANALYSIS This may result in either gain or loss in weight, according to whether the greater part of the oxygen is retained in solid com- pounds or is lost as volatile ones. The usual tendency is toward a gain in weight so that this error to some extent compensates the loss first mentioned. Such compensation is, however, not to be depended upon and such a determination is therefore not ideal. A method that removes some of these objections is that of drying at ordinary temperatures over sulphuric acid, under diminished pressure. The partial pressure of oxygen being reduced to a negligible quantity, oxidation is entirely prevented. There is also no tendency toward breaking down of the non-vola- tile organic constituents with the production of simpler volatile ones. The method seems to give low and somewhat variable results, however, due to the low rate at which moisture is lost by the coal toward the last of the drying process. The report of the Eighth International Congress of Applied Chemistry, Sub-Committee on Standardization of Methods for Determining Water in Coals and other Fuels 1 favors the method of drying in air in an oven kept at a temperature between 104 and 111, or, for coals thought to oxidize easily, in an oven filled with nitrogen. The committee also recommends that the moisture of the original sample be determined, after it has been subjected to the minimum amount of preliminary mechanical treatment (thorough mixing, with rapid crushing of lumps) by exposing 100 to 500 gm in a metal tray to the atmosphere for 24 hours, at the temperature of the sampling room. The loss in weight is calculated as mois- ture lost by "air drying" (gross moisture) and this provides a basis for calculating the total moisture in the coal. Without this preliminary drying an accurate calculation is often impossible because of the rapid loss of moisture during the preparation of the sample for analysis. Volatile Combustible Matter, Fixed Carbon and Coke. When coal is subjected to dry distillation out of contact with air variable Quantities of volatile products are expelled and a residue of inorganic matter and non-volatile carbon is left. This residue is the "coke," the carbonaceous portion of the coke being called "fixed carbon." It should be understood that the original coal did not consist of free carbon and volatile organic compounds, but that heating at high temperatures resulted in the formation of 1 Orig. Comm. Eighth Intern. Cong. Appl. Chem., 25, 41 (1912). ANALYSIS OF INDUSTRIAL PRODUCTS 255 such substances by a decomposition of the non-volatile bitumens composing the coal. Such a decomposition of complex com- pounds into simpler and more volatile compounds is technically known as " cracking." Cracking is a somewhat indefinite proc- ess and the products depend to a large extent upon the tempera- ture and time of heating. The volatile products of the distilla- tion of coal have a very important application in the industries. The non-volatile coke is also an industrial material, being exten- sively used as a fuel for reducing ores, for the manufacture of producer gas and water gas and for many other purposes. The relative quantities of volatile matter, fixed carbon and mineral matter are of importance in the determination of the fitness of various coals for various industrial uses. On account of the variation in results obtained by variation in the manner of heating it becomes difficult or impossible to make an intelligent comparison of different coals unless a standard method is adopted for their examination. The measurement of the temperature at which they are heated is impracticable without expensive apparatus if many analyses are to be made. It is also difficult entirely to exclude air during the heating and a variable oxidation occurs. The method suggested by the Joint Coal Committee already referred to is being followed by most chemists although it is not at all ideal. The student is again reminded that volatile combustible matter, fixed carbon and coke are arbitrary classifications of substances produced by an arbitrary method and that they have little scientific meaning except as they provide a basis for comparisons. Ash. Most of the inorganic matter contained in coal is left behind as ash when the coal is burned. The compounds con- tained in the ash are not the same as those in the unburned coal, carbonates of all but the alkali metals being left as oxides, sulphides being converted into oxides, etc. If the percent of ash is subtracted from that of coke the remainder does not accu- rately represent the percent of fixed carbon. The error is, how- ever, not large unless a considerable quantity of sulphur is present as pyrite, in which case a certain correction may be made from the percent of sulphur. Ultimate Analysis. The complete ultimate analysis of coal will include the determination of all elements. The com- plete analysis is not often made, the determination of sulphur, 256 QUANTITATIVE ANALYSIS carbon, hydrogen and nitrogen being all that is usually required. Sulphur. Sulphur may exist in coal in one or more of four forms: elementary sulphur, inorganic sulphides (principally iron pyrite) inorganic sulphates and organic compounds. The accurate determination of the amount present in the different forms is difficult and not often required. The determination of total sulphur may be made by the Carius method, 1 decomposing the coal by heating in a sealed tube with fuming nitric acid, or by Eschka's method. In either method' the sulphur is oxidized to sulphuric acid and precipitated by barium chloride. It may then be determined gravimetrically or more rapidly by photometric methods. 2 The latter depend upon the observation of a standard light, shining through a standard column of the liquid containing suspended barium sulphate. Eschka's method 3 depends upon oxidation of the coal by gentle heating in contact with magnesium oxide and sodium carbonate and later precipitation of the sul- phates formed, by barium chloride. The function of the magne- sium oxide is chiefly that of providing a light porous medium which, when mixed with coal, permits the free entrance of air. It also probably acts to some extent as a carrier of oxygen, being alter- nately reduced and reoxidized, and serves to prevent the escape of any sulphur dioxide or trioxide by forming magnesium sulphite or sulphate. Sodium carbonate is, however, added for the latter purpose. Alcohol burners should be used for heating the mixture because all fuel gas contains some hydrogen sulphide and the sul- phur dioxide resulting from its combustion may be absorbed by the basic substances, magnesium oxide and sodium carbonate. When the oxidation of the coal is complete the mixture is boiled with water and bromine, the latter to oxidize to sulphates any sulphites that may have been formed. After filtration the solu- tion is acidified and boiled, whereby bromates or hypobromites are decomposed and the bromine expelled from the solution: NaBrOs+HCl NaCl+HBrO 3 , NaBrO + HCl-NaCl+ HBrO, HBrO 3 +5HBr 3H 2 O+3Br 2 , HBrO+HBr H 2 O+Br 2 . 1 Ber., 3, 697 (1870). 2 Jackson: J. Am. Chem. Soc., 23, 799 (1901). 3 Chem. News, 21, 261 (1870). ANALYSIS OF INDUSTRIAL PRODUCTS 257 It is necessary thus to decompose salts of oxyacids of bromine because of the tendency shown by barium sulphate to occlude these compounds at the moment of precipitation. Nitrogen. Nitrogen may be determined by combustion or by the Kjeldahl method. The principles underlying the latter are discussed on page 429. In the com- bustion method the coal is mixed with fine cupric oxide and is heated in a tube closed at one end. Oxidation occurs and the gases are passed through more heated cupric oxide to complete the oxidation of carbon monoxide or of gaseous hydro- carbons, then over heated copper, the latter to reduce oxides of nitrogen to elementary nitrogen. The mixture of carbon dioxide, water vapor, sulphur dioxide and nitrogen is passed into sodium hydroxide solution where all gases ex- cept nitrogen are absorbed. The latter is collected in a eudiometer and meas- ured, the weight being then calculated. A special form of eudiometer called the " nitrometer," is useful for this purpose. This is shown in Fig. 68. In order to force the gases out of the combustion tube a short space in the closed end is filled with sodium bicarbonate. Carbon dioxide is evolved at will by heating this and it causes the expulsion of other gases from the tube. Carbon and Hydrogen. The determi- nation of carbon and hydrogen is made by combustion and absorption of water vapor and carbon dioxide in weighed tubes containing appropriate absorbents. The method is the same as that used in other connections for organic substances containing sulphur and nitrogen. For the absorption of water vapor calcium chloride or sulphuric acid may be used, the latter giving more nearly complete absorption while the former is more conveniently used. For carbon dioxide, solid soda lime or a solution of potassium 17 FIG. 68. Schiff's nitro- meter. 258 QUANTITATIVE ANALYSIS hydroxide is used, any of the standard forms of absorbing tubes or bulbs serving as containers. A combustion tube of glass, silica or porcelain is used for the decomposition of the coal. The rela- tive merits of these materials were discussed in an earlier section (pages 30 and 31). For the present purpose the silica tube is most satisfactory and any suitable combustion furnace may be used. The tube should be 95 cm long and should project 10 to 15 cm beyond the furnace at each end. The combustion is effected by heating the coal in an atmosphere of oxygen. Since this also produces volatile organic compounds and some carbon monoxide, it is necessary to pass the gases through a solid oxidiz- ing agent in order to complete the oxidation. Cupric oxide is used for this purpose in the analysis of most organic substances, but when sulphur is present, as in all coals, sulphur oxides are not completely retained by cupric oxide and are therefore absorbed in bulbs containing potassium hydroxide. In the com- bustion of such materials lead chromate is substituted for cupric oxide. Lead sulphate is formed and this is not decomposed by heating. A lower temperature is used to avoid fusing the material into the tube. In order to prevent the absorption of nitrogen oxides in the potassium hydroxide a roll of copper is placed in the end of the tube. This reduces such oxides and nitrogen passes through the bulbs without absorption. The presence of halogens in any material being analyzed would also make necessary the insertion of a roll of silver into the combustion tube but this need not be done in the analysis of coal. Oxygen for the combustion may be made from manganese dioxide and potassium chlorate and stored in a large " gasometer" or it may be purchased in steel cylinders. Oxygen made by the first method named always contains chlorine oxides and it must be purified by passing through a concentrated solution of potas- sium hydroxide. Most of the commercial oxygen formerly obtainable in compression cylinders contained chlorine oxides, carbon dioxide and hydrogen, and such oxygen must be properly purified before it enters the combustion tube. Since the develop- ment of the manufacture of oxygen from liquid air it is possible to obtain gas having a high degree of purity. It is then only necessary to pass the oxygen through potassium hydroxide or soda lime in order to remove traces of carbon dioxide. In order to be able to control the flow a " gasometer" should be filled from ANALYSIS OF INDUSTRIAL PRODUCTS 259 the high-pressure cylinder and this used as the supply for the combustion. Oxygen. The direct determination of oxygen is difficult and is seldom attempted. The percent is sometimes estimated by subtracting the sum of percents of all other elements, water and ash, from 100. This is but a rough approximation but is usually all that is required. Fuel Value. After a decision has been reached as to the class of coal that is most suitable for a given industrial purpose the inquiry that is next in importance concerns the number of heat units that can be obtained from unit weight of the various grades of coal entering into that class. The custom of purchasing coal upon a tonnage basis at a contract price, with nothing more than the variety of coal named, is rapidly being displaced, by large consumers, by the method of purchasing upon a heat unit basis. A contract price is made, based upon a specified number of heat units per pound 'or ton and any deviation from this fuel value involves a corresponding alteration in the price. In scientific work the fuel value is always calculated as calories per gram or kilogram of fuel, while in industrial work it is gener- ally calculated as " British thermal units" per pound of fuel. The calorie is the quantity of heat required to raise 1 gm of water 1 C. in temperature. The British thermal unit (B. T. U.) is the quantity of heat necessary to raise 1 Ib of water 1 F. in temperature. The calculation of fuel values in either system involves equal weights of water and fuel and the relation cal r\gj* 2m p-Tpryy ifr is therefore the relation of the centigrade degree g to the Fahrenheit degree. That is, cal per gm X^ = B. T. U. per Ib, and B. T. U. per lbX^ = cal per gm. Fuel values may be calculated from the proximate analysis or the ultimate analysis, or may be determined directly by means of a calorimeter. Calculation of Fuel Value from the Ultimate Analysis. The cal- culation from the ultimate analysis is based upon the assumption that the heat of oxidation of a compound is equal to the sum of the heats of oxidation of the elements composing it. The ele- ments of coal that are oxidizable are carbon, hydrogen, and sul- phur. Nitrogen is mostly evolved in the free state and inorganic 260 QUANTITATIVE ANALYSIS matter other than sulphides is of little or no value for producing heat. Water is incombustible and absorbs heat in becoming vaporized, thus reducing the available heat energy. Oxygen is not only not combustible but, because of the fact that it is already combined with carbon and hydrogen it reduces the per- cent of these elements still available for combustion and therefore reduces the amount of heat that is available when the fuel is burned. The fuel value of elementary carbon is 8080, that of hydrogen is about 34,500 and that of sulphur is 2162 calories per gram, if it is understood that the products of combustion are cooled to ordinary temperature after combustion. This assump- tion is not realized in practice but it is customary to make the assumption in calculating fuel values. There is, of course, no way of knowing the manner in which the elements are combined in the organic compounds making up the coal substances. The arbitrary assumption is, however, made that all oxygen that is already contained in the dry coal is in combination with hydrogen. One-eighth of the percent of oxygen would then be the percent of hydrogen not available as fuel. The complete statement for calorific value, based upon these assumptions, is 34500(H-JO) + 8080-C + 2162S q = - ~~~~ ~~ cal per gm ' where H, O, C and S represent percents of hydrogen, oxygen, carbon and sulphur respectively. This formula is far from being a complete statement of avail- able heat. Before any compound can be burned to the oxides of the constituent elements the compound must be dissociated into its elements. This change will involve absorption or liberation (usually absorption) of heat energy and the amount of energy change will depend upon the method of combustion. This is not known for coal and the correction cannot be applied. There is also a fuel value (positive or negative) for the mineral matter contained in the coal, since the ash left upon burning is not the same as the original matter. This heat must also be omitted from the calculation because its quantity is not known. The changes taking place during the conversion of wood into the various forms of coal are represented by the following approxi- mate figures for the composition: ANALYSIS OF INDUSTRIAL PRODUCTS 261 Carbon Hydrogen Oxygen Wood 43 6 51 Lignite . 70 5 25 Bituminous coal 86 4 5 9 5 Anthracite coal 95 2 3 The gradual loss of volatile matter involves a relatively large loss of oxygen. This fact explains the steady rise in fuel value as the coals progress toward the anthracite, although the decrease in the ratio of hydrogen to carbon {remaining would lead one to expect a fall in fuel value. Calculation of Fuel Value from the Proximate Analysis. Many attempts have been made to devise a formula that will serve for calculating fuel value from the results of the proximate analysis. Such a formula must necessarily be purely empirical and must fail in many cases because coals of practically identical proxi- mate composition may vary widely in ultimate composition and constitution. The chief value of all such formulas lies in mak- ing possible an approximate valuation of the fuel where neither the ultimate analysis nor calorimetric determinations can be obtained. Following are a few examples of these formulas. Formula of Haas: 156.75 [100- (% ash+% S+%H 2 0)]+40.5X % S = B.T.U. perlb. Formula of Goutal: 1 82 C+ AM = cal per gm when C = % fixed .carbon, M = volatile combustible matter and A = a coefficient whose value is fixed by the volatile combustible matter as follows: M = 2-15 15-30 30-35 35-40 A =130 100 95 90 Formula of Gmelin: [100- (% H 2 O+% ash)] 80-6CX% H 2 O = cal per gm, in which C is a coefficient which varies with the percent of mois- ture as follows: 1 Compt. rend., 136, 477 (1902). 262 QUANTITATIVE ANALYSIS Percent moisture C <3 - 4 3-4.5 + 6 4.5-8.0 + 12 8.5-12.0 + 10 12-20 + 8 20-28 + 6 >28 + 4 If fixed carbon is calculated upon a basis of true coal, dry and ash free, the following table may be used: FC | B.T.U. perlb || FC B. T. U. per Ib 100 14,500 68 15,480 97 14,760 63 15,120 94 15,120 60 14,580 90 15,480 57 14,040 87 15,660 54 13,320 80 15,840 51 12,600 72 15,660 50 12,240 Determination of Fuel Value by Means of the Calorimeter. The best laboratory method for the determination of fuel value is by the use of one of the standard calorimeters. Practically all of these depend upon the measurement of rise in tempera- ture of water, caused by the combustion of fuel within a closed vessel or "bomb" immersed in the water. Combustion is best effected by electrical ignition in an atmosphere of compressed oxygen since oxidation is complete and no reactions can occur other than those of ideal combustion. The original bomb calorimeter of Berthelot 1 has been improved and changed in such a manner as to make it a practical instrument for industrial as well as for purely scientific laboratories. Successful modifications are those of Mahler 2 and Emerson. 3 Parr 4 has also perfected a fuel calorimeter in which the oxidizing agent is sodium peroxide. The advantages of this instrument for industrial testing are chiefly due to the fact that it dispenses with the use of compressed oxygen and much of the accessory apparatus for filling the bomb. Charging and firing become a comparatively simple matter and the instrument may 1 Ann. chim. phys. [5] 23, 160 (1881); [6] 10, 433 (1887). 2 Chem. Zentr., 63, 889 (1892). 3 J. Ind. Eng. Chem., 1, 17 (1909). 4 J. Am. Chem. Soc., 22, 646 (1900); 29, 1606 (1907). ANALYSIS OF INDUSTRIAL PRODUCTS 263 be operated by persons who have limited scientific training. The great fault of this and similar instruments comes from the re- action of the products of combustion with the excess of sodium peroxide or with sodium monoxide formed. Such reactions as the following occur: H 2 O+Na 2 2NaOH, CO 2 +Na 2 0Na 2 C0 3 , SO 3 +Na 2 Na 2 S0 4 . Materials in the ash also react and form sodium salts. All of these reactions involve heat liberation or absorption and this cannot be exactly calculated because the composition of the coal is never exactly known. The best that can be done is to deter- mine experimentally approximate corrections which will apply to different classes of coal. The sum of these corrections may often be as large as 10 percent of the total rise in temperature during a determination and the uncertainty is so great as to render the instrument of questionable value for any but the most approxi- mate determinations. The instruments using compressed oxygen, while usually more expensive and more difficult to operate, are best for accurate fuel testing, even for the works laboratory. A description of the Emerson calorimeter and directions for its use will be found on page 26.9. Sampling. The correct sampling of coal is a very difficult process. In a substance showing such a lack of uniformity in composition it is obvious that the sample must be selected with extreme care if the results of the analysis are to express the average composition. For a thorough discussion of this matter, and es- pecially with regard to the selection of samples from cars, heaps, mines, etc., the student is referred to a paper by Bailey. 1 The chemist usually does not have supervision over the taking of the first large sample although he may prescribe rules for taking it. The whole subject will resolve itself into an intelligent application of principles of sampling earlier discussed (page 8). When the sample reaches the laboratory it is treated to the usual progressive crushing and quartering until the proper fine sample is obtained. During this process there is a continued loss of moisture so that the fine sample finally obtained has not the same percentage composition as the sample as received. It is not easy to decide 1 J. Ind. Eng. Chein., 1, 161 (1909). 264 QUANTITATIVE ANALYSIS upon what basis the percents should be calculated. If the coal as received is taken as a basis its composition will not be the same as that of another coal of different moisture content but other- wise identical with the first. The only scientifically correct method is to calculate all other percents to a dry coal basis, reporting the moisture in the sample as received. It is sometimes necessary, however, to base all calculations upon the coal as received. In this case moisture must be determined at once, as well as after crushing and preparing the sample for analysis, as already men- tioned. Loss of moisture occurs at all times when the coal is exposed. The sample is ground fine enough to serve for the analy- sis and a determination of moisture is made upon this sample. The difference between the percent of moisture in the original coarse sample and the fine sample provides a basis for the cal- culation of the analytical results, to the original coal basis. It is understood that the fine sample has been analyzed. Since mois- ture has been lost all other percents are correspondingly high. Each percent will be corrected as follows: where P = experimental percent, p = corrected percent, g = percent of gross moisture and r = percent of residual moisture in the fine sample. In order to correct the analytical results to the basis of dry coal the following formula is used: 100 Proximate Analysis of Coal. Procure a sample as directed in the preceding discussion. Determination of moisture, volatile combustible matter, fixed carbon and ash may be made upon the same portion or a different portion may be used for the moisture determination. Moisture. Determine the gross moisture in the sample as received, as follows: In the least possible time thoroughly mix and quarter the sample until a portion weighing 100 to 500 gm is obtained, breaking the larger lumps if necessary. Weigh the quantity in a shallow metal pan and expose to the air at the temperature of the room for 24 hours. Weigh and calculate gross moisture. The dried sample is then broken, mixed and quartered in the usual way until about 1 gm is obtained . This is ground to pass a 100-mesh sieve and residual moisture determined upon the sample as follows: ANALYSIS OF INDUSTRIAL PRODUCTS 265 Weigh a crucible, then add to it about 1 gm of the pulverized coal, cover and weigh again. If this portion is to be used also for the deter- minations named above the crucible must be of platinum and must have a well fitted cover. If only moisture is to be determined upon this portion a porcelain crucible may be used. Weigh duplicate portions. Dry in the uncovered crucible for one hour in an oven at a temperature of 104 to 111, cool and weigh, or dry for 24 hours over sulphuric acid in an exhausted desiccator and reweigh. Calculate the loss as residual moisture in the fine sample. The sum of gross and residual moisture is to be reported as the total moisture of the original coal. Volatile Combustible Matter. Use 1 gm of the fine sample. If the dried portion that was used for the moisture determination is in a platinum crucible, proceed as directed below. If a new portion is to be taken, weigh the crucible then add as nearly as possible 1 gm of fine coal, cover and weigh again. Regulate the flame of a No. 4 Me"ker burner so that its extreme height is 15 cm. Support the closely covered crucible by a platinum or other metal triangle so that the bottom is 1 cm above the top of the burner. Heat for exactly 7 minutes, cool and weigh. Subtract the total weight after heating from the total weight before heating. If the undried sample has been used the loss is " total volatile matter." From this the percent of moisture in the fine sample is to be subtracted and the remainder recorded as " volatile com- bustible matter." If the dried sample has been used the loss gives, di- rectly, the volatile combustible matter. In either case the percent is to be corrected to a basis of either the original coal (represented by the coarse sample) or to coal free from moisture. Record both uncorrected and corrected percents. An alternative method for the determination of volatile combustible matter is to heat 1 gin of coal in a 10-gm platinum crucible having a capsule cover (fitting inside the crucible instead of upon the top). The crucible is heated for 7 minutes in a muffle that is maintained at 950, the cover being tapped into place after the luminous flame above the crucible disappears. The crucible should be supported upon an alloy triangle. Ash. Incline the crucible containing the coke (the dried coal or another sample of undried coal may be used as well), adjust the cover to assist combustion and burn all organic matter and carbon. The ash must be free from black particles but will not be white, chiefly on account of the presence of iron oxide. Subtract the weight of the empty crucible from the weight next obtained and calculate the percent of ash. Correct as for volatile combustible matter. Fixed Carbon, Subtract from 100 the sum of the percents of mois- ture, volatile combustible matter and ash. The remainder is the percent 266 QUANTITATIVE ANALYSIS of fixed carbon in the original sample. Fixed carbon in the dry sample is obtained by subtracting from 100 the sum of the percents (corrected to this basis) of volatile combustible matter and ash. Coke. The sum of the percents of ash and fixed carbon is the per- cent of coke. Ultimate Analysis of Coal : Sulphur. In a platinum dish of 50 to 75 cc capacity mix intimately, by means of a stiff platinum wire, 0.5 gm of fine coal, 1 gm of dry, powdered sodium carbonate and 0.5 gm of light, well powdered magnesium oxide. Heat, by means of an alcohol lamp, gently at first to avoid loss of material through escaping gases, then more strongly, mixing occasionally to bring new portions into contact with the dish. The decomposition is re- garded as finished when no black particles can be detected in the mixture. Cool the dish and add 50 cc of distilled water and 5 cc of saturated bromine water. Boil for 5 minutes, allow to settle and decant upon a filter. Add two more portions of water of 10 cc each, boil after each addition and decant. Finally wash the entire residue' upon the filter with hot water until free from bromides. To the filtrate add 5 cc of concentrated hydrochloric acid and boil until all bromine is expelled. The expulsion of bromine may be hastened by passing a stream of carbon dioxide through the solution. Precipitate the sulphuric acid as barium sulphate by the method already learned (page 77) or determine photometrically. Calculate the percent of sul- phur in the coal and correct as in the other determinations. Carbon and Hydrogen. A tube combustion furnace of any of the approved types and about 75 to 80 cm long is necessary. The combus- tion tube may be of hard glass, silica or porcelain. It should be long enough to project for at least 10 cm at each end of the furnace, in order to prevent heating of the rubber stoppers that must be inserted in the ends. The internal diameter of the tube should be 12 to 15 mm. Since coal always contains sulphur, lead chroma te must be used in the combustion tube and this is prepared by fusing, cooling and crushing about 100 gm. The largest pieces should be small enough to easily enter the tube. By sifting the crushed material, using a 40-mesh sieve, a finer grade will be obtained and this is used for mixing with the powdered coal. The combustion tube should have well rounded ends. It is filled according to the following directions, assuming that the length of the furnace is 75 cm and that of the tube is 95 cm. Into one end of the tube insert a closely fitting roll of copper gauze, 5 cm long, and push this in until a space of 10 cm is left at the end of the tube. Into the other end pour the coarsely crushed lead chromate until a space 50 cm long is filled. The material should be well settled ANALYSIS OF INDUSTRIAL PRODUCTS 267 but not packed in such a way as to obstruct the passage of gases. Insert another roll of copper gauze like the first, to hold the lead chromate in place. Another roll of copper gauze, 10 cm long, is inserted in such a way as to leave a space of 10 cm at the end of the tube, and a space of 5 cm between the two rolls. The latter space is for the boat containing the coal. The ends of the tube are closed by rubber stoppers carrying short glass tubes for connecting with the rest of the appa- ratus. The method of filling the tube is shown in Fig. 69. Fig. 70 shows the method of assembling the complete appa- ratus. Entering oxygen and entering air are passed through cylinders A and A', respectively, containing a good quality of soda lime. The gases are next passed through U-tubes, B and B', containing fused, granular calcium chloride. These tubes are connected with the combustion tube by means of a three-way stopcock. Gases leaving the combus- tion tube first pass through two U-tubes C and D (prefer- ably glass stoppered) containing calcium chloride, then through the carbon dioxide absorption bulbs E containing potassium hydroxide solution and calcium chloride. Follow- ing these tubes is a guard tube F containing calcium chloride, also an aspirator G. For the detailed directions for filling and connecting the various absorption tubes and aspirator, refer to the discussion of the determination of carbon dioxide in carbonates. When the combustion tube and all parts of the appara- tus are in order start a slow but steady current (three bub- bles per second in the bulbs) of air by means of the aspi- rator, then heat gradually the entire length of the combustion tube. The drying tubes C and D and the potassium hydrox- ide bulbs need not be in the train because this preliminary heating is for the purpose of thoroughly drying the contents of the combustion tube and oxidizing any organic matter with which the tube might be contaminated. They are therefore removed, carefully wiped clean and dry, and are then closed and placed in the balance case. After they have stood for 15 minutes, if ready to proceed with the blank test, these pieces are weighed. The temperature of the part of the tube containing lead chromate must not be higher than is indicated by dull redness although other parts may be heated to any temperature under the softening point of the tube. When moisture has been expelled from the tube to the extent that 268 QUANTITATIVE ANALYSIS no condensation is noticed on the forward end the calcium chloride tubes C and D and the potassium hydroxide bulbs are weighed and placed in the absorption train. The flow of air is continued for 20 minutes, when the aspirator is stopped and the absorption tubes are again re- moved, stoppered, placed in the balance case and weighed after stand- ing for 15 minutes. If there is a gain of more than 0.5 mg in either the weight of the potassium hydroxide bulbs or the combined weights of the two calcium chloride tubes the entire operation must be repeated until there is no greater gain than 0.5 mg. When this is the case, that part of the combustion tube which is at the left of the lead chromate is allowed to cool. uimj > C D E F || FIG. 70. Diagram of connections for combustion apparatus. Provide a porcelain or platinum boat, about 5 cm long and of the proper width for insertion into the combustion tube. In the bottom of this place a layer of powdered lead chromate 1 mm deep and then weigh into the boat about 0.5 gm of powdered coal which has been properly sampled and dried. Mix with a platinum wire. Remove the rubber stopper at the left end of the tube and quickly remove the roll of copper gauze (now largely oxidized to cupric oxide) by means of a wire hook and insert the boat, pushing the latter in until it touches the roll of cupric oxide which confines the lead chromate. Replace the first roll and the rubber stopper as quickly as possible, start a current of oxygen through the tube and gradually heat the cooled portion of the tube. Volatile matter will escape from the coal but this will be com- pletely oxidized by the lead chromate in the forward part of the tube. Backward diffusion of volatile combustible matter will occasion no loss by condensation because the roll of cupric oxide behind the boat will serve to oxidize a small quantity of such gases. When all glowing of the coal has ceased, turn the three-way stop cock so that air is drawn into the tube, gradually lower the temperature of the left end so as to avoid cracking and continue the passage of air until about 1000 cc more of water has run out of the aspirator. Remove the absorption tubes and bulbs, close and allow to stand 15 ANALYSIS OF INDUSTRIAL PRODUCTS 269 minutes and then weigh. From the total gain in weight of the two calcium chloride tubes, due to absorption of water vapor, calculate the percent of hydrogen in the coal. From the weight of carbon dioxide absorbed in the potassium hydroxide bulbs calculate the percent of car- bon in the coal. Duplicate determinations should be made. If many samples are to be analyzed much economy of time will result from the use of two boats and two sets of absorption tubes. As each experiment is finished another can be started and the combustion will proceed while the first set of tubes is standing in the balance and being weighed. The determination of nitrogen may be made by the Kjeldahl method, described on pages 429 to 433, or by the method of Dumas, already discussed as the method of combustion. Fuel Value. If a calorimeter is available determine the heat units experimentally. Calculate also the heat units from the FIG. 71. Emerson's calorimeter. analytical results, using the formulas already given as well as others that have been proposed. Comparison with calorimetric data will indicate the degree of usefulness of the formulas. Following is a description of the Emerson calorimeter and also directions for making the determination of fuel value. Bomb. The bomb is made of steel, consisting of two cups joined 270 QUANTITATIVE ANALYSIS by means of a heavy steel nut. The two cups are machined at their contact faces with a tongue and groove, the joint being made tight by means of a lead gasket inserted in the groove. The lin- ing is of sheet metal spun in to fit. The bomb is made tight with a milled wrench or spanner. The pan holding the combusti- ble is of platinum or nickel, and the supporting wire of nickel. The fuse wire should be platinum in general fuel testing. In standardizing the calorimeter by means of cane sugar, benzoic acid, etc., and in burning hard coal or coke without an auxiliary combustible, it is necessary to use iron fuse wire. FIG. 72. Emerson's calorimeter in section. At the right, bomb connected with oxygen tank. Calorimeter. The jacket is a double walled copper tank between the walls of which water is inserted. The calorimeter can is made as light as is possible, of sheet brass. Stirring Device. The stirrer is directly connected to a small motor and is enclosed in a tube to facilitate its action in circulating the water. The stirrer is mounted on a post on the calorimeter jacket as is the thermometer holder. The motor is driven by a 110-volt circuit, and should be placed ANALYSIS OF INDUSTRIAL PRODUCTS 271 in series with a 16-c.p. lamp. The motor may be driven by either direct or alternating current. Oxygen Piping. The piping for the insertion of oxygen under pressure is made especially strong and durable. The piping of small internal bore is made of heavy brass. The system is fitted with a hand nipple on the end, to make the connection with the bomb, and the other end has a special fitting to grasp the oxygen supply tank, with nipple to fit. Iron Plate Holder. The plate holder or vise is to be used when tightening the nut of the bomb with the spanner. Swivel Table. The table with the rotating top is to hold the bomb when this is connected to the oxygen piping. Figs. 71 and 72 illustrate the entire apparatus and method of filling the bomb. FIG. 73. Method of wiring the Emerson bomb. Determination: Heat of Combustion of Solid Fuels. Place the lower half of the bomb in the holder, and the fuel pan in the wire support, after having wired the fuse wire according to Fig. 73. To place the fuse wire, twist one end of it to the small hole at the edge of the pan, leaving the short end of sufficient length to pinch the same between the pan and its supporting ring. Make sure of good contact. Extend the wire across the pan through the hole in the mica, allowing it to dip sufficiently to be in contact with the fuel, which is afterward placed in the pan. After passing through the mica, the wire is led to the side of the bomb, where it is grounded at the binding post. The wire must in no case touch the pan except at the edge where the twisted contact is made. The fuse wire should be placed in series with two 32-c.p. lamps in parallel when the 110-volt power circuit is used for firing. 272 QUANTITATIVE ANALYSIS The fuel used is sampled and powdered according to directions already given. Fill a weighing bottle with the prepared sample, and weigh accurately to one-tenth of a milligram. Pour from this into the pan in the bomb, until the pan is approximately half full. Weigh the bottle again, and the difference between the above weighings gives the net quantity of the fuel in the bomb. This weight should be greater than 0.5 gm and not more than 1.2 gm. For hard coal the maximum charge should be not greater than 1 gm. Hard coal should not be as finely divided as soft coal. (Through an 80-mesh sieve is sufficient.) The upper half of the bomb is placed in position and the nut is screwed down as far as may be by hand, care being taken not to cross the threads. The shoulder on the upper half of the bomb, over which the nut makes bearing contact, should be thoroughly lubricated with oil. Extreme care should be taken that no oil or grease is deposited on the lead gasket, as the bomb, when working properly, closes without the upper half turning on the gasket on account of the contact friction of the nut. Any oil on the lead gasket would tend to hinder the proper action in this respect. The bomb is now ready to be filled with oxygen and this is accom- plished by means of the spindle valve at the top of the bomb. The nipple is coupled to the oxygen piping by means of the attached hand union, the bomb resting on the swivel table, and after the connection of the bomb to the oxgyen piping is accomplished the hand set screw on the table is tightened. (The oxygen piping should be properly located and screwed fast to the bench. The screw holes in the feet of the swivel table are left large, and are made for round-head screws, so as to allow for adjustment relative to the oxygen piping. Both should be fixed in position.) In handling the bomb, care should be taken not to tip or jar it, as fuel may be thrown from the pan. The spindle valve on the bomb need be opened only one turn, and then the valve on the oxygen supply tank is very cautiously opened. The pressure gauge should be carefully watched and the tank valve so regulated that the pressure in the system shall rise very gradually. When the pressure reaches 300 Ib per square inch, the tank valve is closed and then the spindle valve immediately after. The bomb should be immersed in water immediately to detect any possible leakages. The bomb is now ready for the calorimeter, which is prepared as follows : 1900 gm of distilled water is placed in the calorimeter can at a temperature about 1.5 below the jacket temperature (which tempera- ture should be in the proximity of the room temperature). The bomb is then placed in the calorimeter and the stirrer and thermometer are lowered into position as indicated by the illustration. The thermometer ANALYSIS OF INDUSTRIAL PRODUCTS 273 is immersed about 3 inches in the water. The bulb of the ther- mometer should not touch the bomb. The terminals of the electric circuit used for firing should now be attached, one to the bomb and the other to the can, the can making contact with the pin in the plug at the bottom of the bomb. Care should be taken that neither the bomb nor the stirrer touches the sides of the can. The stirrer is now started and allowed to run 3 or 4 minutes to equal- ize the temperature throughout the calorimeter. Readings of the thermometer are now taken for 5 minutes (reading to 0.001 or 0.002 every half minute) at the end of which time the switch is turned on for an instant only, which will be found sufficient to fire the charge. In course of a few seconds the temperature begins to rise rapidly and readings are taken as before, every half minute from the time of firing. After a maximum temperature is reached and the rate of change of temperature is evidently due only to radiation to or from the calorimeter, the readings are continued for an additional 5 minutes, reading every half minute. These readings before the firings and after the maximum temperatures are necessary in the computa- tion of the cooling correction. The time elapsed from the time of firing to the maximum temperature should be, in no case, more than 6 minutes. When through with the run, replace the bomb in the holder and allow the products of combustion to escape through the valve at the top of the bomb. Unscrew the large nut and clean the interior of the bomb. The inside of the nut should be' kept greased, also the threaded part at the top of the lower cup. Immediately after each run, the inside of the bomb should be washed out with a cloth moistened with a dilute solution of sodium hydroxide and then with water. The lining of the lower cup is removed by with- drawing the fuse wire binding post, which is held in place with a taper fit and is easily removed. The lining to the upper cup is held in place by the small screw at the top. When the apparatus, after using, is to be left for several hours or more before making another test, the linings should be removed and the inner surface of the bomb slightly coated with oil. This oil under the linings should be removed when next preparing the bomb for use, as an excess of it may be ignited with a possible resulting injury to the linings. Heavy Oils, Coke, Hard Coal, Etc. The determination of the heat of combustion of heavy oils, such as crude petroleum, and also of coke and extremely hard coals, is best made by mixing with a ready burning combustible, such as a high-grade bituminous coal or pure carbon. This auxiliary combustible facilitates the complete combustion of the 18 274 QUANTITATIVE ANALYSIS whole mixture in the case of coke and hard coal, and with the heavy oil it acts as a holder and prevents rapid evaporation of the oil. The auxiliary combustible should be placed at the bottom of the pan and the coke, coal or oil sprinkled over it. The carbon or other auxiliary combus- tible should be dried with extreme care and carefully standardized as to the resulting rise in temperature per gram in the calorimeter when completely burned. Calculation. The data obtained during the experiment are used as follows: The difference between the temperature at maximum and the temperature at firing gives directly the apparent rise in tem- perature in the calorimeter. To this apparent rise, however, a cooling correction must be applied, which is computed as follows : The change in temperature during the preliminary 5 minutes of reading, divided by the time (5 minutes) gives the rate of change of temperature per minute, due to radiation to or from the calorimeter, and also any heating due to stirring, etc. This factor will be called RI and in like manner the readings taken after final temperature give R 2 . The two rates of change of tem- perature give the existing conditions in the calorimeter at the start and at the finish of the run. Therefore, the algebraic sum of the two rates, divided by two, will give the mean (or average) value of the rate of change of temperature during the entire run, due to radiations to and from the calorimeter. This value multi- plied by the time from firing to maximum will give the total cooling correction. The cooling correction thus determined has been found by long experience to be a very close approximation to the radiation effects encountered when working under these conditions. The latter quantity is either added to, or subtracted from, the apparent rise taken from the data of the run, accordingly as the balance of heat radiation is to the surroundings or from the surroundings. This is at once determined from an inspection of the data. Cooling correction is thus expressed: T> _|_T> ^ Xtime from firing to maximum temperature. The corrected rise of temperature divided by the weight of fuel used, will give directly the rise per gram of fuel. This rise per gram multiplied by the weight of water plus the ANALYSIS OF INDUSTRIAL PRODUCTS 275 " water equivalent" (this figure is furnished by the manufacturers) will give immediately the calories per gram of fuel, which is the result to be obtained. The result in calories per gram of fuel, multiplied by the factor 1.8 gives B.T.U. per pound of fuel. Example : Weight of tube and coal = 7. 9379 gm Weight of tube alone = 7.0713 gm Weight of coal =0.8666 gm Weight of water = 1900 gm The thermometer readings were as follows: Time | Temp., degrees || Time Temp., degrees Time | Temp., degrees 20.348 30 21.000 11 23.182 30 20.350 6 22.600 30 23.178 1 20.352 30 22.900 12 23.174 30 20.356 7 23.100 30 23.170 2 20.358 30 23.150 13 23.166 30 20.360 8 23.194 30 23.162 3 20.362 30 23.196 (max. 14 23.158 temp.) 30 20.364 9 23.196 30 23.154 4 20.368 30 23.194 15 23.150 30 20.374 10 23.194 5 20.376 (firing 30 23.190 temp.) Apparent rise in temperature = 2. 820. Rate of change of temperature before firing = . 0056 = RI. Rate of change of temperature after maximum temperature = 0.0088 = R 2 . 0032 Average rate of change of temperature during run= '~ 0032 Total cooling correction = ^ X 3. 5 = 0.006 (additive). Total corrected rise in temperature = 2. 826. Rise per gram of sample = 3. 261. The water equivalent of bomb, calorimeter can, stirrer, etc. = 490. Calories per gram of sample = (1900+490) X 3. 261 = 7794. British Thermal Units per pound of sample = 7794 X 1 . 8 = 14,030. GAS MIXTURES The separation and exact determination of gases may be accomplished by using various gravimetric and volumetric 276 QUANTITATIVE ANALYSIS methods. Certain gases may be absorbed in suitable reagents, the absorption product being precipitated and determined gravi- metrically. Examples of this class of methods have been met in the determination of the halogens (page 99). Sulphur dioxide may be absorbed in a basic solution, oxidized by bromine and precipitated as barium sulphate. Carbon dioxide has already been determined by absorption in a weighed solution of potassium hydroxide. Numerous other examples will suggest themselves. On the other hand many gases can be absorbed by reagents in which they can be determined volumetrically. For example chlorine may be absorbed by a solution of potassium iodide and the liberated iodine titrated by standard sodium thiosulphate (page 228) ; carbon dioxide may be absorbed in a standard solu- tion of a base and the solution titrated by a standard acid in presence of phenolphthalein; sulphur dioxide may be absorbed and titrated by standard iodine solution, etc. For commercial mixtures of gases these methods are not often used because the time required for a complete analysis is too long. The analysis of such mixtures as illuminating gas, natural gas, producer or water gas, or chimney or mine gas must be made by more rapid methods even at a sacrifice of a degree of accuracy. The gases from a measured volume of the original mixture are absorbed in suitable reagents and the volume loss is measured. The results of the analysis are computed in percents by volume. A standard type of apparatus for gas volumetry and one that is to be found in most laboratories is that of Hempel. Gas Burette. The gas burette, in which the gas mixture is measured, is shown in Fig. 74. The measuring tube (a) is con- nected with a levelling tube (b), the gas being confined over water. In making a reading the water is brought to the same level in the two tubes so that the gas is measured at atmospheric pressure. A complete analysis may usually be completed in a time suffi- ciently short that no serious error is caused by barometric changes. Changes in temperature during the course of an analysis con- stitute the most serious sources of error. To make the method even commercially accurate great care must be exercised in this regard. A quiet room in which no other work is being performed should be used. The operator must at all times avoid touching the burette or levelling tube directly with the hands or breathing upon them more than is necessary. Sometimes the burette is ANALYSIS OF INDUSTRIAL PRODUCTS 277 enclosed in a water jacket to guard against any but very slow changes. The burette may have a simple rubber tip and pinch cock at the top or it may be closed by a glass cock. The latter is desir- I a m FIG. 74. Gas burette with levelling tube. able but is liable to become stuck by contact with basic reagents. It must be well lubricated and frequently used or loosened. The glass three-way cock at the bottom of the tube is not often used and is not required. 278 QUANTITATIVE ANALYSIS Absorption Pipette. The apparatus in which the gases are absorbed is known as an " absorption pipette." The simplest form of the Hempel pipette is illustrated in Fig. 75. The reagent fills the lower bulb, the bent tube and the capillary tube. The latter is connected with the gas burette by means of a short bent capillary tube and when the gas mixture is forced into the pipette the absorbent fills the upper bulb. Some reagents are rapidly altered and rendered inefficient by contact with air. Protection from such action is afforded by the compound pipette (Fig. 76), the second pair of bulbs being fl FIG. 75. Hempel's simple absorption pipette. FIG. 76. Hempel's double or "com- pound" absorption pipette. filled with water. It is sometimes necessary to insert solid rea- gents, such as sticks of yellow phosphorus, copper wires for reducing cupric chloride, etc., or rolls of iron gauze or glass tubes for giving greater absorbing surface to the reagent. A pipette for solids and liquids has an opening at the bottom of the first bulb for the insertion of such materials. (Fig. 77.) In order to increase the rate of absorption modifications of the original pipette have been introduced. The gas is caused to bubble through the reagent instead of being forced down over the latter. After absorption has been completed the remaining gas is drawn from the top by turning the three-way cock to com- municate with the upper part of the bulb. (See Fig. 78.) When transferring gases from the burette to the pipette it is necessary to avoid mixing the water of the burette with the ANALYSIS OF INDUSTRIAL PRODUCTS 279 reagent in the pipette because the latter is thereby diluted. It is still more important that the entrance of reagents into the burette should be prevented because such contamination of the water would cause premature absorption of gases. In order that such mixing may be avoided it is necessary that there be a neutral zone in the connecting tubes, into which neither water nor re- agent shall enter. If this part of the tube has any but a very small capa- city there will be an appreciable error, due to the gas that is left in the tube each time. For this reason the con- necting tubes are of capillary dimen- sions. FIG. 77. HempeFs double pipette, modified to admit solids. FIG. 78. Bubbling absorp- tion pipette. One of the most serious disadvantages in the use of Hempel pipettes comes from the necessity for connecting and discon- necting each pipette in turn as the different gases are absorbed. To obviate this inconvenience many modifications have been made in the direction of a composite apparatus that does not require the interchange. The most important feature of such forms of apparatus is a permanent connection of the burette with the several absorption pipettes, communication being estab- lished with each in turn by special forms of stop cocks. This usually involves the use of longer capillary tubes and this in- 280 QUANTITATIVE ANALYSIS creases the error already mentioned as inherent in connecting tubes. In the Barnhart-Randall apparatus 1 the absorption pipettes are arranged concentrically around the burette, the stop cock being turned as each reagent is to be used. In apparatus designed for the analysis of chimney gases the feature of permanent connection must be combined with port- ability because the analysis must usually be conducted at the 1DL1 Pipette for Stick Phosphorus Section of Stopper and Capillary Tubes Seal for Reagents FIG. 79. Earnhardt-Randall apparatus for analysis of illuminating gas. plant. A recent modification of the Orsat apparatus is here illustrated (Fig. 80). Solubility of Gases in Reagents. When water is used as the confining liquid in the gas burette and water solutions are used as absorbents in the absorption pipettes it is impossible to avoid small errors, due to the solubility of the components of the 1 Electrochem. Met. Jnd., 6, 350 (1907). ANALYSIS OF INDUSTRIAL PRODUCTS 281 gas mixture in water. If the gases are taken into a burette con- taining pure water, each gas dissolves and the volume is dimin- ished after the total volume has been read. In order to avoid the disappearance of a part of the gases in this way, the water must have been previously saturated by allowing the gas to bubble through it. This does not entirely obviate the error because, as the mixture is drawn back into the burette for measurement after the removal of each constituent, the partial pressure of that constituent being reduced to zero, a part passes out of the solution in the burette and mixes with the re- maining gases, the total ob- served volume being rendered too large. To illustrate this ac- tion, suppose that a mixture of oxygen, carbon dioxide and car- bon monoxide is being analyzed. The water in the burette is first saturated with the mixture but the amount of each dissolved is a function of its partial pressure (concentration) in the mixture. The measured gases are passed into a pipette containing potas- sium hydroxide where the car- bon dioxide is completely ab- sorbed, its partial pressure in the gases being reduced to (prac- tically) zero. Upon passing the mixture of carbon monoxide and oxygen back into the burette a certain amount of dis- solved carbon dioxide will be given up by the water and the volume will be somewhat larger than the sum of the volumes of the other two gases. Also where the mixture was confined over potassium hydroxide 'solution the latter dis- solved small amounts of carbon monoxide and oxygen and some of these gases may be given up to mixtures later being analyzed. The calculation of the amount of error may thus become a com- plicated matter. The error is negligible, from the industrial FIG. 80. Orsat's apparatus (modi- fied) for analysis of chimney gases. 282 QUANTITATIVE ANALYSIS standpoint, if the analysis is completed within a short period of time. Fuel and Lighting Gases. In illuminating gas the following constituents are determined: carbon dioxide, ethylene and its homologues, oxygen, carbon monoxide, hydrogen, methane and nitrogen. They are absorbed, in the order named,, one after another, and the contraction in volume noted after each absorp- tion. Hydrogen and methane are determined by combustion and nitrogen is computed by subtracting the sum of the other gases from 100. The various absorbents for these gases will be discussed. Carbon Dioxide. A solution of any of the strong bases may be used for absorbing carbon dioxide. Potassium hydroxide possesses the advantage of large solubility and rapid absorption of gas and is almost always used for this purpose in gas analysis. A solution made by dissolving solid potassium hydroxide in twice its weight of water (ab ">ut 33 percent, by weight) is suitable, this being the same strength as that employed for gravimetric deter- minations. 100 cc of a 33 percent solution will absorb about four liters of carbon dioxide before it becomes inefficient. The potas- sium hydroxide used should not be that which has been purified from alcohol solution, because traces of alcohol are retained in the solid base and alcohol vapor or other organic vapors are given up to the gas. The solution may be used in either the single or double Hempel pipette or in any of the modified pipettes. If the Hempel pipette is used it should contain rolls of iron gauze in order to increase the surface of solution exposed. As the solution is forced down, leaving the gauze exposed, the film of solution retained upon the surface of the wires greatly increases the rate of absorption. Hydrogen sulphide will be included in the fraction absorbed by potassium hydroxide unless it has been otherwise removed. Its quantity is usually small. Carbon Monoxide. The most conveniently used absorbent for carbon monoxide is a solution of cuprous chloride. This salt is only slightly soluble in water and must be dissolved in either hydrochloric acid or ammonium hydroxide. Either solu- tion absorbs carbon monoxide with the formation of a rather unstable compound whose exact nature is unknown. The acid solution is made as follows: Mix 86 gm of cupric oxide and ANALYSIS OF INDUSTRIAL PRODUCTS 283 17 gm of finely divided copper and slowly add to 1000 cc of a mixture of equal volumes of concentrated hydrochloric acid and water. Stir until the solid matter has dissolved, then place in bottles having bundles of copper wire reaching from top to bottom. Stopper the bottles and allow to stand until colorless. Cupric chloride, formed by dissolving cupric oxide in hydro- chloric acid, is reduced by copper to cuprous chloride: CuO+2HCl CuCl 2 +H 2 O, CuCl 2 +Cu 2CuCl. 100 cc of this solution will efficiently absorb about 400 cc of carbon monoxide. Absorption takes place slowly and the gas must be shaken with the solution for some time or be allowed to bubble through it. The double pipette must be used because cuprous chloride is readily oxidized in contact with air, cupric chloride being formed. Oxygen. Oxygen is absorbed by a solution of potassium pyrogallate or by yellow phosphorus. The former solution is prepared by dissolving 120 gm of potassium hydroxide in 80 cc of water, cooling and placing in the double absorption pipette then adding 15 cc of a 25 percent solution of pyrogallic acid and mix- ing. Potassium hydroxide purified by alcohol should not be used. The solution will readily absorb about 200 cc of oxygen for each 100 cc of solution. It does not act rapidly at temperatures below 15. Yellow phosphorus may be used in the form of sticks which are placed in the double pipette for solids and liquids and kept covered with water. This absorbent possesses the great advan- tage of retaining its capacity for absorbing oxygen until the sticks have become completely used up. The product of the union of phosphorus and oxygen, phosphorus pentoxide, dissolves in water so that the surface of the sticks is always fresh. Absorp- tion becomes slow below 15, and traces of unsaturated hydrocar- bons of the ethylene series partially inhibit the absorption. For the latter reason phosphorus is not suitable for use in those forms of assembled apparatus for the analysis of chimney gases in which the ethylene hydrocarbons are not determined at all. Ethylene and Its Homologues. These gases give higher illumi- nating power to the mixture of methane, hydrogen and carbon mon- oxide, gases which burn with a non-luminous flame. For this reason 284 QUANTITATIVE ANALYSIS they are collectively known as "illuminants." Fuming sulphuric acid or bromine water may be used. Fuming sulphuric acid reacts with members of the ethylene series of hydrocarbons, forming addition products as well as condensation products. These are either liquids or soluble solids and are therefore removed from the .gas mixture. The absorption is not rapid and the acid should be shaken with the gas if the Hempel pipette, or one FIG. 81. Fuming sulphuric acid pipette for unsaturated hydrocarbons. Gill's modifica- tion. FIG. 82. Hempel's explosion pipette. 'similar to it, is used. The single pipette is used, since a water seal in the second bulbs is inadmissible. In order to increase the contact of gas with acid the pipette contains a third small bulb which is filled with glass beads. Contact of the acid with rubber connections must be avoided. Gases exposed to the action of fuming sulphuric acid will always contain oxides of sulphur. These must be absorbed by passing into the potassium hydroxide pipette before measuring the residual gas. 100 cc of fuming sulphuric acid will absorb about 800 cc of ethylene. Bromine water absorbs ethylene and its homologues with formation of bromine addition compounds : C 2 H 4 4-Br 2 C 2 H4Br 2 . It is somewhat more convenient to use than fuming sulphuric acid but does not absorb with great readiness. If excess of bromine ANALYSIS OF INDUSTRIAL PRODUCTS 285 is placed in the pipette the absorbing power is undiminished until all of this bromine has been dissolved. Hydrocarbon Vapors. Gases formed by distilling coal often contain vapors of liquid hydrocarbons, chiefly benzene. These are partly absorbed by fuming sulphuric acid but may not be entirely removed. They may be absorbed in absolute alcohol and so determined. The absorbing power of absolute alcohol is not large and gases coming from the ordinary gas burette, being saturated with water vapor, soon diminish the efficiency of the alcohol by imparting moisture to it. Dennis and O'Neill suggested 1 the use of a solution of nickel sulphate in ammonium hydroxide. Neither this solution nor absolute alcohol is an entirely satisfactory absorbent. The determination of hydro- carbon vapors is frequently omitted, these vapors then being absorbed along with unsaturated hydrocarbons. Hydrogen. The determination of hydrogen is made by burn- ing with oxygen, measuring the resulting contraction in volume, or by absorption in palladium. The combustion may be carried out by exploding the mixture of hydrogen and oxygen over mercury in a suitable pipette or the burning may be made to proceed more slowly. If a mixture of hydrogen with an excess of oxygen or air is burned the resulting water vapor condenses and only the excess of oxygen or air remains as gas. From the equation 2H 2 +O 2 2H 2 O it is seen that two-thirds of the volume of the disappearing gas is that of hydrogen. Therefore, two-thirds of the contraction measured after cooling equals the volume of hydrogen. The explosion pipette is shown in Fig. 82. The confining liquid should not be water since larger quantities of gases will dissolve in it at the moment of explosion because of the momen- tary increase in pressure. Mercury is substituted for water and the pipette is so constructed as to permit altering at will the difference in level between the mercury in the two bulbs. If the ordinary single pipette were used it would be impossible to force gas into the pipette because of the great density of mercury and the consequent back pressure. Ignition is effected by con- necting with the secondary of an induction coil. i J. Am. Chem. Soc., 26, 503 (1903). 286 QUANTITATIVE ANALYSIS If pure hydrogen is mixed with pure oxygen and burned the explosion is too violent for safety. If the gas to be burned is rich in hydrogen it is mixed with air instead of with oxygen. One volume of hydrogen requires more than two and one-half volumes of air for complete combustion, allowing a small excess. Dilution with air is not necessary if the residual gas is poor in combustibles. On the other hand it may be necessary to enrich the gas, before burning, by adding a measured volume of pure hydrogen. This is conveniently generated from zinc and sul- phuric acid in a special pipette (Fig. 83). FIG. 83. Pipette for the prepara- tion of hydrogen. FIG. 84. Pipette for slow com- bustion. Combustion may also be effected by passing the mixture with oxygen through a heated capillary tube or by exposing the mixture to a glowing platinum wire in the pipette arranged for slow combustion (Fig. 84). In using this pipette either of two methods of procedure may be followed: The hydrogen is placed in the pipette, the wire made to glow by the passage of a current and a measured volume of oxygen led in, or the hydrogen and oxygen are mixed in the burette and slowly brought into the pipette, in which the wire is glowing. In either case combustion occurs without explosion. Hydrogen may be separated from nitrogen and methane by absorption in palladium sponge which has been superficially coated with oxide. Absorption readily takes place at 100 and ANALYSIS OF INDUSTRIAL PRODUCTS 287 the hydrogen may be later removed by passing oxygen through the palladium, the hydrogen being thereby oxidized and palla- dium oxide again formed on the surface. A tube of the form shown in Fig. 85 is used. The enlarged part is filled with asbestos which has been coated with spongy palladium and the tube is connected directly with the burette at one side and with a pipette filled with water at the other. Upon passing the gases through two or three times the hydrogen is quantitatively ab- sorbed, a small amount being oxidized by the trace of palladious oxide, and a certain amount is also burned by oxygen of air which was already in the tube. Except for this small amount of oxygen, the shrinkage in volume gives directly the volume of FIG. 85. Palladium tube. hydrogen. The amount of air in the tube must be known. This may be determined by connecting with the gas burette, and measuring the expansion between two temperatures. One- fifth of the total volume is taken as the contraction due to con- tained oxygen. The absorption of hydrogen by palladium is hindered by traces of hydrochloric acid. On this account the ammoniacal solution of cuprous chloride should be used for the absorption of carbon monoxide if this is to be followed by palladium absorption of hydrogen. The explosion pipette is accurate and not difficult to manipu- late but requires a battery and an induction coil. It is subject to the disadvantage that only a small amount of gas may be used, on account of the relatively large volume of air that must be mixed with it in the pipette, so that the error in reading volumes is relatively large. Gill has devised a pipette 1 which overcomes this objection. The bulb in which the explosion is to take place i J. Am. Chem. Soc., 17, 771 (1895), 288 QUANTITATIVE ANALYSIS is large enough to hold the entire residue from 100 cc of gas, together with the necessary oxygen for the combustion, and is made of quite heavy glass. Both the slow combustion pipette and the palladium tube permit the use of larger quantities of gas. Methane. Methane is determined by combustion, the pro- cedure being the same as for hydrogen. In the analysis of natural gas and illuminating gas, as well as many other com- mercial gas mixtures, hydrogen and methane will both occur in the residue after other gases have been absorbed. They must therefore be burned together unless hydrogen is to be absorbed by palladium. According to the equation CH 4 +20 2 C0 2 +2H 2 one volume of methane with two volumes of oxygen will produce one volume of carbon dioxide, the rest of the oxygen disappear- ing as condensed water vapor. The contraction is therefore twice the volume of the methane. Since a volume of carbon dioxide equal to that of the methane is produced a measurement of the former by absorption in potassium hydroxide will give a direct determination of the volume of methane. For the residue of hydrogen and methane, therefore, the procedure is as follows : An excess of air or oxygen is mixed with the gases and the mix- ture exploded. The gases are cooled and measured in the burette, the contraction being noted. Carbon dioxide is then determined by absorption in potassium hydroxide. The volume of carbon dioxide is equal to the volume of methane. Twice this volume is the contraction due to the combustion of methane. This contraction subtracted from the total contraction leaves the contraction due to the combustion of hydrogen. Two- thirds of this contraction is equal to the volume of hydrogen. The volumes of hydrogen and methane so determined, multi- plied by the ratio of the total residue to the volume taken for explosion, gives the volumes of hydrogen and methane in the original gas. The following example will illustrate the calcula- tions involved : 100 cc of illuminating gas gave, after all absorbable gases had been removed, 65.2 cc of residue, this consisting of hydrogen, methane and nitrogen. 15 cc of the residue was mixed with air, the total volume then being 90.5 cc. After explosion the volume ANALYSIS OF INDUSTRIAL PRODUCTS 289 was 71.0 cc. Carbon dioxide was absorbed, the volume of the remaining gases being then 66.6 cc. Volume methane = volume carbon dioxide = 71. 066.6 = 4.4 cc. Contraction due to combustion of methane = 2X4.4 = 8.8 cc. Total contraction = 90.5-71.0=19.5 cc. Contraction due to combustion of hydrogen = 19.5 8.8 = 10.7 cc. 2 Volume of hydrogen=~X 10.7 = 7.1 cc. 65 2 Volume of methane in original gas = n ' X4.4 = 19.1 cc. lo 65 2 Volume of hydrogen in original gas = -^- X 7. 1 = 30.9 cc. lo ( volume of sample Volume of nitrogen in original gas = \ sum of volumes [ of all other gases. Since 100 cc of gas was taken for analysis, the volume of the con- stituents will also be these percents by volume. Analysis of Illuminating Gas. For this exercise the Hempel appara- tus may be used or any of the modified pipettes or burettes may be substituted. The method of manipulation is not essentially different for the different forms of apparatus except in minor details and such variations will readily suggest themselves. Throughout the analysis avoid touching the body of the burette or the bulbs of the pipettes with the hands, or breathing upon them. Allow the sides of the burette to drain thoroughly each time before reading. Prepare water for the gas burette by allowing the gas to bubble through it for ten minutes. Fill the burette with this water, raise the levelling tube until the water flows out of the top of the burette, then close the upper cock. Place a rubber tube on a gas cock and allow gas to escape through it until all air is displaced. With the gas still running connect the tube with the top of the burette, open the burette cock and fill with gas until the 100 cc mark has been passed. Close the upper cock and detach from the gas supply. It is desirable that ex- actly 100 cc of gas be taken, measured at the prevailing pressure of the atmosphere. In order to do this allow the water to drain down the sides of the burette for 1 minute then raise the levelling tube, compressing the gas until the 100 cc mark is exactly reached. Now close the cock at the bottom of the burette or close a pinch cock which is placed on the 19 290 QUANTITATIVE ANALYSIS rubber connecting tube. Open the upper cock momentarily and close again. This permits gas to escape until the pressure within the burette is the same as that of the atmosphere. Hydrocarbon Vapors. Place the pipette filled with absolute alco- hol on the stand by the burette and connect with the burette by a bent capillary tube, having previously caused the alcohol to fill the lower bulb and the capillary up to a point near the top. Force the gas into the pipette, detach the latter and shake for 1 minute. Return the gas to the burette, allow the water to drain down the sides, adjust the levelling tube to provide atmospheric pressure and measure. Record the difference as " hydrocarbon vapors." Carbon Dioxide (and Hydrogen Sulphide). Attach the burette to the pipette containing potassium hydroxide solution, pass the gas into the pipette and directly back again. Measure and record the percent of carbon dioxide (including also hydrogen sulphide if present). Illuminants. Determine illuminants by absorption in fuming sul- phuric acid or bromine water, drawing back to the burette at once. Avoid the entrance of any water into the fuming sulphuric acid. Pass the gas into the potassium hydroxide pipette to absorb sulphur trioxide and then measure. Oxygen. Absorb oxygen by yellow phosphorus, allowing three minutes, or by potassium pyrogallate, shaking the pipette for three minutes. If pyrogallate is used in a pipette containing rolls of iron gauze the shaking may be omitted. Carbon Monoxide. Absorb carbon monoxide in either acid or basic solution of cuprous chloride. The gas should be shaken with the cu- prous chloride solution for three minutes, then passed into the pipette containing potassium hydroxide to absorb vapors of hydrochloric acid. Hydrogen, Methane and Nitrogen. Pass all of the gas residue into the cliprous chloride pipette for storage, pour out the water from the burette and replace with water that has been saturated with air. Deter- mine hydrogen and methane by one of the following described methods. Combustion by Explosion. Return 10 to 12 cc of the gas to the burette, measure accurately, then draw air into the burette until a total volume of nearly 100 cc is obtained. Do not attempt to obtain exactly 100 cc as there is danger of loss of gas during the adjustment of volume. Measure, then transfer the mixture of air and gas to the ex- plosion pipette, allowing water from the burette to enter and fill the capillary of the explosion pipette. Close the rubber connecting tube (which should have thick walls and be securely wired in place) with a screw clamp. Place the mercury reservoir bulb so that the mercury is at the same level as inside the explosion bulb, then connect the ter- minal wires with the secondary of an induction coil and cause a spark to ANALYSIS OF INDUSTRIAL PRODUCTS 291 pass. A flash will pass across the bulb and mercury will almost imme- diately begin to flow into the bulb, on account of the contraction of gas volume resulting from the combustion. At all times when the gas is in the explosion pipette the mercury must be so adjusted in level that a pressure much greater or less than that of the atmosphere is avoided. Return the gas to the burette, allow the water to drain down the sides, then measure. Absorb the carbon dioxide and remeasure. In order to be sure that an excess of oxygen was present the gas should be passed into the phosphorus or pyrogallate pipette. If no oxygen is found the explosion must be repeated with another sample of gas, using a larger proportion of air. Calculate the percents of hydrogen, methane and nitrogen by the method already discussed. Repeat with another portion of the residue in the cuprous chloride pipette. Slow Combustion. Use the pipette shown in Fig. 84. Measure about half of the residue which is stored in the cuprous chloride pipette and transfer this to the combustion pipette. If the residue is known to be chiefly methane not more than 25 cc should be used. If it is chiefly hydrogen more may be taken since hydrogen requires for com- bustion only half its own volume of oxygen. Fill the burette with pure oxygen, made by the electrolysis of water, and measure accurately. Connect the terminals of the platinum wire of the pipette with a current source and heat the coil to bright redness. Pass the oxygen into the combustion pipette but not so rapidly as to cause an explosion. When the combustion is completed transfer the entire gas mixture to the burette and record the volume and contraction. Determine carbon dioxide, test for excess of oxygen and calculate exactly as in the case of explosion. Absorption of Hydrogen by Palladium, Followed by Combustion of Methane. If the palladium tube is to be used for absorption of hydro- gen the solution of cuprous chloride in ammonium hydroxide must have been used for absorption of carbon monoxide. The entire gas residue is used. Connect the palladium tube with the burette on one side and a pipette filled with water on the other. The palladium tube should dip into a beaker of water which is kept nearly boiling. Pass the gases through the tube and back, repeating two or three times. Replace the hot water with water at the temperature of the room and again pass the gas through the tube to cool it. Determine the internal volume of the palladium tube as already directed and subtract one-fifth of this volume from the total contraction. The remainder is the volume of hydrogen. Determine methane by combustion by either of the methods already described. Chimney Gases. Ideal combustion of fuel gases or of coal should yield waste gases containing only carbon dioxide, water 292 QUANTITATIVE ANALYSIS vapor, and nitrogen. In practice complete combustion is not secured without a considerable excess of air, and oxygen is there- fore found in the chimney gases. The presence of carbon monoxide is an indication of imperfect draught and incomplete combustion while a large excess of oxygen shows that heat has been wasted in raising the temperature of unused air. For FIG. 86. Aspirator for sampling chimney gases. control work the determination of oxygen, carbon dioxide and carbon monoxide is sufficient and the portable form of apparatus (Fig. 80) is used. It contains a burette and three pipettes for these determinations. Many modifications of this apparatus will be found illustrated and described in the scientific journals and trade catalogues. To obtain the sample for analysis a porcelain or iron tube is inserted into the stack at the proper point. An aspirator is caused to draw a continuous stream of gas from the stack, the ANALYSIS OF INDUSTRIAL PRODUCTS 293 sample being removed by the burette as often as desired. The determination of the three gases is made as with the Hempel apparatus. Potassium pyrogallate should be used for the absorption of oxygen because of the possible presence of traces of ethylene. BURNING OILS The chemist's examination of fuel oils usually has more to do with the determination of certain physical constants than with the actual analysis. Petroleum products are cheaper than animal or vegetable oils and are, consequently, seldom adulterated with the latter. Animal and vegetable oils are rarely used for burning. The examination of the fuel oil, therefore, usually resolves itself into a determination of the fitness of the oil for the purpose for which it is to be used. The determinations may include specific gravity, flash point, burning point and fractional distillation. Specific Gravity. The relation between the specific gravity and the volatility of petroleum fractions is fairly definite, so that it is often possible to secure the correct oil by specifying only the specific gravity. This may be determined by means of a Westphal balance or a floating hydrometer. The latter is most conveniently used and is sufficiently accurate for most purposes. The specific gravity may be expressed in relation to water or in degrees Baume. The system of Baume is much used in commercial testing. In this system two scales are used, one being for liquids lighter than water, the other for liquids heavier than water. The first is applicable to all petroleum products and to most other oils and fats. In the original Baume* scale for liquids heavier than water the point to which the hydrometer sinks in a solution of sodium chloride, 15 percent by weight and at 15 C., was taken as 15. The corresponding point for pure water was taken as and all other points were located by these two. For liquids lighter than water the scale has the point 10 for the density of pure water at 15 C. and the point corresponds to the density of a 10 percent solution of sodium chloride. Several modifications of these scales have come into use and much confusion has resulted thereby. As the system is at present used in American industrial laboratories the following for- 294 QUANTITATIVE ANALYSIS mulas may be used for converting specific gravity into Baume degrees and vice versa. For liquids heavier than water: 144300 = where B = degrees Baume and S = specific gravity. For liquids lighter than water: 146000 " On account of the complexity of this system and the fact that it is entirely unnecessary it is unfortunate that it has become so generally used in chemical industries. Flash Point. The " flash point" is the temperature at which the oil gives off vapor rapidly enough that the mixture with air becomes explosive and will flash if a small flame is brought into the mixture. This is one of the most important tests to be applied to burning oils because it determines the degree of safety attending the use of the oil in enclosed vessels, such as lamps and burners of various kinds. In most of our States the lower limits of flash and burning points are specified for kerosene by legal restriction. The location of the flash point depends to a great extent upon the manner of confining and heating the oil. The mixture of vapor with air is explosive at any temperature if the concentration of vapor is sufficiently great. Under ordinary circumstances the vapor is evolved so slowly that it escapes by diffusion before an inflammable mixture is obtained and it is only when the tem- perature is raised that rapid evolution of vapor results in the pro- duction of a mixture that will ignite. From this it will readily be seen that the flash point is lowered by rapid heating, by con- finement of the vapor by covering the tester, as well as by too close contact of the test flame with the surface of the oil. It has therefore become necessary to regulate by law not only the tern- ANALYSIS OF INDUSTRIAL PRODUCTS 295 peratures of the flash point but also the exact form of the tester and the manner of heating. The following extract from the Indiana law of March 11, 1901, relates to this matter. " . . . , . The test shall be made in a test cup of metal or glass, cylindrical in shape, two and one-quarter inches in diameter and four inches deep (both measurements being made inside the cup) and this cup shall be filled to within one-quarter of an inch of the brim with the oil or other substance to be tested. The cup shall be placed in a water bath sufficiently large to leave a clear space of one inch under the cup and three-eighths of an inch around it, and in such a manner as to project about one-quarter of an inch above the water bath. The space between the cup and the water bath shall be filled with water of medium temperature and shall be heated by an alcohol lamp, with its flames so graduated that the rise in temperature, from 60 degrees Fahrenheit to the highest test temperature, shall not be less than two degrees per minute and shall, in no case, exceed four degrees per minute. A Fahrenheit thermometer shall be suspended in such a manner that the upper surface of its bulb shall be, as near as practicable, one-quarter of an inch below the surface of the oil undergoing the test. As soon as the temperature reaches the point of ninety-eight degrees Fahrenheit, the lamp shall be removed from under the water bath, and the oil shall then be allowed to rise to the temperature of one hundred degrees Fahrenheit by the residual heat of the water, and at that point the first test for flash shall be made as follows : A taper (hereinafter described) shall be lighted and the surface of the oil shall be touched with the flame of the taper (and it shall be lawful to apply this flame either to the center of the oil surface or to any or all parts of it) but the taper itself shall not be plunged into the oil. If no flash takes place at the temperature of one hundred degrees Fahrenheit, the lamp shall be placed under the water bath, and the temperature raised to one hundred and three degrees Fahrenheit, when the lamp shall be again withdrawn and the oil allowed to rise to one hundred and five degrees by the residual heat of the water, when the test shall be made by again applying the flame of the taper as hereinbefore specified; if no flash occurs the test shall be repeated as often as the oil gains five degrees in temperature, three degrees with the lamp under the water bath, and two with the lamp removed. These tests shall be repeated until a flash is obtained. The one making the test shall further test the oil by applying the taper at every two degrees rise without removing the lamp or stirring; but if a flash is obtained by this means, by a less rise in temperature than five degrees herein re- quired, he shall at once remove the lamp, stir the oil, and immediately apply the flame. The taper used for testing may be of any wood 296 QUANTITATIVE ANALYSIS giving a clear flame, and it shall be made as slender as possible, and with a tip no more than one-sixteenth of an inch in thickness. No taper or match with sulphur on it shall be used, unless the sulphur is first removed before lighting. When a taper is first lighted, it shall be applied to the oil immediately (that is to say, before an ash or coal has had time to form on the end of the taper beyond the end of the flame) and the flame shall be made to touch the oil, but the taper itself shall not be brought in contact with the oil; provided, that if the taper be so brought in contact with the oil, but not held there longer than for the space of one second, and the oil flashes, the test shall not thereby be vitiated, but the Supervisor of Oil Inspection shall immediately remove the lamp, and again test the oil by the flame without allowing the body of the taper to touch the oil. No oil or other substance, which, by the test herein described, flashes at any temperature below one hundred and twenty degrees Fahrenheit, shall be allowed to be sold, offered for sale, or consumed for illuminating purposes in this State. And it shall be lawful to sell for illuminating purposes any oil or oils herein described, to be consumed within this State, which shall bear a flash test of one hundred and twenty degrees Fahrenheit, as shown by said apparatus." The Indiana law is not specific in the matter of covering the tester and the inference is that the open tester is permitted. Burning Point. The burning point ("fire test") is the tem- perature at which vapor is evolved with sufficient rapidity to sustain a continuous flame. It is determined by continuing the heating after the flash point has been passed, applying the test flame until a temperature is reached where continuous flame results. The thermometer bulb is immersed in the oil and the temperature is always noted just before the application of the test flame, which should be as small as possible. On page 297 are given the legal requirements 1 regarding the minimum flash and fire tests as specified by several of the differ- ent states of the United States, also the names of the instruments, where these are indicated in the statutes. The temperatures in- dicated are degrees Fahrenheit. It is to be noted that the tem- peratures given are not directly comparable except when the same instrument is used in the tests. Besides the state requirements several of the larger cities specify different temperatures. Examination of Kerosene. Determine the specific gravity at 15 C. with a hydrometer float or a Westphal balance, reporting in the usual 1 Gill: Oil Analysis, 6th ed., 159. ANALYSIS OF INDUSTRIAL PRODUCTS 297 State Flash Fire Instrument Arkansas - 130 Tagliabue Connecticut 110 Florida Georgia . . 130 120 Tagliabue Illinois 150 Tagliabue Indiana 120 Indiana, open Iowa Kansas 105 110 Elliott Tagliabue Kentucky 130 Louisiana 125 Tagliabue Maine 120 Tagliabue, open Massachusetts 100 Tagliabue, open Michigan 120 148 Foster Minnesota 110 Minnesota Missouri 150 Tagliabue Montana 110 Nebraska New Hampshire 100 100 120 Foster Tagliabue New Jersey New Mexico 100 115 150 New York 110 Elliott North Carolina 100 Foster North Dakota Ohio 100 120 Foster Pennsylvania 110 Tagliabue Rhode Island 110 South Dakota 110 Foster Tennessee 120 Open cup Vermont . 110 Tagliabue Wisconsin 120 Wisconsin units and also in degrees Baume*, using the formula given on page 294 for calculation of degrees Baume*. Determine the flash and burning points, using, preferably, the tester specified by the law of the state in which the oil was sold and following in detail the directions furnished with the instrument. If no such tester is available construct one as follows: Upon a small sand bath place a 3-inch porcelain dish, pressing the dish into the sand until the latter is within 1/4 inch of the top of the dish. Fill to the same height with the oil to be tested and suspend in the middle of it a thermometer. Cover the dish with a watch glass having a perforation for the ther- mometer and a notch at the side for the application of the test flame. Heat the oil so that the temperature shall rise at the rate of about 2 per minute. When the temperature has reached 85 F. begin testing and test for each two degrees rise in temperature by inserting a small 298 QUANTITATIVE ANALYSIS flame (a gas flame 1/4 inch long) and immediately withdrawing it- The experiment should be performed where the light of the room is not strong and in a place free from air, currents. The flash point is reached when a flash passes entirely across the dish. Remove the cover and continue the heating and testing until a permanent flame is sustained. This temperature is the "fire point" or "burning point." The method used with the form of apparatus just described will not give the same flash point as that obtained by another form of apparatus and cannot be used as a legal check where another form of tester is specified by law. It is here described because it will afford practice in the determination when no other tester is available. Fractional Distillation. Any fraction of petroleum now ap- pearing in commerce includes many different chemical com- pounds and can itself be separated by fractional distillation into other fractions having boiling points within still more narrow limits. The determination of the amount distilling between certain specific limiting temperatures yields information regard- ing the composition of the mixture. The results have little significance, however, unless the distillation is conducted in a standard apparatus and by a standard method. LUBRICATING OILS For purposes of lubrication either mineral, animal or vege- table oils or mixtures of these are used. Such an oil should have the proper viscosity for the purpose, should be free from acidity, should produce the minimum of gumming under continued use, and, if to be used as a lubricant for cylinders of internal com- bustion engines, it must be capable of undergoing distillation without the deposition of more than a very small percent of free carbon. This is analogous to the "fixed carbon" of coal. In many cases specifications provide against the presence of more than small amounts of animal or vegetable oils or even against any quantity, because of the gumming action that occurs by oxidation and because of the development of acids through par- tial hydrolysis of the oil. Viscosity. Viscosity is usually expressed either as a specific property with the viscosity of water considered as unity, or in terms of an arbitrary scale of one of the standard instruments. The exact determination of viscosity is a difficult process. For ANALYSIS OF INDUSTRIAL PRODUCTS 299 commercial purposes an approximate determination is all that is necessary. The various instruments that are used for the determination of viscosity of oils do not give the same results but when the arbitrary scale of a given instrument is used, com- parative results are obtained for different oils. The Engler FIG. 87. Engler's viscosimeter. viscosimeter 1 is illustrated in Fig. 87. The principle used in this and many other viscosimeters is that of measuring the time required for a given quantity of oil to flow through a standard orifice. Determination. Pour the oil into the inner cup until the points marking the required level are reached. Fill the outer cup with water 1 Z. angew. Chem., (1892) 725; J. Soc. Chem. Ind., 12, 291 (1893). 300 QUANTITATIVE ANALYSIS to the mark on the inside and heat by the ring burner until both water and oil are at the desired temperature. A wooden plug closes the gold- lined orifice in the bottom of the oil cup. When this is lifted note the time on a stop watch and allow the oil to flow out until 100 cc or 200 cc is measured in the graduated flask, noting the time when the gradua- tion is reached. The viscosity is the number of seconds required for 200 cc of oil to flow out. The instru- ment is standardized by measuring the time necessary for 200 cc of water to flow at 20. This should be 50 to 52 seconds. The specific viscosity is the ratio of the time required for the oil to that for water at the same temperature. Specific Gravity. Determine as with burning oils unless the viscosity is too high to permit the use of either of these methods. In the latter case a picnometer is to be used or the viscosity is determined at higher temperatures. The special hydrometer designed by Sommer 1 may also be used for the deter- mination of the specific gravity of 'highly viscous oils. This is illustrated in Fig. 88. The brass cup has a capacity of exactly 10 cc. It is filled with the oil, the cap is screwed on and the cup is then suspended from the hydrometer float, which is placed in pure water at 20. The specific gravity is read on the stem of the float, at the position of the meniscus. Acidity. Shake a small amount of oil in a test-tube with warm water and test the water with litmus. If acidity is shown a weighed sample of oil is shaken with alcohol and the acids titrated with a standard alcoholic solution of a base which is preferably decinormal potassium hydroxide. Fixed Carbon. Weigh a dry retort or distilling flask having a capacity of 100 cc and weigh into it 40 to 50 gm of oil. Close with a cork and distill over a flame at the rate of 4 to 5 drops per minute. Near the end of the process the upper part of the apparatus is also heated until 1 J. Ind. Eng. Chem., 2, 181 (1910). FIG. 88. Sommer's hy- drometer for asphalt and vis- cous oils. ANALYSIS OF INDUSTRIAL PRODUCTS 301 the last drops have been distilled. Heat over a strong flame to a point just short of the melting-point of the glass but do not burn the carbon residue. Cool and weigh. Cylinder oils for gas or gasoline engines should not leave more than 0.50 percent of fixed carbon and the better oils will leave less than 0.20 percent. Separation of Saponifiable from Mineral Oils. The method of separation depends upon the difference in chemical nature between mineral oils and those of animal or vegetable origin. The former are mostly hydrocarbons while the latter are esters derived from glycerine and small quantities of other higher alco- hols with fatty acids. The esters are saponifiable by bases and the resulting soaps are soluble in water while the unsaponified mineral oils easily dissolve in petroleum ether. Determination. Weigh a 100 cc Erlenmeyer flask, add about 10 gm of the oil and weigh again. Add 50 cc of an approximately half- normal solution of potassium hydroxide in alcohol, place in the neck of the flask a funnel having a stem not more than 5 cm long and warm on the water bath for 30 minutes. Remove the funnel and evaporate, frequently blowing out the vapor, until the odor of alcohol disappears. The evaporation of alcohol may be hastened by inserting a glass tube in the flask so that the end is four or five centimeters above the liquid, attaching a pump and drawing air through the flask. The tube must be slanted downward outside the flask in order to prevent condensed alcohol from returning to the flask. Cool, add 50 cc of petroleum ether, stir thoroughly with the soap and rinse into a separatory funnel with petroleum ether, disregarding any soap that may adhere to the flask. Add to the ethereal solution in the separatory funnel an equal volume of water, shake and allow to completely separate. The water will dissolve the soap that was produced from animal or vegetable oils while the petroleum ether containing the mineral oil will form the upper layer. Separate and discard the water solution and then rinse the ethereal solution into the flask in which saponification was accomplished, having previously washed and dried the flask. Evapo- rate the petroleum ether by placing the flask in a water bath from which the flame has been removed. The evaporation may be hastened by the same device as was used in evaporating alcohol from the soap. After all ethereal odor has disappeared the flask is cooled and weighed. This gives directly the percent of mineral oil in the sample, and this percent subtracted from 100 gives the percent of saponifiable oil. The method gives somewhat high results for saponifiable oils because some loss of mineral oil accurs during the extraction of the soap. 302 QUANTITATIVE ANALYSIS Chill Test. -The chill test is the determination of the tempera- ture at which turbidity appears because of the formation of crystals. This is the temperature at which the oil would tend to clog oil holes in bearings. It has little significance except where saponifiable oils are present because mineral oils do not crystallize upon cooling. Determination. A 4-ounce bottle having a wide mouth is half filled with the oil and a thermometer placed in it. The bottle is placed in a freezing mixture and stirred continuously with the thermom'eter. When the liquid ceases to be perfectly clear the temperature is noted as the " chill test." Cold Test. This is the determination of the temperature at which the oil ceases to flow freely and at which it will therefore fail to be delivered to bearings from the oil cup. From a con- sideration of the composite nature of all oils it will be seen that both chill and cold tests can give results which are only approxi- mately constant. They are of service, however, in forming a basis for judging the fitness of oils for use within known temperature ranges. Determination. The bottle containing the oil that was used in the chill test is placed in the freezing mixture and cooled until the oil becomes solid. It is then removed and allowed to warm by contact with the air, being stirred with the thermometer meanwhile. At intervals of two degrees rise in temperature the bottle is inverted. When the oil has become sufficiently fluid to flow from one end of the bottle to the other the temperature is noted as the "cold test." EDIBLE FATS AND OILS For an interesting discussion of the fat and oil industries reference may be made to a recent address by Lewkowitsch. 1 Composition. The chief constituents of animal and vegetable oils are esters derived from fatty acids and the triatomic alcohol, glycerine. Of the former the most important are palmitic, stearic and oleic acids, the first two being saturated acids, the last an unsaturated acid. The glycerides of these acids are re- spectively known as palmitin, stearin and olein and they have the following composition: i Bull. soc. chim., [4] 5, 1 (1909); Am. Chem. J., 43, 428 (1910). ANALYSIS OF INDUSTRIAL PRODUCTS 303 Palmitin Stearin Olein In addition to these are esters of higher alcohols other than glycerine and of other saturated and unsaturated fatty acids, also in certain cases small amounts of free higher alcohols. The chief differences in properties of different oils are caused by varia- tions in the proportions of the constituent esters. Vegetable oils contain much palmitin while stearin predominates in animal oils. The more liquid oils contain more olein and esters of acids having smaller molecular weights. The true waxes differ chemically from the oils and fats in that they are not glycerides but are esters of mono- or diatomic alcohols with the higher fatty acids. These alcohols are either aliphatic or aromatic. Some examples of such esters are as follows : Cetyl palmitate, derived from palmitic acid and cetyl alcohol, Ci 6 H 33 OH; this is the chief constituent of spermaceti. Ceryl palmitate, the chief constituent of opium wax, is derived from palmitic acid and ceryl alcohol, C27H 55 OH. Myricyl palmitate occurs in beeswax. It is an ester of palmitic acid and myricyl alcohol, C 30 H 6 iOH. Ceryl cerotate is the chief consitu- ent of Chinese wax. It is an ester of cerotic acid, C 2 4H 49 COOH, and ceryl alcohol. The most important aromatic alcohols occur- ring in waxes are the isomeric alcohols cholesterol and phytos- terol, C 26 H 43 OH. These are found as esters of palmitic, stearic and oleic acids. Notwithstanding the differences in composition the task of separating and determining the percent of different oils in a mixture is a difficult and often impossible one, because of the fact that the same general compounds constitute the greater proportion of all fats and oils. The chemist must usually be satisfied if he can recognize single oils or, with the nature of a single oil known, determine the approximate extent and nature of adulteration. The differences in molecular weight and degree of saturation, the presence and percent of free alcohols or acids and the occasional occurrence of traces of unusual substances, characteristic of certain oils, constitute the bases of the tests used in the effort to identify an oil. The examination becomes there- fore not an analysis, in the usual sense, but a series of tests applied in order to gain information regarding the identity of a pure oil and, so far as is possible, the composition of a mixture. Certain 304 QUANTITATIVE ANALYSIS physical and chemical " constants" are determined and compared with the constants obtained from oils of known purity. The chief obstacle to the use of such figures lies in the fact that, for a given kind of oil they are actually variable within certain limits. These limits may be very narrow, but since they do include a certain range it sometimes happens that the ranges for two or FIG. 89. Abb6's refractometer. more oils overlap. Thus olive oil from Italy is not chemically identical with olive oil from California. The soil, climate, variety of plant and method 'Of expressing from the olive have their influence upon the properties of the various glycerides and other substances present in the oil. It is only when the ranges of variation do not overlap that it is easy to determine the identity ANALYSIS OF INDUSTRIAL PRODUCTS 305 of a single oil, although it often happens that while overlapping occurs with a single constant it does not occur with others. The significance of the various constants and their methods of determination will be described. Specific Gravity. This subject has already been discussed in connection with burning and lubricating oils and the directions there given will apply. Index of Refraction. This is a valu- able indication in many cases and often serves as a means of identification of oils. The index of refraction is best determined at 25 for oils and at 40 for fats by the use of any of the standard instruments, such as the Abbe, Pul- f rich, Zeiss butyro-ref ractometer or the immersion refractometer. Of those named the Abbe refractometer is prob- ably the most generally useful instru- ment for the laboratory because it may be used with either solids or liquids covering a wide range of refractive in- dices and because it does not require the use of monochromatic light. This instrument is shown in Fig. 89. A layer of the oil is enclosed between two prisms in such a manner that light rays enter it at an angle different from the normal, refraction resulting (Fig. 90). The instrument measures the angle of total reflection of the emerg- ing ray, the field being a divided light and dark one. Dispersion is corrected by a " compensator" consisting of two similar Amici prisms, of direct vision for the D-line and rotated simultane- ously, though in opposite directions, around the axis of the telescope by means of the screw head. In this process of rotation the dispersion of the com- pensator passes through every value from zero (when the refracting edges of the two prisms are parallel and on differ- 20 FIG. 90. Section of prisms, objective and field of the Abbe refractometer. 306 QUANTITATIVE ANALYSIS ent sides of the optical axis) to double the amount of dispersion of a single Amici prism (the refracting edges being parallel and on the same side of the optical axis). The dispersion produced by the oil in the refractometer may thus be annulled by rotating the screw head of the compensator until the latter produces a dispersion equal to that of the oil but in the opposite direction. The border line between light and dark fields then becomes sharp and distinct, even when white light is used for illum- ination of the refractometer prisms. The scale is graduated to read directly the index of re- fraction. The prisms are en- closed in such a manner that water at any desired temper- ature may be circulated about them. The heating arrange- ment for the water is a special feature of the Zeiss instru- ments. This is shown in Fig. 91. Water is caused to pass through the heating spiral and the refractometer under a constant pressure, the temperature being con- trolled by regulating the size of the burner flame and the rate of flow through the clamp C. Since the pressure of water in the laboratory mains is not constant the pressure is made independent by fixing the upper and lower levels by means of the overflow tubes in the vessels A and B. The Zeiss " butyro-ref ractometer " is an instrument which uses the same arrangement of prisms as that of the Abbe instru- ment. It is made especially for use in the examination of butter and has a purely arbitrary scale. Readings of the butyro-ref rac- tometer can be converted into indices of refraction by use of a table furnished with the instrument. The chief disadvantage of FIG. 91. Zeiss' apparatus for heating refractometer prisms. ANALYSIS OF INDUSTRIAL PRODUCTS 307 this instrument is the absence of the compensator. The prisms are achromatized for pure butter and give no dispersion of white light when this fat is used. For all other liquids the line of division between the light and dark fields is indistinct and con- sists of a prismatic series of colors unless monochromatic light is used. Determination by Means of the Abbe* Refractometer. Set up the refractometer in front of a window or a source of sodium light. Con- nect the heating apparatus as shown in the figure and adjust the flow of water and the height of the flame until the desired temperature is attained. Open the prism so that the lower half is in a horizontal position and place two or three drops of oil or melted fat upon it, using a glass rod or pipette but avoiding scratching the prisms. Quickly close and lock the prisms, allow time for the temperature to become constant then adjust the compensator until the line of division of the field is sharply defined and bring this line to the cross hairs. Read the index of refraction upon the scale. Clean the prisms by applying a mixture of equal volumes of alcohol and ether, using a tuft of absorbent cotton. (Ordinary cotton may con- tain grit.) Iodine Absorption Number. The iodine absorption number is the percent of iodine absorbed by the fat or oil when subjected to the action of an iodine solution under certain specified con- ditions. Halogens are absorbed by oils because of the presence of glycerides of unsaturated acids, which form halogen addition products. The most important of such acids is oleic acid, Ci8H 34 O2. Other unsaturated acids occurring combined as esters are elaidic acid, CjgHs^, isomeric with oleic acid, erucic acid, C 2 2H 42 02, ricinoleic acid, CigH^Os, linolic acid, Ci8H 32 O2, linolenic acid and isolenolenic acid, Ci 8 H 30 02. The last four acids are found in rather large quantities as glycerides in linseed oil, an oil of pronounced " dry ing" properties. Unsaturated acids also slowly absorb oxygen, forming oxygen addition prod- ucts which are generally resinous in their nature. This is the action known as "drying," although the term is here misapplied since no real drying occurs. Iodine (or other halogen) absorp- tion is then a measure of the "drying" properties of an oil and is one of the most valuable properties for characterizing the dif- 308 QUANTITATIVE ANALYSIS ferent oils. Oils in general are, for purposes of description, divided into " drying," " semi-drying " and " non-dry ing," ac- cording to whether the iodine number is high, medium or low. There is no sharp distinction between these classes and there is no animal or vegetable oil known that does not contain at least a small proportion of halogen-absorbing constituents. Free iodine acts but slowly or not at all upon most oils. The most active solutions contain an unstable compound of iodine with either chlorine or bromine, this instability being the cause of the activity as an oxidizing agent. Many of the methods for determining iodine absorption num- ber have been open to the objection that they permit more or less substitution in saturated compounds as well as addition to unsaturated compounds. HiibPs 1 method, formerly much used, is especially faulty in this respect. HiibFs solution is made by dissolving 26 gm of iodine in 500 cc of alcohol and 30 gm of mercuric chloride in 500 cc of alcohol, the two solutions being then mixed. The resulting solution probably contains 2 some mercuric chloriodide and iodine monochloride, the reaction being expressed as follows: HgCl 2 + 1 2 HgICl+ IC1. The latter is the active constituent of the solution but its con- centration is relatively small, which accounts for the fact that much time is required for the absorption. The oil is dissolved in chloroform and allowed to stand with a measured volume of the solution for three hours, after which the excess of iodine is titrated. Substitution takes place to a considerable extent and the amount of iodine absorbed varies with the time allowed. When hydrogen in a saturated ester is substituted by iodine, hydriodic acid is also formed. In the case of palmitin: By determining the amount of hydriodic acid so formed the amount of substitution may be determined but it is better to 1 Dingl. polyt. J., 263, 281 (1884); J. Soc. Chem. Ind., 3, 641 (1884). 2 Ephriam: Z. angew. Chem., (1895), 254. ANALYSIS OF INDUSTRIAL PRODUCTS 309 use one of the solutions suggested by Hanus and Wijs because these cause very little substitution. Hanus' 1 solution is made by dissolving iodine in glacial acetic acid and adding an equivalent weight of bromine. The active constituent is iodine monobromide, IBr. The oil is dissolved in chloroform. Wijs' 2 solution contains iodine monochloride, IC1, and is made by adding an equivalent amount of chlorine to a solution of iodine in glacial acetic acid. Either chloroform or carbon tetrachloride is used as the solvent for the oil. Both Hanus' and Wijs' solutions are more active than that of Hiibl, the absorption being completed in thirty minutes. The solutions are also more stable and need not be so frequently restandardized. The amount of substitution taking place is also much less and is practically zero with many oils. The solu- tion of Hanus is more conveniently made than that of Wijs and the method of Hanus will therefore be described. 3 Determination. Dissolve 13.2 gm of powdered iodine in 1000 cc of glacial acetic acid. The acetic acid should contain no substances capa- ble of reducing an acid solution of potassium dichromate. Titrate the iodine with a sodium thiosulphate solution and add an equivalent quantity of bromine to the cold solution, titrating by sodium thiosul- phate to determine the percent. Keep in a glass-stoppered bottle and out of direct sunlight. Prepare starch paste, also prepare and standardize a decinormal solution of sodium thiosulphate according to the directions given on page 227. Half fill a 20 cc weighing bottle with oil, place in it a piece of glass rod and weigh without the stopper. Carefully pour about 0.25 gm of the oil into a 500 cc bottle having a ground glass stopper, using the glass rod to assist in the transference. Reweigh and prepare two more samples in the same manner. Dissolve the weighed sample of oil in 10 cc of chloroform then add 25 cc of iodine monobromide solution, measuring from a burette. Stop- per the bottle, mix and allow to stand for thirty minutes, shaking occasionally. The bottle should not be left in strong light. At the time that the iodine monobromide solution is measured into 1 Z. nahr. Genussm., 4, 913 (1901). 2 Ber., 31, 750 (1898). 3 See a comparison of methods by Tolman and Munson: J. Am. Chem. Soc., 26, 244 (1903); 26, 826 (1904). 310 QUANTITATIVE ANALYSIS the oil solution, measure the same amount of solution into two bottles, containing the chloroform but no oil. Treat these in exactly the same manner as the solution containing oil. At the end of the absorption period add 10 cc of the potassium iodide solution which was used in standardizing the sodium thiosulphate solu- tion. Stopper the bottle, shake thoroughly to permit the removal of most of the iodine from the chloroform solution then add 100 cc of water, washing down any iodine that may be on the stopper. Titrate the unabsorbed iodine with standard sodium thiosulphate, shaking con- stantly. When only a faint yellow remains add 1 cc of starch solution and finish the titration. At the last the bottle should be closed and shaken until all iodine remaining in the chloroform has been extracted by the potassium iodide. The temperature should be kept as nearly constant as possible throughout the experiment. From the volume of sodium thiosulphate required for the iodine solu- tion alone subtract that required for the oil and iodine solutions. The remainder is the volume corresponding to the absorbed iodine. Calcu- late the percent of iodine absorbed. Acid Value. Fresh oils sometimes contain small amounts of free fatty acids produced during the process of extraction. Rancid fats and oils contain free acids as products of hydrolysis of the glycerides composing them. The acid value is defined as the number of milligrams of potassium hydroxide required to neutralize the free fatty acids in 1 gm of oil or fat. The determination of acid value is made for the purpose of determining the condi- tion of the oil and its fitness for a given use, rather than for the purpose of identifying it, since the acid value is a variable within rather wide limits for any oil. Determination. Weigh 20 gm of oil or fat into a 200 cc flask and add 50 cc of 95 percent alcohol which has been made neutral to phenolphthalein by a dilute solution of sodium hydroxide. Heat to the boiling-point in a water bath and agitate thoroughly. Titrate with a decinormal solu- tion of sodium or potassium hydroxide using phenolphthalein. Shake vigorously during the titration and add the standard solution until the pink color persists. Saponification (Kottstorfer) Number, Soluble Acids and Insoluble Acids (Hehner Value). The saponification number 1 is the number of milligrams of potassium hydroxide required to saponify 1 gm of oil or fat. Different oils show different saponi- 1 Z. anal. Chem., 18, 199 and 431 (1878). ANALYSIS OF INDUSTRIAL PRODUCTS 311 fication numbers because of variation in the molecular weight of the esters contained in them, those of relatively low average molecular weights requiring more base for the saponification of a given weight of oil than those of relatively higher molecular weights. The variation is, however, not as great as is the case with iodine absorption numbers and the saponification number is consequently not as valuable for use in identifying oils as is the iodine number. The comparatively small variation in saponi- fication number is due to the relatively small variation in the average molecular weight of the esters entering into the compo- sition of the oils. Of the whole list of the more common oils nearly all are chiefly composed of palmitin, stearin and olein, the molecular weight of these being 890, 806 and 884, respectively. The saponification numbers of the pure esters would be as follows: for palmitin ^ X 1000 =189, for stearin J^ X 1000 = 208, ^j z 56 and for olein ^-^ X 1000 = 190. The greatest possible variation in the proportions of these three esters could make a difference of but 19 in the saponification numbers. The occurrence of appreciable quantities of esters of lower acids in certain oils causes a much greater deviation from the numbers given above. For example, butter fat is chiefly composed of the following glycerides in the approximate proportions indicated: butyrin, CsH^C-iHyC^a, 7.0 percent; caproin, C 3 H5(C6Hii0 2 ) 3 , caprylin, CsI^CCsHuC^s and caprin, CsH^CioHigC^s, 2.3 percent; olein 37. 7 percent; palmitin, stearin and glycerides of small quantities of other acids 53.0 per- cent. The calculated saponification number of butyrin is 554, of caproin 434, of caprylin 356 and of caprin 302. The presence of these esters of small molecular weight raises the saponification number to about 227, a number which serves to distinguish butter fat from a large number of other fats, particularly from oleomar- gerine which has a saponification number of about 195. Water solutions of potassium hydroxide will act upon oils but very slowly because of the small solubility of most oils in water. Hot alcohol dissolves oils more readily and alcoholic solutions of potassium hydroxide are therefore used for the saponification. Commercial alcohol contains aldehydes which are changed by potassium hydroxide into resinous bodies, a dark red solution being produced and the basic concentration being diminished. 312 QUANTITATIVE ANALYSIS The alcohol should therefore be purified by first heating with a stick of potassium hydroxide in a flask fitted with a reflux con- denser and placed on a water bath, then distilling. The determination of the saponification number may be con- veniently combined with the determination of soluble acids and insoluble acids. Among the most important of the acids of smaller molecular weight' than oleic acid combined as glycerides are butyric, caproic, caprylic and capric acids, discussed above. These acids are soluble in water, the solubility decreasing as the molecular weight increases, so that, while butyric acid is infi- nitely soluble, capric acid dissolves to the extent of 1 part in 1000 parts of boiling water. The next acid in the series, lauric acid, is almost insoluble while the next member, myristic acid, is practically insoluble. An approximate separation of the lower acids from the higher ones may be accomplished by saponifying the oil, decomposing the resulting soap with sulphuric acid and Washing the fatty acids with water. The percent of insoluble acids is called the Hehner value. 1 Determination. Prepare a tenth-normal solution of sodium or potas- sium hydroxide in water, using phenolphthalein in standardizing. Purify two liters of alcohol by heating on a water bath for thirty minutes with about 10 gm of potassium hydroxide, using a reflux condenser. Distill and make 1000 cc of a half -normal solution of potassium hydroxide in the alcohol. The potassium hydroxide should be as nearly free from carbonate as is possible. Allow the solution to stand until the small amount of potassium carbonate that is always present has settled out, then decant into another bottle. The concentration does not remain constant for long and the solution need not be standardized until it is used for saponifying the oil. Prepare also a half-normal solution of hydrochloric acid in water. Saponification Number. Select two ordinary flasks of 250 cc capacity having, if possible, necks of slightly larger diameter at the top than at the bottom, though this feature is not essential. Clean with alcohol. Weigh into each flask about 5 gm of oil or fat, using a small bottle and glass rod as in the determination of iodine number. Add to each flask 50 cc of the alcoholic solution of potassium hydroxide from a calibrated pipette or burette, place in the neck of the flask a funnel having a short stem and warm on the water bath until the alcohol boils, though it should not be evaporated more than is necessary. The oil is saponified by the potassium hydroxide and the evidence of complete saponification 1 Z. anal. Chem., 16, 145 (1877). ANALYSIS OF INDUSTRIAL PRODUCTS 313 is the formation of a homogeneous solution so that no separation occurs when boiling is interrupted. Measure 50 cc of the alcohol solution of potassium hydroxide into each of two other flasks, for standardization. While saponification of the oils is proceeding titrate these solutions with the half-normal acid, using phenolphthalein. Cool the flasks in which the oil was saponified, add a drop of phenolphthalein and titrate the excess of base with half-normal acid, deduct the value used for 50 cc of the base in the standardization and calculate the saponification num- ber. Preserve the neutral solution for the determination of soluble and insoluble acids. Soluble Acids. Evaporate the alcohol by placing the flasks upon a water bath and drawing air through them as explained on page 301. When the odor of alcohol has entirely disappeared add enough standard acid to make the total amount present 1 cc more than the volume that is equivalent to the 50 cc of potassium hydroxide used in saponifying the oil. Connect a reflux condenser and warm on the water bath until the insoluble fatty acids have melted and separated from the water solution. Add hot water to bring the liquid within about 2 cm of the top of the neck of each flask, again allow the insoluble acids to separate, then cool in ice water. Carefully detach the cake of insoluble acids and pour the cold solution through a filter into a flask of 1000 cc capacity. Replace the cake of acids in the flask, fill with hot water, separate as before and filter. Repeat the treatment once more and do not discard the insoluble acids. The combined filtrates now contain the excess of half-normal acid (1 cc) and the soluble acids of the oil, besides potassium chloride, glycerine and other alcohols, etc. Titrate the acids with tenth-normal base in presence of phenolphthalein. From the volume of standard base used deduct the volume equivalent to 1 cc of the standard acid and cal- culate the percent of soluble acids as butyric acid. The arbitrary assumption that butyric acid is the only soluble acid present is merely a convenience. Insoluble Acids (Hehner Value}. Allow the cake of insoluble acids to dry on the filter paper for twelve hours at the temperature of the room, then transfer to a small weighed dish. Wash thoroughly with warm alcohol the paper and the flask in which saponification was accomplished, allowing the solution to run into the dish. Evaporate the alcohol on the water bath and dry to constant weight at 100. From 2 to 5 hours dry- ing will usually be required. Calculate the percent of insoluble acids. Reich ert Number and Reich ert-Meissl Number. There is no sharp line of division between the fatty acids volatile with steam and those not volatile and it is not possible to effect more than a very approximate separation by a method of distillation unless 314 QUANTITATIVE ANALYSIS this is continued for a very long time. On the other hand fairly constant proportions of acids may be distilled if the method is rigidly standardized. In this way figures may be obtained that have a certain value in identifying oils and fats. The determination is made chiefly in the examination of butter and its substitutes. Pure butter contains volatile acids to the extent of nearly 10 percent of the total fatty acids. The saturated acids that have been classed as " soluble" (to and including capric acid) are the only ones of the series that may be distilled without decomposition. They are therefore known as " volatile" acids while the higher acids (above lauric) decom- pose when distilled and are therefore called " non-volatile." Lauric acid distills with steam but is slightly decomposed. Al- though the volatile acids boil at temperatures higher than 100 they can be distilled with steam. The boiling points of the more commonly occurring " volatile" acids are as follows: Acid Boiling point, degrees Butyric 162.3 Caproic 200 Caprylic Capric 236 270 The method proposed by Reichert 1 and the modifications of ihis method by Meissl 2 have been extensively adopted. It should be understood that neither method gives the correct percent of volatile acids but simply the proportion that will be distilled under certain stated conditions. The Reichert Number is the number of cubic centimeters of tenth-normal base required to titrate the acids obtained from 2.5 gm of oil or fat by Reichert 's distillation process. The Reichert-Meissl number is the same as the Reichert number except that 5 gm of oil or fat is used. The Reichert-Meissl number is not exactly double the Reichert number. The Reichert-Meissl number of most oils, fats and waxes is less than 1. The following oils are exceptional in this respect. 1 Z. anal. Chem., 18, 68 (1879). 2 Dingl. polyt. J., 233, 229 (1879); Chem. Zentr., 10, 586 (1879). ANALYSIS OF INDUSTRIAL PRODUCTS 315 Oil or fat Reichert-Meissl number Butter fat Cocoanut Croton .".... 28 7 13 Mocaya Palmnut Porooise. . 7 5 47 Determination. -Prepare the following reagents: (a) Sodium hydroxide solution in water, 50 percent by weight. (b) Alcohol, 95 percent redistilled from sodium or potassium hydroxide. (c) Sulphuric acid, 1 part concentrated acid in 5 parts water. (d) Potassium hydroxide, approximately tenth-normal, standardized against standard acid, using phenolphthalein as indicator. If the sample is either real or imitation butter it will contain water and curd. Melt and keep at 60 until the fat has separated and, if necessary, filter the fat through a dry paper placed in a hot- water funnel. If the sample is an oil it may usually be weighed without treatment. Ordinary flasks of 200 cc capacity, are cleaned and dried. The oil or melted fat is dropped in from a weighed bottle until 5 gm, measured to within one drop, is obtained. The oil must not be left on the neck of the flask. Record the exact weight. Add 10 cc of alcohol and 2 cc of 50 percent sodium hydroxide solution, connect with a reflux condenser and heat upon the water bath until the oil is saponified. Remove the con- denser and evaporate the alcohol as in the determination of soluble acids. Add 135 cc of water and warm on the water bath until solution is com- plete, then cool. Add two or three pieces of pumice stone or about 1 gm of crushed porcelain to prevent bumping, then add 5 cc of the diluted sul- phuric acid. Again attach the reflux condenser and heat on the water bath until the acids form a clear layer. Connect the flask with a Kjeldahl or Hopkins distilling tube and a condenser and distill over a flame at such a rate that 110 cc shall be obtained in approximately thirty minutes. The distillate is received in a flask which is graduated to contain 110 cc. Mix the distillate, and filter through a dry filter to remove traces of insoluble acids carried over by the steam, receiving the filtrate in a flask graduated to contain 100 cc. Titrate 100 cc of the filtrate with standard potassium hydroxide. Make the proper correc- tion for the fact that only 100 cc of the distillate was used, also correct the number of cubic centimeters of standard potassium hydroxide used, in case this solution was not tenth-normal. The result is the Reichert-Meissl number. 316 QUANTITATIVE ANALYSIS Polenske Value. 1 One of the most important adulterants of butter is cocoanut oil, a pure white vegetable fat having a pleas- ant taste and a consistency which is about the same as that of butter. Its Reichert-Meissl number is somewhat lower than that of butter, as is shown in the table on page 315, but it may be mixed with fats having higher Reichert-Meissl numbers in such a way that it cannot be detected by a determination of this value. Polenske noticed that the volatile acids obtained from cocoanut FIG. 92. KjeldahFs distilling tube. \J FIG. 93. Hopkins' distilling tube. oil in the Reichert-Meissl distillation contained much larger quantities of acids insoluble at 15 than do the volatile acids from butter. Butyric acid comprises from 60 percent to 70 percent of the volatile acids from butter and this acid is soluble in water in all proportions. The volatile acids from cocoanut oil contain larger quantities of caproic and lauric acids, these being almost insoluble at 15. The Polenske value (called by its author the "new butter value") is the number of cubic centimeters of deci- 1 Z. Nahr. Genussm., 7, 273 (1904); J. Soc. Chem. Ind., 23, 387 (1904). ANALYSIS OF INDUSTRIAL PRODUCTS 317 normal base required to titrate the insoluble acids obtained in the Reichert-Meissl distillation. The Polenske value for pure butter varies from 1.5 to 3.0, while that for cocoanut oil varies from 16 to 18. It is necessary to avoid the use of alcohol in the saponification of the fat and therefore the determination of Reichert-Meissl number must be modified if the two determinations are to be combined. Polenske's modification is essentially as follows: Determination. Saponify 5 gm of the fat by heating in a 300-cc round flask, using a reflux condenser. For the saponification use 20 gm of glyc- erol and 2 cc of a 50 percent solution of sodium hydroxide in water. When saponification is complete dissolve the soap in 90 cc of recently boiled water and add 50 cc of dilute sulphuric acid (25 cc of concentrated acid in 1000 cc of solution) and a small amount of crushed porcelain or pum- ice. Connect with a Condenser by means of a Kjeldahl or Hopkins distilling tube and distill into a flask which is graduated at 100 cc and 110 cc; the distillation should proceed at such a rate that 110 cc passes over in 19 to 21 minutes and the water in the condenser should have a temperature of 18 to 22. When the distillate reaches the 110 cc mark on the flask replace the latter by a 25 cc cylinder and stop the distilla- tion. Immerse the flask in water at 10 and allow to remain for 10 min- utes. The level of the water must be above the 110 cc mark on the flask. Mix the contents of the flask and pass through a dry, 8 cm filter and, if desired, determine the Reichert-Meissl number, using 100 cc of the filtrate. Wash the filter three times with 15 cc of water, this water having previously been used for washing the condenser, cylinder and flask. Dissolve the insoluble acids from the condenser, cylinder and filter, using three successive portions of 90 percent alcohol and allow- ing the solution to run into the 110 cc flask. Titrate the alcoholic solution with decinormal potassium hydroxide solution, using phenol- phthalein, and calculate the Polenske value. Acetyl Value. Compounds containing a hydroxyl group will readily combine with acetic anhydride, acetic acid and an acetyl compound being produced. This takes place with an oil con- taining free higher alcohols or hydroxy-acids, the latter either in the form of esters or of free acids. The general reaction may be thus shown: RCHOHCOOH+(CH 3 CO) 2 0->RCHOCH 3 COCOOH+ CHaCOOH, ROH+(CH 3 CO) 2 ROCH 3 CO+CH 3 COOH. 318 QUANTITATIVE ANALYSIS For example lanopalmic acid forms acetolanopalmic acid: CH 3 COOH. After washing out the excess of acetic anhydride the amount absorbed may be determined by saponifying the oil with an alcohol solution of potassium hydroxide, evaporating the alcohol, adding standard sulphuric or hydrochloric acid to liberate the acetic and fatty acids and either distilling the acetic acid or washing out with water, then titrating. The reactions illustrated by the case of aceto-lanopalmitin are 3 COCO) 3 +6KOH C 3 H 5 (OH) 3 + 3Ci 5 H 30 OHCOOK+3CH 3 COOK, 2Ci 5 H 3 oOHCOOK+H 2 SO 4 2Ci 5 H 3 oOHCOOH+K 2 S0 4; 2CH 3 COOK+H 2 S0 4 CH 3 COOH+K 2 S0 4 , It should be noticed that whether the distillation or the nitration process is employed, the standard base required to finally titrate the acid will include that equivalent to acids other than acetic. That is, the distillation process will yield a distillate of acetic acid and volatile organic acids while the filtration process will yield a filtrate containing acetic acid and soluble organic acids. The close relation between soluble acids and volatile acids has already been discussed (page 314). To correct for the presence of these acids in the solution containing the acetic acid one may either subtract the volume of base used in the determination of soluble (or volatile) acids, or a different method may be used. Benedikt and Ulzer 1 proposed first saponifying the oil and then liberating the fatty acids by the addition of sulphuric acid. After washing the fatty acids they are acetylated and the excess of acetic anhydride removed. The acids are then titrated in cold alcoholic solution, under which circumstances the carboxyl alone reacts with the base. The acetylated soap is then heated with alcoholic potassium hydroxide when the acetyl radical is saponified. A titration of the excess of base gives the acetyl value. This method avoids the interference of soluble fatty acids but, as was shown by Lewkowitsch 2 it is sub- 1 Monatsh. 8, 41 (1887). 2 Proc. Chem. Soc., 6, 72 (1890). ANALYSIS OF INDUSTRIAL PRODUCTS 319 ject to another error in the fact that acetic anhydride also reacts, to a small extent, with non-hydroxylated fatty acids forming acetic acid and fatty acid anhydride: 2Ci 5 H,iCOOH+(CH,CO) ? (Ci 5 H 3 iCO) 2 O+2CH 3 COOH. Palmitic acid Acetic anhydride Palmitic anhydride Acetic acid In cold alcoholic solution these anhydrides are not at once saponi- fied, part remaining until the treatment with hot potassium hydroxide solution, being then saponified and giving rise to a positive error in the calculation of acetyl value. The most desirable method is to acetylate the oil, wash free from acetic acid, saponify, liberate the fatty acids and acetic acid from the soap and then either distill or filter, titrating the acids of the distillate or filtrate and making the proper correction for volatile or soluble acids. The " acetyl value" is defined to be the number of milligrams of potassium hydroxide required to combine with the acetic acid liberated from 1 gm of acetylate d fat or oil. Certain oils are characterized by unusually high acetyl values. Castor oil is the most noteworthy of these, having a value of about 150. Another class of oils having high acetyl values is composed of " blown" or " oxidized" oils. By blowing air through oils at somewhat elevated tem- peratures (70 to 115) the viscosity and specific gravity are considerably increased and they become suitable for use as lubricating oils. The chemical changes that take place are not thoroughly understood but oxidation is known to occur. This is partly due to combination with unsaturated acids (evidenced by a diminished iodine absorption number) and partly to the forma- tion of hydroxyl radicals from hydrogen. The latter change results in an increased acetyl value and this may even reach a number as great as that for castor oil. The large variation in acetyl values recorded in the table on page 320, adapted from a similar table by Lewkowitsch 1 , will indi- cate the value of this determination for the identification of certain oils and fats. The method here described for the determination of acetyl value is essentially that of Lewkowitsch 2 and adopted as a pro- visional method by the Association of Official Agricultural 1 J. Soc. Chem. Ind., 16, 503 (1897). 2 LOG. cit. 320 QUANTITATIVE ANALYSIS Oil or fat Acetyl value (average) Butter fat . Castor 149 5 Colza 16 6 Cotton seed. ....... 21.5 Croton Fish 19.9 41 Linseed 6 9 Maize Olive iShark liver 8.2 13.5 17.8 Chemists. 1 It involves the acetylation of the oil before saponi- fication and includes the soluble or volatile fatty acids, if calcu- lated according to the " official" method. Much confusion would be avoided if the true acetyl value were recorded instead of this acetyl-soluble acid value. In the following exercises the true acetyl value will be calculated. Determination. Place about 20 gm, approximately weighed, of oil or fat in a 100 cc flask, add an equal volume of acetic anhydride, con- nect with a reflux condenser and boil gently for two hours. Cool and pour into 500 cc of water contained in a beaker. Pass a current of car- bon dioxide into the beaker through a fine orifice of a glass tube and boil for 30 minutes. At the end of this time siphon out the water layer and repeat the treatment with water and boiling until the water is no longer acid, as shown by a litmus test. Separate the acetylated oil in a separa- tory funnel, filter in a drying oven and dry. Weigh accurately 2 to 4 gm of the acetylated oil into a flask and saponify according to the method used in determining the saponification number, measuring the alcohol solution of potassium hydroxide accu- rately and running blank determinations for standardization. Evap- orate the alcohol and dissolve the soap in water. Add standard sul- phuric acid in a quantity exactly equivalent to the potassium hydroxide added, warm to melt the fatty acids and filter through a wet paper. Wash with boiling water until the washings are no longer acid, testing with litmus paper by barely touching a corner to the bottom of the funnel. The combined filtrate and washings are titrated with tenth- normal base. Subtract the volume of base already found to be equiva- lent to soluble acids and calculate the true acetyl value according to the definition of this number. U. S. Dept. of Agr., Chem. Bull. 107, 142. ANALYSIS OF INDUSTRIAL PRODUCTS 321 Maumene Number and Specific Temperature Reaction. All oils and fats react with concentrated sulphuric acid, heat being evolved. The reactions are complex and cannot be expressed by a simple equation but oxidation occurs to a considerable degree. The heat evolution varies with different oils and is, to some extent, characteristic. The Maumene number 1 is the number of centigrade degrees rise in temperature caused by mixing 10 cc of concentrated sulphuric acid with 50 gm of oil. A small variation in the proportion of water in the acid causes a con- siderable variation in the heat evolved and to this extent the figures recorded by different investigators are not comparable because " concentrated sulphuric acid," as obtained commer- cially, is not a substance with any definite percent of water. In order to eliminate the errors due to variation in water a determination may be 'made, using the same amount of acid but substituting 50 gm of water for the oil. The ratio Rise in temperature with oil Rise in temperature with water is known as the " specific temperature reaction." 2 That this number is not subject to variation as is the Maumene number is shown by the following table in which the specific temperature reaction is multiplied by 100. Kind of oil Sulphuric acid of 95.4 percent Sulphuric acid of 96.8 percent Sulphuric acid of 99 percent Maumene' No. Sp. temp, reaction Maumene No. Sp. temp, reaction Maumen6 No. Sp. temp, reaction Olive Rape Castor Linseed. . . . 36.5 49 34 104.5 95 127 88 270 39.4 95 89 44.8 58 125.2 96 124 269 37 Water 38.6 100 41.4 100 46.5 100 Determination. Place a beaker, about 5X1.5 inches, inside one that is about 6X3 inches and pack the open space between with wool, asbestos or cotton Cover the beakers with a piece of cardboard through which passes a thermometer. Weigh into the inner heaker 50 gm of oil. Bring concentrated sulphuric acid to the same temperature as that 1 Compt. rend., 35, 572 (1852). 2 Thomson and Ballantyne: J. Soc. Chem. Ind., 10, 233 (1891). 21 322 QUANTITATIVE ANALYSIS of the oil and then add, under a hood, 10 cc of this acid, stirring thoroughly with the thermometer. When the acid is all in, place the thermometer in the center of the oil-acid mixture and note the highest point attained by the mercury. The total rise in temperature is the Maumene* number. Determine also the specific temperature reaction as follows: Clean the inner beaker and introduce 50 cc of water. Add 10 gm of acid as before and note the rise in temperature. The Maumene" number divided by this rise is the specific temperature reaction. The drying oils often develop so much heat that active foaming results. Such oils should be first diluted with petroleum oils or olive oil and the proper correction made in the temperature rise. Qualitative Reactions. If simple and reliable qualitative tests were known for all of the oils, it is not likely that the work outlined in the preceding pages would often be carried out. It has already been explained that comparatively few such tests are known because of the similarity in the composition of the various animal and vegetable oils. Aside from the mere varia- tion in the proportion of the various glycerides, free alcohols and free acids, there are certain constituents of certain oils that will give color reactions which are characteristic. A few of those that are reliable will be described. In most cases these tests should accompany the determination of the analytical constants, rather than be substituted for them. Resin Oil. Polarize the oil in a 200-mm tube. If the oil is too dark in color for this purpose it may be diluted with petroleum ether and the proper correction made in the reading. Resin oil has a polarization in a 200-mm tube of from +30 to +40 on the sugar scale while other oils read between +1 and 1. Cotton Seed Oil: Halphen Test. 1 Mix carbon disulphide containing about 1 percent of sulphur in solution, with an equal volume of amyl alcohol. Mix equal volumes of this reagent and the oil and heat in a bath of boiling, saturated solution of sodium chloride for 1 to 2 hours. In the presence of as little as 1 percent of cotton seed oil a character- istic red color is produced. Lard and lard oil from animals fed on cotton seed meal will give a faint reaction for cotton seed oil. The unknown constituent which gives the color apparently is assimilated by the animal without change. A negative result does not prove the absence of cotton seed oil because heating the oil for 10 minutes renders it incapable of giving the color. i J. pharm. Chim., [6], 6, 390 (1897). ANALYSIS OF INDUSTRIAL PRODUCTS 323 Cotton Seed Oil: Bechi Test. 1 Dissolve 2 gm of silver nitrate in 200 cc of 95 percent alcohol and add 40 cc of ether and 1 drop of nitric acid. Mix 10 cc of the oil or melted fat, 5 cc of reagent and 10 cc of amyl al- cohol in a test-tube. Divide and heat one portion in a boiling water bath for 10 minutes and compare with the portion not heated. Any blackening due to reduced silver shows the presence of cotton seed oil. Other oils which have become rancid and lards which have been steamed or heated at high temperatures contain certain decomposition products which reduce silver nitrate. Some salad oils which do not respond to the Halphen test give a brown coloration with the Bechi reagent and in some cases reduce silver. Such oils should be purified before testing by heating 20 to 30 gm of the sample on a water bath for a few minutes with 25 cc of 95 percent alcohol. Shake thoroughly, decant as much of the alcohol as possible, wash with 2 percent nitric acid -and finally with water. Arachis (Peanut) Oil : Renard 2 -Tolman* Test. Weigh 20 gm of the oil into a 250-cc Erlenmeyer flask. Saponify with a solution of potas- sium hydroxide in alcohol as directed in the discussion of the determina- tion of saponification number. Add a drop of phenol phthalein and exactly neutralize with 5 percent acetic acid and wash into a 500-cc flask containing a boiling mixture of 100 cc of water and 120 cc of a 20 percent lead acetate solution. Boil for a minute and then cool the precipitated lead soap by immersing the flask in water, occasionally giving it a whirl to cause^the soap to stick to the sides of the flask. After the flask has cooled, the water and excess of lead acetate can be poured off and the soap washed with cold water and with 90 percent (by volume) alcohol. Add 200 cc of ether, cork and allow to stand for some time until the soap is disintegrated; heat on the water bath, using a reflux condenser, and boil for about five minutes. In the oils most of the soap will be dissolved, while in lards which contain much stearin, part will be left undissolved. Cool the ether solution of soap to from 15 to 17 and let stand until all the insoluble soaps have crys- tallized out (about twelve hours). Filter and thoroughly wash the precipitate with ether. Wash the soap on the filter back into the flask by means of a stream of hot water acidified with hydrochloric acid. Add an excess of dilute hydrochloric acid, partially fill the flask with hot water, and heat until the fatty acids form a clear oily layer. Fill the flask with hot water, allow the fatty acids to harden and separate from the precipitated lead chloride, wash, 1 Chem. Ztg., 11, 1328. See also Tolman: J. Am. Chem. Soc., 24, 396 (1902); Gill and Dennison: Ibid., 24, 397 (1902). 2 Z. anal, chem., 12, 231 (1871). 3 U. S. Dept. of Agr., Chem. Bull., 107, 145. 324 QUANTITATIVE ANALYSIS drain, repeat washing with hot water, and dissolve the fatty acids in 100 cc of boiling 90 percent (by volume) alcohol. Cool to 15, shaking thor- oughly to aid crystallization of the fatty acids. From 5 to 10 percent of peanut oil can be detected by this method, as it effects a complete separation of the soluble acids from the insoluble, which interfere with the crystallization of the arachidic acid. Filter, wash the precipitate twice with 10 cc of 90 percent alcohol, and then with alcohol 70 percent by volume. Dissolve off the filter with boiling absolute alcohol, evaporate to dryness in a weighed dish, dry and weigh. Add to this weight 0.0025 gm for each 10 cc of 90 percent alcohol used in the crystallization and washing if done at 15; if done at 20 add 0.0045 gm for each 10 cc. The melting-point of arachidic acid thus obtained is between 71 and 72. Twenty times the weight of arachidic acid will give the approximate amount of peanut oil present. No examination for adulterants in olive oil is complete without making the test for pea- nut oil. Arachidic acid has a characteristic structure and can be de- tected by the microscope. Sesame Oil: Baudouin Test. 1 Dissolve 0.1 gm of finely powdered sugar in 10 cc of hydrochloric acid (sp. gr. 1.20), add 20 cc of the oil to be tested, shake thoroughly for a minute, and allow to stand. The aqueous solution separates almost at once. In the presence of even a very small admixture of sesame oil this is colored crimson. Some olive oils give a slight pink coloration with this reagent, but they are not hard to distinguish if comparative tests with sesame oil are made. The color was thought by Villavecchia to be due to a reaction of a constituent of sesame oil with furfurol, the latter being produced by the interaction of sugar with hydrochloric acid. Furfurol was accordingly substituted for sugar and hydrochloric acid and the method somewhat modified as follows: Sesame Oil: Villavecchia Test* Add 2 gm of furfurol to 100 cc of alcohol (95 percent) and mix thoroughly 0.1 cc of this solution, 10 cc of hydrochloric acid (sp. gr. 1.20) and 10 cc of oil by shaking them together in a test-tube. The same color is developed as when sugar is used, as in the Baudouin test. Villavecchia explained this reaction on the basis that furfurol is formed by the action of levulose and hydro- chloric acid and he therefore substituted furfurol for sucrose. As fur- furol gives a violet tint with hydrochloric acid it is necessary to use the very dilute solution specified in this method. Solubility in Absolute Alcohol. Castor oil, croton oil and olive kernel oil dissolve easily in cold alcohol. Most other oils !U. S. Dept. of Agr., Chem. Bull. 107, 146. 2 Z. angew. Chem., 17, 505 (1893), ANALYSIS OF INDUSTRIAL PRODUCTS 325 dissolve to a limited extent and the determination of solubility will often serve as an important adjunct to the chemical examina- tion. Girard 1 gives the following statement of solubilities in absolute alcohol at 15. Oil Gm soluble in 1000 gm alcohol Oil Gm soluble in 1000 gm alcohol Rape 15 Walnut 44 Colza, 20 Beechnut 44 Mustard seed 27 Poppy seed 47 Hazelnut 33 Hemp seed 53 Olive 36 Cotton seed 64 Almond 39 Arachis 66 Sesame . ... 41 Linseed 70 Anricot kernel. . 43 Cameline. . 78 Castor oil might be confused with croton oil or olive kernel oil on account of its large solubility in alcohol. It is distinguished from these by its insolubility in petroleum ether, which dissolves all other oils. Solubility of Acetic Acid. The determination of solubility of acetic acid in the oils has been applied by Jean 2 as follows: Place 3 cc of the oil in a graduated tube 1 cm in diameter and im- merse the tube in water at a temperature of 50. Remove, by means of a pipette, so much oil that exactly 3 cc remains, measured at this temperature. Add exactly 3 cc of acetic acid (sp. gr. 1.0565 at 15), measured from a pipette or burette at 22, warm the tube in the water for a few minutes, then stopper and mix thoroughly. Replace the tube in the bath, allow complete separation and then read the volume of un- dissolved acetic acid, calculating the percent dissolved in the oil. This is a modification of Valenta's test, 3 which is a measure of the solubility of the oil in glacial acetic acid. Jean's table of .solubility is given on page 326. Examination of an Oil or Fat Whose Identity is Unknown. For the purpose of identifying an oil or fat of unknown character the complete chemical and physical examination is made unless its identity can be determined unmistakably by a qualitative test. With all of the constants determined a comparison is then made with all available data contained in published analyses of 1 Mon. sci., 34, 937 (1889). 2 Lewkowitsch: Chem. Anal, of Oils, Fats and Waxes, 273. 3 Dingl. polyt. J., 253, 418 (1884); J. Soc. Chem. Ind., 3, 643 (1884). 326 QUANTITATIVE ANALYSIS Oil or fat Percent acetic acid dissolved Oil or fat Percent acetic acid dissolved. Arachis (Boulam) 41.65 Poppy seed (France) . . 43.3 Arachis (Gambia) .... Colza 43.66 30.00 Neat's foot Sheep's foot 43.3 36.66 33 30 Horse fat 30 08 33 00 Lard 26.66 Olive 35 00 Veal tallow 26.66 Walnut 36.60 Butter 63.33 Cameline 36.60 Cotton seed stearin . . . 40.00 Castor 100 00 Buttprine . . . 31.60 100 70 iVlargerine 26.66 53 3 Palm 100.00 Poppy seed (Indies) . . . 63.3 Cocoanut . 100.00 oils and fats and a reasonable agreement with such data will generally fix the identity of the unknown oil. In making com- parisons the most important figure is the iodine number because this will serve to classify the oil at once as a drying, semi-drying, or non-drying oil, the approximate ranges for these somewhat arbitrary divisions being as follows: Oils Iodine Number 200 and higher to 120 120 to 95 95 to 70 and lower Drying Semi-drying Non-drying The choice will, by this means, be narrowed down to a limited list of oils or fats. The remaining constants are then considered, one by one, and each comparison will narrow the choice still further. At the last all available qualitative tests are made in order to confirm the results of comparative tests or to aid in making a final decision. It sometimes happens that the figures obtained in the examination of the unknown oil will not all correspond, even to a reasonable degree, with the recorded data for any of the common oils. This may be the result of (1) errors in the determinations, (2) adulteration, or (3) a real abnor- mality of the oil which is being examined. The first case should be at once excluded by repeating the determination of constants in which lack of agreement is observed. A careful inspection of all data may serve to indicate certain oils which, by addition to one that most nearly resembles the oil under examination, would change the " constants" in the manner observed. The matter of commercial values should also be considered in this connection, since any commercial material is adulterated, if at all, by a cheaper material. ANALYSIS OF INDUSTRIAL PRODUCTS 327 a M H i . cq Jj 00 05 a ; ; ; ; ; i? 11 :S :o i|^ :7 rH 1 05 O CD T* 05 00 CO IO 1 co | i 10 - LO T* | '. CO 1 i 1 fc : 8 : i :S i i TH rH LO 00 > a Vegetable dryii cu ! *H * ! a? . 00 O5 CO OO t-o j : : : : : J-) O -CO " (N Reichert- Meissl 00 **! 00 O 00 rH CO II- II (N t^ . 10 >0 d co ^ o O T*( (N 10 00 CO rH rH U} O5 CO O CO O (N (N O - O CO ? " CO CN CO d r-i d d Tj* 10 o - | : : :3 : : 10 : O Tj< >0 1 d co 1> O5 ~* co d 10 T O5 O5 O5 CO 00 O5 O5 lO I | . 00 00 Saponi- ication T" | TH 00 05 rH 05 rH O 00 (NO sSSSH C^ ?J ^ rH Ol ?} ; S o ^ o oa >O CO (N oc o co co o C5 CO lO OO l> C4 O} O5 a 1 CO O O CO 1> 1C (N rH CO CN t^ r (N 00 O5 O rH CO CO rH lO rH nnnin 1> *~* 00 rH 10 : : 10 000 -00 CO^CO T*H ~~O CO 5 : : 5 O O O 10 O 00 00 00 rH Tf o o X O O (N t^ 00 1-1 O rH CO rH <** : : >O rH lO lO rH rh 00 00 CO t 7 !g 55 ^ 7 CO 00 & ^ : : 5J r-; lO rH rH lO CO rH rH rH 1> 2 rH rH rH . . rH rH rH d 10 100^0*010^0 .010U51010101010 ' >O lO O lO I s " r- rH rH ^ rH rH rH rH rH o rt cG ^ bC CO ^ CO b- lO O CM rH (M 1 OS O5 O5 rH O3 1 rH lO rH IN 1 05 | O5 O5 O5 >o o i> d d d d d <* IN d d d d co co C5 O5 O5 05 d d d d O O5 O CO >O iiiii CO O5 O OO ** 05 05 05 00 O5 d d d d d Animal oils. (Continued.) Neat's foot Porpoise (jaw) Sheep's foot Whale Vegetable fats ft >-l "1 '3 Beef tallow Butter Chicken Goose Horse Human Lard Mutton tallow Oleomargerine True waxes Beeswax Carnaiiba wax. ... Spermaceti Sperm oil Wool wax ANALYSIS OF INDUSTRIAL PRODUCTS 329 After the decision as to the identity of the oil has been made or has been limited to two or three possible oils, consult a good reference book for a complete discussion of these oils and make any additional tests that may be there suggested. For this purpose are to be recommended the descriptive part of Lewko- witsch's Chemical Analysis of Oils, Fats and Waxes, and Allen's Commercial Organic Analysis, volume 2, part 1. The figures in the tables on pages 327 and 328 are given for the purpose of comparison. They are gathered from various published analyses of the more common oils and fats. Exact agreement should not be expected. For more extensive tables consult Lewkowitsch: Chemical Analysis of Oils, Fats and Waxes. WATER The chemical examination of water may be made to determine its fitness for drinking or for industrial uses, such as steam pro- duction, laundering, textile industries, etc. It is not necessary that a complete analysis should be made for all of these purposes because not all substances occurring in water are equally impor- tant in the different applications of the water. Natural waters often contain substances that are objectionable if they are to be used industrially and these substances are, for the most part, inorganic salts and, occasionally, acids. Most of such inorganic materials are without appreciable effect upon the human system and the examination for potability is rather directed toward the detection of pollution by sewage. On this account it becomes necessary to treat the subject of water analysis in two distinct divisions. Industrial Analysis. By far the largest industrial consumption of water is for the production of steam and for this reason the chemist is more often called upon for the analysis of water to determine its fitness for steaming than for any other industrial purpose. Pure water, however desirable it may be for use in the steam boiler, is not a natural product. Water from streams and other surface origins contains mineral and organic substances derived from the surface soil as well as inorganic compounds derived from springs which feed the stream. Water from wells contains whatever mineral matter is common to the region 330 QUANTITATIVE ANALYSIS through which it has flowed. Even rain water contains organic matter and ammonia and may develop organic acids when stand- ing. Some of the compounds contained in water are com- paratively unobjectionable because their action is slight. It is to be remembered, however, that in steam boilers the tempera- ture is higher than 100 because of the increased pressure. At a pressure of 100 pounds per square inch the boiling-point of water is 164 and at 200 pounds per square inch the boiling-point is 194. At these temperatures the chemical activity of many dissolved substances is very much augmented. According to their effects upon boiler steel the constituents of natural waters may be classified as corrosives, incrustants and foam producers. Corrosives. Any soluble compound that can dissolve iron at high temperatures will give rise to pitting of the boiler, especially when the steel is not of uniform composition. Corrosives commonly occurring in water are chlorides, nitrates, and sulphates, particularly of the alkaline earth metals, and free carbonic acid. Free inorganic acids are of rare occurrence and absolutely unfit a water for steaming without preliminary treatment. A small amount of acid will cause corrosion for an indefinite period because of the ready hydrolysis of iron salts. A cycle of re- actions takes place as follows: Fe+2HCl->FeCl 2 +H 2 , 6FeCl 2 +30-4FeCl 3 +Fe 2 O 3 , FeCl 3 +3H 2 Fe(OH) 3 +3HCl. A metal chloride which is easily hydrolyzed will also produce continuous corrosion: MgCl 2 +2H 2 0- Mg(OH) 2 +2HCl, Fe+2HCl FeCl 2 +H 2 , etc. Nitrates, are equally injurious, although they seldom occur in more than small concentration. Sulphates are somewhat less corrosive and free carbonic acid still less so. Incrustants. Any substance that can be precipitated by heat- ing or evaporation of water is, in a sense, an incrustant. The steam boiler as a power producer is also a machine for continuous concentration of water solutions, since fresh, impure water is ANALYSIS OF INDUSTRIAL PRODUCTS 331 continually added and only vapor is removed. Strictly speaking only those substances which adhere to the boiler plate when they are precipitated are classed as incrustants because only these are particularly objectionable. These are carbonates of calcium and magnesium and calcium sulphate. In presence of considerable quantities of these materials certain other compounds, such as silicic acid, iron oxide and aluminium oxide, may be included with the scale and then become incrustants. Calcium and magnesium carbonates are not dissolved as such in water but are present as bicarbonates, having been dissolved from the mineral carbonates by carbonic acid. CaCO 3 +H 2 C0 3 Ca(HC0 3 ) 2 , MgC0 3 +H 2 C0 3 Mg(HC0 3 ) 2 . When the water is heated reactions which are the reverse of these take place and the normal carbonates are precipitated: Ca(HC0 3 ) 2 -CaC0 3 +H 2 + C0 2 , Mg(HC0 3 ) 2 MgC0 3 +H 2 + C0 2 . These carbonates adhere to the boiler plate, the greatest amount of precipitation occurring over the heating surface. The scale thus formed, although comparatively loose, hinders the trans- mission of heat from the steel to the water and causes local super- heating. The result is a loss of efficiency and injury to the boiler. Although these substances occur in the water as bicar- bonates, they are arbitrarily calculated as normal carbonates because the latter are precipitated when the water is heated. Calcium sulphate precipitates only when continued evaporation of the water concentrates it to the point of saturation. Precipi- tation then causes the formation of a scale that is much more serious in its effects than the scale of carbonates, because it is compact and adheres firmly to the boiler. While carbonate scale can be largely removed by occasionally blowing off the water, calcium sulphate can be loosened only by the use of hammer and chisel. On this account calcium sulphate is one of the most objectionable incrustants of all compounds found in natural waters. Foam Producers. Carbonates of sodium and potassium in- crease the surface tension of water to such an extent that the result is foaming or " priming" as steam is taken from the boiler. 332 QUANTITATIVE ANALYSIS Some of the alkali waters of the West contain large quantities of these salts. Expression of Results. The systems used in the calculation of results of water analysis were discussed in connection with the determination of hardness of water (page 198). It is convenient to work with 1000 cc of water or simple fractions of this quantity and to express results as milligrams of dissolved substance per liter of water. These figures may be changed to grains per gallon by multiplying by the factor 0.0583+. The analysis of the water solution will be made by means of methods which give metals and acid radicals as the result of separate determinations. It was at one time customary to calcu- late these as basic and acid anhydrides, as is still done in the analysis of minerals. There arises the same difficulty that is experienced in mineral analysis, viz.: that salts of hydracids cannot be expressed as oxides. A much better rule is to calculate all constituents as positive or negative radicals. There is still current among industrial chemists and engineers a custom of making a second calculation of compounds supposed to exist in the water. Most natural waters are highly dilute solutions of mineral matter. In such a solution most of the compounds are highly ionized and all possible combinations of radicals as compounds are present to some extent, no matter what compounds were originally dissolved by the water. It is evident, therefore, that any list of compounds calculated from the results of the analysis will be entirely fanciful, so far as the actual condition of the solution is concerned. The basis of such a calculation was formerly the supposed affinity possessed by the different radicals for each other. If the radicals commonly occurring in water are arranged in order of decreasing base and acid character the following series will be obtained: Positive Radicals Negative Radicals Potassium Sodium Calcium Magnesium Chloride Nitrate Sulphate Carbonate Based' upon the assumption that the combination of these radicals will follow from their relative affinities the mathematical ANALYSIS OF INDUSTRIAL PRODUCTS 333 procedure would be to calculate the maximum amount of potas- sium chloride that could be formed, taking the excess of either potassium or chlorine as combined with the next radical of oppo- site sign (either sodium or the nitrate radical) and so on, down the list. If the analysis has been accurately carried out and if all substances present have been determined the positive and negative radicals should be found in equivalent quantities, with a very slight excess of either magnesium or of the carbonate radical after the calculation is finished. This excess is the result of cumulative errors in the determination of the various radicals existing in the water and also of the occasional omission of small quantities of radicals other than those above named. Silicic acid, iron and aluminium are not included in the calcula- tion of hypothetical compounds because the colloidal nature of their hydroxides causes nearly complete, though indefinite, hy- drolysis of any salts that might originally have been present. They are therefore, according to custom, reported as oxides and this conventional method is sometimes responsible for the appear- ance of a slight excess of negative radicals in the final report. The customary method of calculating hypothetical com- pounds is not based upon scientific principles, as has already been shown. There is a certain justification for such a calculation, on account of the fact that when a water is heated and evapo- rated there will be produced the least soluble compounds of all that might be formed from the various radicals present. Through a certain coincidence, this would leave the radicals combined in about the same manner as is indicated by the conventional cal- culation. Heating in the boiler will produce the maximum possible quantities of normal carbonates of calcium and magne- sium. Which of these carbonates is least soluble at high tem- peratures is not definitely known because of difficulties encoun- tered in the determination of solubility. It is assumed, however, that if the radical of carbonic acid is not present in quantity sufficient to form carbonates with all of the calcium and magne- sium, calcium, rather than magnesium, will ultimately remain to form the sulphate as the water is evaporated. Calcium sulphate is certainly next to .the carbonates of calcium and magnesium with respect to its insolubility and it will precipitate when evaporation within the boiler concentrates it to the point of saturation. After these three compounds have been formed, 334 QUANTITATIVE ANALYSIS the method of combining the remaining radicals is quite im- material because they will not precipitate in any form, on ac- count of the large solubility of salts of the alkali metals. In order to emphasize the real basis for any calculation of compounds we shall reverse the order of radicals given on page 332 and calculate combinations in the following order: Positive Radicals Negative Radicals Magnesium Calcium Sodium Potassium Carbonate Sulphate Nitrate Chloride This will give precisely the same result as the calculations from the original order, unless there is found to be an excess of either positive or negative radicals. In this case the excess will be found to be either potassium or the chloride radical instead of magnesium or the carbonate radical, but the excess should be small enough to be insignificant in either case. The conventional method of calculating hypothetical com- pounds is illustrated in the example given below. The analysis of a ground water gave the following results : Silica Oxides of iron and aluminium . Sodium Potassium Calcium Magnesium Chloride radical Nitrate radical Sulphate radical Carbonate radical. . Milligrams per liter 5.01 3.53 6.02 5.26 75.41 24.19 4.52 0.34 32.91 160.21 Following is the calculation of compounds : = (C0 3 )"=c=Mg; 30 12.16 24.19+59.65 = 83.84 = MgCO 3 . 160.21-59.65 = 100.56 = (CO 3 )" remaining; oU remaining; ANALYSIS OF INDUSTRIAL PRODUCTS 335 100.56+67.20= 167.76 = CaCO 3 . 75.41 -67. 20 = 8.21 = Ca remaining; 48 OS 20^3 X8 ' 21 = 19 ' 69 = ( S 4 ) //=c=Ca remaining; 8.21 + 19.69 = 27.90 = CaSO 4 . 32.91 -19.69 = 13.22= (S0 4 )" remaining; -||-X6.02 = 12.56 = (SO 4)"* Na; 6.02+ 12.56 = 18.58 = Na 2 SO 4 . 13.22-12.56 = 0.66 = (SO 4 )" remaining; OQ in remanng; 0.66 + 0.53 = 1. 19 = K 2 SO 4 . 5.26 -0.53 = 4.73 = K remaining; 0.34+0.21 = 0.55 = KN0 3 . 4.73-0.21 =4.52 = K remaining; O pT A f* 00^X4.52 = 4.10 = Cr^K remaining; 4.52+4.10 = 8.62 = KCl. 4.52 4.10 = 0.42 = Cl' remaining. This excess of chlorine represents experimental errors as is explained above. Since "milligrams per liter" multiplied by 0.0583 gives "grains per gallon" the complete statement of compounds is as follows: Formula for compounds Milligrams per liter Grains per gallon SiO 2 5 01 292 3.53 0.216 KC1 KN0 3 K 2 S0 4 Na 2 SO 4 CaSO 4 8.62 0.55 1.19 18.58 27 90 0.502 0.032 0.069 1.082 1 626 CaCO 3 MeCOa.. 167.76 83.84 9.775 4.885 Determination: Industrial Analysis of Water. Measure accurately in a calibrated flask enough water to give, upon evaporation, 0.5 gm to 1 gm of residue. Add 2 cc of concentrated hydrochloric acid and evapo- 336 QUANTITATIVE ANALYSIS rate in a platinum or porcelain dish. Heat the residue at a temperature below redness until organic matter is removed. Silica or Insoluble Matter. Add 1 cc of concentrated hydrochloric acid to the residue and then add about 20 cc of hot water. Warm and stir until all soluble matter has dissolved then filter on an extracted filter paper. Wash well with hot water until the combined nitrate and wash- ings amount to about 75 cc. Fold the filter paper and burn in a weighed platinum crucible. The residue is reported as " insoluble matter" if it is white and does not weigh more than 5 mg, otherwise it may contain appreciable amounts of metal oxides or silicates. If the weight is greater than 5 mg, add to the residue a drop of sulphuric acid and then volatilize the silica by warming with 1 cc (more if necessary) of hydro- fluoric acid. Ignite the residue and weigh. Report the loss as silica. Dissolve any remaining residue in concentrated hydrochloric acid, or by heating to redness with 1 gm of potassium acid sulphate, and add to the main solution. Oxides of Iron and Aluminium. Drop into the solution a very small bit of litmus paper and carefully add dilute ammonium hydroxide until the solution is slightly basic. Boil gently to flocculate the hydroxides of iron and aluminium and to remove any unnecessary excess of ammo- nium hydroxide. Filter and wash with hot water until free from chlorides. Ignite and weigh and report as oxides of iron and aluminium. Usually iron is not present in quantity sufficiently large to make its separate determination important. If this is desired the method given on page 245 may be used and the equivalent amount of ferric oxide sub- tracted from the combined oxides, the remainder being aluminium oxide. Calcium. Add 1 cc of dilute ammonium hydroxide to the filtrate and washings from the iron and aluminium hydroxides and then precipitate and determine the calcium with the precautions mentioned in the discus- sion on page 245. Report as calcium. Add to the solution 2 cc of concentrated sulphuric acid and evaporate in a weighed platinum dish to dryness. Heat the residue carefully to evaporate excess of sulphuric acid and then more strongly to expel ammonium salts, finally heating for a short time until the dish is dull red. If sulphuric acid fumes do not appear upon heating, excess is not present and more should be added before the stronger heating. Weigh and record the weight of sulphates of lithium, sodium, potassium and magnesium. Dissolve the combined sulphates and dilute the solution to 250 cc in a calibrated volumetric flask. Magnesium. Fill a dry 100-cc volumetric flask with the solution of sulphates and rinse into a Jena beaker. Determine magnesium as directed on page 91. Notice that only 0.4 of the original sample was used for this determination. ANALYSIS OF INDUSTRIAL PRODUCTS 337 Lithium. Evaporate 100 cc of water to dryness in a platinum dish and test the residue qualitatively for lithium by means of a spectroscope. Lithium will rarely be found in more than traces. Many of the com- mercial "lithia waters" do not contain even a trace of lithium. If it is found and a quantitative determination is required, use 100 cc of the solution of sulphates already prepared, making the determination of lithium, potassium and sodium in this portion, which represents 0.4 of the original sample. Evaporate the solution to about 50 cc and then add barium hydroxide solution (saturated) until no further precipitate of magnesium hydroxide forms. Filter and wash well with water, then evaporate to dryness in a platinum dish, heating for a short time almost to redness. Dis- solve in the least practicable quantity of water and filter into a weighed platinum dish. Add 3 drops of concentrated hydrochloric acid and evaporate to dryness. Heat to a temperature just below redness for 5 minutes, cool in the desiccator and weigh quickly, on account of the hygroscopic character of lithium chloride. If the residue of sulphates was weighed just before the determination of magnesium the second weighing of chlorides of sodium, potassium and lithium is not essential, although it serves as a useful check. This residue is now suitable for the determination of lithium, which is made by the method of Gooch. 1 This method depends upon the relatively large solubility of lithium chloride in anhydrous amyl alcohol, as compared with the solubility of chlorides of sodium and potassium. The boiling-point of amyl alcohol is 132 and boiling will remove all but traces of water. The solubilities of the chlorides of sodium and potassium in amyl alcohol were found by Gooch and others to be as follows : Gm soluble in 100 cc amyl alcohol Potassium chloride Sodium chloride 0.0056 0.0051 Lithium chloride 6.6 The residue of chlorides is dissolved in the smallest possible quantity of hot water and transferred to a 50-cc flask, rinsing the dish. Evaporate until nearly dry, then add 30 cc of amyl alcohol, insert a thermometer and boil cautiously, to avoid bumping, until the boiling-point rises to 132. Continue the evaporation until about 15 cc of alcohol remains. Decant and filter on a small paper and wash the flask and residue two or three times with amyl alcohol whose boiling-point indicates that it is anhydrous. Receive the filtrate in a platinum dish and evaporate to dryness but not directly over a flame. Add a drop or two of concen- 1 Am. Chem. J., 9, 33 (1887). 22 338 QUANTITATIVE ANALYSIS trated sulphuric acid to the contents of the dish and heat until white fumes of sulphuric acid are expelled, finally heating to a temperature of dull redness. Weigh the lithium sulphate and subtract from this 0.0013 gm for each 10 cc of amyl alcohol in the filtrate from sodium and potas- sium chlorides. (This figure is obtained by supposing that the solu- tion was saturated with sodium chloride and potassium chloride, these being then changed into sulphates.) Calculate lithium in the water. Evaporate the amyl alcohol from the flask and paper containing chlo- rides of sodium and potassium and dissolve these chlorides in hot water, using the solution for the determination of potassium. Potassium. If lithium has been determined use the solution from which lithium has been separated, otherwise use 100 cc of the sulphate solution that was obtained just before the determination of magnesium. In either case evaporate in a platinum dish, adding the necessary quan- tity of chlorplatinic acid before crystallization of salts begins. Determine potassium by the Lindo-Gladding method, page 83. Here also 0.4 of the original quantity of sample was used. Sodium. Calculate the weight of potassium sulphate equivalent to the potassium chlorplatinate found and also the weight of magnesium sulphate equivalent to magnesium pyrophosphate found. Multiply the sum of these weights and of lithium sulphate by 2.5 and subtract the product so obtained from the total weight of combined sulphates already found. The remainder is sodium sulphate. From this calculate sodium. Sulphates. Use 100 cc of water unless a qualitative test shows the presence of only a small concentration of sulphates, in which case 500 cc or more should be evaporated to about 100 cc. Add 0.5 cc of concen- trated hydrochloric acid and precipitate by barium chloride, carrying out the precipitation and treatment of the precipitate as directed on page 77. Calculate milligrams per liter of the sulphate radical. Chlorides. Make 500 cc of a standard solution of pure sodium chloride, 1 cc of which contains 0.001 gm of chlorine. Make 1500 cc of a solution of silver nitrate such that 1 cc is calculated to be equivalent to about 0.00101 gm of chlorine and standardize against the sodium chloride solution as follows : Measure 30 cc of the standard sodium chlo- ride solution into a 4-inch porcelain dish or into a beaker placed on a white background. Add 1 cc of a 5 percent solution of potassium chromate (from which chlorides have been precipitated by the addition of a slight excess of silver nitrate) and then titrate with silver nitrate solution until the first permanent red tint of silver chromate appears. This is the end point of the reaction between silver nitrate and sodium chloride. Calculate the dilution necessary to make 1 cc equivalent to ANALYSIS OF INDUSTRIAL PRODUCTS 339 0.001 gm of chlorine and dilute 1000 cc to the required volume by adding distilled water from a burette. Determine chlorine in the water by titrating 100 cc or more of water by standard silver nitrate, using potassium chromate as the indicator. In case chlorides are present in very small concentration it may be necessary to use more than 100 cc and to evaporate to this volume. The end point of the titration is much more easily observed if a rather heavy precipitate of silver chloride is present at the end. If, therefore, the concentration of chlorine in the water is small it is advantageous to add first a measured volume (10 to 25 cc) of standard sodium chloride solution, deducting this volume from the total volume of silver nitrate required. Calculate milligrams per liter of the chloride radical. Nitrates. Use one of the methods described on pages 363 and 364 for the sanitary examination of water. Calculate milligrams per liter of the nitrate radical. The concentration of nitrates in most waters is too small to be of consequence in steaming but this is not always the case and the determination should not be omitted. Carbonates. Titrate 100 cc of the water by fiftieth-normal hydro- chloric acid, using methyl orange as indicator. More or less than 100 cc of water may be necessary if the concentration of carbonates is very small or very large. Enough should be used to require 25 to 45 cc of standard acid. Calculate milligrams per liter of the carbonate radical (00,)'.' Calculate milligrams per liter of the hypothetical compounds using the method illustrated on page 334. Calculate the corresponding grains per U. S. gallon. The determination of carbonates by titration with standard acid is the determination of temporary hardness as described on page 201 except that hardness is arbitrarily calculated as calcium carbonate while in this connection the carbonate radical is calculated. Treatment. If the chief incrustants of a water are bicarbonates of calcium and magnesium these may be largely precipitated by heating the feed water by means of the exhaust steam. This kind of treatment is limited in its application on account of the short time allowed for settling. Other incrustants than those mentioned are not removed by this process. Calcium hydroxide will precipitate bicarbonates and this is the cheapest agent available for this purpose. It also possesses the advantage of leaving no by-products in the water: Ca(HC0 3 ) 2 +Ca(OH) 2 2CaC0 3 +2H 2 0. Mg(HC0 3 ) 2 +Ca(OH) 2 -CaC03+MgC0 3 +2HA 340 QUANTITATIVE ANALYSIS Sodium carbonate will also precipitate bicarbonates as well as other salts of calcium and magnesium. It, however, leaves in the water the corresponding salt of sodium and this is objection- able. Examples of the reactions are expressed by the following equations: 8 CaC0 8 +2NaHC0 8 , . CaSO 4 +Na 2 C0 3 -> CaCO 3 +Na 2 SO 4 , CaCl 2 +Na 2 CO 3 -> CaCO 3 +2NaCl. The best procedure is to treat the water first with the amount of calcium hydroxide necessary to react with bicarbonates, a'lowing a small excess, and then with sufficient sodium carbonate to precipitate all remaining calcium and magnesium. The reactions should be carried out in tanks large enough to provide water for the plant, allowing the necessary time for settling. The initial cost of the purifying plant is almost the only cost because of the relatively low cost of the small amount of lime and soda ash necessary. In calculating the necessary treatment the purity of the lime or lime water must be known and also that of the soda ash. The results may be expressed according to custom as pounds of reagent per 100,000 gallons of water. The quantity of (CO 3)", expressed as grains per gallon, is multiplied by the fraction equivalent weight of CaO , i ^u7 F7?s7v\//, the result being grains of lime re- equivalent weight of (COs) quired for one gallon of water. One pound avoirdupois contains , grains CaO per gallon X 100,000 7000 grains. Therefore- %nnn - = pounds of calcium oxide required for 100,000 gallons of water. Instead of this the calculation may be made to gallons of lime water per 100,000 gallons of water, where a saturated solution of calcium hydroxide is first made and its concentration determined. The amount of sodium carbonate necessary for the precipitation of other salts of calcium and magnesium may be calculated in a similar manner. An inspection of the statement of the analysis as given on page 370 will show that calcium and magnesium as carbonates may be precipitated by lime water, while calcium as sulphate may be removed by sodium carbonate. Since no other bicarbonates are ANALYSIS OF INDUSTRIAL PRODUCTS 341 present the amount of calcium oxide required may be calculated directly from the amount of carbonate radical. 56X160.21X0.0583X100,000 60X7000 = 124.6 Therefore 124.6 pounds of calcium oxide will be required to precipitate calcium and magnesium carbonates. 53X30.89X0.0583XlOO,OOO^g 68X7000 Therefore 20.1 pounds of sodium carbonate will be required to remove calcium sulphate from 100,000 gallons of water. The remaining salts are classed as corrosives and they cannot be removed by chemical treatment. Exercise. Calculate the treatment for the water already analyzed in the laboratory. Boiler Compounds. Many commercial mixtures are now on the market, designed to be mixed with feed water as it enters the boiler to prevent the formation of scale or to loosen scale already formed. Most of these mixtures are solutions of sodium carbon- ate or sodium hydroxide with or without the addition of tannin or some other colloidal organic compound. The base precipitates bicarbonates but this precipitation occurs within the boiler where it would occur if no base had been added. The base is a strong corrosive and may be highly injurious to the boiler if added in excess. Colloidal bodies, such as tannin, starch or dextrine, have a certain loosening effect upon scale already present and to some extent prevent the formation of compact scale. This action can produce only temporary relief, and a preliminary treatment such as has been already described is much cheaper and better. The chief objection to the use of " boiler compounds" may thus be summarized: 1. They are usually corrosive. 2. They are inefficient. 3. They are generally solutions of cheap materials in water and as such are expensive. Examination of Water for Sanitary Purposes. The examina- tion of water to be used for drinking may be made to determine the quantity of mineral salts that have a supposed medicinal 342 QUANTITATIVE ANALYSIS value or to determine its potability. The examination for the first purpose will follow the lines of methods already described for boiler water or it may be extended to include other substances, such as lithium, free carbonic acid, hydrogen sulphide, etc. It is practically certain that none of the mineral waters that are exploited in a commercial way contains enough of any salt or gas to have any appreciable effect upon the human system and that the beneficial effects that are noticed by those who take treatment at the mineral springs are due largely or entirely to other causes, such as enforced dieting, bathing, relaxation from care, good exercise and the drinking of plenty of water. This being the case it is apparent that the chemist's report on the analysis of mineral waters can have little value except as a commercial document. If this analysis is demanded the methods already given may be used, proper modifications being made in quantity of water and reagents according to large variations in the percent of mineral matter present. Additional determinations will be described. Free Carbonic Acid. Water containing free carbonic acid will rapidly lose carbon dioxide and it therefore becomes necessary to make this determination as soon as the water sample is taken or to preserve the sample in tightly closed bottles entirely filled with water. The determination depends upon the fact that as sodium carbonate is added to carbonic acid in presence of phenol- phthalein a color change occurs at the moment that all of the carbonic acid has been used in forming sodium bicarbonate, sodium carbonate having a basic reaction toward phenolphthalein. The end point is shown when the following reaction is completed: NaaCOa+HaCOs-* 2NaHC0 3 . Determination. Calculate the normality of a solution of sodium car- bonate so made that 1 cc is equivalent to 0.001 gm of carbon dioxide. Make 1000 cc of such a solution, using sodium carbonate prepared by heating recrystallized sodium bicarbonate to about 300 until it ceases to lose weight. Do not allow the solution to come into contact with air more^than is necessary. Titrate 100 cc of water as rapidly as possible with the standard sodium carbonate solution and report milligrams per liter of carbon dioxide. The report is often made as volume of gas per unit volume of water at some specified temperature as, for example., cubic inches of gas per gallon ANALYSIS OF INDUSTRIAL PRODUCTS 343 of water, at 60 F. Reference to tables of density of carbon dioxide will give the necessary data for this calculation. Hydrogen Sulphide. The determination of hydrogen sulphide must be made as soon as the water is taken and for the same reasons that apply to carbon dioxide. Titration is made with standard iodine solution according to the reaction: 2I+H 2 S 2HI+S. Determination. Make a solution of iodine, 1 cc of which is equivalent to 0.001 gm of hydrogen sulphide. Standardize by titrating with standard sodium thiosulphate. Titrate 100 cc of the water with the standardized iodine solution, using starch solution as indicator. Calcu- late milligrams per liter of hydrogen sulphide, also cubic inches per gallon. Iron. The quantity of iron in water is usually too small to make ordinary volumetric methods desirable for its determina- tion and a more sensitive colorimetric method is substituted. The iron is obtained in the ferric state and is treated with potassium thiocyanate. The red color so produced is compared in tubes with that formed by a standard iron solution. A distinction may be made between ferrous and ferric iron by using potassium ferricyanide instead of potassium thiocyanate. In this case a blue color is produced by ferrous iron and no visible color results from the reaction of the small amount of ferric iron usually present. Determination of Total Iron. Prepare the following reagents: 1 . Standard Iron Solution . Calculate the weight of ferrous ammonium sulphate required for 1000 cc of solution, 1 cc of which shall contain 0.0001 gm of iron. Dissolve this quantity of the salt in about 50 cc of distilled water and add 20 cc of dilute sulphuric acid. Warm slightly and add potassium permanganate solution until the iron is completely oxidized, using the smallest possible excess. Dilute to 1000 cc. 2. Potassium Thiocyanate. Dissolve 20 gm of the salt in 1000 cc of distilled water. 3. Dilute Hydrochloric Acid. Dilute the concentrated acid with an equal volume of distilled water. The acid must be free from nitric acid. 4. Potassium Permanganate. Make 500 cc of a solution approxi- mately fifth-normal. Evaporate 100 cc of the sample to dryness, or use the residue left after the determination of solids. With silt-bearing waters the quantity of iron is sometimes so great that it is necessary to use a s 344 QUANTITATIVE ANALYSIS little as 10 cc of the sample. With such waters evaporation should be made in the presence of 5 to 10 cc of concentrated hydrochloric acid to effect complete solution of the iron. If the sample of water contains much organic matter, destroy this by ignition, taking care not to pro- long the ignition so as to render the iron too difficultly soluble. Cool the dish and add 5 cc of dilute hydrochloric acid to moisten the whole of the inner surface of the dish. Place the dish on the steam bath for two or three minutes and again moisten the whole inner surface by allowing the hot acid to flow over it. Add 5 to 10 cc of distilled water to rinse down the sides of the dish, and again place on the water bath for about three minutes. The hot acid solution is washed from the dish with distilled water into a 100-cc Nessler tube (see page 355) . Filter the sample if necessary, carefully washing the filter paper with hot water. Add a drop or two of potassium permanganate solution to oxidize the iron to the ferric condi- tion. The color of the permanganate should persist for at least 5 minutes; if not, add more permanganate solution, a drop at a time. To the cooled solution 10 cc of potassium thiocyanate solution is added, and the volume made up to 100 cc and well mixed. Immediately compare the resulting color with that in a series of standards prepared side by side with the sample in 100-cc Nessler tubes in which amounts of standard iron solution ranging from 0.05 cc to 4 cc are first diluted with water to about 50 cc. 5 cc of dilute hydro- chloric acid and a drop or two of potassium permanganate are added to each tube of standard solution and all are diluted to 100 cc. The number of standards needed is governed by the quantity of iron likely to be present in the sample examined. Potassium thiocyanate is added to each of the standard solutions at the same time that this reagent is added to the samples of water under examination. Comparison of the sample with the standards, which are made up to 100 cc after adding the thiocyanate and mixing, should be made immediately. Potability. The examination of water to determine its suit- ability for drinking (potability) is practically always directed toward the question as to whether pollution by sewage has occurred. This examination is quite different in principle from any of the processes already studied, in that the substances actually determined are nearly or quite harmless and are sig- nificant only as they point to the probable presence of patho- genic bacteria. The chemical examination goes no farther than the determination of certain chemical compounds which always accompany sewage and which therefore indicate a danger in the ANALYSIS OF INDUSTRIAL PRODUCTS 345 use of the water for drinking because disease-producing micro- organisms also generally accompany sewage. Since this is the case it might be supposed that the examina- tion might be more properly made by the bacteriologist, who determines directly the presence or absence of bacteria in ab- normal numbers and who also makes a direct test for B. coli communis, an organism that practically always accompanies faecal discharges and is therefore found in all water polluted by waste matter from human organisms. If the results of the bacteriological examination were unfailing this examination would probably suffice for all cases. It should be noted, however, that the bacteriologist is also striving for indirect rather than direct results. It is not practicable to make a direct examina- tion of the species of every organism found in order to test for the presence of actual pathogenic forms and reliance is generally placed upon the two factors noted above, i.e., the concentration of bacteria (number per cubic centimeter) and the presence or absence of B. coli communis. If, for some reason, conditions were temporarily unfavorable to the growth of bacteria at the time the sample was taken, the number of organisms might be so reduced as to cause no suspicion of the real condition of the water. It is conceivable that the chance entrance of antiseptic substances into sewers or streams or the action of sunlight and air should bring about this result and B. coli communis might also be entirely absent. This might, for instance, happen as a result of mixing with factory wastes such as those from the manu- facture of paper and textiles and from plating, bleaching and other chemical industries. In such a case the water would be passed by the bacteriologist when it should be condemned. On the other hand such influences as those mentioned would not eliminate the chemical products of putrescent sewage and the chemical examination would be likely to show pollution. From this examination the water would be condemned. The conclu- sion is that neither method of examination is infallible and both should be used wherever much importance attaches to the results. If one method must be omitted it is preferable that this should be the bacteriological method, provided that sufficient data are at hand for the proper interpretation of the results of the chemical examination. 346 QUANTITATIVE ANALYSIS Interpretation. In order to properly interpret the results of the analysis it is necessary to know the normal condition of the particular class of water under examination because all of the substances occur normally in practically all waters and their proportions vary with the source of the water. For example, chlorides occur in all ground and surface waters and the amount is governed largely by soil conditions. Near the sea shore chlorides occur in ground waters in large quantities. Similar conditions obtain for nitrogen in all of its forms and for organic matter and total solids. The necessary data for the interpreta- tion of a single analysis cannot well be collected by individuals without great expenditure of time and labor. They are gener- ally obtained as a result of organized efforts of state and city boards of health and of scientific societies. The details of manipulation in the analytical work later described, as well as the directions for taking samples and making the physical examination, are essentially those recommended by the Committee on Standard Methods of Water Analysis of the American Public Health Association. The report of this Com- mittee forms a most valuable contribution to the scientific phases of the subject, not only in the matter of unification of practice but also in the guidance that it affords in a selection of the best methods now available. The entire report is printed in the Journal of Infectious Diseases Supplement No. 1, May, 1905, and is reprinted as a special volume to be obtained from The American Public Health Association. For the directions for determinations other than those here described reference should be made to the complete report. Collection of Samples : Quantity. The minimum quantity necessary for making the ordinary physical, chemical and microscopical analyses of water or sewage is one gallon; for the bacteriological examination, two ounces. In special cases larger quantities may be required. Bottles. The bottles for the collection of samples must be of hard, clear, white glass, and they must have glass stoppers. Cork stoppers are not permitted except when physical or microscopical examinations only are to be made. Earthen jugs or metal con- tainers must not be used. Sample bottles must be carefully cleansed each time before using. This may be done by treating with sulphuric acid and ANALYSIS OF INDUSTRIAL PRODUCTS 347 potassium dichromate, or with a basic solution of potassium per- manganate, and afterward with a mixture of oxalic and sulphuric acids, then thoroughly rinsing with water and draining. When clean, the stoppers and necks of the bottles are protected from dirt by tying cloth or thick paper over them. For shipment they are packed in cases with a separate com- partment for each bottle. Wooden boxes may be lined with indented fiber paper, felt or some similar substance, or provided with spring corner strips, to prevent breakage. Lined wicker baskets also may be used. Bottles for bacterial samples, besides being washed, must be sterilized with dry heat for one hour at 160 or in an autoclave at 115 for fifteen minutes. For transportation they may be wrapped in sterilized cloth or paper, or the necks may be covered with tin foil and the bottles put in tin boxes. When bacterial samples must of necessity stand for twelve hours before plating, bottles holding more than four ounces must be used. The bottles used for chemical samples may be sterilized and the samples so collected used for the bacteriological analysis. When bacterial samples are not plated at the time of collection they are kept on ice at a temperature not higher than 15 and preferably as low as 10. Time Interval between Collection and Analysis. Generally speaking, the shorter the time elapsing between the collection and the analysis of a sample, the more reliable will be the analytical results. Under many conditions, analyses made in the field are to be commended, as data so obtained are frequently preferable to those made in a distant laboratory after the composition of the water has changed en route. The allowable time that may elapse between the collection of a sample and the beginning of its analysis cannot be stated defi- nitely, as it depends upon the character of the sample and upon other conditions, but the following may be considered as fairly reasonable maximum limits under ordinary conditions: Physical and Chemical Analysis. Ground waters 72 hours Fairly pure surface waters 48 hours Polluted surface waters 12 hours Sewage effluents 6 hours Raw sewages 6 hours 348 QUANTITATIVE ANALYSIS Microscopical Examination. Ground waters 72 hours Fairly pure surface waters 24 hours Waters containing fragile organisms . Immediate exam- ination Bacteriological Examination. Ground waters 6 hours Fairly pure surface waters 6 hours Polluted surface waters 6 hours Sewage effluents Immediate plating Raw sewages Immediate plating If sterilized by the addition of chloroform, formaldehyde? mercuric chloride, or some other disinfectant, samples for chemical and microscopical examination may be allowed to stand for longer periods than those indicated, but as this is a matter which must vary according to local circumstances, no definite procedure is recommended. If unsterilized samples of sewage, sewage effluents, and highly polluted surface waters are not analyzed on the day of their collection, caution must be used in regard to the organic contents, which frequently change materially upon standing. The gaseous contents of samples, especially dissolved oxygen and carbonic acid, should be obtained immediately, in accordance with the directions given in connection with each determination. Representative Samples. Care should be taken to secure a sample which is truly representative of the liquid to be analyzed. In the case of sewages this is especially important, in view of the marked variations in composition which occur from hour to hour. Frequently satisfactory samples can be obtained only by mixing together several portions collected at different times or in different places the details as to collecting and mixing depending upon local conditions. Physical Examination: Temperature. The temperature of the sample should be taken at the time of collection, and should be preferably expressed in Centigrade degrees, to the nearest 0.5 degree or closer if for any reason more exact data are required. For obtaining the temperature of water at various depths below the surface the thermophone is recommended. ANALYSIS OF INDUSTRIAL PRODUCTS 349 Turbidity. The turbidity of water is due to suspended matter, such as clay, silt, finely divided organic matter, microscopic organ- isms, etc. The increasing use of filters for the purification of water and sewage has made this determination one of great importance. The standard of turbidity is that adopted by the United States Geological Survey, namely, a water which contains 100 parts of silica per million in such a state of fineness that a bright platinum wire one millimeter in diameter can just be seen when the center of the wire is 100 millimeters below the surface of the water and the eye of the observer is 1 . 2 meters above the wire, the observation being made in the middle of the day, in the open air, but not in sunlight, and in a vessel so large that the sides do not shut out the light so as to influence the results. The turbidity of such water is taken as 100. Coefficient of Fineness. The number obtained by dividing the weight of suspended matter in the sample (in parts per million) by the turbidity is called the coefficient of fineness. If greater than unity it indicates that the matter in suspension in the water is coarser than the standard; if less than unity, that it is finer than the standard. Preparation of Silica Standard. Use diatomaceous earth as free as possible from sponge spicules and amorphous silica. Wash with water to remove soluble salts; dry, and ignite to re- move organic matter; warm with dilute hydrochloric acid; wash with distilled water until free of acid, and dry thoroughly. . Grind in an agate mortar, sifting through a 200-mesh sieve in order to separate mats obtained by grinding, and dry in a desiccator. 1 gm of this preparation in one liter of distilled water makes a stock suspension which contains 1000 parts per million of silica and which should have a turbidity of 1000. Test this suspen- sion, after diluting a portion of it with nine times its volume of distilled water, with a wire to ascertain if the silica has the neces- sary degree of fineness and if the suspension has the necessary degree of turbidity. If not, correct by adding more silica or more water as the case demands. Standards for comparison are prepared from this stock sus- pension by dilution with distilled water. For turbidity readings below 20, standards of 0, 5, 10, 15 and 20 are kept in gallon bottles made of clear white glass; for readings above 20, standards 350 QUANTITATIVE ANALYSIS of 20, 30, 40, 50, 60, 70, 80, 90 and 100 are kept in 100-cc Nessler tubes approximately 20 mm in diameter. Comparison of the water under examination with the stand- ards is made by viewing them sidewise toward the light, looking at some object and noting the distinctness with which the mar- gins of the object can be seen. The standards must be kept stoppered and both sample and standards throughly shaken before making the comparison. In order to prevent any bacterial or algal growths from appear- ing in the standards, small amount of mercuric chloride may be added to them. Platinum Wire Method. This method requires a rod with a platinum wire having a diameter of 1 mm or 0.04 inch inserted in it about one inch from the end of the rod, and projecting from it at least one inch at a right angle. Near the end of the rod, at a distance of 1.2 meters (about four feet) from the platinum wire, a wire ring is placed directly above the wire through which, with his eye directly above the ring, the observer shall look when making the examination. The rod is graduated as follows: The graduation mark of 100 is placed on the rod at a distance of 100 mm from the center of the wire. Other graduations are made according to the table on page 351. These graduations are the ones used to construct what is known as the U. S. Geological Survey Turbidity Rod of 1902. Procedure. Push the rod down into the water vertically as far as the wire can be seen and then read the level of the surface of the water on the graduated scale. This will indicate the turbidity. The following precautions should be taken to insure correct results : Observations should be made in the open air, preferably in the middle of the day and not in direct sunlight: The wire must be kept bright and clean. Waters which have a turbidity above 500 are diluted with clear water before the observations are made, but in case this is done the degree of dilution used should be stated and should form a part of the report. The wire method is used for testing the degree of fineness of the standard silica, and this degree of fineness shall be such that when added to distilled water in an amount equal to 100 parts per million, the wire observed under standard conditions ANALYSIS OF INDUSTRIAL PRODUCTS 351 Turbidity, parts Vanishing depth Turbidity, parts per million of wire, mm per million Vanishing depth of wire, mm 7 1095 70 138 8 971 75 130 9 873 80 122 10 794 85 116 11 729 90 110 \ 12 674 95 105 13 627 100 100 14 587 110 93 15 551 120 86 16 520 130 81 17 493 140 76 18 468 150 72 19 446 160 68.7 20 426 180 62.4 22 391 200 57.4 24 361 250 49.1 26 336 300 43.2 28 314 350 38.8 30 296 400 35.4 35 257 500 30.9 40 228 600 27.7 45 205 800 23.4 50 187 1000 20.9 55 171 1500 17.1 60 158 2000 14.8 65 147 3000 12.1 can be just seen at a depth of 100 mm below the surface of the water. Expression of Results. The results of turbidity observations are expressed in whole numbers which correspond to parts per million of silica, and recorded as follows: Turbidity between 1 and 50, recorded to nearest unit. Turbidity between 51 and 100, recorded to nearest 5 Turbidity between 101 and 500, recorded to nearest 10 Turbidity between 501 and 1000, recorded to nearest 50 Turbidity between 1001 and above, recorded to nearest 100 352 QUANTITATIVE ANALYSIS Color. The color of water may form an important indication of pollution. There is little of value to be obtained from a quantitative measurement of color although the determination is discussed at length in the report of the Committee on Standard Methods. Odor. The observation of the odor of cold and hot samples of surface waters is very important, as the odors are usually con- nected with some organic growths or with sewage contamination or both. The odor of ground waters is often caused by the earthy con- stituents of the water bearing strata. The odor of a contami- nated well water is often decisive evidence of its pollution. A study of the organisms of water is an invaluable adjunct to the physical and chemical examination of water. Certain organ- isms can be distinguished by their odor, as, for example, the " fishy" odor of Uroglena, the " aromatic" or "rose geranium" odor of Asterionella and the " pig-pen" odor of Anabsena. Determination. Observe and record the odor, both at room tempera- ture and at just below the boiling-point, as follows : Cold Odor. Shake the sample violently in one of the gallon collect- ing bottles, when it is about half or two-thirds full and when the sample is at room temperature (about 20). Remove the stopper and test the odor at the mouth of the bottle. Hot Odor. Into a tall 400 cc beaker without lip pour about 150 cc of the sample. Cover the beaker with a well-fitting watch glass, place on a hot plate and bring the water to just below boiling. Remove the beaker from the plate and allow it to cool for not more than five min- utes. Then shake with a rotary movement, slip the watch glass to one side and test the odor. Expression of Results. Express the quality of the odor by some such descriptive epithet as the following, which for purposes of record may be abbreviated: v vegetable m moldy a aromatic M musty g grassy d disagreeable f fishy p peaty e earthy s sweetish. Express the intensity of the odor by a numeral prefixed to the term expressing quality, which may be defined as follows: ANALYSIS OF INDUSTRIAL PRODUCTS 353 Numerical value Term Approximate definition None 1 2 Very faint Faint An odor that would not be ordinarily detected by the average consumer, but that could be detected in the laboratory by an experienced observer. 3 4 Distinct Decided were called to it, but that would not otherwise attract attention. An odor that would be readily detected and that might cause the water to be regarded with disfavor. An odor that would force itself upon the attention and 5 Very strong. . . that might make the water unpalatable. An odor of such intensity that the water would be absolutely unfit to drink. (A term to be used only in extreme cases.) Chemical Examination. The following determinations may be made : Total solids, chlorine of chlorides, organic nitrogen, nitrogen of ammonia or ammonium salts, nitrogen of nitrities, nitrogen of nitrates, total organic matter, dissolved oxygen and poisonous metals. Besides the chemical analysis certain purely physical tests may be made such as temperature, color, odor and turbidity. These determinations have just been described. Total Solids. This is taken as the residue obtained when a measured volume of water is evaporated. The general character of the solids may be sometimes noted, also the amount of loss suffered by igniting in air and the odor and amount of charring afford an indication as to the quantity and character of solid organic matter. Determination. Ignite and weigh a platinum dish, then evaporate in it 100 cc of water, using the water bath. Heat the residue at about 103 for one-half hour. Cool in the desiccator and weigh. Report the increase in weight as milligrams per liter of total solids. Heat the dish over a free flame until all organic matter is burned. The change in weight is loss on ignition. Chlorine of Chlorides. Chlorine occurs to some extent in all natural waters. It is found to a much larger extent in sewage where it enters chiefly as sodium chloride of urine and fseces. Sewage polluted streams or wells, therefore, always 'carry abnorm- ally large quantities of chlorine. Determination. Use the method described on page 338. milligrams per liter of chlorine, 23 Report 354 QUANTITATIVE ANALYSIS Nitrogen in Various Forms. Human faeces contains large quantities of nitrogen while urine has a normal content of about 0.85 percent of nitrogen. The entrance of sewage therefore imparts abnormal concentrations of nitrogen to water. This nitrogen is at first practically all in the form of organic compounds and of urea. Part of the organic nitrogen is readily converted into am- monia by oxidizing with potassium permanganate in basic solution. This part is known as "albumenoid nitrogen" because it is contained in albumenous bodies. The action of certain forms of bacteria (chiefly anaerobic) causes the putrefaction of organic matter and this cleavage of complex compounds results in the formation of ammonia from the nitrogen. Part or all of this ammonia may combine with acids to form ammonium salts. All such nitrogen is known as " nitrogen of free ammonia/' whether this be of really free ammonia or of ammonium salts. Where sewage or water polluted by it is exposed to air and sunlight the simpler organic compounds produced by putrefaction are subjected to oxidation, this being promoted by other forms of bacteria (aerobic). Ultimately the organic compounds are completely oxidized. The two processes, putrefaction and oxidation, are made the basis of the septic process of water puri- fication. In the operations of water analysis the changes in the forms of nitrogen are most important. "Free" ammonia is oxidized to nitrous acid which usually remains combined as nitrites of metals or of ammonium. Further oxidation produces nitric acid or nitrates, the final stage in the series. The analytical estimation of the nitrogen in different forms in water offers a valuable indication, not only as to the probability of pollution but also concerning the present condition. The presence of abnormal quantities of "albumenoid nitrogen" indicates the presence of unchanged sewage and the probable presence of dangerous micro-organisms in their most virulent condition. "Free ammonia" in considerable quantities shows that the raw sewage has become fermented and that it must have been largely diluted in the time that has elapsed since the entrance of sewage. Nitrites are very readily oxidized and will not be found in more than traces unless free ammonia is also present. Abnormal quantities of nitrates, unless these are of inorganic origin, are the result of complete oxidation of organic ANALYSIS OF INDUSTRIAL PRODUCTS 355 matter and this must have required time and continued action of air and sunlight. If all forms of nitrogen are found in abnormal quantities continuous pollution is occurring. All of these figures are highly significant in view of the fact that the same influences that promote the decomposition and oxidation of nitrogenous organic matter also combine for the partial or complete steriliza- tion of the water. It is not, by any means, to be concluded that water which has been polluted by sewage but in which the latter has become completely oxidized is necessarily safe for drinking. Indeed if the analysis shows pollution, even at a remote source or time, the water should be condemned as dangerous. The degree of danger is still indicated and the indication will prove of value. As compared with most other substances ordinarily considered in quantitative analysis the different forms of nitrogen occur in extremely slight concentrations. Unusually delicate reactions must be used in order to cause the figures to have any value. Ordinary gravimetric or volumetric processes are rarely used in this connection but very sensitive color reactions are made the basis for the comparison of the water with color standards. Free Ammonia is made evident by the brown color produced when a solution of potassium mercuriodide, K^Hgl^ is added. This solution is known as "Nessler's reagent," from the name of the discoverer of the reaction. 1 The compound that is produced when ammonia is added is a complex substance, thought to have the composition Hg 2 NI. It is an intensely colored brown substance of small solubility and gives a visible color in water containing one part of nitrogen as ammonia in ten million parts of water. The process of determining free ammonia is one of comparing the color produced by adding Nessler's reagent to water with that produced by the reagent with a standard solution of ammon- ium chloride. The comparison is made in tubes of colorless glass, the two that are being compared having the same cross section so that the same length of column is placed in the line of vision. The color is observed by looking vertically downward through the tubes at a white surface placed at an angle in front of a window so as to reflect the light upward If Nessler's reagent is added directly to water containing organic matter, iron or aluminium, a precipitate is produced and *Z . anal. Chem., 7, 415 (1868). 356 QUANTITATIVE ANALYSIS an accurate color comparison is impossible. One of two pre- liminary treatments may be used. The free ammonia may be separated by distillation and the distillate then "Nesslerized" or reagents may be added to the water sample to precipitate inter- fering substances and "direct Nesslerization " may be employed. Distillation is preferable, but where apparatus or time is limited direct Nesslerization is useful. Direct Nesslerization. For precipitating organic matter use is made of the power of flocculating colloids for adsorbing this material, which is itself chiefly colloidal. Cupric sulphate is added to the water which is then made basic by the addition of potassium hydroxide. The precipitating cupric hydroxide so clarifies the water that direct Nesslerization is practicable. Instead of adding cupric sulphate a solution of magnesium chlor- ide may be substituted. Colloidal magnesium hydroxide accom- plishes the same result as does cupric hydroxide. If the wa.ter already contains much magnesium it is unnecessary to add even magnesium chloride. Boiling the water with potassium hydrox- ide will cause the precipitation of magnesium hydroxide. If hydrogen sulphide is present it will cause the precipitation of mercuric sulphide when Nessler's reagent is added. This inter- ference is prevented by the addition of lead acetate before the removal of colloids by cupric hydroxide. The chief objection to direct Nesslerization is the tendency of the precipitating colloids to adsorb small amounts of ammonia. Whether direct Nesslerization or distillation processes are used for free ammonia either an accurately prepared standard solution of an ammonium salt or a standard color solution of a permanent nature is required. This must have a very slight concentration and it is best made by successive dilutions of a more concentrated solution. The solvent used is water that has been shown to be free from ammonia by a test with Nessler's reagent. The labor- atory supply of distilled water is often free from ammonia. If it is not it may be purified by the addition of basic potassium permanganate solution and distilling. After the distillate no longer gives a test for ammonia it is collected and kept in well- stoppered bottles. For a permanent color standard mixtures of potassium chlor- platinate and cobalt chloride are recommended. By properly varying the relative concentrations of the two salts, solutions are ANALYSIS OF INDUSTRIAL PRODUCTS 357 obtained in which the color accurately corresponds with that of Nesslerized ammonia solutions of known concentrations. These solutions are to be preferred to the standard ammonium chloride solutions in laboratories where many determinations are to be made, because of their permanency. Albumenoid Nitrogen cannot be determined by direct Nessleri- zation. It is determined after the distillation of free ammonia by adding to the residue a basic solution of potassium permanganate and distilling. The organic matter is oxidized and remaining nitrogen is distilled and Nesslerized. All determinations of nitrogen must be made in a laboratory in which the air is free from ammonia. Determination. -Prepare the following reagents: 1. Nessler's Reagent. Dissolve 25 gm of potassium iodide in the minimum quantity of cold water. Add a saturated solution of mercuric chloride until a slight but permanent precipitate persists. Add 200 cc of 50 percent solution of potassium hydroxide made by dissolving the potassium hydroxide and allowing it to clarify by sedimentation before using. Dilute to 500 cc, allow to settle and decant. This solution should give the required color with ammonia within five minutes after addi- tion, and should not precipitate with small amounts of ammonia within two hours. 2. Basic Potassium Permanganate. 'Pour 600 cc of distilled water into a porcelain dish holding 1500 cc, boil 10 minutes and turn off the gas. Add 8 gm of potassium permanganate and stir until dissolved. Add 400 cc of 50 percent clarified solution of potassium or sodium hydroxide and enough distilled water to fill the dish. Boil down to 1000 cc. Test each lot of this solution for albuminoid ammonia by making a blank determination. Correction should be made accordingly. 3. Ammonia-free Water. -Test the laboratory supply of distilled water by filling a clean Nessler tube to the mark and adding 2 cc of Nessler's reagent. Cover and allow to stand for 5 minutes. If the color produced at the end of this time is more intense than that of the diluted Nessler's reagent at first, the water must be purified. In this case add 10 cc of basic potassium permanganate solution to each 1000 cc of distilled water and distill, using a tin or aluminium condenser if one is available. After the distillate ceases to give a test for ammonia it is collected in a clean, glass-stoppered bottle. 4. Standard Solution for Color Comparisons. Use either (a) or (b). a. Ammonium Chloride Solution. Dissolve 3.82 gm of ammonium chloride in 1000 cc of distilled water; dilute 10 cc of this to 1000 cc with ammonia-free water. One cc contains 0.00001 gm of nitrogen. 358 QUANTITATIVE ANALYSIS b. Platinum Solution and Cobalt Solution. Weigh 2 gm of potassium chlorplatinate, dissolve in a small amount of distilled water, add 100 oc of concentrated hydrochloric acid and make up to 1000 cc. Weigh 12 gm of cobalt chloride and dissolve in distilled water; add 100 cc of concentrated hydrochloric acid and make up to 1000 cc. Nitrogen of Free Ammonia. A metal or glass flask, connected with a tin or aluminium condenser in such a way that the distillate may be conveniently delivered from the condenser tube directly into the Nessler tubes, is freed from ammonia by boiling distilled water in it, until the distillate shows no further traces of free ammonia. When this has been done, empty the distilling flask and measure into it 500 cc of the sample, or a smaller portion diluted to 500 cc. Apply heat so that the distilla- tion will be at the rate of not more than 10 cc nor less than 6 cc per minute. Collect three Nessler tubes of the distillate, 50 cc to each portion; these contain the free ammonia to be measured as described below. If the sample is acid, or if the presence of urea is suspected, add about 0.5 gm of sodium carbonate previous to distillation. Omit this when possible, as it tends to increase "bumping." Use only Nessler tubes which do not show a variation of more than 6 mm (0.25 inch) in the distance which the graduation mark (50 cc) is above the bottom. The tubes should be of clear white glass, with pol- ished bottoms. The residue from the distillation is immediately used for the determination of albuminoid nitrogen as described below. The measurement may be made either by (1) comparison of the Ness- lerized solutions containing known quantities of nitrogen as ammonium chloride, or (2) comparison of the Nesslerized distillates with permanent standards. Comparison with Ammonia Standards. Prepare a series of 16 Nessler tubes which contain the following numbers of cubic centimeters of the standard ammonium chloride solution, diluted to 50 cc with ammonia- free water, namely: 0.0, 0.1, 0.3, 0.5, 0.7, 1.0, 1.4. 1.7, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, and 6.0. These will contain 0.00001 gm of nitrogen for each cc of the standard solution used. Nesslerize the standards and also the distillates by adding approxi- mately 2 cc of Nessler's reagent to each tube. Do not stir the contents of the tubes. Have the temperature of the tubes practically the same as that of the standards, otherwise the colors will not be directly comparable. Compare the color produced in these tubes with that in the standards by looking vertically downward through them at a white surface placed at an angle in front of a window so as to reflect the light upward. Allow ANALYSIS OF INDUSTRIAL PRODUCTS 359 the tubes to stand for at least 10 minutes after Nesslerizing before mak- ing the comparison. In case the color obtained by Nesslerizing the distillates is greater than that of the darkest tube of the standards, mix the contents of the tube thoroughly and pour out half of the liquid, making up the remainder to the original volume with ammonia-free water, then make the color comparison and multiply the result by two. If, after pouring out half of the liquid, the color is still too dark, repeat this process of division until a reading can be made. In case the color of the distillates is too high, this process may be shortened by mixing together all of the distillates from one sample before making the comparison, subsequently taking an aliquot portion for comparing with the standards. After the readings have been made and recorded, add together the results obtained by Nesslerizing each portion of the entire distillate from each sample. Calculate parts per million of nitrogen as free am- monia in the sample. Comparison with Permanent Standards. Prepare standards by put- ting varying amounts of potassium chlorplatinate and cobalt chloride solutions (page 358) in Nessler tubes, filling up to the mark with distilled water as follows : Equivalent volume of standard ammonium chloride, cc Platinum solution, cc Cobalt solution, cc 0.0 1.2 0.0 0.1 1.8 0.0 0.3 2.8 0.0 0.5 4.7 0.1 0.7 5.9 0.2 1.0 7.7 0.5 1.4 9.9 1.1 1.7 11.4 1.7 2.0 12.7 2.2 2.5 15.0 3.3 3.0 17.3 4.5 3.5 19.0 5.7 4.0 19.7 7.1 4.5 19.9 8.7 5.0 20.0 10.4 6 20.0 15.0 7.0 20.0 22.0 It is necessary to use tubes which have the 50 cc mark not-less than 360 QUANTITATIVE ANALYSIS 20 nor more than 22 cm above the bottom. These standards may be kept for several months if protected from dust. The method of cal- culating results is practically the same as with the ammonia standards. Albumenoid Nitrogen. Interrupt the distillation (made as already described) after the collection of the distillate for free ammonia. Add 40 cc or more of basic potassium permanganate solution and conduct this distillation until at least four portions of 50 cc each and preferably five portions of the distillate have been collected in separate tubes. Have enough permanganate solution present to insure the maximum oxidation of the organic matter. These distillates contain the albu- menoid nitrogen as ammonia, measurement of which will be made as described in connection with nitrogen as free ammonia. Free Ammonia by Direct Nesslerization. Prepare the following solutions. 1. A 10 percent solution of cupric sulphate. 2. A 50 percent solution of sodium hydroxide. If hydrogen sulphide is present in the water prepare also : 3. A 10 percent solution of lead acetate. 50 cc of the sample to be tested is mixed with an equal volume of water, placed in a Nessler tube and a few drops of cupric sulphate solution added. After thorough mixing 1 cc of the potassium hydroxide solution is added and the solution again thoroughly mixed. The tube is then allowed to stand for a few minutes, when a heavy precipitate should fall to the bottom leaving a colorless supernatant liquid. Ness- lerize an aliquot portion of this liquid. If hydrogen sulphide is present add a few drops of lead acetate solu- tion before the addition of potassium hydroxide. Nitrogen as Nitrites. It has already been explained that nitrites will not normally occur in more than traces in water because of the readiness with which they oxidize. In order to give this determination any significance it is necessary to use a very delicate test. Use is made of the ready action of nitrous acid with aromatic amines, forming diazo compounds, and of the latter with naphthylamine, forming azo dyes of intense coloring power. When water containing nitrites is acidified and sulphanilic acid (p-amidobenzenesulphonic acid) is add 3d there is formed the anhydride of p-diazobenzenesulphonic acid, thus: /NH 2 /N = N X HN0 2 +C 6 H/ C 6 H 4 < > +2H 2 0. \ S0 3 / ANALYSIS OF INDUSTRIAL PRODUCTS 361 If to this solution a-amidonaphthalene is added, an azo dye, a-amidonaphthaleneazobenzene-p-sulphonic acid, is produced. /N = N x /N = NC 10 H 6 NH 2 C 6 H 4 < > +C 10 H 7 NH 2 -C 6 H4< X S0 3 / \S0 8 H This dye possesses a very intense red color and one part of nitro- gen as nitrite can be detected in 1,000,000,000 parts of water. The amino compounds entering into these reactions are not .easily soluble and their soluble salts are used. Formerly the hydrochlorides were employed but the reactions are found to proceed more rapidly if the acetates are used. Determination.- -Prepare the following reagents : 1. Sulphanilic Acid. Dissolve 4 gm of the purest sulphanilic acid in 500 cc of 5-normal acetic acid (sp. gr. 1.041). This is a practically saturated solution. 2. a-amidonaphthalene Acetate. Dissolve 2.5 gm of solid -naph- thylamine in 500 cc of 5-normal acetic acid. Filter the solution through washed absorbent cotton. 3. Sodium Nitrite, Stock Solution. Dissolve 1.1 gm of silver nitrite in nitrite-free water. Precipitate the silver with sodium chloride solution and dilute the whole to 1000 cc. 4. Standard Sodium Nitrite Solution. Dilute 100 cc of solution (3) to 1000 cc and dilute 10 cc of the resulting solution to 1000 cc with sterilized nitrite-free water. Add 1 cc of chloroform and preserve in a sterilized bottle. Calculate and record the weight of nitrogen in 1 cc of this last solution. Measure 50 cc or 100 cc of the water to be tested into a Nessler tube. These Nessler tubes should be of clear, white glass, with the graduation mark not varying more than 6 mm in its distance above the bottom. At the same time make a set of standards by diluting various volumes of the standard nitrite solution in Nessler tubes to 50 or 100 cc with nitrite-free water, for example, 0, 1, 3, 5, 7, 10, 14, 17, 20 and 25 cc. Add 2 cc of reagents (1) and (2) to each 100 cc of the sample and to each 1 * standard. Mix and allow to stand 10 minutes. Compare the samples with the standards. Do not allow the samples to stand over one-half hour before being compared. Make a blank determination in all cases to correct for the presence of nitrites in the air, the water and the reagents. Dilute all samples which develop more color than the 30 cc standard before comparing. Mixing is important. Calculate milligrams per liter of nitrogen as nitrites. 362 QUANTITATIVE ANALYSIS Nitrogen as Nitrates. When a soluble aromatic sulphonic acid is mixed with nitrates and sulphuric acid the nitric acid so liberated acts upon the aromatic compound and produces nitro- derivatives which are faintly yellow in most cases. If a base is now added the sulphonate is formed and this is much more in- tensely colored. These reactions are applied to the determination of nitrates in water by what was originally known as Sprengel's method. 1 The method was further modified by Grandval and Laj oux. 2 A measured volume of water is evaporated to dryness, sodium carbonate having been first added if the water contains free acid. The dry residue is treated with a small amount of a phenolsul- phonic acid, the mono-nitro derivative being formed. The reagent is made by heating phenol with sulphuric acid in the pro- portions indicated below. These interact with the formation phenol-o-p-disulphonic acid. The reaction of this acid with ni- tric acid results in the formation of o-nitrophenol-o-p-disulphonic acid: C 6 H3.OH.(S0 3 H) 2 +HNO3 C 6 H 2 .OH.NO 2 .(SO 3 H) 2 +H 2 O. Treatment with a base produces the highly colored sulpho- nate, e.g.? C 6 H 2 .OK.N0 2 .(S0 3 K) 2 . The phenolsulphonic acid method fails in at least two cases, when applied to waters. If the water contains any considerable amount of organic matter, as is always the case with surface streams, sewage and waters contaminated by sewage, the addi- tion of the solution of phenolsulphonic acid in concentrated sul- phuric acid causes a charring of organic matter and the color comparison cannot be accurately made because of the resulting brown coloration. Organic matter also causes the reduction of some of the nitrates during evaporation. The second cause of failure is the occurrence of unusual quantities of chlorides in the water. In this case nitrates and chlorides react during evapora- tion and part of the nitrate is reduced. 1 Fogg. Ann., 121, 188 (1863). 2 Compt. rend., 62, 101 (1885). 3 Chamot and Pratt: J. Am. Chem. Soc., 31, 922 (1909); 32, 630 (1910); 33, 366 and 381 (1911). ANALYSIS OF INDUSTRIAL PRODUCTS 363 The only satisfactory method for the determination of nitrates in water containing much organic matter or chlorides is based upon the reduction of nitrates by nascent hydrogen, ammonia being 'formed. Subsequent Nesslerization gives the ammonia produced from nitrates and nitrites, as well as the free ammonia originally present. After making the proper correction for nitrites and free ammonia the amount of nitrogen as nitrates is calculated. Determination. For the phenolsulphonic acid method prepare the following reagents : 1. Phenolsulphonic Acid. Mix 30 gm of synthetic phenol with 370 gm of concentrated sulphuric acid in a flask having a round bottom. Place the flask in a water bath in such a way that it shall be completely immersed in the water. Heat for 6 hours at a temperature near 100. 2. Ammonium Hydroxide Solution! Dilute 200 cc of ammonium hydroxide having a specific gravity of 0.90 with an equal volume of water. If the determination of nitrates must be made in the room where am- monia is being determined substitute a 10 percent solution of the purest obtainable potassium hydroxide for the ammonium hydroxide. 3. Standard Nitrate Solution. Dissolve 0.72 gm of pure recrystal- lized potassium nitrate and dilute to 1000 cc with distilled water. Evaporate cautiously 10 cc of this solution in a dish placed on a water bath. Moisten the residue quickly and thoroughly with 2 cc of phenol- sulphonic acid, dissolve and dilute to 1000 cc. Calculate the weight of nitrogen in 1 cc of the last solution. Evaporate 20 cc of the water sample in a small dish placed on the water bath. More or less than 20 cc should be used if the proportion of nitrates is exceptionally small or large. Remove the dish from the bath just before the residue has become dry and allow the last few drops of water to evaporate at room temperature in a place that is protected from dust. Add 1 cc of phenolsulphonic acid and rub this quickly and thor- oughly over the residue with a glass rod. Add about 10 cc of distilled water and stir until all soluble matter has dissolved. Add enough ammonium hydroxide or potassium hydroxide solution to render the solution basic. Transfer to a Nessler tube and fill to the mark with dis- tilled water. The yellow color is compared with that of permanent standards, made by measuring the quantities of standard nitrate solu- tion as indicated below, into Nessler tubes, adding 5 cc of ammonium hydroxide or potassium hydroxide solution to each tube and diluting to the mark with distilled water. The Nessler tubes must be selected so that the capacity mark is at the same height on all. Numbers of cubic centimeters of standard solution to be used are as follows : 364 QUANTITATIVE ANALYSIS 1.0, 3.0, 5.0, 7.0, 10.0, 15.0, 20.0, 25.0, 30.0, 35.0, 40.0, or as many of these as will extend over the working range with the classes of water being examined. The tubes containing the standards should be kept stoppered except when making color comparisons. Calculate milligrams per liter of nitrogen as nitrates. For the reduction method for nitrates prepare the following reagents : 1. Sodium Hydroxide Solution. Dissolve 50 gm of the purest ob- tainable sodium hydroxide in 250 cc of distilled water and evaporate to 200 cc by boiling. 2. Aluminium Foil. Use strips about 5 cm long, 0.012 mm thick and of such width thai each strip weighs about 0.35 gm. Place 50 cc (or a smaller amount diluted to 50 cc) of the sample in a test-tube about 30 cm long and 15 mm in diameter. Add 5 cc of the sodium hydroxide solution and a strip of the aluminium foil. Place a loose stopper in the tube and allow to stand without heating for twelve hours. Rinse the contents of the tube into a 500-cc flask and distill with steam, using a tin condenser. Nesslerize the distillates, correct for free ammonia and nitrites and calculate milligrams per liter of nitro- gen as nitrates. Required Oxygen. Besides the indirect estimation of organic matter through the determination of nitrogen in its various forms a more direct estimation may be made by oxidizing with standard potassium permanganate. It is readily seen that no calculation of organic matter can be made as a result of such a titration because the great variety of organic substances present in polluted water gives rise to a great variety of reactions. On the other hand the calculation of the oxygen required to oxidize organic matter gives a fair, though inexact idea of the amount of organic pollution. Differences in. procedure will cause the reduc- tion of varying quantities of potassium permanganate and it is therefore necessary to rigidly standardize the method. It is also necessary to correct the results according to the amount of nitrites, ferrous salts and hydrogen sulphide, if these are found in considerable quantities. The determination is carried out by treating a measured volume of water with an excess of standard potassium perman- ganate solution, at a specified temperature, and titrating the excess after a stated period. If the solution is heated during the treatment the excess of permanganate is determined by adding a measured ex cess of a standard solution of oxalic acid or ammonium oxalate, titrating the excess by standard potassium permanganate ANALYSIS OF INDUSTRIAL PRODUCTS 365 solution. If cold treatment is used the excess of potassium per- manganate cannot be determined in this manner because oxalic acid reduces permanganates very slowly unless heated to at least 60. In this case the excess of potassium permanganate is reduced by adding potassium iodide and titrating the liberated iodine with standard sodium thiosulphate solution: 2KMnO 4 +10KI+8H 2 S0 4 6K 2 SO 4 +2MnS0 4 +8H 2 O + 10I; 2Na 2 S 2 8 +2I-*Na 2 S 4 O 6 +2NaI. The method involving digestion at 100 will be described Determination. Prepare the following reagents: 1. Sulphuric Acid. Dilute the concentrated acid with three volumes of distilled water. Add potassium permanganate until a faint pink color persists after standing for several hours. 2. Standard Potassium Permanganate Solution. Calculate the weight of crystallized potassium permanganate required for 1000 cc of a solu- tion, 1 cc of which shall be equivalent to 0.0001 gm of oxygen. Dis- solve this weight of salt in 1000 cc of distilled water. Standardize by titrating against solution (3), adding 10 cc of solution (1) and heating to 60-65 before titrating. Calculate the value of the solution in milligrams of available oxygen per cubic centimeter. 3. Ammonium Oxalate Solution. Use the purest obtainable salt. Make a solution of which 1 cc is equivalent, as a reducing agent, to 0.0001 gm of oxygen. Measure 100 cc or less of the water into a 200-cc flask, add 10 cc of solution (1) and 10 cc (exactly measured) of solution (2) and immediately place the flask in a bath of boiling water, the water level of which is kept above the level of the contents of the flask. Digest for exactly 30 minutes. Remove the flask and add exactly 10 cc of solution (3). Titrate with standard potassium permanganate solution and calculate milligrams per liter of required oxygen. The following discussion is quoted from the report of the Committee of the American Public Health Association. Putrescibility. "This test, sometimes called the 'incubator test,' is of fairly recent English origin. It is now quite generally used in connection with sewage works analyses. Its purpose is to ascertain whether or not the quantity of organic matter in a sewage effluent of an unstable or putrescible character is in excess of that which can be oxidized by the oxygen which it contains in the form of dissolved oxygen or oxygen available from nitrates, nitrites, and perhaps sulphates. While it is a very useful and important test in connection with sewage purification, the object 366 QUANTITATIVE ANALYSIS of which is the elimination of gross nuisances, and in studying the details of a highly polluted stream, it is a test which obviously has no direct bearing in the regular field of water analysis, or even in the purification of sewage the effluent of which without further treatment is intended to enter a stream used a short distance below for drinking purposes. "Procedure. A round, glass-stoppered bottle of good quality having a capacity of at least four ounces is completely filled with the sample, and after being tightly stoppered is placed in an incubator at 37 C. As the sample is collected determinations are made of the dissolved oxygen, nitrogen as nitrites and nitrates, and the oxygen consumed by digestion in an acid solution with potassium permanganate at the room temperature for a period of three minutes. After the sample has been incubated 24 hours (or more), observations are carefully made as to the appearance of the sample, that is, whether it has turned black or not, and particular attention is given to the presence or absence of well- defined odors of putrefaction. Sometimes a qualitative test for the presence of sulphuretted hydrogen may be made with advantage, by suspending in the mouth of the bottle a strip of filter paper saturated with lead acetate. Some make this test quantitatively. "Samples which after incubation are black in appearance, due to ferrous sulphide, and which possess foul odors, may be unquestionably regarded as putrescible without making any further tests. "Samples which at the end of the incubation period still contain an appreciable quantity of dissolved oxygen or oxygen available from ni- trates, and are free from sulphuretted hydrogen or other odors result- ing from putrefaction, may be generally regarded with safety as non- putrescible. "Samples in which dissolved oxygen and nitrogen in the form of nitrates are absent, or nearly so, with more or less nitrogen in the form of nitrites, and in which the oxygen consumed when determined for three minutes in the cold has increased on incubation, require more careful consideration before recording definitely the result of the putrescibility test. The best procedures by which any additional information can be obtained appears to vary under different local conditions as to character of sewage treated, the method of treatment, season of the year, etc., and it seems inadvisable now to specify in precise terms further pro- cedures for use under all circumstances. "As the applicability of this test is studied in various laboratories, it is recommended that reports set forth distinctly the procedures by which conclusions have been arrived at with reference to putrescibility. "It is to be noted that this test is a very rigid one for practical condi- tions, inasmuch as sewage effluents almost invariably are diluted more ANALYSIS OF INDUSTRIAL PRODUCTS 367 or less by the oxygen-containing waters of the streams which they enter. In some laboratories the practice is adopted of making the tests as above outlined and also repeating the same process with samples diluted with equal or varying volumes of river water. Where facilities permit, the adoption of this test also on a diluted sample is to be recommended. "The results shall be recorded simply as positive or negative, stating clearly whether the sample was diluted, and if so, to what degree." IRON AND STEEL The impurities contained in iron and steel usually form a very small portion of the total mass. Wrought iron and s.teel often contain a total of less than one percent of elements other than iron while even pig iron does not often contain as much as ten percent of other elements. It is therefore not customary to make determinations of the percent of iron but rather of the small amounts of other elements, which give certain very important properties to the metal in which they are contained. Elements occurring in iron and steel and commonly determined are carbon, silicon, phosphorus, sulphur, manganese and titanium. In alloy steels for special purposes determinations are also made of tungsten, nickel, chromium, molybdenum, vanadium and copper. For the determination of each element there are available certain well known methods and these are continually being revised and supplemented by other newer methods. Consider- able experience is therefore necessary if the analyst is to be able to intelligently select the method best adapted to his purpose. There is a certain distinction to be made between what may be classed as " exact " methods and others that are more properly called "rapid" methods. Thus a determination of carbon in steel, made by an approved exact method, may require at least two hours and sometimes longer while a less exact determination might be made by another method in ten or fifteen minutes. Conversations with works chemists will often give the student the erroneous impression that the longer methods are impracticable and are taught in colleges but not used in practice, while the rapid methods are improperly neglected in the students' college courses. It is true that more emphasis is laid upon the exact method, as a rule. If the science and careful manipulation involved in the longer method are properly appreciated and 368 QUANTITATIVE ANALYSIS learned the student will have no difficulty in learning the shorter and less exact method after he enters his professional career. It is highly important that one should understand the proper place of each class of methods. In the steel works samples may be taken from the melted iron as it runs from the blast furnace or from the steel ladles which receive the product of the steel furnaces. These samples are taken directly to the works labora- tory where the analysis must be made very quickly in order to furnish information which will serve as a guide in mixing charges for the steel furnace or for properly disposing of or modifying the product -of a given furnace. The results of such an analysis do not often serve as a guarantee to the steel consumer but rather as a check upon the various stages in the process of steel manufac- ture. For this reason rapid methods are quite suitable for the purpose. When the steel is placed upon the market as a finished product the case is quite different. Modern industrial development has created new and rigorous requirements regarding the quality of steel entering into machinery and structural work. It becomes necessary for the steel manufacturer to guarantee the percents of the elements in his steel within very narrow limits and a method of analysis that will not give results having a high degree of ac- curacy is quite useless for this purpose. An inspection of the methods for analysis of steel as practiced in the various works laboratories will show that while the several standard methods are quite universally used, many variations have been introduced by individual chemists. Each laboratory usually has its methods described and specified and these must be rigidly followed by all chemists working in that laboratory. There is, of course, much difference of opinion concerning the relative merits of different methods and it is inevitable indeed it is even desirable that modifications should be made whenever any improvement is seen to be possible. It is also true, however, that many modifications of good methods have made poor methods because the modifications have been made without an adequate knowledge of the scientific principles underlying the analytical process. Many chemists have confidence in their methods when this confidence is based upon little more than the ability to obtain close agreement of duplicate determinations. The error involved in such conclusions has been discussed in an im ANALYSIS OF INDUSTRIAL PRODUCTS 369 earlier section. Many of the analytical methods for iron .and steel have been in use for a long time. Some of these have been retained in practically their original form and still bear the names of the chemists who first proposed them. Others have been so modified that they bear little resemblance to the original method. Sampling. Analysis may be required of either works samples, taken from the metal as it runs from the furnace, or of the finished product. Samples of the first class are dipped from the melted metal by means of a small ladle and are poured onto a clean iron plate or into a small iron mold. The sample is crushed, if a brittle product like pig iron, or drilled if steel. Pieces of already solidi- fied metal are drilled to obtain a sample for analysis. The outer case should be first removed because it may contain iron oxide or sand, or the percent of carbon may have been lowered by oxida- tion. The drill should be set to make as fine drillings as possible and if powder is at the same time produced it should be well mixed with the larger pieces before weighing for analysis. Carbon. The most important element occurring in steel is carbon. This is because it is the element which makes possible the formation of steel by imparting to iron the capability of being hardened by suitable heat treatment. The development of alloy steels has lately brought nickel, chromium, vanadium, tungsten and other metals into prominence as constituents of special steels, but without carbon, alloys of these elements with iron would be of little value. The effect of carbon upon iron with which it is combined is to increase the tensile strength and hardness and to decrease the ductility. Carbon is present in steel chiefly as a carbide, Fe 3 C, although small quantities may occur as free carbon. In cast iron large quantities of carbon are free, particularly in gray cast iron. A more extended discussion of the properties of steel, as dependent upon the condition of the carbon, will be taken up later (page 400) . The determination of total carbon is the only carbon determina- tion that is usually required in steel analysis. In the analysis of cast iron determinations also of free and combined carbon may be required. The determination of total carbon is generally made by a combustion process, the carbon being oxidized and the resulting carbon dioxide determined, although oxidation in solu- tion has been employed, the oxidizing agent being chromic acid. The details of the combustion processes vary widely. Fine 24 370 QUANTITATIVE ANALYSIS drillings of steel may be burned directly or a preliminary separa- tion of the carbon may be made. The apparatus for combustion may be a furnace and combustion tube or a special form of closed crucible, through which air and oxygen may be passed. The resulting carbon dioxide may be measured at an accurately observed temperature and pressure and its weight calculated or it may be absorbed in a basic solution and either weighed or the excess of base determined by titration. Direct Combustion. Fine particles of iron or steel may be com- pletely burned in oxygen, but with larger particles combustion is incomplete. The method is desirable because it avoids a rather tedious process of preliminary solution, filtration and wash- ing. It is not to be recommended, however, except with such irons and steels as can be obtained in a fine state of division. For the combustion there is required a tube of quartz, porcelain or glass, 24 inches long and having an inside diameter of 3/4 inch. The combustion furnace may be heated by gas or electricity and should be 12 to 14 inches in length. A quartz tube and an electrically heated furnace are to be preferred. The combustion is carried out in a manner quite similar to the combustion of coal (page 266). The long furnace and tube there used are not required in this case because no volatile hydrocarbons are pro- duced and long contact of the gases with cupric oxide is not neces- sary. The small amount of carbon monoxide that may be formed at first is completely oxidized by passing the mixture with oxygen over a small amount of cupric oxide or through platinized asbestos. The platinum black in the latter case catalyzes the combination of carbon monoxide with oxygen. The train of apparatus neces- sary for the gravimetric determination is as follows: (1) Oxygen tank, (2) bubble tube of 30 percent potassium hydroxide solution, (3) empty tube to retain spray from the potassium hydroxide solution, (4) combustion tube containing (4a) space of about 3-1/2 inches for the combustion boat, (4b) platinized asbestos to the end of the furnace, leaving the projecting ends of the tube empty, (5) U-tube filled with granular zinc or with glass beads moistened with chromic acid solution for the retention of oxides of sulphur, (6) U-tube filled with calcium chloride, (7) absorption bulbs filled with potassium hydroxide solution and carrying a tube of calcium chloride and (8) guard tube of calcium chloride. The method of preparing and assembling this apparatus will be clear from the ANALYSIS OF INDUSTRIAL PRODUCTS 371 discussion of the determination of carbon dioxide in carbonates (page 109) and of the determination of carbon and hydrogen in coal (page 266). If the volumetric determination of the carbon dioxide is to be made, the absorption bulbs are replaced by a^ottle or flask which can be easily rinsed and the solution used is a fifth-normal solu- tion of potassium hydroxide or barium hydroxide. In this case the U-tube (6), filled with calcium chloride, is omitted since absorption of moisture in the absorption flask will occasion no error. Combustion, Preceded by Solution of the Iron and Separation of the Carbon. Iron or steel dissolves easily in a solution of the double chloride of potassium and copper. The cupric chloride is the active agent and the double salt is used only because it is more easily purified and preserved. The reactions are: Fe+2CuCl 2 FeCl 2 +2CuCl and Fe 3 C+6CuCl 2 3FeCl 2 +6CuCl+C. Free carbon is also left undissolved. The residue contains organic compounds, formed during the process of solution, and the total residue cannot, therefore, be weighed directly for the determina- tion of total carbon. If the solution is not well stirred or if not enough cupric potassium chloride is used, copper will separate, returning to the solution upon stirring or addition of more of the solution of cupric salt: Fe+CuCl 2 FeCl 2 +Cu, Cu+CuCl 2 -*2CuCl. These reactions illustrate the principle of replacement of one metal of a salt solution by another which has a higher solution tension. The solution of cupric potassium chloride must contain hy- drochloric acid in order to prevent the precipitation of cuprous chloride, a substance having very small solubility in water. If too much acid is present carbon may be lost through the forma- tion of hydrocarbons during the process of solution. This is typified by the hypothetical reaction shown by the following equation: 2Fe 3 C+12HCl 6FeCl 2 +C 2 H 2 +5H 2 . 372 QUANTITATIVE ANALYSIS Concerning the choice of methods it may be said that direct combustion is much more rapid and is accurate if the metal is in a state of fine division. The method of preliminary solution is fully as accurate, except for high-speed tool steels, and is safer if the nature of the steel is not accurately known. There is little to choose between the gravimetric method, weighing the absorp- tion bulbs before and after the absorption of carbon dioxide, and the volumetric method. The so-called " moist combustion" processes and the methods involving measuring the volume of carbon dioxide evolved, while attractive in principle, are trouble- some in execution and are subject to large errors unless great care is exercised. These are therefore little used. Determination by Direct Combustion. -Set up the apparatus as al- ready directed and test for leaks. Weigh into a combustion boat of porcelain or alundum 0.5 gm of fine drillings or powder of iron or steel. If a layer of powdered alundum is placed in the bottom of the boat the latter will be protected from the absorption of iron oxide and will last longer. Insert the absorption bulbs, heat the combustion tube to dull redness and pass a stream of oxygen through the apparatus until all air is expelled. Without allowing the tube to cool, interrupt the flow of oxygen, remove and plug the absorption bulbs and place these in the balance case. If the carbon dioxide is to be determined volumetrically the weighing of the bulbs is not necessary, 50 cc of fifth-normal potassium hydroxide or barium hydroxide being measured into an absorption flask just before the insertion of the combustion boat. If the absorp- tion bulbs have been weighed, replace them in the train, quickly insert the combustion boat, close the tube and again pass oxygen through at a rate of about three bubbles per second in the absorption bulbs. Bub- bling will become slower when the carbon dioxide reaches the absorbing solution. When this occurs the amount of gas entering the tube may be increased and the supply diminished only when increased bubbling indicates that nearly all of the carbon dioxide has reached the absorbing solution. Continue the flow of gas 15 minutes longer then remove the absorption bulbs, stopper and place in the balance case, weighing after ten minutes. If an absorption flask has'been used, and the solution is to be titrated, add a drop of phenolphthalein solution and titrate at once with fifth- normal acid solution. After the end point with phenolphthalein has been reached add a drop of methyl orange and continue the titration. Make blank determinations by passing oxygen through the heated com- bustion tube and absorbing solution, exactly as in the real determination, omitting the steel. Titrate as above and repeat until constant results ANALYSIS OF INDUSTRIAL PRODUCTS 373 are obtained. The blank will serve for all determinations made by means of the solution so standardized, unless opportunity has been given for the latter to change its concentration or to absorb carbon dioxide from the air. Subtract the volume of acid used in the methyl orange titration of the blank from that used in the combustion experi- ment with the same indicator and calculate the percent of carbon in the sample. In the calculation of the equivalent weight of carbon refer to the discussion of the analysis of mixtures of bases and carbonates (page 197). Determination by Combustion, Preceded by Solution. Prepare a solution of cupric potassium chloride containing 500 gm of the crystal- lized salt and 75 cc of concentrated hydrochloric acid in 1000 cc of solu- tion. Filter through ignited asbestos into the bottle. Weigh 1 gm of the steel or iron drillings, place in a 250-cc beaker and add 100 cc of the cupric potassium chloride solution. Stir until the metal is all dissolved, warming to about 65. If many determinations are to be made a stirring machine is desirable. Filter the solution through a Gooch crucible or a carbon tube (shown in Fig. 65, page 214). The asbestos used in the filter must have been previously ignited to remove all organic matter. Wash with warm (50) dilute hydrochloric acid until the washings are free from color, then with cold water until free from chlorides. It is desirable that most of the water be removed from the filter and carbon, although the latter need not be absolutely dry. After partial drying by means of the pump and dry- ing oven the asbestos and carbon are carefully removed and placed in a combustion boat, using a small pair of forceps. This operation should be performed over a sheet of white, glazed paper. The inside of the crucible or carbon tube is carefully wiped clean, using a tuft of ignited asbestos. The combustion and subsequent determination are carried out as in the direct combustion process except that directly following the combus- tion tube there is inserted a U-tube containing a saturated solution of ferrous sulphate, acidified by sulphuric acid. If traces of hydrochloric acid are retained by the filter or carbon this is partly oxidized, chlorine being produced, and partly carried over without change. Chlorine is absorbed and reduced by the ferrous sulphate while small quantities of hydrochloric acid are absorbed by the water of the solution. Free (Graphitic) Carbon. When steel or iron containing both free and combined carbon is dissolved in nitric acid of specific gravity 1.2 the combined carbon passes into solution as hydro- carbons, the graphitic carbon being left as an insoluble residue. The latter may be separated by filtration and used for the deter- mination of free carbon. If it is to be weighed directly the silica 374 QUANTITATIVE ANALYSIS which is also left must be removed by washing with potassium hydroxide solution, then with water. A better method is to wash the carbon and silica free from iron salts and acids and then deter- mine by combustion, exactly as in the case of total carbon. Graphitic carbon may also be determined by difference, sub- tracting the combined carbon from the total carbon. Determination. Weigh 1 gm of pig iron or 10 gm of steel and dis- solve in nitric acid, specific gravity 1.2, using 15 cc of acid for each gram of sample. Filter through ignited asbestos in a Gooch crucible or a carbon tube and wash with dilute hydrochloric acid, then with hot water until free from chlorides. Burn in the combustion tube used for total carbon and determine in the same way. Combined Carbon. The percent of combined carbon may be determined indirectly by subtracting the percent of graphite from that of total carbon. The only reliable method for the direct determination of combined carbon is that of Eggertz. 1 This method depends upon the fact, noted in the discussion of free carbon, that when steel or iron containing combined carbon is dissolved in dilute nitric acid the combined carbon forms soluble organic compounds which impart a color to the solution, the inten- sity of which varies with the percent of combined carbon. The solution is then compared in tubes with the solution of a standard steel whose carbon content is known, the unknown percent being then calculated. It will be seen later, when a more extended study of carbon conditions is taken up, that combined carbon may exist in more than one physical state, although probably always present as the carbide Fe 3 C. This difference in physical state is influenced by the presence of other elements and also by the mechanical and thermal treatment which the steel has received. The color of the acid solution is affected by all of these factors and it therefore becomes necessary to use for a standard steel one in which not only the percent of combined car- bon is known to be approximately the same as that of the steel being examined but also one that has nearly the same percents of other impurities and that has been subjected to the same thermal and mechanical treatment. All of these factors cannot well be known in general testing and the method is therefore of little value for this class of work. Its chief value is to the steel works chemist who knows in every case the nature of the steel with which he is 1 Z. anal. Chem., 2, 433 (1862); Chem. News, 7, 254 (1863). ANALYSIS OF INDUSTRIAL PRODUCTS 375 dealing and who is thereby enabled to select his standard steel with due regard to all of the variable factors. Determination!, Treat the standard steel and the steel being ex- amined as follows : Weigh 1 gm of the drillings and dissolve in a beaker in 30 cc of nitric acid whose specific gravity is 1.2 and which is free from chlorine. Warm the acid toward the end of the process, to complete the solution. Filter to separate free carbon and silica, receiving the nitrate in a 100-ce volumetric flask. Wash the residue, dilute to the mark and mix. Transfer 30 cc of the solution of lighter color to an Eggertz tube, which is a tube graduated from 1 cc to 30 cc and having an internal diameter of about 1 cm. Add the darker solution to another similar tube until the color of the two tubes appears to be equal, viewed from above. In case the color is very dark, less solution must be used or the color observed from one side, or else the color of the darker solution is lightened by dilution. Calculate the percent of combined carbon. Silicon. Silicon occurs in all steels, generally in quantities less than 0.3 percent. Certain silicon steels contain as much as 5 per- cent. Cast iron contains as much as 3 percent of silicon. Silicon occurs as a silicide which is probably to be represented by the formula FeSi, this forming a solid solution with the remainder of the iron. Silicon has little effect upon the mechanical properties of steel but is desired in cast iron because of its tendency toward throwing carbon out of its combination with iron, thus forming gray iron which has a greater fluidity when melted than does white iron, and which is therefore better suited for foundry purposes When iron silicide is dissolved in nitric acid the silicon is entirely converted into silicon dioxide, largely in the state of colloidal silicic acid. If the silicic acid is dehydrated and the resulting silicon dioxide made insoluble by heating with acids it may be separated by filtration. As obtained from pig iron the silicon dioxide so obtained contains all of the free carbon of the iron. This is removed by ignition. In the adaptation of this process to the quantitative determination of silicon in iron and steel the chief difficulties encountered are due to the tendency of silica to change from the gel to the sol and also to incomplete washing of the silica. In order to assist in the separation of silica in an insoluble condition Drown suggested 1 the addition of i Trans. Am. Inst. Min. Eng., 7, 346 (1879). 376 QUANTITATIVE ANALYSIS sulphuric acid to the solution during the evaporation to render silica insoluble. This materially shortens the time required for a determination as otherwise the solution must be evaporated and Iheated for some time in order to completely separate the silica. During the washing of the silica, if pure water is used, iron salts ihydrolyze and insoluble basic salts are retained by the filter. Alternate washing with water and hydrochloric acid will remove all but traces of iron salts and a correction may be made for these by the common process of volatilization of silica by hydrofluoric acid. Determination. Prepare a mixture of 375 cc of concentrated nitric acid, 125 cc of concentrated sulphuric acid and 500 cc of water. Weigh 0.5 gm of pig iron or 1 gm of steel and dissolve by warming in a casserole or platinum dish with 25 cc of the acid mixture. The solu- tion is evaporated by agitation over a flame until pronounced fumes of the sulphuric acid appear. Allow the solution to cool, then add 10 cc of dilute hydrochloric acid and 50 cc of water. Warm until iron salts are dissolved then filter and wash alternately with hot dilute hydro- chloric acid and water until free from iron and chlorides. Ignite in a platinum crucible, cool and weigh. Volatilize the silica by treatment with sulphuric acid and hydrofluoric acid (see page 250), and from the loss in weight calculate the percent of the element silicon. Sulphur. Sulphur occurs in iron and steel as ferrous sulphide, PeS, unless manganese is present, in which case it forms mangan- ous sulphide MnS. Ferrous sulphide is itself brittle. It also shows a tendency toward the formation of envelopes surrounding the crystalline grains of steel, reducing their cohesion and result- ing in " shortness," particularly when hot. Sulphur is therefore said to cause "red shortness" of steel. Manganese sulphide is much less objectionable than ferrous sulphide and the steel maker therefore relies upon manganese to correct largely the bad effects of sulphur, although the latter should not be present in steel in quantities greater than 0.05 percent. In the best steel its quantity is much less than this. The determination of sulphur may be accomplished by oxida- tion to sulphuric acid, followed by precipitation as barium sul- phate, or by evolution methods; in the latter the metal is dis- solved in hydrochloric acid, ferrous sulphide or manganous sul- phide forming hydrogen sulphide. The latter is distilled into ANALYSIS OF INDUSTRIAL PRODUCTS 377 some absorbing solution and subsequently determined by gravi- metric or volumetric methods. Oxidation Method. Steel or iron dissolves more readily in dilute nitric acid than in the concentrated acid and the former is therefore used for dissolving the sample for nearly all other deter- minations. This acid will not serve for dissolving the metal as a preliminary to the gravimetric determination of sulphur because a part of the sulphur will be evolved as hydrogen sulphide and will then escape. Concentrated nitric acid completely oxidizes the sulphide to sulphate. 6FeS + 24HN0 3 2Fe 2 (S0 4 )3+2Fe(NO 3 )3+18NO + 12H 2 0. The sulphur is then precipitated as barium sulphate. The separation from the large amount of iron involves some difficulty. Unless a considerable excess of acid is present basic ferric salts, products of hydrolysis, are retained by the precipitate of barium sulphate. If too much acid is present the precipitation of barium sulphate is incomplete. For the solubility of barium sulphate in hydrochloric acid, see page 76. Nitric acid must not be present at all because of its effect upon the occlusion of iron salts by the precipitate. Silica must be separated by evaporation and filtration before the precipitation of barium sulphate, and during the evaporation and heating that are necessary for this purpose there is danger of loss of sulphur through decomposition of ferric sulphate: Fe 2 (SO 4 )3Fe 2 03+3SO 3 . In order to prevent loss of sulphur trioxide by this means, a small amount of sodium carbonate is added before the evapora- tion. This immediately forms sodium nitrate or chloride (hy- drochloric acid also having been added) and this reacts during the evaporation, thus: Fe 2 (S0 4 ) 3 +6NaCl 2FeCl 3 +3Na 2 S0 4 . Sodium sulphate is not decomposed by moderate heating. Determination. Weigh 5 gm of drillings or powder into a casserole. Place under a hood and add 50 cc of concentrated nitric acid which is free from sulphuric acid. Action may not begin at once unless the cas- serole is warmed but after the metal begins to dissolve the action may become violent. In this case the casserole should be placed in cold 378 QUANTITATIVE ANALYSIS water. In the later stages it may again be necessary to heat the cas- serole. Add 1 gm of sodium carbonate, free from sulphate, and evapo- rate to dryness, holding the casserole over the flame and giving it a rotary motion to prevent bumping and to hasten evaporation. When the residue is dry, heat for 15 minutes at a temperature just below red- ness, then add 30 cc of concentrated hydrochloric acid, and again evaporate to dryness and heat as before. Cool, add 30 cc of concen- trated hydrochloric acid, warm until air iron salts are in solution, then evaporate in the same manner as before until ferric chloride begins to crystallize. Add a very small amount of hydrochloric acid to redissolve these crystals, then add 25 cc of water, filter and wash with water, then with a very small amount of hot, dilute hydrochloric acid, repeating the water and acid washing until the paper, silica and carbon are free from the red or brown stains of ferric chloride. Finally wash with hot water until the volume of the filtrate is about 200 cc. If this residue is large in quantity it will contain an appreciable amount of sulphur. In this case transfer the paper with the residue to a platinum crucible, burn until paper and carbon have been removed and fuse with 2 gm of sodium carbonate. Cool, dissolve the fusion in dilute hydrochloric acid, using no more than is necessary, and wash into the main solution. Heat to boiling and add, a drop at a time and stirring continuously, 10 cc of 10 percent barium chloride solution. Digest at a temperature near the boiling-point for 30 minutes, then allow to stand for 12 hours. Filter and wash, alternately with dilute hydrochloric acid and water, until the filter and precipitate are white and finally with water until free from chlorides. In this washing use as little hydrochloric acid as possible. Ignite the paper and precipitate in a platinum crucible and weigh the barium sulphate. If the ignited precipitate is not white some iron oxide is contained in it. In this case add 1 gm of sodium carbonate, fuse, dissolve the fusion in water and dilute hydrochloric acid and precipitate as before. Calculate the percent of sulphur in the sample. . Evolution Method. The determination of sulphur by evolu- tion depends upon the decomposition of metallic sulphides by hydrochloric acid, the resulting hydrogen sulphide being distilled and absorbed in another solution. The absorbing solution may form an insoluble sulphide with the hydrogen sulphide or it may oxidize the latter to sulphuric acid which is then determined gravimetrically. Absorbents of the first class are basic solutions of salts of lead, cadmium or silver. Absorbents of the second class are bromine in hydrochloric acid, potassium permanganate and hydrogen peroxide. A solution of cadmium chloride in excess ANALYSIS OF INDUSTRIAL PRODUCTS 379 of ammonium hydroxide is to be preferred. The precipitate of cadmium sulphide may be washed, dried and weighed, but it is better to decompose it with hydrochloric acid and titrate the liberated hydrogen sulphide with standard iodine solution. The solubility of cadmium sulphide in hydrochloric acid is not large unless the resultant hydrogen sulphide is removed, as in this case by oxidation. The evolution method may be performed in less time than the oxidation method. It has been shown by Phillips to be inaccu- rate, 1 however, for white pig iron because of the formation of organic sulphur compounds, of which methyl sulphide, (CH 3 ) 2 S, was isolated. Such sulphides are difficult to expel from the evolution flask and require as much as two hours of boiling, during which time air, carbon dioxide or hydrogen is drawn through the apparatus. Phillips found that the organic sulphides could be decomposed by passing the vapors through a tube, heated to redness. The additional time necessary for the ex- pulsion of organic sulphides from the evolution flask makes the method impracticable for white pig iron. For steel and gray pig iron, containing relatively low percents of combined carbon, the method is fairly satisfactory although it is less accurate than the oxidation method. It is therefore suited for rapid con- trol work but is not a method of precision, suitable for final testing. Determination. Use a 300-cc flask, having a round bottom, for the evolution flask. Connect, through a 2-hole rubber stopper, a 100- cc separatory funnel and a short tube, bent at a right angle with the flask. The separatory funnel should reach to the bottom of the flask and should have the bottom turned up, as in the apparatus for the determination of carbon dioxide in carbonates. The short exit tube is connected with another tube which reaches to the bottom of a bottle having a capacity of about 100 cc, in which is placed 50 cc of a solution made as follows: Cadmium chloride 5 gm, water 375 cc, concentrated ammonium hydroxide 625 cc. Weigh into the evolution flask 10 gm of steel or iron drillings, close the flask and place 75 cc of hydrochloric acid (1 : 1) in the separatory funnel. Admit the acid fast enough to cause a rapid evolution of hydrogen. Finally add all but about 5 cc of the acid and warm to assist the solution. When the metal is all dissolved boil for 5 minutes at a rate that will permit absorption of the hydrogen sulphide.' This 1 J. Am. Chem. Soc., 17, 891 (1895). 380 QUANTITATIVE ANALYSIS boiling should completely expel hydrogen sulphide and hydrogen from the flask. Remove the source of heat, disconnect the delivery tube and rinse, allowing the washings to run into the absorption bottle. If the tube contains any cadmium sulphide wash with dilute hydrochloric acid and then with water but do not agitate the solution. Rinse the contents of the absorption bottle into a 500-ec beaker, dissolving adhering precipi- tate by means of dilute hydrochloric acid, allowing this solution to run immediately into the main body of solution. Add water until the volume is about 300 cc, then add dilute hydrochloric acid until the liquid is distinctly acid in character, stirring gently meanwhile. Rapid stirring and undue agitation will cause a loss of hydrogen sulphide. Add 1 cc of starch solution and titrate at once with decinormal iodine solution. Calculate the percent of sulphur in the sample. Phosphorus. The proportion of phosphorus in steel of satisfactory quality is not usually higher than 0.1 percent and is frequently required to be less. Acid open hearth and acid Bessemer steel contain larger quantities of phosphorus than steel made by basic processes. Phosphorus occurs in steel as the phosphide Fe 3 P. Its effect is to cause brittleness of the steel, this being at least partly due to the promotion of coarse granulation. , The determination of phosphorus in iron or steel may follow either gravimetric or volumetric methods. In any case the final determination must be preceded by separation from the relatively large excess of iron. The separation is usually made by either a modification of the method of Fresenius 1 known as the "acetate method," or the molybdate method of Sonnenschein. 2 Acetate Method. This method of separating iron and phos- phorus depends upon the relatively large solubility of ferrous acetate as compared with that of basic ferric acetate and ferric phosphate. The iron is first reduced entirely to the ferrous condition by sulphurous acid, then either a small amount reoxi- dized by bromine or a small amount of ferric chloride is added. The solution is now made slightly basic, then an excess of acetic acid and water is added. A precipitate forms, consisting of ferric phosphate and basic ferric acetate, the lattter being present 1 J. prakt. Chem., 45, 258 (1848). 2 Ibid., 53, 339 (1851). ANALYSIS OF INDUSTRIAL PRODUCTS 381 in very small quantity. The larger part of the iron has remained in solution as ferrous acetate and is separated by nitration. This method necessarily leaves a small amount of iron in the phosphorus precipitate. In order to separate this, advantage is taken of the fact that small quantities of the iron are not pre- cipitated by ammonium hydroxide if organic acids are present. Either citric acid or ammonium citrate is added and the phos- phorus is precipitated by " magnesia mixture" in presence of ammonium hydroxide. The ionization of ferric citrate is so small that the solubility product of neither ferric hydroxide nor basic ferric citrate is attained. The acetate method is accurate if carefully performed, but is complicated in detail and is more liable to fail than the next method to be described. Molybdate Method. The molybdate method of separating iron and phosphorus depends upon the insolubility of ammonium phosphomolybdate and the solubility of iron in nitric acid. The iron or steel is dissolved in nitric acid, carbon is oxidized by potas- sium permanganate, the solution is nearly neutralized and a solu- tion of ammonium molybdate in nitric acid is added. The pre- cipitate of ammonium phosphomolybdate is separated by filtration and is then treated according to the method which has been selected for the final determination. The removal of carbon by oxidation is necessary in order that precipitation shall be complete. The determination of phosphorus may now be made (1) by drying and weighing the yellow precipitate of ammonium phos- phomolybdate, (2) by measuring its volume, (3) by titrating its molybdic oxide by means of a standard base, (4) by reducing its molybdic oxide to molybdenum sesquioxide and titrating by standard potassium permanganate solution, or (5) by dissolving the yellow precipitate in ammonium hydroxide and precipitating as magnesium ammonium phosphate. If method (1) is to be followed it is necessary that care be exercised in precipitating the ammonium phosphomolybdate in order that its composition may be constant. Precipitated under the conditions later described its composition is represented by the formula (NH 4 ) 3 P04.12Mo0 3 . The composition is somewhat altered by variation in temperature, excess of ammonium molybdate, excess of nitric acid and time of precipitation. ^ It also may contain small amounts of free molybdic acid, especially 382 QUANTITATIVE ANALYSIS if too much nitric acid is present, or of ammonium silicomolybdate if silicon has not been removed. The method of direct weighing is not often followed. Method (2) is a rapid but inaccurate method. The precipita- tion is carried out in a pear-shaped bulb having a graduated stem. The precipitate is packed into the stem by centrifugal action and its volume is read and converted into weight percent by a previously determined factor. Method (3) was suggested by Pemberton. 1 In this method the yellow precipitate is dissolved in an excess of a standard solution of potassium hydroxide or sodium hydroxide, the excess being then titrated by a standard acid solution, phenolphthalein being used as the indicator. The reaction between the phosphomolyb- date and the base is as follows: 12H 2 O. Upon the addition of standard acid, phenolphthalein changes color when the excess of base has been neutralized and the following reaction has occurred: (NH 4 ) 3 P0 4 +HC1 (NH 4 ) 2 HP0 4 +NH 4 C1. Twenty-three equivalents of base have therefore apparently been used at the end point and in order to express this fact in one equa- tion the reaction is often represented as follows : 2(NH 4 ) 3 P0 4 .12Mo0 3 +46KOH -> 2(NH 4 ) 2 HP04+(NH 4 )2Mo04 +23K 2 Mo0 4 +22H 2 O. This is seen to be really a direct titration of molybdic acid instead of a titration of phosphorus and it is therefore an indirect estima- tion of phosphorus and can be correct only in case the composition of the precipitate is constant. It is also essential that no free molybdic acid should be present with the phosphomolybdate. There is some difference in opinion concerning the accuracy of this method. If the precipitation is carefully performed it is probably nearly as accurate as the gravimetric method (5) . Determination by Pemberton's Method. Prepare the following reagents : (a) Acid Solution of Ammonium Molybdate. Dissolve 100 gm of * J. Chem. Soc., 15, 382 (1893); 16, 278 (1894). ANALYSIS OF INDUSTRIAL PRODUCTS 383 molybdic acid in a mixture of 144 cc of ammonium hydroxide (specific gravity 0.90) and 271 cc of water. Pour this solution, slowly and with vigorous stirring, into a mixture of 590 cc of concentrated nitric acid (specific gravity 1.42) and 1148 cc of water. Allow to stand at a tem- perature of about 40 for several days and then decant from sediment and preserve in glass-stoppered bottles. (6) Standard Potassium Hydroxide Solution, 1 cc of which is equiva- lent to 0.0001 gm of phosphorus. This should be as nearly free from carbonates as possible and is made as follows : Dissolve 2 percent more than the calculated quantity for 1000 cc, dilute to 100 cc and add 1 cc of a saturated solution of barium hydroxide. Stopper the flask and allow to stand until the precipitate of barium carbonate has settled. Decant and dilute to 1000 cc. Standardize by titration against solution (c), using phenolphthalein. (c) Standard Hydrochloric Acid Solution, equivalent in concentra- tion to the standard base; use boiled water. (d) Potassium Permanganate Solution, 1.5 gm in 100 cc. (e) Potassium Nitrate Solution, 1.0 percent. Weigh 2 gm of iron or steel into a 250-cc Erlenmeyer flask and add 100 cc of nitric acid (specific gravity 1.13) and warm until the sample is dissolved (see note on page 385: " Interference of Titanium"). Boil to expel oxides of nitrogen, then add 10 cc of solution (d) and boil until the combined carbon is completely oxidized and the excess of potassium permanganate is decomposed, as is made evident by the disappearance of the pink color. Dissolve the precipitated manganese dioxide by warming with about 1 gm of ferrous ammonium sulphate. Add dilute ammonium hydroxide very slowly and with vigorous stirring. This operation must be conducted with care because if much ferric hydroxide is allowed to form it will not readily redissolve, even though the solution still contains an excess of acid. Redissolve the precipi- tate by the addition of the least necessary quantity of nitric acid. Warm to a temperature of 60 to 65 by placing the flask in a water bath and then add 40 cc of freshly filtered ammonium molyb.date solu- tion, stir well and allow to stand 15 minutes. Filter immediately and wash with solution (e) until the washings are neutral to phenolphthalein. Transfer the paper and precipitate to a beaker and add enough standard solution of potassium hydroxide to dissolve the precipitate. Dilute to about 75 cc with recently boiled water, add a drop of phe- nolphthalein and titrate the excess of base with standard acid solution. Calculate the percent of phosphorus in the steel. Method (4) . This is also an indirect method for the determina- tion of phosphorus, since it also depends upon reactions of molyb- denum oxides, rather than of phosphorus. The precipitate of 384 QUANTITATIVE ANALYSIS ammonium phosphomolybdate, obtained as in method (3), is dissolved in ammonium hydroxide, the solution is acidified with sulphuric acid and zinc is then added. Molybdenum trioxide, Mo0 3 , is reduced to molybdenum sesquioxide Mo 2 C>3, which is again oxidized by titration with standard potassium perman- ganate solution. Method (5). This is one of the most reliable of all methods if carefully performed, since its accuracy does not depend, in any way, upon the composition of the yellow precipitate. A some- what less acid solution of ammonium molybdate may be used and this keeps better than the solution required for volumetric proc- esses. The yellow precipitate of ammonium phosphomolybdate is dissolved in ammonium hydroxide and the phosphorus is then precipitated as ammonium magnesium phosphate by the addition of a solution of magnesium chloride. The ammonium mag- nesium phosphate is ignited and weighed as magnesium pyro- phosphate. Potassium permanganate cannot be used for the oxidation of carbon since it would later form a precipitate of ammonium manganese phosphate. Determination. Prepare the following solutions: (a) Acid Solution of Ammonium Molybdate. Dissolve 100 gm of molybdic acid in a mixture of 144 cc of ammonium hydroxide (specific gravity 0.90) and 271 cc of water. Pour this solution slowly and with vigorous stirring into a mixture of 490 cc of concentrated nitric acid (specific gravity 1.42) and 1148 cc of water. Allow to stand at a tem- perature of about 40 for several days and then decant and preserve in glass-stoppered bottles. (6) Ammonium Citrate Solution. Dissolve 50 gm of citric acid in water, add 350 cc of ammonium hydroxide (specific gravity 0.90) and dilute to 1000 cc. (c) Ammonium Hydroxide Solution containing 2.5 percent of ammonia. (d) "Magnesia Mixture." Dissolve 55 gm of crystallized magne- sium chloride and 140 gm of ammonium chloride in water, add 130 cc of ammonium hydroxide (specific gravity 0.90) and dilute to 1000 cc. (e) Ammonium Nitrate Solution, 10 percent. Dissolve 1 to 2 gm of steel in 20 cc of nitric acid (specific gravity 1.2) in a casserole, cover and boil until nitrogen oxides are expelled. Evaporate to dryness on the steam bath or by agitating over a flame. Heat for 15 minutes over the direct flame in order to oxidize organic matter, formed from combined carbon. Cool, add 30 cc of concentrated hydrochloric acid and heat to dissolve iron oxide. Evaporate with ANALYSIS OF INDUSTRIAL PRODUCTS 385 stirring until ferric chloride begins to crystallize but do not allow salts to dry on the sides of the casserole. Add 10 cc of concentrated nitric acid, boil to expel chlorine, dilute to 75 cc and filter into a 250-cc flask. (If titanium is present, see note below: "Interference of Titanium.") Wash the silica and carbon on the paper with 2 per- cent nitric acid and water until the iron is all removed, as made evident by the disappearance of brown stains. Dilute the filtrate to about 100 cc and add dilute ammonium hydrox- ide solution very slowly and with vigorous stirring until a small amount of precipitate remains undissolved. Redissolve this in concentrated nitric acid, immerse the flask in water and warm to about 60. Add 50 cc of ammonium molybdate solution (a), shake and allow to remain at a temperature of 65 for an hour. Filter and wash with solution (e) until no brown stains remain on the paper. It is not necessary to re- move all of the precipitate from the sides -of the flask at this point, but it must be well washed. Place the flask in which precipitation was made under the funnel and dissolve the precipitate by adding about 25 cc of ammonium citrate solution (6). Wash the paper thoroughly with hot water. Rotate the flask un- til all of the precipitate is dissolved from the sides then nearly neutralize with hydrochloric acid. Dilute to 100 cc, add 10 cc of magnesia mix- ture, slowly and with vigorou's stirring. After the solution has stood for 30 minutes add ammonium hydroxide of specific gravity 0.90 in quantity, equal to 1/9 of the total volume of solution. Allow to stand for two hours, filter and wash with dilute ammonium hydroxide solution. Ignite in a platinum or porcelain crucible until white and weigh the magnesium pyrophosphate. Calculate the percent of phosphorus in the steel. Interference of Titanium. If titanium is present, as it fre- quently is in pig iron and sometimes in steel, the phosphorus will not all be recovered by any of the methods already described because the action of acids upon iron leaves an insoluble double salt of phosphoric acid, titanic acid and iron. In this case the residue of silica, carbon, ferric phosphotitanate, etc., obtained by filtration of the acid solution of iron, is ignited in a platinum crucible to burn organic matter, the silica is volatilized by mois- tening with a drop of sulphuric acid and adding 1 cc of hydrofluoric acid, and the residue is then fused with about 2 gm of sodium carbonate. Sodium phosphate, ferric oxide and sodium titanate, Na 2 Ti03, are formed. Sodium phosphate is dissolved in water and added to the principal solution of the iron in nitric acid. Sodium titanate and ferric oxide are insoluble in water. 25 386 QUANTITATIVE ANALYSIS Titanium. Titanium is often present in pig iron as an impur- ity, being derived from the iron ores. There is now an increasing use of titanium, in the form of iron- titanium " alloys," as an agent to promote sound castings and sound steel. Its effect is to reduce oxides, combine with nitrogen and sulphur and thus to prevent blow holes and flaws by the formation of a solid oxide or nitride which enters the slag. If this action were ideal there should be no titanium remaining in the metal at the end of the process but this is not always the case and determinations of titanium may be required. Titanium is now also used to some extent as an essential constituent of finished alloy steels. It has already been stated that much of the titanium will remain as an insoluble compound with iron and phosphorus when iron is dissolved in acids. This is freed from carbon by ignition and from silica by treatment with sulphuric acid and hydrofluoric acid. The titanium in the acid solution of the sam- ple is recovered by neutralizing the excess of acid, reducing the iron to the ferrous state by sodium thiosulphate or sulphurous acid and precipitating titanic acid by boiling. Titanic acid is an irreversible colloid (see page 17) and becomes insoluble when its solution is boiled for some time. This precipitate is removed by filtration and added to the residue already in the crucible. The whole is then fused with sodium carbonate and the sodium acid titanate, NaHTi0 3 , and ferric oxide are separated from sodium phosphate by dissolving the latter and filtering. One of two methods of procedure may now be adopted: (a) The insoluble sodium acid titanate is fused with potassium acid sulphate, forming titanic acid. Sulphuric acid and water are added, the titanic acid forming a colloidal sol. The iron also passes into solution as ferric sulphate. This is reduced to the ferrous condition by sulphurous acid or ammonium acid sulphite, the solution is largely diluted and boiled, when the sol is floccu- lated, titanic acid again passing into the irreversible gel. This is separated by filtration, washed and ignited and weighed as titan- ium dioxide. (6) The sodium titanate in the crucible is dissolved in hot, dilute sulphuric acid, transferred to a color comparison tube (a Nessler cylinder or similar tube) and treated with hydrogen peroxide. Titanium is oxidized by hydrogen peroxide to the hexavalent condition and forms an intensely yellow solution. ANALYSIS OF INDUSTRIAL PRODUCTS 387 The color is compared with that produced by a standard titanium solution in a similar tube. Determination. Weigh from 2 to 5 gm of iron or steel and place in a casserole. Add 50 cc of concentrated hydrochloric acid, cover and warm until the metal is dissolved. Filter, wash twice with hot water, transfer to a platinum crucible and burn the paper and all carbon. Add a drop of sulphuric acid and about 3 cc of hydrofluoric acid and finally heat to expel the acids and silicon tetrafluoride. To the filtrate containing most of the iron add dilute ammonium hydroxide, slowly and with continuous stirring, until a small amount of ferric hydroxide remains undissolved. Redissolve this in hydro- chloric acid, leaving the solution with a small excess of acid. Add a 20-percent solution of sodium thiosulphate until the red color of ferric chloride disappears and sulphur begins to precipitate. Dilute to about 400 cc, add 20 gm of sodium acetate and 50 cc of 30-percent acetic acid and boil for 15 minutes or until precipitation of titanic acid seems to be complete. Filter and wash two or three times with hot, 1-percent acetic acid and place the paper and precipitate in the crucible contain- ing the main portion of titanium. Burn the paper and carbon then add about 5 gm of sodium carbonate and thoroughly fuse. Cool, place the crucible in a beaker and cover with hot water. When the fusion is entirely disintegrated, filter and wash the sodium titanate and iron oxide with 1-percent sodium carbonate solution. Proceed by method (a) or (), below. (a) Gravimetric Method. 1 Return the paper containing the washed residue of sodium titanate to the crucible in which the fusion was made. Add 10 gm of potassium acid sulphate and heat gently, avoiding loss by effervescence. Gradually raise the temperature until the crucible is finally red and keep at this temperature until all the iron oxide is dis- solved. Cool, add 15 cc of concentrated sulphuric acid and heat until the entire contents of the crucible have become liquid. Cool and pour, slowly and with stirring, into 400 cc of water contained in a 500-cc beaker. If basic ferric salts precipitate, redissolve in hydrochloric acid. Add 50 cc of a 20-percent solution of sodium thiosulphate. Filter if not clear, nearly neutralize with ammonium hydroxide, redissolve any precipitate that may have formed and add a clear solution contain- ing 20 gm of sodium acetate and 150 cc of 30-percent acetic acid. Boil and filter the titanic acid. Wash three times with 1-percent acetic acid, transfer the paper and precipitate to a porcelain or platinum crucible and burn the carbon, finally igniting for five minutes over the blast lamp. Weigh the titanic oxide, Ti0 2 , and calculate the percent of titanium. 1 Blair: The Chemical Analysis of Iron, 7th ed., 184. 388 QUANTITATIVE ANALYSIS (&) Colorimetric Method. Dissolve * the residue in the crucible by heating with dilute sulphuric acid, place the filter paper in a beaker and pour the sulphuric acid upon it. Heat until the sodium titanate is dissolved then remove the paper, rinse thoroughly and rinse the contents of the crucible into the beaker. Transfer to a Nessler tube, filtering if not clear, and dilute to the mark. Prepare a standard solution of titanic acid as follows: Ignite 1 gm of the purest obtainable titanic acid in a weighed platinum crucible, cool and weigh. Dissolve the titanic acid in dilute sulphuric acid, rinse into a 1000-cc volumetric flask and dilute to the mark. Mix well, transfer to a dry glass-stop- pered bottle and record the concentration of the solution. Into four similar tubes, having the same capacity and the mark at the same height as the first tube, containing the titanium from the sample, place 1 cc, 3 cc, 5 cc and 10 cc, respectively, of the standard titanium solu- tion and dilute these to the mark. Add to each of the five tubes so prepared 5 cc of hydrogen peroxide. Compare the color of the tube containing the sample with that of the four tubes of standard, looking vertically downward through the tubes toward a white surface, placed near a window. As a result of these comparisons, limits will be found for the concentration of the sample tube. Prepare other tubes of solutions whose concentrations lie between these limits, until an equality of intensity is obtained. Calculate the percent of titanium in the sample. Manganese. Manganese is found in certain quantities in practically all iron and steel. At least traces of this metal are derived from iron ores while larger quantities are intentionally added, either to correct the undesirable effects of other elements or to add desirable properties of its own. It has already been stated that manganese overcomes the tendency of sulphur to render steel " red-short." Manganese also has an effect upon car- bon, exactly the opposite of that of silicon, which is to increase the formation of graphitic, or free, carbon. Manganese, on the other hand, increases the tendency of carbon to remain combined with iron as the carbide, Fe 3 C. With cast iron it thus favors the formation of " white" iron. Part of the carbon also combines with manganese to form a carbide, Mn 3 C, which is very hard and brittle. On this account the addition of manganese to steel in quantities above 0.50 percent renders the steel increasingly hard. Such steel is frequently used for apparatus that must resist abrasion, such as ore crushers and grinders. Several excellent methods are in use for the determination of ANALYSIS OF INDUSTRIAL PRODUCTS 389 manganese in iron and steel. The underlying principles of the most important of these will be discussed and details will be given for some. All methods involve (a) the separation of the manganese from the large excess of iron and (b) the determi- nation of the separated manganese. Bismuthate Method. Schneider 1 discovered the reaction upon which this method is based and the details of the method have since been modified. The method is now recognized as one of the most accurate and easily applied of all methods now in use. It was adopted as a standard method in 1907 by the Committee on Standard Methods for. the Analysis of Iron, of the American Foundrymen's Association. 2 This method is based upon the oxidation of bivalent manganese to heptavalent manganese by sodium bismuthate. The solution of pig iron or steel in an acid contains manganese as a mangan- ous salt. This is oxidized to sodium permanganate by sodium bismuthate, the bismuth being reduced to the trivalent condition. Sodium bismuthate is derived from bismuth pentoxide and is a salt of the hypothetical acid HBi0 3 . The reaction between sodium bismuthate and manganous nitrate may be represented thus: 2NaMnO 4 +5Bi(N0 3 )3+ 3NaNO 3 +7H 2 O. The method usually involves a solution in nitric acid. Metzger and McCrackan 3 proposed a sulphuric acid solution, the reaction taking place as had already been shown by Schneider. Determination. Prepare a solution of potassium permanganate, 1 cc of which is equivalent to 0.001 gm of manganese. Standardize against ferrous ammonium sulphate by the method given on page 214. Prepare also a solution of ferrous ammonium sulphate in recently boiled water, equivalent in concentration to the permanganate solution and containing 50 cc of concentrated sulphuric acid in each 1000 cc. This solution will slowly oxidize, even if stoppered when not in use, and it will be necessary to obtain its relation to the permanganate solu- tion at the time the determination of manganese is made,, by a blank determination. 1 Dingl. polyt. J., 269, 224 (1888); Monatsh. 9, 242 (1888). 2 J. Am. Chem. Soc., 29, 1372 (1907). 3 Ibid., 32, 1250 (1910). 390 QUANTITATIVE ANALYSIS For pig iron, which contains much silicon and free carbon, weigh i gm of drilled or crushed sample, dissolve in 50 cc of nitric acid of specific gravity 1.13 and filter, receiving the filtrate in a 250-cc flask. Wash the paper and residue with dilute nitric acid until free from iron. For steel, weigh 1 gm of drillings, placing in a 250-cc flask, and dissolve in 50 cc of nitric acid. For either pig iron or steel proceed with the clear solution as follows: Add about 0.5 gm of sodium bismuthate which is free from manganese. Heat until the permanganate, which is formed at first, is decomposed by nitric acid and the pink color disappears. This insures the oxida- tion of organic matter from combined carbon. Add enough powdered ferrous ammonium sulphate to redissolve any precipitated manganese dioxide and boil until all nitrogen oxides are expelled. Cool and add 0.5 gm of sodium bismuthate and agitate. Add 50 cc of 3-percent nitric acid and filter through an ignited asbestos filter in a Gooch crucible or a carbon tube, washing the excess of sodium bismuthate with 50 cc of 3-percent nitric acid. Immediately add from a burette the ferrous ammonium sulphate solution until the permanganate is reduced and an excess of the ferrous salt is present. Titrate this ex- cess by means of standard potassium permanganate. The ratio of concentrations of ferrous ammonium sulphate and potassium permanganate solutions should be determined each day, or more often if it changes rapidly. A direct titration should not be made because the presence of sodium bismuthate exerts a slight disturbing effect due to partial oxidation of iron. In order to correct this error the blank determination is made by treating the solutions as they are treated in the determination of manganese. Measure 25 cc of nitric acid having the same concentration as the acid used in dissolving the sample and 50 cc of 3-percent nitric acid. Add 0.25 gm of sodium bismuthate. Heat to boiling, cool and filter through asbestos into a 250-cc flask, washing with 50 cc of 3-percent nitric acid. Add 35 cc, accurately measured, of ferrous ammonium sulphate solution and titrate immediately with standard potassium permanganate solution. Calculate the number of cubic centimeters of potassium permanganate solution equivalent to 1 cc of ferrous ammo- nium sulphate solution and record this as the value of the secondary standard. Calculate the percent of manganese in the sample. Ford's Method. 1 The separation of manganese from most of the iron is accomplished, in this method, by precipitating the former metal from an acid solution, using nitric acid and potas- 1 Trans. Am. Inst. Min, Eng., 9, 397 (1879). ANALYSIS OF INDUSTRIAL PRODUCTS 391 slum chlorate. The nitric acid must be quite free from nitrous acid as the latter will redissolve manganese dioxide : Mn0 2 +HN0 2 +HN0 3 Mn(N0 3 ) 2 + H 2 0. Manganese is oxidized to the dioxide and precipitates, carrying a small amount of iron with it. The precipitate is filtered on asbes- tos that has been washed with acids and ignited, is washed and dissolved in sulphurous acid or ammonium acid sulphite and the excess of sulphur dioxide is removed by boiling. The iron is reoxidized by bromine and is then precipitated as basic acetate by boiling with acetic acid or ammonium acetate and water. In the filtrate from this precipitate the manganese is precipitated as manganese ammonium phosphate and ignited to the form of manganese pyrophosphate by a method described on page 92. The Ford-Williams Method. Williams proposed 1 the precipi- tation of manganese dioxide by Ford's method, following this by the volumetric determination of the manganese by adding a measured excess of ferrous ammonium sulphate and titrating the excess by means of standard potassium permanganate solution. The time necessary for a complete determination is thereby much shortened and the method is fully as accurate as Ford's method. In neither method is the iron precipitated, so that large samples may be used when the percent of manganese is low. The reaction between manganese dioxide and ferrous sulphate is expressed by the following equation: Mn0 2 +2FeSO 4 +2H 2 SO 4 MnSO 4 +Fe 2 (S0 4 ) 3 +2H 2 O. An objection to both the Ford and the Ford- Williams methods is in the difficulty that is experienced in filtering the manganese dioxide when the steel or iron contains much silicon. A. P. Ford and Bregowsky showed 2 that the addition of a few drops of hydrofluoric acid to the solution before filtering eliminated the silica without materially injuring the beaker. If this is not done it is necessary to evaporate such solutions to dry ness with hydrochloric acid in order to render silica insoluble. Determination. Prepare the following solutions: (a) Potassium Permanganate Solution, 1 cc of which is equivalent to 0.001 gm of manganese by the reaction just given and assuming that 1 Trans. Am. Inst. Min. Eng., 10, 100 (1880). 2 J. Am. Chem. Soc., 20, 504 (1898). 392 QUANTITATIVE ANALYSIS the excess of ferrous sulphate is to be titrated by potassium perman- ganate. (6) Ferrous Ammonium Sulphate Solution, approximately equivalent in concentration to the potassium permanganate solution and contain- ing 50 cc of concentrated sulphuric acid in each 1000 cc. Dissolve about 5 gm of sample in a 250-cc beaker in 75 cc of nitric acid of specific gravity 1.2. Evaporate until the solution becomes viscous then add 75 cc of concentrated nitric acid and 5 gm of potassium chlorate. The nitric acid must be free from nitrous acid as indicated by the absence of a brown coloration. If not perfectly colorless, draw a current of air through the acid until all oxides are removed. After the addition of concentrated nitric acid and potassium chlo- rate, boil the solution for 15 minutes, remove the flame and add about 5 or 6 drops of hydrofluoric acid, dropping it near the center of the beaker and mixing it well with the solution at once^ Boil for 10 minutes to expel silicon tetrafluoride and the excess of hydrofluoric acid, then add 1 gm of potassium chlorate and boil again until chlorine oxides are no longer evolved. Filter on a pad of acid-washed and ignited asbestos and wash two or three times with concentrated nitric acid which is free from nitrous acid. The precipitate need not all be removed from the beaker but all must be washed. Remove as much acid as possible from the filter by suction, then wash the beaker and residue on the filter with cold water until the washings are free from acid. Remove the asbestos pad and manganese dioxide to the beaker in which the latter was precipitated, wiping the interior of the Gooch cru- cible or filtering tube with a tuft of asbestos. Measure into the beaker enough ferrous ammonium sulphate solution to dissolve completely the manganese dioxide and stir until solution is complete. Titrate at once with standard potassium permanganate solution. Measure 35 cc of ferrous ammonium sulphate solution into another beaker and titrate with standard potassium permanganate solution. Calculate the number of cubic centimeters of potassium permanganate solution equivalent to 1 cc of ferrous ammonium sulphate solution and record this as the value of the secondary standard. Calculate the percent of manganese in the sample. Volhard's Method. This method is discussed on page 217. Applied to iron and steel it is somewhat difficult of execution because of the large amount of ferric hydroxide that is produced when zinc oxide is added. Either the bismuthate or the Ford- Williams method is to be preferred to it. Acetate Method. This title really applies only to the sepa- ANALYSIS OF INDUSTRIAL PRODUCTS 393 ration of manganese and iron and it is, in practice, followed by any one of several methods of determination. In the discussion of the acetate method for separating iron and phosphorus (page 380) it was noticed that the object is to precipi- tate the phosphorus as ferric phosphate, along with the least possible excess of basic ferric acetate, leaving the greater portion of the iron in solution in the ferrous condition. In the acetate method for manganese separation the object is to precipitate all of the iron as basic ferric acetate, leaving the manganese in solu- tion to be subsequently determined as manganese pyrophosphate or manganese tetroxide, or by one of the volumetric processes. The method is not applicable to the use of more than 1 gm of sample because of the difficulty that is experienced in the filtra- tion of the large quantity of colloidal basic ferric acetate and in washing this precipitate free from manganese. Walters' Method. Marshall showed 1 that ammonium per- sulphate, in presence of silver nitrate, oxidizes manganese from the bivalent to the heptavalent condition, thus producing per- manganates from manganese salts. His interpretation of the reaction is as follows: (NH 4 ) 2 S 2 8 +2AgN0 3 Ag 2 S 2 8 +2NH 4 N0 3 , Ag 2 S 2 O 8 +2H 2 0- 2H 2 30 4 +Ag 2 O 2 . According to this the silver peroxide, formed momentarily in small amount, is responsible for the oxidizing action: 5Ag 2 2 +2Mn(NO 3 ) 2 +6HNO 3 2HMn0 4 +10AgNO 3 +2H 2 0. Walters applied these reactions to the quantitative determina- tion of manganese in iron and steel. 2 The sample is dissolved in nitric acid, silver nitrate and ammonium persulphate are added and the intensity of color is compared with that produced by a standard steel in which the manganese has been determined by another method. The relative volumes required to produce the same intensity of color in the two provide a basis for the calcula- tion of the percent of manganese in the sample. Determination. Weigh 0.2 gm of the sample and the same amount of a standard steel of known manganese content, placing in different test tubes having a capacity of 50 cc. Add to each 10 cc of nitric acid 1 Chem. News, 83, 76 (1901). 2 Ibid., 84, 239 (1901). 394 QUANTITATIVE ANALYSIS (specific gravity 1.2) and immerse the tubes in hot water until solution is complete and brown oxides of nitrogen are expelled. Add 15 cc of a solution containing 0.02 gm of silver nitrate. Immediately add 1 gm of ammonium persulphate and continue warming until the pink tint of permanganic acid is fully developed, which will require about 1 minute. Remove the tubes from the bath while oxygen is still being evolved and place in cold water. When the solutions are cool rinse into 50-cc volumetric flasks, dilute to the mark and mix. If free carbon is present allow it to settle, then pour the solution whose color is less intense into a color comparison tube (a Nessler or similar tube). Fill a burette with the other solution and measure this into a second tube until the color has the same intensity, viewed from above when placed over a white surface near a window. From the relative volumes of the two solutions calculate the percent of manganese in the sample. Peter's Method. 1 This is an older method than that of Wal- ters and is also a colorimetric method, the manganese being oxi- dized to permanganic acid by lead peroxide in presence of nitric acid. It is necessary to remove the excess of lead peroxide before comparing the colors and this constitutes the greatest objection to the method. The reaction is expressed as follows: 5Pb0 2 +2Mn(NO 3 ) 2 +6HN0 3 5Pb(NO 3 ) 2 +2HMnO 4 +2H 2 O Deshay's Method. Manganese is here oxidized by lead per- oxide, as in Peter's Method, followed by titration of the perman- ganic acid by a standard solution of sodium arsenite. 5Na 3 As0 3 +2KMn0 4 +6HNO 3 5Na 3 AsO 4 +2Mn(NO 3 ) 2 + 3H 2 0+2KN0 3 . Moore and Miller suggested 2 the separation of iron and man- ganese by precipitating iron as ferric hydroxide by the addition of pyridine. The separation is quantitative, although it has not yet been applied to iron and steel analysis. Tungsten. When tungsten is alloyed with iron in steel it has the effect of retarding the change from hard to annealed steel. If sufficient tungsten is present (3 or 4 percent or more) the change to soft steel is almost entirely prevented. " Self-hardening'' steels, invented by Mushet and often given his name, contain 4 to 12 percent of tunsgten, 2 to 4 percent of manganese and 1.50 to 1 Chem. News, 33, 35 (1876). 2 J. Am. Chem. Soc., 30, 593 (1908). ANALYSIS OF INDUSTRIAL PRODUCTS 395 2.50 percent of carbon. They are called self-hardening because, when subjected to the ordinary process of slow cooling for anneal- ing, they retain their hardness, even though they have been cooled from a temperature near the melting-point. Taylor and White, in 1906, developed a process for imparting a remarkable degree of toughness to self-hardening steels, so that they can be used for steel-cutting tools that are used for such rapid cutting that they become red hot and yet do not lose their hardness or toughness. Such steels are known as " high-speed tool steels." The carbon percent is usually less than 0.75 and they contain from 5 to 25 percent of tungsten, 2 to 10 percent of chromium and less than 0.50 percent of manganese. The thermal treatment of " high-speed" steels consists of heating to near the melting-point and then cooling in a blast of air. The determination of tungsten in steel must include separation from iron, silicon, carbon, phosphorus and usually chromium, since the latter metal is now generally associated with tungsten in tool steels. A gravimetric method is usually employed, tung- sten being weighed as tungstic oxide, WO 3. The details of the following method are mainly those given by Johnson. 1 Determination. dissolve 2 gm of sample in 30 cc of sulphuric acid (1 : 3) in a 250-cc beaker or casserole, heating to hasten solution. Add 60 cc of nitric acid (specific gravity 1.2) and digest at a temperature near the boiling-point until the residue is yellow and is free from black particles. Filter and wash free from iron by means of dilute sulphuric acid. The filter paper now contains the main portion of the impure tungstic acid. Transfer to a weighed platinum crucible and burn the paper at a low temperature. Recover the tungsten from the filtrate by precipitating by means of cinchonine solution (25 gm of cinchonine in 200 cc of 1 : 1 hydro- chloric acid). Filter and wash free from iron by means of the cincho- nine solution. Bum the paper in the crucible containing the main portion of tungstic oxide and weigh, then add potassium acid sulphate to the extent of about twenty times the weight of residue in the crucible. Heat, gradually at first, finally raising the temperature until the cru- cible is dull red and keep at this temperature until yellow particles of tungstic oxide are dissolved. Silica will remain undissolved. Cool and place the crucible in a 250-cc beaker containing 100 cc of 10-percent 1 Chemical Analysis of Special Steels, Steel-Making Alloys and Graphite, 64 396 QUANTITATIVE ANALYSIS ammonium carbonate solution and warm until disintegration of the mass is complete. Filter and wash the residue with 1-percent ammonium carbonate solution until the washings are free from sulphates. Ignite the paper in the same crucible as that used first and weigh. This gives the weight of ferric oxide, chromium oxide and silica and this subtracted from the weight of the total residue gives the weight of tungstic oxide, WOs. Calculate the percent of tungsten in the sample. Chromium and Nickel. Alloy steels con- taining nickel and chromium, also chromium and tungsten, are now of considerable com- mercial importance. Both nickel and chro- mium increase the hardness, tensile strength and elastic limit of steel and decrease the ductility but slightly, if at all. Nickel also lowers the temperature at which quenched steel is softened by slow cooling. In commer- cial nickel steels the nickel is not present to the extent of more than 3.50 percent, the carbon content being not more than 0.50 per- cent. Chrome steels usually contain not more than 3 percent of chromium and less than 1 percent of carbon. Various combinations of chromium, nickel, carbon, and iron produce chrome-nickel steels of great strength and hardening power. The separation of nickel and chromium from iron may be conveniently made by the ether method of Rothe. 1 This is based upon the fact that from a solution in hydrochloric acid having a specific gravity between 1.100 and 1>1Q5 ( contaming 2 1 to 22 percent of acid) ether will extract all but traces of FIG. 94. Appara- tus for separation of iron from other metals by solution in ferric chloride, leaving chlorides of chromium, ether. nickel, copper, manganese, aluminium and cobalt in the water solution. The apparatus shown in Fig. 94 may be used for the separation. The manipu- lation of the apparatus is described below. After the separation of iron the solution in water is boiled 1 Mitt. kgl. tech. Versuchs., 1892, 132; J. Soc. Chem. Ind.,~ll, 940 (1892). ANALYSIS OF INDUSTRIAL PRODUCTS 397 to remove dissolved ether, after which several methods are available for the determination of the metals in this solution. The method described below is probably as easy of execution and as accurate as any of these. In this method chromium is precipi- tated as chromium hydroxide which is ignited and weighed as chromium sesquioxide, Cr 2 3 . Nickel is deposited electrolyt- ically. If copper is present it may also be separated electrolyt- ically before nickel is deposited. Determination. The quantity of steel to be taken for analysis will depend upon the percents of nickel and chromium present. If 1 to 3 percent of either metal is contained in the steel about 2.5 gm will be sufficient. If one metal is present in about this proportion and the other in but traces it may be necessary to use two samples, making the determination of the metal whose percent is small upon a larger sample. Weigh the proper quantity of sample and place in a casserole. Dis- solve in 30 cc of concentrated hydrochloric acid and 10 cc of concen- trated nitric acid, adding the latter acid cautiously. Evaporate to dry- ness and heat for a short time then redissolve in dilute hydrochloric acid and filter to remove silica. Evaporate until the liquid thickens, due to the separation of ferric chloride crystals. This gives an acid of approxi- mately correct composition for the ether separation. Transfer the solu- tion to bulb A of the apparatus shown in Fig. 94, the lower cock being turned to close both bulbs; rinse the casserole with hydrochloric acid having a specific gravity 1.100, until the bulb is nearly half full of solu- tion. Nearly fill the bulb with ether, close the upper cock and mix gradually by shaking, cooling under the tap to avoid rise in temperature, which would cause reduction of ferric chloride by ether. Allow the apparatus to stand until perfect separation of ether and water has occurred, then carefully turn the lower cock to establish communica- tion between the bulbs, allowing the lower water solution to run into bulb B. Introduce about 10 cc more of hydrochloric acid into A, mix and separate as before. Turn the lower cock to allow the aqueous solution in B to run into a 250-cc beaker. Boil the solution until the odor of ether has disappeared. Add a few drops of bromine to oxidize iron and manganese, make slightly basic with ammonium hydroxide and boil. The precipitate consists of a small amount of ferric hydroxide together with chromium hydroxide, aluminium hydroxide and manganese peroxide, but it is diffi- cult to wash it free from the soluble nickel and copper salts. To make the separation complete, redissolve the precipitate in hydrochloric acid and reprecipitate by means of bromine and ammonium hydroxide. Set aside the solution for the determination of nickel. 398 QUANTITATIVE ANALYSIS Chromium. Transfer the paper and precipitate to a platinum cru- cible and burn the paper. Add 5 gm of sodium carbonate and 0.5 gm of potassium nitrate and fuse until effervescence occurs. Place the cru- cible on its side in a beaker and cover with water. Heat until the mass is disintegrated, filter and wash with 1-percent sodium carbonate solu- tion. The residue consists of ferric oxide, some manganese oxide and aluminium oxide and it is discarded. The solution contains sodium chromate and potassium chromate and some sodium manganate. Evaporate the solution nearly to dryness after adding 5 gm of potas- sium nitrate and enough ammonium hydroxide to give a distinct odor. Dilute to 100 cc and filter, thus removing aluminium hydroxide and man- ganese dioxide. Boil the filtrate to remove ammonia then add a slight excess of hydrochloric acid and 5 gm of sodium acid sulphite. This reduces sodium chromate to chromium chloride. Boil to remove all sulphur dioxide, add a slight excess of ammonia, boil for one minute and filter the precipitate of chromium hydroxide. Wash with water until free from chlorides, burn the paper, ignite the precipitate and weigh as chromium sesquioxide, Cr 2 03. Calculate the percent of chromium in the sample. Nickel. The solution obtained above contains nickel and copper if these were present in the steel. Add 10 cc of concentrated sulphuric acid and evaporate until the heavy fumes of sulphuric acid appear, thus removing chlorides. Cool, dilute and deposit the copper and nickel by means of the current, using the method described in connection with the analysis of a nickel coin, page 135. If copper is known to be absent the solution may be made basic with ammonium hydroxide im- mediately after evaporating with sulphuric acid and diluting, the nickel then being deposited at once. Calculate the percent of copper and of nickel in the steel. Nickel may -also be determined by precipitating the almost insoluble nickel dimethylglyoxime by the addition of an alcohol solution of dimethylglyoxime to the slightly basic solution containing nickel. 1 Oxygen. Steel in the liquid state dissolves considerable quantities of oxygen and nitrogen from the air, as well as hydro- gen from dissociated water vapor. Upon cooling these gases are released from the solution and give rise to flaws known as "blow holes." Oxygen, however, chemically combines with the iron as it cools, forming ferrous and ferric oxides and it is in this way retained. This is even more objectionable than nitrogen or hydrogen because the oxides of iron render the steel brittle and ^bbotson: Chem. News., 104, 224 (1911); Bogoluboff: Stahl u. Eisen (1910), 458. ANALYSIS OF INDUSTRIAL PRODUCTS 399 they also form gases when the steel is heated, by combination with carbon of the steel. The only satisfactory method for the determination of oxygen is Ledebur's method. 1 This consists in heating the finely di- vided sample in a current of pure, dry hydrogen, absorbing and weighing the water produced. Several important sources of error must be avoided. The hydrogen must be absolutely free from water, carbon dioxide, hydrogen sulphide, and oxygen. Traces of the latter gas are removed by passing through a pre- liminary heating tube containing platinized asbestos, then through a water absorbent. Carbon dioxide and hydrogen sul- phide are removed by passing through potassium hydroxide solution. Moisture is also absorbed before the gas enters the tube containing the sample. The sample must be quite free from oil and should be taken by very slowly drilling or milling. If the metal is heated by cutting it will be superficially oxidized. The sample is placed in a combustion tube for heating with hydrogen. A second furnace may be used for the preliminary heating of the hydrogen or the tube containing platinized asbestos may be placed in the furnace containing the main tube. For the sample a silica tube 3/4 inch by 30 inches is suitable. For the preliminary heating a silica tube 1/4 inch by 12 inches will serve unless it is to be placed in the furnace with the larger tube, in which case its length must be the same as that of the larger tube. The apparatus is set up in the following order: (D Hydrogen generator of the Kipp or similar type, charged with zinc and hydrochloric acid (1:1). (2) Absorption bottle half filled with 33-percent potassium hydroxide solution. (3) Absorption bottle half filled with concentrated sulphuric acid. (4) Silica tube, 1/4 inch in diameter, containing platinized asbestos for 6 inches of its length. (5) U-tube filled with dry calcium chloride. (6) Silica tube, 3/4 inch in diameter, to hold the boat containing the sample. (7) Two U-tubes filled with dry calcium chloride, the first (a) for absorption of the water produced by the combina- tion of the oxygen of the sample with hydrogen, the second (6) to act as a guard tube. (8) An aspirator similar to that used for carbon 1 Leitfaden fur Eisenhuttenlaboratoiien, 6th ed., 122; Stahl u. Eisen, 2, 193 (1882). 400 QUANTITATIVE ANALYSIS dioxide determinations. Prepare another U-tube (9) containing calcium chloride, close the side tubes and do not connect in the apparatus. For the directions for filling absorption tubes and setting up the apparatus, see the determination of carbon dioxide in carbonates, page 105. Determination. Set up the apparatus as already indicated but do not connect the aspirator. Pass a rapid stream of hydrogen through the apparatus for 30 minutes to displace all of the oxygen, then weigh and insert the absorption tube (7 a) and heat the tubes to bright redness. Continue the passage of gas for 30 minutes then remove the tube (7a), wipe clean, close the side tubes with rubber tubes and plugs, leave in the balance case for 10 minutes, remove the plugs and weigh. The increase in the weight of the tube in the blank determination should not be more than 2 or 3 mg. Run a second blank determination or more if the gain is not constant within 0.5 mg. Allow the combustion tube to cool and, in the meanwhile, weigh 20 to 30 gm of finely divided steel sample and place in a boat which is 6 inches long and 1/2 inch wide. With the stream of hydrogen still passing quickly open the end which is nearest the absorption tube 7a, insert the boat, push it to the middle of the tube, and quickly insert the stopper. By opening this end of the tube the current of hydrogen prevents the entrance of oxygen, which would entirely vitiate the results of the determination. Insert the weighed absorption tube, heat the silica tubes to redness and main- tain this temperature for 30 minutes with the hydrogen passing through at the rate of about six bubbles per second in the tube containing sul- phuric acid. At the end of this period disconnect {he absorption tubes (7 a) and (76) from the silica tube, connect tube (9) with the unguarded end of (7a) and connect (76) with the aspirator. Draw air through the tubes rapidly until about 500 cc has passed, as measured by the water that has run out of the aspirator. Close the side tubes of (7a) with rubber tubes and plugs and stand in the balance case. At the end of 10 minutes remove the rubber tubes and plugs and weigh the absorp- tion tube. From the weight of water so fdund calculate the percent of oxygen in the sample. Treatment of Steel. The property which makes iron the most generally useful of all metals is the property of combining with various quantities of other elements in such a way that its physical and mechanical properties are varied over a wide range. Scarcely second to this property is that of undergoing important changes in character as the compounds with carbon are subjected to dif- ANALYSIS OF INDUSTRIAL PRODUCTS 401 ferences in thermal treatment. The latter property is possessed by all irons containing carbon. These include the crude products of the blast furnace, "pig iron/' and the various grades of the more refined product, known as "steel/' The fact that a very small percent of carbon gives iron the capability of being hardened by suddenly cooling from high tem- peratures has been known for a long time. The beginning of an understanding of why this is true came when it was found that certain structural changes in steel take place with thermal treatment. The entrance of the microscope into the field of metal testing marked the beginning of a new age for steel, an age of the development of the scientific principles underlying thermal and mechanical treatment. The chemist's analysis is no longer expected to tell the entire history of the steel. As a result of the analysis we know the composition but this tells us only what the steel may be made to do. The microscope, following this, tells what the steel has been made to do. This is a develop- ment of vast importance to the user of steel. When the manufac- turer of steel articles buys his bars, sheet, rods or billets from the manufacturer of the steel itself, he is chiefly concerned with the composition, with respect to the various elements that are com- bined with iron to make the commercial steel, because he knows the composition that is necessary for his particular purpose. When he has made the steel into the forms necessary for his manufac- tured article of commerce he is still more seriously concerned with the properties that have been given, or are to be given, by the careful thermal treatment that is necessary. The analysis can be of little or no assistance at this point. The percentage composition is not materially changed by thermal treatment, except in those forms of combined chemical and thermal treat- ment, known as "case-hardening" or " cementing." The micro- scopic anatomy of the steel is, on the contrary, very profoundly changed and these recognizable changes are so intimately asso- ciated with changes in physical and mechanical properties that the microscope is, at this point, the most important instrument available for testing. This branch of testing is known as " metallography." A thorough discussion of the principles of thermal treatment and of the metallography of steel would require a volume in itself. In the next following pages a brief outline of these principles 26 402 QUANTITATIVE ANALYSIS will be given, with directions for a limited number of experiments, which will serve to illustrate the main points. Thermal Changes. If steel containing about 0.05 percent of carbon is allowed to cool from a high temperature and the rate of cooling is followed by means of a sensitive pyrometer, it will be noticed that the previously uniform rate of cooling is interrupted at about 875, the temperature remaining stationary for a short time or the cooling being at least retarded. At about 750 another interruption is noticed and at 690 still a third. The exact location of these points depends upon the composition of 0.4 1.4 1.6 1.8 0.6 0.8 1.0 1.2 Percent of Carbon FIG. 95. Recalescence curve for steel. the steel and also upon the rate of cooling, more rapid cooling lowering the point of change. These interruptions in the cooling process are due to certain internal changes which involve the evolution of heat and the temperatures at which they occur are therefore called points of "recalescence" or " critical points." If a series of steels having gradually increasing percents of carbon is treated as just described it will be noticed that the upper critical point is lowered as the carbon percent increases, while the lower two critical points remain practically unaltered until a steel containing about 0.42 percent of carbon is reached, when the upper two points merge. As the carbon percent is further increased this new point is still further lowered until the steel ANALYSIS OF INDUSTRIAL PRODUCTS 403 having 0.85 percent carbon is reached when all three changes are merged. In order to distinguish the various points of recalescence the lowest is denoted by Ari, the second by Ar 2 and the highest by Ar 3 . The relations between carbon percent and points of recales- cence are shown in Fig. 95, which is somewhat idealized. In steels containing more carbon than 0.85 percent, Ar\ remains at the same temperature while a new higher point is again noticed, increasing carbon raising the location of this point. Sauveur denotes this by the symbol Ar cm - Allotropism of Iron. The occurrence of these points of recal- escence shows that internal changes are taking place while the steel is cooling. These changes might, conceivably, be either physical or chemical, or both. The fact that Ar 3 and Ar 2 are noticed in the purest iron that can be manufactured indicates that at least a part of the heat evolution is due to allotropic changes. That such changes are not the only ones occurring at the critical points of steel is shown by the fact that the recal- escence in pure iron is very faint while in medium and high carbon steel it is very pronounced, actual glowing being noticed at Ar\. Iron is now believed to exist in at least three allotropic modifica- tions, as follows: f-iron exists from the melting-point down to Ar 3 . It crystal- lizes in the cubic system, the prevailing forms of crystals being octahedra. Its electrical resistance is about ten times that of a-iron and it is non-magnetic. Its hardness is somewhat less than that of /?-iron but greater than that of a-iron It dissolves iron carbide to the extent of 25.5 percent, corresponding to 1.7 percent of carbon. /?-iron is normally present between Ar s and Ar 2 until these points merge, when it disappears. It crystallizes in the cubic system with cubes as the prevailing forms. It is feebly magnetic and is very hard. It has little or no solvent power for carbon. a-iron exists below Ar%. It also crystallizes in the cubic system. Its electrical resistance is greater than that of either ,#-iron or f-iron and it is strongly magnetic. It is the softest form of iron. Influence of Sudden Cooling. The influence of carbon upon the magnitude and location of the thermal changes of iron would indicate that it also plays a part in the changes occurring 404 QUANTITATIVE ANALYSIS at the points of recalescence. This is confirmed by many careful chemical and microscopical examinations of polished and etched steel samples. The question very naturally arises as to how a microscopical or chemical examination can be made of any but a-iron, since the other forms do not normally exist at ordinary temperatures. Such examination is made possible by the fact that sufficiently sudden cooling partially or entirely arrests the change from f-iron to /?-iron and from /?-iron to a-iron, with the accompanying changes in the form of carbon. This, as will later be shown, is the property upon which all thermal treatment of steel is based. If, then, a sample if iron is heated to a temper- ature above Ar 3 and suddenly cooled by quenching in cold water, mercury, liquid air, etc., f-iron is retained as an abnormal substance because at ordinary temperatures the molecular mobility of iron is too small to permit any changes in structure which would have taken place at higher temperatures if sufficient time had been allowed. Similarly /?-iron may be retained by quenching from a temperature between Ar 3 and Ar%. It has also been shown that carbon is retained in the form which is nor- mal to the quenching temperature. This method of prevent- ing changes that normally occur, retaining abnormal structures and physical states, opens the way for an investigation of the condition of steel at high temperatures. Proximate Constituents of Slowly Cooled Steel. Pure iron is never obtained in industrial practice and is seldom desired, since steel is the form which so readily lends itself to modification in its properties to suit the most varied requirements. Ferrite. The commercial article that probably most nearly approaches pure iron in character is what is now known as " ingot iron." If a small section of such iron is given a high polish and is then subjected to the action of an etching agent, such as nitric acid or tincture of iodine, and the sample is then placed under a microscope it will be found that the substance that to the naked eye appeared quite bright and homogeneous is really made ,up of numbers of granules somewhat resembling crystals in form. The structure is really crystalline but the crystals are so hampered in the process of formation that the word " granule" better expresses their appearance. The action of the etching solution has been to attack the grains more vigorously at the lines of juncture and therefore to bring their outlines into relief. A pho- ANALYSIS OF INDUSTRIAL PRODUCTS 405 tomicrograph of such a section is shown in Fig. 96. The carbon- less iron thus appearing as granules is found to be a distinct con- stituent of all steel containing less than 0.85 percent of carbon. This constituent is called "ferrite" and it is what has already been described as a-iron. Some of the properties of ferrite have been described in the discussion of a-iron. In addition it may be said that it is quite ductile, that it has the power of combining with carbon to form other constituents of steel and that its tensile strength is about 50,000 pounds per square inch and its elongation is about 40 per- cent. It cannot be hardened by sudden cooling except to the (extent that is indicated by the partial retention of /'-iron. Cementite. If a piece of steel containing more than 0.85 per- cent of carbon is similarly polished, etched and examined under somewhat high magnification it will exhibit certain shaded areas and also bright areas somewhat similar in appearance to the granules of ferrite, except that their extent will be small with a small excess of carbon over 0.85 percent and that it will increase with increasing carbon percent. Such a section is shown in Fig. 97. The bright constituent is iron carbide, FeaC, already mentioned in connection with the analysis of steel. As a distinct crystal- line constituent of steel, iron carbide is known as "cementite." The properties of cementite are quite different from those of ferrite. Cementite cannot be obtained in the pure condition because, as its formula indicates, it contains 6.67 percent of carbon, and steel of this composition cannot be prepared without the presence of manganese or other elements or without the separation of graphitic carbon. According to Sauveur 1 the ten- sile strength of cementite is about 5000 pounds per square inch .and its ductility (represented by elongation) is practically zero. It is the hardest constituent of steel and cannot be made appre- ciably harder by sudden cooling. Pearlite. In the description of cementite it was stated that steel containing more than 0.85 percent of carbon shows certain shaded areas in addition to the bright cementite, when it is polished, etched and highly magnified. (Fig. 97.) These shaded areas increase in extent as the carbon approaches 0.85 percent from either above or below this figure. If the carbon 1 The Metallography of Iron and Steel, Lesson V, 15. 406 QUANTITATIVE ANALYSIS X 550 X 550 FIG. 96. Ingot iron, slowly cooled. FIG. 97. Steel, carbon 1.3 percent. Ferrite granules, pitted. Heated for 3 hours at 800 and slowly cooled. Pearlite and cementite. X1200 FIG. 98. Steel, carbon 0.85 per- cent. Cooled in air from above the critical range. Pearlite. X700 FIG. 99. Steel, carbon 0.88 per- cent. Cooled in air from above the critical range. Pearlite, sorbite and troostite. X650 FIG. 100. Steel, carbon 0.6 percent. Cooled in air from above the critical range. Pearlite, sorbite and ferrite. X130 FIG. 101. Steel, carbon 0.1 per- cent. Slowly cooled. Ferrite and pearlite. ANALYSIS OF INDUSTRIAL PRODUCTS 407 content is practically 0.85 percent the relatively bright areas of cementite and of ferrite are both absent. If the carbon content is less than 0.85 percent ferrite appears in addition to the shaded portion. Both ferrite and cementite are distinct, homogeneous entities, the one an element, the other a definite chemical com- pound. The darker substance is neither element nor compound but is a composite of both, so constituted that it reflects light in a way suggesting mother-of-pearl. On this account it is known as "pearlite." The peculiar iridescence of pearlite is evident without magnification of the polished specimen. If an unhardened steel is prepared as already described and examined under high magnification the structure of pearlite is shown quite clearly to be that of alternating plates or laminae of a dark and a light material. This is shown in Figs. 98, 99 and 100. These plates have been shown to be chemically free iron and iron carbide. That is, they are the materials that have been described as ferrite and cementite, although the latter names are generally reserved for distinct granules of these materials, not constituents of pearlite. The laminations of pearlite show dis- tinctly because the etching agent attacks free iron more readily than iron carbide, thus bringing the former into relative shadow. The laminae are usually bent or contorted. The properties of pearlite are between those of ferrite and cementite, as might be predicted from a knowledge of its com- posite nature. One of the most characteristic of the properties of pearlite is its constancy in composition, the percent of free iron being 87.26 and that of iron carbide 12.74. This is calcu- lated from the percent of carbon in cementite (6.67) and in pearl- ite (0.85). The percent of iron carbide in pearlite is therefore ~a~7^, = 12.74. Pearlite has a hardness between that of b.o7 ferrite and of cementite; its tensile strength is about 125,000 pounds per square inch and its ductility is represented by an elongation of about 10 percent. It is the only one of the three constituents of unhardened steel that possesses the power of hardening when suddenly cooled and it is due to pearlite that steel exhibits this remarkable and useful property. Relation between Structure and Carbon Percent. It would follow from what has been said concerning the chemical and physical composition of ferrite, cementite and pearlite that the 408 QUANTITATIVE ANALYSIS relative areas of these substances appearing in the polished and etched section would give at least a fair indication of the percent of carbon. This is seen to be the case by an inspection of Figs. 101, 102, 103, 104, and 105. An estimate of the carbon percent may be made from the microscopic appearance, with an accuracy of 0.10 percent for steels containing less than about 0.6 percent of carbon. With more carbon than this it becomes more difficult to judge the percent. For this purpose a magnification of 50 to 150 diameters is suitable. Austenite. If a small piece of steel is heated to a temperature considerably above Ar s and cooled very suddenly by quenching, the microscope does not reveal the presence of either ferrite, pearl- ite or cementite. Instead the mass has assumed a fairly definite crystalline appearance, in which no constituent can be distin- guished as different from another. The steel has also been made much harder than formerly, the degree of hardness depending upon the percent of carbon. The single crystalline substance composing the mass of hardened steel is called "austenite," after Roberts-Austen the English metallurgist. Austenite contains carbon in any proportion up to 1.7 percent. It is non-magnetic and thus contains iron in the f modification. It is not the hardest 'constituent of hardened steel but is harder than either ferrite or pearlite. Relation of Ferrite, Cementite, Pearlite and Austenite to the Critical Points of Steel. It has been shown that slowly cooled steel contains either ferrite and pearlite, pearlite alone, or cemen- tite and pearlite, according to whether the steel contains less than 0.85 percent, exactly 0.85 percent or more than this amount of carbon, also that steel cooled with sufficient rapidity contains only austenite, no matter what the percent of carbon may be. The change of austenite into the other constituents when the steel is slowly cooled begins normally at the temperature denoted by the line ABCD of Fig. 95, that is at the point Ar 3 , for the steel of any particular composition. If less than 0.85 percent of carbon is contained in the steel the formation of ferrite begins at Ar 3 and continues until Ar t is reached, when the austenite then remaining changes completely into pearlite. Since pearlite has a definite and constant composition it will be seen that austenite must have lost enough iron between Ar s and Ar\ to leave austenite contain- ing 0.85 percent of carbon. If more than this percent of carbon ANALYSIS OF INDUSTRIAL PRODUCTS 409 is contained in the steel austenite loses iron carbide at the point Ar cm and continues to lose it until Ar t is reached, when again pearlite is formed of what remains. Steel as a Solid Solution. This behavior of slowly cooling steel suggests the behavior of ordinary solutions where separation of the constituents occurs at tolerably definite temperatures, depending upon the percentage composition of the solution. For a solution of any two substances in each other there exists a percentage composition for which the lowest possible freezing point is noticed. If more of either constituent is present there is a higher freezing point and at this point the excess of this constituent freezes and separates, leaving a solution which also freezes at or near the lower point already noticed. At the temperatures of separation there also occur thermal changes which are usually in the nature of evolution of heat. The solu- tion having the lowest freezing point of all solutions of a given pair of components is known as the "eutectic" solution. In these and a great many other respects, steel resembles liquid solutions. In fact the resemblance is so close that it can no longer be doubted that austenite is a solution of f-iron and iron car- bide in each other. To be sure, austenite is solid while most com- mon solutions are liquid, but from the physical standpoint the mere state of aggregation is a point of secondary importance in the consideration of solutions. Austenite possesses the property of physical homogeneity, common to liquid and all other solutions. It changes completely into the resolution product, pearlite, at Ari if of eutectic composition. If not of this composition it begins to lose excess of either iron or iron carbide, as ferrite or cementite, at Ar s , Ar 3)2 , or Ar cm , just as the liquid solution would lose the excess of solvent or solute if not of eutectic composition. Coincident with the separation of ferrite or of cementite allotropic changes also occur in the iron. Martensite. It is difficult to prevent entirely the change of austenite into the constituents normal to lower temperatures by the process of sudden cooling. If the quenching medium is one that will cause cooling of extreme rapidity the change may be almost entirely arrested. Such a medium is ice-water, ice and salt solution or liquid air. These media are not generally used in commercial processes of hardening and therefore austenite does not make up the mass of commercially treated steels. In- 410 QUANTITATIVE ANALYSIS X130 FIG. 102. Steel. Carbon 0.3 per- cent. Slowly cooled. Ferrite and pearlite. XI 30 FIG. 103. Steel. Carbon 0.5 per- cent. Slowly cooled. Ferrite and pearlite. X130 FIG. 104. Steel. Carbon 6.6 per- cent. Slowly cooled. Ferrite and pearlite. X130 FIG. 105. Steel. Carbon 0.72 per- cent. Slowly cooled. Ferrite and pearlite. X650 FIG. 106. Steel. Carbon 0.8 per- cent. Quenched at 725. Marten- site. I X650 FIG. 107. Steel. Carbon 0.8 per- cent. Quenched from above the critical range. Martensite. (More deeply etched than section in Fig. 106.) ANALYSIS OF INDUSTRIAL PRODUCTS 411 stead a new formation, called "martensite" (after Martens, Ger- man metallurgist), makes its appearance. The true nature of martensite is not, even now, thoroughly understood. It was formerly thought to be the stable solid solution itself, but this view is no longer held. While opinions still differ, martensite is probably a transition product between austenite, on the one hand, and pearlite on the other and it is a solid solution of iron carbide in iron; the iron is not here in the f condition, as in aus- tenite, but in either the /?, the a or both /? and OL conditions, so that martensite is magnetic and is even harder than austenite. Like austenite, its hardness varies with the percent of carbon in the steel, i.e., with the percent of dissolved iron carbide in the martensite. The crystalline appearance of martensite is rather characteristic, as is shown in Figs. 106 and 107. It consists of flat plates of intersecting needles which usually arrange them- selves as equilateral triangles. Being a transition substance and not in real equilibrium at any temperature, but merely a metastable condition retained by rapid cooling, martensite is frequently associated with austenite, or with troostite or sorbite, two other transition constituents to be presently described. Relation between Location of the Thermal Critical Points and the Rate of Cooling or Heating. Reference has already been made to the influence of rapid cooling upon the resolution of austenite into its components, the change being entirely pre- vented by sufficiently rapid cooling. Even when the rate of cooling is comparatively slow a certain lag is noticed in the trans- formation, so that the critical point is lowered as the rate of cool- ing is increased. The exact location of Ar\, Ar 2 and Ar s , there- fore, depends upon this rate of cooling, as well as upon the percent of carbon. Similarly the changes in structure observed upon heating steel are affected by the rate of heating, the critical points being raised. This " thermal hysteresis" is sometimes large, as in the case of steels containing nickel or chromium which retard the transformation. In ordinary work the difference between the points observed upon heating and those upon cooling is not greater than 30. In order to distinguish between the two points, the transformation temperatures observed when the steel is heated are denoted by the symbols Aci, Ac 2 , Ac s and Ac cm For the theoretical point where Ac and Ar would coincide 412 QUANTITATIVE ANALYSIS if the heating and cooling were infinitely slow, the symbols A\, A 2, etc., are used. Relation between Hardening and Annealing of Steel and the Constituents Already Described. The following summary of the properties of the five constituents of steel already described will form a basis for an understanding of the true nature of harden- ing and annealing of steel. Above A 3 steel is essentially austenite, a solid solution of iron carbide in 7-iron. Austenite is a stable substance at all tempera- tures above A 3 and is very hard. Between A s and A\ austenite spontaneously loses ferrite or cementite, according to the percent of carbon in the steel, these substances forming definite masses or granules, separate from the remaining austenite. Upon cool- ing to Ari the austenite remaining changes spontaneously into pearlite which remains mixed with the granules of ferrite or cementite already formed. Ferrite is relatively very soft and ductile, cementite is very hard and pearlite combines the prop- erties of ferrite and cementite, being moderately hard and tough. Normally, then, steel is hard above Aa and soft below A i, the de- gree of hardness in both regions being conditioned by the per- cent of carbon. The latter condition should be easily understood. Below AI low carbon steel is largely ferrite, the softest of all constituents of steel. Medium carbon steel contains more pearlite while high carbon steel contains cementite and no ferrite. The hard- ness of these three substances increases in the order named. Above AZ austenite exists and, being a solution of a very soft and a very hard substance in each other, its hardness will naturally increase with the percent of the hard constituent. Steel possesses the properties normal to temperatures below AI only in case 'it has cooled very slowly. Even ordinary liquid solutions may be considerably supercooled without the normal dissolution taking place. Steel, already solid, possesses much less molecular mobility and requires even more time for such changes to take place. If this time is denied a metastable condition is obtained and this persists at ordinary temperatures because the molecular rigidity of the mass makes structural changes impossible. It is not, however, possible to cool austenite with sufficient rapidity to prevent absolutely its transformation and an intermediate product, martensite, always appears to some ANALYSIS OF INDUSTRIAL PRODUCTS 413 extent. This is, however, even harder than austenite and also contains /?- and a-iron. The effect of the commercial hardening process is now clear. It consists in heating steel to a temperature above Ac z and sud- denly cooling by quenching, austenite or martensite or both being retained, according to the nature of the quenching medium. The annealing or softening process is simply reheating a hardened steel to a temperature just above Ac s , then cooling slowly so that the softest constituents may make their appearance. The details of these processes will naturally vary according to the degree of hardness desired. Some of these details are later dis- cussed along with tempering and with quenching media. Troostite. Since the transformation of austenite into its segregated components requires an appreciable period of time it is but natural to expect that variation in the rapidity of cooling would result in varying degrees of imperfection in the formation of these separate components. It does not necessarily follow that these more or less imperfectly formed components will have dis- tinct physical properties, other than in the matter of crystal formation, but two fairly distinct transformation products in addition to martensite have been recognized. "Troostite" (after the French chemist, Troost) is the stage following marten- site in the transformation of austenite. Its constitution is not known and much difference of opinion still exists with regard to it. The Committee on Nomenclature of the Microscopic Sub- stances and Structures of Steel and Cast Iron of the International Association for Testing Materials 1 says of troostite that it is "an uncoagulated conglomerate of the transition stages. The degree of completeness of the transformation represented by it is not definitely known and probably varies widely." Its hard- ness varies with the percent of carbon but, in general, it lies between that of martensite and that of sorbite (the next stage in the transformation). Its tensile strength is greater than that of sorbite or pearlite, its ductility less. It occurs in granular masses, usually associated with martensite and sorbite, some- times also with pearlite. It is distinguished by its property of coloring more darkly than either martensite or sorbite; the lack of crystalline character also distinguishes it from martensite. 1 Sauveur: The Metallography of Iron and Steel, Appendix II. 414 QUANTITATIVE ANALYSIS X166 FIG. 108. Steel. Carbon 0.4 per- cent. Quenched in oil within the critical range. Ferrite, troostite and sorbite. X650 FIG. 1 10. Steel. Carbon 1 .0 per- cent. Cooled in air from 850, Cementite, pearlite, sorbite and troostite. X400 FIG. 109. Steel. Carbon 0.5 per- cent. Cooled in air from 775. Fer- rite, troostite and sorbite. X650 FIG. 111. Steel. Cooled in air from 900. Sorbite and pearlite. X130 FIG. 112. Steel. Carbon 0.25 per- cent. Heated for three hours at 900 and cooled in the furnace. Coarse granulation. X130 FIG. 113. Steel. Same piece as shown in Fig. 112 but reheated to 825 and immediately cooled in air. Grain refined. ANALYSIS OF INDUSTRIAL PRODUCTS 415 Troostite is shown as the very dark portions of Figs. 99, 108 and 109. Sorbite. The next stage after troostite, in the resolution of the solid solution into its components, is sorbite, so called in honor of Sorby. Sorbite is to be regarded as imperfectly formed pearlite. The distinctly laminated appearance of the latter is lacking, sorbite being granular and apparently amorphous. It is somewhat less ductile than pearlite but its higher tensile strength and elastic limit make it a desirable constituent of struc- tural steels. Sorbite appears as the more granular portions of Figs. 99, 100, 110 and 111. Influence of Method of Cooling. Quenching Media. If the resolution of austenite into its ultimate transformation products were an instantaneous change, requiring, for instance, a very small fraction of a second, it is doubtful whether any quenching medium could cool it so suddenly as to prevent the practically complete resolution. That the transformation does require an appreciable period of time is shown by the retention of austenite in very suddenly cooled steel. That the rapidity of cooling depends largely upon the nature of the quenching medium is shown by the appearance of the transition stages, martensite, troostite and sorbite, in steel cooled from above, or within, the critical range by quenching in such media as warm water, oil or air. Considering the gradation of properties of the series beginning with austenite and ending with pearlite it will be understood that the properties of hardened steel may be varied at will over a wide range by a proper selection of the quenching medium. Thus heating to a temperature above Ac 3 and quenching in cold water will produce little more than martensite, with its high degree 'of hardness, tensile strength, and elastic limit, but great brittleness. Quenching in cold water from a temperature near Ar 3 , but below it, will produce martensite, associated with more or less ferrite or cementite and also some troostite or sorbite or both, the latter two products imparting a higher degree of toughness, as opposed to the brittleness of martensite, but being also responsible for a decreased tenacity and increased ductility. Quenching in the same medium from a temperature somewhat above Ar L , but nearer to this point than to Ar^ will increase the proportion of 416 QUANTITATIVE ANALYSIS sorbite and f errite and decrease that of martensite and troostite, re- sulting in a further change in properties in the direction indicated. If the quenching medium is oil, slower cooling results and the constitution of the quenched steel is such as to indicate greater transformation than is the case with water. In other words, the tendency is now toward the sorbite end of the series and away from austenite. Cooling in quiet air or in an air blast will result in still slower cooling and a still greater percent of sorbite will now be formed, even pearlite appearing if the piece is large enough to cool slowly. Similar considerations will apply to various other quenching media, "such as ice water, liquid air, mercury, salt solutions, alco- hols, mixtures of alcohols and water, etc The exact reasons for the different rates of cooling produced by different media are not definitely known. Different scientists have held that the rate of cooling depends upon (a) the temperature of the cooling bath, (b) its specific heat, (c) its conductivity for heat or (d) its heat of vaporization. The number of substances commercially used for cooling baths is not as large as might be expected, water, oil and air supplying most of the needs. Tempering. Extremely hard steel is demanded for certain purposes but for most structural work, tools, etc., the brittleness accompanying extreme hardness is undesirable and it becomes necessary to sacrifice a certain degree of hardness and tenacity for a greater degree of toughness. From what has been said concerning the effect of varying the rate of cooling a method would here seem to be available for securing the desired combination of properties. To " temper" the steel (the property of hardness is " tempered") it should be necessary to slowly cool the piece to the correct temperature and then to cool by quenching in the medium that is found by experience to produce the desired result. This can, in fact, be done and this method is actually used to a certain extent. In practice, however, it is found to be rather difficult to determine the exact necessary conditions or to accurately reproduce these conditions if they are determined. The modern demands upon steel are so severe that seemingly trivial depar- tures from a standard composition and a standard thermal treatment will often serve to unfit a piece of steel for the work that must be done by it. ANALYSIS OF INDUSTRIAL PRODUCTS 417 The method of tempering that is found to be more easily controlled is that of reheating the thoroughly hardened piece to a temperature where the metastable austenite (or in commercial practice, usually martensite) is partially converted into the transi- tion products, troostite or sorbite. While the metastable aus- tenite or martensite may be retained fdr an indefinite period at ordinary temperatures, it becomes partially converted into its more stable products long before the temperature reaches Aci. Transformation begins as low as 100 and takes place to a greater extent as the temperature is raised toward Ac\ which, it will be remembered, is about 700. If it be supposed that the hardened steel is never brought quite to Aci, the degree of tempering pro- duced will vary directly with the temperature of the piece dur- ing the tempering process. After the proper temperature is reached the steel may be permanently fixed in its tempered con- dition by again cooling. In practice it is usually quenched but this is not essential to the tempering process and is practiced only for the purpose of saving time. The advantages of this method of tempering over the method of partial hardening 'will readily be seen. The rate of cooling, in the latter method, must be so accurately related to the degree of hardening required that it is a practical impossibility to regu- late the process with the required refinement. On the other hand there is no difficulty in first completely hardening the piece, subse- quently raising its temperature to a point, experimentally deter- mined to be correct, then cooling. The method of tempering by reheating possesses another advantage, in that no ferrite, pearlite or cementite can be formed and the structure and properties of the steel must therefore be more uniform than is the case when regulated cooling from above the critical range has occurred. The best method of observing the temperature is by a good py- rometer, preferably of the thermoelectric type. In fact, scientific tempering must be regulated in this way. The method of "tem- pering by color" has been much practiced in the past and con- tinues to be practiced in cases where extreme accuracy is not required. This method depends upon the peculiar progressive change in color, noticed upon the surface of a previously polished piece of steel as it is heated to various temperatures below 500. The colors are due to films of oxides of different composition and stability at different temperatures. 27 418 QUANTITATIVE ANALYSIS Granulation. Up to this point in the discussion of thermal treatment emphasis has been placed upon the identity and physical properties of the constituents of steel. " Scarcely second to these in importance is the question of size of granules. No matter what may be the strength of the individual particles composing a piece of material, the strength of the piece as a whole Will also depend largely upon the degree of coherence of the particles. If the various surfaces separating adjacent grains (cleavage planes) are relatively large in area the piece will suffer permanent rupture more readily than if they are small. This being the case, the strength of the piece will vary inversely as the size of the granules composing it. The temperature of steel and the length of time during which a given temperature is maintained determine the size of the gran- ules. At temperatures below A\ any existing granulation will remain unchanged for an indefinite period of time. If the tem- perature is now raised to Aci, the destruction of existing granula- tion is begun by the partial formation of martensite. Complete reformation of grains does not take place at this point unless the steel is of eutectic composition ("eutectoid steel") because until Ac s is reached a certain amount of free ferrite or cementite is normal to the steel. For eutectoid steel, complete destruction of existing granules occurs and new granules begin to form, Ac s and Aci coinciding for such steel. For steel containing less than 0.85 percent of carbon (" hypoeutectoid steel") or more than this amount 'of carbon ("hypereutectoid s"teel") new granulation of martensite begins the moment Aci is reached and these gran- ules continue to grow in size until A c 3 is passed, complete absorp- tion of free ferrite or cementite being here accomplished. This explains the fact that it is difficult to produce thoroughly satis- factory refinement of grain in steel which is very low or very high in carbon. Ac s and Ac\ are so widely separated (Fig. 95) that the new system of granules has grown to an undesirable extent by the time Acs, the point of complete destruction of old granules, is attained. The growth of granules increases in speed at higher tempera- tures and granules continue to grow so long as the steel is held at a temperature above A i. From these considerations the fol- lowing rules of procedure will be understood: 1. To refine the grain of a coarsely granulated piece of steel, ANALYSIS OF INDUSTRIAL PRODUCTS 419 reheat the piece untilJ.c 3 is passed, then cool immediately, slowly if the piece is to be annealed, suddenly if it is to be hardened, whether or not hardening is to be followed by tempering. 2. All thermal treatment for the purpose of hardening or an- nealing should be carried out at the lowest temperature that will permit the desired constitutional changes and the time consumed in the treatment should be as short as possible, in order to avoid coarse granulation and consequent weakness. The effect of long heating at high temperatures and of reheat- ing to refine the grain is shown in Figs. 112 and 113. These photomicrographs clearly show that a good piece of steel may be rendered absolutely unfit by careless or ignorant treatment, and also that many of such pieces may be restored by reheating. "Burnt Steel." If a piece of medium or high carbon steel is heated to a temperature near its melting point and held at this X130 FIG. 114. Steel. Heated for three hours at 1000 and quenched in water. Burnt. (Not deeply etched.) temperature for some time it becomes extremely brittle. The brittleness is partly due to the very coarse granulation that is acquired by such treatment, but that it is not entirely due to this cause is shown by the fact that its former malleability cannot be fully restored by reheating to Ac^, or indeed, by any other process except remelting. Such a steel is said to be " burnt" because it was long understood that considerable iron oxide was produced in the interior of the piece thus destroying the cohesion of the grains. The formation of oxide is, undoubtedly, part of the cause of the brittleness of burnt steel and oxide is probably what prevents rewelding the grains by hot or cold working. Weakness of structure also results from the evolution of gases, 420 QUANTITATIVE ANALYSIS such as nitrogen or carbon monoxide, at high temperatures. The only cure for burnt steel is remelting. Fig. 114 illustrates the effect of " burning." Proper Temperature for Thermal Treatment Indicated by Composition. Throughout the discussion of thermal treatment reference has been made to the critical points of steel as indicated by Fig. 95. On account of the pronounced slope of the graph of As it will be seen that a knowledge of the percent of carbon is absolutely essential to the proper application of thermal treat- ment, unless a delicate pyrometer can be used for locating the critical points. It is quite easy to determine the location of A\ by the latter method but at the upper critical points the evolution of heat is small and only a delicate instrument will detect it. An inspection of the figure will show that there is a difference of nearly 200 between the proper temperatures for hardening or annealing two steels of 0.05 percent and 0.90 percent, respectively, of carbon. Case Hardening. The treatment of steel to give it its maxi- mum hardness also produces brittleness. This may be of small importance if the piece is sufficiently massive or if it is to be sub- jected to no great shock or stress in service. It often happens, however, that small pieces are required to resist abrasion on the surface and still must have sufficient toughness to withstand shock or other stresses. Such is the case with gears and other small pieces of machinery. No combination of hardening and temper- ing is suitable for producing a piece of steel that will comply with both requirements. It will be remembered that either hardened or annealed high carbon steel is harder and more brittle than low carbon steel treated in the same manner. If a piece of low carbon steel could be given a surface or case containing a higher percent of carbon the problem would be solved. This can be done by making use of the well known power of iron for absorbing carbon when the two materials are at temperatures above the upper critical point, A 3. This power is probably due to the penetration of gases, such as carbon monoxide or cyanogen, derived from carbonaceous materials with which the steel is in contact. The piece of " mild ' ' steel is packed in the carburizing material and heated to a temper- ature just above Ac s , the length of exposure depending upon the depth of case required and varying from 30 minutes to several ANALYSIS OF INDUSTRIAL PRODUCTS 421 hours. The percent of carbon acquired by the case may be as high as 2.5 but it is not usually higher than 0.90. The materials used for case hardening are wood charcoal, bone charcoal, ground raw bone, leather scraps, scraps of horns and hoofs, etc. Potassium cyanide or ferrocyanide is sometimes used for quickly producing a very thin and hard case. Thermal Treatment of Case-hardened Articles. The temper- ature necessary for case hardening depends upon the percent of carbon already contained in the steel, since the piece must be heated above Ac^. The subsequent thermal treatment is even more important. Protracted heating above the critical range has given both case and core a coarsely granulated structure. In order to refine the grain the piece must be reheated to just above A c 3 and then quenched but this process, which would have been simple before the production of the case, is now complicated by the fact that Ac 3 for the core is considerably higher than for the case. If, for example, the core contains 0.10 percent and the case 0,90 percent of carbon, reference to Fig. 95 will show that Ac s for the core is more than 100 higher than for the case. If the piece is heated to refine the core the case, momentarily refined as its critical range is passed, becomes again coarse through exposure to the higher temperature. To remedy this defect a double treatment is employed. The piece is first reheated to refine the core and then quenched in oil or water. It is then reheated to the lower temperature for refining the case and again quenched. Any desired tempering may follow. Effect of Working. The mere act of forging or rolling steel into the required form does not, in itself, alter the constitution of the steel. The temperature at which such work is performed has, on the other hand, an important influence upon the size and form of the component granules. "Hot working" is carried out at temperatures above the critipal range, "cold working" below it. Cold working has no other effect upon the granules than to flatten and elongate them. Hot working, on the other hand, having to do with steel in the form of the solid solution, has the effect of breaking up existing granulation and of preventing the growth of other granules. The granulation that will be observed in the finished piece must have been produced after work was finished and this makes the finishing temperature the really important consideration. Reference to the paragraph on " Granulation," 422 QUANTITATIVE ANALYSIS page 418, will point the rule that the finishing temperature should be slightly above Ar s but as near to this point as is possi- ble, if subsequent reheating is not to be applied. If the article is to be reheated for hardening, annealing or refinement, the finish- ing temperature is not particularly important. Apparatus for Work in Metallography. A detailed description of the apparatus, or detailed directions for manipulation of the apparatus that is necessary for metallographic investigation of the results of thermal treatment would be entirely beyond the scope of this book. In the following discussion and the directions for a limited number of experiments it is assumed that the labora- tory is fitted with the necessary apparatus and that the instructor will provide detailed instructions. An outline of the necessary steps in the preparation and examination of samples is given and some experiments illustrating the important principles of thermal treatment are described. Experimental Furnace. A small furnace which may be quickly heated and cooled and whose temperature may be easily con- trolled is suitable for experimental treatment. Electrically heated furnaces are most convenient and the temperature should be observed by means of a sensitive pyrometer which may be of the recording or the indicating type. The furnace chamber need not be larger than 2X2X3 inches. Cutting and Polishing Machines. It is not often possible to polish and examine large specimens and a hack saw is almost a necessity in the metallographic laboratory. This may be a small hand saw or one of the more expensive mechanical saws which may be purchased at a nominal price. Before the microscopic examination can be made it is necessary that a high polish be given to the surface examined. Here again, polishing may be done by hand, but if much work is to be done a polishing machine is almost a necessity. Such a machine should provide at least four polishing surfaces. The final polish is given by a very fine powder, such as rouge, but it is quite impracticable to polish the original, rough surface with such a powder. Instead, the piece is successively polished with powders of increasing fineness, each polish being applied at a direction perpendicular to the next preceding one and the application being continued until all scratches made by the preceding opera- tion have been removed. The following series of polishes may ANALYSIS OF INDUSTRIAL PRODUCTS 423 be used, proceeding toward the finest: (1) fine grinding surface of alundum, emery or carborundum, (2) fine emery powder on broadcloth or canvas, (3) tripoli on broadcloth, (4) rouge on broadcloth. The polishing powders are kept in water in separate dishes and are applied to the polishing cloth, wet, by means of brushes while the machine is running. It is highly important that the powders should never be mixed or applied to the wrong surfaces. The dishes should be kept covered to exclude dust. Etching. The microscopic examination will reveal little if applied to the brightly polished surface of steel, although graphitic carbon may easily be seen in cast iron by this means. It is then necessary to bring the components into direct relief by the use of some agent which will color them differently or attack the grain boundaries. Very many such etching agents have been used, examples being alcoholic solution of nitric acid and of picric acid, concentrated nitric acid immediately followed by running water, tincture of iodine and others. The period of application will depend upon the agent used and upon the nature of the sample. High carbon steels etch more rapidly than low carbon steels and darken rapidly on account of the preponderance of pearlite. The etching solution must be thoroughly removed by rinsing, at the end of the etching process, and the sample dried by rinsing with alcohol and holding in an air blast. If the sample is to be preserved it must be protected from oxidation by a thin coat of lacquer, this being removed by alcohol before the microscopic examination. Examination. The microscope that is to be used for metallo- graphic purposes must be of a special type because reflected, instead of transmitted, light must be used. A reflector is in- serted in the microscope, just above the objective, and illumina- tion is produced by artificial light from a Welsbach or arc light. For the examination to determine the approximate carbon per- cent, as well as to note the condition of granulation, it is best to use the low-power lenses. For this purpose a magnification of 50 to 130 diameters is convenient. This is sufficient to show readily the separate granules of pearlite and ferrite or cementite, or to determine whether the steel is hardened, and to indicate the relative area of pearlite in annealed pieces. For the closer examination to determine the identity, form of crystallization, lamination, etc., of the separate constituents a magnification 424 QUANTITATIVE ANALYSIS of 400 to 600 diameters or, in special cases, even higher magnifi- cation is desirable. If a record of the results of the examination is to be kept a photographic camera should be attached to the microscope in such a manner that it may be swung into place after the visual examination has been made. A special contrast or "process" plate works best for making the negative. The light must usually be passed through ray filters but the nature of these will depend entirely upon the nature of the light and of the specimen under examination. Exercise: Determination of Structure with Variation in Carbon Percent. Select a series of annealed simple carbon steels having ap- proximately the following percents of carbon: 0, 0.10, 0.30, 0.50, 0.70, 0.90, 1.20, or as many of these as may be obtained. Cut a small piece from each sample, of such form as to provide one plane surface 1/2 to 3/4 inch square, although the form of outline of this surface is not im- portant. Grind this surface until it is plane, also slightly round its edges to prevent cutting the polishing cloth. Polish by hand or on a machine, beginning with the coarsest of the surfaces and finishing on the finest, polishing with each powder in a direction perpendicular to the last polishing and continuing each operation until all scratches left by the preceding operation have disappeared. The polishing heads must be kept wet and gentle pressure used, to avoid heating. The appearance of a film of oxide on the polished surface is an indication of too little water or too great pressure. The powder and water should always be applied in such a way that the sample does not "drag" on the polishing surface. Wash the sample and the hands with each change to a finer polishing surface. When the polishing is finished wash the sample, but without rubbing the polished surface, then rinse with alcohol. If drops of water are permitted to remain on the surface, spots of oxide will appear in a few minutes. Use an etching solution of 10 percent concentrated nitric acid in absolute alcohol. Pour this solution into a shallow dish and immerse the specimen in the solution with the polished surface up. The length of exposure necessary will vary with the percent of carbon, 3 to 10 seconds being required. The steels containing more than about 0.50 percent of carbon will visibly darken. The lower carbon steels will simply become frosted on the surface. The exposure is easily learned by experience and if some are etched too little they may be re- treated. If etched too deeply the etched surface is removed by polish- ing and a new application of the solution is made. At the end of the etching process remove the specimen and at once ANALYSIS OF INDUSTRIAL PRODUCTS 425 rinse thoroughly with running water. Rinse off the water by alcohol and dry in a blast of clean air. The piece is now ready for examination. Examine the pieces in order of carbon content and, if possible, make photomicrographs under the direction of the instructor. Use first a magnification of about 100 diameters and observe the increase of the darker pearlite as the percent of carbon increases, ferrite decreasing and finally disappearing when eutectoid steel is reached, cementite appear- ing in steels of hypereutectoid composition. Under a magnification of 500 to 1000 diameters, carefully study the structure of the individual granules of ferrite, pearlite and cementite. If the sample of nearly carbonless iron is wrought iron, particles of slag will be noticed. Also either wrought iron or " ingot iron" will show numerous "etching pits" within the granules of ferrite. The latter are due to the crystalline character of ferrite and indicate the boundaries of smaller crystals within the granules. Hardened Steel. Heat all of the samples to temperatures just above Ac- and immediately quench in cold water. The addition of 10 percent of alcohol will improve the quenching bath. To determine the location of Ac 3 refer the carbon content to Fig. 95. The samples containing about 0.50 percent and 0.90 percent, respectively, of carbon are now to be examined. Polish, etch, and examine these samples as in the case of the un- hardened pieces. Note the disappearance of granules, as observed by low-power magnification, and the characteristic crystalline appearance of martensite. Austenite may be produced by heating the steel to a higher temperature and cooling in ice water. Properly Annealed Steel. Reheat the one of the hardened pieces which contains 0.50 percent of carbon to just above Ac 3 and immediately cool in the furnace. When 600 has been reached the piece may be removed and quenched. Polish, etch, and examine, noting size of gran- ules. Photograph if possible. Improperly Annealed Steel. Reheat the annealed sample to a tem- perature between 900 and 1100 and keep it at this temperature from 1 to 2 hours, then cool in the furnace. If desired it also may be quenched after Ar\ is passed. Examine and note the excessively coarse granula- tion, cracks frequently appearing. If the temperature has been allowed to reach 1400 or 1500 the characteristics of burnt steel may be ob- served. Photograph, if possible, and compare with the photograph of the same piece, properly annealed. Refinement of Grain of Improperly Annealed Steel. Reheat the coarsely granulated (but not burnt) steel to a temperature just above Ac 3 and immediately cool in the furnace, quenching, if desired, after Ari is passed. Examine and note the refinement of grain. 426 QUANTITATIVE ANALYSIS Tempered Steel. Reheat the various other pieces of hardened steel to about 200, 250, 300 and 350, respectively, quenching in water as soon as the required temperature is reached. Polish, etch and examine and make an effort to identify troostite and sorbite in these tempered pieces. Case-hardened Steel. Cut a small piece of low carbon steel (about 0.02 percent to 0.05 percent carbon) and pack in raw bone, bone charcoal or any of the other case-hardening materials, using a small cast-iron box which may be covered. Heat to 900 for about 3 hours then cool in the furnace in order to leave the piece soft enough to permit cutting by the hack saw. Cut entirely across the piece, so that both case and core will appear in the section, polish, etch and examine. Note the differ- ent appearances of core and case, indicating more carbon in the latter. The case will contain more pearlite or it may even contain cementite. Physical and Mechanical Tests. If the laboratory is equipped with the various appliances for physical and mechanical tests of metals the work may be made even more interesting and instruc- tive by comparing and correlating these properties with thermal treatment, chemical composition and microscopic structure. FERTILIZERS Many elements naturally occurring in soils are extracted and used in small quantities by plants. Certain others are necessary to the growth of plant life and are demanded in greater abundance. With the growth of the knowledge of soil chemistry the addition of deficient elements to the soil has become a commercial matter and the analysis of fertilizers has become a necessary part of the chemist's work, not only for the purpose of placing a correct estimate upon the commercial value of the fertilizer but also to provide a basis for the intelligent application of the fertilizer to the soil that lacks it. Substances added to the soil to promote plant growth belong either to the class of plant foods or to that of correctives. The most valuable elements belonging to the first class are nitrogen, phosphorus and potassium. An example of the second class is calcium carbonate, added to correct excessive acidity of certain soils. It is generally true that substances of the second class also provide plant food but this is usually incidental to the main purpose for which they are added. ANALYSIS OF INDUSTRIAL PRODUCTS 427 Preparation of Samples. Grind the sample to pass through a sieve having circular perforations 1 mm in diameter, then thoroughly mix by rolling. Perform the grinding, sifting and mixing as rapidly as possible, to avoid undue loss or gain of moisture. Moisture. The moisture of a fertilizer is, in most cases, a substance without any value whatever and its determination is made with this in view. Determination. Heat 2 gm, or 5 gm if the sample is very coarse, for 5 hours at 100. If the sample is of potassium salts, sodium nitrate or ammonium sulphate, heat from 1 to 5 gm at about 130 until the sample ceases to lose weight. Calculate the percent of moisture. Nitrogen. Although nitrogen is so abundant as an elementary constituent of the atmosphere it is the most costly of all the elements that are required for plant growth. This is because its chemically inert character makes its fixation and assimilation by plants a difficult matter. Plants do not abstract gaseous nitrogen from the atmosphere 1 although the roots of leguminous plants support certain bacteria whose action is to oxidize at- mospheric nitrogen to nitric acid, this being then fixed in the soil by forming nitrates with such basic materials as calcium carbonate. Nitrogen may be added as an artificial fertilizer in the form of ammonium salts, nitrites, nitrates or organic ni- trogenous materials. Crude chloride' and sulphate are the more common forms of ammonium salts used, ammonium sulphate being obtained during the process of gas manufacture. Nitrites are little used as commercial fertilizers. Nitrates are the most common and probably the most valuable of the various nitrog- enous materials. In the past, the chief source of nitrates has been the "Chile saltpetre" beds of South America. The artifi- cial fixation of atmospheric nitrogen has been successfully accomplished by two processes : (1) When calcium carbide is heated with pure nitrogen to 700-800, in the presence of a small amount of calcium chloride or calcium fluoride, calcium cyanamide, CaNCN, is formed. This substance decomposes in the soil, forming first cyanamine, then urea and finally ammonium carbonate. CaNCN+C0 2 +H 2 0->CaCO 3 +H 2 NCN, H 2 NCN+H 2 O-CO(NH 2 ) 2 , CO(NH 2 ) 2 +2H 2 0-(NH 4 ) 2 C0 3 . 428 QUANTITATIVE ANALYSIS (2) Under the influence of an electrical discharge nitrogen and oxygen directly combine, forming nitric oxide. This occurs to some extent during electrical storms, which explains the occur- rence of nitric acid in rain water. The reaction is now used on a large scale for the production of nitric acid which is then converted into nitrates and used for fertilizers and other purposes. The most important nitrogenous organic materials that are used for fertilizers are dried blood and tankage obtained from the packing houses, also fish scraps, guano and ordinary stable manure. Dried blood is a very valuable fertilizer because of the large percent of nitrogen which it contains (12 to 14 per- cent) and because it is readily available for assimilation by plants. Tankage consists of scraps of refuse meat, skin, etc., from which the oil has been removed by steaming and pressing. Fish scrap, guano and farm manures are valuable as nitrogenous fertilizers but are limited in quantity. Certain other nitrogenous materials that are sometimes added to mixed fertilizers because they are rich in nitrogen are useless on account of the fact that their nitrogen is only very slowly available for assimilation by plants. Such materials are hair, horns, hoofs, leather scrap, and peat. When these are finely ground and mixed with other fertilizing material they yield rela- tively high percents of nitrogen in the analytical process but are of little use to the plant life. Their detection is often possible only by means of the microscope and the determination of their quantity in a mixed fertilizer is a very difficult matter and for these reasons their admixture with commercial fertilizers is forbidden by many state laws. The determination of nitrogen in organic compounds has already been discussed in connection with the analysis of coal, page 257. Fertilizers may contain nitrogen in only one form or in a mixture of two or more classes of compounds, as ammonium salts, nitrates, or organic compounds. The determination of the percent of nitrogen in each form is occasionally demanded but usually the determination of total nitrogen is all that is required, it being understood that hair, leather scraps and other such mate- rials are excluded. The method of Dumas or the soda lime method may be used for the determination of total nitrogen but the method of Kjeldahl, or one of the modifications of this method, is better suited to this class of work. ANALYSIS OF INDUSTRIAL PRODUCTS 429 Kjeldahl's Method. Kjedahl's original process 1 consists in digesting the organic material with boiling concentrated sul- phuric acid until complete decomposition has been effected. The exact course of the reactions cannot be traced but the carbon and hydrogen are completely oxidized and nitrogen is converted into ammonia, which immediately combines with sulphuric acid and remains as ammonium sulphate. The completion of decom- position is insured by the final addition of a small amount of potassium permanganate. The solution is then diluted with water, an excess of sodium hydroxide is added and the resultant ammonia is distilled into a measured quantity of standard acid solution. To determine the excess of standard acid, potassium ' iodate and potassium iodide are added and the liberated iodine is titrated by a standard solution of sodium thiosulphate. Iodine is liberated according to the following equation: KIO 3 +5KI+3H 2 SO 4 -> 3K 2 SO 4 +6I+3H 2 O. Instead of this method of determining the excess of standard acid it is now customary to titrate the excess by means of a standard basic solution. The method is inapplicable to the determination of nitrogen of nitrates because of the loss of nitric acid which occurs as soon as the material is treated with sulphuric acid. Modifications of the method to suit the analysis of nitrates will presently be dis- cussed. The digestion with sulphuric acid is best accomplished in a pear-shaped flask with a long neck, like that shown in Fig. 115. The concentrated sulphuric acid of commerce boils at tempera- tures ranging from 210 to 340, depending upon the percent of water contained in it. Such a temperature is high enough to permit condensation of nearly all of the vapor without the use of a water condenser, the long neck of the digestion flask serving for this purpose. If the solution is to be transferred to a special distilling flask the capacity of the digestion flask need not be greater than 200 cc. It is more convenient, however, to distill from the flask in which digestion is accomplished, in which case the capacity of the flask should be 500 cc. The digestion must be performed under a hood or some other provision must be made for carrying away the fumes. An excellent arrangement for i Z. anal. Cbem., 22, 366 (1883). 430 QUANTITATIVE ANALYSIS this purpose is a lead pipe, 6 inches in diameter and with holes in the side so that the necks of a number of digestion flasks may be inserted with the flask in an inclined position. The end of the lead pipe leads to a chimney. Wilfarth showed 1 that the addition of mercuric oxide, copper oxide or ferric oxide to the mixture of the organic material and sulphuric acid considerably accelerates the reactions that occur during digestion. The action is of a catalytic nature and de- pends upon the capability of the metal of existing in more than one state of oxidation. The metal is thus alternately reduced by organic matter and oxidized by sulphuric acid, somewhat as follows: 2HgSO 4 +2H 2 0+S0 2 . Of the three metals named, mercury serves best because its salts are colorless and do not obscure the end point of the oxida- tion. It is necessary in this case to precipitate the mercury by the addition of potassium sulphide, before distillation, in order to prevent the formation of mercurammonium compounds which are not readily decomposed by sodium hydroxide. During the distillation of ammonia, after the addition of excess of sodium hydroxide, there is usually a tendency toward bumping. In order to prevent this the " official" method of the Association of Official Agricultural Chemists 2 directs the addi- tion of granulated zinc or pumice stone to the contents of the flask before distillation. The reaction of zinc with sodium hydrox- ide produces a continuous evolution of hydrogen and this effectually prevents bumping. There is, however, a disadvantage con- nected with the use of zinc which is sometimes serious, in that the sodium zincate that is formed by the reaction so increases the surface tension of the solution that troublesome frothing occurs. An excellent substitute for both zinc and pumice is a small amount (0.5 gm) of crushed porcelain from which the dust has been removed by sifting. Sulphuric acid nearly always contains a small amount of ammonium sulphate. Distilled water may also contain a small quantity of ammonium hydroxide. In order to make the 1 Z. anal. Chem., 24, 455 (1885); Chem. Zentr., [3] 16, 17 and 113 (1885). 2 U. S. Dept. Agr., Bur. Chem., Bull. 107, 6. ANALYSIS OF INDUSTRIAL PRODUCTS 431 proper correction for the ammonia that will be derived from the reagents a " blank" determination must be made, omitting the sample of fertilizer but carrying out the operations exactly as in the real determination. The reactions of digestion may be imitated by the use of sugar as the organic material although there seems to be little or no real need of this. Determination. Prepare a fifth-normal solution of hydrochloric acid or sulphuric acid, a tenth-normal solution of potassium hydroxide and a saturated solution of crude sodium hydroxide. In standardizing the base against the acid use cochineal as indicator. Prepare also a 4 percent solution of potassium sulphide. If the approximate percent of nitrogen in the sample is known, cal- culate the weight that will yield ammonia equivalent to about 35 cc of the standard acid. If nothing is known of the nitrogen content use about 1 gm of sample. The sample must contain no nitrates, nitrites or nitro-compounds. Place two weighed samples in 500 cc Kjeldahl digestion flasks, holding the latter in a vertical position to prevent the sample from sticking to the sides of the neck. The neck should be dry, for the same reason. Weigh 1 gm of sugar into another flask and treat the same as the fertilizer sample. Add about 0.7 gm of mercuric oxide or of mercury, and 25 cc of concentrated sulphuric acid. Incline the flask in a hood or with the neck inserted into a lead-pipe ventilator and heat gently until the violence of the reactions has moderated, then grad- ually raise the temperature until the acid is boiling. The flask may be heated without protection by a gauze if it is of Jena glass or a similar resistance glass and if it is placed over a hole in a stand of sheet iron in such a manner that the flame cannot come into contact with the sides of the flask above the liquid. (See Fig. 115.) Digest by gently boiling until the solution is nearly colorless. This may occur after a short time or the digestion may require several hours. Remove the flame and at once drop into the flask small quantities of powdered potassium permanganate until the solution acquires a green or purple tint which persists after shaking. Allow the flask to stand until cool. (Do not cool under a tap.) Carefully add 200 cc of distilled water and mix by rotating the flask. Add about 0.5 gm of crushed porce- lain and 25 cc of 4 percent potassium sulphide solution, shaking as the latter is added . Have the connections with a tin condenser ready and have 50 cc of standard acid measured into a 400-cc flask into which the delivery tube (of glass) dips. Most laboratories in which much work of this kind is done will be equipped with a special form of apparatus for carry- ing on several distillations at once. The flask should be in a vertical position and some kind of trap should be used to prevent spray from 432 QUANTITATIVE ANALYSIS being carried over by the steam. The delivery tube should be capable of being detached from the condenser for the purpose of cleaning and rinsing it. The entire condenser must be thoroughly rinsed before each distillation, to insure freedom from basic solutions. Pour 50 cc of saturated sodium hydroxide solution (which should contain but little carbonate) down the inclined flask in such a way that mixing does not occur. Immediately connect with the condenser, care- fully mix the contents of the flask by shaking, then distill until about 150 cc of distillate has been collected. It sometimes happens that too much sulphuric acid has been added to hasten a difficult digestion or that the FIG. 115. Kjeldahl flask, stand and lead pipe ventilator. sodium hydroxide solution is not saturated. The consequence is that the solution still contains an excess of acid when ready for distillation. This will not be the case if the directions have been carefully followed but the addition of a drop of phenolphthalein to the solution will serve to indicate the fact. It should be remembered, however, that a con- centrated solution of a base soon decolorizes phenolphthalein and this action may be mistaken for an indication of an excess of acid. When the distillation is finished lower the receiving flask until the delivery tube is above the liquid, then remove the flame. Disconnect the delivery tube from the condenser and rinse inside and outside, allow- ing the rinsings to run into the flask. Add enough cochineal to tint the solution, then titrate with standard base. Subtract the excess of acid thus indicated and calculate the percent of nitrogen in the sample, ANALYSIS OF INDUSTRIAL PRODUCTS 433 making proper correction for any nitrogen found in the reagents by the blank determination with sugar. Gunning Method. It was observed by Gunning 1 that in the ordinary Kjeldahl process the water produced by the oxidation of organic matter dilutes the sulphuric acid and retards its action. Gunning proposed the addition of potassium sul- phate which forms acid sulphates which lose water much more readily than the hydrates of sulphuric acid so that the solution does not become diluted. A mixture of one part of potassium sulphate and two parts of sulphuric acid is heated together and finally allowed to cool. This mixture is measured into the diges- tion flask, where the digestion is performed as in the Kjeldahl process except that no mercury is added and, consequently, no potassium sulphide is needed before the distillation. In the method as now carried out the required amounts of potassium sulphate and sulphuric acid are added directly to the flask without preliminary heating. Determination. Calculate the weight of sample required, as in the Kjeldahl method; and weigh this amount into digestion flasks. Add to the sample in the digestion flask 10 gm of potassium sulphate and 25 cc of concentrated sulphuric acid. Digest as in the Kjeldahl process except that no mercury, mercuric oxide or potassium permanganate is to be added. When the solution is colorless, cool, dilute and conduct the distillation as in the Kjeldahl process, omitting, however, the potas- sium sulphide solution. Make a blank determination as in the Kjeldahl process. Calculate the percent of nitrogen in the sample. Modifications to Include the Nitrogen of Nitrates. It has already been noted that most of the nitrogen of nitrates is lost by directly heating with sulphuric acid. Asboth 2 modified the Kjeldahl method by adding benzoic acid, nitrobenzoic acid being formed and later oxidized by potassium permanganate. Jodl- bauer 3 substituted-phenolsulphonic acid for benzoic acid and reduced the resultant nitrophenolsulphonic acids to aminophenol- sulphonic acids by zinc dust. The amino compound was then oxidized by heating with sulphuric acid. The addition of phos- phoric acid was also found to hasten the oxidation. Both ben- zoic acid and phenolsulphonic acid are now generally substituted by salicylic acid and the reducing agent for the nitro compound 1 Z. anal. Chem., 28, 188 (1889). 2 Chem. Zentr., [3] 17, 161 (1886). 3 Ibid., [3] 17, 433 (1886); Z. anal. Chem., 26, 92 (1887), 28 434 QUANTITATIVE ANALYSIS is either zinc dust or sodium thiosulphate. The reactions may be represented as follows: 2KNO 3 +H 2 SO 4 -*K 2 SO 4 +2HNO 3 , /OH /OH HNO 3 +C 6 H/ C 6 H 3 -COOH+H 2 O. \N0 2 The nitro compound is then reduced by nascent hydrogen: /OH /OH 6H+C 6 H 3 -COOH-C 6 H 3 -COOH+2H 2 0, \NO 2 \NH 2 or by sodium thiosulphate: Na 2 S 2 3 +H 2 SO 4 Na 2 S0 4 +H 2 SO 3 +S, /OH /OH 6H 2 SO 3 +2C 6 H 3 -COOH+2H 2 O 6H 2 SO 4 +2C 6 H 3 -COOH. \N0 2 \NH 2 The oxidation of the amino-salicylic acid by sulphuric acid is not well enough understood to be represented by an equation. Determination by the Modified Kjeldahl Method. Weigh the sam- ple of fertilizer and place in a digestion flask. Add 30 cc of concentrated sulphuric acid to which has been added 1 gm of salicylic acid and mix by shaking. After 10 minutes add 5 gm of sodium thiosulphate. Heat gently until frothing has ceased then boil until white fumes are ex- pelled. Add about 0.7 gm of mercury or of mercuric oxide and continue the digestion, distillation and titration as in the. Kjeldahl method. Make blank determinations of nitrogen in the reagents. Calculate the percent of nitrogen in the fertilizer. Determination by the Modified Gunning Method. To the weighed sample in a digestion flask add 30 cc of concentrated sulphuric acid containing 1 gm of salicylic acid, mix and allow to stand for 10 minutes. Add 5 gm of sodium thiosulphate and heat for 5 minutes. Cool, add 10 gm of potassium sulphate and heat until nearly or quite colorless. Dilute, distill and titrate as in the original Gunning method. Make a blank determination of nitrogen in the reagents. Calculate the percent of nitrogen in the fertilizer. Availability of Nitrogen. Mention has already been made of the low fertilizing value of certain nitrogenous materials because ANALYSIS OF INDUSTRIAL PRODUCTS 435 of the slow decomposition that results when the fertilizer is added to the soil. Nitrogen is probably directly assimilated by plants only in the most highly oxidized form, i.e., that of nitrates. Ammonium salts and certain organic materials, such as dried blood, have almost as great value because they readily decom- pose and oxidize in the soil, forming nitrates. Hoofs, hair, leather and hide do not so decompose, except very slowly and a method of differentiating between available and non-available forms of nitrogen is desirable. The microscope will detect ground hair and other similar materials but it can give only qualitative results. Fortunately qualitative results are all that are necessary in states where the addition of such materials is contrary to law, but for scientific purposes a quantitative dis- tinction between available and non-available nitrogen may be of great practical use. An exact analytical method for such a pur- pose seems to be impossible because there is no sharp distinction to be made between the classes of fertilizer materials. ^ Great reliance is placed upon culture experiments, comparing the effect of using different fertilizers with plants under otherwise identical conditions. Such experiments are slow and have no value what- ever for analytical purposes. An approximate distinction can be made by the use of potassium permanganate in either neutral or basic solution. Readily decomposable materials are oxidized and the nitrogen is converted into ammonia. It has not yet been determined how much reliance is to be placed upon these methods but they have been adopted as provisional methods by the Association of Official Agricultural Chemists. 1 Determination: Neutral Permanganate Method. Weigh 2 gm, or less, of the sample into a 300-cc beaker. Add 125 cc of a 1.6 percent solution of potassium permanganate, cover and digest in a steam bath for 30 minutes. Stir twice at intervals of 10 minutes during the diges- tion. Remove from the bath, add 100 cc of cold water and filter through a heavy 15-cm folded filter. Wash with cold water until the total volume of the filtrate is 400 cc. Dry the residue on the filter and deter- mine nitrogen by the Kjeldahl or Gunning method. This gives the percent of non-available nitrogen. Determine total nitrogen by the Kjeldahl or Gunning method (modified if nitrates are present) and calculate the percent of available nitrogen. 1 U. S. Dept. Agr., Bur. Chem., Bull. 107, 10, 11. 436 QUANTITATIVE ANALYSIS ' Determination : "Alkaline" Permanganate Method. Weigh a quan- tity of sample containing approximately 0.045 gm of nitrogen and trans- fer to a 600-cc distilling flask. Connect with a condenser, which should be of tin, and measure 50 cc of fifth-normal acid into a 400-cc flask which is placed under the delivery tube. To the fertilizer in the flask add 100 cc of a solution containing 1.6 gm of potassium permanganate and 15 gm of sodium hydroxide and digest for 30 minutes at a temperature near the boiling point but without boiling. Raise the temperature to the boil- ing point and distill 85 cc into the standard acid. If the material ad- heres to the sides of the flask an occasional gentle rotation is necessary during distillation. Titrate the unused excess of acid and calculate the percent of available nitrogen. Phosphorus. The most important sources of phosphorus for fertilizing purposes are mineral phosphates (chiefly apatite, which is calcium orthophosphate, Ca3(PO4)2, but contains some mag- nesium phosphate) ground raw and steamed bone, slag from basic Bessemer steel furnaces (known as " Thomas slag") and, to a less extent, fish scrap, oil cake and tankage. The normal calcium orthophosphate of mineral deposits has a very small solubility in water or in any salt or acid solutions commonly occurring in soils. It is, in consequence, generally considered as a source of non-available phosphorus. In order to change it to a soluble form so that it may be used as a fer- tilizer, the mineral phosphate is treated with sulphuric acid, there being formed calcium acid phosphates and sometimes free phosphoric acid if an excess of sulphuric acid has been used: Ca 3 (P0 4 ) 2 +H 2 SO 4 CaSO 4 +2CaHP0 4 , Ca 3 (PO 4 ) 2 +2H 2 S0 4 2CaS0 4 +Ca(H 2 PO 4 )2, Ca 3 (P0 4 ) 2 +3H 2 SO 4 -> 3CaS0 4 +2H 3 PO 4 . Normal calcium phosphate (tricalcium phosphate) is almost insoluble in water. Dicalcium phosphate, CaHPO 4 , dissolves in water to the extent of only 0.136 gm in 1000 cc at 25, but dis- solves quite easily in certain salt solutions, as ammonium citrate. Monocalcium phosphate, Ca(H 2 PO 4 ) 2 , is easily soluble in water. Salt solutions existing in soils dissolve dicalcium phosphate in a manner similar to that shown by ammonium citrate solution. On this account the analyst speaks of " water soluble," " citrate soluble," and " insoluble" phosphate. Water soluble and citrate soluble forms are taken together as "available" phos- ANALYSIS OF INDUSTRIAL PRODUCTS 437 phate so that a distinction between the two forms composing this class is now seldom required. When a mixture of the compounds formed by " acidulating" phosphate rock is allowed to stand a reaction occurs between normal calcium phosphate and monocalcium phosphate: Ca 3 (PO 4 )2+Ca(H 2 PO4)2 4CaHP0 4 . The dicalcium phosphate formed in this way is known as " re- verted" phosphate. This term is gradually falling into disuse because it is not possible to distinguish between truly reverted phosphate and dicalcium phosphate formed by the action of sulphuric acid upon the normal calcium phosphate. The phosphorus of bones is in the form of normal calcium phosphate but it is in a condition which makes it possible for soil acids to readily convert it into acid phosphates. It also dissolves in ammonium citrate solution for the same reason and it is therefore properly classed as citrate soluble and as available. In reporting the analysis of fertilizers phosphorus is calculated as phosphorus pentoxide, which is often improperly called by fertilizer chemists " phosphoric acid." It is therefore customary to speak of total " phosphoric acid/' and of water soluble, citrate soluble and available " phosphoric acid," meaning by these terms phosphorus in the various forms already described, but calcu- lated as the pentoxide. These terms will not, however, be used in the following paragraphs. Total Phosphorus. The various methods for the determina- tion of phosphorus have already been discussed in connection with steel analysis, pages 380 to 385. The same methods are used in the analysis of fertilizers but the preliminary treatment will, of course, be quite different from that of steel. This treat- ment must include (a) destruction of organic matter and (b) solution of the phosphate. Since most methods for oxidizing the organic matter will involve the use of solvents for phosphates, these matters need not be considered separately. The details of the following method are essentially those of the " official" method of the A. 0. A. C., 1 although several addi- tional methods for dissolving the fertilizer are approved by the association. 1 U. S. Dept. Agr., Bur. Chem., Bull. 107, 21. 438 QUANTITATIVE ANALYSIS Gravimetric Determination. The following reagents are necessary: Ammonium molybdate solution, ammonium nitrate solution and "mag- nesia mixture," all to be prepared as directed for the determination of phosphorus in steel, page 384. Weigh about 2 gm of the fertilizer and place in a Kjeldahl digestion flask. Add 30 cc of concentrated sulphuric acid and 0.7 gm of mercury or mercuric oxide and digest as in the Kjeldahl method for the deter- mination of nitrogen. Do not add potassium permanganate (this would form manganese ammonium phosphate in the final precipitation) but after the solution has become colorless, cool, add 100 cc of water and then boil for a short time. Cool, transfer to a 250-cc volumetric flask, dilute to the mark and mix. Fill a dry, 100-cc volumetric flask with the phosphate solution and rinse into a 250-cc flask of Jena or other resistance glass. Add 15 gm of ammonium nitrate, warm to 65 and add molybdate solution, 50 cc for every decigram of phosphorus pentoxide that is thought to be present. Immerse in water and disgest at 65 for an hour, then filter and wash several times with 10-percent ammonium nitrate solution. During the washing the precipitate that adheres to the flask need not be removed but it must be washed. To the filtrate add 50 cc more of ammonium molybdate solution and digest. If more precipitate forms, filter and wash as before. Place the flask in which precipitation was made under the funnel and dissolve the precipitate on the filter in concentrated ammonium hydroxide, allowing the solution to run into the flask, thus dissolving the precipitate that adheres to the flask. Wash the paper thoroughly with hot water. Transfer the entire solution and washings to a 250-cc Jena beaker. The total volume of the solution should not be greater than 100 cc. Nearly neutralize with hydrochloric acid, test- ing by touching a corner of a piece of litmus paper to the stirring rod. Add, very slowly and with vigorous stirring, 25 cc of "magnesia mixture" and after 15 minutes add ammonium hydroxide (specific gravity 0.90) equal to one-ninth of the total volume of the solution. Cover and allow to stand for 2 hours. Filter and wash with ammonium hydroxide (containing 2.5 percent of ammonia) until nearly free from chlorides. The precipitate must be white. A Gooch filter may be substituted for the paper filter, in which case ignition to whiteness will be less trouble- some. Ignite the precipitate, observing all precautions earlier learned (see page 90), and weigh the magnesium pyrophosphate. Calculate the percent of total phosphorus pentoxide. Water Soluble Phosphorus. The phosphorus of untreated phosphate rock and of bone and other organic sources is in- soluble in water and this determination is omitted. In " acidu- ANALYSIS OF INDUSTRIAL PRODUCTS 439 lated" samples it is sometimes required although, as has already been stated, citrate insoluble phosphorous subtracted from total phosphorus gives available phosphorus and there is little object in making the determination in any case. Following is the " official" method. Determination. Place 2 gm of the sample on a 9-cm filter, wash with successive small portions of water, allowing each portion to pass through before adding more, until the volume of the filtrate is about 250 cc. Preserve the residue on the filter for the determination of citrate in- soluble phosphorus. If the filtrate is turbid add nitric acid until clear. Dilute to 200 cc in a volumetric flask, mix and determine the phos- phorus in 50-cc portions by the method above described for total phos- phorus. Calculate the percent of water soluble phosphorus, expressed as phosphorus pentoxide. Citrate Insoluble Phosphorus. Phosphates that are insoluble in neutral ammonium citrate solution, and therefore in soil solutions, are so slowly assimilated by plants that they are, for fertilizing purposes, practically valueless. The greatest diffi- culty exists, however, in making an accurate determination of insoluble phosphate because of the difficulty that is encountered in the preparation of a neutral ammonium citrate solution. In the discussion of indicators it was shown that no indicator is sufficiently sensitive to both acids and bases as to indicate accu- rately the point of neutralization if both acid and base are weakly ionized. It is therefore extremely difficult to neutralize exactly the weak citric acid by the weak base, ammonium hydroxide, with the aid of any organic indicator. The action of ammonium citrate upon monocalcium phosphate is due to the presence of citric acid of hydrolysis : It is therefore highly important that the concentration of citric acid in the solution should be the same in all cases if the ana- lytical results are to possess any significance, as the action of the solution is, at best, but an arbitrary and approximate imitation of the action of solutions found in soils. Two methods for the preparation of neutral ammonium citrate solution are approved by the A. O. A. C. 1 The first method is that of neutralizing a stated quantity of citric acid in 1 Loc. cit. 440 QUANTITATIVE ANALYSIS solution by ammonium hydroxide, the indicator being corallin (commercial rosolic acid). In the second method the solution is nearly neutralized and then a measured volume of a solution of calcium chloride in water and alcohol is added. Calcium citrate is at once precipitated. If the solution was neutral, ammonium chloride is the only other product of the reaction; if an excess of citric acid was present hydrochloric acid is also produced: Ca 3 (C6H 5 7 )3+6HCl. If the solution was basic instead of neutral or acid, ammonium hydroxide remains after the precipitation. By testing the solu- tion with cochineal after nitration and then adding either am- monium hydroxide or citric acid, as is indicated as being neces- sary by the reaction with cochineal, and by repeating this process as often as is necessary, the solution may finally be brought to a neutral condition. The advantage of this method over the first official method is in the substitution of an equivalent amount of a strong acid (hydrochloric acid) for the weak citric acid, so that an indicator may now be chosen of sufficient sensibility toward ammonium hydroxide to give indication of real neutral- ity. Even by this method, however, the ratio of ammonia to citric acid is found to vary. McCandless 1 found that this ratio for solutions made by nine different chemists varied between the limits 1:3.775 and 1:4.189. The calculated raito for normal ammonium citrate is 1:3.766. The excess of acid indicated by the above ratios would give the solution a greater solvent power for calcium phosphate and would give rise to incorrect results in the determination of insoluble phosphorus. Patten and Marti have devised a method 2 which they call the "titration method" and have shown that strictly neutral solu- tions can easily be made. The solution is first made approxi- mately neutral and then both ammonia and citric acid are deter- mined. The determination of ammonia is made by treating 5 cc of the solution with magnesium oxide and distilling the ammonia into standard acid: 1 U. S. Dept. Agr., Bur. Chem., Bull. 122, 147. 2 J. Ind. Eng. Chem., 5, 567 (1913). ANALYSIS OF INDUSTRIAL PRODUCTS 441 The excess of standard acid is titrated as in the Kjeldahl method for the determination of nitrogen. The determination of total citric acid in the solution depends upon the reaction of formalde- hyde with ammonia, either free or combined in salts, to form hexamethylenetetramine, a substance so weakly basic that it does not affect phenolphthalein. The reaction is represented by the following equations: 4NH 4 OH+6HCHO N(CH 2 NCH 2 ) 3 +10H 2 O, 4(NH 4 ) 3C 6 H 5 O7+ 18HCHO 3N(CH 2 NCH 2 ) 3 + 18H 2 0+ The free citric acid is then titrated by a standard solution of a strong base, as sodium hydroxide, in presence of phenolphthalein. This device is somewhat similar to that used in the calcium chloride method where the weak acid of the citrate solution is substituted by a strong acid and the question of neutrality is qualitatively decided by the use of an indicator that is sensitive to weak bases. In the Patten and Marti method the weak base of the citrate solution is destroyed and the remaining acid is titrated by a strong base, using an indicator that is sensitive to weak acids. The ratio of ammonia to citric acid having been now quantita- tively determined the solution is exactly neutralized by the addi- tion of the calculated quantity of either ammonium hydroxide or citric acid. Determination. In addition to the solutions used for the determina- tion of total phosphorus, prepare a neutral solution of ammonium citrate by the method of Patten and Marti as follows : Dissolve 370 gm of commercial citric acid in 1500 cc of water; nearly neutralize with ammonium hydroxide, testing with phenolphthalein. Measure 50 cc of this solution into a 250-cc volumetric flask, dilute to the mark and mix thoroughly. From a burette measure 5 cc of the diluted solution into a beaker, add 4 cc of a neutral 40-percent solution of formaldehyde and titrate with decinormal sodium hydroxide or potassium hydroxide solution in presence of phenolphthalein. The pink color should remain after the solution is boiled; if it does not the ammonia has not been entirely decomposed and another titration should be made, using more formaldehyde. Determine the total (free and combined) ammonia in the solution as follows: Carefully measure 5 cc of the di- luted solution into a 500-cc Kjeldahl digestion flask, add 0.5 gm of 442 QUANTITATIVE ANALYSIS magnesium oxide and at once distill into 50 cc of fifth-normal acid. Titrate the excess of acid, using cochineal as indicator. From the titration of citric acid and of ammonia calculate the amount of citric acid or of a standard solution of ammonium hydroxide that must be added to 1450 cc of the stronger solution of ammonium citrate in order to make an exactly neutral solution. After neutralization dilute the solution to 2000 cc. The specific gravity of the solution as finally diluted should be 1.09. Place 100 cc of ammonium citrate solution in a 250-cc flask which is immersed in warm water. Heat to 65 then drop into the solution the paper containing the washed residue from the determination of water-soluble phosphorus, close the flask with a rubber stopper and shake vigorously until the paper is reduced to a pulp. (If a deter- mination of water soluble phosphorus has not been required the un- treated sample may be used for the determination of citrate insoluble phosphorus.) Replace the flask in the water bath and keep at 65, shaking every 5 minutes. At the expiration of exactly 30 minutes from the time that the material was introduced remove the flask from the bath and filter the contents as rapidly as possible. ' Wash thor- oughly with water which is at a temperature of 65, then determine nitrogen in the residue by the method used for total phosphorus. From the percent thus obtained for citrate insoluble phosphorus and the per- cent of total phosphorus calculate the percent of available phosphorus. Also, if water soluble phosphorus has been determined, calculate the percent of citrate soluble phosphorus. Express all results as percents of phosphorus pentoxide. The volumetric method described on page 382 may also be used for the final determination of either total, water soluble or citrate insoluble phosphorus, the methods for making the fer- tilizer solution remaining unchanged. Potassium. All soils contain certain minerals, one component of which is potassium, and these are continually undergoing decomposition yielding potassium in a soluble form. The most important of such minerals belong to the class of felspars. Soils are frequently deficient in potassium and require fertiliza- tion with some material containing this metal. It is interesting to note that sodium, so nearly allied to potassium in chemical properties, is able to replace it in the plant organism very little, if at aU. The most important natural sources of potassium for fertilizer are the Stassfurt deposits from which are obtained kainite, ANALYSIS OF INDUSTRIAL PRODUCTS 443 KCl.MgS0 4 .3H 2 O, carnallite, KCl.MgCl 2 .6H 2 O, potassium sul- phate and a number of other potassium salts. Certain quantities of potassium are also derived from wood ashes, rich in potassium carbonate, and from certain organic materials, as tobacco stems and cotton-seed hulls. The various potassium fertilizers may be sold singly or they may be components of mixed fertilizers. Organic matter may or may not be present. If it is present it must be destroyed by oxidation and the earth and alkaline earth metals must be separated. The separation of organic matter and interfering metals and the determination of potassium are best accomplished by the Lindo-Gladding method, the principles of which have already been discussed (page 78). The reagents there described are necessary in this connection. Following is the official method of the A.O.A.C., 1 slightly modified. Determination. Prepare the potassium solution from the fertilizer as follows; (a) Potassium Salts and Mixed Fertilizers. Boil 10 gm of the sample with 300 cc of water for 30 minutes. In the case of mixed fertilizers add to the hot solution enough ammonium hydroxide to make slightly basic and then enough powdered ammonium oxalate to precipitate all of the calcium that is present. Cool, dilute to exactly 500 cc in a volumet- ric flask, mix and pass through a dry filter, rejecting the first 50 cc. In the case of potassium chloride, potassium sulphate, magnesium potas- sium sulphate and kainite, dissolve and dilute to 500 cc without the addition of ammonium hydroxide and ammonium oxalate. Evaporate 50 cc of the solution nearly to dryness, add 1 cc of dilute sulphuric acid (1:1), evaporate to dryness and ignite until white. Dissolve and dilute to about 50 cc. (b) Organic Compounds. Saturate 10 gm of sample with concentra- ted sulphuric acid and evaporate and ignite at a temperature indicated by dull redness, to destroy organic matter. A muffle furnace will be found to be convenient for this operation. Add a little concentrated hydrochloric acid and warm slightly in order to loosen the mass from the dish. Dissolve in hot water, dilute to 500 cc and mix thoroughly. To the solution of sulphate obtained in (a) or to 50 cc of the solution obtained in (b) add enough chlorplatinic acid to precipitate all of the potassium and to leave about 1 cc of platinum solution in excess. If the percent of potassium in the fertilizer is approximately known the quantity of platinum solution that is necessary should be calculated. 1 U. S. Dept. Agr., Bur. Chem., Bull. 107, 11. 444 QUANTITATIVE ANALYSIS Contamination with ammonia vapor must be avoided. Evaporate the solution on the water bath to a thick paste and add to the residue 25 cc of 80-percent alcohol. Stir thoroughly and allow to stand for 15 minutes. Filter through a Gooch crucible or a paper filter. If the filtrate is not colored, sufficient chlorplatinic acid is not present and the analysis must be begun again with another portion of the solution, increasing the amount of chlorplatinic acid. Wash the precipitate on the filter with 80 percent alcohol, continuing the washing after the filtrate is colorless. Remove the filtrate to the bottle for platinum waste and wash the precipitate five times with 10 cc portions of 10 percent ammonium chloride solution which has been saturated with potassium chlorplatinate. Wash again thoroughly with 80 percent alcohol, using particular care to remove all ammonium chlo- ride from the upper part of the filter. Dry the precipitate for 30 minutes at 100. For the procedure in case a paper filter has been used, see page 83. If the fertilizer is a nearly pure potassium salt instead of a mixed fertilizer, use only 5 gm of sample. The potassium is usually reported as percent of potassium oxide (called by the fertilizer chemist "potash") instead of the element. MILK The milk of most mammals has been analyzed and its composi- tion determined but, for practical purposes, the analyst rarely has to do with any other than cow's milk and human milk. The analysis of cow's milk may be made for purely scientific purposes as, for instance, the determination of the relation be- tween the composition of milk and the breed of animal, the season of the year or the rations upon which the animal is fed, or the determination of the changes that occur in composition during the period of storage, and other similar questions. The analysis may also be made for purposes of legal control to detect sophis- tication. The analysis of woman's milk is usually made for hygienic purposes, in order to provide a basis for modification of the mother's diet, etc., in cases where the infant is not thriving. The percentage composition of milk vBries rather widely al- though the same substances are found in practically all milk from a given species of animal. It is therefore not possible to fix, by legal enactment, the exact composition of milk that is to become an article of commerce, but certain minimum figures ANALYSIS OF INDUSTRIAL PRODUCTS CO 05 1 4 1 CO 00 CO CO .'c o D 00 IT CO Tt > i < d Glycerides of in- r,~l,,Ul~ ~J non-volatile . , 14 < \ 3 1 $ p 5,1 < B = j Ij 5*1 volatile acids Containing ' nitrogen (proteids) ^ CO CO O o : o O i-H o d O 1 1 1 1 d d "05 s H? ^ o> { Olein 1 Palmitin ll o3 '-C -t-3 PQ PQ c 1 o Caprylin (trac > Caprinin (trac [ Casein Albumin | ? i fi a 'o .g g JoU il 2 a 1 gg ' * G Sodium oxide. Calcium oxide Magnesium oxide Ferric oxide. . . Sulphur trioxic Phosphorus pentoxide Chlorine Water.. 445 o 446 QUANTITATIVE ANALYSIS are usually established by law and any milk containing a con- stituent in quantity below the legal minimum is considered to be adulterated. The average composition of cow's milk is given by Babcock in the foregoing table: 1 The comparative figures for cow's milk and human milk are given in the following table, compiled by Leach. 2 No. of analyses Sp. gr. Water Casein Albu- min Total proteids Fat Lac- tose Ash Cow's milk Minimum Maximum 800 1 . 0264 1 . 0370 80.32 90 32 1.79 6 29 0.25 1 44 2.07 6 40 1.67 6 47 2.11 6 12 0.35 I 21 Average 1.0315 87.27 3.02 0.53 3 55 3 64 4 88 71 Human milk 200 1 027 81 09 18 32 69 1 43 3 88 12 Maximum 1 032 91 40 1 96 2 36 4 70 6 83 8 34 1 90 Average 87.41 1.03 1.26 2.29 3.78 6.21 0.31 Adulteration of Milk. Adulteration is practiced by skimming or watering, or by both methods. There is a rather widely disseminated popular belief that the only part of milk that has much value is the cream. Many municipalities control the com- position of milk only by specifying a minimum for fat and the milk inspection will often include little else than a determina- tion of the percent of fat. Milk containing a high percent of butter fat, such as that from Jersey cows, could then be watered in such a way as to leave the legal minimum of fat but the solids not fat would thereby be lowered. Proper control of adulteration can be secured by a considera- tion of the relations between total solids, solids not fat and fat. Watering has the effect of lowering the percent of total solids, solids not fat and fat, proportionately. Skimming lowers the percent of fat and total solids and slightly raises the percent of solids not fat. If both watering and skimming are practiced the ratio of solids not fat to total solids is slightly increased, while the ratio of fats to total solids is abnormally low, all three per- cents being lowered. The methods for the analysis of dairy products as used in most laboratories are the official and provisional methods of the A. 1 Leach : Food Inspection and Analysis, 90. 2 Ibid., 91. ANALYSIS OF INDUSTRIAL PRODUCTS 447 O. A. C. 1 These are given, with certain modifications, in the following pages. It should be noted that the fat of milk quickly rises when the milk is quiet and the sample must be thoroughly mixed by pour- ing from one vessel to another immediately before the removal of any sample for a determination. Violent agitation must be avoided as this will result in coalescence of the fat globules. The analysis should be completed within the shortest possible time after the sample is received, in order to avoid the inter- ference of fermentative or putrefactive changes. If it is impos- sible to make all of the determinations within a short time the sample for each determination should be removed and weighed, after which the work may be interrupted for a reasonable period at the following stages in the various determinations: Determination Specific gravity Total solids. . Stage after which work may be interrupted Must be completed at once Evaporation of sample Ash Weighing the sample Total nitrogen Addition of sulphuric acid Casein and albumin. . Addition of sulphuric acid to filtered proteids Lactose. Addition of mercuric salt solution Fat (centrifugal) ! Addition of sulphuric acid Fat (gravimetric) . . . Formaldehyde Drying of paper coil with milk Test must be made at once. In case it is impossible to begin any of the determinations while the sample is fresh, add formaldehyde (1 part of 40 percent solu- tion to 2500 parts of milk) and place on ice. Specific Gravity. The specific gravity of milk may be deter- mined by means of a hydrometer, a Westphal balance or a pyc- nometer, or by any method used for other liquids. It is most convenient, and sufficiently accurate, to use a hydrometer, which should be standardized on account of the large errors that are frequently made in graduating these instruments. It is unfortu- nate that arbitrary scales have come into use for the expression of the density of milk. The instrument called the " lactometer" is simply a hydrometer having an arbitrary scale of its own, sometimes also reading directly in specific gravity. Quevenne's lactometer is graduated in degrees from 15 to 40, correspond- ing to specific gravity 1.015 to 1.040. The New York Board of 1 U. S. Dept. Agr., Bur. Chem., Bull. 107, 117. 448 QUANTITATIVE ANALYSIS Health lactometer is graduated also in arbitrary degrees, corresponding to a specific gravity of 1 and 100 to a specific gravity of 1.029, the latter figure being considered as the specific gravity of pure average milk. Degrees on the New York lac- tometer would thus roughly indicate the percent of whole milk in a milk and water mixture. Such an indication can have but slight value in milk testing. Determination. Mix the sample of milk by pouring from one vessel to another several times, avoiding violent agitation. Immediately determine the specific gravity at 15.5, using a lactometer. Record both specific gravity and lactometer degrees. Total Solids. A statement of the chemical nature of the solids of milk has already been given in the table on page 445. The fatty constituents are present in the form of an emulsion of minute globules which can be made to coalesce by agitation, such as occurs during churning. The proteids are chiefly in the form of sols from which they are flocculated by such means as the addition of acids, boiling, etc. The well-known " curdling " of milk when it " sours" is chiefly due to the development of lactic acid by fermentation of lactose. Lactose itself is present as a true solution, as are also the inorganic constituents, although calcium phosphate combines in some manner with casein. The determination of total solids must be made at a tempera- ture not higher than 100 in order to avoid chemical changes. Great difficulty is often experienced in the attempt to remove completely the water, on account of the formation of a skin of flocculated proteids. This effect may be minimized by placing a small amount of sand in the dish and stirring occasionally. The drying surface is, in this way, very much increased. Alu- minium dishes are suitable for this determination. They should be not less than 5 cm in diameter and should be flat on the bottom. Determination. Clean, dry and weigh a dish containing 15 to 20 gm of clean sand and a short glass rod, then add about 5 gm of milk. The sample may be weighed in the dish or it may be accurately measured after the specific gravity has been determined. In either case it must be well mixed as directed for the determination of specific gravity. Dry at 100 until the weight is constant, stirring occasionally with the glass rod, which is left in the dish. Cool in a desiccator and weigh rapidly, in order to avoid absorption of moisture. Calculate the loss in weight as percent of water. ANALYSIS OF INDUSTRIAL PRODUCTS 449 Ash. The ash of milk contains the inorganic constituents in the form in which they are left after burning the solids and it does not represent the original combinations of these constituents. Determination. Weigh a platinum dish, add about 20 gm of milk and quickly reweigh. Add 6 cc of concentrated nitric acid, evaporate to dryness over a steam bath and ignite at a temperature just below redness until free from carbon. Weigh and calculate the percent of ash. Total Nitrogen. The nitrogenous constituents of milk are usually calculated as proteids, although traces of other nitroge- nous compounds are present. While the molecular constitu- tion of the proteids is not known the percentage composition is established as varying within fairly narrow limits for the different proteids. The following table expresses the approximate composition. Element Percent Carbon 50 to 55 Hydrogen 6.9 to 7.3 Nitrogen 15 Oto 19.0 Oxygen 19 to 24 Sulphur 0.3 to 2.4 Both casein and albumin contain slightly less than 15.7 percent of nitrogen and, since these compounds make up more than 95 percent of the total proteids of milk, the percent of total proteids is found with sufficient accuracy by multiplying the percent of total nitrogen by 6.38 ( = T^) Nitrogen is determined by the Kjeldahl or Gunning method, as already discussed in connection with fertilizers, the sample of milk being digested without previous evaporation. Determination. Having accurately determined the specific gravity of the sample, mix well and measure 5 cc into a Kjeldahl digestion flask, using a calibrated pipette, and calculate the weight. Without evaporating the milk proceed to determine nitrogen by either the Kjeldahl or Gunning method, described on pages 431 and 433. Mul- tiply the percent of nitrogen by 6.38 and record the result as pcreent of total proteids. Casein. Casein is present in milk as a sol and it has a high 29 450 QUANTITATIVE ANALYSIS degree of molecular association. It is flocculated by dilute acids and will not thereafter redissolve, so that it is to be classed as an irreversible colloid. It dissolves in concentrated acids but its chemical nature is thereby changed. Casein is approximately separated from the other proteids by warming to 40 after the addition of water and acetic acid. Casein flocculates and is separated by nitration. Nitrogen in this residue is determined and casein is calculated. Casein may also be precipitated by heating to 40 with a solution of potassium aluminium sulphate. Determination. Weigh a 125-cc beaker, add 10 cc of milk and quickly reweigh. Add 90 cc of water which is at a temperature of 40 to 42 and add, at once, 1.5 cc of a 10 percent solution of acetic acid. Stir with a glass rod and allow to stand for 5 minutes longer. Filter and wash three or four times with cold water, saving the filtrate and washings, the total volume of which should not be greater than 125 cc. The filtrate should be quite clear. Place the paper and casein in a Kjeldahl digestion flask and deter- mine nitrogen by the Kjeldahl or the Gunning method. Multiply the percent of nitrogen by 6.38 and record the result as the percent of casein. Albumin. Albumin is flocculated and separated from the remaining proteids by the use of a more dilute solution of acetic acid but at a higher temperature. The determination of nitrogen in the precipitate gives a basis for the calculation of the percent of albumin. Determination. Add a drop of phenolphthalein to the filtrate from casein and neutralize with an approximately decinormal solution of sodium hydroxide or potassium hydroxide. Add G.3 cc of 10-percent acetic acid solution, immerse the beaker in boiling water and heat until the albumin is completely flocculated. Filter, wash and deter- mine nitrogen in the precipitate as in the determination of casein. The percent of nitrogen multiplied by 6.38 gives the percent of albumin. Lactose. The only carbohydrate occurring in milk in suffi- cient quantity to be of any importance is lactose. It may be de- termined by either optical methods, depending upon its power to rotate the plane of polarization of plane-polarized light, or by reduction methods, based upon its power to reduce a cupric salt in a basic solution containing a tartrate. Optical Methods. With the exception of the proteids lac- tose is the only optically active substance in milk and it is only ANALYSIS OF INDUSTRIAL PRODUCTS 451 necessary to remove the proteids in order to apply the polari- scope to the determination of the concentration of lactose. It is assumed that the student is familiar with the physical prin- ciples upon which the construction of the various types of polar- iscopes is based. The Laurent and the Ventzke polariscopes, which are among the best-known instruments used for this pur- pose, are provided with scales reading in angular degrees and also with special sugar scales, upon which each degree indicates 1 percent of sugar if a certain specified weight of material is con- tained in 100 cc of solution and polarized in a 200-mm tube. This specified weight is the "normal" weight for a given instru- ment. The normal weight of cane sugar for the Laurent polar- iscope is 16.19 gm. For the Ventzke instrument the normal weight is 26.048 gm. These weights are, however, based upon dilution to 100 Mohr cubic centimeters. (See page 144.) Since volumetric apparatus is now generally calibrated upon a basis of the true cubic centimeter it is necessary to correct these weights by multiplying by the ratio of the volume of the true cubic cen- timeter to that of the Mohr cubic centimeter. This ratio is: Density of water at 17.5 _ T7j _ , x Densityof-water at 20 so that 1 ounce Troy of silver has a coinage value of $1.29. For the final weighing of the gold and silver a balance of high sensibility and precision is necessary. Differences as small as 0.01 mg should be perceptible. If the sample used were as small as 0.1 A. T., as is frequently the case, even this amount of error would involve a difference of 0.1 ounce per ton and this, in a gold ore, would involve an uncertainty in value of $2.06 per ton of ore. In order to produce a balance of such sensibility the knife edges must be very finely and accurately ground and the moving parts must be very light. The pans are only 2 to 3 cm in diameter and instead of the usual supporting bows the pans are suspended from a single wire. This type of assay er's balance is called a " button balance" because it is used only for weighing the "buttons" of metal. Decomposition of the ore and extraction of the gold and silver may be accomplished by either the crucible process or the scorifi- cation process. Crucible Process. In this process the finely ground ore is intimately mixed with fluxes and lead oxide and a small amount of an organic reducing agent is also added unless the ore contains enough material of a reducing character. Three essential changes take place when the mixture is heated in a crucible: (1) The ore combines with the fluxes and a fused slag is produced, ANALYSIS OF INDUSTRIAL PRODUCTS 471 (2) the lead oxide is reduced to finely divided lead, and (3) the particles of reduced lead, settling in a fine spray through the whole mass of slag, alloy with the particles of mechanically freed silver and gold, later collecting at the bottom of the crucible. For the fusion of the mixture of ore and fluxes there is required a large crucible of fire clay, of the form shown in Fig. 118. This is placed in a furnace large enough to heat two, four, six or even more crucibles at a time. Such a furnace is found in all assaying laboratories. It may be heated by coke, gas, gasoline, kerosene or crude oil. A detailed descrip- tion of such furnaces will be found in any of the special books dealing with assaying. The reagents that are used in the crucible process may be classified as fluxes, reducing agents, oxidizing agents and lead compounds, although some re- agents fall in more than one of these classes. Fluxes. Fluxes must be chosen with regard to the nature of the ore that is being decomposed. The formation of a fusible slag is due to a chemical combination of acid and basic materials to form a salt FIG. 118. Assay crucible. or mixture of salts. If the gangu\e of the ore is of an acid nature, as is the case with quartz and many silicates, a basic flux will be required. If, on the other hand, the ore contains much iron oxide, limestone or similar metallic oxides or carbonates it will require a flux having an acid character. Many materials, either ore refractories or fluxes, are classed as acid or basic when they have the composition of salts. This is because they have the power of combining with other substances to form salts of lower fusing points, or because they change in such a way as to become acid or basic when heated. Thus clay, a silicate of aluminium, is classed as an acid refractory because it will combine with calcium oxide, sodium oxide, etc. (added as carbonates and 472 QUANTITATIVE ANALYSIS changed by heating), forming double silicates, fusing at lower temperatures. Carbonates of the alkali and alkaline earth metals while they have the composition of normal salts, lose carbon dioxide when heated with acid materials and thus act the same as though the oxides had been originally added. They are there- fore classed as basic fluxes. The principal fluxes that are used in assaying are as follows: Acid Fluxes. Silica, borax and borax glass are the common members of this class although a great many other compounds might be used. Silica combines readily with metallic oxides or carbonates forming silicates of various melting points, the latter depending upon the proportions of flux and metallic oxide. Borax or sodium pyroborate, Na 2 B407.10H 2 0, and borax glass, which is the anhydrous salt, Na 2 B 4 07, act as acid fluxes because of their power of combining with metallic oxides to form ortho- borates and metaborates: Na 2 B407+5CaO-*2NaCaBO8+Ca 8 (BOs)2. This action may be more easily understood if oxide formulas are used. Na 2 0.2B 2 3 -h5CaO-Na 2 O.2CaO.B 2 O3+3CaO.B 2 O 3 . The reactions of crystallized and anhydrous sodium pyroborate are identical but the swelling of borax which accompanies the loss of water of crystallization often causes a loss of a part of the fusion mixture. On this account the borax should first be fused, cooled and powdered. This gives the product known as borax glass. Basic Fluxes. Fluxes which are basic or which become basic when heated are sodium carbonate, sodium bicarbonate, potas- sium carbonate, potassium bicarbonate, calcium carbonate, calcium oxide, lead oxide, ferric oxide and argols. The alkali carbonates and bicarbonates become oxides when heated, espe- cially when an acid substance is present to combine with the oxides as formed. The bicarbonates are somewhat cheaper than the normal carbonates but they evolve twice as much carbon dioxide for a given amount of metallic oxide formed and the normal salts are therefore preferable. This difference is shown by the following equations: ANALYSIS OF INDUSTRIAL PRODUCTS 473 Na 2 CO 3 +Si0 2 ->Na 2 SiO 3 + CO 2 ; 2NaHCO 3 +Si0 2 -* Na 2 Si0 3 +2CO 2 +H 2 O. For similar reasons calcium oxide is preferable to calcium carbon- ate, although neither of these fluxes is extensively used in assay- ing. Lead oxide is an excellent flux for silica and silicates as most lead silicates fuse easily and become more fluid than many other slags. On account of its relatively high cost it is substi- tuted, as far as possible, by some of the cheaper fluxes already named. It is, however, always added to the mixture to serve as a source of finely divided lead and it therefore always acts to some extent as a basic flux. The amount which will actually enter the slag is diminished by increasing the proportion of other basic fluxes, such as sodium carbonate. Argols or crude potassium bitartrate, KHC 4 H 4 C>6, is not added to serve as a flux but to reduce lead oxide. However, it acts as a basic flux by virtue of the potassium which it contains, heating causing decomposition: 2KHC 4 H 4 6 K 2 0+5H 2 O+6CO+2C. Carbon monoxide and carbon thus formed reduce lead oxide. Reducing Agents. Metallic lead must be very intimately mixed with the ore after fusion has taken place, in order that it may alloy with all particles of gold and silver. It is not practica- ble to mix elementary lead with the ore before fusion because of the difficulty involved in reducing the metal to a sufficiently fine state of division and because lead so mixed would settle out of the mixture before complete decomposition and fusion of the ore. Instead, it is better to mix finely powdered lead oxide with the charge, providing a reducing agent that will act upon the oxide somewhat slowly, in this way providing intimate contact of the minute particles of lead with every portion of the ore. Reducing agents may constitute a part or all of the gangue of the ore. The most important of such reducing agents occurring in ores are sulphides and arsenides. Reactions like the following may occur : 5PbO+FeS 2 5Pb+FeO+2S0 2 . 2PbO+PbS->3Pb+S0 2 . 3PbO+ZnSZnO+SO 2 +3Pb. Many ores, on the other hand, contain no reducing agents or even contain oxidizing agents, such as ferric oxide or cupric 474 QUANTITATIVE ANALYSIS carbonate. In such cases its becomes necessary to add a reducing agent to the crucible charge. Reducing agents used for this purpose are argols, charcoal, flour and starch. Many other organic compounds might be used with equal success but the cost would generally be higher and no advantage would be gained. The exact reducing power of none of the common reducing agents can be calculated because they are not usually uniform in corn- position. A preliminary fusion with lead oxide and suitable fluxes will establish this value. 1 gm of argols of average purity will reduce about 10 gm of lead, under the average conditions that obtain in the crucible fusion. Oxidizing Agents. It has already been stated that some ores, particularly those containing sulphides, reduce lead oxide with the production of metallic lead. They may even reduce more oxide than is desirable, in which case the quantity of reduced lead must be diminished by the addition of an oxidizing agent. Whether this acts upon the sulphides or upon the lead after reduction is immaterial. Both actions occur to some extent but the ultimate effect of either is to cause less lead to be finally obtained. Instead of adding an oxidizing agent the ore may be subjected to a preliminary roasting or heating in air, oxidation of sulphides and arsenic leaving the ore almost entirely free from reducing power, but this is an operation that requires additional time and it is usually simpler to add an oxidizing agent to the mixture which is to be heated in the crucible. The only oxidizing agent that is commonly used in assaying is potassium nitrate. Sodium nitrate would serve equally well but its tendency to deliquesce in moist air hinders proper mixing with the other constituents of the crucible charge. As a substi- tute for an oxidizing agent metallic iron is often used, usually in the form of nails that are long enough to reach through the entire mixture in the crucible and to protrude above. Iron acts as a "desulphurizer," by forming ferrous sulphide, a compound which does not reduce lead oxide : FeS 2 +Fe 2FeS. PbS+Fe Pb+FeS. The last reaction produces lead but not as much as would be ANALYSIS OF INDUSTRIAL PRODUCTS 475 formed if lead sulphide (of galena) were allowed to react with lead oxide: Pb+2PbO3Pb+S0 2 . Lead Compounds. Lead oxide has already been mentioned in connection with fluxes. In the form of litharge this is the only lead compound that is ever used for this purpose in the crucible process for decomposing ores. On account of the almost uni- versal association of lead with silver in nature it is difficult to obtain litharge that is entirely free from silver. It is often furnished by manufacturers under the label " silver free" but this is, by no means, to be taken as an assurance that it contains no silver. A preliminary assay of litharge should be made with each lot purchased and the proper correction made in the crucible assays of ores. This corresponds to the "blank" determinations often made in connection with other analytical processes. Crucible Charge. The correct charge for the crucible fusion can be made only upon the basis of a knowledge of the composition of the ore. The experienced assay er can usually decide from the general appearance of the ore as to the approximate mixture that is necessary for the proper fusion and extraction of the gold and silver. This is true, however, only in case the unground ore can be inspected, since powdering destroys the characteristic appearance of most minerals. Fortunately a certain latitude in the proportion of the ingredients is permissible. Small varia- tions from the composition of the best possible mixture will often work no more serious harm than production of a slag requiring a somewhat higher temperature for complete lique- faction or the production of a lead button that is somewhat larger than is desirable. The fact that some variation is per- missible and also that experienced assayers do not often make pre- liminary analyses of ores often leads the student to the conclusion that the mixing of charges is largely a matter of guess work in any case. This is very far from being true. To enter into a detailed discussion of all classes of ores is not possible in this brief treatment of the subject of assaying. Cer- tain typical ores will be considered and the approximate crucible charge for each class stated. A knowledge of the fundamental principles of assaying will thereby be gained and the proper 476 QUANTITATIVE ANALYSIS treatment pf other ores will be known as a result of more extensive work in the assaying laboratory. Silicious Ores Containing No Reducing or Oxidizing Com- pounds. The chief gangue of this class of ores is quartz or sili- cates such as clay or the felspars. The crucible charge must contain the proper fluxes for the production of a liquid slag, also litharge and a reducing agent. The only fluxes required are sodium carbonate and litharge. Argols is probably the best reducing agent. After the materials are thoroughly mixed they are placed in the crucible and covered with a layer of sodium chloride about 1/4 inch thick. The salt fuses at a comparatively low temperature (776) and the liquid cover thus formed prevents the loss of powdered ore by action of escaping gases. Preliminary Assay. In order to determine the exact propor- tions of the various components of the crucible charge necessary for the best results, a preliminary assay may be made, using smaller quantities of the materials than will be used in the final assay. This serves the purpose of a preliminary analysis and the degree of fluidity of the slag and the size of the lead button obtained will indicate the necessary modification of the charge for the final assay. The preliminary assay is usually omitted unless the available sample of ore is small or unless the ore is already powdered so that its chemical nature cannot be deter- mined by inspection. It is usually preferable to use the full quantities of ore and reagents. If the assay is successful it need not be repeated. If it is unsuccessful it serves all of the purposes of a preliminary and a new assay may then be made. The slag should be perfectly fluid and the lead button obtained should weigh 25 to 30 gm. Silicious Ores. A typical charge for a silicious ore would be as follows: Ore 1 A, T. Litharge 50 gm Sodium carbonate 50 gm Borax glass 2 gm Argols 2.5 gm Salt cover Borax glass is here added to increase the fluidity of the slag by the formation of lead borate. If crystallized borax is used, ANALYSIS OF INDUSTRIAL PRODUCTS 477 nearly twice as much will be required, as will be seen from the molecular weights of the two substances (202 and 382). In this, as in all other charges suggested in the following pages, the stated proportions of the components are merely average proportions for ores of the general characters indicated. Varia- tion in the charges will often be necessary but these will generally be made upon the advice of the instructor. No condensed state- ment of this kind could meet all of the conditions brought about by small variations in the composition of ores. It is well to remember that increasing the proportion of sodium carbonate somewhat increases the size of the lead button, also that too much borax gives a thick and viscous slag. The variation in the size of the lead button is due to the fact that with small quantities of sodium carbonate present more litharge enters the slag as a basic flux, leaving the argols to be oxidized to some extent by the air. Silicious and Carbonate Ores. For an ore in which limestone occurs with the silicious gangue the following mixture will serve : Ore 1 A. T. Litharge 35 gm Sodium carbonate 35 gm Borax glass 5 gm Silica 5 gm Argols 2.5 gm Salt cover Oxidizing Ores. Oxidizing ores contain certain reducible oxides or carbonates such as those of iron or manganese, ferric oxide being the most common. If the main portion of the gangue is still silicious the charge will be similar to that stated above for a silicious ore, but modified by increasing the amount of argols and by the addition of more borax glass, borax or silica to serve as a flux for the metallic oxides. The following charge may be used for a silicious ore containing about 50 percent of hematite: Ore 1 A. T. Litharge . . 40 gm Sodium carbonate 30 gm Borax glass 10 gm Silica 5 gm Argols 7 gm Salt.. cover 478 QUANTITATIVE ANALYSIS If the slag is sticky instead of fluid increase the porportion of sodium carbonate. If the lead button is too small increase the amount of argols, calculating the amount to be added from the previously determined reducing power of the argols. If the ore contains more than 50 percent of hematite increase the borax glass, silica and argols accordingly. If it contains less than this quantity decrease the amount of these substances. Reducing Ores. Ores containing sulphides or other reducing agents capable of reducing lead oxide must first be roasted or else an oxidizing agent or a desulphurizer must be added. The addition of potassium nitrate is recommended. The production of nitrogen oxides and sulphur dioxide causes some disturbance and the preliminary heating must be moderated accordingly. The addition of iron as a desulphurizer is not suitable for ores containing arsenic on account of the formation of a "speiss" or arsenide of iron. This speiss separates from both slag and lead and it usually carries gold if this metal is present in the ore. If iron nails are used they must be removed from the crucible before pouring and they sometimes cause trouble on account of the adherence of small globules of lead, this causing a loss of gold and silver. For a pyritic ore the following charges are suggested for trial assays. Unless the reducing power of the ore is exactly known it is impossible to predict the weight of lead that will be obtained but the charge may be modified, if necessary, after the first trial. Nitrate Method Iron Method Ore 0. 5 A. T. 0. 5 A. T. Litharge 70 gm 30 gm Sodium carbonate 25 gm 50 gm Borax glass 2.5 gm 5 gm Silica 2 gm 2 gm Potassium nitrate 5 gm Nails, cut, 20d 4 Salt cover cover If the lead button obtained by the nitrate method is too small decrease the amount of potassium nitrate; if it is too large increase the potassium nitrate. If the button from the iron method is too small add a calculated weight of argols. Ores Containing Copper, Arsenic or Antimony. If ^ores con- taining compounds of copper, arsenic or antimony are treated ANALYSIS OF INDUSTRIAL PRODUCTS 479 by the methods already described, without modification, these metals will be reduced and will enter the lead button as constitu- ents of the alloy. The button will thereby be rendered brittle and difficult to free from slag. This action must not be prevented by the addition of potassium nitrate because this will prevent also the formation of a lead button of sufficient size. It can be prevented without this interference by largely increasing the proportion of litharge, which keeps the interfering elements in the form of their oxides, these entering the slag by combination with fluxes. Copper oxide will, of course, combine with acid fluxes and the proportion of borax glass or silica is, on this account, increased. Oxides of arsenic or antimony will require basic fluxes. The following charge may be used for an ore which is largely chalcopyrite, Cu 2 S.Fe 2 S 3 . Ore 0.5 A. T. Litharge 125 gm Sodium carbonate 30 gm Borax glass - 10 gm Silica 5 gm Salt cover It is here assumed that sufficient lead will be reduced by the sulphides present without the necessity for the addition of another reducing agent. If this is not the case add the required amount of argols in the next fusion. Copper in a slag is indicated by a red or green color, the former being due to cuprous silicate and borate, the latter to cupric salts of the same acids. Cupellation. After the ore has been decomposed and the alloy of lead, gold and silver has been obtained the button must be treated in such manner as to completely remove the lead. This is easily accomplished because of the readiness with which lead oxidizes when heated in contact with air. The alloy is placed in a small vessel called a "cupel," which is made of bone ash and is shaped as shown in Fig. 119. Bone ash is chiefly composed of calcium phosphate and this has the very valuable property of being able to absorb lead oxide at high temperatures. In making the cupel the ash is moistened and pressed together. This produces a mass which is quite porous so that the lead oxide is readily absorbed. Cupels thus made are very fragile and 480 QUANTITATIVE ANALYSIS must be handled carefully. Laboratories in which large numbers of assays are made usually make their own cupels by means of an inexpensive hand or power press. Manufacturers of cupels for shipping usually add to the water a small amount of glue or molasses which serves as a binding material. This chars and blackens when the cupel is heated but it soon burns out. The cupel must be large enough to easily contain the lead alloy after the button is melted. It should weigh at least as much as the button in order to efficiently absorb the necessary quantity of lead oxide. The furnace in which the cupel is heated is of the muffle type and it must have a good draught in order that the lead may be quickly oxidized. The muffle is an arched chamber of fire clay, varying from 4 to 12 inches wide, 8 to 20 inches long and 3 to 6 inches high. It is heated by a furnace in FIG. 119. Cupel which the fuel may be any of those used for the crucible furnace. As cupellation proceeds the lead is oxidized, a part is volatilized and drawn into the chimney and the remainder is absorbed by the cupel. When this process is finished the button of gold and silver is weighed and it is then prepared for the process of parting. Inquartation. The gold and silver of the button are to be sepa- rated by dissolving the silver in nitric acid. If gold constitutes more than about one-fourth of the weight of the button the silver will dissolve very slowly. In this case it is necessary to increase the proportion of silver after the button has been weighed. This process is known as "inquartation." The button is wrapped in the necessary quantity of pure silver foil and is then placed in a clean cupel and melted by means of a blowpipe. It is kept in a fused condition until the added silver has thoroughly dissolved and it is then allowed to cool. With some experience it is not difficult to determine whether it will be necessary to inquart the button before parting, the depth of yellow indicating the approxi- mate percent of gold. In the beginning it is better to inquart if the button is appreciably yellow. Parting. The addition of nitric acid (specific gravity 1.2) to a button which contains not more than about 25 percent of gold will cause the solution of all of the silver, leaving the gold in the ANALYSIS OF INDUSTRIAL PRODUCTS 481 form of a brown skeleton which later usually falls to a coarse powder. The nitric acid must be free from chlorine as otherwise gold will be dissolved to some extent. Annealing. Gold left from the parting process has not the characteristic yellow color of massive gold but acquires it upon being heated. The mere change of color is of little or no impor- tance but the heating that is necessary for complete drying causes the change. This process is called " annealing." After annealing the gold is brushed into the pan of the button balance and weighed. Determination. Sample the ore according to the usual plan but exercise extraordinary care in the operations of mixing and dividing. The last portion obtained should weigh at least 100 gm and it must be ground to pass a 100-mesh sieve without forcing by the brush. Mix the sifted sample by rolling and leave in a flattened pile on the paper. Take out the sample for weighing by means of a spatula, dipping from various parts of the pile and taking the entire depth of the pile at each dipping. Weigh in the pulp balance the amount of ground ore that is required for the fusion, using one of the charges already suggested or a modification made by the instructor according to known variations in the character of the ore. (If the ore is known to be a very rich one the quantity of sample used will be less than 1 A. T. and the other com- ponents of the charge will be reduced accordingly.) Before removing the weighed ore from the pulp balance weigh, on the ordinary laboratory balance, the other components of the charge with the exception of the salt, in the order named in the statement of the charge, beginning with the litharge. All of the reagents must be free from lumps. Place these substances in a flattened pile on a piece of mixing paper or oilcloth and finally brush the ore sample onto the top of the pile and mix well by rolling. Empty the charge into an assay crucible which is 6 to 8 inches in height, brush the paper to remove the last of the mixture and tap the crucible slightly to settle the charge. Lastly cover the mixture with a layer of salt about | inch thick and place the crucible in the furnace, which should not be hot enough to crack the crucible. Raise the temperature gradually, using a moderate temperature until violent effervescence has ceased. After the fusion is quiet heat to a temperature of bright redness for a period lasting from ten minutes to one hour, according to the difficulty experienced in obtain- ing complete decomposition. The pouring mould, which is made of iron and has conical depressions, should be warmed meanwhile to prevent sudden chilling of the button and slag when pouring. Lift the crucible from the furnace, tap lightly to settle globules of lead that may be suspended in the slag and pour, 31 482 QUANTITATIVE ANALYSIS quickly but steadily, into the mould. This mould is never made large enough to contain all of the slag, so that the latter will always run over. A mould which would contain the entire contents of the crucible would require an inconveniently long time for cooling. The attention should be fixed, not upon the slag but upon the lead alloy, which appears as a bright stream near the end of the pouring. This stream must be directed toward the center of the mould and it must be poured without splashing. The lead immediately sinks to the bottom where it later solidifies as an inverted cone under the slag. After pouring, the crucible must be free from masses of imperfectly fused slag and from particles of lead. Allow the mould to stand quietly until the slag and lead button are perfectly solid then invert the mould, when the contents will easily drop out. Carefully break the slag from the button by means of a hammer. Examine the slag in order to detect any particles of lead that may have been caught by it. Such particles may be saved but the assay is not reliable in such a case and it is better to begin again, changing the con- ditions as may be necessary to obtain perfect separation. The button should be quite malleable and it should separate easily without leaving a crust of lead on the slag. Carefully free the button from all adhering particles of slag by hammering on a small, clean anvil. This operation should be performed in such a manner as to leave the button in the form of a cube, finally truncating the corners to prevent later injury to the cupel. If detached particles of lead have been recovered place these on the clean cube and weld into the latter by a stroke of the hammer. The lead button should weigh from 25 to 30 gm although a button as light as 20 gm will often contain all of the gold and silver of the ore if the fusion has been normal. If a heavier button than 30 gm is obtained do not discard it but reduce its size by scorification, a process to be later described (page 484). If the button is too small or if the slag does not pour and separate well, make another charge, properly modified in accordance with the principles already discussed. Cupellation. Place a cupel in the already heated muffle furnace and bring to a temperature of bright redness, then carefully drop in the button by means of long tongs provided for the purpose. Close the muffle door and raise the temperature to about 700 (bright redness) when the black crust of lead suboxide, Pb 2 0, changes to the yellow monoxide, PbO, and this begins to volatilize. Remove the top of the muffle door, the lower half being left in to shield the cupel from the entering current of cold air. If the door is in one piece remove and place two or three empty cupels in front of the one that contains the alloy. Much of the lead oxide is vaporized and is drawn into the chimney 1 ANALYSIS OF INDUSTRIAL PRODUCTS 483 while the remainder is absorbed by the cupel. If the temperature is too high the lead will boil, with consequent mechanical loss. If the temperature is too low the button will freeze and again loss will occur through "sprouting." The latter action is due to the contraction of the cooled and solidified skin of lead, the liquid alloy from the interior breaking through and often being thrown out of the cupel. A sprouted button should be at once discarded as the results obtained from it will be unreliable. The correct temperature for cupellation is indicated by the presence of a ring of litharge crystals upon the inner surface of the cupel, just above the liquid alloy. As cupellation proceeds the button becomes smaller. Toward the end of the process the temperature must be raised somewhat, on account of the rise in the melting point of the alloy which is now richer in gold and silver, metals of higher melting point than that of lead. So long as lead remains the surface of the alloy is covered by a thin, iridescent layer of oxide which is in continual motion over the surface. As the last of the lead is oxidized the iridescence suddenly disappears and the surface brightens or "blicks." To insure the removal of the last trace of lead close the door of the muffle and raise the temperature for about one minute. Remove the cupel from the muffle and cover the former in order to prevent too rapid cooling and consequent sprouting of the button. When the button is cold take it out by means of a pair of strong, pointed pliers and brush with a stiff brush to remove particles of oxide or cupel material. Weigh on the button balance and record the weight in milligrams. Inquartation. If the button is silver white or only faintly yellow it may be parted without previous inquartation. If the intensity of yellow indicates more than about 25 percent of gold cut a piece of pure silver foil weighing from one to three times as much as the button, according to the indicated composition of the button. Wrap the button in this foil and place in a new, clean cupel. Carefully fuse by means of a blowpipe and keep in the fused condition for .one minute, then allow to cool. Parting. Place the button in a No. 1 porcelain crucible and nearly fill the latter with nitric acid whose specific gravity is 1.2. Warm gently until action begins. The silver should dissolve rapidly enough to cause a moderate evolution of gas. If it does not do so it contains too much gold and parting will be imperfect. In this case remove the button, wash, dry and fuse with more silver. When all action of the acid has ceased and only a brown skeleton of gold remains carefully decant the acid solution of silver nitrate into a porcelain dish, allowing no particles of gold to escape. Wash by decantation, using distilled water which has been tested and found to be free from chlorides. The 484 QUANTITATIVE ANALYSIS washing process must be performed with extreme care as small particles of gold are easily detached and lost. A white porcelain dish is used for receiving the washings because of the consequent ease in detecting lost gold particles. Wash until all silver is removed, as shown by test- ing with hydrochloric acid. After the washing is completed dry the crucible on the hot plate. Annealing. Heat the crucible over the ordinary burner until the gold changes from brown to yellow, then allow to cool, brush into the pan of the button balance and weigh. Calculate the ounces per ton of gold and silver upon the basis of milligrams obtained per assay ton of ore, making the proper correction in the weight of the silver for any silver that has been found in the litharge. Scorification. For the treatment of high-grade silver ores, and especially those containing copper, arsenic, antimony or zinc, the scorification process is sim- pler than the crucible process. In the scorification process the ore is mixed with granulated lead, usually with the addition of a small amount of borax glass or silica, and is heated FG 120 Scorifier in an oxidizing atmosphere in a shal- low vessel of fire clay called a "scori- fier." This is shown in Fig. 120. Its size varies between 3/4 inch and 4-1/2 inches, inside diameter, but the size ordinarily used is about 2 inches in diameter. In the crucible process the reactions are between the ore and added fluxes, litharge being one of these, and enough lead is reduced to form a button of the correct weight. Air plays little or no part in this process. In the scorification process the chief flux is lead oxide but it is formed by the oxidation of lead which is added in relatively large quantities, the unused excess being vapor- ized as oxide. Because of this possibility of expelling the unused excess of flux there is a rather large permissible latitude in the quantity of lead that may be taken. Because the process may be continued until the required quantity of lead is left for the button, no reducing agent being used, no preliminary calculation concerning this matter is necessary. Because the process is an oxidizing one copper, arsenic, antimony and zinc are easily driven into the slag and do not contaminate the button. In ANALYSIS OF INDUSTRIAL PRODUCTS 485 other words, the adjustment of the charge involves little more than the addition of a certain minimum amount of lead, with a small quantity of borax glass or silica to aid in the formation of a liquid slag if the ore contains little acid gangue. These features give the scorification process a decided advantage over the crucible process with ores to which it will apply. It is not suitable for low-grade silver ores because of the small amount of ore that must necessarily be used. Not more than 0.2 A. T. can conveniently be scorified and 0.1 A. T. will usually give better results if a scorifier of ordinary dimensions is used. The process does not work well with gold ores because of appreciable losses of gold in the slag. The following charges are suitable for the scorification of typ- ical ores of the classes named. Silicious ores: Ore 0.1 A. T. Lead 40 gm Borax glass 2 gm Ores containing arsenic and antimony: Ore 0.1 A. T. Lead 50 gm Borax glass 5 gm Ores containing copper: Ore 0.1 A: T. , Lead 65 gm Borax glass 1 gm Silica 1 gm Ores containing iron (pyritic ores) : Ore 0.1 A. T. Lead 50 gm Borax glass 3 gm The lead that is used for this purpose (known as "test lead") is a finely granular form and it should be as nearly as possible free from silver. The same difficulty is encountered in obtaining silver-free lead as was noted in the case of litharge and for the same reason. It is therefore necessary to make a preliminary assay of each lot of test lead and to correct the results of the assay of ores according to the amount of silver found in the lead. 486 QUANTITATIVE ANALYSIS Determination. Sample and weigh the ore according to the direc- tions given for the assay by the crucible process. Weigh the materials for the scorifier charge according to one of the statements given above. Place one-half of the lead in the scorifier, add the weighed sample of ore and mix well with the lead by means of a platinum wire, then add the rest of the lead. Place the borax glass and silica on the top and then place the scorifier in the muffle, which should be hot. Close the door of the muffle and raise the temperature to the point where the lead is melted and the black suboxide changes to the yellow, more volatile, monoxide. Open the door of the muffle and admit a full supply of air. The lead now rapidly oxidizes and a part of the oxide vaporizes but most of it attacks the ore and decomposes it with the formation of a liquid slag. This process of oxidation and slag formation will now continue until the ore is completely decomposed, a perfectly fluid slag forming a ring around the circumference of the scorifier leaving the lead exposed over a large circle in the center. As lead is thus used in slag formation and through vaporization of the oxide the exposed circle of the metal (the "bull's eye") becomes smaller on account of the descent of the sur- face toward the narrower part of the scorifier. When the process is finished the slag entirely covers the lead. After the "bull's eye" has disappeared close the muffle door and raise the temperature for a short time in order that the slag may become so thoroughly liquefied that it will not become viscous during pouring, then pour into the mould. The mould that was used in the crucible process may be used here also but a more shallow mould which has hemispherical depressions is preferred. When the slag and button are cold, remove from the mould and free from slag exactly as was done with the button obtained from the crucible fusion. Cupel, part and weigh as already directed. TABLE OF LOGARITHMS 488 QUANTITATIVE ANALYSIS LOGARITHMS Natural Numbers 1 2 3 4 5 6 7 8 9 Proportional Parts 1 2 | 3 4 5 6 7 |8 |9 10 00000043 0086:01280170 0212:0253 0294 0334 '0374 4 812 17 2L25'29 33 37 11 0414 0453 0492 0531 0569 0607 0645 0682 0719 0755 4 8 11 15 19J23 26 30 34 12 0792 0828 0864 0899 0934 0969 1004 1038 1072 1106 3 7 10 11 17|2l;24 28;31 13 1139 1173 1206 1239 1271 1303 1335 1367 1399 1430 3 6 10 13 16 19i23i26 29 14 1461 1492 1523 1553 1584 1614 1644 1673 1703 1732 3 6 9 12 15 18 21 24 27 15 1761 1790 1818 1847 1875 1903 1931 1959 1987 2014 3 6 8 11 14 17 20 22 25 16 2041 2068 20952122 2148 2175 2201 2227 2253 2279 3 5 8 11 13 16 : 1821 24 17 2304 2330 2355 2380 2405 2430 2455 '2480 2504 2529 2 5 7 10 12 15 172022 18 2553 ! 2577 2601 2625 2648 2672|2695;2718 2742 2765 2 5 7 <> 12il4J16 19 21 19 2788 2810 2833 2856 2878 2900 2923! 2945 2967 2989 2 4 7 <) 11 13 16 18 20 20 3010 3032 3054 3075 3096 3118 3139 3160 3181 3201 2 4 6 8 11 13 15 17 Lfl 21 3222 3243 3263 3284 3304 3324 3345 3365 3385 3404 2 4 6 s 10 12 14 10 18 22 3424 3444 3464 3483 3502 3522 3541 3560 3579 3598 2 4 6 8 10 12 14J15 17 23 3617 3636 36553674 3692 3711 3729 3747 3766 3784 2 4 6 7 9 11 13 15 17 24 3802 3820 3838 3856 3874 3892 3909 3927 3945^3962 2 4 5 7 9 11 12 14 16 25 3979 3997 4014 4031 4048 4065 4082 4099 41164133 2 3 5 7 9 10 12 14 15 26 4150 4166 4183 4200 4216 4232 4249 4265 4281 4298 2 3 5 7 8 10 11 r.i 15 27 4314 4330 4346 4362 4378 4393 4409 4425 4440 4456 2 31 5 8 9 11 13 14 28 4472 4487 4502 4518 4533 4548 4564 4579 4594 4609 2 3 5 (i 8 9 11 12 14 29 4624 4639 4654 4669 4683 4698 4713 4728 4742 4757 1 3 4 (i 7 9 10 12 13 30 4771 4786 48004814 4829 4843 4857 4871 4886 4900 1 3 4 6 7 9 10 11 13 31 4914 4928 4942 4955 4969 4983 4997 5011 5024.5038 1 3 4 6 7 8 10 11 12 32 5051 5065 5079 5092 5105 5119i5132!5145 5159-5172 1 3 4 5 7 8 9 11 12 33 5185 5198 5211 5224 5237 5250 5263 5276 5289 5302 1 3 4 5 6 8 9 10 12 34 5315 5328 5340 5353 5366 5378 5391 5403 5416 5428 1 3 4 5 6 8 9 10 11 35 5441 5453 5465 5478 5490 5502(5514 5527 5539 5551 1 2 4 5 6 7 9 10 11 36 5563 5575 5587 5599 5611 5623 5635 5647 5658 5670 1 2 4 5 6 7 8 10 11 37 5682 5694 5705 5717 5729 5740 5752 5763 5775*5786 1 2 3 5 6 7 8 9 10 38 5798 5809 5821 5832 5843 5855 5866 5877 5888 5899 1 2 3 5 6 7 8 9 10 39 5911 5922s 5933 5944 5955 5966 5977 5988 5999 6010 1 2 3 4 5 7 8 9 10 40 6021 6031 6042 6053 6064 6075:6085 6096 6107 6117 1 2 3 4 5 6 8 9 10 41 6128 6138 6149 6160 6170 6180 6191 6201 6212 6222 1 2 3 4 5 6 7 8 9 42 6232 6243 6253 6263 6274 6284 6294 6304 6314 6325 1 2 3 4 5 6 7 8 9 43 6335 6345 6355 6365 6375 6385 6395 6405 6415 6425 1 2 3 4 5 (3 7 8 9 "44 6435 6444 6454 6464 6474 6484 6493 6503 6513 6522 1 2 3 4 5 6 7 8 9 45 6532 6542 6551 6561 6571 6580 6590 6599 6609 6618 1 2 3 4 5 6 7 8 9 46 6628 6637 6646 6656 6665 6675 668416693 6702 6712 1 2 3 4 5 6 7 7 8 47 6721 6730 6739 1 6749 6758 6767'6776 6785 6794 6803 1 2 4 5 5 6 7 8 48 6812 6821 6830 6839 6848 6857 6866 6875 6884 '6893 1 2 4 4 5 6 7 8 49 6902 6911 69206928 6937 6946 6955 6964 6972 6981 1 2 4 4 5 el 7 8 50 6990 6998 7007 7016 7024 7033 7042 7050 7059 7067 1 2 3 4 5 6 7 8 51 7076 7084 7093 7101 7110 7118 7126 7135 7143 7152 1 2 2 4 5 e 7 8 52 7160 7168 7177 7185 7193 7202 7210 7218 7226 7235 i| 2 3 -) 5 6 7 7 53 7243 7251 7259 7267 7275 7284 7292 7300 7308 7316 ij 2 3 i 5 6 6 7 54 7324 7332 j 7340 7348 7356 7364 7372 738017388 7396 li 2 5 4 566 7 TABLES 489 LOGARITHMS Natural Numbers 1 2 3 4 5 6 7 s|. Proportional Parts 1 2 3 4|5 6789 55 7404 7412i7419j7427!7435 443 7451 7459 7466 7474 li 2| 2 3 4 5 5 6 7 56 7482 749017497 7505 7513 52075287536175437551 I 1 2 2 3 4 5 5 6 7 57 75597566757475827589 597 7604 761217619^7627 li 2 2 3 4 5 5l 6 7 58 7634 7642 7649 1 7657 7664 672 7679 76867694 7701 1 1| 2 3 4 4 5 6 7 59 7709 7716 7723 7731 7738 745 7752 7760 7767 7774 1 1 2 3 4 4 5 6 7 60 7782 7789 7796 7803 7810 818 7825 7832 7839 7846 1 1 2 3 4 4 5 a 6 61 7853 78607868 7875 7882 889 7896 7903 7910 7917 1 1 2 a 4 4 5 6 6 62 7924 7931 7938 7945 7952 959 7966 7973 7980 7987 1 1 2 a 3 4 5 6 6 63 7993;8000'8007 8014 8021 028 8035 8041 8048 8055 1 1 2 a 3 4 6 5 6 64 80628069 8075 8082 8089 096 8102 8109 8116 8122 I I 2 a 3 4 5 5 6 65 8129 8136 8142 8149 8156 162 8169 8176 8182 8189 1 I 2 3 3 4 5 6 6 66 8 195 1 8202 s 8209 8215 8222 228 8235 8241 8248 8254 1 1 2 a 3 4 5 5 8 67 8261 8267 8274(8280 1 8287 293 8299 8306 8312 8319 1 1 2 a 3 4 5 51 6 68 8325 8331 8338 8344 8351 357 8363 8370 8376 8382 1 1 2 3 3 4 4 5 6 69 8388 8395 8401 8407 8414 8420 8426 8432 8439 8445 1 1 2 3 4 4 5 6 70 8451 8457 8463 8470 8476 8482 8488 8494 8500 8506 1 1 2 4 4 5 6 71 72 8513|8519 8525 857385798585 853118537 85918597 8543 8549 8603 8609 8555 8615 8561 8621 8567 8627 1 1 1 1 2 4 4 4 4 5 5 5 & 73 8633 \ 8639 8645 i 8651 8657 8663 8669 8675 8681 8686 1 1 4 4 5 5 74 8692 8698 8704 8710 8716 8722 8727 8733 8739 8745 1 1 4 4 5 8 75 8751 8756 8762 8768 8774 8779 8785 8791 8797 8802 1 3 4 5 5 76 8808 8814 8820 8825 8831 8837 8842 8848 8854 8859 1 3 4 '5 S 77 8865 88718876 8882 8887 8893 8899 8904 8910 8915 1 2 4 4 5 78 8921 8927 8932 8938 8943 8949 8954 8960 8965 8971 1 2 4 4 5 79 8976 8982 8987 8993 8998 9004 9009 9015 9020 9025 1 2 4 4 5 80 9031 9036 9042 9047 9053 9058 9063 9069 9074 9079 1 1 2 4 4 5 81 9085 9090 9096 9101 9106 9112 9117 9122 9128 9133 1 1 j 4 4 5 82 9138 9143 9149 9154 9159 9165 9170 9175 9180 9186 1 1 o j 4 5 83 9191 9196 9201 9206 9212 9217 9222 9227 9232 9238 1 4 4 5 84 9243 9248 9253 9258 9263 9269 9274 9279 9284 9289 1 2 3 4 4 5 85 9294 j 9299 9304 9309 9315 9320 9325 9330 9335 9340 1 3 4 4 5 86 9345 J9350 9355 9360 9365 9370 9375 9380 9385 9390 ] 2 2 3 4 4 5 87 9395 9400*9405 9410 9415 9420 9425 9430 9435 9440 ( ] j 3 3 4 4 88 9445^45094559460 9465 9469 9474 9479 9484 9489 1 2 3 3 4 4 89 9494 9499 9504 9509 951 9518 9523 9528 9533 9538 ( 1 1 2 3 3 4 4 90 9542 9547 9552 9557 956 9566 9571 9576 9581 9586 1 1 2 3 3 4 4 91 9590 9595 9600 9605 960 9614 9619 9624 9628 9633 1 1 2 3 3 4 4 92 9638 9643 9647 9652 965 9661 9666 9671 9675 968C 1 1 2 3 3 4 4 93 9685 9689 9694 9699 970 9708 9713 9717 9722 972 ( 1 I 2 3 3 4 4 94 9731 9736 9741 9745 975 9754 9759 9763 9768 977 c 1 ] 2 3 3 4 4 95 9777 9785 978e 9791 979 980C 9805 9809 9814 981 1 1 2 a a 4 4 96 9823 9827 9832 9836 984 9845 9850 9854 9859 986 I 1 ] 2 a a 4 4 97 9868987298779881988 9890,9894 9899 9903 990 ( 1 1 2 a 3 4 4 98 9912991799219926993 993419939 9943 9948 995 c 1 ] 1 a 3 4 4 99 199569961 996599691997 9978 9982 9987 9991 999 1 l| 1 3 3 3 4 TABLE OF ANTILOGARITHMS 492 QUANTITATIVE ANALYSIS ANTILOGARITHMS Logarithms 1 2 3 4 5 6 7 8 9 Proportional Parts 1~ 2 | 3 | 4 | 5\ ~Q\Y\8 9 .00 1000 1002 005 007 1009 1012 1014 1016 1019 1021 i 1 1 1 2 2 2 .01 1023 1026 1028 030 1033 1035 1038 1040 1042 1045 i 1 1 1 2 2 2 .02 1047 1050 1052 054 1057 1059 1062 1064 1067 1069 i 1 1 1 2 2 2 .03 1072 1074 1076 1079 1081 1084 1086 1089 1091 1094 i 1 1 1 2 2 2 .04 1096 1099 1102 1104 1107 1109 1112 1114 1117 1119 1 i 1 I 2 2 2 2 .05 1122 1125 1127 1130 1132 1135 1138 1140 1143 1146 1 i 1 1 2 2 .t 2 .06 1148 1151 1153 1156 1159 1161 1164 1167 1169 1172 1 i 1 1 2 3 2 2 .07 1175 1178 1180 1183 1186 1189 1191 1194 1197 1199 1 i 1 1 2 2 2 2 .08 1202 1205 1208 1211 1213 1216 1219 1222 12251227 1 i 1 1 2 2 2 3 .09 1230 1233 1236 1239 1242 1245 1247 1250 1253 1256 1 i 1 1 2 2 2 3 .10 1259 1262 1265 1268 1271 1274 1276 1279 1282 1285 1 i 1 1 2 2 2 3 .11 1288 1291 1294 1297 1300 1303 1306 1309 1312 1315 1 1 2 2 2 2 3 .12 1318 1321 1324 1327 1330 1334 1337 1340 1343 1346 1 1 2 2 2 .2 3 . 13 1349 1352 1355 1358 1361 1365 1368 1371 1374 1377 1 1 2 2 2 3 3 .14 1380 1384 1387 1390 1393 1396 1400 1403 1406 1409 1 1 2 2 2 * 3 .15 1413 1416 1419 1422 1426 1429 1432 1435 1439 1442 1 1 2 2 2 3 3 .16 1445 1449 1452 1455 1459 1462 1466 1469 1472 1476 1 1 2 . 2 2 3 3. .17 1479 1483 1486 1489 1493 1496 1500 1503 1507 1510 1 1 2 2 2 3 3 .18 1514 1517 1521 1524 1528 1531 1535 1538 1542 1545 1 i 1 2 2 2 3 3 .19 1549 1552 1556 1560 1563 1567 1570 1574 1578 1581 1 i 1 2 2 3 3 3 .20 1585 1589 1592 1596 1600 1603 1607 1611 1614 1618 1 i 1 2 2 3 3 3 .21 1622 1626 1629 1633 1637 1641 1644 1648 1652 1656 1 i 2 2 2 3 3 3 .22 1660 1663 1667 1671 1675 1679 1683 1687 1690 1694 1 i 2 2 2 3 3 3 .23 1698 1702 1706 1710 1714 1718 1722 1726 1730 1734 1 i 2 2 3 3 4 .24 1738 1742 1746 1750 1754 1758 1762 1766 1770 1774 1 i 2 2 3 3 4 .25 1778 1782 1786 1791 1795 1799 1803 1807 1811 1816 1 2 2 3 3 4 .26 1820 1824 1828 1832 1837 1841 1845 1849 1854 1858 1 2 3 3 3 4 .27 1862 1866 1871 1875 1879 1884 1888 1892 1897 1901 1 2 3 3 3 4 .28 1905 1910 1914 1919 1923 1928 1932 1936 1941 1945 1 2 3 3 4 4 .29 1950 1954 1959 1963 1968 1972 1977 1982 1986 1991 1 2 3 3 4 4 .30 1995 2000 2004 2009 2014 2018 2023 2028 2032 2037 1 i 2 3 3 4 4 .31 2042 2046 2051 2056 2061 2065 2070 2075 2080 2084 1 i 2 3 3 4 4 .32 2089 2094 2099 2104 2109 2113 2118 2123 2128 2133 p 1 i 2 3 3 4 4 .33 2138 2143 2148 2153 2158 2163 2168 2173 2178 2183 1 i 2 3 3 4 4 .34 2188 2193 2198 2203 2208 2213 2218 2223 2228 2234 1 1 3 3 4 4 6 .35 2239 2244 2249 2254 2259 2265 2270 2275 2280 2286 1 1 2 3 3 4 4 5 .36 2291 2296 2301 2307 2312 2317 2323 2328 2333 2339 1 1 2 8 3 4 4 5 .37 2344 2350 2355 2360 2366 2371 2377 2382 2388 2393 1 1 J 3 3 4 4 5 .38 2399 2404 2410 2415 2421 2427 2432 2438 2443 2449 1 1 2 ;; 3 4 4 5 .39 2455 2460 2466 2472 2477 2483 2489 2495 2500 2506 1 1 2 2 3 3 4 5 5 .40 2512 2518 2523 2529 2535 2541 2547 2553 2559 2564 1 1 2 2 3 4 4 5 5 .41 2570 2576 2582 2588 2594 2600 2606 2612 2618 2624 1 1 j t 3 4 4 5 5 .42 2630 2636 2642 2649 2655 2661 2667 2673 2679 2685 1 1 $ 3 4 4 5 6 .43 2692 2698 2704 2710 2716 2723 2729 2735 2742 2748 1 1 t 3 8 4 4 5 6 .44 2754 2761 2767 2773 2780 2786 2793 2799 2805 2812 1 1 i) 3 3 4 4 5 6 .45 2818 2825 2831 2838 2844 2851 2858 2864 2871 2877 1 i 2 : 3 4 5 6 6 .46 2884 2891 2897 2904 2911 2917 2924 2931 2938 2944 1 i 2 3 a 4 5 5 .47 2951 2958 2965 2972 2979 2985 2992 2999 3006 3013 ] i 2 8 3 4 6 5| 6 .48 3020 3027 3034 3041 3048 3055 3062 3069 3076 3083 1 i \ 3 4 4 5 6 6 .49 309C 3097 3105 3112 3119 3126 3133 3141 3148 3155 1 i 2 | 4 4 5 6 6 TABLES ANTILOGARITHMS 493 Logarithms 1 2 3 4 5 6 7 8 9 Proportional Parts 1 2 3 4| 5 6| 7| 8| 9 .50 3162 3170 3177 3184 3192 3199 32063214 3221 3228 1 1 2 3 4 4 5 6 7 .51 3236 3243 3251 3258 3266 3273 3281 3289 3296 3304 1 2 2 3 4 5 5 6 7 .52 331113319 3327 3334 3342 3350 3357 3365J3373 3381 1 2 2 3 4 5 5 6 7 .53 3388 3396 3404 3412 3420 3428 3436 344313451 3459 1 2 2 3 4 5 6 6 7 .54 3467 3475 3483 3491 3499 3508 3516 3524 3532 3540 1 2 2 3 4 5 6 6 7 .55 3548^3556 3565 3573 3581 3589 3597 3606 3614 3622 2 2 3 4 5 6 7 7 .56 3631 3639 3648 ! 3656 3664 3673 3681 369036983707 2 3 3 4 5 6 7 8 .57 3715 3724 3733 3741 3750 3758 3767 37763784 3793 2 3 3 4 6 6 7 8 .58 3802i3811 38193828 3837 3846 3855 3864 3873 3882 2 8 4 4 5 9 7 8 .59 3890 3899 3908 3917 3926 3936 3945 3954 3963 3972 2 3 4 6 5 6 7 8 .60 3981 3990 3999 4009 4018 4027 4036 4046 4055 4064 2 3 4 6 6 7 V 8 .61 4074 4083 4093 4102 4111 4121 4130 4140 4150 4159 1 2 3 4 5 6 7 8 9 .62 4169 4178 4188 4198 4207 4217 4227 4236 4246 4256 1 2 3 4 5 6 7 8 9 .63 4266 4276 4285 4295 4305 4315 4325 4335 4345 4355 1 2 3 4 5 6 7 8 9 .64 4365 4375 43854395 4406 4416 4426 4436 4446 4457 1 2 3 4 5 6 7 8 9 .65 4467 4477 4487 4498 4508 4519 4529 4539 4550 4560 2 3 4 5 6 7 8 9 .66 4571 4581 45924603 4613 4624 4634 4645 4656 4667 2 3 4 5 7 9 10 .67 4677 4688 46994710 4721 4732 4742 4753 4764 4775 2 3 4 5 7 8 9 10 .68 4786 4797 4808 4819 4831 4842 4853 4864 4875 4887 2 3 4 6 7 8 9 10 .69 4898 4909 4920 4932 4943 4955 4966 4977 4989 5000 2 3 5 6 7 8 9 10 .70 5012 5023 503515047 5058 5070 5082 5093 5105 5117 2 4 5 6 7 8 9 11 .71 5129 5140 5152 5164 5176 5188 5200 5212 5224 5236 2 4 5 G 7 8 10 11 .72 5248 5260 5272 5284 5297 5309 5321 5333 5346 5358 2 4 5 6 8 9 10 11 .73 5370 5383 53955408 5420 5433 5445 5458 5470 5483 1 3 4 5 6 8 9 10 11 .74 5495 5508 5521 5534 5546 5559 5572 558515598 5610 1 a 4 5 6 8 9 10 12 .75 5623 5636 5649 5662 5675 5689 5702 5715 5728 5741 1 3 4 5 7 8 9 10 12 .76 5754 5768 5781 5794 5808 5821 5834 5848 5861 5875 1 3 4 5 7 8 9 11 12 .77 5888 5902 5916 5929 5943 5957 5970 5984 5998 6012 1 3 4 5 7 8 10 11 12 .78 6026 6039 6053 6067 6081 6095 6109 6124 6138 6152 1 3 4 6 7 8 10 11 13 .79 6166 6180 6194 6209 6223 6237 6252 6266 6281 6295 1 3 4 G 7 9 10 11 13 .80 6310 6324 6339 6353 6368 6383 6397 6412 6427 6442 1 3 4 6 7 9 10 12 13 .81 6457 6471 6486 6501 6516 6531 6546 6561 6577 6592 2 3 5 6 8 9 11 12 14 .82 6607 6622 663716653 6668 6683 6699 6714 6730 6745 2 3 5 G 8 9 11 13 14 .83 6761 6776 679216808 6823 6839 6855 6871 6887 6902 2 3 5 G 8 9 11 13 14 .84 6918 6934 69506966 6982 6998 7015 7031 7047 7063 2 3 5 G 8 10 11 13 15 .85 7079 7096 7112(7129 7145 7161 7178 7194 7211 7228 2 3 5 7 8 10 12 13 15 .86 7244 7261 7278 7295 7311 7328 7345 7362 7379 7396 2 3 5 7 8 10 12 13 15 .87 7413 7430 7447 ; 7464 7482 7499 7516 75347551 7568 & 3 5 7 9 10 12 14 1G .88 7586 7603 7621 7638 7656 7674 7691 7709 j 7727 7745 2 4 5 7 9 11 12 14 1G .89 7762 7780 7798 j 78 16 7834 7852 7870 7889 7907 7925 2 4 5 7 9 11 13 14 1G .90 7943 7962 79807998 8017 8035 8054 8072 8091 8110 2 4 6 7 9 11 13 15 17 .91 8128 8147 8166 ! 8185 8204 8222 8241 8260 8279 8299 2 4 8 9 11 13 15 17 .92 8318 8337835683758395 8414 8433 84538472 8492 2 4 6 8 10 12 14 15 17 .93 8511 8531855185708590 8610^8630 8650 8670 8690 2 4 6 8 10 12 14 16 18 .94 8710 8730 8750 8770 8790 8810 8831 8851 8872 8892 2 4 6 8 10 12 14 16 18 .95 8913 8933j8954 8974 8995 9016 9036 90579078 9099 2 4 6 8 10 12 15 17 19 .96 .97 9120 9333 9141'91629183'9204 9354937693979419 9226 9247 94419462 9268 9484 9290 9506 9311 9528 2 2 4 6 4 7 8 9 11 11 13 13 15 15 17 17 19 20 .98 9550 9572 9594 9616 9638 96619683 9705 9727 9750 2 4 7 9 11 13 16 18 20 .99 9772 9795 9817 9840 9863 98869908 9931 9954 9977 2 5 7 9 11 14 16 18 20 INDEX Abbe refractometer, 304, 307 Accuracy, limit of, 1 Acetate method for determination of manganese, 392 of phosphorus, 380 Acetic acid, solubility in oils, 325 Acetyl value of oils, 317, 320 Acid value of oils, 310 determination, 310 Acids, standard, 191 standardization by direct weigh- ing, 193 Acree, standardization by direct weighing, 193 Adsorption, 24, 25 Adulteration of milk, 446 Aerobic bacteria in water, 354 Albumen in milk, 450 ' determination, 450 Albumenoid nitrogen in water, 354, 357 determination, 360 Alcohol, solubility of oils in, 325 Aliquot parts, 165 Allotropism of iron, 403 Alloy triangles, 39 Almond oil, constants, 327 solubility in alcohol, 325 of acetic acid in, 326 Aluminium, 71 gravimetric determination, 74 hydroxide, ionization and solu- bility, 72 in carbonate minerals, 244 in silicate minerals, 249 in water, 336 Alundum, 23 crucibles, 37 Amidobenzenesulpbonic acid, 360 Amidonaphthalene, a, 361 Ammeters, 125, 127 Ammonium chlorplatinate, solubil- ity, 80 Ammonium citrate, reagent, 439 Amyl alcohol, solubility of alkali chlorides, 337 Anaerobic bacteria in water, 354 Analysis, definition, 4 Analyzed chemicals, 58 Andrews, separation of bromine and chlorine, 98 Annatto in butter, 467 Annealing of gold, 481, 484 of steel, 412, 425 Antilogarithms, tables of, 491 Antimony in gold ores, 478, 485 Apparent valence, 208 Apricot kernel oil, solubility in alcohol, 325 Arachis oil, constants, 327 Renard-Tolman test, 323 solubility in alcohol, 325 of acetic acid in, 326 Archibald, Wilcox and Buckley, solubility of potassium chlorplatinate, 80 Arms, balance, to correct for in- equality of, 55 Arsenic, 91, 224 in gold ores, 478, 485 Asboth, determination of nitrogen, 433 Aschoff, see Jannasch Ashless filter paper, 20 Ash of coal, 255, 265 of condensed milk, 461 of milk, 449 Assay of gold and silver ores, 468 annealing, 481 crucible process, 470, 481 cupellation, 479 fluxes, 471 inquartation, 480 interference of copper, arsenic and antimony, 478, 485 oxidizing agents, 474 495 496 INDEX Assay, parting, 480 preliminary assay, 476 reducing agents, 473 scofification, 484 weighing, 469, 470 Assay ton, system of weights, 469 Atomic weights, table of, 7 Austenite, 408, 412 Availability of nitrogen in fertilizers, 434 Available chlorine in bleaching pow- der, 232 oxygen, 219 Babcock, composition of milk, 446 determination of fat in milk, 458 Bacteria in water, 345 Bailey, sampling of coal, 263 Balance, analytical, 42 adjustment, 49 beam, to set in motion, 49 button, 470 inequality of arms, 55 manipulation, 48 relative lengths of arms, 51 sensibility, 44, 51 zero point, 42, 49, 53 Ballantyne, see Thomson Bancroft, oxidation potentials, 96 Banthisch, solubility of barium sul- phate in hydrochloric acid, 76 Barium, 75 carbonate, solubility of, 75 chromate, solubility of, 75 gravimetric determination, 77 sulphate, occlusion of salts by, 76 solubility of, 19, 75, 76 Barnhart-Randall, gas apparatus, 280 Bases and carbonates, analysis of mixtures, 197 standard, 202 standardization, 203 Baudouin, test for sesame oil, 324 Baume system, 293 B. coli communis, 345 Beam rests, circular action, 43 vertical action, 43 Bechi, test for cotton seed oil, 323 Beech nut oil, solubility in alcohol, 325 solubility of acetic acid in, 326 Beeswax, constants, 328 Benedikt and Ulzer, acetyl value, 318 Berthelot, calorimeter, 262 Bicarbonates and carbonates, anal- ysis of mixtures, 198 Bismuthate method for determina- tion of manganese, 389, 391, 393 Blair, titanium in steel, 388 Blast lamps, 41 Bleaching powder, 232 Blow holes in steel, 398 Blown oils, 319 Boiler compounds, 344 Borda, method of weighing, 55 Boric acid, 205 Bottger, solubility of silver halides, 16, 69 Bregowsky, see Ford Bromine and chlorine, separation, 94, 97, 98 and iodine, separation, 94 Bube, solubility of magnesium am- monium phosphate, 86 Buckley, see Archibald Bugarszky, separation of bromine and chlorine, 98 Burckhardt, see Nietzki Burdick, see Gooch Burettes, 142, 143, 148, 149, 160, 202 calibration by standard bulbs, 160 by weighing, 160 gas, 276 Burners, 40 Burning filter paper, 32 oils, 293 burning point, 296, 297 fire test, 296, 297 flash point, 294, 297 specific gravity, 293, 296 INDEX 497 Burnt steel, 419 Butter, 463 annatto in, 467 artificial coloring of, 467 casein, 465 fat, 464 acetyl value, 320 constants, 328 Polenske value, 316 Reichert-Meissl value, 315 saponification number, 311 solubility of acetic acid in, 326 foam test, 467 microscopic examination, 466 moisture, 464 Polenske value, 316 saffron in, 467 salt in, 465 Butterine, solubility of acetic acid in, 326 Button balance, 470 Butyro-ref ractometer, 304, 306 Calcium, 64, 216 carbonate, as incrustant, 331 solubility, 64 chloride, drying agent, 30, 108 cyanamide, 427 gravimetric determination, 66 in carbonate minerals, 245 in silicate minerals, 250 in water, 336 oxalate, solubility, 64 oxide, for water treatment, 339 volumetric determination, 217 Calculations of volumetric analysis, 163 C aid well, filtering crucible, 23 Calibration by standard bulbs, 153, 158, 160 by weighing, 151, 159, 160, 161 of burettes, 160 of flasks, 158, 159 of volumetric apparatus, 151 of weights, 56 Calorimeter, Berthelot, 262 Emerson, 262, 269 Mahler, 262 32 Calorimeter, Parr, 262 Cameline oil, solubility in alcohol, 325 solubility of acetic acid in, 326 Carbide of iron, 369, 405 Carbonate minerals, 240 aluminium in, 244 calcium in, 245 carbon dioxide in, 243 iron in, 244 magnesium in, 245 manganese in, 245 potassium in, 245 silica in, 243 sodium in, 245 Carbonates, acid for decomposing, 107 and bases, analysis of mixtures, 197 and bicarbonates, analysis of mixtures, 198 in water, 339 titration, 197 Carbon dioxide, 103 absorbent for, 108 dissociation tension in sodium bicarbonate, 192 gravimetric determination , 109 in carbonate minerals, 243 in carbonates, 104 determination by absorp- tion, 105 by loss, 104 in gas mixtures, 103, 282 determination, 290 free or graphitic, 373 in coal, 257, 266 in steel, 369 combined, 374, 375 combustion after solution, 371, 373 direct combustion, 370, 372 free, 373 monoxide in gas mixtures, 282 determination, 290 Carbonic acid, 103 in solutions, 103 498 INDEX Carbonic acid in water, 342 Carius, determination of halogens, 102 determination of sulphur, 256 Carnaiiba wax, constants, 328 Case hardening, 420, 421, 426 Casein in butter, 465 in milk, 449 Castor oil, acetyl value, 320 constants, 327 Maumene number, 321 solubility in alcohol, 325 of acetic acid in, 326 specific temperature reaction, 321 Cathode, crucible, 123 dish, 123, 136 mercury, 122, 137 Cells, electrical, primary, 124 secondary, 125 Cementite, 405, 408, 412 Chamot and Pratt, determination of nitrates, 362 Chemist's slide rule, 6 Chicken fat, constants, 328 Chill test, 302 Chimney gases, 291 Chlorides, in water, 338, 353 Chlorine and bromine, separation, 94, 97, 98 and iodine, separation, 93 available, in bleaching powder, 234 Chromite, 223 Chromium, 223 in steel, 396, 397, 398 Chromophors, 177 C. P., 57 Citric acid, 204 Classen dish cathode, 123, 136 Classes of methods, 3 Clark, determination of hardness, 200 Cleaning solution, 157 Cleanliness and care, 1 Coal, 252 ash, 255, 265 carbon, 257, 266 Coal, fixed carbon, 254, 265 fuel value, 259, 269 hydrogen, 257, 266 moisture, 253, 254 nitrogen, 257 oxygen, 259 proximate analysis, 242, 253, 264 sampling, 263 sulphur, 256, 266 ultimate analysis, 255, 266 volatile combustible matter, 254, 265 Cobalt, 75 and platinum solution, reagent, 358 Cochineal, 182, 185 Cocoanut oil, constants, 328 Polenske value, 316 Reichert-Meissl number, 315 solubility of acetic acid in, 326 Cod liver oil, constants, 327 Coefficient of fineness, 349 Cold test, 302 Colloids, 17 hydrosols, 17 irreversible, 17 reversible, 17 sols, 17 Color change, of indicators, 176 of water, 352 Colza oil, acetyl value, 320 solubility in alcohol, 325 of acetic acid in, 326 Condensed milk, 461 ash, 461 fat, 462 lactose, 462 proteids, 461 solids, 461 sucrose, 462 Cooling of iron and steel, 403, 411, 415 Copper, 75, 130, 230 electrolytic determination, 130, 138 in gold and silver ores, 478, 485 volumetric determination, 231 INDEX 499 Corrosives in water, 330 Cotton seed oil, acetyl value, 320 Bechi test, 323 constants, 327 Gill and Dennisen test, 323 Halphen test, 322 solubility in alcohol, 325 of acetic acid in, 326 Cream, 460 Critical points of steel, 402 Croton oil, acetyl value, 320 constants, 327 Reichert-Meissl number, 315 solubility in alcohol, 325 Crucible, cathode, 123 process for assaying, 470, 481 Crucibles, alundum, 23, 37 Caldwell, filtering, 23 gold, 37 Gooch, filtering, 21, 23 platinum, 35 porcelain, 34 silica, 37 Cupel, 479 Cupellation, 479, 482 Cuprous chloride, reagent for gas analysis, 282 Current density, 120 production for electro-analysis, 124 DeBruyn, solubility of sodium sulphate, 80 Decimal, number of places to be reported, 3 system, 173 Decomposition voltage, 115, 119 Dennis and O'Neill, determination of hydrocarbon vapors, 285 Dennisen, see Gill Deshay, determination of man- ganese, 394 Desiccator, 29 DeVille, demonstration of adsorp- tion, 24 Digestion of precipitates, 19 Distillation, fractional, of oils, 298 Distilled water, 59 Dittmar and McArthur, factor for potassium, 81 Double layer, 116, 117 Drown, determination of silicon in steel, 375 Drying, agents, 30 of precipitates, 27 oils, 308, 326, 327 ovens, 27 Dynamo current, 125 Ebermayer, solubility of magnesium ammonium phosphate, 85 Edible fats and oils, 302 Eggertz, determination of combined carbon, 374 Electro-analysis, 113 apparatus for, 124 current for, 124 laboratory for, 125 separations by graded poten- tials, 119 solvents, 115 temperature, 115 Electrodes, 121 gauze, 123 graphite, 122 moving, 135 nature of, 121 platinum, 121, 122, 123 Electrolytes, influence upon electro- analysis, 114 Electrolytic pressure, 115 Electromagnetic apparatus for elec- tro-analysis, 137 Electronic vibration, absorption of light, 178 Emerson calorimeter, 262, 269 Engler viscosimeter, 298 Ephriam, composition of Hubl's solution, 308 Equivalent weights, 166 Erythrosin, 182, 185 Eschka, determination of sulphur, 256, 266 Estimation, 4 Etching of metals, 404, 423 Ethyl orange, 182, 185 500 INDEX Ethylene in gas mixtures, 283 determination, 290 Eutectic solutions, 409 Evolution methods for sulphur, 378, 379 Ewell, see Wiley Factors, 5, 8 Faraday's law, 120 Fat, butter, 311, 315, 316, 320, 326, 328 chicken, 328 cow, 328 goose, 328 hog, 326, 328 horse, 326, 328 human, 328 in butter, 464, 465 in condensed milk, 462 in milk, 455, 457, 459 mutton, 328 veal, 326 Felspars, 246 Ferrite, 404, 412 Ferrous ammonium sulphate, pri- mary standard, 212 sulphate, primary standard, 212 Fertilizers, 426 moisture in, 427 nitrogen in, 429 phosphorus in, 436 potassium in, 442 Filter paper, 19 ashless, 20 burning of, 32 folding of, 31 platinum, 23 Filters, inorganic, 21 Filtration, 19 Fire test of oils, 296 Fish oil, acetyl value, 320 constants, 327 Fixed carbon, 254, 265 Flames, regulation, 41 Flash point of oils, 294, 297 Flasks, volumetric, 141, 147, 149 calibration by standard bulbs, 158 Flocculation of colloids, 17 Fluxes, acid, 472 basic, 472 Foam producers in water, 331 test for butter, 467 Folding filter papers, 31 Ford, determination of manganese, 390 and Bregowsky, determination of manganese, 391 Ford- Williams method for deter- mination of manganese, 391 Formaldehyde in milk, 460 Fractional distillation of oils, 298 Frary, electromagnetic apparatus , 137 Free ammonia in water, 354, 355, 358, 360 Fresenius, determination of phos- phorus, 380 solubility of ammonium chlor- platinate, 80 Fuel value of coal, 259, 269 calculation from analysis, 259, 261 Fuming sulphuric acid, reagent for gas analysis, 284 Funnels, selection of, 20 Gas absorption pipette, 278 burette, 276 chimney, 291 illuminating, 289 mixtures, 275 carbon dioxide in, 282, 290 monoxide in, 282, 290 ethylene in, 283, 290 hydrocarbon vapors in, 285, 290 hydrogen in, 285, 290 methane in, 288, 290 oxygen in, 283, 290 Gases, solubility in reagents, 280 Gauss, method of weighing, 55 Gauze electrodes, 123 Geissler, tests for azo colors, 467 Gibbs, precipitation of magnesium ammonium phosphate, 87 INDEX 501 Gill, explosion pipette, 287 and Dennisen, test for cotton seed oil, 323 Girard, solubility of oils in absolute alcohol, 325 Gladding, determination of potas- sium, 81 Glycerine method for determination of boric acid, 205 Gmelin, calculation of fuel value, 261 Gold, crucibles, 37 occurrence of, 468 ores, assay of, 468 Gooch, determination of lithium, 337 filtering crucible, 21, 23 oxidizing agents for halogen separation, 96 and Burdick, electrodes, 122 Goose fat, constants, 328 Goutal, calculation of fuel value, 261 Grandval and Lajoux, determination of nitrates, 362 Granulation of steel, 418 Granules of steel and iron, 404 Grape seed oil, constants, 327 Graphite electrodes, 122 Guldberg and Waage, mass law, 14 Gunning, determination of nitrogen, 433 Haas, calculation of fuel value, 261 Halogen compounds, organic, 101 oxy acids, 101 Halogens, 68, 93 free, 101 gravimetric determination, 100 indirect separation, 93 separation by graded oxidation, 94 volumetric determination, 238 Halphen test for cotton seed oil, 322 Hantzsch, color change of indicator?, 178 Hanus, iodine absorption number, 309 Hardening of steel, 412, 420, 421, 425, 426 Hardness of water, 198 Hazel nut oil, constants, 327 solubility in alcohol, 325 Hehner, hardness of water, 199 value of oils, 310 Helmholtz, double layer, 116, 117 Hemp seed oil, constants, 327 solubility in alcohol, 325 Henbane oil, constants, 327 Hillebrand, analysis of carbonate minerals, 242 Horse fat, constants, 328 solubility of acetic acid in, 326 Hiibl, iodine absorption number, 308 Hulett, solubility increased by sub- division, 19 Human fat, constants, 328 Hydrocarbon vapors in gas mixtures, 285 determination, 290 Hydrochloric acid, oxidation by po- tassium permanganate, 211 standard solution, 194 Hydrofluoric acid, decomposition of silicates, 249 Hydrogen in coal, 257, 266 in gas mixtures, 285, 290 sulphide in water, 343 Hydrometer, Sommer, 300 Hydrosols, 17, 25 Hypothetical compounds in water, 332, 334 Hysteresis, thermal, of steel, 411 Ibbotson, determination of nickel, 398 Ignition of precipitates, 31 Illuminating gas, 289 Incrustants, of water, 330 Index of refraction, 304 Indicators, classification, 180 cochineal, 182, 185 color change, 176 erythrosin, 182, 185 ethyl orange, 182, 185 lacmoid, 182, 185 litmus, 182, 184 methyl orange, 177, 178, 182, 184 502 INDEX Indicators, paranitrophenol, 182, 184 phenolphthalein, 176, 177, 179, 181, 182, 184 rosolic acid, 182, 184 Indirect method of separation, 93 Industrial analysis of water, 329, 335 Ingot iron, 404 Inquartation, 480, 483 Insoluble acids of oils, 310, 313 International atomic weights, table of, 7 Interpretation of results of water analysis, 346 Iodine, absorption number, 307, 309 and bromine, separation, 94 and chlorine, separation, 93 monobromide, 309 monochloride, 308, 309 standard solution, 223, 224 Iron, 133, 210, 367 allotropism, 403 and steel, 367 foreign elements, 367 sampling, 369 carbide, 369, 405 cooling of, 403, 411, 415 electrolytic determination, 133, 139 in carbonate minerals, 244 in silicates, 250 in water, 343 magnetic properties, 403 oxide, in water, 336 primary standard, 211 reduction, 212 volumetric determination, 214, 223 Irreversible colloids, 17 Jackson, photometric determination of sulphur, 256 Jannasch and Aschoff, separation of bromine and chlorine, 97 Japan wax, constants, 328 Jean, solubility of acetic acid in oils, 325 Jodlbauer, determination of nitro- gen,**^ Johnson, determination of tungsten in steel, 395 Kerosene, 296 Kjeldahl, determination of nitrogen, 429 Kohlrausch, solubility of strontium sulphate, 77 and Rose, solubility of barium salts, 75 of calcium salts, 64 of silver chloride, 16 of silver halides, 69 Kottstorfer number, of oils, 310 Kuster, titration of carbonates, 197 Labels, 61 Lacmoid, 182, 185 Lactose in condensed milk, 462 in milk, 450 optical methods, 450, 453 reduction methods, 453, 454 Lajoux, see Grandval Lard, constants, 328 solubility of acetic acid in, 326 Lead, electrolytic determination, 134 _tgat, 485 Leblanc, decomposition voltage, 119 Ledebur, determination of oxygen in steel, 399 Lescoeur, dissociation tension of carbon dioxide in sodium bicarbonate, 192 Lewkowitsch, determination of acetyl value, 318, 319 fat and oil industries, address, 302 Lindo, determination of potassium, 80 Lindo-Gladding, determination of potassium, 82 Linseed oil, acetyl value, 320 constants, 327 Maumene number, 321 solubility in alcohol, 325 specific temperature reaction, 321 INDEX 503 Liter, Mohr, 144 true, 144 Lithium chloride, solubility in amyl alcohol, 337 determination in water, 337 Litmus, 182, 184 Logarithms, tables of, 487 use of, 6 Low, determination of copper, 230 test for azo colors, 467 Lubricating oils, 298 acidity, 300 chill test, 302 cold test, 302 fixed carbon, 300 separation of saponifiable from mineral oils, 301 viscosity, 298 Magnesium, 85 ammonium phosphate, precipi- tation of, 87, 90 ignition, 90 solubility, 85, 86 carbonate, incrustant, 331 gravimetric determination, 91 in carbonate minerals, 245 in silicate minerals, 250 in water, 336 orthophosphate, 86 Magnetic properties of iron, 403 Mahler, calorimeter, 262 Maize oil, acetyl value, 320 constants, 327 solubility of acetic acid in, 326 Malic acid in vinegar, 204 Manchot, oxidation of hydrochloric acid by potassium perman- ganate, 211 Manganese, 92, 217 gravimetric determination, 92 in carbonate minerals, 245 in silicate minerals, 250 * in steel, 389 reduction, 97 volumetric determination, 218 Manganous acid, 218 Margerine, solubility of acetic acid in, 326 Marshall, determination of man- ganese in steel, 393 Martensite, 409 Marti, see Patten Mass law, 14 Maumene number, 321 Me Arthur, see Dittmar McCandless, ammonium citrate solu- tion, 440 McCrackan, see Metzger Medicinal value of water, 342 Mercury cathode, 122, 137 Metallography, 401 Methane in gas mixtures, 288 determination, 290 Methyl orange, 177, 178, 182, 184 Metzger and McCrackan, deter- mination of manganese in steel, 389 Microscope for metallography, 423 in steel testing, 401 Microscopic examination of butter, 466 Mineral oils, separation from saponi- fiable oils, 301 Milk, 444 adult, 446 albumen, 450 ash, 449 casein, 449 composition, 445 condensed, 461 fat, 455 formaldehyde, 460 lactose, 450 nitrogen, 449 solids, 448 specific gravity, 447 Mocaya oil, 315 Mohr liter, 144 Moisture in butter, 464 in coal, 253, 264 in fertilizers, 427 Molybdate method for phosphorus, 381 Moments, principle of, 45 504 INDEX Moody, standardization by direct weighing, 193 Morse, decomposition of potassium permanganate, 210 and Blalock, standardizing bulbs, 155 Moving electrodes, 135 Munroe, platinum filter, 23 Mushet, self-hardening steel, 394 Mustard seed oil, solubility in alco- hoJ, 325 Myrtle wax, constants, 328 Naphthylamine, a, reagent for ni- trites, 360 Neat's foot oil, constants, 327 solubility of acetic acid in, 326 Nernst, electrolytic solution tension, 116 Nesslerization of ammonia, 356, 360 direct, 356 Nessler's reagent, 355 Nickel, 75, 135 electrolytic determination, 135 in steel, 396, 397, 398 Nietzki and Burckhardt, tetrabrom- phenolphthalein, 177 Nitrates, in water, 339, 362 Nitric acid, oxidation of halogen hydracids by, 98 Nitrites, in water, 360, 361 Nitrogen, albumenoid, in water, 354, 357, 360 as free ammonia in water, 354, 355, 358, 360 as nitrates in water, 362, 363, 364 fixation in soils, 427 in coal, 257 in fertilizers, 427, 431, 434, 435, 436 in milk, 449 in water, 354 Nitrometer, 257 Nitrous acid, as oxidizing agent, 96 Non-drying oils, 308, 326, 327 Normal density, 121 system, 170 No system, 170 Note books, 61 Odor of water, 352 Oil, almond, 325, 326, 327 apricot kernel, 325 arachis, 323, 325, 326, 327 beechnut, 325, 326 cameline, 325, 326 castor, 320, 321, 325, 326, 327 cocoanut, 315, 316, 326, 328 cod liver, 327 colza, 320, 325, 326 cotton seed, 320, 322, 323, 325, 326, 327 croton, 315, 320, 325, 327 fish, 320, 327 grape seed, 327 hazel nut, 325, 327 hemp seed, 325, 327 henbane, 327 Unseed, 320, 321, 325, 327 maize, 320, 326, 327 mocaya, 315 mustard seed, 325 neat's foot, 326, 327 olive, 320, 321, 325, 326, 327 palm nut, 315, 326, 328 peanut, see oil, arachis pistachio, 327 poppy seed, 325, 326 porpoise, 315, 328 quince, 327 rape seed, 321, 325, 327 ravison, 326 resin, 322 sesame, 324, 325, 327 shark liver, 320 sheep's foot, 326, 328 sperm, 328 sunflower, 327 tung, 327 walnut, 325, 326, 327 whale, 328 Oils, acetyl value, 317 acid value, 310 blown, 319 INDEX 505 Oils, burning, 293 point, 296, 297 chill test, 302 cold test, 302 drying, 308, 326, 327 edible, 302 fire test, 296, 297 flash point, 294, 297 Hehner value, 310 index of refraction, 304 insoluble acids, 310 iodine absorption number, 307, 309 lubricating, 298 Maumene number, 321 mineral, separation from saponi- fiable oils, 301 non-drying, 308, 326, 327 Polenske value, 316 Reichert-Meissl number, 313 Reichert number, 313 saponifiable, separation from mineral oils, 301 saponification number, 310 semi-drying, 308, 326, 327 soluble acids, 310, 314 solubility in absolute alcohol, 325 of acetic acid in, 325 specific gravity, 304 temperature reaction, 321 table of constants, 327, 328 viscosity, 298 Olein, 303 Oleomargerine, 311, 466 constants, 328 Olive oil, acetyl value, 320 constants, 327 Maumen6 number, 321 solubility in alcohol, 325 of acetic acid in, 326 specific temperature reaction , 321 O'Neill, see Dennis Orsat, apparatus for gas analysis, 281 Ostwald, W., solution tension of crystals, 19 Ostwald, W., theory of indicators, 176 Ostwald W., total surface of colloids, 25 Ovens, drying, 27 Overvoltage, 119 Oxalic acid, primary standard, 212 Oxidation and reduction titrations, 207 method for determination of sulphur, 377 potentials, 95 Oxidizing agents for assaying, 474 Oxygen, available, 219 in coal, 259 in gas mixtures, 283 in steel, 398, 400 Palladium tute, for hydrogen ab- sorption, 287 Palmitin, 303 Palm nut oil, constants, 328 Reichert-Meissl number, 315 solubility of acetic acid in, 326 Paranitrophenol, 182, 184 Parr calorimeter, 262 Parting of gold and silver, 480, 482 Patten and Marti, ammonium citrate solution, 440 Peanut oil, see Arachis oil Pearl ash, 196 Pearlite, 405, 408, 412 Pemberton, determination of phos- phorus, 382 Percent, direct reading from burette, 169 Permanent hardness, 201 Peroxides, 227 determination of oxidizing power, 229 Peters, determination of manganese, 394 Phenolphthalein, 176, 177, 179, 181, 182, 184 Phenolsulphonic acid, reagent for nitrates, 362 Phillips, errors in evolution methods for sulphur, 379 Phosphates, decomposition by heat- ing, 88 506 INDEX Phosphoric acids, 30, 85 derivation of, 88 Phosphorus in fertilizers, 436, 438, 439, 441 in steel, 380, 382, 384, 385 reversion, 435 Physical examination of water, 348 Pipette, gas absorption, 278 gas explosion, 287 Pipettes, 141, 147, 148, 149, 161 Pistachio oil," constants, 327 Platinum and cobalt solution, for direct Nesslerization, 358 committee, report, 36 crucibles, 35 electrodes, 121, 122 filter, 23 gauze electrodes, 123 recovery from waste, 84 triangles, 39 ware, defects, 36 wire method for turbidity, 350 Polenske value, 316 Polishing machines, 422 Poppy seed oil, solubility in alcohol , 325 solubility of acetic acid in, 326 Porcelain crucibles, 34 triangles, 38 Porpoise oil, constants, 328 Reichert-Meissl number, 315 Potability of water, 344 Potassium, 78 arsenate, as oxidizing agent, 96 chloride, solubility in amyl alcohol, 337 chlorplatinate, 78, 80 cobaltinitrite, 78 dichromate, standard solution, 208, 209, 220, 221, 223, 225 ferricyanide, indicator, 221 gravimetric determination, 83 hydroxide, standard solution, 203 in carbonate minerals, 245 in fertilizers, 442 in silicate minerals, 250 in water, 338 Potassium iodate, oxidizing agent, 98 perchlorate, 78 permanganate, decomposition, 210 oxidation of halogen hy dracids, 97 of hydrochloric acid, 211 standard solution, 210, 214, 217, 220 tetroxalate, primary standard, 203 Potential difference, 117, 118 Potentials, oxidation, 95 Pratt, see Chamot Precipitates, digestion, 19 drying, 27 enlargement of particles, 18 ignition, 31 washing, 23 Precipitation, 5, 13 Predetermined weights, 54 Preliminary assay, 476 Pressure, electrolytic, 115 Primary cells, 124 standards, 189 Priming in water, 331 Principle of moments, 45 Proteids in condensed milk, 461 in milk, 449 Proximate analysis of coal, 242, 253 Pulfrich refraotometer, 304 Putrescibility of water, 365, 366 Quartering of samples, 10, 11 Quenching media, 415 Quince oil, constants, 327 Quinone ring, 179 Radicals, combination in water, 332 Randall, see Barnhart Rape seed oil, constants, 327 Maumen6 number, 321 solubility in alcohol, 325 specific temperature reaction, 321 Ravison oil, solubility of acetic acid in, 326 INDEX 507 Reagents, 57 Recalescence of steel and iron, 402, 404, 408 Records, 60 Reducing agents for assaying, 473 Reduction and oxidation titrations, 207 method for determination of nitrogen as nitrate, 364 of iron, 212 - of manganese, 97 Refinement of grains, of steel, 418, 425 Refractometer, Abbe", 304, 307 Pulfrich, 304 Zeiss, butyro-, 304, 306 Reich ert-Meissl number, 313, 315 Reichert number, 313 Reinhardt, titration of iron, 211 Renard, test for arachis oil, 323 Required oxygen in water, 364 Resin oil, polarization, 322 Reversible colloids, 17 Reversion of phosphorus, 437 Rheostats, 125, 127, 128 Richards, occlusion of salts by barium sulphate, 76 Rider carrier, 47 Rose, see Kohlrausch Rosolic acid, 182, 184 Rothe, separation of iron from other metals by ether, 396 Saffron, in butter, 467 Salt, in butter, 465 Salting of samples, 9 Salts, corrosive action upon steam boilers, 330 Samples, mixing, 9 of water collection, 346 preparation, 8 quartering, 11 salting, 9 solution, 13 Sampling, maximum size of particles, 11 of coal, 263 of iron and steel, 369 Sand, separation by graded cathode potential, 119 Sanitary examination of water, 341 Saponifiable oils, separation from mineral oils, 301 Saponification number, 310 Saunders, properties of alundum, 37 Sauveur, tensile strength of cemen- tite, 405 Schellbach, burette, 143 Schneider, determination of man- ganese in steel, 389 Scorification, 484, 486 Scorifier, 484 Secondary cell, 125 Self-hardening steel, 394 Semi-drying oils, 308, 326, 327 Sensibility, 44 determination, 51 Sesame oil, Baudouin test, 324 constants, 327 solubility in alcohol, 325 Villavecchia test, 324 Seubert, atomic weight of potas sium, 81 Shark liver oil, acetyl value, 320 Sheep's foot oil, constants, 328 solubility of acetic acid in, 326 Shortness of steel, 388 Silica, crucibles, 37 in carbonate minerals, 243 in silicate minerals, 249 in water, 336 standards for turbidity, 349 Silicate minerals, 246 aluminum in, 249 calcium in, 250 iron in, 250 magnesium in, 250 manganese in, 250 potassium in, 250 silica in, 249 sodium in, 250 Silicates, 246 analysis, 249 decomposition, 248, 249 Silicon in steel, 376 Silver, 68, 131, 236, 468 508 INDEX Silver chloride, solubility, 16 electrolytic determination, 131, 132, 139 gravimetric determination, 70 halides, solubility, 69 occurrence, 468 ores, assay, 468 volumetric determination, 236 Slide rule, chemist's, 6 . Smith, decomposition of silicates, 248 Snelling, platinum filter, 23 Soda ash, 195 lime, 107 reagent for determination of hardness of water, 201 Sodium, 78 carbonate, primary standard, 194 treatment of water, 340 chloride, solubility in amyl alcohol, 337 gravimetric determination, 83 in carbonate minerals, 245 in silicates, 250 in water, 338 sulphate, solubility, 80 .thiosulphate, standard solution, 223, 227, 229, 230 Solids, in condensed milk, 461 in milk, 448 in water, 353 Solid solution, 409 Solubility of acetic acid in oils, 325 of compounds in water at high temperatures, 333 product, 15 reduction by excess of reagent, 15 Soluble acids of oils, 310, 313, 314 Sols, 17 Solution, eutectic, 409 solid, 409 tension, electrolytic, 116 of crystals, 19 Solvents for electro-analysis, 115 Sommer, hydrometer, 300 Sonnenschein, determination of phosphorus, 380 Sorbite, 415 Soxhlet, table for determination of lactose, 456 Specific gravity, Baume" system, 293 of burning oils, 293, 296 of edible fats and oils, 304 of lubricating oils, 300 of milk, 447 temperature reaction, 321 Speed in analytical work, 1 Spermaceti, constants, 328 Sperm oil, constants, 328 Sprengel, determination of nitrates, 362 Standard acids, 191 gravimetric standardization, 192 standardization by direct weigh- ing, 193 bases, 202 Standardization of solutions, 186 Standardizing bulbs, 155 Standard methods for water analysis, 346 Stannous chloride, reduction of iron by, 212 . Stearin, 303 Steel, annealing, 412, 425 blow holes, 398 . burnt, 419 carbon in, 369 case hardening, 420, 421, 426 chromium in, 396 combustion of carbon after solution, 371 cooling of, 403, 411, 415 critical points, thermal, 402 direct combustion of carbon, 370 effect of working, 421 etching 404, 423 free carbon in, 373 granulation, 418 graphitic carbon in, 373 hardening, 412, 420, 421, 425, 426 high speed, tool, 395 manganese in, 389 microscope for examination, 423 INDEX I Steel, microscope in testing, 401 nickel in, 396 oxygen in, 398 phosphorus in, 380 polishing machine for, 422 quenching media, 415 recalescence, 402, 404, 408 self-hardening, 394 shortness, 388 silicon in, 375 sulphur in, 376 tempering, 416, 426 thermal and mechanical treat- ment, 400 thermal changes, 402 hysteresis, 411 titanium in, 386 total carbon in, 369 tungsten in, 394 Steel and iron, 367 granules, 404 Sterilization, temporary, of water, 345 Stieglitz, color change of indicators, 178 Stolberg, determination of calcium, 65 Strontium, gravimetric determina- tion, 77 sulphate, solubility, 77 Sucrose, in condensed milk, 462 Sulphanilic acid, reagent for de- termination of nitrites, 360 Sulphates, in water, 338 Sulphur, in coal, 256, 266 in steel, 376 Sulphuric acid, 75 drying agent, 30 Sunflower oil, constants, 327 Supersaturation, 18 Tallow, constants, 328 mutton, constants, 328 Taylor and White, high-speed steel, 395 Temporary hardness of water, 201 Temperature, effect upon electroly- sis, 115 Ten 01 .Vc, J 509 Test lead foHassaying, 485 Tetrabromphenolphthalein, 177 Thermal changes in steel, 402 critical points of steel, 411 hysteresis, 411 Thomson, glycerine method for boric acid, 205 and Ballantyne, specific tem- perature reaction, 321 Time between collection and analysis of water, 347 Titanium in steel, 386 interference in phosphorus de- termination, 385 Tolerance, 145, 150 Tolman, test for arachis oil, 323 Total solids, of milk, 448 of water, 353 Transference of liquids, 59 Treatment of steel, 400 of water, 339 Triangles, 37 alloy, 39 platinum, 39 porcelain, 38 Troostite, 413 Tung oil, constants, 327 Tungsten in steel, 394 Turbidity of water, 349 Turrentine, graphite electrodes, 122 Ultimate analysis of coal, 242, 255, 266 Ulzer, see Benedikt Units of volume, 144 Vacuum desiccator, 29 Valence, apparent, 208 Valenta, solubility of oils in acetic acid, 325 Veal tallow, solubility of acetic acid in, 326 Vertical action of beam rests, 43 Villavecchia, test for sesame oil, 324 Vinegar, 204 Viscosimeter, Engler, 298 510 } Visr r DEX d&vsolubility, 16^ "ia" Volhard, aeiermma- on of man- ganese, 217, 392 determination of silver, 238 Voltmeters, 125, 127 Volumetric analysis, 140 Walnut oil, constants, 327 solubility in alcohol, 325 of acetic acid in, 326 Walters, determination of man- ganese, 393 Wash bottle, 26 Washing precipitates, 23 Water, 329 albumenoid ammonia in, 354, 357 ammonia in, 356 analysis, expression of results, 201, 332 aluminium in, 336 bacteria, 345, 354 calcium, 336 carbonates, 339 carbon dioxide, 342 carbonic acid, 342 chlorine, 338, 353 collection of samples, 346 color, 352 corrosives, 330 density, 152 distilled, 59 foam producers, 331 hardness, 198 hydrogen sulphide, 343 hypothetical compounds, 332, 334 incrustants, 330 industrial analysis, 329, 335 interpretation of results of analysis, 346 iron, 336, 343 lithium, 337 magnesium, 336 medicinal value, 342 f.er. nitrates, 339, 362 nitrites, 360 nitrogen, 354 odor, 352 physical examination, 348 potability, 344 potassium, 338 priming, 331 putrescibility, 365 relation of pressure and boiling point, 330 required oxygen, 364 sanitary examination, 341 silica, 336 sodium, 338 solubility of compounds at high temperatures, 333 standard methods for analysis, 346 sulphates, 338 temperature, 348 temporary sterilization, 345 time between collection and analysis, 347 total solids, 353 treatment, 339 turbidity, 349 Wax, bee's, 328 carnaiiba, 328 Japan, 328 myrtle, 328 spermaceti, 328 wool, 328 Weber, recovery of platinum from scrap, 84 Weighing, 41, 52 Weights, analytical, 47 calibration, 56 predetermined, 54 Wenze, solubility of potassium per- chlorate, 79 Whale oil, 328 Wijs, iodine absorption number, 309 Wilcox, see Archibald Wiley, precipitation of proteids from milk, 452 recovery of platinum from waste, 84 steel, 41^ * - DATE THIS BOOK ON THE A ON TH E FOURTH OVERDUE. SEVENTH O A V APR LD21 _100m-7,'39(402s) YC 21886 UNIVERSITY OF CALIFORNIA LIBRARY