/ HUBERT DYER. A SYSTEMATIC HANDBOOK OF VOLUMETRIC ANALYSIS, V\ 6 R A R y OF THE SYSTEMATIC HANDBOOK OP VOLUMETRIC ANALYSIS; OR, THE QUANTITATIVE ESTIMATION OF CHEMICAL SUBSTANCES BY MEASURE, APPLIED TO LIQUIDS, SOLIDS, AND GASES, ADAPTED TO THE REQUIREMENTS OF PURE CHEMICAL RESEARCH, PATHOLOGICAL CHEMISTRY, PHARMACY, METALLURGY, MANUFACTURING CHEMISTRY, PHOTOGRAPHY, ETC., AND FOR THE VALUATION OF SUBSTANCES USED IN COMMERCE, AGRICULTURE, AND THE ARTS. BY FRANCIS BUTTON, F.I.C., F.C.S., |) PUBLIC ANALYST FOR THE COUNTY OF NORFOLK ; LATE MEMBER OF COUNCIL OF THE SOCIETY OF PUBLIC ANALYSTS ; LATE MEMBER OF COUNCIL OF THE PHARMACEUTICAL SOCIETY OF GREAT BRITAIN; CORRESPONDING MEMBER OF THE IMPERIAL PHARMACEUTICAL SOC. OF ST. PETERSBURGH CORRESPONDING MEMBER OF THE AUSTRIAN APOTHEKER VEREIN, VIENNA; CONSULTING CHEMIST TO THE NORFOLK CHAMBER OF AGRICULTURE; ETC., ETC. SIXTH EDITION, FNU^iEfl PHILADELPHIA : P. BLAKISTON, SON, & CO., 1012 WALNUT STREET. 1890. [All rights reserved.] PREFACE. THE fifth edition of this book, and the largest yet issued, having been absorbed in a shorter space of time than any former edition, is an evidence of the increasing demand for volumetric methods of analysis, and due, most likely, to the necessity for shorter technical methods in connexion with manufacturing processes of various kinds. The present book is enlarged by about fifty pages of new matter, with twelve new illustrations, comprising fuller descriptions of the indicators used in saturation analyses and additions to methods of titration in almost every section. There is also added a table of co-efficients and logarithms for use in volumetric analyses, taken partly from some of the earlier editions of Mohr's Titrirmethode, from Dr. W. H. Ince's tables published in the Analyst, and from Mr. A. E. Johnson's valuable Analyst's Laboratory Companion, issued by Messrs. J. & A. Churchill. Professor McLeod has kindly furnished me with revised and corrected tables of logarithms required in the analysis of gases supplementary to his original article, and which are now inserted at the end of the book, as was the case in the second edition. I shall be happy to supply separate 331983 VI PREFACE. copies of these tables for laboratory use, and also those required for Frankland's and Armstrong's method of water analysis, to any one who desires them, on receipt of address. I am indebted to Mr. J. Lunt, B.Sc., for a condensed report on Schiitzenberger's method of estimating oxygen in waters, etc., worked out by Sir Henry Eoscoe and himself, as shown in 68. Professor P. P. Bedson has also kindly sent me photographs of his modified gas pipettes (see fig. 94) ; and last, but not least, I am indebted to my friend Mr. W. Thorp, B.Sc., for his kind supervision of the proof sheets, and for valuable suggestions throughout the entire book. FKANCIS SUTTON. NORWICH, October, 1890. CONTENTS. PART I. Sect. Page 1. General Principles . . . . .1 2. The Balance . . . . . 5 3. Volumetric Analysis without "Weights ; . . .5 4. Volumetric Analysis without Burettes ... 6 5. The Burette . . . . . .6 6. The Pipette ..... 14 7. The Measuring Flasks . . . . .15 8. The Correct Reading of Graduated Instruments . . 16 9. The Weights and Measures to be adopted in Volumetric Analysis 18 10. Preparation of Normal Solutions in General . . 23 11. Direct and Indirect Processes of Analysis . . .28 PART II. 12. Alkalimetry . . . . . .30 13. Indicators used in Saturation Analyses ... 30 14-. Normal Alkaline and Acid Solutions . . .40 15. Correction of Abnormal Solutions ... 46 Table for the Systematic Analysis of Acids, Alkalies, and Alkaline Earths . . . . .49 16. Tit-ration of Alkaline Salts . . . .50 17. Titration of Alkaline Earths . . . .65 18. Ammonia . . . . . . .67 19. Acidimetry . . . . . 80 20. Acetic Acid . . . . " . .82 21. Citric Acid .... 85 22. Oxalic Acid . . . . . .86 23. Phosphoric Acid . . . . 86 24. Sulphuric Anhydride . . . . .87 25. Tartaric Acid . . . . . 88 26. Carbonic Acid and Carbonates . . . .91 27. Estimation of Combined Acids in Neutral Salts . . 101 28. Extension of Alkalimetric Methods 102 Vlll CONTENTS. PAET III. Sect. Page 29. Analysis by Oxidation or Eeduction . . . 105 30. Permanganic Acid and Perrons Oxide . .. . 106 31. Titration of Perric Salts by Permanganate . . . .109 32. Calculation of Permanganate Analyses . . . ' 109 33. Chromic Acid and Perrons Oxide . . . .111 34. Iodine and Thiosulphate . . . .113 35. Analysis of Substances by Distillation with Hydrochloric Acid . 117 36. Arsenious Acid and Iodine 121 PAET IV. 37. Analysis by Precipitation .... 123 38. Indirect Analyses by Silver and Potassic Chromats . 125 39. Silver and Thiocyanic Acid . ' . . .127 40. Precision in Colour Eeactions . . . 128 41. The Colorimeter 129 PAET V. 42. Antimony . . . . . . 134 43. Arsenic . . . . . . 136 44. Barium . . . v . . 141 45. Bismuth . . -. . . 142 46. Bromine . . . . . .144 47. Cadmium ...... 147 48. Calcium . . . . . .148 49. Cerium ...... 149 50. Chlorine . . . . . .149 51. Chlorine Gas and Bleach . . . .151 Chlorates, lodates, and Bromates . . . .154 52. Chromium . . . . .154 53. Cobalt . . . . . .157 54. Copper ...... 160 65. Cyanogen ...... 173 56. Ferro- and Perri-Cyanides .... 175 Sulphocyanides ...... 177 57. Gold . . . . . 178 58. Iodine . . . . . .179 59. Perrous Iron . . . . . 186 60. Perric Iron . . . .. . 190 61. Iron Ores . . . . . . 195 62. Lead . . . . . . . 201 63. Magnesia and 63A Alumina . . . . ; . 204 64. Manganese .' . .. i . . 206 65. Mercury . . . ,' ' ;. . 219 CONTENTS. IX Sect. Page 66. Nickel . . ... . .> .223 67. Nitrogen as Nitrates and Nitrites . SkV * ^^ 68. Oxygen and Hydrogen Peroxide .... 254 69. Phosphoric Acid and Phosphates . . . 269 70. Silver ' '* . . : . .284 71. Sugar . . .. . . .291 72. Sulphur ... . 305 73. Sulphuric Acids and Sulphates . . . 310 74. Sulphuretted Hydrogen . . . . .314 75. Tannic Acid . . . 316 76. Tin . . . . . . 321 77. Uranium ..... 323 78. Zinc .... . . . 323 79. Vanadium . . . . ... 332 APPENDIX TO PAET V, 80. Boric Acid and Borates ..... 334 81. Oils and Tats ..... 336 82. Glycerin . . . . . .345 83. Phenol (Carholic Acid) . . . .348 84. Carhon Disulphide ' . . . . . 349 85. Molybdenum, Tungsten, and Lead . . . 350 PAET VI, 86. Analysis of Urine ..... 352 87. Analysis of Potable Waters and Sewage . . 373 88. Analytical Processes for "Water . . . .381 89. Interpretation of Eesults of Water Analysis . . 419 90. Water Analysis without Gas Apparatus . . . 429 91. Eeagents and Processes employed . . . 435 92. Oxygen Dissolved in Water .... 446 Table for Calculations and Logarithms . . . 448 PAET VII. 93. Volumetric Analysis of Gases and Construction of Apparatus . 452 94. List of Gases Estimated Directly and Indirectly . 466 95. Hydrochloric, Hydrobrornic, and Hydriodic Acids . . 466 96. Analysis of Air, Carbonic Anhydride, SH 2 , and SO 2 . 468 97. Indirect Determinations ..... 474 98. Improvements in Gas Apparatus . . . 489 99. Simpler Methods of Gas Analysis .... 519 100. The Nitrometer 529 [*] Names of Elementary Substances occurring- in Volumetric Methods, with their Symbols and Atomic Weights. Name. Symbol. Exact Atomic Weight as found by the latest researches. Atomic Weight adopted in this Edition. Aluminium Al 27-3 27-3 Antimony Arsenic Sb As 119-6 74-9 120-0 75-0 Barium Ba 136-8 136-8 Bismuth . Bi 208-0 208-0 Bromine Br 79-75 80-0 Cadmium . Cd 111-6 111-6 Calcium Ca 39-9 40-0 Carbon C 11-97 12-0 Cerium Ce 141-2 141-2 Chlorine . Cl 35-37 35-37 Chromium Cr 52-4 52-4 Cobalt . Co 58-6 59-0 Copper Gold Cu Au 63-18 196-2 63-0 196-5 Hydrogen Iodine H I 1-0 126-5 1-0 126-5 Iron . Fe 55-88 56-0 Lead Pb 206-4 206-4 Magnesium . Manganese Mercury ' . Molybdenum Nickel Mg Mn Hg Mo Ni 23-94 55-0 199-8 95-8 58-6 24-0 55-0 200-0 95-8 59-0 Nitrogen . Oxygen Phosphorus Platinum N O P Pt 14-01 15-96 30-96 194-3 14-0 16-0 31-0 194-3 Potassium K 39-04 39-0 Silver Sodium . Ag Na 107-66 22-99 107-66 23-0 Strontium Sr 87-2 87-2 Sulphur . Tin . S Sn 31.98 117-8 32-0 118-0 Tungsten . Uranium W Ur 184-0 239-8 184-0 240-0 Vanadium Va 51-2 51-2 Zinc . . g Zn 64-9 65-0 [xi] Abbreviations and Explanations. The formulae are constructed on the basis H = l. The normal temperature for the preparation and use of standard solutions is 16 C., or about 60 Fahr. c.c. denotes cubic centimeter. gm. gram =15-43235 grains English. grn. grain. dm. decem= 10 fluid grains at 16 C. 1 liter = 1000 c.c. at 16 C. 1 c.c. = 1 gm. distilled water at 16 C. 1 dm. = 10 grn. Distilled water is to be used in all the processes, unless other- wise expressed. Normal Solutions are those which contain one gram atom of reagent (taken as monobasic), or an equivalent in some active constituent (e.g. oxygen) in the liter (see page 23). Deciiiormal Solutions are one-tenth of that strength = T ^-. Centinormal, one hundredth = T Jg-. Empirical Standard Solutions are those which contain no exact atomic proportion of reagent, but are constructed generally so that 1 c.c. = O'Ol gm. (one centigram) of the substance sought. A Titrated Solution (from the French word titre, title or power) denotes a solution whose strength or chemical power has been accurately found by experiment. When a chemical substance or solution is directed to be titrated, the meaning is, that it is to be quantitatively tested for the amount of pure substance it contains by the help of standard or titrated solutions. The term is used in preference to tested or analyzed, because these expressions may relate equally to qualitative and quantitative examinations, whereas titrations can only apply to quantitative examination. J. C. S. denotes Journal of the Chemical Society (Transactions only). J. S. C. I. Journal of the Society of Chemical Industry. Z. a. C. Zeitschrift fiir Analytische Chemie. C. N. Chemical News. Other book-references are given in full. ERltATA AND ADDENDA. Page 108. Line 12 from top, substitute for the words " new gasvolumeter " " modified nitrometer." Page 130. The figure should be numbered 32 instead of 30. Page 153. Line 12 from top, read "modified nitrometer" for "new gas- volumeter.'* Page 332. Line 15 from top, the words "and water" should be omitted. Page 332. Line 20, after the words "one hour," insert, "the digestion may be carried on without heating with practically the same results." Page 332. Line 25, add " or 1 part of pure zinc should theoretically liberate 0'7799 part of iodine." Page 354. Line 10 from top, instead of the words " one place " read " two places." VOLUME TRIG ANALYSIS OF LIQUIDS AND SOLIDS, PART I. GENERAL PRINCIPLES. 1. QUANTITATIVE analysis by weight, or gravimetric analysis, consists in separating out the constituents of any compound, either in a pure state or in the form of some new substance of known composition, and accurately weighing the products. Such opera- tions are frequently very complicated, and occupy a long time, besides requiring in many cases elaborate apparatus, and the exercise of much care and experimental knowledge. Volumetric processes, on the other hand, are, as a rule, quickly performed ; in most cases are susceptible of extreme accuracy, and need much simpler apparatus. The leading principle of the method consists in sub- mitting the substance to be estimated to certain characteristic reactions, employing for such reactions solutions of known strength, and from the volume of solution necessary for the pro- duction of such reaction, determining the weight of the substance to be estimated by aid of the known laws of chemical equivalence. Volumetric analysis, or quantitative chemical analysis by measure, in the case of liquids and solids, consequently depends upon the following conditions for its successful practice : 1. A solution of the reagent or test, the chemical power of which is accurately known, called the "standard solution." 2. A graduated vessel from which portions of it may be accurately delivered, called the "burette." 3. The decomposition produced by the test solution with any given substance must either in itself or by an indicator be such, that its termination is unmistakable to the eye, and thereby the quantity of the substance with which it has combined accurately calculated. ANALYSIS. 1. Suppose, for instance, that it is desirable to know the quantity of pure silver contained in a shilling. The coin is first dissolved in nitric acid, by which means a bluish solution, containing silver, copper, and probably other metals, is obtained. It is a known fact that chlorine combines with silver in the presence of other metals to form silver chloride, which is insoluble in nitric acid. The pro- portions in which the combination takes place are 3 5 '3 7 of chlorine to every 107 '66 of silver; consequently, if a standard solution of pure sodic chloride is prepared by dissolving in water such a weight of the salt as will be equivalent to 35*37 grains of chlorine ( = 58*37 grains NaCl) and diluting to the measure of 1000 grains; every single grain measure of this solution will combine with '107 6 6 grain of pure silver to form silver chloride, which is precipitated to the bottom of the vessel in which the mixture is made. In the process of adding the salt solution to the silver, drop by drop, a point is at last reached when the precipitate ceases to form. Here the process must stop. On looking carefully at the graduated vessel from which the standard solution has been used, the operator sees at once the number of grain measures which has been necessary to produce the complete decomposition. .For example, suppose the quantity used was 520 grain measures ; all that is necessary to be done is to multiply 520 by the coefficient for each grain measure, viz. 0*10766, which shows the amount of pure silver present to be 55*98 grains. This method of determining the quantity of silver in any given solution occupies scarcely a quarter of an hour, whereas the estimation by weighing could not be done in half a day, and even then not so accurately as by the volumetric method. It must be understood that there are certain necessary precautions in conducting the above process which have not been described ; those will be found in their proper place ; but from this example it will at once be seen that the saving of time and trouble, as compared with the older methods of analysis, is immense ; besides which, in the majority of instances in which it can be applied, it is equally accurate, and in many cases much more so. The only conditions on which the volumetric system of analysis are to be carried on successfully are, that the greatest care is exercised with respect to the graduation of the measuring instru- ments, the strength and purity of the standard solutions, and the absence of other matters which would interfere with the accurate estimation of the particular substance sought. The fundamental distinction between gravimetric and volumetric analysis is, that in the former method, the substance to }ye estimated must be completely isolated in the purest possible state or combination, necessitating in many instances very patient and discriminating labour ; whereas, in volumetric processes, such com- plete separation is very seldom required, the processes being so contrived as to admit of the presence of half a dozen or more 1. GENERAL PRINCIPLES. 3 other substances which have no effect upon the particular chemical reaction required. The process just described for instance, the estimation of silver in coin, is a case in point. The alloy consists of silver and copper, with small proportions of lead, antimony, tin, gold, etc. Xone of these things affect the amount of salt solution which is chemically required to precipitate the silver, whereas, if the metal had to be determined by weight it would be necessary to first filter the nitric acid solution to free it from insoluble tin, gold, etc. ; then precipitate with a slight excess of sodic chloride ; then to bring the precipitate upon a filter, and wash repeatedly with pure water until every trace of copper, sodic chloride, etc., is removed. The pure silver chloride is then carefully dried, ignited separately from the filter, and weighed ; the filter burnt, residue as reduced metallic silver and filter ash allowed for, and thus finally the amount of silver is found by the balance with ordinary weights. On the other hand the volumetric process has been purely chemical, the burette or measuring instrument has taken the place of the balance, and theoretical or atomic weights have supplanted ordinary weights. The end of the operation in this method of analysis is in all cases made apparent to the eye. In alkalimetry it is the change of colour produced in litmus, turmeric, or other sensitive colouring matter. The formation of a permanent precipitate, as in the estimation of cyanogen. A precipitate ceasing to form, as in chlorine and silver determination. The appearance of a distinct colour, as in iron analysis by permanganate solution, and so on. I have adopted the classification of methods used by Mohr and others, namely : 1. Where the determination of the substance is effected by saturation with another substance of opposite properties generally understood to include acids and alkalies, or alkaline earths. 2. Where the determination of a substance is effected by a reducing or oxidizing agent of known power, including most metals, with their oxides and salts ; the principal oxidizing agents being potassic permanganate, potassic bichromate, and iodine ; and the corresponding reducing agents, ferrous and stannous compounds, and sodic thiosulphate. 3. Where the determination of a substance is effected by precipitating it in some insoluble and definite combination, an example of which occurs in the estimation of silver described above. This classification does not rigidly include all the volumetric processes that may be used, but it divides them into convenient .sections for describing the peculiarity of the reagents used, and their preparation. If strictly followed out, it would in some cases necessitate the registration of the body to be analyzed under two B 2 4 VOLUMETRIC ANALYSIS. 1. or three heads. Copper, for instance, can be determined residually by potassic permanganate ; it can also be determined by precipitation with sodic sulphide. The estimation of the same metal by potassic cyanide, on the other hand, would not come under any of the heads. It will be found, therefore, that liberties have been taken with the arrangement ; and for convenient reference all analytical processes applicable to a given body are included under its name. It may be a matter of surprise to some that several distinct volumetric methods for one and the same substance are given ; but a little consideration will show that in many instances greater convenience, and also accuracy, may be gained in this way. The operator may not have one particular reagent at command, or he may have to deal with such a mixture of substance as to preclude the use of some one method ; whereas another may be quite free from such objection. The choice in such cases of course requires judgment, and it is of the greatest importance that the operator shall be acquainted with the qualitative composition of the matters with which he is dealing, and that he should ask himself at every step why such and such a thing is done. It will be apparent from the foregoing description of the volumetric system, that it may be successfully used in many instances by those who have never been thoroughly trained as analytical chemists ; but we can never look for the scientific development of the system in such hands as these. In the preparation of this work an endeavour has been made to- describe all the operations and chemical reactions as simply as possible, purposely avoiding abstruse mathematical expressions, which, though they may be more consonant with the modern study of chemical science, are hardly adapted to the technical operator. 2. INSTRUMENTS. THE INSTRUMENTS AND APPARATUS. THE BALANCE. $ 2. STRICTLY speaking, it is necessary to have two balances in order to carry out the volumetric system completely ; one to carry about a kilogram in each pan, and turn when loaded with about five milligrams. This instrument is used for graduating flasks, or for testing them, and for weighing large amounts of pure reagents for standard solutions. The second balance should be light and delicate, and to carry about fifty grams, and turn easily and quickly when loaded with one or two-tenths of a milligram. This instrument serves for weighing small quantities of substances to be tested, many of which are hygroscopic, and need to be weighed quickly and with great accuracy ; it also serves for testing the accuracy of pipettes and burettes. For all technical purposes, however, a moderate-sized balance of medium delicacy is quite sufficient, especially if rather large quantities of substances are weighed and brought into solution then further subdivided by means of measuring flasks and pipettes. The operator also requires, besides the balance and the graduated instruments, a few beakers, porcelain basins, flasks, funnels, stirring rods, etc., as in gravimetric analysis ; above all he must be practically familiar with proper methods of filtration, washing of precipitates, and the application of heat. VOLUMETRIC ANALYSIS WITHOUT WEIGHTS. 3. THIS is more a matter of curiosity than of value ; but, nevertheless, one can imagine circumstances in which it might be useful. In carrying it out, it is necessary only to have (1) a correct balance, (2) a pure specimen of substance to use as a weight, (3) an accurate burette filled with the appropriate solution. It is not necessary that the strength of this should be known ; but the state of concentration should be such as to permit the necessary reaction to occur under the most favourable circumstances. If a perfectly pure specimen of substance, say calcic carbonate, be put into one scale of the balance, and be counterpoised Avith an impure specimen of the same substance, and both titrated with the same acid, and the number of c.c. used for the pure substance be called 100, the number of c.c. used for the impure substance will correspond to the percentage of pure calcic carbonate in the specimen examined. The application of the process is, of course, limited to the use of such substances as are to be had pure, and whose weight is not variable by exposure ; but where even a pure substance of one kind cannot be had as a weight, one of another kind may be used as a substitute, and the required result obtained by calculation. For 6 VOLUMETRIC ANALYSIS. 4. instance, it is required to ascertain the purity of a specimen of sodic carbonate, and only pure calcic carbonate is at hand to use as a weight ; equal weights of the two are taken, and the impure speci- men titrated with acid. To arrive at the required answer, it is necessary to find a coefficient or factor by which to convert the number of c.c. required by the sodic carbonate, weighed on the calcic, into that Avhicli should be required if weighed on the sodic, basis. A consideration of the relative molecular weights of the two bodies will give the factor thus- Calcic carbonate 100 Sodic carbonate 106~~ If, therefore, the c.c. used are multiplied by this number, the percentage of pure sodic carbonate will be obtained. The method may be extended to a number of substances, on this principle, with the exercise of a little ingenuity. L. de Koningh has communicated to me a similar method devised by himself and Peacock, in which the same end is attained without the aid of a pure substance as standard, thus : Say a specimen of impure common salt is to be examined, a moderate portion is put on the balance and counterpoised with silver nitrate ; the latter is then dissolved up to 100 c.c. and placed in a burette. The salt is dissolved in water, a few drops of chromate added and titrated with the silver solution, of which 10 c.c. is required ; the salt is therefore equal to 10 per cent, of its weight of silver nitrate, then 16-96 : 58-37 : : 10 = 3'44 % NaCl Or, in the case of an impure soda ash, an equal weight of oxalic acid is taken and made up to 100 c.c. ; the soda requires, say, 50 c.c. for saturation, or 50 per cent., then 126 : 106 : : 50 = 42 % IS T a 2 C0 3 It may happen that, in some cases, more than one portion of the reagent is required to decompose the substance tested, and to provide against this two or more lots should be weighed in the first instance. VOLUMETRIC ANALYSIS WITHOUT BURETTES OR OTHER GRADUATED INSTRUMENTS. 4. THIS operation consists in weighing the standard solutions on the balance instead of measuring them. The influence of variation in temperature is, of course, here of no consequence. The chief requisite is a delicate flask, fitted with a tube and blowing ball, as in the burette fig. 7, or an instrument known as Schuster's alkalimeter may be used. A special burette has been devised for this purpose by Casamajor ((7. -ZV. xxxv. 98). The INSTRUMENTS. method is capable of very accurate results, if care be taken in preparing the standard solutions and avoiding any loss in pouring the liquid from the vessel in which it is weighed. It occupies much more time than the usual processes of volumetric analysis, but at great extremes of temperature it is far more accurate. THE BURETTE. 5. THIS instrument is used for the delivery of an accurately measured quantity of any particular standard solution. It invari- ably consists of a long glass tube of even bore, throughout the Fig. 1. Fig. 2. length of which are engraved, by means of hydrofluoric acid, certain divisions corresponding to a known volume of fluid. 8 VOLUMETKIC ANALYSIS. It may be obtained in a great many forms, under the names of their respective inventors, such as Mohr, Gay Lussac, Binks, etc., but as some of these possess a decided superiority over others, it is not quite a matter of indifference which is used, and therefore a slight description of them may not be out of place here. The burette, with india-rubber tube and clip, contrived by Mohr, is shown in figs. 1 and 2, and with stop-cock in fig. 3. Fig. 3. Fig. 4. The advantages possessed by this form of instrument are, that its fixed upright position enables the operator at once to read off the number of degrees of test solution used for any analysis. The quantity of fluid to be delivered can be regulated to the greatest nicety by the pressure of the thumb and finger on the spring clip or by the tap ; and the instrument not being held in the hand, there is no chance of increasing the bulk of the fluid by the heat of the body, and thus leading to incorrect measurement, as is the case with Binks' or Gay Lussac's form INSTRUMENTS. 9 of instrument. The principal disadvantage, however, of these two latter forms of burette is, that a correct reading can only be obtained by placing them in an upright position, and allowing the fluid to find its perfect level. The preference should, therefore, unhesitatingly be given to Mohr's burette, wherever it can be used ; the greatest drawback to its original form is, that it cannot be used for permanganate, and some other solutions affected by the india-rubber. This defect is avoided by using the stop-cock burette, fig. 3. This tap burette may of course be used not only for permanganate but for all other solutions, and may also be arranged so as to deliver the solution in drops, leaving both the hands of the operator disengaged. A new and ingenious arrange- ment of tap is shown in fig. 4, the tap being placed obliquely through the spit, so as to avoid its dropping out of place; the floats shown are very small thermometers. Owing to the action of 10 VOLUMETRIC ANALYSIS. 5. caustic alkalies upon glass, such a burette does not answer well for strong solutions of potash or soda, unless emptied and washed immediately after use. Two convenient forms of stand for Mohr's burettes are shown in figs. 5 and 6 ; in the latter, the arms carrying the burettes revolve. We are indebted to Mohr for another form of instrument to avoid the contact of permanganate and india-rubber, viz., the foot burette, with elastic ball, shown in fig. 7. The flow of liquid from the exit tube can be regulated to a great nicety by pressure upon the ball, which should be large, and have 'two open- ings, one cemented to the tube with marine glue, and the other at the side, over which the thumb is placed when pressed, and on the removal of which it refills itself with air. Gay Lussac's burette, supported in a wooden foot, may be used instead of the above form, by inserting a good fitting cork into the open end, through which a small tube bent at right angles is passed. If the burette is held in the right hand, slightly inclined to- wards the beaker or flask into which the fluid is to be measured, and the mouth applied to the tube, any portion of the solution may be emptied out by the pressure of the breath, and the disadvantage of holding the instrument in a horizontal position, to the great danger of spilling the contents, is avoided ; at the same time the beaker or flask can be held in the left hand and shaken so as to mix the fluids, and by this means the end of the operation be more accurately determined (see fig. 8). There is an arrangement of Mohr's burette which is extremely serviceable, when a series of analyses of the same character have to be made, such as in alkali works, assay offices, etc. It consists in having a ~]~ piece of glass tube inserted between the lower end of the burette and the spring clip, which communicates with a reservoir of the standard solution, placed above so that the Tig. 7. Fig. 8. 5. INSTKUMENTS. 11 burette may be filled as often as emptied, by a syphon, and in so gradual a manner that 110 air bubbles occur, as in the case of filling it with a funnel, or pouring in liquid from a bottle ; beside which, this plan prevents evaporation or dust in the standard solution either in the burette or reservoir. Figs. 9 and 10 show this arrangement in detail. HUBERT DYER rig. 9. It sometimes happens that a solution requires titration at a hot or even boiling temperature, such as the estimation of sugar by copper solution : here the ordinary arrangement of Mohr's burette will not be available, since the steam rising from the liquid heats the burette and alters the volume of fluid. This may be avoided either by using a special burette, in which the lower end is extended at a right angle with a stop-cock, or by attaching to an ordinary burette a much longer piece of india-rubber tube, so that the burette stands at the side of the capsule or beaker being heated, and the 12 VOLUMETRIC ANALYSIS. 5. elastic tube is brought over its edge ; the pinch-cock is fixed midway ; no heat can then reach the body of fluid in the burette, since there can be no conduction past the pinch-cock. If this plan is not adopted, a Gay Lussac or ball burette should be used. Gay Lus sac's burette, shown in fig. 11, should have a wooden Fig. 10. Fig. 11. support or foot into which it may be inserted (fig. 8), so as to lie read correctly. By using it in the following manner, its natural disadvantages may be overcome to a great extent. Having fixed the burette into the foot securely, and filled it, take it up by the foot with the left hand, and resting the upper end upon the edge 5. INSTRUMENTS. 13 of the beaker in which the solution to be tested is placed, drop the test fluid from the burette, meanwhile stirring the contents of the beaker with a glass rod held in the right hand ; by a slight elevation or depression of the left hand, the flow of test liquid is regulated until the end of the operation is secured, thus avoiding the annoyances which arise from alternately placing the instrument in an upright and horizontal position. Binks', or, as it is sometimes called, the English burette, is well known, and need not be described ; it is the least recommendable of all forms, except for very rough estimations. It is most convenient to have burettes graduated to contain 25 or 30 c.c. in y 1 ^ c.c., 50 or 60 c.c. in 4 c.c., and 100 or 110 c.c. in J or y c.c. The pinch-cock generally used in Mohr's burette is shown in fig. 1 . These are made of brass and are now generally nickel-plated to prevent corrosion ; another form is made of one piece of steel wire, as devised by Hart ; the wire is softened by heating and coiled round, as shown in fig. 12. When the proper shape has been attained, the clip is hardened and tempered so as to convert it into a spring. Another useful pinch-cock is shown in fig. 12. It may be made of hard wood, horn, or preferably, of flat glass rod. The levers should be long. A small piece of cork, of the same thickness as the elastic tube of the burette when pressed close, should be fastened at the angles of the levers as shown in the engraving. The use of any kind of pinch-cock may be avoided, and a very delicate action obtained, by simply inserting a not too tightly fitting piece of solid glass rod into the elastic tube, between the end of the VOLUMETEIC ANALYSIS. burette and the spit, a firm squeeze being given by the finger and thumb to the elastic tube surrounding the rod, a small canal is opened, and thus the liquid escapes, and of course can be controlled by the operator at his will (see fig. 13). 50 CC 10CC Pig. 13. Fig. 14. THE PIPETTE. 6. THE pipettes used in volumetric analysis are of two kinds, viz., those which deliver one certain quantity only, and those which are graduated on the stem, so as to deliver various quantities at the discretion of the analyst. In the former kind, or whole pipette, the graduation may be either that in which the fluid is suffered to run out by its own weight, or in which it is blown out by the breath. The best form is that in which the liquid flows out by its INSTRUMENTS. 15 own weight, but in this case the last few drops empty themselves very slowly ; if, however, the lower end of the pipette be touched against the moistened edge of the beaker or the surface of the fluid into which it is emptied, the flow is hastened considerably, and in graduating the pipette, it is preferable to adopt this plan. In both the whole and graduated pipettes, the upper end is narrowed to about -J inch, so that the pressure of the finger is sufficient to arrest the flow at any point. Pipettes are invariably filled by sucking the upper end with the mouth, unless the liquid contain volatile or other highly poisonous matter, in which case the instrument may be dipped completely into the fluid, but if so the outside liquid must be wiped off before measuring. Beginners invariably find a difficulty in quickly filling the pipette above the mark, and stopping the fluid at the exact point. Practice with pure water is the only method of over- coming this. Fig. 14 shows two whole pipettes, one of small and the other of large capacity, and also a graduated pipette of medium size. It must be borne in mind that the pipette graduated throughout the stem is not a reliable instrument for actual analysis, owing to the difficulty of stopping the flow of liquid at any given point, and reading off the exact measurement. Its chief use is in the approximate estimation of the strength of any standard solution in the course of preparation. THE MEASURING FLASKS. 7. THESE indispensable instru- ments are made of various capacities ; they serve to mix up standard solu- tions to a given volume, and also for the subdivision of the substance to be tested by means of the pipettes. They should be tolerably wide at the mouth, and have a well-ground glass stopper, and the graduation line should fall just below the middle of the neck, so as to allow room for shaking up the fluid. Convenient sizes are 100, 200, 250, 300, 500, and 1000 c.c., all graduated to contain the respective quantities. If required to deliver these volumes they must have a second higher mark in the neck, obtained by weighing into the wetted and drained flasks the respective number of grams of dis- tilled water at 16 C. A liter flask is shown in fig. 15. Fig. 15. 16 VOLUMETRIC ANALYSIS. ON THE CORRECT READING OF GRADUATED INSTRUMENTS. 8. THE surface of liquids contained in narrow tubes is always curved, in consequence of the capillary attraction exerted by the sides of the tube, and consequently there is a difficulty in obtaining a distinct level in the fluid to be measured. If, how- ever, the lowest point of the curve is made to coincide with the graduation mark, a correct proportional read- ing is always obtained, hence this method of reading is the most satisfactory (see fig. 1 6). The eye may be assisted materially in reading the divisions on a graduated tube by using a small card, Fig. 16. the lower half of which is blackened, the upper re- maining white. If the line of division between the black and white be held about an eighth of an inch below the surface of the liquid, and the eye brought on a level with it, the meniscus then can be seen by transmitted light, bounded below by a sharply denned black line. A card of this kind, sliding up and down a support, is of great use in verifying the graduation of the burettes or pipettes with a cathetometer. Another good method is to use a piece of mirror, upon which are gummed two strips of black paper, half an inch apart ; apply it in contact with the burette so that the eye can be reflected in the open space. The burette or pipette is filled with water at the proper temperature, and the contents of each division of 10 c.c. or so carefully read off with the telescope and weighed. In order to do this with pipettes they must be fixed in a burette support, and have over their upper end a tightly fitting elastic tube closed with a pinch-cock. The operator may here consult with advantage the details of gradu- ating and verifying measuring instruments for the analysis of gases as described in Part 7. In taking the readings of burettes, pipettes, and flasks, the graduation mark should coincide as nearly as possible with the level of the operator's eye. Erdmann's Float. This useful little instrument to accompany Mohx's burette, gives the most accurate reading that can be obtained ; one of its forms is shown in fig. 17, another, containing a thermometer, is shown in fig. 4. It consists of an elongated Fig. 17. COllliECT READINGS. glass bulb, rather smaller in diameter than the burette itself, and weighted at the lower end with a globule of mercury, like an hydrometer. It is drawn to a point at the upper end, and the point is bent round so as to form a small hook, by means of which it can be lifted in and out of the burette with a bent wire ; a line is made with a diamond round its middle by means of a lathe, and the coincidence of this line with the graduation mark of the burette is accepted as the true reading. The actual height of the liquid in the burette is not regarded, because if the operator begins with the line on the float, opposite the graduation mark on the burette, the same proportional division is always maintained. It is essential that the float should move up and down in the burette without wavering, and the circle upon it should always be parallel to the graduations of the burette. One great value of this float is found in testing the accuracy of the burette itself; it enables a strict comparison to be made between say each 10 c.c., with very slight differences in weigh- ing, supposing the instrument to be .correct. It is always well, however, to bear in mind that absolute accuracy cannot be obtained in graduating in- struments ; 5 or 10 milligrams of water either way in 10 c.c. may safely be disregarded. To prevent evaporation and the entrance of dust in Mohr's burette, while in use, a small beaker or wide test tube should be dropped over its orifice. In burettes containing caustic al- kaline solutions, a cork with carbonic acid tube should be used if the solution is allowed to remain in them for any length of time. Besides the measuring flasks it is necessary to have graduated Fig. 18. vessels of cylindrical form, for the purpose of preparing standard solutions, etc. Fig. 18 shows a stoppered cylinder for this purpose, generally called a test mixer. 18 VOLUMETRIC ANALYSIS. 9. Filter for ascertaining: the end-reaction in certain pro- cesses. This is shown in fig. 19, and the instrument is known as Beale's filter. It serves well for taking a few drops of clear solution from any liquid in which a pre- cipitate w r ill not settle readily. To use it, a piece of filter paper is tied -over the lower end, and over that a piece of fine muslin to keep the paper from being broken. When dipped into a muddy mixture, the clear fluid rises and may be poured out of the little spout for testing. If the process in hand is not completed, the contents are washed back to the bulk, and the operation repeated as often as may be required. Fig. 19. ON THE SYSTEM OF WEIGHTS AND MEASURES TO BE ADOPTED IN VOLUMETRIC ANALYSIS. 9. IT is much to be regretted that the decimal system of weights and measures used on the Continent is not universally adopted, for scientific and medicinal purposes, in England. Its great advantage is its uniformity throughout. The unit of weight is the gram ( = 15'43235 grains troy), and a gram of distilled water at 4 C., or 39 Fahr., measures exactly a cubic centimeter. The kilogram contains 1000 grams, the liter 1000 cubic centi- meters. It may not be out of place here to give a short description of the origin of the French decimal system, now used exclusively for scientific purposes in that country, and also in Prussia, Austria, Holland, Sweden, Denmark, Belgium, and Spain. The commission appointed in France for the purpose of instituting a decimal system of weights and measures, founded their standard on the length of the meridian arc between the pole and equator, the ten-millionth part of which was called the metre ( = 39 '37 10 English inches), although the accuracy of this measurement has been disputed. It would have been preferable, as since proposed, that the length of a pendulum vibrating exactly 86,400 times in twenty-four hours, or one second for each vibration, equivalent to 39 '137 2 English inches, should have been taken as the standard metre, in which case it would have been much easier to verify the standard in case it should be damaged or destroyed. However, the actual metre in use is equal to 3 9 '3 71 inches, and from this standard its multiples and subdivisions all proceed decimally ; its one-tenth part being the decimetre, one-hundredth the centimetre, and one- thousandth the millimetre. In accordance with this, a cube of distilled water at its greatest density, viz., 4 C., or 39 Fahr., whose side measures one decimeter, has exactly the weight of one kilogram, or 1 000 grams, and occupies the volume of one liter, or 1000 cubic centimeters. This simple relationship between liquids and solids is of great 9. WEIGHTS AND MEASURES. 19 value in a system of volumetric analysis, and even for ordinary analysis by weight ; for technical purposes it is equally as applicable as the grain system, the results being invariably tabulated in percentages. With these brief explanations, therefore, I have only to state that the French decimal system will be mainly used throughout this treatise ; but at the same time, those who may desire to adhere to the ordinary grain weights, can do so without interfering with the accuracy of the processes described. As has been before stated, the true cubic centimeter contains one gram of distilled water at its greatest density, viz., 4 C., or 39 Fahr. ; but as this is a degree of temperature at which it is impossible to work for more than a month or two in the year, it is better to take the temperature of 16 C., or about 60 Fahr., as the standard ; because in winter most laboratories or rooms have furnaces or other means of warmth, and in summer the same localities ought not, under ordinary circumstances, to have a much higher degree of heat than 16 C. In order, therefore, that the graduation of instruments on the metrical system may be as uniform as possible with our own fluid measures, the cubic centimeter should contain one gram of distilled water at 16 C. The true c.c. (i.e. 1 gm. at 4 C., or 39 Fahr.) contains only 0-999 gm. (strictly 0'998981) at that temperature; but for con- venience of working, and for uniformity with our own standards of volume, it is better to make the c.c. contain one gram at 16 C. The real difference is one-thousandth part. The operator, there- fore, supposing he desires to graduate his own measuring flasks, must weigh into them 250, 500, or 1000 grams of distilled water at 16 C., or 60 Fahr. Fresenius and others have advocated the use of the strict liter by the graduation of instruments, so that they shall contain 999 gm. at 16 C. Mohr, on the contrary, uses a 1000 gm., at the temperature of 17 '5, the real difference being 1'2 c.c. in the liter, or about one eight-hundredth part. It will be seen above that I have advocated a middle course on two grounds: (1) That in testing instruments it is much easier to verify them by means of round numbers, such as 5 or 10 gm. (2) That there are many thousands of instruments already in use varying between the two extremes ; and as these cannot well be annihilated, the adoption of a mean will give a less probable amount of error between the respective instruments ; and, moreover, the difference between the liter at 4 and 16 being one-thousandth part, it is easy to correct the measurement for the exact liter. It matters not which plan is followed, if all the instruments in a particular set coincide with each other; but it would be manifestly wrong to use one of Mohr's burettes with one of Fresenius' measuring flasks. Operators can, however, without much difficulty re-mark their measuring flasks to agree with their c 2 20 VOLUMETRIC ANALYSIS. smaller graduated instruments, if they are found to differ to any material extent. Variations of Temperature. 111 the preparation of standard solutions, one thing must especially be borne in mind ; namely, that saline substances on being dissolved in water have a consider- able effect upon the volume of the resulting liquid. The same is also the case in mixing solutions of various salts or acids with each other (see Gerlach, " Specifische Gewichte der Salzlosungen ; " also Gerlach, " Sp. Gewichte von wasserigen Lb'sungen," Z. a. C. viii. 245). In the case of strong solutions, the condensation in volume is as a rule considerable : and, therefore, in preparing such solutions for volumetric analysis, or in diluting such solutions to a given volume for the purpose of removing aliquot portions subsequently for examination, sufficient time must be given for liquids to assume their constant volume at the standard temperature. If the strength of a standard solution is known for one temperature, the strength corresponding to another temperature can only be calculated if the rate of expansion by heat of the liquid is known. The variation cannot be estimated by the known rule of expansion in distilled water; for Gerlach has shown that even weak solutions of acids and salts expand far more than water for certain increments of temperature. The rate of expansion for pure water is known, and may be used for the purpose of verifying the graduation of instruments, where extreme accuracy is required. The following short table furnishes the data for correction. The weight of 1000 c.c. of water at t C., when determined by means of brass weights in air of t C., and at 0*76 m.m. pressure, is equal to 1000 - x gin. Slight variations of atmospheric pressure may be entirely disregarded. t 10 11 12 13 14 15 16 17 18 19 X 1-34 1-43 1-52 1-63 1-76 1-89 2-04 2-2 2-37 2-55 t 20 21 22 23 24 25 26 27 28 29 30 X 2-74 2-95 317 3-39 3-63 3-88 4-13 4-39 4-67 4-94 5'24 x is the quantity to be subtracted from 1000 to obtain the weight of 1000 c.c. of water at the temperature t. Thus at 20 2-74 must be deducted from 1000 = 997'26. Bearing the foregoing remarks in mind, therefore, the safest plan in the operations of volumetric analysis, so far as measurement is concerned, is to use solutions as dilute as possible. Absolute accuracy in estimating the strength of standard solutions can only WEIGHTS AND MEASURES. 21 be secured by weight, the ratio of the weight of the solution to the weight of active substance in it being independent of temperature. Casamajor (C. N. xxxv. 160) has made use of the data given by Matthiessen in his researches on the expansion of glass, water, and mercury, to construct a table of corrections to be used in case of using any weak standard solution at a different temperature to that at which it was originally standardized. The expansion of water is different at different temperatures ; the expansion of glass is known to be constant for all temperatures up to 100. The correction of volume, therefore, in glass burettes, must be the known expansion of each c.c. of water for every 1 C., less the known expansion of glass for the same temperature. It is not necessary here to reproduce the entire paper of Casamajor, but the results are shortly given in the following table. The normal temperature is 15 C. ; and the figures given are the relative contractions below, and expansions above, 15 C. Deg. C. 7- 000612 8 000590 9 000550 10 - 000492 11 000420 12 000334 13 000236 14 - 000124 15 Normal 16 + 000147 17 + 000305 18 + 000473 19 + 000652 20 + 000841 21 + 001039 22 + 001246 23 + 001462 Deg. C. 24 + 001686 25 + 001919 26 + 002159 27 + 002405 28 + 002657 29 + 002913 30 + 003179 31 + 003453 32 + 003739 33 + 004035 34 + 004342 35 + 004660 36 + 004987 37 + 005323 38 + 005667 39 + 006040 40 + 006382 By means of these numbers it is easy to calculate the volume of liquid at 15 C. corresponding to any volume observed at any temperature. If 35 c.c. of solution has been used at 37 C., the table shows that 1 c.c. of water in passing from 15 to 37 is increased to 1 '005323 c.c. ; therefore, by dividing 35 c.c. by 1 '005323 is obtained the quotient 34 '8 19 c.c., which represents the volume at 15 corresponding to 35 c.c. at 37; or the operation can be simplified by obtaining the factor, thus : 1-005323 = 0-994705 A table can thus be easily constructed which would show the factor for each degree of temperature. These corrections are useless for concentrated solutions, such as 22 VOLUMETRIC ANALYSIS. 9. normal alkalies or acids ; with great variations of temperature these solutions should be used by weight. The accurate graduation of burettes and pipettes can only be done by carefully constructed machines, and is, therefore, generally speaking, beyond the compass of the analyst himself ; nevertheless, they should be carefully tested by him before being used, as, unfortunately, they do not always possess the accuracy to which they pretend. In the verification of both burettes and pipettes, it is only necessary to allow ten cubic centimeters of distilled water to flow from the instrument to be tested into a dry and accurately tared flask or beaker. If the weight at 16 C., or 60 Fahr., is 10 grams, it is sufficient; the next 10 c.c. may be tried in like manner, and so on until the entire capacity is proved ; differences of 5 or 10 milligrams may be disregarded. Thorpe (Quant if. Chem. Anal. p. 119) attaches the burette or pipette with elastic tube and pinch-cock to a balance, and thus weighs the respective volumes of fluid delivered. Graduated flasks are extremely serviceable in dividing small quantities of substance into still smaller proportional parts. Suppose, for instance, it is desired to take the tenth part of a solution for the purpose of separating any single constituent, let it be put into a 200 c.c. flask, which is then filled to the mark with water or other appropriate liquid, and well shaken; 20 c.c. taken out with a pipette will at once give the quantity required. Instruments graduated on the Grain System. Burettes, pipettes, and flasks may also be graduated in grains, in which case it is best to take 10,000 grains as the standard of measurement. In order to lessen the number of figures used in the grain system, so far as liquid measures are concerned, I propose that ten fluid grains be called a decem, or for shortness dm. ; this term corresponds to the cubic centimeter, bearing the same proportion to the 10,000 grain measure as the cubic centimeter does to the liter, namely, the one-thousandth part. The use of a term like this will serve to prevent the number of figures, which are unavoidably introduced by the use of a small unit like the grain. Its utility is principally apparent in the analysis for percentages, particulars of which will be found hereafter. The 1000 grain burette or pipette will therefore contain 100 decems, the 10,000 gr. measure 1000 dm., and so on. The capacities of the various instruments graduated on the grain system may be as follows : Flasks : 10000, 5000, 2500, and 1000 grs. = 1000, 500, 250, and 100 dm. Burettes : 300 grs. in 1-gr. divisions, for very delicate purposes = 30 dm. in -^ ; 600 grs. in 2-gr. divisions, or \ dm. ; 1100 grs. in 5-gr. divisions, or -| dm. ; 1100 grs. in 10-gr. divisions, or 1 dm. The burettes are graduated above the 500 or 1000 grs. in order to allow of analysis for percentages by the residual method. 10. SYSTEMATIC SOLUTIONS. 23 Whole pipettes to deliver 10, 20, 50, 100, 200, 500, and 1000 grs., graduated ditto, 100 grs. in -^ din. ; 500 grs. in J dm. ; 1000 grs. in 1 dm. Those who may desire to use the decimal systems constructed on the gallon measure = 70,000 grains, will bear in mind that the "septem" of Griffin, or the " decimillem " of Ac land are each equal to 7 grs., ; and therefore bear the same relation to the pound = 7000 grs., as the cubic centimeter does to the liter, or the decem to the 10,000 grs. An entirely different set of tables for calculations, etc., is required for these systems; but the analyst may readily construct them when once the principles contained in this treatise are understood. VOLUMETRIC ANALYSIS BASED ON THE SYSTEM OF CHEMICAL EQUIVALENCE AND THE PREPARATION OF NORMAL TITRATED SOLUTIONS. 10. WHEN analysis by measure first came into use, the test solutions were generally prepared so that each substance to be tested had its own special reagent; and the strength of the standard solution was so calculated as to give the result in percentages. Consequently, in alkalimetry, a distinct standard acid was used for soda, another for potash, a third for ammonia, and so on, necessi- tating a great variety of standard solutions. Griffin and Ure appear to have been the first to suggest the use of standard test solutions based on the atomic system ; and following in their steps, Mohr has worked out and verified many methods of analysis, which are of great value to all who concern themselves with scientific and especially technical chemistry. Not only has Mohr done this, but in addition to it, he has enriched his processes with so many original investigations, and improved the necessary apparatus to such an extent, that he may with justice be called the father of the volumetric system. Normal Solutions. It is of great importance that no misconcep- tion should exist as to what is meant by a normal solution ; but it does unfortunately occur, as may be seen by reference to the chemical journals, also to Muir's translation of Fleischer's book (see Allen, C. N. xl. 239, also Analyst, xiii. 181). Normal solutions as originally devised are prepared so that one liter at 16 C. shall contain the hydrogen equivalent of the active reagent weighed in grams (H=l). Seminormal, quintinormal, decinormal, and centinormal solutions are also required, and may be shortly designated as |- --$ and T Jy- solutions.* * It is much to be regretted that the term "normal," originally based on the equivalent system, should now be appropriated by those who advocate the use of solutions based on molecular weights, because it not only leads to confusion between the two systems, but to utter confusion between the advocates of the change themselves. In Fleischer's German edition of his Maasanalijse the molecular system 24 VOLUMETRIC ANALYSIS. 10. In the case of uiiivalent substances, such as silver, iodine, hydrochloric acid, sodium, etc., the equivalent and the atomic (or in the case of salts, molecular) weights are identical ; thus, a normal solution of hydrochloric acid must contain 36 '37 grams of the acid in a liter of fluid, and sodic hydrate 40 grains. In the case of bivalent substances, such as lead, calcium, oxalic acid, sulphurous acid, carbonates, etc., the equivalent is one-half of the atomic (or in the case of salts, molecular) weight ; thus, a normal solution of oxalic acid would be made by dissolving 63 grams of the crystallized acid in distilled water, and diluting the liquid to the measure of one liter. Further, in the case of trivalent substances, such as phosphoric- acid, a normal solution of sodic phosphate would be made by weighing -^f-= 119*3 grams of the salt, dissolving in distilled water, and diluting to the measure of one liter. One important point, however, must not be forgotten, namely, that in preparing solutions for volumetric analysis the value of a reagent as expressed by its equivalent hydrogen-weight must not always be regarded, but rather its particular reaction in any given analysis ; for instance, tin is a quadrivalent metal, but when using stannous chloride as a reducing agent in the analysis of iron, the half, and not the fourth, of its molecular weight is required, as is shown by the equation Fe 2 Cl + Sn Cl 2 = 2 Fe Cl 2 + Sn Cl 4 . In the same manner with a solution of potassic permanganate Mn KO 4 when used as an oxidizing agent, it is the available oxygen which has to be taken into account, and hence in constructing a normal solution one-fifth of its molecular weight i?*- = 31'6 grams must be contained in the liter. Other instances of a like kind occur, the details of which will be given in the proper place. is advocated, but, as the old atomic weights are used, the solutions are really, in the main, of the same strength as those based on the equivalent system. Pattinson Muir, however, in his translation, has thought proper to use modern atomic weights, and the curious result seems that one is directed to prepare a normal solution of caustic potash with 39'1 grams K to the liter, while a normal potassic carbonate is to contain 138'2 grams K 2 CO 3 , or 78'2 grams K, in the same volume of solution. Again, Muter, in his Manual nf Analytical Chemistry, defines a normal solution as having one molecular weight of the reagent in grams per liter ; then follows the glaring inconsistency, among others, of directing that a decinormal solution of iodine should contain 127 grams of I per liter, whereas, if it was strictly made according to the original definition, it should contain 25'4 grams in the liter. If the unit H be adopted as the basis or standard, everything is simplified, and actual normal solutions may be made and used ; but, on the molecular system, this is, in many cases, not only unadvisable but impossible, besides leading to ridiculous inconsistencies. As Allen points out in the reference above, it is, to say the least of it, highly inconvenient that the nomenclature of a standard solution should be capable of two interpretations. I have given the term a;;stematic to this handbook, and I maintain that the equivalent system used is the only systematic and consistent one; it was adopted originally by Mohr, followed by Fresenius, and continued by Classen in the new edition of Mohr's Titrii methode. Allen himself has unhesitatingly preferred to use it in his Organic Analysis, and these, together with this treatise, being all text-books having a wide circulation, ought to settle definitely the meaning of the t< rm normal as applied to systematic standard solutions. Anyhow, it is to be hoped that those who communicate processes to the chemical journals, or abstractors of foreign ai tides for publication, will take care to distinguish between the conflicting systems. 10. SYSTEMATIC SOLUTIONS. 25 A further illustration may be given in order to show the method of calculating the results of this kind of analysis. Each c.c. of Y^- silver solution will contain y^^nr of the atomic weight of silver = 0*010766 gm., and will exactly precipitate T _i.__ of the atomic weight of chlorine = 0*003537 gm. from any solution of a chloride. In the case of normal oxalic acid each c.c. will contain ^-^-5 of the molecular weight of the acid = 0*0 6 3 gm., and will neutralize __i__ of the molecular weight of sodic monocarbonate = 0*053 gm., or will combine with o-^Vo" f the atomic weight of a dyad metal such as lead = '1032 gm., or will exactly saturate __!__ of the molecular weight of sodic hydrate = 0*040 gm., and so on. Where the 1000 grain measure is used as the standard in place of the liter, 63 grains of oxalic acid would be used for the normal solution; but as 1000 grains is too small a quantity to make, it is better to weigh 630 grains, and make up the solution to 10,000 grain measures = 1000 dm. The solution would then have exactly the same strength as if prepared on the liter system, as it is pro- portionately the same in chemical power ; and either solution may be used indiscriminately for instruments graduated on either scale, bearing- in mind that the substance to be tested with a c.c. burette must be weighed on the gram system, and vice versa, unless it be desired to calculate one system of weights into the other. The great convenience of this equivalent system is, that the numbers used as coefficients for calculation in any analysis are familiar, and the solutions agree with each other, volume for volume. We have hitherto, however, looked only at one side of its advantages. For technical purposes the plan allows the use of all solutions of systematic strength, and simply varies the amount of substance tested .according to its equivalent weight. Thus, the normal solutions say, are Crystallized oxalic acid = 63 gm. per liter. Sulphuric acid =49 gm. ,, Hydrochloric acid =36.37 gm. Nitric acid =63 gm. ,, Anhydrous sodic carbonate =53 gm. ,, Sodic hydrate =40 gm. ,, Ammonia =17 gm. ,, 100 c.c. of any one of these normal acids should exactly neutralize 100 c.c. of any of the normal alkalies, or the corresponding amount of pure substance which the 100 c.c. contain. In commerce we continually meet with substances used in manufactures which are not pure, and it is necessary to know how much pure substance they contain. Let us take, for instance, refined soda ash (sodic carbonate). If it were absolutely pure, 5*3 gm. of it should require exactly 100 c.c. 26 VOLUMETRIC ANALYSIS. 10. of any normal acid to saturate it. If we therefore weigh that quantity, dissolve it in water, and deliver into the mixture the normal acid from a burette, the number of c.c. required to saturate it will show the percentage of pure sodic carbonate in the sample. Suppose 90 c.c. are required = 90 %. Again a manufacturer buys common oil of vitriol, and requires to know the exact percentage of pure hydrated acid in it ; 4*9 grams are weighed, diluted with water, and normal alkali delivered in from a burette till saturated ; the number of c.c. used will be the percentage of real acid. Suppose 58 '5 c.c. are required = 58 '5 %. On the grain system, in the same way, 53 grains of the sample of soda ash would require 90 dm. of normal acid, also equal to 90 %. Or, suppose the analyst desires to know the equivalent percentage of dry caustic soda, free and combined, contained in the above sample of soda ash, without calculating it from the carbonate found as above, 3'1 gm. is treated as before, and the number of c.c required is the percentage of sodic oxide. In the same sample 52 '6 c.c. would be required = 52 '6 per cent, of sodic oxide, or 90 per cent, of sodic carbonate. Method for percentage of Purity in Commercial Substances. The rules, therefore, for obtaining the percentage of pure substance in any commercial article, such as alkalies, acids, and various salts, by means of systematic normal solutions such as have been described are these 1. With normal solutions ~ or ^ (according to its atomicity) of the molecular weight in grams of the substance to be analyzed is to be weighed for titration, and the number of c.c. required to produce the desired reaction is the percentage of the substance whose atomic weight has been used. With decinormal solutions yj-^ or ^^ of the molecular weight in grams is taken, and the number of c.c. required will, in like manner, give the percentage. Where the grain system is used it will be necessary, in the case of titrating with a normal solution, to weigh the whole or half the molecular weight of the substance in grains, and the number of decems required will be the percentage. With decinormal solutions, y 1 ^ or -j^ of the molecular weight in grains is taken, and the number of decems will be the percentage. It now only remains k) say, with respect to the system of weights and measures to be used, that the analyst is at liberty to choose his own plan. Both systems are susceptible of equal accuracy, and he must study his own convenience as to which he will adopt. The normal solutions prepared on the gram system are equally applicable for that of the grain, and vice verm, so that there is no necessity for having distinct solutions for each system. Factors, or Coefficients, for the Calculation of Analyses. It frequently occurs that from the nature of the substance, or from 10. PRESERVATION OF SOLUTIONS. 27 its being in solution, this percentage method cannot be conveniently followed. For instance, suppose the operator has a solution con- taining an unknown quantity of caustic potash, the strength of which he desires to know ; a weighed or measured quantity of it is brought under the acid burette and exactly saturated, 32 c.c. being required. The calculation is as follows : The molecular weight of potassic hydrate being 56 : 100 c.c. of normal acid will saturate 5*6 gm. ; therefore, as 100 c.c. are to 5*6 gm., so are 32 c.c. to g, 5 ' 32 = 1*792 gm. KHO. The simplest way, therefore, to proceed, is to multiply the number of c.c. of test solution required in any analysis, by the IFDTT ( or T^Vo ^ bivalent) of the molecular weight of the substance sought, which gives at once the amount of substance present. An example may be given 1 gm. of marble or limestone is taken for the estimation of pure calcic carbonate, and exactly saturated with standard nitric or hydrochloric acid (sulphuric or oxalic acid are, of course, not admissible) 17 '5 c.c. are required, therefore 17 "5 x O'OSO (the TT^QO" f the molecular weight of CaCO 3 ) gives 0*875 gm., and as 1 gm. of substance only w r as taken = 87 '5% calcic carbonate. Preservation of Solutions. There are test solutions which, in consequence of their proneness to decomposition, cannot be kept at any particular strength for a length of time ; consequently they must be titrated on every occasion before being used. Stannous chloride and sulphurous acids are examples of such solutions. Special vessels have been devised for keeping solutions liable to alter in strength by access of air, as shown in figs. 20 and 21. Fig. 20 is especially applicable to caustic alkaline solutions, the tube passing through the caoutchouc stopper being filled with dry soda-lime, resting on cotton wool. Fig. 21, designed by Mohr, is a considerable improvement upon this, since it allows of the burette being filled with the solution from the store bottle quietly, and without any access of air whatever. The vessel can be used for caustic alkalies, baryta, stannous chloride, permanganate, and sulphurous acids, or any other liquid liable to undergo change by absorbing oxygen. The corks are dried and soaked in melted paraffine ; or, still better, may be substituted by caoutchouc stoppers ; and a thin layer of rectified paraffin oil is poured on the top of the solution, where, of course, owing to its low specific gravity, it always floats, placing an impermeable division between the air and the solution ; and as this body (which should always be as pure as possible) is not affected by these reagents in their diluted state, this form offers great advantages. Solutions not affected chemically by contact with air should nevertheless be kept in bottles, the corks or stoppers of which are perfectly closed, and tied over with india-rubber or 28 VOLUMETRIC ANALYSIS. 11. bladder to prevent evaporation, and should further be always shaken before use, in case they are not quite full. The influence of bright light upon some solutions is very detrimental to their Fig. 21. chemical stability ; hence it is advisable to preserve some solutions not in immediate use in the dark, and at a temperature not exceeding 15 or 16 C. ON THE DIRECT AND INDIRECT PROCESSES OF ANALYSIS AND THEIR TERMINATION. 11. THE direct method includes all those analyses where the substance under examination is decomposed by simple contact with a known quantity or equivalent proportion of some other body capable of combining with it, and where the end of the decomposition is manifested in the solution itself. It also properly includes those analyses in which the substance reacts upon another body to the expulsion of a representative equivalent of the latter, which is then estimated as a substitute for the thing required. 11. DIRECT AND INDIRECT METHODS. 29 Examples of this method are readily found in the process for the determination of iron by potassic permanganate, where the beautiful rose colour of the permanganate asserts itself as the end of the reaction. The testing of acids and alkalies conies, also, under this class, the great sensitiveness of litmus, or other indicators, allowing the most trifling excess of acid or alkali to alter their colour. The indirect method is exemplified in the analysis of manganese ores, and also other peroxides and oxygen acids, by boiling with hydrochloric acid. The chlorine evolved is estimated as the equivalent of the quantity of oxygen which has displaced it. We are indebted to Buns en for a most accurate and valuable series of processes based on this principle. The residual method is such that the substance to be analyzed is not estimated itself, but the excess of some other body added for the purpose of combining with it or of decomposing it ; and the quantity or strength of the body added being known, and the con ditions under which it enters into combination being also known, by deducting the remainder or excess (which exists free) from the original quantity, it gives at once the proportional quantity of the substance sought. An example will make the principle obvious : Suppose that a sample of native calcic or baric carbonate is to be tested. It is not possible to estimate it with standard nitric or hydrochloric acid in the exact quantity it requires for decomposition. There must be an excess of acid and heat applied also to get it into solution ; if, therefore, a known excessive quantity of standard acid be first added, and solution obtained, and the liquid then titrated backward with an indicator and standard alkali, the quantity of free acid can be exactly determined, and consequently that which is combined also. In some analyses it is necessary to add a substance which shall be an indicator of the end of the process ; such, for instance, is litmus or the azo colours in alkalimetry, potassic chromate in silver and chlorine, and starch in iodine estimations. There are other processes, the end of which can only be determined by an indicator separate from the solution ; such is the case in the estimation of iron by potassic bichromate, where a drop of the liquid is brought into contact with another drop of solution of red potassic prussiate on a white slab or plate ; when a blue colour ceases to form by contact of the two liquids, the end of the process is reached. 30 VOLUMETRIC ANALYSIS. 12. PART II. ANALYSIS BY SATURATION. ALKALIMETRY. 12. GAY LUSSAC based his system of alkalimetry upon -a titrated solution of sodie carbonate, Avith a corresponding solution of sulphuric acid. It possesses the recommendation, that a pure standard solution of sodic carbonate can be more readily obtained than any other form of alkali. Mohr introduced the use of caustic alkali instead of a carbonate, the strength of which is established by a standard solution of oxalic or sulphuric acid. The advantage in the latter system is, that in titrating acids with a caustic alkali, the well-known interference produced in litmus by carbonic acid is avoided : this difficulty is now overcome with carbonates by the new indicators to be described. INDICATORS USED IN ALKALIMETRY. 1 3. 1 . Litmus Solution. This well- known, indicator is best prepared as follows : Extract all soluble matters from the solid litmus by repeated quantities of hot water; evaporate the mixed extracts to a moderate bulk, and add acetic acid in slight excess to de- compose carbonates ; evaporate to a thick extract, transfer this to a beaker, and add a large proportion of hot 85- per-cent. alcohol or methylated spirit : by this treatment the blue colour is precipitated, and the alkaline acetates, together with some red colouring matter, remain dissolved ; the fluid with pre- cipitate is thrown on a filter, washed with hot spirit, and the pure colouring Fig. 22. matter finally dissolved in warm distilled water, through the filter, for use."* Another method gives very good results. The crushed litmus is extracted with boiling methylated spirit, three or four times, to * It is preferable to keep the pasty extract, still wet with spirit, in a bottle, and dissolve a little in warm water when required, adjusting the tint with very dilute hydrochloric acid or soda as the case may be. It has been recommended by some to dissolve the extract in glycerol,as a preservative, but the use of any foreign matter for this purpose, such as alcohol, boric or salicylic acid, &c., ought to be avoided. 13. INDICATORS. 31 remove the red colouring matter, the residue is digested for some time with cold water, allowed to settle, the clear liquid decanted, acidified with sulphuric acid and boiled to expel CO 2 . It is then cautiously neutralized with baryta water, and either filtered or allowed to settle clear for use. Unless a solution of litmus has been sterilized by mercuric chloride, or some such agent, it speedily loses its colour when kept in closed bottles, becomes offensive in smell, and a rapid fungoid growth occurs, due to a micrococcus. If kept in a vessel open to the air, such as is shown in fig. 22, the tendency to this change is much lessened, and whenever the colour may have been lost, it can be restored by exposing the solution to the air in an open dish. Free carbonic acid interferes considerably with the production of the blue colour, and its interference in titrating acid solutions with alkaline carbonates can only be got rid of by boiling the liquid during the operation, in order to displace the gas from the solution. If this is not done, it is easy to overstep the exact point of neutrality in endeavouring to produce the blue colour. The same difficulty is also found in obtaining the pink-red when acids are used for titrating alkaline carbonates, hence the great value of the caustic alkaline solutions free from carbonic acid when this indicator is used. It sometimes occurs that titration by litmus is required at night. Ordinary gas or lamp light is not adapted for showing the reaction in a satisfactory manner; but a very sharp line of demarcation between red and blue may be found by using a monochromatic light. With the yellow sodium flanue the red colour appears perfectly colourless, while the blue or violet appears like a mixture of black ink and water. The transition is very sudden, and even sharper than the change by daylight. The operation should be conducted in a perfectly dark room ; and the flame may be best obtained by heating a piece of platinum coil sprinkled with salt, or a piece of pumice saturated with a concentrated solution of salt, in the Bunsen flame. 2. Litmus Paper. Is simply made by dipping strips of calendered unsized paper in the solution and drying them ; the solution used being rendered blue, red, or violet as may be required. 3. Cochineal Solution. This indicator possesses the advantage over litmus, that it is not so much modified in colour by the presence of carbonic acid, and can be used by gas-light. It can also IDC used with the best effect with solutions of the alkaline earths, such as lime and baryta water ; the colour with pure alkalies and earths is especially sharp and brilliant. The solution is made by digesting 1 part of crashed cochineal with 10 parts of 25-per-cent. alcohol. Its natural colour is yelloAvish-red, which is turned to violet by. alkalies; mineral acids restore the original colour; it is not so 32 VOLUMETRIC ANALYSIS. 18. easily affected by weak organic acids as litmus, and therefore for these acids the latter is preferable. It cannot be used in the pre- sence of even traces of iron or alumina compounds or acetates, which fact greatly limits its use. 4. Turmeric Paper. Pettenkofer, in his estimation of car- bonic acid by baryta water, prefers turmeric paper as an indicator. For this purpose it is best prepared by digesting pieces of the root, first in repeated small quantities of water to remove a portion of objectionable colouring matter, then in alcohol, and dipping strips of calendered unsized paper into the alcoholic solution, drying and preserving them in the dark. Thompson in continuance of his valuable studies on various indicators found that turmeric paper is of very little use for ammonia, or the alkaline carbonates, or sulphides and sulphites, but he prepared a special paper of a light red-brown colour, by dipping it into the alcoholic tincture of turmeric rendered slightly alkaline by caustic soda. If this paper is wetted with water the eolour is intensified to a dark red-brown ; when partly immersed in a very dilute solution of an acid, the wetted portion becomes bright yellow, while immediately above this a moistened dark red-brown band is formed, and the upper dry portion retains its original colour. This appearance only occurs in the titration of a com- paratively large proportion of an acid, when the latter is nearly all neutralized, and thus serves to indicate the near approach to the end-reaction. When neutral or alkaline, the colour of the immersed portion of paper is simply intensified as already described. This intensification is quite as decided as a change of tint. This red- brown paper is equally as sensitive as phenolphthalein for the titration of citric, acetic, tartaric, oxalic and other organic acids by standard soda or potash, and can be used for highly coloured solutions. It is also available, like phenolphthalein, for the estimation of small quantities of acid in strong alcohol. Indicators derived from the Azo Colours, etc. An immense stride has been taken in the application of these modern indicators, and the best thanks of all chemists are due to E. T. Thompson for his valuable researches on them, read before the Chemical Section of the Philosophical Society of Glasgow, and published in their Transactions; also reprinted (C. N. xlvii. 123, 185; xlix. 32, 119; J. S. C. I. vi. 195). The experiments recorded in these papers are most carefully carried out, and the truthfulness of their results has been verified by Lunge and other practical men as well as by myself. Space will only permit here of a record of the results, fuller details being given in the publications to which reference has been made. 13. INDICATORS. 33 5. Methyl Orange, or para-sulpho-benzene azo-dimethylaniline, is prepared by the action of diazotized sulphanilic acid upon dimethylaniline, the commercial product being an ammonium or sodium salt of the sulphonic acid thus produced. If carefully prepared from the purest materials it possesses a bright orange colour, perfectly soluble in water ; but the commercial product is a powder possessing a dull orange-brown appearance, probably due to slight impurities in the substances from which it is produced, and often not completely soluble in water. Complaints have been made by some operators that the commercial article is sometimes unreliable as an indicator; it may be so, but although I have examined many specimens, I have not yet found any in which the impurities sensibly affected its delicate action when used in the proper manner. The common error is the use of too much of it; again, there is the personal error of observation, some eyes being much more sensitive to the change of tint than others. The great value of this indicator is that, unlike litmus and some other agents, it is totally unaffected by carbonic acid, sulphuretted hydrogen, hydrocyanic, silicic, boric, arsenious, oleic, stearic, palmitic, and carbolic acids, etc. It must not be used for the organic acids, such as oxalic, acetic, citric, tartaric, etc., since the end-reaction is indefinite ; nor can it be used in the presence of nitrous acid or nitrites, which decompose it. It may safely be used for the estimation of free mineral acids in alum, ferrous sulphate or chloride, zinc sulphate, cupric sulphate or chloride. The acid radical (and consequently its equivalent metal) in cupric sulphate and similar salbs may be estimated with accuracy by precipitating the solution with sulphuretted hydrogen, filtering, and titrating the filtrate at once with normal alkali and methyl orange. The earthy carbonates dissolved in natural waters (and which constitute the temporary hardness) may immediately be estimated by simple titration with ~ mineral acid and this indicator. Nothing can exceed the value of methyl orange for the accurate standardizing of any of the mineral acids by means of pure sodic carbonate in the cold, the liberated carbonic acid having practically no effect, as is the case with many indicators. Its effect is also admirable with ammonia or its salts. A convenient strength for the indicator is 1 gram of the powder in a liter of distilled water ; a single drop of the liquid is sufficient for 100 c.c. or more of any colourless solution the colour being faint yellow if alkaline, and pink if acid ; if too much is used the end-reaction is slower and less definite. All titrations with methyl orange should be carried on at ordinary temperatures. There are two other azo-compounds in use, more especially by continental chemists, which possess the same properties and give much the same effect as methyl orange, namely, Fischer's dimethylamido-azobenzene and trop03olin 00. My experience is 34 VOLUMETEIC ANALYSIS. 13. that these preparations are less sensitive than methyl orange, and wherever they are recommended for any process of titration the latter may be substituted with advantage. 6. Phenacetolin. This substance is prepared by boiling together for several hours equal molecular proportions of phenol, acetic anhydride, and sulphuric acid. The product is well washed with water to remove excess of acid and dried for use ; it is soluble only in alcohol, and a convenient strength is 2 gm. per liter. The solution is dark brown, which gives a scarcely perceptible yellow Avith caustic soda or potash, when a few drops are used with the ordinary volumes of liquid. With ammonia and the normal alkaline carbonates it gives a dark pink, with bicarbonate a much more intense pink, and with mineral acids a golden yellow. This indicator may be used to estimate the amount of caustic potash or soda in the presence of their normal carbonates if the proportion of the former is not very small, or of caustic lime in the presence of carbonate. Practice however is required with solutions of known composition, so as to acquire knowledge of the exact shades of colour. 7. Phenolphthalein. This substance is obtained by heating together at 120 C., for ten or twelve hours, five parts of phthalic anhydride, ten of phenol, and four of sulphuric acid ; the product is boiled with water, and the residue dissolved in dilute soda and filtered. The filtrate contains the phenolphthalein, which may be precipitated by neutralizing with acetic and hydrochloric acids, and purified by solution in alcohol, boiling with animal charcoal and re-precipitating with boiling water ; it is of a resinous nature, but quite soluble in 50-per-cent. alcohol. A convenient strength is 5 gm. per liter. A few drops of the indicator show no colour in the ordinary volumes of neutral or acid liquids ; the faintest excess of caustic alkalies, on the other hand, gives a sudden change to purple-red. This indicator is useless for the titration of free ammonia, or its compounds, or for the fixed alkalies when salts of ammonia are present.* It is also very easily affected by CO 2 and SH 2 . It may however be used like phenacetolin for estimating the proportions of hydrate and carbonate of soda or potasli in the same sample where the proportion of hydrate is not too small. Unlike methyl orange, this indicator is especially useful in titrating all varieties of organic acids; viz., oxalic, acetic, citric, tartaric, etc. One great advantage possessed by phenolphthalein is, that it may be used in alcoholic solutions, or mixtures of alcohol and * Long (Artier. Chem. Jour, xi, 84) states, and my own experiments support him, that this disadvantage may he remedied in many cases hy using a large proportion of the indicator and a low temperature. 13. INDICATOKS. 35 ether,* and therefore many organic acids insoluble in water may be accurately titrated by its help ; in addition to this it may be used to estimate the acid combined with many organic bases, such as morphia, quinia, brucia, etc., the base having no effect on the indicator. 8. Rosolic Acid, also known as Aurin or Corallin, is soluble in 50-per-cent alcohol, and a convenient strength is 2 gm. per liter. Its colour is pale yellow, unaffected by acids, but turning to violet- red with alkalies. It possesses the advantage over litmus and the other indicators, that it can be relied upon for the neutralization of .sulphurous acid with ammonium to normal sulphite (Thompson). Its delicacy is sensibly affected by salts of ammonia and by carbonic acid. It is excellent for all the mineral, but not reliable for the organic acids, excepting oxalic. 9. Lacmoid. This indicator is a product 'of resorcin, and is therefore somewhat allied to litmus ; nevertheless, it differs from it in many respects, and has a pronounced and valuable character of its own, especially when used in the form of paper. It may be prepared by heating gradually to 110 C. a mixture of 100 parts resorcin, five parts sodic nitrite, and five parts water ; after the violent reaction moderates, it is heated to 120 C. until evolution of ammonia ceases. The residue is dissolved in warm water, and the lacmoid precipitated therefrom by hydrochloric acid; it is well washed from free acid and dried for use. Lacmoid is soluble in dilute alcohol, and the indicator is best made by dissolving 2 gm. to the liter. 10. Lacmoid Paper. This is prepared by dipping slips of calendered unsized paper into the blue or red solution, and drying them. Thompson states that, in nearly every particular, lacmoid paper, either blue or red, is an excellent substitute for methyl orange, and may be employed in titrating coloured solutions where the latter would be useless. Solution of lacmoid, on the other hand, is not so valuable as the paper, inasmuch as it is more easily affected by weak acids such as carbonic, boric, etc. There are a host of other indicators belonging to the same category as those mentioned above, such as Congo red, Porrier's blue, nuorescin, etc. ; but as they have no special advantages over them, and indeed are practically inferior in delicacy, no description of them will be given here. * H. N. and C. Draper (C. N. lv. 143) have shown that this indicator is rapidly decomposed by atmospheric carbonic acid, which is more readily absorbed by alcohol than by water. Fortunately thig is less the case with hot solutions than with cold ; titrations of this kind should therefore be quickly done, and with not too small a quantity of the indicator. D 2 36 VOLUMETRIC ANALYSIS. 13. Special indicators for certain purposes, such as potassic chromate for silver, ferric sulphate for sulphocyanides, etc., will be described in their proper place. SHORT SUMMARY OF THOMPSON'S RESULTS WITH INDICATORS AND PURE SALTS OF THE ALKALIES AND ALKALINE EARTHS. The whole of the base or acid in the following list of substances may be estimated with delicacy and precision unless otherwise mentioned. Litmus Cold. Hydrates of soda, potash, ammonia, lime, baryta, etc. ; arsenites of soda and potash, and silicates of the same bases ; nitric, sulphuric, hydrochloric, and oxalic acids. Litmus Boiling. The neutral and acid carbonates of potash, soda, lime, baryta, and magnesia, the sulphides of sodium and potassium, and the silicates of the same bases. Methyl Orange Cold. The hydrates, carbonates, bicarbonates, sulphides, arsenites, silicates, ancl borates of soda, potash, ammonia, lime, magnesia, baryta, etc., all the mineral acids, sulphites, half the base in the alkaline and earthy alkaline phosphates and arseniates. Rosolic Acid Cold. The whole of the base or acid may be estimated in the hydrates of potash, soda, ammonia, and arsenites of the same ; the mineral and oxalic acids. Rosolic Acid Boiling. The alkaline and earthy hydrates and carbonates, bicarbonates, sulphides, arsenites, and silicates. Phenacetolin Cold. The hydrates, arsenites, and silicates of the alkalies ; the mineral acids. Phenacetolin Boiling. The alkaline and earthy hydrates, car- bonates, bicarbonates, sulphides, arsenites, and silicates. Phenolphthalein Cold. The alkaline hydrates, except ammonia ; the mineral acids, oxalic, citric, tartaric, acetic, and other organic acids. Phenolphthalein Boiling. The alkaline and earthy hydrates, carbonates, bicarbonates, and sulphides, always excepting ammonia and its salts. Lacmoid Cold. The alkaline and earthy hydrates, arsenites and borates, and the mineral acids. Many salts of the metals which are 13. INDICATORS. 37 more or less acid to litmus are neutral to lacmoid, sucli as the sulphates and chlorides of iron, copper, and zinc ; therefore this indicator serves for estimating free acids in such solutions. Lacmoid Boiling-. The hydrates, carbonates, and bicarbonates of potash, soda, and alkaline earths. Lacmoid Paper. The alkaline and earthy hydrates, carbonates, bicarbonates, sulphides, arsenites, silicates, and borates ; the mineral acids ; half of the base in sulphites, phosphates, arseniates. This indicator re-acts alkaline with the chromates of potash or soda, but neutral with the bichromates, so that a mixture of the two, or of bichromates with free chromic acid, may be titrated by its aid, which could also be done with methyl orange were it not for the colour of the solutions. The following substances can be determined by standard alcoholic potash, with phenolphthalein. as indicator. One c.c. normal caustic potash (1 c.c. = '056 gm. KHO) is equal to (Hehner and Allen) 088 gn 282 , 256 , 284 , 410 , 329 , i. butyric acid. '1007 gm. tributyrin. oleic acid. '2947 triolein. palmitic acid. '2687 tripalmitin. stearic acid. '2967 ,, tristearin. cerotic acid. '6760 myricin. resin acids (ordinary colophony, chiefly sylvic acid). General Characteristics of the Foregoing 1 Indicators. It is interesting to notice the different degrees of sensitiveness shown by indicators used in testing acids and alkalies. This is well illustrated by Thompson's experiments, where he used solutions of the indicator containing a known weight of the solid material, and so adjusted as to give, as near as could be judged, the same intensity of colour in the reaction. It was found that lacmoid, rosolic acid, phenacetolin, and phenolphthalein were capable of showing the change of colour with one-fifth of the quantity of acid or alkali which was required in the case of methyl orange or litmus ; that is to say, in 100 c.c. of liquid, where the latter took 0*5 c.c., the same effect with the former was gained by O'l c.c. Another important distinction is shown in their respective behaviour with mineral and organic acids. It is true the whole of them are alike serviceable for the mineral acids and fixed alkalies ; but they differ considerably in the case of the organic acids and ammonia. Methyl orange and lacmoid appear to be most sensitive to alkalies, while phenolphthalein is most sensitive to acids; the others appear to 38 VOLUMETKIC ANALYSIS. 13. occupy a position between these extremes, each showing, however, special peculiarities. The distinction, however, is so marked, that, as Thompson says, it is possible to have a liquid which may be acid to phenolphthalein and alkaline to lacmoid. The presence of certain neutral salts has, too, a definite effect on the sensitiveness of certain indicators. Sulphates, nitrates, chlorides, etc., retard the action of methyl orange slightly, while in the case of phenacetolin and phenolphthalein they have no effect. On the other hand, neutral salts of ammonia have such a disturbing influence on the latter as to render it useless, unless with special precautions. Nitrous acid alters the composition of methyl orange ; so also do nitrites when existing in any quantity. Forbes Carpenter has noted this effect in testing the exit gases of vitriol chambers (/. S. C. I. v. 287). Sulphites of the fixed alkalies and alkaline earths are practically neutral to phenolphthalein, but alkaline to litmus, methyl orange, and phenacetolin. Sulphides, again, can be accurately titrated with methyl orange in the cold, and on boiling off the H 2 S a tolerably accurate result can be obtained with litmus and phenacetolin, but with phenolphthalein the neutral point occurs when half the alkali is saturated. The phosphates of the alkalies, arseniates, and arsenites, also vary in their effects on the various indicators. Again, boric acid and the borates can be very accurately titrated by help of methyl orange or lacmoid paper, but the other indicators are practically useless. Thompson classifies the usual neutrality indicators into three groups. The methyl orange group, comprising that substance, together with lacmoid, dimethylamidobenzene, cochineal and Congo red; the phenolphthalein group, consisting of itself and turmeric; the litmus group, including litmus, rosolic acid, and phenacetolin. The methyl orange group are most susceptible to alkalies, the phen- olphthalein to acids, and the litmus somewhat between the two. This classification has nothing to do with delicacy of reaction, but with the special behaviour of the indicator under the same circum- stances ; for instance, saliva, which is generally neutral to litmus paper, is always strongly alkaline to lacmoid or Congo red, and acid to turmeric paper. Fresh milk reacts very much in the same way. ]STo absolutely hard and fast line can however be drawn. Thompson gives the following table as an epitome of the results obtained with indicators, and on which several processes have been based. The figures refer to the number of atoms of hydrogen displaced by the monatomic metals, sodium or potassium, in the form of hydrates. Where a blank is left it is meant that the end- reaction is obscure. The figures apply also to ammonia, except where phenolphthalein is concerned, and when boiling solutions are used. Calcic and baric hydrates also give similar results, except INDICATORS. 39 where insoluble compounds are produced. Lacmoid paper acts in every respect like methyl orange, except that it is not affected by nitrous acid or its compounds. Turmeric paper behaves exactly like phenolphthalein with the mineral acids and also with thio- sulphuric and organic acids. Acids. Methyl Orange. Phenolphthalein. Litmus. Name. Formula. Cold. Cold. Boiling, Cold. Boiling. Sulphuric . . H 2 SO 4 2 2 2 2 2 Hydrochloric . HC1 I 11 1 1 Nitric . . . HNO 3 1 1 1 1 1 Thiosulphuric . H 2 S 2 O 3 2 2 2 2 2 Carbonic . . H 2 C0 3 1 dilute Sulphurous H 2 S0 3 1 2 Hydrosulphurio Phosphoric H 2 S H 3 P0 4 1 1 dilute 2 Arsenic . H 3 AsO 4 1 2 Arsenious H 3 AsO 3 Nitrous . HNO 2 indicator destroyed 1 1 Silicic . . H 4 SiO 4 Boric . . H 3 B0 3 . . Chromic H 2 CrO 4 1 2 2 Oxalic . H 2 C 2 4 2 2 2 2 Acetic . HC 2 H 3 O 2 1 . 1 nearly Butyric . . HC 4 H7O 2 1 1 nearly Succinic H 2 C 4 H 4 O 4 2 2 nearly Lactic . . HC 3 H 5 :J 1 1 Tartaric . H 2 C 4 H 4 O f 2 2 Citric . H 3 C 6 H 5 O 7 1 3 Allen (Pharm. Jour., May llth, 1889) clearly points out that the acid which enters into the composition of an indicator must be weaker than the acid which it is required to estimate by its means. The acid of which methyl orange is a salt is a tolerably strong one, since it is only completely displaced by the mineral acids; the organic acids are not strong enough to overpower it completely, hence the uncertainty of the end-reaction. The still weaker acids, such as carbonic, hydrocyanic, boric, oleic, etc., do not decompose the indicator at all, hence their salts may be titrated by it, just as if the bases only were present. On the other hand the acid of phenolphthalein is extremely weak, hence its salts are easily decomposed by the organic and carbonic acids. A combination of the two indicators is frequently of service ; say, for instance, in a mixture of normal and acid sodic carbonate, if first titrated with 'phenolphthalein and standard mineral acid, the rose colour dis- appears exactly at the point when the normal carbonate is saturated, the bicarbonate can then be found by continuing the operation with methyl orange. The study of these new indicators is still imperfect, and requires further elucidation ; meantime there can be 40 VOLUMETEIC ANALYSIS. 14. no question that the use of such as have been described is an immense advance upon the old-fashioned litmus. PREPARATION OF THE NORMAL ACID AND ALKALINE SOLUTIONS. 14. IT is quite possible to carry out the titration of acids and alkalies with only one standard liquid of each kind ; but it frequently happens that standard acids or alkalies are required in other processes of titration beside mere saturation, and it is therefore advisable to have a variety. Above all things it is absolutely necessary to have, at least, one standard acid and alkali prepared with the most scrupulous accuracy, to use as foundations for all others. I prefer sulphuric acid for the normal acid solution, inasmuch as there is no difficulty in getting the purest acid in commerce. The normal acid made with it is totally unaffected by boiling, even when of full strength, which cannot be said of either nitric or hydrochloric acid. Hydrochloric acid is however generally pre- ferred by alkali makers, owing to its giving soluble compounds with lime and similar bases. Nitric and oxalic acids are also sometimes convenient. Sodic carbonate, on the other hand, is to be preferred for the standard alkali, because it can readily be obtained in a perfectly pure state, or can be easily made by heating pure bicarbonate to a temperature of 150 or 200 C. for some hours in an air bath, or gently igniting over the gas for ten or fifteen minutes. The chief difficulty hitherto with sodic carbonate has been, that with litmus as indicator, the titration must be carried on at a boiling heat in order to get rid of carbonic acid, which hinders the pure blue colour of the indicator, notwithstanding the alkali may be in great excess. This difficulty is now set aside by the use of methyl orange. In case the operator has not this indicator at hand, litmus gives perfectly accurate results, if the saturation is conducted by rapidly boiling the liquid in a thin flask for a minute after each addition of acid until the point is reached when one drop of acid in excess gives a pink-red colour, which is not altered by further boiling. As has been previously said, these two standards must be pre- pared with the utmost care, since upon their correct preparation and preservation depends the verification of other standard solutions. It may, however, be remarked, that in place of a standard solution of sodic carbonate, which is of limited use for general purposes, the pure anhydrous salt may be used for the rigid adjustment of normal acid. In this case about 2 or 3 grams of pure ISVCO 3 are freshly heated to dull redness for a few minutes in a weighed 14. NORMAL SOLUTIONS. 41 platinum crucible, cooled under an exsiccator, the exact weight taken, then transferred to a flask by means of a funnel, through which it is washed and dissolved with distilled water, methyl orange added, and the operation completed by running the acid of unknown strength from a burette divided into ~ c.c. into the soda solution in small quantities until exact saturation occurs. A second portion of sodic carbonate should now be weighed as before, but not of necessity exactly the same quantity. Its exact weight must be noted ; the saturation is carried out precisely as at first. The data for ascertaining the exact strength of the acid solu- tion by calculation are now in hand. A strictly normal acid should at 16 C. exactly saturate sodic carbonate in the proportion of 100 c.c. to 5*3 gm. Suppose that 2*46 gm. sodic carbonate required 41 '5 c.c. of the acid in the first experiment, then 2-46 : 5-3 : : 41'5 : x = 89'4 c.c. Again: 2'153 gm. sodic carbonate required 36*32 c.c. of acid, then 2-153 : 5-3 : : 36'32 : x = 89'4 c.c. The acid may now be adjusted by measuring 890 c.c. into the graduated liter cylinder, adding 4 c.c. from the burette, or with a small pipette, and filling to the liter mark with distilled water. Finally, the strength of the acid so prepared must be proved by taking a fresh quantity of sodic carbonate, or by titration with a normal sodic carbonate solution previously made by direct weighing of 53 gm. to the liter, and using not less than 50 c.c. for the' titration, so as to avoid as much as possible the personal errors of measurement in small quantities. If the measuring instruments all agree, and the operations are all conducted with due care, a drop or two in excess of either acid or alkali in 50 c.c. should suffice to reverse the colour of the indicator. 1. Normal Sodic Carbonate. 53 gm. :N T a 2 C0 3 per liter. This solution is made by dissolving 53 gm. of pure sodic monocarbonate, previously gently ignited and cooled, in distilled water, and diluting to 1 liter at 16 C. If the pure salt is not at hand, the solution may be made as follows : About 85 gm. of pure sodic bicarbonate, free from thiosulphate, are heated to dull redness (not to fusion) in a platinum crucible, for fully ten minutes, to expel one-half of the carbonic acid, then placed under an exsiccator to cool ; when placed upon the balance it will be found that very little more than 53 gm. remains. The excess is removed as quickly as possible, and the contents of the crucible washed into a beaker, and as soon as the salt is dissolved 42 VOLUMETKIC ANALYSIS. 14?. the solution is decanted into a liter flask and filled up to the mark with distilled water at 16 C. 2. Normal Potassic Carbonate. 69 gin. K 2 C0 3 per liter. This solution is sometimes, though rarely, preferable to the soda salt, and is of service for the estimation of combined acids in certain cases, where, by boiling the compound with this agent, an inter- change of acid and base occurs. It cannot be prepared by direct weighing of the potassic carbonate, and is therefore best established by titrating a solution of unknown strength with strictly normal acid. 3. Normal Sulphuric Acid. 49 gm. H 2 SO* per liter. About 30 c.c. of pure sulphuric acid of sp. gr. 1*840, or there- abouts, are mixed with three or four times the volume of distilled water and allowed to cool, then put into the graduated cylinder and diluted up to the liter. The solution may now be roughly tested by normal alkali, which is best done by putting 20 c.c. into a small beaker or flask with methyl orange, and allowing the acid to flow from a burette, divided into -^ c.c., until the point of neutrality is reached. If more than 20 c.c. are required, the acid is too weak ; if less, too strong. If the acid from which the solution was made was of the sp. gr. mentioned, it will generally be too strong, which is preferable. The final adjustment with sodic carbonate may now be made as before described. The solution may also be controlled by precipitation with baric chloride, in which case 10 c.c. should produce as much baric sulphate as is equal to 0'49 gm. of sulphuric acid, or 49 gm. per liter". 4. Normal Oxalic Acid. 63 gm. C 2 4 H 2 ,2H 2 0, or 45 gm. C 2 4 H 2 per liter. This solution cannot very well be established by direct weighing, owing to uncertain hydration ; hence it must be titrated by normal alkali of known accuracy. The solution is apt to deposit some of the acid at low tempera- tures, but keeps well if preserved from direct sunlight, and will bear heating without volatilizing the acid. Very dilute solutions of oxalic acid are very unstable; therefore, if a decinornial or centinormal solution is at any time required, it should be made when wanted. Some operators prefer potassic tetra-oxalate to oxalic acid, because the actual strength of the normal solution may at any time be verified by evaporating a measured volume to 14. NORMAL SOLUTIONS. 43 dryness, igniting and weighing the resulting potassic carbonate ; as this salt is, however, extremely hygroscopic, an error may easily occur. 5. Normal Hydrochloric Acid. 36-37 gm. HC1 per liter. It has been shown by Roscoe and Dittmar (/. C. S. xii. 128, 1860) that a solution of hydrochloric acid containing 20 "2 per cent, of the gas when boiled at about 760 m.m. pressure, loses acid and water in the same proportion, and the residue will therefore have the constant composition of 20 '2 per cent., or a specific gravity of I'lO. About 181 gm. of acid of this gravity, diluted to one liter, serves very well to form an approximate normal acid. The actual strength may be determined by precipitation with silver nitrate, or by titration with an exactly weighed quantity of pure sodic monocarbonate. Hydrochloric acid is useful on account of its forming soluble compounds with the alkaline earths, but it has the disadvantage of volatilizing at a boiling heat. Dittmar says that this may be prevented by adding a few grams of sodic sulphate. In many cases this would be inadmissible, for the same reason that sulphuric acid cannot be used. The hydrochloric acid from which standard solutions are made must be free from chlorine gas or metallic chlorides, and should leave no residue when evaporated in a platinum vessel. 6. Normal Nitric Acid. 63 gm. HNO 3 per liter. A rigidly exact normal acid should be established by sodic carbonate, as in the case of normal sulphuric and hydrochloric acids. The nitric acid used should be colourless, free from chlorine and nitrous acid, sp. gr. from 1'35 to 1*4. If coloured from the presence of nitrous or hyponitrous acids, it should be mixed with two volumes of water, and boiled until white. When cold it may be diluted and titrated as previously described. \ 7. Normal Caustic Soda or Potash. 40 gm. tfaHO or 56 gm. KHO per liter. Pure caustic soda made from metallic sodium may now be readily obtained in commerce, and hence it is easy to prepare a standard solution of exceeding purity, by simply dissolving the substance in distilled water till of about T05 sp. gr., or about 50 gm. to the liter, roughly estimating its strength by normal acid and methyl orange ; then finally adjusting the exact strength by titrating 50 c.c. with normal acid. However pure caustic soda or potash may otherwise be, they are both in danger of absorbing carbonic acid, and hence in using 44 VOLUMETRIC ANALYSIS. 14. litmus the titratioii must be conducted with boiling. Methyl orange permits the use of these solutions at ordinary temperature notwithstanding the presence of CO 2 . Soda and potash may both be obtained in commerce sufficiently pure for all ordinary titration purposes, but in case they are not at hand the requisite solutions may be prepared as follows : Two parts of pure sodic or potassic carbonate are to be dissolved in twenty parts of distilled water, and boiled in a clean iron pot ; during the boiling, one part of fresh quick-lime, made into a cream with water, is to be added little by little, and the whole boiled until all the carbonic acid is removed, which may be known by the clear solution producing 110 effervescence on the addition of dilute acid ; the vessel is covered closely, and set aside to cool and settle ; when cold, the clear supernatant liquid should be poured or drawn off and titrated by normal acid, and made of the proper strength as directed for sulphuric acid. Soda solutions may be freed from traces of chlorine, sulphuric, silicic, and carbonic acids, by shaking with Millon's base, trimercur-ammomum (C, N. xlii. 8). In preparing these alkaline solutions, they should be exposed as little as possible to the air, and when the strength is finally settled, should be preserved in one of the bottles shown in fig. 19 or 20. 8. Semi-normal Ammonia. 8-5 gm. NH 3 per liter For some years past I have used this strength of standard ammonia for saturation analyses, and have been fully satisfied with its behaviour ; it is cleanly, does not readily absorb carbonic acid, holds its strength well for two or three months when kept in a cool place and well stoppered ; and can at any time be prepared in a few minutes, by simply diluting strong solution of ammonia with fresh distilled water. A normal solution cannot be used with safety, owing to evapora- tion of the gas at ordinary temperatures. It is necessary to add that, even in the case of strength, the solution should be titrated from time to time against correct normal acid. ^ ammonia keeps its strength for a long time in well-closed bottles. 9. Standard Caustic Baryta (Pettenkofer). Especially serviceable for free carbonic acid and coloured acid liquids, such as commercial vineg-ars, etc. The solution of caustic baryta is best made from the crystallized hydrate. It is not advisable to have the solution too concentrated, since, when it is near complete saturation it is apt to deposit the 14. NORMAL SOLUTIONS. 45 hydrate at low temperature. The corresponding acid may be either Y^- oxalic, nitric, or hydrochloric. Oxalic acid is recommended by Pettenkofer for carbonic. acid estimation, because it has no effect upon the baric carbonate suspended in weak solutions ; but there is the serious drawback in oxalic acid, that in dilute solution it is liable to rapid decomposition ; and as in my experience -j-j- hydro- chloric acid in dilute mixtures has no effect upon the suspended baric carbonate, it is preferable to use this acid. The baryta solution is subject to constant change by absorption of carbonic acid, but this may be prevented to a great extent by preserving it in the bottle shown in fig. 20. A thin layer of petroleum oil on the surface of the liquid preserves the baryta at one strength for a long period. The reaction between baryta and yellow turmeric paper is very delicate, so that the merest trace of baryta in excess gives a decided brown tinge to the edge of the spot made by a glass rod on the turmeric paper. If the substance to be titrated is not too highly coloured, litmus may be used as an approximate indicator in the mixture ; this enables the operator to find the exact point of saturation more conveniently. 10. Normal Ammonio-Cupric Solution for Acetic Acid and free Acids and Bases in Earthy and Metallic Solutions. This acidimetric solution is prepared by dissolving pure cupric sulphate in warm water, and adding to the clear solution liquid ammonia, until the bluish-green precipitate which first appears is nearly dissolved ; the solution is then filtered into the graduated cylinder, and titrated by allowing it to flow from a pipette graduated in i or y'p- c.c. into 10 or 20 c.c. of normal sulphuric or nitric acid (not oxalic). While the acid remains in excess, the bluish-green precipitate which occurs as the drop falls into the acid rapidly disappears ; but so soon as the exact point of saturation occurs, the previously clear solution is rendered turbid by the precipitate remaining insoluble in the neutral liquid. The process is especially serviceable for the estimation of the free acid existing in certain metallic solutions, i.e. mother-liquors, etc., where the neutral compounds of such metals have an acid reaction on litmus such as the oxides of zinc, copper, and magnesia, and the protoxides of iron, manganese, cobalt, and nickel; it is also applicable to acetic and the mineral acids. If cupric nitrate be used for preparing the solution instead of sulphate, the presence of barium, or strontium, or metals precipitable by sulphuric acid is of no consequence. The solution is stand- ardized by normal nitric or sulphuric acid ; and as it slightly alters by keeping, a coefficient must be found from time to time by titrating with normal acid, by which to calculate the results systematically. Oxides or carbonates of magnesia, zinc, or other 46 VOLUMETRIC ANALYSIS. 15. admissible metals, are dissolved in excess of normal nitric acid, and titrated residually with the copper solution. Example : 1 gm. pure zinc oxide was dissolved in 27 c.c. normal acid, and 2"3 c.c. normal copper solution required to produce the precipitate = 24*7 c.c. acid; this multiplied by 0'0405, the coefficient for zinc oxide, = 1-000 gm. ESTIMATION OF THE CORRECT STRENGTH OF STANDARD SOLUTIONS NOT STRICTLY NORMAL OR SYSTEMATIC. 15. IN discussing the preparation of the foregoing standard solutions, it has been assumed that they shall be strictly and absolutely correct; that is to say, if the same measure be rilled lirst with any alkaline solution, then with an acid solution, and the two mixed together, a perfectly neutral solution shall result, so that a drop or two either way will upset the equilibrium. Where it is possible to weigh directly a pure dry substance, this approximation may be very closely reached. Sodic monocarbonate, for instance, admits of being thus accurately weighed. On the other hand, the caustic alkalies cannot be so weighed, nor can the liquid acids. An approximate quantity, therefore, of these substances must be taken, and the exact power of the solution found by experiment. In titrating such solutions it is exceedingly difficult to make them so exact in strength, that the precise quantity, to a drop or two, shall neutralize each other. In technical matters a near approxima- tion may be sufficient, but in scientific investigations it is of the greatest importance that the utmost accuracy should be obtained ; it is therefore advisable to ascertain the actual difference, and to mark it upon the vessels in which the solutions are kept, so that a slight calculation will give the exact result. Suppose, for instance, that a standard sulphuric acid is prepared, which does not rigidly agree with the normal sodic carbonate (not at all an uncommon occurrence, as it is exceedingly difficult to hit the precise point) ; in order to find out the exact difference it must be carefully titrated as in 14. Suppose the weight of sodic carbonate to be 1'9 gm., it is then dissolved and titrated with the standard acid, of which 36*1 c.c. are required to reach the exact neutral point. If the acid were rigidly exact it should require 35 '85 c.c. ; in order, therefore, to find the factor necessary to bring the quantity of acid used in the analysis to an equivalent quantity of normal strength, the number of c.c. actually used must be taken as the denominator, and the number which should have been used, had the acid been strictly normal, as the numerator, thus 35-85 - 0-993; FTHE COLLEGE OF 15. NOKMAL SOLUTIONS. 0'993 is therefore the factor by which it is necessary to multiply the number of c.c. of that particular acid used in any analysis in order to reduce it to normal strength, and should be marked upon the bottle in which it is kept. On the other hand, suppose that the acid is too strong, and that 35 "2 c.c. were required instead of 35*85, 35-85 35310184; 1-0184 is therefore the factor by which it is necessary to multiply the number of c.c. of that particular acid in order to bring it to the normal strength. This plan is much better than dodging about with additions of water or acid. Under all circumstances, it is safer to prove the strength of any standard solution by experiment, even though its constituent has been accurately weighed in the dry and pure state. Further, let us suppose that a solution of caustic soda is to be made by means of lime as described previously. After pouring off the clear liquid, water is added to the sediment to extract more alkaline solution ; by this means we may obtain two solutions, one of which is stronger than necessary, and the other weaker. Instead of mixing them in various proportions and repeatedly trying the strength, we may find, by two experiments and a calculation, the proportions of each necessary to give a normal solution, thus : The exact actual strength of each solution is first found, by separately running into 10 c.c. of normal acid as much of each alkaline solution as will exactly neutralize it. We have, then, in the case of the stronger solution, a number of c.c. required less than 10. Let us call this number V. In the weaker solution the number of c.c. is greater than 10, represented by v. A volume of the stronger solution = x will saturate 10 c.c. of normal acid as often as V is contained in x. A volume of the weaker solution = y will, in like manner, saturate ^ c.c. of normal acid ; both together saturate y + - and the volume of the saturated acid is precisely that of the two liquids, thus- 1() x 1Q _ + _-* = X + y m Whence IQ v x + 10 V y = V v x + V v y v x (10 - V) = V y(v - 10). And lastly, ^ T . _ AX as V (v - 10) ~j = v (10 - V) An example will render this clear. A solution of caustic soda was taken, of which 5 -8 c.c. were required to saturate 10 c.c. normal 48 VOLUMETRIC ANALYSIS. 15. acid ; of another solution, 12 '7 c.c. were required. The volumes of each necessary to form a normal solution were found as follows : 5-8 (12-7 -10)= 15-66 12-7(10 -5-8) -53-34 Therefore, if the solutions are mixed in the proportion of 15 -6 6 c.c. of the stronger with 53'34 c.c. of the weaker, a correct solution ought to result. The same principle of adjustment is, of course, applicable to standard solutions of every class. Again : suppose that a standard solution of sulphuric acid has been made, approximating as nearly as possible to the normal strength, and its exact value found by f precipitation with baric chloride, or a standard hydrochloric acid with silver nitrate, and such a solution has been calculated to require the coefficient 0'995 to convert it to normal strength, by the help of this solution, though not strictly normal, we may titrate an approximately normal alkaline solution thus : Two trials of the acid and alkaline solu- tions show that 50 c.c. alkali =48'5 c.c. acid, having a coefficient of 0*995 = 48*25 c.c. normal ; then, according to the equation, x 50 = 48*25 is the required coefficient for the alkali. = And here, in the case of the alkaline solution being sodic carbonate, we can bring it to exact normal strength by a calculation based on the equivalent weight of the salt, thus 1 : 0-965 : : 53 : 5M45. The difference between the two latter numbers is 1'855 gm., and this weight of pure sodic carbonate, added to one liter of the solution, will bring it to normal strength. 15. SYSTEMATIC TABLE. TABLE FOB THE SYSTEMATIC ANALYSIS OF ALKALIES, ALKALINE EARTHS AND ACIDS. Substance. Formula. Atomic Weight. Quantity to be weighed so that 1 c.c. Normal Solu- tion^ per cent. of substance. Normal Factor.* Soda Na 2 O 62 3'1 gm. 0'031 Sodic Hydrate . . NaHO 40 4'0 gm. 0*040 Sodic Carbonate . . Na 2 CO 3 106 5'3 gm. 0-053 Sodic Bicarbonate NaHCO 3 84 8'4 gm. 0-084 Potash K 2 94 4'7 gm. 0-047 Potassic Hydrate . . KHO 56 5'6 gm. 0-056 Potassic Carbonate . K 2 C0 3 138 6'9 gm. 0-069 Potassic Bicarbonate KHCO 3 100 lO'O gm. O'lOO Ammonia .... NH 3 17 1*7 gm. 0-017 Animonic Carbonate (NH 4 ) 2 C0 3 96 4*8 gm. 0-048 Lime (Calcic Oxide) . CaO 56 2'8 gm. 0-028 Calcic Hydrate . . CaH 2 O 2 74 3'7 gm. 0-037 Calcic Carbonate . . CaCO 3 100 5"0 gm. O'OSO Baric Hydrate . . BaH 2 2 171 8'55 gm. 0-0855 Do. (Crystals) BaO 2 H 2 (H 2 0) 8 315 15-75 gm. 0-1575 Baric Carbonate . . BaCO 3 197 9'85 gm. 0-0985 Strontia SrO 103'5 5-175 gm. 0*05175 Strontic Carbonate . SrCO 3 147-5 7-375 gm. 0-07375 Magnesia .... MgO 40 2-00 gm. 0-020 Magnesic Carbonate . MgCO 3 84 4'20 gm. 0-042 Nitric Acid . . . HNO 3 63 6'3 gm. 0-063 Hydrochloric Acid . HC1 36-37 3'637 gm. 0-03637 Sulphuric Acid . . H 2 SO 4 98 4'9 gm. 0-049 Oxalic Acid . . . C 2 4 H 2 (H 2 0) 2 126 6-3 gm. 0-063 Acetic Acid . . . C 2 2 H 4 60 6'0 gm. 0*060 Tartaric Acid . . . C 4 6 H 6 150 7*5 gm. 0-075 Citric Acid .... C0'H 8 +H 2 210 7'0 gm. 0-070 Carbonic Acid . . . CO 2 44 0-022 * This is the coefficient by which the number of c.c. of normal solution used in any analysis is to be multiplied, in order to obtain the amount of pure substance present in the material examined. If grain weights are used instead of grams, the decimal point must be moved one place to the risrht to give the necessary weight for examination ; thus sodic carbonate, instead of 5 '3 gm., would be 53 grains ; the normal factor in this case would also be altered to 0'53. 50 VOLUMETEIC ANALYSIS. 16. THE TITRATION OF ALKALINE SALTS. 1. Caustic Soda or Potash, and their Neutral or Acid Carbonates. 16. THE necessary quantity of substance being weighed or measured, as the case may be, and mixed with distilled water to a proper state of dilution (say about one per cent, of solid material), an appropriate indicator is added, and the solution is ready for the burette. Normal acid is then cautiously added from a burette till the change of colour occur. In the case of caustic alkalies free from CO 2 , the end-reaction is very sharp with any of the indicators ; but if CO 2 is present, the only available indicators in the cold are methyl orange or lacmoid paper. If the other indica- tors are used, the CO 2 must be boiled off after each addition of acid. In examining carbonates of potash or soda, or mixtures of caustic and carbonate, where it is only necessary to ascertain the total alkalinity, the same method applies. In the examinations of samples of commercial refined soda or potash salts, it is advisable to proceed as follows : Powder and mix the sample thoroughly, weigh 10 gm. in a platinum or porcelain crucible, and ignite gently over a spirit or gas lamp, and allow the crucible to cool under the exsiccator. "Weigh again, the loss of weight gives the moisture ; w r ash the contents of the crucible into a beaker, dissolve and filter if necessary, and dilute to the exact measure of 500 c.c. with distilled water in a half -liter flask ; after mixing it thoroughly take out 50 c.c. = 1 gm. of alkali with a pipette, and empty it into a small flask, bring the flask under a burette containing normal acid and graduated to 4- or T V c.c., allow the acid to flow cautiously as before directed, until the neutral point is reached : the process may then be repeated several times if necessary, in order to be certain of the correctness of the analysis. Residual Titration : As the presence of carbonic acid with litmus and the other indicators, except methyl orange, always tends to confuse the exact end of the process, the difficulty is best overcome, in the case of not using methyl orange, by allowing an excess of acid to flow into the alkali, boiling to expel the CO 2 , and then cautiously adding normal caustic alkali, drop by drop, until the liquid suddenly changes colour; by deducting the quantity of caustic alkali from the quantity of acid originally used, the exact volume of acid necessary to saturate the 1 gm. of alkali is ascertained. This method of re-titration gives a very sharp end-reaction, as- there is no carbonic acid present to interfere with the delicacy of the indicator. It is a procedure sometimes necessary in other cases, owing to the interference of impurities dissipated by boiling, e.g. sulphuretted hydrogen, which would otherwise bleach the indicator, except in the case of methyl orange and lacmoid paper, either of which are indifferent to H 2 S in the cold. An example will make the plan clear : Example : 50 c.c. of the solution of alkali prepared as directed, equal to 1 gm. of the sample, is put into a flask, and 20 c.c. of normal acid allowed to flow into it ; it is then boiled and shaken till all CO 2 is expelled, and normal caustic alkali added till the neutral point is reached ; the quantity required is 3*4 c.c., which deducted from 20 c.c. of acid leaves 1G'6 c.c. The follow- 8 16. ALKALINE SALTS. 51 o ing calculation, therefore, gives the percentage of real alkali, supposing it to be soda : 31 is the half molecular weight of dry soda (Na 2 O*) and 1 c.c. of the acid is equal to 0'031 gin., therefore 16'6 c.c. is multiplied by 0'031, which gives 0'5146 ; and as 1 gm. was taken, the decimal point is moved two places to the right, which gives 51*46 per cent, of real alkali ; if calculated as carbonate, the 16'6 would be multiplied by 0'053, which gives 0'8798 gm. = 87*98 per cent. 2. Mixed Caustic and Carbonated Alkaline Salts. The alkaline salts of commerce, and also alkaline lyes used in soap, paper, starch, and other manufactories, consist often of a mixture of caustic and carbonated alkali. If it be desired to ascertain the proportion in which these mixtures occur, the total alkaline power of a weighed or measured quantity of substance (not exceeding 1 or 2 gm.) is ascertained by normal acid and noted; a like quantity is then dissolved in about 150 c.c. of water in a 200 c.c. flask, and enough solution of baric chloride added to remove all carbonic acid from the soda or potash. Watson Smith has shown (J. S. C. I. i. 85) that whenever an excess of baric chloride is used in this precipitation so as to form baric hydrate, there is an invariable loss of soda : exact precipita- tion is the only way to secure accuracy. The flask is now filled up to the 200 c.c. mark with distilled water, securely stoppered, and put aside to settle. When the supernatant liquid is clear, take out 50 c.c. with a pipette, and titrate with normal hydrochloric acid to the neutral point. The number of c.c. multiplied by 4 will be the quantity of acid required for the caustic alkali in the original weight of substance, because only one-fourth was taken for analysis. The difference is calculated as carbonate, or the precipitated baric carbonate may be thrown upon a dry filter, washed well and quickly with boiling water, and titrated with normal acid, instead of the original analysis for the total alkalinity ; or both plans may be adopted as a check upon each other. The principle of this method is, that when baric chloride is added to a mixture of caustic and carbonated alkali, the carbonic acid of the latter is precipitated as an equivalent of baric carbonate, while the equivalent proportion of caustic alkali remains in solution as baric hydrate. By multiplying the number of c.c. of acid required to saturate this free alkali with the l0 1 6o atomic weight of caustic potash or soda, according to the alkali present, the quantity of substance originally present in this state will be ascertained. As caustic baryta absorbs CO 2 very readily when exposed to the atmosphere, it is preferable to allow the precipitate of baric carbonate to settle in the flask as here described, rather than to filter the solution as recommended by some operators, especially also as the filter obstinately retains some baric hydrate. * The commercial standard (so called English test) often used is 32 (being based on the old erroneous equivalent of Na=24). v 9 52 VOLUMETRIC ANALYSIS. 16. A very slight error, however, occurs in all such cases, in consequence of the volume of the precipitate being included in the measured liquid. K. Williams (/. S. C. I. vi. 346) estimates the caustic soda in soda ash by digesting a weighed quantity in strong alcohol in a tightly stoppered flask with frequent shaking and finally allowing to stand overnight; the undissolved carbonate is filtered off, washed with alcohol until a drop gives no alkaline reaction the filtrate and washings are then titrated with normal acid and phenolphthalein. Peter Hart recommends the following method of ascertaining the relative proportions of caustic and carbonated soda in soda ash : 50 grains of the sample is dissolved in 10 ounces of water, phenolphthalein added, and the standard acid (1 dm. = 0*5 grn. Na 2 0) slowly run in until the colour disappears. At this point all the caustic soda and one-half the carbonate has been neutralized, say 30 dm. has been used. To the same solution (in which the soda now exists as bicarbonate) methyl orange is added, and the titration continued until pink : the burette now reads 50 dm. Then 5030 = 20 as NaHCO 3 , but as this originally existed in the sample as Na 2 C0 3 , this figure must be doubled = 40, which deducted from 50 leaves 10 dm. as representing the caustic soda in the sample. 3. Estimation of Hydrates of Soda or Potash with small proportions of Carbonate. This may be accomplished by means of phenacetolin (Lunge, J. S. C. I. i. 56.) The alkaline solution is coloured of a scarcely perceptible yellow with a few drops of the indicator. The standard acid is then run in until the yellow gives place to a pale rose tint : at this point all the caustic alkali is saturated, and the volume of acid used is noted. Further addition of acid now intensifies this red colour until the carbonate is decomposed, when a clear golden yellow results. The neutralization of the NaHO or the KHO is indicated by a rose tint permanent on standing ; that of ]N"a 2 C0 3 or K 2 C0 3 by the sudden passage from red to yellow. Practice is required with solutions of known composition to accustom the eye to the changes of colour. Phenolpthalein may also be employed for the same purpose as follows : Add normal acid to the cold alkaline solution till the red colour is discharged, taking care to use a very dilute solution, and keeping the spit of the burette in the liquid so that no CO 2 escapes. The point at which the colour is discharged occurs when all the hydrate is neutralized and the carbonate converted into bicarbonate ; the volume of acid is noted, and the solution heated to boiling, with small additions of acid, till the red colour produced by the decom- position of the bicarbonate is finally destroyed. 16. ALKALINE SALTS. 53 In both these methods it is preferable, after the first stage, to add excess of acid, boil off the CO 2 , and titrate back with normal alkali. The results are quite as accurate as the method of precipi- tation with barium. 4. Estimation of Alkaline Bicarbonates in presence of Normal Carbonates (Lunge, J. S. C. I. i. 57). To a weighed quantity of the solid bicarbonate, or a measured quantity of a solution, there is added an excess of |- ammonia, followed by an excess of solution of baric chloride. The mixture is made in a measuring flask, and the whole diluted with hot distilled water to the mark. A portion of the clear settled liquid, or filtered through a dry filter, is then titrated with normal acid : the alkaline strength due to the excess of ammonia, above that required to convert the bicar- bonate into normal carbonate, deducted from the total ammonia added, gives the equivalent of the bicarbonate present. Example (Lunge) : 20 gin. sodic bicarbonate in the course of manufac- ture were dissolved to a liter. 50 c.c. of this solution required 12'1'o.c. normal acid = 0'3751 gm. Na 2 O ; 50 c.c. were then mixed with 50 c.c. of standard ammonia (50 c.c. = 24'3 normal acid) and the whole treated with excess of baric chloride. One half of the clear liquid required 6'25 c.c. normal acid = 11*8 c.c. corrected for strength and double quantity: this is, therefore, the equivalent of the CO 2 as bicarbonate. NaHCO 3 : ll'Sx '084= '9912 gm. Na 2 CO 3 : (12'1 11'8) x "053 = '0159. A simpler plan than the above has been devised by Thompson, which gives good results when carefully carried out. To the cold solution of the sample, an excess of normal caustic soda, free from CO 2 , is added, the CO 2 is then precipitated with neutral solution of baric chloride, and the excess of sodic hydrate found by standard acid, using phenolphthalein as indicator. The precipitate of baric carbonate has no effect on the indicator in the cold. The calculation is the same as before. 5. Estimation of small quantities of Sodic or Potassic Hydrates in presence of Carbonates. This method, by Thompson, has just been alluded to, and consists in precipitating the carbonates by neutral solution of baric chloride in the cold : the baric carbonate being neutral to phenol- phthalein, this indicator can be used for the process. When the barium solution is added, a double decomposition occurs, resulting in an equivalent quantity of sodic or potassic chloride, while the baric carbonate is precipitated, and the alkaline hydrate remains in solution. 54 VOLUMETKIC ANALYSIS. 16. Example (Thompson): 2 gm. of pure sodic carbonate were mixed in solution with '02 gm. of sodic hydrate ; excess of baric chloride was then added, together with the indicator, and the solution titrated with -^ acid, of which in three trials an average of 5 c.c. was required; therefore, 5 x '004 ='02 gm. exactly the quantity used. In this process the presence of chlorides, sulphates, and sulphites do not interfere ; neither do phosphates, as baric phosphate is neutral to the indicator. With sulphides, half of the base will be estimated; but if hydrogen peroxide be added, and the mixture allowed to rest for a time, the sulphides are oxidized to sulphates, which have no effect. If silicates or aluminates of alkali are present, the base will of course be recorded as hydrate. .Thompson further says: " The foregoing method can also be applied to the determination of hydrate of sodium or potassium in various other compounds, which give precipitates with baric chloride neutral to phenolph- thalein, such as the normal sulphites and phosphates of the alkali metals. An illustration of the use to which the facts I have stated in this and former papers may be put will be found in the analysis of sulphite of sodium. Of course sulphate, thiosulphate, and chloride are determined as usual, but to estimate sulphite, carbonate and hydrate, or bicarbonate of sodium by methods in ordinary use is rather a tedious operation. To find the proportion of hydrate, all that is necessary is to precipitate with baric chloride and titrate with standard acid, as above described. Then, by simple titration of another portion of the sample in the cold, using phenolphthalein as indicator, the hydrate and half of the carbonate can be found, and finally, by employment of methyl orange as indicator, and further addition of acid, the other half of the carbonate and half of the sulphite can be estimated. By simple calculations, the respective proportions of these three compounds can be obtained, a result which can be accomplished in a few minutes. It must be borne in mind that if a large quantity of eodic carbonate is in the sample the proportion of that compound found will only be an approximation to the truth, as the end-reaction is only delicate "with small proportions of sodic carbonate. If there is no hydrate found, bicarbonate of sodium can be tested for, and determined by Lunge's method described above "(1 6 - 4 ). 6. Estimation of Alkalies in the presence of Sulphites. It is not possible to estimate the alkaline compounds in the presence of sulphites by titration with acids, as a certain quantity of acid is taken up by the sulphite, SO 2 being evolved. This difficulty may be completely overcome by the aid of hydrogen peroxide, which speedily converts the sulphites into sulphates (Grant and Cohen, /. S. C. I. ix. 19). These operators proved 16. ALKALINE SALTS. 55 that neither caustic or carbonated alkali were affected by H 2 2 , nor had the latter any prejudicial effect on methyl orange in the cold. The quantity of H 2 2 required in any given analysis must depend on the amount of sulphite present; for instance, the caustic salts of commerce contain about 50% of sulphite, and it suffices to take 10 c.c. of ordinary 10 vol. H 2 2 for every O'l gin. of the salts in solution. In the case of mixtures containing less or more sulphite the quantity may be varied. The Analysis : A measured volume of the peroxide is run into a beaker, and three or four drops of methyl orange added. As the H 2 O 2 is invariably faintly acid, the acidity is carefully corrected by adding drop by drop from a pipette T-^jy caustic soda. The required quantity of salt to be analyzed is then added in solution, and the mixture gently boiled, during the boiling the methyl orange will be bleached. The liquid is then cooled, a drop or two more of methyl orange added, and the titration for the proportion of alkali carried out with normal acid in the usual way. The results are very satisfactory. 7. Estimation of Caustic Soda or Potash, by standard Bichromate of Potash. This process was devised by Richter, or rather the inverse of it, for estimating bichromate with caustic alkali by the aid of phenolphthaleiii. Exact results may be obtained by it in titrating soda or potash as hydrates, but not ammonia as recommended by Richter. For the process there are required a decinormal solution of bichromate con- taining 14*74 gm. per liter, and T ^ soda or potash solution titrated against sulphuric acid. A comparison liquid containing about 1 gm. of monochro- niate of potash in 150 200 c.c. water is advisable for ascertaining the exact end of the reaction ; 50 c.c. of the alkali being diluted with the same volume of water, is coloured with phenolphthalein, and the bichromate run in from a burette ; the fine red tint changes to reddish yellow, which remains till the neutral point is nearly reached, when the yellow colour of the mono- chromate is produced ; the change is not instantaneous as with mineral acids, so that a little time must be allowed for the true colour to declare itself. 8. Estimation of Potash in Neutral Salts free from Soda. Stolba precipitates the potash from a tolerably concentrated solution of the substances with hydrofluosilicic acid and strong alcohol. The method is also applicable to the estimation of potash in potassic platinum chloride. To ensure complete decomposition, it is well to warm the mixture for a little time before adding the alcohol, which must be of about the same volume as the liquid itself. After some hours, when the precipitate has settled, the solution is filtered off, the beaker and precipitate well washed with equal mixtures of alcohol and water, the whole transferred to a white porcelain basin, water rather freely added, and heated to boiling, a few drops of litmus added, and normal or semi-normal alkali run in until exact saturation occurs ; or a known excess of alkali may be added, and the amount found by residual titration with normal acid. The results are generally about 1/ too low, owing to the difficulty of fully decomposing the pre- cipitate. 2 eq. alkali = 1 eq. potash. 56 VOLUMETRIC ANALYSIS. 16. The process is very limited in its use, and is not applicable when sulphates are present, nor in the presence of any great amount of free acid. Sulphuric acid may be previously removed by calcic acetate and alcohol ; other acids by moderate ignition previous to precipitation. Large proportions of ammonia salts must also be removed ; and, of course, all other matters precipitable by hydro- nuosilicic acid, especially soda. 9. Direct estimation of Potash in the presence of Soda. Fleischer recommends the following method; and my own experiments confirm his statements, so far at least as the pure salts are concerned. The solution must contain no other bases except the alkalies, nor any acids except nitric, hydrochloric, or acetic. This can almost invariably be easily accomplished. Earthy alkalies are removed by ammonic carbonate or phosphate; sulphuric, chromic, phosphoric, and arsenic acids by baric chloride, followed by ammonic carbonate. The solution should be tolerably concentrated, and the volume about 25 or 30 c.c. ; 10 15 c.c. of neutral solution of ammonic acetate of sp. gr. 1'035 are added ; followed by finely powdered pure tartaric acid in sufficient quantity to convert the potash into acid tartrate, with an excess to form some ammonic tartrate, but not enough to decompose the whole. This is the weak part of the method ; however, as a guide, it is not advisable to add more than 5 gni. tartaric acid for 10 c.c. of ammonic acetate. If the quantity of potash is approximately known, it is best to add about one-third more than is sufficient to convert the whole into acid tartrate. After adding the tartaric acid the mixture must be well stirred for five or ten minutes, without rubbing the sides of the beaker ; a like volume of 95- per-cent. alcohol is added, and again well stirred. The precipitate contains the whole of the potash as tartrate, and a portion of ammonium tartrate. After standing half an hour with occasional stirring, the precipitate is collected on a porous filter, and repeatedly washed with alcohol and water in equal parts until clean. When the washing is finished the precipitate will be entirely free from soda ; filter and precipitate are transferred to a porcelain basin, treated with sufficient hot water to dissolve the tartrates, then exactly neutralized with normal alkali and litmus, and the volume so used noted. A like volume, or preferably, a larger known volume of normal alkali is now added, and the mixture boiled to expel all ammonia; the end maybe known by holding litmus paper in the steam. The excess of normal alkali is now found by titration with normal acid ; the amount so found must be deducted from that which was added in excess after the exact titration of the tartrate ; the difference equals the ammonia volatilized. By deducting this difference from the volume of normal alkali originally required, the volume corre- sponding to potash is found. Example : 29'4 c.c. of normal alkali were required in the first instance to neutralize a given precipitate ; 40 c.c. of the same alkali were then added, the boiling accomplished, and 22'5 c.c. normal acid used for the excess ; then 40 22'5 = 17'5 c.c., and again 29'4 17'5 = 11'9, which multiplied by the factor for K^O=0'056 gives 0'6664 gm. The soda in filtrate may be obtained by evaporation with hydro- chloric acid as sodic chloride, and estimated as in 38. 16. ALKALINE SALTS. 57 10. Mixed Caustic Soda and Potash. This process depends upon the fact, that potassic bitartrate is almost insoluble in a solution of sodic bitartrate. Add to the solution containing the mixed salts a standard solution of tartaric acid till neutral or faintly acid this produces neutral tartrates of the alkalies now add the same volume of standard tartaric acid as before they are now acid tartrates, and the potassic bitartrate separates almost completely, filter off the sodic bitartrate and titrate the filtrate with normal caustic soda ; the quantity required equals the soda originally in the mixture the quantity of tartaric acid required to form bitartrate with the soda subtracted from the total quantity added to the mixture of the two alkalies, gives the quantity required to form potassic bitartrate, and thus the quantity of potash is found. This process is only applicable for technical purposes. Mixtures of potash and soda in the form of neutral chlorides are estimated by J. T. White as follows (C. N. Ivii. 214) : 20 c.c. of the solution containing about 2 gm. of the mixed salts are placed into a 100 c.c. flask, and 5 c.c. of a hot saturated solution of ammonic bicarbonate added ; the mixture is cooled, and alcohol added in small quantities, with shaking, until the measure is made up to 100 c.c. After three or four hours, 10 c.c. of the clear liquid are removed with a pipette, evaporated and ignited, the residue is moistened with a few drops of ammonic chloride solution and again ignited ; the sodic chloride so obtained is then titrated with standard silver solution, 1 c.c. of which represents - 001 gm. Cl ; this is calculated to NaCl and the KC1 found by difference. HUBERT RY^gg^, ag p latino _ chloride> Ill cases where potash exists in combination as a neutral salt, such as kainit or kieserit, etc., or as a constituent of minerals, it has to be first separated as double chloride of potassium and platinum. The method usually adopted is that of collecting the double salt upon a tared filter, when the weight of the dry double salt is obtained, the weight of potash is ascertained by calculation. It may, however, be arrived at by volumetric means as follows : The potash having been converted into double chloride in the usual way is dried, collected, and mixed with about double its weight of pure sodic oxalate, and gently smelted in a platinum crucible ; this operation results in the production of metallic platinum, chlorides of sodium and potassium, with some carbonate of soda. The quantity of potash present is, however, solely measured by the chlorine ; in order to arrive at this, the fused mass is lixiviated with water, filtered, nearly neutralized with acetic acid, and the chlorine estimated with & silver and chromate, the number of c.c. of silver required is multiplied by the factor 0'00157, which gives at once the weight of potash. This factor is used because 1 molecule of double chloride contains 3 atoms chlorine, hence the quantity of ^ silver used is three times as much as in the case of sodic or potassic chloride. 12. Separation of the Potash as Bitartrate. The mixed salts being rendered as nearly neutral as possible, a saturated solution of sodic bitartrate is added in excess, and the whole evaporated to dryness in the water bath. The dry mass is then deprived of the excess of 58 VOLUMETRIC ANALYSIS. 16 sodic bitartrate by washing it on a filter with a saturated solution of potassic bitartrate ; when all the soda salt has been removed, the potash salt is dissolved in hot water, and titrated with normal alkali, of which 1 c.o. represents 0'039 gm. K. In cases where potash is to be separated as bitartrate, the operator should consult 25. 2 and 3. 13. Indirect Estimation of Potash (Dubenard). This process is ingenious, but, like all attempts at estimating tliis base volumetrically, is not very exact, except with extreme precautions. The Analysis : The potassic salt, acidified with nitric acid, is precipitated by a standard solution of sodic chloroplatinate, the excess of which is reduced by zinc, and then determined by means of standard silver nitrate. The process is applicable to sulphates, chlorides, nitrates, etc. 12 to 15 parts of sodic chloroplatinate are dissolved in 100 of alcohol, and 12 to 15 parts of silver nitrate in 1000 of water. The solutions are standardized as follows : 10 c.c. of the platinum solution are reduced by boiling for a minute with a small quantit} 7 - of zinc powder, the whole of the platinum being precipitated, and all the chlorine remains in solution as chlorides of zinc and of sodium ; the solution is made up to 100 c.c. and filtered. In 50 c.c. of the filtrate the chlorine is determined by titrating with the silver solution, of which, say, 40 c.c. are required. Then 40 x 4 = 160 = the number of c.c. which corresponds to 20 c.c. of the platinum solution. 0'5 gm. of potassic nitrate or sulphate is then dissolved in a few c.c. of water, acidified with nitric acid, the potassium precipitated with 20 c.c. of the platinum solution, and the volume made up to 100 c.c. with alcohol (95 per cent.) ; the solution is filtered, and 50 c.c. of it reduced by boiling with a little zinc, again made up to 100 c.c., and filtered ; 50 c.c. of this filtrate is titrated with the silver solution, of which, say, 12 c.c. are required ; then 12 c.c. x 4 = 48 c.c., subtracted from 160 c.c., represents the amount of chlorine precipitated as potassic chloroplatinate by the potassic nitrate; thus (16048) c.c. = 112 c.c. corresponds to 0'5 gm. of potassic nitrate, i.e., to 0'232 K-O. To determine now the amount of potassium in any salt the process just described is followed, taking 10 c.c. of a solution containing 50 gm. of the salt per liter. The amount of chlorine present in the salt, as a chloride, must be determined by titration, and allowed for before calculating the amount of potassium. Thus, if the 10 c.c. of the solution (i.e., 0'5 gm. of the sample) required 8 c.c. of the silver solution and 27 c.c. were required of it after the reduction with zinc, the calculation would be (27 x 4 8) c.c. = 100 c.c. and TOO v O'9^9 - = 0'207 gm. of potash in 0'5 gm., i.e., the sample contained 41-40 per cent, of K 2 O. This process has been critically examined under the direction of Dr. Wiley (C. N. liii. 176), who states, that owing to uncertainty in estimating the chlorine, the conclusions are not satisfactory. This has given me no difficulty when using potassic chromate as indicator, taking the precaution to add a little pure calcic carbonate to insure neutrality. It has given me good approximate results with tolerably pure and concentrated potassium salts, viz., :iitrate, sulphate, and chloride. I doubt its value however for estimation of small percentages of potash in manures, etc. The process is rapid and easy, but care should be taken in filtering the 16. ALKALINE SALTS. 59 alcoholic mixture of the double chloride to avoid loss by evaporation before measuring. The solutions precipitated by zinc need no filtering, as they settle rapidly. ^ silver solution is available for the clilorine estimation ; the weak point in the process is probably due to the strong saline liquids affecting the delicacy of the chromate indicator. TECHNICAL EXAMINATION OF SOME ALKALINE COMPOUNDS FOUND IN COMMERCE OB OCCURRING IN COURSE OF MANUFACTURE. There is now considerable unanimity among English and foreign manufacturers of alkaline compounds, as to methods of analysis to be adopted either for guidance in manufacture or commercial valuation. Lunge has contributed important papers on the subject (/. S. C. I. i. 12, 16, 55, 92), also in conjunction with Hurter in the Alkali Makers' Pocket Book* which contains valuable tables and processes of analysis. So far as volumetric methods are concerned, the same processes will be given here with others. 14. Soda Ash, Black Ash, Mother-liquors, etc. Soda Ash or Refined Alkali. 5 or 10 gm. are dissolved in about 150 c.c. of warm distilled water, and any insoluble matter filtered off (German chemists do not filter), and the volume diluted to ^ or 1 liter. The total quantity of alkali is determined in 50 c.c. by normal sulphuric, nitric, or hydrochloric acid, as in 16. l.f The quantity of caustic alkali present in any sample is determined as in 16. 2 or 5. The presence of sulphides is ascertained by the smell of sulphuretted hydrogen when the alkali is saturated with an acid, or by dipping paper steeped in sodic nitro-prusside into the solution : if the paper turns blue or violet, sulphide is present. The quantity of sulphide and sulphite may be determined by saturating a dilute solution of the alkali with a slight excess of acetic acid, adding starch and titrating with -f^ iodine solution till the blue colour appears. The quantity of iodine required is the measure of the sulphuretted hydrogen and sulphurous acid present. The proportion of sulphide is estimated as follows : 13'820 gm. of pure silver are dissolved in dilute nitric acid, and the solution, together with an excess of liquid ammonia, made up to a liter. Each c.c. =0'005 gm. Na 2 S. The Analysis : 100 c.c. of the alkali liquor is heated to boiling, some ammonia added, and the silver solution dropped in from a burette until no further precipitate of A^S is produced. Towards the end filtration will be necessary, in order to ascertain the exact point, to which end the Beales filter is serviceable (fig. 19). The amount of Na 2 S so found is deducted from the total sulphide and sulphite found by iodine. Sodic chloride (common salt) may be determined by carefully neutralizing 1 gm. of the alkali with nitric acid, and titrating with decinormal silver * Bell & Sons, York Street, Covent Garden. t This gives a slight error, owing to traces of aluminate of soda and lime, which consume acid. 60 VOLUMETRIC ANALYSIS. 16. solution and potassic chromate. Each c.c. represents 0'005837 gm. of common salt. Since the quantity of acid necessary to neutralize the alkali has already been found, the proper measure of ^ nitric acid may at once be added. Sodic sulphate is determined, either directly or indirectly, as in 73. Each c.c. or dm. of normal baric chloride is equal to 0'071 gm. or 0'7l grn. of dry sodic sulphate. Examination of Crude Soda Lyes and Red Liquors. Kalmann and Spuller (Dingl. polyt. J. } 264, 456 459) recommend a process based on the insolubility of baric sulphite and the solubility of baric thiosulphate in alkaline solutions. The estimation is performed in the following manner: 1. The total alkalinity is determined in a measured volume of the liquor under examination by titration with normal acid, methyl orange being used as indicator. The acid consumed equals sodic carbonate + sodic sulphide, + sodic hydroxide, + one-half sodic sulphite (Na'-SO 3 is alkaline and NaHSO 3 neutral to methyl orange). 2. An equal volume of the liquor is titrated with ^ solution of iodine, the volume consumed corres- ponding with the sodic sulphide + the sodic sulphite, + the sodic thiosulphate. 3. Twice the volume of liquor as that used in (1) and (2) is precipitated with an alkaline zinc solution, and the mixture made up to a certain measure, one-half of which is filtered, acidified, and titrated with Y^ iodine. The iodine consumed equals sodic sulphite + sodic thiosulphate. 4. Three or four times the volume of liquor used in (1) and (2) is treated with an excess of a solution of baric chloride, the mixture made up to a known volume with water, and filtered, (a.) One-third or one-fourth (as the case may be) of the filtrate is titrated with normal acid, the amount used corresponding with the sodic hydroxide + the sodic sulphite. (b.) A new third or fourth of the filtrate is acidified and titrated with ^ iodine, the iodine consumed being equal to sodic sulphite + sodic thio- sulphate. The calculation is made as follows : 2 45 ......... = A c.c. j^- iodine corresponding to ...... Na 2 S0 3 2 3 ......... B c.c. ^ iodine corresponding to ...... Na 2 S 46 (23) . . . = C c.c. T *V iodine corresponding to ...... Na 2 S 2 3 4a T VB ...... = D c.c. normal acid corresponding to ... NaOH 1 (4a + T VA) = E c.c. normal acid corresponding to ... Na 2 CO 3 Black Ash. Digest 50 gm. with warm water in a i liter flask, fill up to mark, and allow to settle clear. (1) Total Alkali existing as carbonate, hydrate, and sulphide, is found by titrating 10 c.c. = l gm. of ash with standard acid and methyl orange in the cold. (2) Caustic Soda. 20 c.c. of the liquid is put into a 100 c.c. flask with 10 c.c. of solution of baric chloride of 10 per cent, strength, filled up with hot water, well shaken, and corked after settling a few minutes. The clarified liquid is filtered, and 50 c.c. = l gm. ash, titrated with standard acid and methyl orange; or it may be titrated without filtration if standard oxalic acid and phenolphthalein are used, this acid having no effect on the baric carbonate. Each c.c. normal acid = 0'031 Na 2 O. This includes sulphides. (3) Sodic Sulphide. Put 10 c.c. of liquor into a flask, acidulate with acetic acid, dilute to about 200 c.c. and titrate with ^ iodine and starch. Each c.c. = 0-0039 Na 2 S, or 0'0031 Na 2 O. (4) Sodic Chloride. 10 c.c. are neutralized exactly with normal nitric acid, and boiled till all H 2 S is evaporated. Any sulphur which may have been precipitated is filtered off, and the filtrate titrated with ^ silver and chromate. Each c.c. = 0'005837 gm. NaCl. (5) Sodic Sulphate. This is best estimated by precipitation as baric sulphate, and weighing, the quantity being small. If, however, volumetric estimation is desired, it may be done as in 73, taking 50 c.c. of liquor. 1G. ALKALINE COMPOUNDS. 61 For other methods of examining the various solid and liquid alkali wastes used for soda and sulphur recovery, etc., the reader is referred to the Alii all Makers' Pocket Bool- already mentioned. 15. Salt Cake Is the impure sodic sulphate used in alkali manufacture or left in the retorts in preparing hydrochloric acid from sulphuric acid and salt, or nitric acid from sodic nitrate. It generally contains free sulphuric acid existing as sodic bisulphate, the quantity of which may be ascertained by direct titration with normal alkali. The common salt present is estimated by decinormal silver solution and chromate; having first saturated the free acid with pure sodic carbonate, 1 c.c. or 1 dm. silver solution is equal to 0'005837 gm. or 0'05837 grn. of salt. Sulphuric acid, combined with soda, is estimated either directly or indirectly as in 73 ; 1 c.c. or 1 dm. of normal baryta solution is equal to 0'07l gm. or 0'71 grn. of dry sodic sulphate. Iron is precipitated from a filtered solution of the salt cake with ammonia in excess, the precipitate of ferric oxide re-dissolved in sulphuric acid, reduced to the ferrous state with zinc and titrated with permanganate. Grossman adopts a method suggested by Bohlig (see 28), and has worked out the process in the case of salt cake in careful detail (C. N. xli. 114) as follows : The neutral solution of salt cake (3'55 gm.) is put into a 500 c.c. flask, 250 c.c. of a cold saturated solution of baric hydrate added, the flask filled with water, and shaken up. Of the filtered clear liquid 250 c.c. are put in an ordinary flask, carbonic acid passed through for about ten minutes, and then the contents of the flask boiled so as to decompose any baric bicarbonate which may be in solution. After cooling, the contents of the flask are again transferred to the 500 c.c. flask, the latter filled up with water to the mark, shaken up, and filtered. 250 c.c. of the filtrate i.e., one-fourth of the original quantity used are then titrated with one-fourth normal sulphuric acid. The number of c.c. of one-fourth normal acid used multiplied by two will give the percentage of sodic sulphate. There are, however, sources of error in the experimental working of this method which make certain corrections necessary. They arise (1) Prom the impurities of the caustic baryta. (2) From the precipitate formed in the measured liquid. (3) From certain constant losses. The commercial caustic baryta always contains baric nitrate, and sometimes baric chloride. It is evident that on adding a solution of baric hydrate which contains baric nitrate to a solution of sodic sulphate, a quantity of the latter, equivalent to the quantity of the baric nitrate present, will be converted into sodic nitrate, and thus escape the alkalimetric test, as will be seen by the following equations : Ba(N0 3 ) 2 +Na 2 S0 4 =BaS0 4 +2NaNO 3 . Ba(N0 3 ) 2 +2NaOH+C0 2 =BaCO 3 +2NaN0 3 +H 2 0. It is therefore necessary to measure approximately the quantity of baryta solution used, so as to know the amount of baric nitrate introduced into the process. The latter can be easily ascertained by passing carbonic acid in excess through the cold saturated solution of baric hydrate, boiling, filtering, and precipitating the baryta left in solution by sulphuric acid as usual. 62 VOLUMETRIC ANALYSIS. 16. 250 c.c. of a baryta solution used for experiment yielded 0"0280 gin. BaSO 4 , which corresponds to 0'0l7l gm. Na 2 SO 4 , or 0"96 c.c. of one-fourth normal acid ; and it follows that for every 250 c.c. of this baryta solution was found O'OIYI gm. Na 2 SO 4 too little; or, that there must be added 0'24 c.c. of one-fourth normal acid to the result of the final tilration (of one-fourth of the original quantity). If the baryta contain caustic alkali, a corresponding quantity of baric nitrate will be found less by the test; but it is easily understood, that the calculations will not be influenced as long as the baric nitrate is in excess of the caustic alkali, which is always the case in good commercial baryta. The second error arises from the precipitates of baric sulphate and carbonate taking up some space in the 500 c.c. flask, the final results thus being found too high. If it is assumed that a cold saturated solution of baryta contains about 23 gm. BaO per liter, it will be near enough for all practical purposes if in the experiment, working with 3"55 gm. of Na 2 SO 4 and 250 c.c. of baryta solution, 0"4 per cent, is subtracted from the final results for this error. Three experiments made with 3' 55 gm. of pure ignited sodic sulphate gave the following results : Used one-fourth normal acid ... 49"37 c.c. Add for Ba(NO 3 ) 2 0'24 c.c. 49'61 c.c. = 99'22 per cent. Na 2 SO 4 . II. Used one-fourth normal acid ... 49'21 c.c. Add for Ba(N0 3 ) 2 0'24 c.c. 49-45 c.c. =98-90 per cent. Na 2 S0 4 . III. Used one-fourth normal acid ... 49'37 c.c. Add for Ba(NO 3 ) 2 0'24 c.c. 49-61 c.c. = 99*22 per cent. Na 2 S0 4 . The average of these three experiments gives 99'1 per cent. ; and if 0'4 per cent, be subtracted for the precipitate, the result is 98'7 per cent, instead of 100. Grossman states that this loss of T3 per cent, in working with 3'55 gm. of sulphate in the given dilution is a constant, and by dividing all results by 0*987 correct results are obtained. It now remains to show the applicability of this method to the assay of salt cake and like substances. The following is a complete analysis of a sample of salt cake made in the usual way : Moisture 0'49 Insoluble 0'29 Free sulphuric hydrate 0"38 Aluminium sulphate 0"23 Ferric sulphate . . .' 0'42 Calcic sulphate 1*17 Sodic chloride 2'00 Sodic sulphate (by difference) 95'02 100-00 16. ALKALINE COMPOUNDS. 63 In order to make a good analysis of salt cake by weight it is necessary to estimate seven constituents, to find by difference the quantity of actual sodic sulphate, which is the only constituent wanted. When baric hydrate is added to a solution of salt cake the free acid is precipitated, so are alumina and iron, and the sulphuric acid combined with them and with lime. The lime is partly thrown down as such, and what is left as lime in solution is precipitated as carbonate in the second operation. Thus, whatever other sulphates be present, only the sodic sulphate is given ; and by one simple test we are thus able to get a result which formerly could only be attained by a tedious complete analysis. The salt cake, of which a complete analysis is given above, was tested by the alkalimetric method. 3' 5 5 gm. required One-fourth normal acid 46*95 c.c. Add for Ba(NO 3 ) 2 0'24 c.c. 47*19 c.c. =94-38 per cent. Na 2 SO 4 . (94-38 0-40) -=93-98. 93-98 : 0-987=95-2 per cent. Na 2 SO 4 . Thus, by the alkalimetric test, 95 -2 per cent. Na 2 S0 4 occurs, whereas the analysis gives 95 -02 per cent. If it be considered how difficult it is to wash soda salts completely from precipitates, it is not surprising to find the result too low in the complete analysis, as in five precipitates a very minute quantity will make up 0*2 per cent. It is hardly necessary to point out that none of the figures for the correction of the errors enumerated above can be used by any one else working by this method, but that they must be ascertained in every individual case. It is absolutely necessary -to ascertain after the first operation that there is no sulphate, and after the second (before titrating) that there is no baryta in solution. 16. Raw Salt, Brine, etc. Lime may be estimated by precipitation with ammonic oxalate, and the precipitate titrated with permanganate, as in 48. Sulphuric acid as in 73. Magnesia is precipitated as ammoniacal phosphate, by a solution of sodic phosphate containing ammonia, first removing the lime by ammonic oxalate, the precipitate of double phosphate of magnesia and ammonia is brought on a filter, washed with cold water containing ammonia, then dissolved in acetic acid, and titrated with standard uranium solution, or by the process for P 2 5 ( 23-2). The quantity of real salt in the sample may be ascertained by treating a weighed quantity in solution with caustic baryta, boiling, setting aside that the excess of baryta may precipitate itself as carbonate, or more quickly by adding ammonic carbonate, filtering, evaporating the solution to dryness, and gently igniting the residue is pure salt. The loss of weight between this and the original specimen taken for analysis, will show the percentage of impurities. 17. Silicates of Soda and Potash. A weighed quantity of the substance is gently ignited, until no aqueous 64 VOLUMETRIC ANALYSIS. 16. vapours are given off, and the residue weighed thus the respective per- centages of water and anhydrous material are obtained. Another portion of the substance is dissolved in hot water, and titrated with litmus and normal acid boiling, or with methyl orange after cooling. The volume of acid is calculated to soda br potash. Solid alkaline silicates require to be finely powdered previous to solution in hot water. 18. Soap. The methods here given are a combination of those published by A. R. Leeds (C. N. xlviii. 166) and C. R. A. Wright (Journ. Soc. Arts, 1885, 1117, also J. S. C. I. iv. 631). (1) Moisture and Volatile Matters. 15 gm. are dried to a constant weight, first at 100, then at 120 C. (2) Free Fats. Residue of 1 is exhausted with light petroleum ether, and the extract, after evaporation of the ether, weighed. (3) Fatty Acids, Chlorides, Sulphates, Glycerine, etc. The residue from (2), which has been treated with ether, represents 15 gm. soap ; it is weighed, und two-thirds of it are dissolved in water, and normal nitric acid added in excess to separate the fatty acids. These are collected on a tared filter, dried, and weighed. The acid filtrate is now titrated with normal soda or potash (free from chlorides or sulphates), with phenolphthalein as indicator; the difference between the volumes of acid and alkali used gives roughly the total alkali. The residual neutral liquid is divided into two equal parts, in one of which chlorine is estimated with & silver and chromate, and in the other sulphuric acid by normal baric chloride. If glycerine is present, it may be estimated by Muter 's copper test in the absence of sugar. Sugar is, however, often largely used in transparent soaps in place of glycerine : when both are present, the separate estimation is difficult, but Wright suggests the method of Fehling for the sugar, first inverting with acid ; the copper retained in solution by the glycerine being estimated colorimetrically, using for comparison a liquid containing both sugar and glycerine to known extents, treated side by side with the sample tested. (4) Free and Total Alkali. These are obtained by Wright's alcohol test. Two or three grams of the soap is boiled with 95 per cent, alcohol, the extract filtered off and residue washed with alcohol. The solution so obtained may be either positively alkaline with caustic alkali, or negatively alkaline from the presence of fatty acids or a diacid soap, according to the kind of soap used. Phenolphthalein is added, which shows at once whether free alkali is present, and in accordance with this either standard alcoholic acid or alkali is used for titration. The residue on the filter is then dissolved in water, and titrated with normal or decinormal acid ; the alkali so found may include carbonate, silicate, borate, or aluminate of soda or potash, and also any soluble lime. The sum of the two titrations will be the total alkalinity in case both showed an alkaline reaction ; if otherwise, the alkali used to produce a colour in the alcoholic extract is deducted from the volume of acid used in the water extract. This method of taking the alkalinity of a soap is very fairly exact ; the error ought never to exceed +_ 0'5 per cent. Wilson (C. N. lix. 280) states that the estimation of free caustic alkali in high class soaps, containing no free glycerides, by the alcoholic method is correct, but in the case of common commercial soaps it is entirely misleading. The method of C. Hope is undoubtedly the quickest and best for the examination of the alcoholic solution of soap. Two grams of soap is dissolved in hot absolute alcohol, a drop of phenolphthalein indicator added, and some bubbles of CO 2 passed through till the colour disappears. The liquid is filtered ; the residue, consisting of total impurities, is washed with hot alcohol, 17. ALKALINE EARTHS. 65 weighed and titrated with T \ acid and methyl orange, which gives the alkali not existing as soap. The alcoholic solution is evaporated to dryness at 100 C. and the dry soap weighed. It is then gently ignited, dissolved in water, and titrated with T ^- acid and methyl orange to find the alkali existing as soap. The difference between this and the soap residue, before ignition, gives the fatty anhydrides, which multiplied by 1'03 gives the fatty acids. The water is found by deducting the weights of the impurities and dry soap from 100. Fuller information on this subject may be found in Allen's Organic Analysis and in Lant Carpenter's treatise on Soap and Candles. TITBATION OF ALKALINE EARTHS. 17. STANDARD hydrochloric or nitric acid must in all cases be used for the titration of the caustic or carbonated alkaline earths, as these are the only acids yielding soluble compounds, except in the case of magnesia. The hydrates, such as caustic lime, baryta, strontia, or magnesia, may all be estimated by any of the indicators, using the residual method, i.e., adding a known excess of standard acid, boiling to expel any trace of CO 2 , and re-titrating with standard alkali. The carbonates of the same bases may of course also be estimated in the same way, bearing in mind, that when methyl orange is used, the liquid is best cooled before re-titration. All heating may be avoided when using methyl orange in titrating mixtures of hydrates and carbonates, or the latter only, unless it is impossible to dissolve the substance in the cold. A good excess of acid is generally sufficient. The total amount of base in mixtures of caustic and carbonated alkaline earths is also estimated in the same way. (1) Estimation of Mixed Hydrates and Carbonates. This may be done either by phenacetolin or phenolphthalein as indicator. The former has been recommended by Degener and Lunge : the method, however, requires practice in order to mark the exact change of colour. The liquid containing the compound in a fine state of division is tinted with the indicator so as to be of a faint yellow ; standard acid is then cautiously added until a permanent pink occurs (at this stage all the hydrate is saturated), more acid is now cautiously added until the colour becomes deep yellow, the volume of acid so used represents the carbonate. The method is especially adapted to mixtures of calcic hydrate and carbonate. It is also applicable to barium, but not to magnesium, owing to the great insolubility of magnesic hydrate in dilute acid. If phenolphthalein is used as indicator, the method is as follows : Heat the liquid to boiling, and cautiously add standard acid until the red colour is just discharged. The carbonates of lime and baryta, rendered dense by boiling, are both quite neutral to the indicator. To obtain the whole of the base, excess of standard acid is used, and the mixture re-titrated with standard alkali. 66 VOLUMETRIC ANALYSIS. 17. Magnesia in solution as bicarbonate may be accurately estimated in the cold with methyl orange as indicator. (2) Estimation of Calcium, Barium, and Strontium in Neutral Soluble Salts. The amount of base in the chlorides and nitrates of these alkaline earths may be readily estimated as follows : The weighed salt is dissolved in water, cautiously neutralized if acid or alkaline, phenolphthalein added, heated to boiling, and standard sodic carbonate delivered in from time to time \\lth boiling until the red colour is permanent. Magnesium salts cannot however be estimated in this way, or even mixtures of lime and magnesia, as magnesic carbonate affects the indicator in a different manner to the other carbonates. (3) Precipitation of the Alkaline Earths from their Neutral Salts as Carbonates. Soluble salts of lime, baryta, and strontia, such as chlorides, nitrates, etc., are dissolved in water, and the base precipitated as carbonate, with excess of ammonic carbonate and some free ammonia. The mixture is heated to about 60 C. for a few minutes. The precipitated carbonate is then to be filtered, well washed with hot water till all soluble matters, especially ammonia, are removed, and the precipitate with filter titrated with normal acid, as already described. Magnesia salts require caustic soda or potash instead of ammonic carbonate ; but the process gives results slightly too low, owing to the slight solubility of magnesic hydrate in the alkaline liquid. (4) Lime and Magnesia Carbonates in Waters. The amount of calcic, or calcic and magnesic carbonates, dissolved in ordinary non-alkaline waters may be very readily, and with accuracy, found by taking 200 or 300 c.c. of the water, heating to near boiling, adding phenacetolin or lacmoid, and titrating cautiously with yV nitric or sulphuric acid. An equally accurate result may be obtained by methyl orange in the cold liquid. (5) Hardness of "Water estimated without Soap Solution. As is generally known, the soap-destroying power of a water is ascertained in Clark's process by a standard solution of soap in weak alcohol, titrated against a standard solution of calcic chloride. The valuation is in so-called degrees, each degree being equal to 1 grain of calcic carbonate, or its equivalent, in the imperial gallon. The process is an old and familiar one, but open to many objections from a scientific point of view. The scale of degrees is arbitrary, and is seriously interfered with by the presence of varying proportions of magnesia. We are indebted, primarily to Mohr, and subsequently to Hehner, for an ingenious method of determining both the temporary and permanent hardness of a water without the use of soap solution. The standard solutions required are -j^ sodic carbonate and -T^J- sulphuric acid. Each c.c. of standard acid exactly neutralizes 1 m.gm. of CaCO 3 , and each c.c. of the alkali precipitates the like amount of CaCO 3 , or its equivalent in magnesia, in any given water. 18. AMMONIA. 67 The Analysis : 100 c.c. of the water is tinted with an indicator of suitable character, heated to near boiling, and standard acid cautiously added until the proper change of colour occurs. Hehner recommends phenacetolin ; but my own experiments give the preference to lacmoid, which is also commended by Thompson. Draper (C. N. li. 206) points out the value of lacmoid and carminic acid for such a process, and I fully endorse his opinion with respect to both indicators. Practice is desirable in order to recognize the precise .end-reaction. The number of c.c. of acid used represents the number of Clark's degrees of temporary hardness per 100,000. To obtain degrees per gallon, multiply the number of c.c. by O'V. The permanent hardness is ascertained by taking 100 c.c. of the water and adding to it a rather large known excess of the standard sodic carbonate. The quantity must of course be regulated by the amount of sulphates, chlorides, or nitrates of lime and magnesia present in the water ; as a rule, a volume equal to the water will more than suffice. Evaporate in a platinum dish to dry ness (glass or porcelain will not do, as they affect the hardness), then extract the soluble portion with small quantities of distilled water, through a very small filter, and titrate the filtrate with the standard acid for the excess of sodic carbonate : the difference represents the permanent hardness. Some waters contain alkaline carbonates, in which case there is of course no permanent hardness, because the salts to which this is due are decomposed by the alkaline carbonate. In examining a water of this kind, the temporary hardness will be shown to be greater than it really is, owing to the alkaline carbonate ; and the estimation for permanent hardness will show more sodic carbonate than was actually added. If the difference so found is deducted from the temporary hardness, as first noted, the remainder will be the true temporary hardness. AMMONIA. 18. IN estimating the strength of solutions of ammonia by the alkalimetric method, it is better to avoid the tedious process of weighing any exact quantity, and to substitute for it the following plan, which is applicable to most liquids for the purpose of ascertaining both their absolute and specific weights. Let a small and accurately tared flask, beaker, or other convenient vessel be placed upon the balance, and into it 10 c.c. of the ammoniacal solution delivered from a very accurately graduated 10 c.c. pipette. The weight found is, of course, the absolute weight of the liquid in grams ; suppose it to be 9'65 gm., move the decimal point one place to the left, and the specific weight or gravity is at once given (water being 1), which in this case is 0'965. It must be borne in mind that this system can only be used properly with tolerably delicate balances and very accurate pipettes. The latter should invariably be tested by weighing distilled water at 16 C. The 10 c.c., weighing 9'65 gm., is now titrated with normal acid, of which 49 c.c. are required, therefore 49 x 0'017=0-833 gm. NH 3 =8'64 per cent, of real ammonia; according to Otto's table 0'965 sp. gr. is equal to 8'50 per cent. Ammonic carbonate, and a mixture of the same with bicarbonate, as it most commonly occurs in commerce, may be titrated direct with normal F 2 68 VOLUMETRIC ANALYSIS. 18. acid for the percentage of real ammonia, using methyl orange as indicator. The carbonic acid can be determined by precipitating the solution while hot with baric chloride, and when the precipitate is well washed, dissolving it with an excess of normal acid and titrating backward with normal alkali ; the number of c.c. of acid used multiplied by 0'022 (the | mol. wt. of CO 2 ) will give the weight of carbonic acid present in the sample. 1. Estimation of Combined Ammonia by distillation with Alkalies or Alkaline Earths. This method allows of the expulsion of ammonia from all its- salts. Caustic soda, potash, or lime, may any of them be used where no organic nitrogenous compound exists in the substance ; but should such be the case, it is preferable to use freshly ignited magnesia. The distilling apparatus may conveniently be arranged by con- necting an ordinary well-stoppered small retort to a small Liebig condenser, and leading the distilled gas into a vessel containing an excess of normal acid. After the operation is ended, the excess of acid is ascertained by residual titration with normal alkali or J ammonia, and thus the amount of displaced ammonia is found. The retort must be so supported that its neck inclines well upwards, in order that any alkali mechanically carried into it by the spray which occurs during ebullition shall not reach the condenser. An angle of about 30 suffices ; and in order that a convenient connection may be made with the condenser, the end of the retort is bent downward, and the connection securely made with india-rubber tubing. In like manner, the end of the condenser is elongated by a glass tube and india-rubber joint, so that the tube dips into a two-necked bottle or bulb, containing the measured normal acid; the end of this tube should be cut obliquely, and reach nearly,- but not quite, to the surface of the acid. The outlet of the receiver is fitted with a tube containing glass wool, broken glass, or fibrous asbestos, Avetted with a portion of the normal acid, so that any traces of ammonia which may possibly escape condensation in the bulk of the acid may be retained. The retort containing the ammoniacal compound in solution being securely fixed, and all the apparatus tightly connected, the stopper of the retort is removed, and a strong solution of caustic alkali, or, in case of compounds in which ammonia is quickly released, pieces of solid alkali are rapidly introduced, the stopper inserted, and the distillation forthwith commenced. Lime or magnesia, suspended in water, must be added through a small funnel ; the distillation is continued until the steam has washed all traces of ammonia out of the condenser tube into the normal acid. Cold water is of course run continuously through the condenser as usual. Finally, the tubes connected with the receiver are well washed out into the bulk of normal acid, methyl orange added, and the titration completed with normal alkali or f ammonia. AMMONIA. 69 Each c.c. of normal acid neutralized by the displaced ammonia represents O'OIT gni. XH :5 . The apparatus shown in fig. 23 is of great value in determining accurately all the forms of ammonia which can be displaced by' soda, potash, or lime, and the gas so evolved collected in a known volume in excess of normal acid, the excess of acid being after- wards found by residual titration with normal alkali or | ammonia. Many modifications of this apparatus have been suggested, such as the introduction of a condenser between the two flasks to cool the distillate ; another is the use of a U tube containing some standard acid in place of c. I do not find that any of these modifications are required to secure accuracy, if the apparatus is tightly fitted. It is, however, necessary that a bulb should exist in the distilling tube, just above the cork of the distilling flask, otherwise I have found that the spray from the boiling liquid Is occasionally projected into the tube, and is blown over with the condensed steam. Another precaution is advisable where dilute 70 VOLUMETEIC ANALYSIS. 18. liquids are boiled, and much steam generated, that is, to immerse the condenser flask in cold water. The little flask, holding about 200 c.c. and placed upon the wire gauze, contains the ammoniacal substance. The tube d is filled with strong solution of caustic potash or soda. The large flask holds about half a liter, and contains a measured quantity of normal acid, part being contained in the tube c, which is tilled with glass wool or broken glass, and through which the normal acid has been poured. The stoppers of the flasks should be caoutchouc, failing which, good corks soaked in melted parafline may be used. The substance to be examined is weighed or measured, and put into the distilling flask with a little water, the apparatus then being made tight at every part ; some of the caustic alkali is allowed to flow in by opening the clip, and the spirit lamp is lighted under it. The contents are brought to gentle boiling, taking care that the froth, if any, does not enter the distilling tube. It is well to use a movable gas burner or common spirit- lamp held under the flask in the hand ; in case there is any tendency to boil over, the heat can be removed immediately, and the flask blown upon by the breath, which reduces the pressure in a moment. In examining guano and other substances containing ammoniacal salts and organic matter by this means, the tendency to frothing is considerable ; and unless the above precautions are taken, the accuracy of the results will be interfered with. The distilling tube has both ends cut obliquely; and the lower end nearly, but not quite, reaches to the surface of the acid, to which a little methyl orange may be added. The quantity of normal acid used must, of course, be more than sufficient to combine with the ammonia produced ; the excess is- afterwards ascertained by titration with normal alkali or f ammonia. It is advisable to continue the boiling for say ten or fifteen minutes, then wait a minute or two to allow all the ammonia to be absorbed ; then opening the clip, blow through the pipette so as to force all the remaining gas into the acid flask. The tube c must be thoroughly washed out into the flask with distilled water, so as to carry down the acid with any combined gas which may have reached it ; the distilling tube must also be washed through into the acid flask. The titration then proceeds as usual. This process is particularly serviceable for testing commercial animomacal salts, gas liquor, etc. (see below). The results are extremely accurate. 2. Indirect Method. Ill the case of tolerably pure ammoniacal salts or liquids, free from acid, a simple indirect method can be used, which is as follows : If the ammoniacal salt be boiled in an open vessel with normal caustic alkali, the ammonia is entirely set free, leaving its acid combined with the fixed alkali. If, therefore, the quantity of alkaline solution is known, the excess beyond that, necessary to supplant the ammonia, can be found by the ordinary system of titration. The boiling of the mixture must be continued till a piece of red litmus paper, held in the steam from the flask, is no longer turned blue. Example : 1'5 gm. of purest sublimed ammonic chloride was placed in a wide-mouthed flask with 40 c.c. of normal soda, and boiled till all ammonia was expelled, then titrated back with normal sulphuric acid, of which 11'9 c.c. were required; 2S'l c.c. of normal alkali had therefore been neutralized, which multiplied by 0'05337, the factor for ammonic chloride, gave T499 gm., instead of 1'5 gm. originally taken. 18. GAS LIQUOE. 71 3. Technical Analysis of Gas Liquor, Sulphate of Ammonia, Sal Ammoniac, etc., arranged for the use of Manufacturers. This process depends upon the fact, that when ammoniacal salts are heated with caustic soda, potash, or lime, the whole of the ammonia is expelled in a free state, and can by a suitable apparatus (fig. 24) be estimated with extreme accuracy (see 18.1). The set of apparatus here described consists of a distilling flask B, and condensing flask F, fitted together in such a manner, that no loss of free ammonia can occur ; the whole of the ammonia being liberated from the distilling flask into a measured quantity of free acid contained in the condensing flask, where its amount is after- wards found by the method hereinafter described. Analysis of Gas Liquor. This liquid consists of a solution of carbonates, sulphates, hyposulphites, sulphides, cyanides, and other salts of ammonia. The object of the ammonia manufacturer is to get all these out of his liquor into the form of sulphate or chloride as economically as possible. The whole of the ammonia existing as free or carbonate in the liquor, can be distilled off at a steam heat; the fixed salts, however, require to be heated with soda, potash, or lime (the latter is generally used on a large scale as most economical), in order to liberate the ammonia contained in them. The valuation of gas liquor is almost universally made in Great Britain by Twaddle's hydrometer, every degree of which is taken to represent what is technically called "two-ounce strength;" that is to say, a gallon of such liquor should neutralize exactly two ounces by weight of concentrated oil of vitriol thus 5 degrees, Twaddle, is called " ten-ounce " liquor but experiment has clearly proved, that although the hydrometer may be generally a very convenient indicator of the commercial value of gas liquor, it is not accurate enough for the manufacturer who desires to work with the utmost economy. Sometimes the liquor contains a good deal of free ammonia, and in such case the hydrometer would show it to be weaker than it really is ; on the other hand, sometimes, from accidental causes, other solid matters than ammonia salts occur in the liquor, and the hydrometer shows it to be stronger than it really is. The method of saturation, by mixing standard acid with the liquor, is perhaps more correct than the hydrometer; but this system is entirely at fault in the presence of much fixed ammonia, and is, moreover, a very offensive and poisonous operation. The apparatus here described is exactly the same on a small scale as is necessary in the actual manufacture of sulphate of ammonia in quantities ; and its use enables any manufacturer to tell to a fraction how much sulphate of ammonia he ought to obtain from any given quantity of gas liquor. It also enables him to tell exactly how much ammonia can be distilled off" with heat alone, and how much exists in a fixed condition requiring lime. The measures used in this process are on the metrical system ; HH I I I I I h'"l""l u u 18. GAS LIQUOR. 73 the use of these may, perhaps, at first sight appear strange to English manufacturers ; but as the only object of the process is to obtain the percentage of ammonia in any given substance, it is a matter of no importance which system of measures or weights are used, as when once the percentage is obtained, the tables will at once show the result in English terms of weight or measure. a is a small pipette, holding 10 cubic centimeters to the mark in neck : this is the invariable quantity of liquor used for the analysis, whatever the strength may be. This measure is filled to the mark by suction and transferred, without spilling a drop, to flask B the fittings being previously removed the tube C is then filled in the same manner, with strong caustic soda solution from a clean cup or other vessel, in order to do which, the clip at the top must be opened ; the cork is then replaced, and the flask B is then securely imbedded in perfectly dry sand, in the sand- bath D. The graduated pipette E is then filled in the same manner to the O mark, with standard acid, and 20, 30, 40, or 50 c.c. (according to the estimated strength of the liquor) allowed to flow into the flask F, through the cup G, which is filled with glass wool or fibrous asbestos. The wool should be completely wetted with the acid, so that any vapours of ammonia which may escape the acid in the flask shall become absorbed by the acid. The quantity of standard acid to be used is regulated by the approximately known strength of the liquor, which of course can be told by Twaddle's hydrometer: thus, for a liquor of 3 Twaddle=6-oz. liquor, 20 c.c. 8-oz , 25 c.c. 10-oz., 30 c.c. of acid will be sufficient but there must always be an excess. The required quantity cm always be approx- imately known, since every 10 c.c. of acid represents 1 per cent, of ammonia. The standard acid having been carefully measured through the glass wool, the apparatus is fitted together at H by the elastic tube, and the india-rubber stoppers securely inserted in both flasks; this being done, the lamp is lighted under the sand-bath, and at the same time the spring-clip on C is pressed, so as to allow about two-thirds of the caustic soda to flow into B ; the rest will gradually empty itself during the boiling. The heat is continued to boiling, and allowed to go on till the greater bulk of the liquid in B is boiled away into F. A quarter of an hour is generally sufficient' for this purpose, but if the boiling is continued till the liquid in B just covers the bottom of the flask, all the ammonia will have gone over to F; during the whole operation the distilling tube must never dip into the acid in F. In order to get rid of the last traces of ammonia vapour out of B, the lamp is removed, and the mouth being applied to the tube over the spring clip, the latter is opened, and a good blast of air immediately blown through. The apparatus may then be detached at H ; distilled or good boiled drink- ing water is then poured repeatedly through G in small quantities, till all traces of acid are removed; and some water is also poured down the distilling tube, so as to wash all traces of ammonia which may be hanging about into flask F. This latter now contains all the ammonia out of the sample of liquor, with an excess of acid, and it is necessary now to find out the quantity of acid in excess. This is done by means of the burette I, and the standard solution of soda, which soda is of exactly the same strength as the standard acid. In order to find out how much of the standard acid has been neutralized by the ammonia in the liquor distilled, the burette I is filled to O with standard soda, and one drop of methyl orange, or a sufficiency of any other indicator, other than phenolphthalein, being added to the cooled contents of flask F, the soda is slowly dropped into it from the burette, with constant shaking, until the indicator changes colour. The number of c.c. of soda so used, deducted from th3 number of c.c. of standard acid used, will show the number neutralized by the ammonia in the liquor distilled ; therefore, if 74 VOLUMETRIC ANALYSIS. 18. the number of c.c. of soda used to destroy the pink colour be deducted from the number of c.c. of standard acid originally used, it will show the number of c.c. of standard acid neutralized by the ammonia, which has been distilled out of the liquor, and the strength of the solutions is so arranged that this is shown without any calculation. The following examples will suffice to show this : Suppose that a liquor is to be examined which marks 5 Twaddle, equal to 10-ounce liquor; 10 c.c. of it is distilled into 30 c.c. of the standard acid, and it has afterwards required C c.c. of standard soda to neutralize it; this leaves 24 c.c. as the volume of acid saturated by the distilled ammonia, and this represents 2'4 per cent. ; and on referring to the table it is found that this number corresponds to a trifle more than 11 ounces, the actual figures being 2*384 per cent, for 11-ounce strength. The strength of the standard soda* and acid solutions is so arranged, that when 10 c.c. of liquor is distilled, every 10 c.c. of acid solution represents 1 per cent, of ammonia in the liquor. In like manner 13 c.c. of acid will represent 1*3 per cent, of ammonia corresponding to 6-ounce liquor. The burette is divided into tenths of a cubic centimeter, and those who are familiar with decimal calculations can work out the results to the utmost point of accuracy ; the calculation being, that every 1 per cent, of ammonia requires 4 '61 ounces of concentrated oil of vitriol (sp. gr. 1 '845) per gallon, to convert it into sulphate : thus, suppose that 10 c.c. of any given liquor have been distilled, and the quantity of acid required amounts to 18 - 6 c.c., this is 1*86 per cent., and the ounce strength is shown in ounces and decimal parts as follows : 4-61 1-86 2766 3688 461 8.5746 ounces of oil of vitriol. The liquor is therefore a trifle over 8J-ounce strength. Spent Liquors. It is frequently necessary to ascertain the percentage of ammonia in spent liquors, to see if the workman have extracted all the available ammonia. In this case the same measure, 10 c.c. of the spent liquor, is taken, and the operation conducted precisely as in the case of a gas liquor. Example : 10 c.c. of a spent liquor were distilled, and found to neutralize 3 c.c. of acid ; this represents three-tenths of a per-ceut. equal to 1-oz. and four-tenths of an ounce, or nearly 1| oz. Such a liquor is too valuble to throw away, and should be worked longer to extract more ammonia. * Soda is recommended in preference to ammonia for the standard alkali, as it was found that a solution of ammonia containing *01 gin. per c.c. readily lost strength by keeping. GAS LIQUOR. 75 Analysis of Sulphate of Ammonia, or Sal Ammoniac : An average sample of the salt being drawn, ten grams are weighed, transferred without loss to a beaker or flask having a 100 c.c. mark upon it, distilled or boiled drinking water poured on it, and well stirred till dissolved, and finally water added exactly to the mark. The 10 c.c. measure is then filled with the solution, and emptied into the distilling flask B; 30 c.c. of standard acid are put into flask F and the distillation carried on precisely as in the case of a gas liquor. The number of c.c. of standard acid required shows directly the percentage of ammonia; thus, if 24*6 c.c. are used in the case of sulphate, it contains 24' 6 per cent, of ammonia. The liquors when tested must be measured at ordinary tempera- tures, say as near to 60 F. as possible. The standard solutions must be kept closely stoppered and in a cool place. The following table is given to avoid calculations ; of course, it will be understood that the figures given are on the assumption that the whole of the ammonia contained in the liquor is extracted in the manufacture as closely as it is in the experiment. With the most perfect arrangement of plant, however, this does not as a rule take place ; but it ought .to be very near the mark with proper apparatus, and care on the part of workmen. Approxi- Acid in c c. NH ' and tenths. Ounce strength per gallon. Weight of Sulphuric Acid in pounds and decimal parts required for each gallon of liquor. Yield of Sulphate per gallon in Ibs. and decimal parts. C. O. V. 169 Tw. B. O. V. 144 Tw. Chamber Acid 120 Tw. 2-2 '2168 1 0625 0781 0893 0841 4-3 4336 2 1250 1562 1786 1682 6'5 6504 3 1875 2343 2679 2523 87 8672 4 2500 3124 3572 3364 lO'l 1-0840 5 3125 3905 4465 4205 13'0 1-3000 6 3750 4686 5358 5046 15-2 1-5176 7 4375 5467 6251 5887 17'3 1-7344 8 5000 6248 7144 6728 19-5 1-9512 9 5625 7029 8037 7569 21-7 2-1680 10 6250 7810 8930 8410 23-8 2-3840 11 6875 8591 9823 9251 26-0 I 2-6016 12 7500 9372 1-0716 1-0092 28-2 2-8184 13 8125 1-0153 1-1609 1-0933 30-4 30350 14 8750 1-0934 1-2502 1-1774 32-5 3-2520 15 9375 1-1715 1-3395 1-2615 34-7 3-4688 16 1-0000 1-2496 1-4288 1-3456 36-9 3-6856 17 1-0625 1-3277 1-5181 1-4297 39-0 3-9024 18 1-1250 1-4058 1-6074 1-5138 41-2 4-1192 19 1-1875 1-4839 1-6967 1-5979 43-3 4-3360 20 1-2500 1-5620 1-7860 1-6820 The weight of sulphuric acid being given in decimals renders it very easy to arrive at the weight necessary for every thousand gallons of liquor, by simply moving the decimal point ; thus 8-oz. liquor would require 500 Ibs. of concentrated oil of vitriol, 625 Ibs. 76 VOLUMETRIC ANALYSIS. 18. of brown oil of vitriol, or 71 4 J Ibs. chamber acid for every 1000 gallons, and should yield in all cases 672*8 (say 673) Ibs. of sulphate. 4. Combined Nitrogen in Organic Substances. This process consists in heating the dried substance in a combustion tube with soda lime, by which the nitrogen is con- verted into ammonia; and this latter being led into a measured volume of normal acid contained in a suitable bulb apparatus, combines with its equivalent quantity ; the solution is then titrated residually with standard alkali for the excess of acid, and thus the quantity of ammonia found. As the combustion tube Avith its arrangements for organic analysis is well known, and described in any of the standard books on general analysis, it is not necessary to give a description here. Instead of leading the ammonia through normal acid, hydro- chloric acid of unknown strength may be used, the liquid brought into the distilling apparatus, and the ammonic chloride estimated by the process described in 18.1. When it is necessary to estimate very minute portions of ammonia, it may be brought into the form of chloride, and estimated by decinormal silver solution ( 38) ; or in many cases preferably by Nessler's test, described in the section on Water Analysis. 5. Kjeldahl's Method. This has met with considerable acceptance in lieu of the combustion method, on account of its easy management and accurate results. Moreover, unlike the combustion method, the ammonia .is obtained free from organic matters or colour. It was first described by the author (Z. a. C. xxii. 366), and has since been commented upon by many operators, among whom are Warington (C. N. lii. 162), Pfeiffer and Lehmann (Z. a. C. xxiv. 388), Marcker and others (Z. a. C. xxiii. 553; xxiv. 199, 393; xxv. 149, 155; xxvi. 92; xxvii. 222, 398). The process consists in heating the organic substance in a flask, with concentrated sulphuric acid, to its boiling point, and when the oxidation is nearly completed, adding finely powdered per- manganate of potash in small quantities till a green or pink colour remains constant : the whole of the nitrogen is thus converted into ammonic sulphate. The flask is then cooled, diluted with water somewhat, excess of caustic soda added, the ammonia distilled off into standard acid, and the amount found by titration in the usual way. Some practical difficulties occur in the process : for instance, the final distillation with concentrated alkali gives rise to bumping, with a tendency to spray the liquid into the tube which leads into 18. COMBINED NITROGEN. 77 the acid. To overcome this, it is necessary to put two or three pieces of metallic zinc into the flask, which, by the generation of hydrogen gas, facilitate the operation. Experience has however shown that though this is the case, there is a greater chance of the distillate being contaminated with traces of alkali. This is especially so if a large excess of alkali with much zinc is used. Therefore it is proper to use only a moderate excess of alkali, 'and not much zinc. Another difficulty is, that if nitrates are present in the compound analyzed, their reduction to ammonia is not certain nor regular, and unless this difficulty be overcome the value of the process is limited. Warington has investigated this contingency, and finds that in order to get rid of them before the treatment with sulphuric acid, the material is best digested with ferrous sulphate and strong hydrochloric acid with heat, finally carried to dryness the sulphuric acid is then added, and the process carried out as recommended by Kjeldalil. When this method is adopted the nitrates are separately estimated by some other method. In the modified method given below, the IS r existing as nitrate is converted into- ]S T H 3 , so that one operation estimates tiae_ whole of the nitrogen as ammonia. The experience of many hundreds of operators since this method was first introduced has resulted in rendering it as perfect as need be, and no better arrangement of the process can be given than that adopted by the U. S. Association of Official Agricultural Chemists (1887 8). This arrangement in all essential particulars will now be described, omitting the details as to some of the special forms of apparatus, and which are not absolutely essential. The method requires the following articles : 1 . Standard acid, which may be either sulphuric or hydrochloric ; a convenient strength is semi-normal. 2. Standard alkali, either ammonia, soda, or potash, of corres- ponding strength to the acid. 3. Concentrated sulphuric acid free from nitrates and ammonic sulphate.* 4. Mercuric oxide prepared in the wet way or metallic mercury. 5. Powdered potassic permanganate. 6. Granulated zinc. * Commercial oil of vitriol frequently contains ammonia, owing to the fact that makers sometimes add ammonic sulphate during concentration in order to get rid of nitrous compounds. Meldola and Moritz state that any traces of ammonia may be destroyed by digesting the acid for two or three hours at a temperature below boiling, with sodic or potassic nitrite, in the proportion of 0'5 gm. of the salt to 100 c.c. of acid. Lunge objected to this treatment, because of the probable formation of nitro- sulphuric acid. Experiments have since been made by Moritz which prove that the objection is groundless, provided the digestion is carried on for a period sufficient to expel the nitrous acid (J. S. C. I. ix. 443). The purification of the acid may of course be obviated by ascertaining once for all the amount of ammonia in any given stock of acid, by making a blank experiment with pure sugar and allowing in all cases for the amount of .N H ;i so found. 78 VOLUMETRIC ANALYSIS. 18. 7. Solution of potassic sulphide in water, 40 gm. in the liter. 8. A saturated solution of caustic soda free from nitrates or nitrites. 9. A suitable indicator cochineal is recommended, but any other except phenolphthalein may be used. 10. Digestion flasks with long neck, holding about 200 250 c.c. These flasks should be well annealed and not too thick the neck about f inch wide, and 3J 4 inches long. 11. Distillation flasks of hard Bohemian glass, 550 600 c.c. capacity, fitted with a rubber stopper and a bulb tube above to prevent the spray of the boiling alkaline liquid from being carried over into the condenser. I invariably use a tube of f in. bore, with two bulbs 1 in. diameter just above the stopper, and have proved the absolute security of this tube in preventing the passage of any trace of alkali into the distillate. 12. The condenser. Owing to the undoubted solubility of glass in fresh distilled water containing ammonia, it is advisable to have the condenser tube made of block tin. This should be about half-an-inch wide externally, and is connected with the bulb tube of the distilling flask with stout pure rubber tube. It is surrounded by either a metal or glass casing, through which cold water is passing in the usual manner. It is very easy to fit up such an arrangement with the condenser tubes made entirely of glass sold by the dealers in chemical apparatus. The end of the condenser tube may be simply inserted into the neck of a flask containing the standard acid, or it may have a delivery tube connected by rubber leading into a beaker. There is no necessity in any case for dipping the delivery tube into the acid. The condenser recommended by the Association, when a number of estimations are carried on at the same time, was devised by Professor Johnson, and consists of a copper tank supported by a wooden frame, so that its bottom is 1 1 inches above the work- bench on which it stands. This tank is 16 inches high, 32 inches long, and 3 inches wide from front to back, widening above to 6 inches. It is provided with a water-supply tube which goes to the bottom and a larger overflow pipe above. The block tin condensing tubes, whose external diameter is f of an inch, seven in number, enter the tank through holes in the front side of it near the top, above the level of the overflow, and pass down perpendicularly through the tank and out through rubber stoppers tightly fitted into holes in the bottom. They project about 1J inches below the bottom of the tank, and are connected by short rubber tubes with glass bulb tubes of the usual shape, which dip into glass precipitating beakers. These beakers are 6| inches high, 3 inches in diameter below, somewhat narrower above, and of about 500 c.c. capacity. The titration can be made directly in them. The seven distillation flasks are supported on a sheet-iron shelf attached to the wooden frame that supports the tank in front 18. COMBINED NITROGEN. 79 of the latter. Where each flask is to stand a circular hole is cut, with three projecting lips, which support the wire gauze under the flask, and three other lips which hold the flask in place and prevent its moving laterally out of place while distillation is going on. Below this sheet-iron shelf is a metal tube carrying seven Bunsen burners, each with a stop-cock like those of a gas combustion furnace. These burners are of larger diameter at the top, which prevents smoking when covered with fine gauze to prevent the flame from striking back. 13. The stand for holding the digestion flasks consists of a pan of sheet-iron, 29 inches long by 8 inches wide, on the front of which is fastened a shelf of sheet-iron as long as the pan, 5 inches wide and 4 inches high. In this are cut six holes If inches in diameter. At the back of the pan is a stout wire running lengthwise of the stand, 8 inches high, with a bend or depression opposite each hole in the shelf. The digestion flask rests with its lower part over a hole in the shelf and its neck in one of the depressions in the wire frame, which holds it securely in position. Heat is supplied by low Bunsen burners below the shelf. With a little care the naked flame can be applied directly to the flask without danger. Analysis : Prom 0'7 to 1 gm. of the substance to be analysed is brought into a digestion flask with approximately 0'7 gm. of mercuric oxide or 0'5 gm. metal and 20 c.c. of sulphuric acid.* The flask is placed on wire gauze over a small Bunsen burner in an upright position, or the frame above described in an inclined position, and heated below the boiling-point of the acid for from five to fifteen minutes, or until frothing has ceased. The heat is then raised till the acid boils briskly. No further attention is required till the contents of the flask have become a clear liquid, which is colourless, or at least has only a very pale straw colour. The flask is then removed from the frame, held upright, and, while still hot, permanganate is dusted in carefully, and in small quantity at a time, till after shaking the liquid remains of a green or purple colour. After cooling, the contents of the flask are transferred to the distilling flask with water, and to this 25 c.c. of potassic sulphide solution are added, 50 c.c. of the soda solution, or sufficient to make the reaction strongly alkaline, and a few pieces of granulated zinc. The flask is at once connected with the condenser, and the contents of the flask are distilled till all ammonia has passed over into the standard acid contained in the precipitating flask previously described, and the concentrated solution can no longer be safely boiled. This operation usually requires from twenty to forty minutes. The distillate is then titrated with standard alkali. The use of mercury or its oxide in this operation greatly shortens the time necessary for digestion, which is rarely over an hour and a half in the case of substances most difficult to oxidize, and is more commonly less than an hour. In most cases the use of permanganate is quite unnecessary, but it is believed that in exceptional cases it is required for complete oxidation, and in view of the uncertainty it is always used. Potassic sulphide removes all mercury from solution, and so prevents the formation of mercuro-ammonium * Some albuminoid substances, such as pure isinglass, refuse apparently to yield the theoretical proportion of ammonia by treatment with ordinary sulphuric acid (see Oddy and Cohen, J". S. C. I. ix. 17). This may arise from insufficient heating or from weakness in the acid ; under such circumstances I am inclined to think that the addition of some Nordhausen acid to increase the strength or some potassic sulphate to increase the boiling point of the acid would insure a more perfect decomposition. 80 VOLUMETRIC ANALYSIS. ] 9. compounds which are not completely decomposed by soda solution. The addition of zinc gives rise to an evolution of hydrogen, and prevents violent bumping. Previous to use the reagents should be tested by a blank experiment with sugar, which will partially reduce any nitrates that are present which might otherwise escape notice. The following modification must be used for the determination of nitrogen in substances which contain nitrates when it is desired to use this method. Estimation of Nitrogen, including- the Nitrogen of Nitrates, by a Modified Method of Kjeldahl. Bring from 07 to T4 gm. of the substance to be analyzed into a Kjeldahl digesting flask, add to this 36 c.c. of sulphuric acid containing 2 gm. of salicylic acid, and shake thoroughly. Then add gradually 3 gm. of zinc-dust, shaking the contents of the flask at the same time. Finally, add two or three drops of platinic chloride solution, and place the flask on the stand for holding the digestion flasks, where it is heated over a low flame until all danger from frothing has passed. The heat is then raised until the acid boils briskly, and the boiling continued until white fumes no longer pour out of the flask. This requires about five or ten minutes. Add now about 7 gm. mercuric oxide, and continue the boiling until the liquid in the flask is colourless or nearly so. (In case the contents of the flask are likely to become solid before this point is reached add 10 c.c. more of sulphuric acid.) Complete the oxidation with a little permanganate in the usual way, and proceed with the distillation as described above. The titration of the distillate is made in the usual way with methyl orange or some other indicator other than phenolphthalein. Kjeldahl prefers to titrate a measured small portion of the distillate for excess of acid by the iodine and starch reaction described in 19. The substances available for the accurate estimation of their nitrogen by the Kjeldahl method are: All amides and ammonium bases, the pyridine and chinolin bodies, the alkaloids, the bitter principles, the albumenoids and kindred substances. All nitro, nitroso, azo, diazo, hydrazo, and amido-azo compounds with the compounds of nitric and nitrous acid, however, require previous treatment similar to that suggested by War ing ton, or must be dealt with by the modified process above described. ACIDIMETRY OB THE TITRATION OF ACIDS. 19. THIS operation is simply the reverse of all that has been said of alkalies, and depends upon the same principles as have been explained in alkalimetry. With free liquid acids, such as hydrochloric, sulphuric, or nitric, the strength is generally taken by means of the hydrometer or specific-gravity bottle, and the amount of real acid in the sample ascertained by reference to the tables constructed by Otto, Bineau, or Ure. The specific gravity may very easily be taken with the pipette, as recommended with ammonia, and of course the 19. ACIDIMETRY. 81 real acid may be quickly estimated by normal caustic alkali and an appropriate indicator. In the case of titrating concentrated acids of any kind it is preferable in all cases to weigh accurately a small quantity, dilute to a definite volume, and take an aliquot portion for the analysis. Delicate End-reaction in Acidimetry. If an alkaline iodate or bromate be added to a solution of an alkaline iodide in the presence of a mineral acid, iodine is set free and remains dissolved in the excess of alkaline iodide, giving the solution the well-known colour of iodine. This reaction has been long observed, and is capable of being used with excellent effect as an indicator for the delicate titration of acids, and therefore of alkalies, by the residual method. Kjeldahl, for instance, uses it in his ammonia process, where the distillate contains necessarily an excess of standard acid. The reaction is definite in character, and may be used in various ways in volumetric processes. For instance, potassic bromate liberates iodine in exact proportion to its contained oxygen in the presence of excess of dilute mineral acid, and the iodine so liberated may be accurately titrated with sodic thiosulphate. In acidimetry, however, the method is simply used for its exceeding delicacy as an end-reaction, one drop of T ^Q- sulphuric, nitric, or hydrochloric acid being quite sufficient to cause a deep blue colour in the presence of starch. The adjustment of the standard liquids is made as follows : 2 or 3 c.c. of y^- acid is run into a flask, diluted somewhat with water, and a crystal or two of potassic iodide thrown in ; 1 or 2 c.c. of a 5 per cent, solution of potassic iodate are then added, which at once produces a brown colour, due to free iodine. A solution of sodic thiosulphate is added from a burette, with constant shaking, until the colour is nearly discharged ; a few drops of freshly prepared starch indicator are now poured in, and the blue colour removed by the very cautious addition of thiosulphate. The quantity of thiosulphate used represents the comparative strengths of it and the standard acid, and is used as the basis of calculation in other titrations. The first discharge of the blue colour must be taken in all cases as the correct ending, because on standing a few minutes the blue colour re-occurs, due to some obscure reaction from the thiosulphate. This has been probably regarded as one of the drawbacks of the process, and another is the instability of the thiosulphate solution ; but these by no means invalidate its accuracy, and it moreover possesses the advantage of being applicable to excessively dilute solutions, and may be used by artificial light. The organic acids cannot be estimated by this method, the action not being regular. Neutral alkaline and alkaline earthy salts have no interference, but salts of the organic acids and borates must be absent. 82 VOLUMETRIC ANALYSIS. 20. ACETIC ACID. 20. IN consequence of the anomaly existing between the sp. gr. of strong acetic acid and its actual strength, the hydrometer is not reliable, but the volumetric estimation is now rendered extremely accurate by using phenolphthalein as indicator, acetates of the alkalies and alkaline earths having a perfectly neutral behaviour to this indicator. Even coloured vinegars may be titrated when highly diluted. Where, however, the colour is too much for this method to succeed, recourse must be had to litmus paper, upon which streaks of the liquid should be made from time to time during the titration with a glass rod. Several processes have at various times been suggested for the accurate and ready estimation of acetic acid, among which is that of Greville Williams, by means of a standard solution of lime syrup. The results obtained were very satisfactory. C. Mohr's process consists in adding to a known quantity of the acid a known excessive quantity of baric or calcic carbonate in fine powder. Pure calcic carbonate is preferable, as it dissolves more readily than baric salt. When the decomposition is as nearly as possible complete in the cold, the mixture must be heated to expel the CO 2 , and to complete the saturation; the residual carbonate is then brought upon a filter, washed with boiling water, and titrated with excess of normal acid and back with alkali. This process is applicable in all cases, and however dark the colour may be. In testing the impure brown pyroligneous acid it is especially serviceable. Pettenkofer titrates acetic acid or vinegar with a known excess of baryta water; and estimates the excess of the latter with ~-fj nitric or oxalic acid by the help of turmeric paper. The titration of acetic acid or vinegar may also be performed by the ammonio-cupric solution described in 14.10. 1. Free Mineral Acids in Vineg-ar. Hehner has devised an excellent method for this purpose (Analyst i. 105). Acetates of the alkalies are always present in commercial vinegar; and when such vinegar is evaporated to dryness, and the ash ignited, the alkalies are converted into carbonates having a distinct alkaline reaction on litmus; if, however, the ash has a neutral or acid reaction, some free mineral acid must have been present. The alkalinity of the ash is diminished in exact proportion to the amount of mineral acid added to the vinegar as an adulteration. Hence the following process : 50 c.c. of the vinegar are mixed with 25 c.c. of ^ soda or potash, evaporated to dryness, and ignited at a low red heat to convert the acetates into carbonates ; when cooled, 25 c.c. of ^ acid are added ; the mixture heated to expel CO-, and filtered ; after washing the residue, the filtrate and washings are exactly 20. ACETIC ACID. 83 titrated with yV alkali ; the volume so used equals the amount of mineral acid present in the 50 c.c. of vinegar. 1 c.c. T ^ alkali=0-0049 gm. H 2 SO 4 or 0'003637 gin. HC1. If the vinegar contains more than Qr2 per cent, of mineral acid, more than 25 c.c. of -^ alkali must be used to the 50 c.c. vinegar before evaporating and igniting. 2. Acetates of the Alkalies and Earths. These salts are converted by ignition into carbonates, and can be then residually titrated with normal acid ; no other organic acids must be present, nor must nitrates, or similar compounds decomposable by heat. 1 c.c. normal acid = '06 gm. acetic acid. 3. Metallic Acetates. Neutral solutions of lead and iron acetates may be precipitated by an excess of normal sodic or potassic carbonate, the precipitate well boiled, filtered, and washed with hot water, the filtrate and washings made up to a definite volume, and an aliquot portion titrated with N or yV acid ; the difference between the quantity so used and calculated for the original volume of alkali will represent the acetic acid. If such solutions contain free acetic or mineral acids, they must be exactly neutralized previous to treatment. If other salts than acetates are present, the process must be modified as follows : Precipitate with alkaline carbonate in excess, exactly neutralize with hydrochloric acid, evaporate the whole or part to dryness, ignite to convert the acetates into carbonates, then titrate residually with normal acid. Any other organic acid than acetic will, of course, record itself in terms of acetic acid. 4. Commercial Acetate of Lime. The methods just described are often valueless in the case of this substance, owing to tarry matters, which readily produce an excess of carbonates. Fresenius (Z. a. C. xiii. 153) adopts the following process for tolerably pure samples : 5 gm. are weighed and transferred to a 250 c.c. flask, dissolved in about 150 c.c. of water, and 70 c.c. of normal oxalic acid added ; the flask is then well shaken, and filled to the mark, 2 c.c. of water are added to allow for the volume occupied by the precipitate, the whole is again well shaken, and left to settle. The solution is then filtered through a dry filter into a dry flask : the volume so filtered must exceed 200 c.c. 100 c.c. are first titrated with normal alkali and litmus; or, if highly coloured, by help of litmus or turmeric paper ; the volume used multiplied by 2' 5 will give the volume for 5 gm. Another 100 c.c. are precipitated with solution of pure calcic acetate in slight excess, warmed gently, the precipitate allowed to settle somewhat, then filtered, well washed, dried, and strongly ignited, in order to convert the oxalate into calcic carbonate or oxide, or a mixture of both. The residue so obtained is then decomposed with excess of normal acid, and titrated residually with normal alkali. By deducting the volume of acid used to neutralize the precipitate from that of the alkali used in the first 100 c.c., and multiplying by 2'5, is obtained the volume of alkali expressing the weight of acetic acid in the 5 gm. of acetate. G 2 8-4 VOLUMETRIC ANALYSIS. 20. In the case of very impure and highly coloured samples of acetate, it is only possible to estimate the acetic acid by repeated distillations with phosphoric acid and water to incipient dryness, and then titrating the acid direct with alkali, each c.c. of which represents 0*006 gm. acetic acid. The distillation is best arranged as suggested by Still we 11 and Gladding, or later by Harcourt Phillips (C. N, liii. 181). A 100 to 120 c.c. retort, the tubulure of which carries a small funnel fitted in with a caoutchouc stopper, and the neck of the funnel stopped tightly with a glass rod shod with elastic tube, is supported upon a stand in such a way that its neck inclines upwards at about forty-five degrees : the end of the neck is drawn out, and bent so as to fit into the condenser by help of an elastic tube. The greater part of the retort neck is coated with flannel, so as to prevent too much condensation. 1 gm. of the sample being placed in the retort, 10 c.c. of a 40 per cent, solution of P 2 O 5 is added, together with as much water as will make about 50 c.c. A small naked flame is used, and if carefully manipulated, the distillation may be carried on to near dryness without endangering the retort. After the first operation the retort is allowed to cool somewhat, then 50 c.c. of hot water added through the funnel, another distillation made as before, and the same repeated a third time, which will suffice to carry over all the acetic acid. The distillate is then titrated with alkali and phenolphthalein. By this arrangement the frothing and spirting is of no con- sequence, and the whole process can be completed in less than an hour. The results are excellent for technical purposes. Weber (Z. a. C. xxiv. 614) has devised a ready and fairly accurate method of estimating the real acetic acid in samples of acetate of lime, based on the fact that acetate of silver is insoluble in alcohol. The Analysis : 10 gm. of the sample in powder is placed in a 250 c.c. flask, a little water added, and heated till all soluble matters are extracted, cooled, and made up to the measure : 25 c.c. are then filtered through a dry filter, put into a beaker, 50 c.c. of absolute alcohol added, and the acetic acid at once precipitated with an alcoholic solution of silver nitrate. The silver acetate, together with any chloride, sulphate, etc., separates free from colour. The precipitate is brought on a filter, well washed with 60 per cent, alcohol till the free silver is removed ; precipitate is then dissolved in weak nitric acid, and titrated with ^ salt solution. Each c.c. represents O'OOG gm. acetic acid. Several trials made in comparison with the distillation method with phosphoric acid gave practically the same results. A good technical process has been devised by Grimshaw (Allen's Organic Analysis i. 397). 10 gm. of the sample is treated with water and an excess of sodic bisulphate (NaHSO 4 ), the mixture diluted to a definite volume, filtered, and a measured portion of the filtrate titrated with standard alkali ; a similar portion meanwhile is evaporated to dryness with repeated moisten- ing .with water, to drive off all free acetic acid. The residue is 21. CITRIC ACID. 85 dissolved and titrated with standard alkali, when the difference between the volume now required and that used in the original solution will correspond to the acetic acid in the sample. Litmus paper is the proper indicator. CITRIC ACID. 21. THIS acid in the free state may readily be titrated with pure normal soda and phenolphthalein. 1 c.c. normal alkali = 0*07 gm. crystallized citric acid. 1. Citrates of the Alkalies and Earths. These citrates may be treated with neutral solution of lead nitrate or acetate, in the absence of other acids precipitable by lead. The lead citrate is washed with a mixture of equal parts alcohol and water, the precipitate suspended in water, and H-S passed into it till all the lead is converted into sulphide; the clear liquid is then boiled to remove H 2 S, and titrated with normal alkali. 2. Fruit Juices, etc. If tartaric is present, together with free citric acid, the former is first separated as potassic bitartrate, which can very well be done in the presence of citric acid, as follows : A cold saturated proof spirit solution of potassic acetate is added to a somewhat strong solution of the mixed acids in proof spirit, in sufficient quantity to separate all the tartaric acid as bitartrate, which after stirring well is allowed to stand some hours; the precipitate is then transferred to a filter, and first washed with proof spirit, then rinsed off the filter with a cold saturated solution of potassic bitartrate, and allowed to stand some hours, with occasional stirring; this treatment removes any adhering citrate. The bitartrate is again brought on to a filter, washed once with proof spirit, then dissolved in hot water, and titrated with normal alkali, 1 c.c. of which= 0'15 gm. tartaric acid. The first filtrate may be titrated for the free citric acid present after evaporating the bulk of the alcohol. 3. Lime and Lemon Juices. The citric acid contained in lemon, lime, and similar juices, may be very fairly estimated by Warington's method (J. O. S. 1875, 934). 15 or 20 c.c. of ordinary juice, or 3 4 c.c. of concentrated juice, are first exactly neutralized with pure normal soda, made up, if necessary, to about 50 c.c., heated to boiling in a salt bath, and so much solution of calcic chloride added as to be slightly in excess of the organic acids present. The mixture is kept at the boiling point for about half-an-hour, the precipitate collected on a filter and washed with hot water, filtrate and washings concen- trated to about 15 c.c., and a drop of ammonia added ; this will produce a further precipitate, which is collected separately on a very small filter by help of the previous filtrate, then washed with a small quantity of hot water. Both filters, with their precipitates, are then dried, ignited at a low red heat, and the ash titrated with normal_or ^ acid, each c.c. of which represents respectively 0'07 or 0'007 gm H 3 Ci + H 2 O. 86 VOLUMETRIC ANALYSIS. 22. OXALIC ACID. C 2 H 2 4 2H 2 0=126. 22. THE free acid can be accurately titrated with normal alkali and phenolphthalein. In combination with alkalies, the acid can be precipitated with calcic chloride as calcic oxalate, where no other matters occur precipitable by calcium ; if acetic acid is present in slight excess it is of no consequence, as it prevents the precipitation of small quantities of sulphates. The precipi- tate is well washed, dried, ignited, and titrated with normal acid, 1 c.c. of which=0-063 gm. O. Acid oxalates are titrated direct for the amount of free acid. The reaction continues to be acid until alkali is added in such proportion that 1 molecule acid = 2 atoms alkali metal. The combined acid may be found by igniting the salt, and titrating the residual alkaline carbonate as above. The estimation of oxalic acid in various combinations by permanganate is fully explained in 30.2 (c) and 48. PHOSPHORIC ACID. P 2 5 =142. 23. FREE tribasic phosphoric acid cannot be titrated directly with normal alkali in the same manner as most free acids, owing to the fact, that when an alkaline base (soda, for instance) is added to the acid, a combination occurs in which at one and the same time red litmus paper is turned blue and blue red. This fact has been repeatedly noticed in the case of some specimens of urine, also in milk. In order, therefore, to estimate phosphoric acid, or alkaline phosphates, alkalimetrically, it is necessary to prevent the formation of soluble phosphate of alkali, and to bring the acid into a definite compound with an alkaline earth. Such a method gives tolerably good results when carried out as follows : The solution of free acid, or its acid or neutral combination with alkali in a somewhat dilute state, is placed in a flask, and a known volume of normal alkali in excess added, in order to convert the whole of the acid into a basic salt; a drop or two of rosolic acid is added, then sufficient neutral baric chloride poured in to combine with all the phosphoric acid, the mixture is heated nearly to boiling ; and, while hot, the excess of alkali is titrated with normal acid. The suspended baric phosphate, together with the liquid, possesses a rose-red colour until the last drop or two of acid, after continuous heating and agitation, gives a permanent white or slightly yellowish, milky appearance, when the process is ended. The volume of normal alkali, less the volume of acid, represents the amount of alkali required to convert the phosphoric acid into a chemically neutral salt, e.g. trisodic phosphate. 1 c.c. alkali = 0'02366 gm. P 2 O 5 . In dealing with small quantities of material, it is better to use | or ^5- standard solutions. 24. PHOSPHORIC ACID. 87 Thompson has shown in his researches on the indicators, that phosphoric acid, either in the free state, or in combination with soda or potash, may with very fair accuracy be estimated by the help of methyl orange and phenolphthalein. If, for instance, normal potash be added to a solution of phosphoric acid until the pink colour of methyl orange is discharged, KH 2 P0 4 is formed (112 KHO=H2 P 2 5 ). If now phenolphthalein is added, and the addition of potash continued until a red colour occurs, K 2 HP0 4 is formed. (Again 112 KHO = 142 P 2 5 .) On adding standard hydrochloric or sulphuric acid, until the pink colour of methyl orange reappears, the titration with standard potash may be repeated. Many attempts have been made to utilize these reactions for the accurate estimation of P 2 5 in manures, etc., but, so far as my own experience goes, without adequate success. Titration as Ammonio-magrnesian Phosphate. Stolba (Clietll. Cent. 1866, 727, 728) adopts an alkalimetric method, which depends upon the fact, that one molecule of the double salt requires two molecules of a mineral acid for saturation. The precipitation is made with magnesia mixture, well washed with ammonia, and the latter completely removed by washing with alcohol of 50 or 60 per cent. The precipitate is then dissolved in a measured excess of TTT acid, methyl orange added, and the amount of acid required found by titration with T ^ alkali. Care must be taken that all free ammonia is removed from the filter and precipitate, and that the whole of the double salt is decomposed by the acid before titration, which may always be insured by using a rather large excess and warming. The titration is carried on cold. This method has given me very good results in comparison with the gravimetric method. The same process is applicable to the estimation of arsenic acid, and also of magnesia. 1 c.c. of fV acid = 0-00355 gin. P 2 5 = 0-00575 gm. As 2 3 = 0-002 gm. MgO The reaction in the case of phosphoric acid may be expressed as follows : Mg (XH 4 ) PO 4 + 2HC1 - (XH 4 ) H 2 P0 4 + MgCl 2 . SULPHURIC ANHYDRIDE. SO 3 = 80. 24. XORDHAUSEN or fuming sulphuric acid consists of a mixture of SO 3 and H 2 S0 4 . When it is rich in SO 3 it occurs in a solid form, and being very hygroscopic cannot be weighed in the ordinary manner. Its strength is therefore best taken in the w r ay recommended by Messel as follows : A very thin bulb tube with capillary ends is inserted into a bottle of the melted acid. The 88 VOLUMETRIC ANALYSIS. 25. ends are bent like the letter /, the bulb being in the middle. The bottle should be of such size, that one end of the tube projects out of its mouth. As soon as the bulb is filled, the upper capillary end is sealed, the tube lifted out, wiped, inverted, and the other end sealed ; the tube is then carefully wiped with blotting paper till dry and clean, then weighed. A stoppered bottle, just large enough to allow the tube being placed loosely inside it, is then about one-third filled with water, the tube gently inserted, the stopper replaced, held firmly in by the hand, and a vigorous shake given so as to break the tube. A sudden vibration occurs from contact of the acid with the water, but no danger is incurred. A white cloud is seen on the sides of the bottle, which disappears on shaking for a few minutes. After the bottle is cooled the contents are emptied into a measuring flask. An aliquot portion is then taken out and titrated with -$ iodine for SO 2 , which is always present in small quantity : another portion is titrated with standard alkali and methyl orange for sulphuric acid. TARTARIC ACID. C 4 H 6 6 =150. < 25. THE free acid may be readily titrated with normal alkali and phenolphthalein. 1 c.c. alkali = '07 5 gm. tartaric acid. The amount of tartaric acid existing in tartaric acid liquors is best estimated by precipitation as potassic bitartrate ; the same is also the case with crude argols, lees, etc. Manufacturers are highly indebted to War ing ton and Grosjean for most exhaustive papers on this subject, to which reference should be made by all who desire to study the nature and analysis of all commercial com- pounds of citric and tartaric acids (War ing ton, /. C. S. 1875, 925994; Grosjean, /. C. S. 1879, 341356). Without entering into the copious details and explanations given by these authorities, the methods may be summarized as follows : 1. Commercial Tartrates. In the case of good clean tartars, even though ihey may contain sulphates and carbonates, very accurate results may be obtained by indirect methods. (a) The very finely powdered sample is first titrated with normal alkali, and thus the amount of tartaric acid existing as bitartrate is found ; another portion of the sample is then calcined at a moderate heat, and the ash titrated. By deducting from the volume of acid so used the volume used for bitartrate, the amount of base corresponding to neutral tartrates is- obtained. (b) The whole of the tartaric acid is exactly neutralized with caustic soda, evaporated to dryness, calcined, and the ash titrated with normal acid ; the total tartaric acid is then calculated from the volume of standard acid used; any other organic acid present will naturally be included in this 8 25." TARTAHIC ACID. 89 o amount. In the case of fairly pure tartars, etc., this probable error may be disregarded. Waring ton's description of the first process is as follows : 5 gm. of the finely powdered tartar are heated with a little water to dissolve any carbonates that may be present. If it is wished to guard against crystalline carbonates, 5 c.c. of standard HC1 are added in the first instance, and the heating is conducted in a covered beaker. Standard alkali is next added to the extent of about three-fourths of the amount required by a good tartar of the kind examined, plus that equivalent to the acid used, and the whole is brought to boiling; when nearly cold, the titration is finished. From the amount of alkali consumed, minus that required by the HC1, the tartaric acid present as acid tartrate is calculated. .2 gm. of the powdered tartar are next weighed into a platinum crucible with a well-fitting lid ; the crucible is placed over an argand burner ; heat is first applied very gently to dry the tartar, and then more strongly till inflammable gas ceases to be evolved. The heat should not rise above very low redness. The black ash is next removed with water to a beaker. If the tartar is known to be a good one, 20 c.c. of standard H 2 SO 4 are now run from a pipette into the beaker, a portion of the acid being used to rinse the crucible. The contents of the beaker are now brought to boiling, filtered, and the free acid determined with standard alkali. As the charcoal on the filter under some circumstances retains a little acid, even when well washed, it is advisable when the titration is completed to transfer the filter and its contents to the neutralized fluid, and add a further amount of alkali if necessary. From the neutralizing power of a gram of burnt tartar is subtracted the acidity of a gram of unburnt tartar, both expressed in c.c. of standard alkali, the difference in the neutralizing power of the bases existing as neutral tartrates, and is then calculated into tartaric acid on this assumption.* If the tartar is of low quality, 5 c.c. of solution of hydrogen peroxide (1 volume=10 volumes O) are added to the black ash and water, and immediately afterwards the standard acid ; the rest of the analysis proceeds as already described ; the small acidity usually belonging to the peroxide solution must, however, be known and allowed for in the calculation. By the use of hydrogen peroxide the sulphides formed during ignition are reconverted into sulphates, and the error of excess which their presence would occasion is avoided. The above method does not give the separate amounts of acid and neutral tartrates in the presence of carbonates, but it gives the correct amount of tartaric acid ; it is also correct in cases where free tartaric acid exists, so long as the final results show that some acid existed as neutral salt. Whenever this method shows that the acidity of the original substance is greater than the neutralizing power of the ash, it will be necessary to use the method b, which is the only one capable of giving good results when the sample contains much free tartaric acid. Instead of the alkalimetric estimation in both the above methods, equally good results may be got by a carbonic acid determination * It is obvious that the neutralizing power of the ash of an acid tartrate is exactly the same as the acidity of the same tartrate before burning 1 . In making the calcula- tions, it must be remembered that the value of the alkali in tartaric acid is twice as great in the calculation made from the acidity of the unburnt tartar, as in the calculation of the acid existing as neutral tartrates. 90 VOLUMETRIC ANALYSIS. 25. in the ash with Scheibler's apparatus ( 26.6), or any of the usual methods. 2. Tartaric Acid Liquors. Old factory liquors contain a great variety of substances gradually accumulated, from which the actual tartaric acid can only be separated as bitartrate by the following process : (c) A quantity of liquor containing 24 gm. of tartaric acid, and of 30 40 c.c. volume, is treated with a saturated solution of neutral potassic citrate, added drop by drop with constant stirring. If free sulphuric acid is present no precipitate is at first produced ; but as soon as the acid is satisfied, the bitartrate begins to appear in streaks on the sides of the vessel. "When this is seen, the remainder of the citrate is measured in to avoid an undue excess : 4 c.c. of a saturated solution of potassic citrate will be found sufficient to precipitate the maximum of 4 grams of tartaric acid supposed to be present. If the liquor contain a great deal of sulphuric acid, a fine precipitate of potassic sulphate will precede the formation of bitartrate, but is easily distinguished from it. With liquors rich in sulphuric acid, it is advisable to stir the mixture vigorously at intervals for half an hour, then proceed as in 3 d. Grosjean modifies this process by precipitating the liquor with an excess of calcic carbonate, then boiling the mixture with excess of potassic oxalate. By this means the alumina, iron, phosphoric and sulphuric acids are thrown down with the calcic oxalate, and the precipitate allows of ready filtration. The separation as bitartrate then follows, as in d. 3. Very impure Lees and Argrols. Grosjean (/. C. S. 1879, 341) gives a succinct method for the treatment of these substances, based on War ing ton's original oxalate process, the principle of which is as follows : The finely ground sample (=about 2 gm. tartaric acid) is first moistened with a little water, heated to 100 C., then digested for 15 minutes or so with an excess of neutral potassic oxalate (the excess must not be less than 1*5 gm.), and nearly neutralized with potash. After repeated stirring, the mixture is transferred to a vacuum filter, and the residue washed ; the liquid so obtained contains all the tartaric acid as neutral potassic tartrate ; excess of citric acid is added, which precipitates the whole of the tartaric acid as bitartrate, and the amount is found by titration with standard alkali in the usual way. One of the chief difficulties in treating low qualities of material is the filtration of the nearly neutral mixture above mentioned. Grosjeau adopts the principle of Casamajor's filter (C. N. xxxii. 45), using an ordinary funnel with either platinum, lead, or pumice disc ; but whether this, or Bun sen's, or other form of filter is used, the resulting filtrate and washings (which for 2 gm. tartaric acid should not much exceed 50 c.c.) are ready for the separation of the bitartrate in the following improved way : (d) To the 50 c.c. or so of cold solution 5 gm. of powdered potassic chloride is added, and stirred till dissolved: this renders the subsequent precipitation of bitartrate very complete. A 50-per-cent. solution of citric acid is then mixed with the liquid in such proportion, that for every 2 gm. of tartaric acid an equal, or slightly greater amount of citric acid is present. By continuously stirring, the whole of the bitartrate comes down in ten DYER. 26. CARBONIC ACID. 91 minutes (Grosjean) ; if the temperature is much above 16, it is preferable to wait half an hour or so before filtering. This operation is best done on the vacuum filter, and the washing is made with a 5-per-cent. solution of potassic chloride, saturated at ordinary temperature with potassic bitartrate ; if great accuracy is required, the exact acidity of the solution should be found by ^ alkali, and the washing continued until the washings show no greater acidity, thus proving the absence of citric acid. Finally, the washed precipitate is gently pressed into a cake to free it from excess of liquid, transferred to a beaker with the filter, hot water added, and titrated with standard alkali. The troublesome filtration can be avoided in many cases by taking 30 40 gm. of substance, and after decomposition by oxalate, and neutralizing with potash, making up the volume to 150 or 200 c.c., adding water in corresponding proportion to the bulk of the residue, then taking an aliquot portion for precipitation. A blank experiment made by Grosjean in this way, gave a volume of 375 c.c. for the residue in 10 gm. lees. Other things being equal, therefore, 30 or 40 gm. may respectively be made up to 161 and 215 c.c., then 50 c.c. taken for precipitation. CARBONIC ACID AND CARBONATES. 26. ALL carbonates are decomposed by strong acids ; the carbonic acid which is liberated splits up into water and carbonic anhydride (CO 2 ), which latter escapes in the gaseous form. It will be readily seen from what has been said previously as to the estimation of the alkaline earths, that carbonic acid in combination can be estimated volumetrically with a very high degree of accuracy (see 17). The carbonic acid to be estimated may be brought into combination with either calcium or barium, these bases admitting of the firmest combination as neutral carbonates. If the carbonic acid exist in a soluble form as an alkaline mono- carbonate, the decomposition is effected by the addition of baric or calcic chloride as before directed ; if as bicarbonate, or a compound between the two, ammonia must be added with either of the chlorides. As solution of ammonia frequently contains carbonic acid, this must be removed by the aid of baric or calcic chloride, previous to use. 1. Carbonates Soluble in Water. It is necessary to remember, that when calcic chloride is used as the precipitant in the cold, amorphous calcic carbonate is first formed ; and as this compound is sensibly soluble in water, it is necessary to convert it into the crystalline form. In the absence of free ammonia this can be accomplished by boiling. When ammonia is present, the same end is obtained by allowing the mixture to stand for eight or ten hours in the cold, or by heating for an hour or two to 70 80 C. With barium the precipitation is regular. Another fact is, that when ammonia is present, and the precipi- tation occurs at ordinary temperatures, ammonic carbamate is 92 VOLUMETKI C ANALYSIS. 26. formed, and the baric or calcic carbonate is only partially precipi- tated. This is overcome by heating the mixture to near boiling for a couple of hours, and is best done by passing the neck of the flask through a retort ring, and immersing the flask in boiling water. When caustic alkali is present in the substance to be examined, it is advisable to use barium as the precipitant ; otherwise, for all volumetric estimations of CO 2 calcium is to be preferred, because the precipitate is much more quickly and perfectly washed than the barium compound. Example: 1 gm. of pure anhydrous sodic carbonate was dissolved in water, precipitated while hot with baric chloride, the precipitate allowed to settle well, the clear liquid decanted through a moist filter, more hot water containing a few drops of ammonia poured over the precipitate, which was repeatedly done so that the bulk of the precipitate remained in the flask, being washed by decantation through the filter ; when the washings showed no trace of chlorine, the filter was transferred to, the flask containing the bulk of the precipitate, and 20 c.c. of normal nitric acid added, then titrated with normal alkali, of which 1'2 c.c. was required =18'8 c.c. of acid ; this multiplied by 0'022, the coefficient for carbonic acid, gave 0'4136 gm. CO 2 = 41 '36 per cent., or multiplied by 0'053, the coefficient for sodic carbonate, gave 0'9964 gm. instead of 1 gm. 2. Carbonates Soluble in Acids. It sometimes occurs that substances have to be examined for carbonic acid, which do not admit of being treated as above described ; such, for instance, as the carbonates of the metallic oxides (white lead, calamine, etc.), carbonates of magnesia, iron, and copper, the estimation of carbonic acid in cements, mortar, and many other substances. In these cases the carbonic acid must be evolved from the combination by means of a stronger acid, and conducted into an absorption apparatus containing ammonia, then precipitated with calcic chloride, and titrated as before described. The following form of apparatus (lig. 25) aifords satisfactory results. The weighed substance from which the carbonic acid is to be evolved is placed in b with a little water; the tube d contains strong hydrochloric acid, and c broken glass wetted with ammonia free from carbonic acid. The flask a is about one-eighth filled with the same ammonia : the bent tube must not enter the liquid. When all is ready and the corks tight, warm the flask a gently so as to fill it with vapour of ammonia, then open the clip and allow the acid to flow circumspectly upon the material, which may be heated until all carbonic acid is apparently driven off ; then by boiling and shaking the last traces can be evolved, and the operation ended. ~W hen cool, the apparatus may be opened, the end of the bent tube washed into a, and also a good quantity of boiled distilled water passed through c, so as to carry down any ammonic carbonate that may have formed. Then add solution of calcic chloride, boil, filter, and titrate the precipitate as before described. During the filtration, and while ammonia is present, there is a great avidity for carbonic acid, therefore boiling water should be used for washing, and the funnel kept covered with a small glass plate. CA11BONIC ACID. 93 In many instances CO 2 may be estimated by its equivalent in chlorine with T ^ silver and potassic chromate, as shown in 38. Pig. 25. 3. Carbonic Acid Gas in "Waters, etc. The carbonic acid existing in waters as neutral carbonates of the alkalies or alkaline earths may very elegantly and readily be titrated directly by y^- acid (see 17). Well or spring water, and also mineral waters, containing free carbonic acid gas, can be examined by collecting measured quantities of them at their source, in bottles containing a mixture of calcic and animoiiic chloride, afterwards heating the mixture in boiling water for one or two hours, and titrating the precipitate as before described. Pettenkofer's method with caustic "Baryta or lime is decidedly preferable to any other. Lime water may be used instead of baryta with equally good results, but care must be taken that the precipitate is crystalline. The principle of the method is that of removing all the carbonic acid from a solution, or from a water, by excess of baryta or lime 94 VOLUMETRIC ANALYSIS. 26. water of a known strength ; and, after absorption, finding the excess of baryta or lime by titration with -^5- acid and turmeric paper. The following is the best method to be pursued for ordinary drinking waters not containing large quantities of carbonic acid : 100 c.c. of the water are put into a flask with 3 c.c. of strong solution of calcic or baric chloride, and 2 c.c. of saturated solution of ammonic chloride ; 45 c.c. of baryta or lime water, the strength of which is previously ascertained by means of decinormal acid, are then added, the flask well corked and put aside to settle ; when the precipitate is fully subsided, take out 50 c.c. of the clear liquid with a pipette, and let this be titrated with deciuormal acid. The quantity required must be multiplied by 3 for the total baryta or lime solution, there being 50 c.c. only taken; the number of c.c. so found must be deducted from the original quantity required for the baryta solution added ; the remainder multiplied by 0'0022 will give the weight of carbonic acid existing free and as bicarbonate in the 100 c.c. The addition of the baric or calcic chloride and ammonic chloride is made to prevent any irregularity which might arise from alkaline carbonates or sulphates, or from magnesia. If it be desirable to ascertain the volume of carbonic acid from the weight, 1000 c.c. of gas, at and 076 m.m., weigh 1 '96663 gm. 100 cubic inches weigh 47*26 grains. 4. Carbonic Acid in Aerated Beverages, etc. For ascertaining the quantity of carbonic acid in bottled aerated waters, such as soda, seltzer, potass, and others, the following apparatus is useful. Fig. 26. Fig. 26 is a brass tube made like a cork-borer, about five inches long, having four small holes, two on each side, and about two inches from its cutting end ; 26. CARBONIC ACID. 95 the upper end is securely connected with the bent tube from the absorption flask (fig. 27) by means of a vulcanized tube ; the flask contains a tolerable quantity of pure ammonia, into which the delivery tube dips ; the tube a contains broken glass moistened with ammonia. Everything being ready the brass tube is greased, and the bottle being held in the right hand, the tube is screwed a little aslant through the cork by turning the bottle round, until the holes appear below the cork and the gas escapes into the flask. When all visible action has ceased, after the bottle has been well shaken two or three times to evolve all the gas that can be possibly eliminated, the vessels are quietly disconnected, the tube a washed out into the flask, and the contents of the bottle added also ; the whole is then precipitated with calcic chloride and boiled, and the precipitate titrated as usual. This gives the total carbonic acid free and combined. To find the quantity of the latter, another bottle of the same manufacture must be evaporated to dryness, and the residue gently ignited, then titrated with normal acid and alkali ; the amount of carbonic acid in the mono- carbonate deducted from the total, will give the weight of free gas originally present. The volume may be found as follows : 1000 c.c. of carbonic acid at 0, and 76 m.m., weigh T96663 gm. Suppose, therefore, that the total weight of carbonic acid found in a bottle of ordinary soda water was 2'8 gm., and the weight combined with alkali 0'42 gm., this leaves 2'38 gm. CO 2 in a free state 1-96663 : 2'38 : : 1000 : x = 1210 c.c. If the number of c.c. of carbonic acid found is divided by the number of c.c. of soda water contained in the bottle examined, the quotient will be the volume of gas compared with that of the soda water. The volume of the contents of the bottle is ascertained by marking the height of the fluid previous to making the experiment ; the bottle is afterwards filled to the same mark with water, emptied into a graduated cylinder and measured; say the volume was 292 c.c., therefore 1210 -202- = 4-14 vols. CO 2 . 5. Carbonic Acid in Air. A glass globe or bottle capable of being securely closed by a stopper or otherwise, and holding 4 to 6 liters, is filled with the air to be tested by means of a bellows aspirator ; baryta water is then introduced in convenient quantity and of known strength as compared with T |~j acid. The vessel is securely closed, and the liquid allowed to flow round the sides at intervals during half an hour ; if at the end of that time no great amount of cloudiness in the baryta has occurred, it may be advisable to refill the bottle with air one or more times. This can of course be done with the aspirator as at first, taking care on each occasion to agitate the vessel, so as to bring the baryta in contact with all parts of its surface during the space of half an hour. When sufficient air has thus been treated, the baryta is emptied out quickly into a beaker, the bottle rinsed out with distilled water free from CO 2 , the 96 VOLUMETRIC ANALYSIS. 26. rinsings added to the baryta, and the excess of the latter at once ascertained by T ^y- hydrochloric acid and turmeric paper as described in 14.9 ; or, instead of taking the whole of the baryta, that and the rinsings may be emptied into a stoppered cylinder, made up to a definite measure, and half or one-third taken for titration. The final calculation is of course made on the total baryta originally used, and upon the exact measurement of the air-collecting vessel. It is above all things necessary to prevent the absorption of CO 2 from extraneous sources during the experiment. The error may be reduced to a minimum by carrying on the titration in the vessel itself, which is done by fixing an accurately graduated pipette through the cork or caoutchouc stopper of the air vessel, to the upper end of which is attached a stout piece of elastic tube, closed with a pinch-cock ; and this being filled to the mark with dilute standard acid acts as a burette. The baryta solution is placed in the air bottle, and after absorption of all CO 2 , a few drops of phenolphtlialein are added through the ventilator, and the excess of baryta is found by running in the acid until the colour disappears. The cork or stopper must have a second opening to act as ventilator ; a small piece of glass tube does very well. If a freshly made solution of oxalic acid is used containing 0-2863 gm. per liter, each c.c. represents 1 mgni. CO 2 . The liquid holds its strength correctly for a day, and can be made as required from a strong solution, say 28'636 gm. per liter. Another method of calculation is, to convert the volume of baryta solution decomposed into its equivalent volume in ~ acid, 1 c.c. of which = 0'0022 gm. CO 2 or by measurement at C. and 760 m.m. pressure represents I'll 9 c.c. The method here described is a combination of those of Pettenkofer and Dal ton, and though much used, is liable to considerable error from various causes. Haldane and Pembrey (C. N. lix. 256 269) point out that at the Paris Observatory the practice is to absorb the CO 2 in caustic potash through a series of tubes, then liberate it by acid and measure as gas. They describe in the same paper a gravimetric method of their own, which gives very accurate results, and which depends on absorption of the gas by soda lime and weighing the increase. For rough sanitary purposes Angus Smith, many years ago, described in his Air and liain a minimetric method of estimating CO 2 in air ; it depended on the principle that the purer the air the larger the volume of it is required to produce turbidity with lime or baryta water; this method was open to the objection that it is difficult to tell precisely when turbidity occurs. Lunge and Zeckendorf have made use of this principle with modifications, which appears to give good results (Analyst xiii. 185). They use a -5^ solution of sodic carbonate, tinted with phenolph- thalein, the decolouration of which by formation of bicarbonate is taken as the end-reaction. Figures of the apparatus required and 26. CARBONIC ACID. 97 details are given in the Analyst, being a translation from the original paper in Zeitsclir. f. Ancjew. Chein., 1888. I have serious doubts, however, whether any exact method can be obtained with phenolphthalein as indicator, where the CO 2 is being absorbed. I have not found it to answer in the case of CO 2 in coal gas. Expired Air. Marcet uses a special vessel for the treatment for carbonic acid of air from the lungs (/. (7. S. 1880, 495). The absorption by baryta, and the. analysis, however, do not essentially differ from the method described above. 6. Scheibler's Apparattis for the estimation of Carbonic Acid by Volume. This apparatus is adapted for the estimation of the CO 2 contained in native carbonates, as well as in artificial products, and has been specially contrived for the purpose of readily estimating the CO 2 in the bone-black used in sugar refining. The principle upon which the apparatus is founded is simply this : That the quantity of CO 2 contained in calcic carbonate can be used as a measure of the quantity of that salt itself ; and instead of determining, as has been usually the case, the quantity of gas by weight, this apparatus admits of its estimation by volume ; and it is by this means possible to perform, in a few minutes, operations which would otherwise take hours to accomplish, while, moreover, the operator need scarcely possess any knowledge of chemistry. The results obtained by this apparatus are correct enough for technical purposes. The apparatus is shown in fig. 28, and consists of the following parts : The glass vessel, A, serves for the decomposition of the material to be tested for CO 2 , which for that purpose is treated with dilute HC1 ; this acid is contained, previous to the experiment, in the gutta-percha vessel s. The glass stopper of A is perforated, and through it firmly passes a glass tube, to which is fastened the india-rubber tube r, by means of which communication is opened with B, a bottle having three openings in its neck. The central opening of this bottle contains a glass tube (r) firmly fixed, which is in communication, on the one hand, with A, by means of the flexible india-rubber tube already alluded to, and, on the other hand, inside of B, with a very thin india-rubber bladder, K. The neck (q) of the vessel B is shut off during the experiment by means of a piece of india-rubber tubing, kept firmly closed with a spring clamp. The only use of this opening of the bottle B, arranged as described, is to give access of atmospheric air to the interior of the bottle, if required. The other opening is in communication with the measuring apparatus C, a very accurate cylindrical glass tube of 150 c.c. capacity, divided into 0'5 c.c. ; the lower portion of this tube C is in communication with the tube D, serving the purpose of controlling the pressure of the gas. H 98 VOLUMETRIC ANALYSIS. 26. The] lower part of this tube D ends in. a glass tube of smaller diameter, to which is fastened the india-rubber tube p, leading to E, but the communication between these parts of the apparatus is closed, as seen at p, by means of a spring clamp. E is a water reservoir, and on removal of the clamp at p, the water contained HUBERT LV Tig. 28. in C and D runs off towards E ; when it is desired to force the water contained in E into C and D, this can be readily done by blowing with the mouth into V, and opening the clamp at p. The main portion of the apparatus above described, with the 26. CARBONIC ACID. 99 exception, however, of the vessel A, is fixed by means of brass fittings to a wooden board ; a thermometer is also attached. The filling of the apparatus with water is very readily effected by pouring it through a suitable funnel placed in the open end of the tube D, care being taken to remove, or at least to unfasten, the spring clamp at p ; in this manner the water runs into E, which should be almost entirely filled. Distilled water is preferable for this purpose, especially as the filling only requires to be done once, because the water always remains in E as long as the apparatus is intended to be kept ready for use. When it is required to fill the tubes C and D with water, so as to reach the zero of the scale of the instrument, it is best to remove the glass stopper from A. The spring clamp at p is next unfastened, and air is then blown by means of the mouth into the tube V, which communicates with E ; by this operation the water rises up into the tubes C and D, which thus become filled with that liquid to the same height. Care should be taken not to force the water up above the zero of the scale at C, and especial care should be taken against forcing so much of the fluid up that it would run over into the tube u, and thence find its way to B, whereby a total disconnection of all the parts of the apparatus would become necessary. If by any accident the water should have been forced up above the zero at C, before the operator had closed the spring clamp at p, this is easily remedied by gently opening that clamp, whereby room is given for the water to run off to E in such quantity as may be required to adjust the level of that fluid in C precisely with the zero of the scale. The filling of the tube C with water has the effect of forcing the air previously contained in that tube into B, where it causes the compression of the very thin india-rubber ball placed within B. If it should happen that this india-rubber ball has not become sufficiently compressed and flattened, it is necessary to unfasten the spring clamp at g, and to cautiously blow air into B, through the tube q, by which operation the complete exhaustion of the india-rubber bladder placed within B is readily performed. This operation is also required only once, because during the subsequent experiments the india-rubber bladder K is emptied spontaneously. It may happen, however, that while the filling of the tubes D and C with water is being proceeded with, the india-rubber bladder K has become fully exhausted of air before the water in C reaches the zero of the scale. In that case the level of the water in the tubes D and C will not be the same, but will be higher in D : it is evident, however, that this slight defect can be at once remedied by momentarily unfastening the spring clamp at q. The apparatus should be placed so as to be out of reach of direct sunlight, and should also be protected against the heat of the operator's body by intervention of a glass screen, and is best placed near a north window, so as to afford sufficient light for reading off the height of the water in the tubes. H 2 100 VOLUMETRIC ANALYSIS. 26. In testing carbonates the method is as follows : Put the very finely powdered portion of carbonate into the perfectly dry decomposing glass A, fill the gutta-percha tube with 10 c.c. hydrochloric acid of 1-12 sp. gr., place the tube cautiously in the decomposing glass, and then close the bottle with the well-tallowed stopper. Here the water will sink a little in C and rise in D ; open q for a moment, and the equilibrium will be restored. Now note the thermometer and barometer, grasp the bottle with the right hand round the neck to avoid warming, raise it, incline it slightly so that the hydrochloric acid may mix with the substance gradually, and at the same time with the left hand regulate p, so that the water in the two tubes may be kept at exactly the same height ; continue these operations without intermission, till the level of the water in C does not change for a few seconds. Now bring the columns in C and D to exactly the same height., read off the height of the water, and note whether the temperature has remained constant. If it has, the number of c.c. read off indicates the liberated CO 2 , but as a small quantity has been dissolved by the hydrochloric acid, it is necessary to make a correction. Scheibler has determined the small amount of carbonic acid which remains dissolved in the 10 c.c. hydrochloric acid at the mean temperature, and he directs to add 0'8 c.c. to the volume of the carbonic acid read off. Warington (C. N. xxxi. 253) states that this is not a constant quantity, but is dependent upon the volume of gas evolved, and this ratio he fixes at 7 per cent, of the gas measured. Lastly, the volume being reduced to 0, 760 num., and the dry condition, the weight is found. Under no circumstances can the method be considered actually accurate, but for technical purposes it is convenient, as the operation is performed in a very short time, and is specially suitable for comparative examinations of various specimens of the same material. If it is desired to dispense with all corrections, each set of experiments may be begun by establishing the relation between the CO 2 obtained in the process (i.e. the CO 2 actually yielded + 0'S c.c.) and pure calcic carbonate. This relation is, of course, dependent on the temperature and pressure prevailing on the particular day. For example, from 0*2737 gm. calcic carbonate containing 0*1204 gm. CO 2 , 63*8 c.c. were obtained, including the 0*8 c.c. ; and in an analysis of dolomite under the same circumstances from 0'2371 gm. substance, 57*3 c.c. were obtained, including the 0'8 c.c. Therefore 63 "8 : 57 '3 : : 0'1204 : x, or a=0'1082, consequently the dolomite contains 45'62 per cent, of CO 2 . For the special procedure in testing bone-black, used in sugar refining, the reader is referred to the printed instructions supplied with the apparatus.* Wigner (Analyst, i. 158) has obtained exceedingly good results in the analysis of lead carbonates, etc., with Me Leod's gas apparatus. The nitrometer has also been turned to good account for the same purpose. *It is perhaps almost needless to say that the modern apparatus designed by Hempel, Lunge, and others, for technical gas analysis, practically supersedes that of Scheihler. The methods are all, however, open to the objection that an uncertain portion of CO 2 is lost by aqueous absorption. 27. COMBINED ACIDS. ^ * ' \\ '; ' > ^ ; ^ 10L ESTIMATION OP COMBINED ACIDS IN NEUTRAL SALTS. 27. THIS comprehensive method of determining the quantity of acid in neutral compounds (but not the nature of the acid), is applicable only in those cases where the base is perfectly precipitated by an excess of caustic alkali or its carbonate. The number of bodies capable of being so precipitated is very large, as has been proved by the researches of Langer and Wawnikiewicz (Ann. Chem. u. Phar. 1861, 239), who seem to have worked out the method very carefully. These gentlemen attribute its origin to Buns en; but it does not seem certain who devised it. The best method of procedure is as follows : The substance is weighed, dissolved in water in a 300-c.c. flask, heated to boiling or not, as may be desirable ; normal alkali or its carbonate, according to the nature of the base, is then added from a burette, until the whole is decidedly alkaline. It is then diluted to 300 c.c. and put aside to settle, and 100 c.c. are taken out and titrated for the excess of alkali ; the remainder multiplied by 3, gives the measure of the acid combined with the original salt, i.e. supposing the precipitation is complete. Example : 2 gm. crystals of baric chloride were dissolved in water, heated to boiling, and 20 c.c. normal sodic carbonate added, diluted to 300 c.c. and 100 c.c. of the clear liquid titrated with normal nitric acid, of which 1'2 c.c. were required : altogether, therefore, the 2 gm. required 16"4 c.c. normal alkali ; this multiplied by 0'122 gave 2'0008 gm. Bad 2 2H 2 instead of 2 gm. ; multiplied by the factor for chlorine 0'03537, it yielded 0'58007 gm. Theory requires 0'5809 gm. chlorine. The following substances have been submitted to this mode of examination with satisfactory results : Salts of the alkaline earths precipitated with an alkaline carbonate while boiling hot. Salts of magnesia, with pure or carbonated alkali. Alum, with carbonate of alkali. Zinc salts, boiling hot, with the same. Copper salts, boiling hot, with pure potash. vSilver salts, with same. Bismuth salts, half an hour's boiling, with sodic carbonate. Mckel and cobalt salts, with the same. Lead salts, with the same. Iron salts, boiling hot, with pure or carbonated alkali. Mercury salts, with pure alkali. Protosalts of manganese, boiling hot, with sodic carbonate. Chromium persalts, boiling hot, with pure potash. Where the compound under examination contains but one base precipitable by alkali, the determination of the acid gives, of course, the quantity of base also. Wolcott Gibbs (C. N. 1868, i. 151) has enunciated a new acidimetric principle applicable in cases where a base is precipitable at a boiling temperature by hydric sulphide, and the acid set free so as to be estimated with standard alkali. Of course the method 102 ; ; ;VpLtMETEIC ANALYSIS. 28. can only be used where complete separation can be obtained, and where the salt to be analyzed contains a fixed acid which has no effect upon hydric sulphide. A weighed portion is dissolved in water, brought to boiling, and the gas passed in until the metal is completely precipitated ; which is known by testing a drop of the clear liquid upon a porcelain tile with sulphuretted hydrogen water, or any other appropriate agent adapted to the metallic salt under examination. The liquid is filtered from the precipitate, and the latter well washed, and the solution made up to a definite measure. An aliquot portion is then titrated with normal alkali as usual, with one of the phenol indicators. In the case of nitrates or chlorides, where nitric or hydrochloric acid would interfere with the hydric sulphide, it was found that the addition in tolerable quantity of a neutral salt containing an organic acid (e.cj. sodic or potassic tartrate, or the double salt) obviated all difficulty. The results obtained by Gibbs in the case of copper, lead, bismuth, and mercury, as sulphate, nitrate, and chloride, agreed very closely with theory; but the process would be very objectionable to many, on account of the offensive and poisonous character of the gas necessarily employed in the precipitation. EXTENSION OF ALKALIMETBJC METHODS. 28. BOHLIG (Z. a. C. 1870, 310) has described a method for the estimation of sulphuric acid, baryta, chlorine, iodine, and bromine, which appears worthy of some consideration, since the only standard solutions required are an acid and an alkali. Alkaline sulphates are known to be partially decomposed, in contact with baric carbonate, into alkaline carbonates and baric sulphate. The decomposition is complete in the presence of free carbonic anhydride ; acid carbonates of the alkali-metals are left in solution, together with some acid baric carbonate, which can be removed by boiling. The solution is filtered, and the alkaline carbonate determined by means of a standard acid solution, and the amount of sulphuric acid or alkaline sulphate calculated from the amount of normal acid required. This process has been satisfactorily used by Haubst for sulphates in waters (C. N. xxxvi. 227), and by Grossman for salt cake (C. N. xli. 114). See also 16.15. Neutral chlorides, bromides, and iodides, more especially of the alkali-metals, are most readily decomposed by pure silver oxide into insoluble silver salts, leaving the alkali-metal in solution as hydrate (ammonia salts always excepted), which can then be determined as usual by standard acid. The author treats solutions containing sulphates of the heavy metals, of the earths or alkaline earths, and free from acids whose 28. ALKALIMETKIC METHODS. 103 presence would influence the method, viz., phosphoric, arsenic, oxalic, etc., with a solution of potassic carbonate, so as to precipitate the bases and leave about double or treble the amount of alkaline carbonate in solution. From 1 to 1J gin. of substance is operated upon in a flask. The solution is made up to 500 c.c., well shaken, and the precipitate allowed to subside. 50 c.c. are then filtered, and titrated with standard acid and methyl orange. Another 100 c.c. are filtered in like manner into a strong quarter-liter flask, and diluted with about 100 c.c. of hot water; the requisite quantity of normal acid is then run in at once from a burette ; the solution diluted to 250 c.c. ; and about a gram of dry baric carbonate (free from alkali) added. The flask is next closed, and the liquid well agitated. The decomposition of the alkaline sulphate is complete in a few minutes. The flask should be opened now and then to allow the carbonic anhydride to escape. Finally, about J gm. of pulverized baric hydrate is added, the whole well shaken, and a portion of the rapidly clearing liquid tested qualitatively for barium and sulphuric acid. The result should be a negative one. 50 c.c., corresponding to 20 c.c. of the original solution, are then filtered and titrated with normal acid, and the quantity of sulphuric acid (sulphate) calculated as usual. The source of carbonic anhydride is thus placed in the liquid itself, provided the quantity of potassic carbonate be not too small. Equivalent quantities of K 2 SO + 2K 2 C0 3 + 2HC1 + BaCO 3 when mixed with sufficient water change into BaSO 4 + 2KHC0 3 + 2KC1, and it is therefore more than sufficient to add twice the quantity of potassic carbonate compared with the alkaline sulphate operated upon. Baric hydrate is added with a view of removing any carbonic anhydride left in the liquid after boiling, which would otherwise dissolve some of the excess of baric carbonate contained in the precipitate. Any baric hydrate not required to remove CO 2 is acted upon by the acid potassic carbonate, but does not influence the final result. Phosphoric and oxalic acids the author proposes to remove by means of calcic chloride; chromic acid by deoxidizing agents, such as alcohol and hydrochloric acid. Bohlig recommends this method for estimating sulphuric acid in ashes, crude soda, Stassfurth salts, etc. Solutions containing baryta are estimated in like manner by precipitation as carbonate, and decomposition with potassic sulphate in a solution containing free carbonic acid. Chlorine is determined in solutions by first precipitating any metallic chloride with potassic carbonate added in moderate excess. The filtrate is made up to 250 c.c., and the excess of potassic carbonate determined in 50 c.c. by means of a normal solution of HC1. 125 c.c. of the solution are next treated with excess of silver oxide and made up to 250 c.c., well shaken (out of contact with the light) and filtered. 100 c.c. 104 VOLUMETKIC ANALYSIS. 28. of the filtrate are titrated with normal hydrochloric acid. The difference between the quantity of acid required in the last and that of the first experiment, multiplied by 5, gives the amount of chlorine contained in the original solution. A portion of the filtrate should be tested for chlorine by means of mercurous nitrate. The filtrate is obtained perfectly clear only in the presence of some potassic or sodic carbonate, and by employing argentic oxide free from argentous oxide. A few drops of pure potassic per- manganate added to the argentic oxide preserved in water prevent formation of the latter. The oxide to be employed for each experiment is filtered when required, and thoroughly washed. Bromine and iodine are determined in like manner. The author has not been able, however, to estimate the mixtures of the halogen salts ; but he has made the interesting observation that potassic iodide, when boiled with potassic permanganate, is completely oxidized into iodate. This facilitates the detection of small quantities of chlorine and bromine, in the presence of much iodide. The greater part of iodate may be separated also by precipitation with baric nitrate before determining chlorine. The standard acid solutions which Bohlig employed contained not more than one-third of the equivalent of HC1 or SO 3 per liter. For further particulars the reader is referred to the original paper (Arch. Pharm. 3 cxlv. 113). Siebold (Year Book of Pharmacy, 1878, 518) describes a very ingenious process, devised by himself, for the titration of caustic and carbonated alkalies by means of prussic acid, the principle of which is explained in 55. The process is useful in the case of carbonates, since CO 2 is no hindrance. 0'5 to 1 gm. of the alkali or alkaline carbonate is dissolved in about 100 c.c. of water, and an excess of hydrocyanic acid (say 10 or 20 c.c.) of 5 per cent, solution added; then ^ silver solution cautiously added with constant stirring until a faint permanent turbidity occurs. Each c.c. of ^ silver ^0-0138 gm. K 2 CO 3 , or 0'0106 gm. Na 2 CO 3 . In the case of chlorides being present, their quantity may be determined by boiling down the mixture to about half its volume to expel all free prussic acid, adding a drop or two of potassic chromate as indicator, then titrating with $ silver. Any excess above that required in the first titration will be due to chlorine, and may be calculated accordingly. 29. OXIDATION AND REDUCTION ANALYSES. 105 PART III. AXALYSIS BY OXIDATION OR REDUCTION 29. THE series of analyses wliicli occur under this system are very extensive in number, and not a few of them possess extreme accuracy, such in fact, as is not possible in any analysis by weight. The completion of the various processes is generally shown by a distinct change of colour ; such, for instance, as the occurrence of the beautiful rose-red permanganate, or the blue iodide of starch ; and as the smallest quantity of these substances will colour distinctly large masses of liquid, the slightest excess of the oxidizing agent is sufficient to produce a distinct effect. The principle involved in the process is extremely simple. Substances which will take up oxygen are brought into solution, and titrated with a substance of known oxidizing power ; as, for instance, in the determination of ferrous salts by permanganic acid. The iron is ready and willing to receive the oxygen, the permanganate is equally willing to part with it ; while the iron is absorbing the oxygen, the permanganate loses its colour almost as soon as it is added, and the whole mixture is colourless ; but immediately the iron is satisfied, the rose colour no longer disappears, there being no more oxidizable iron present. In the case of potassic permanganate the reaction is: 10FeO + 2MnK0 4 = 5Fe 2 3 + 2MnO + K 2 0. Oxalic acid occupies the same position as the ferrous salts ; its composition is C 2 4 H 2 + 2H 2 = 126. If perman- ganate is added to it in acid solution, the oxalic acid is oxidized to carbonic acid, and the manganic reduced to manganous oxide, thus Mn 2 7 + 5C 2 4 H 2 + 2H 2 S0 4 = 10C0 2 + 2MnS0 4 + 7H 2 0. When the oxalic acid is all decomposed, the colour of the permanganate no longer disappears. On the other hand, substances which will give up oxygen are deoxidized by a known excessive quantity of reducing agent, the amount of which excess is afterwards ascertained by residual titration with a standard oxidizing solution; the strength of the reducing solution being known, the quantity required is a measure of the substance which has been reduced by it. The oxidizing agents best available are potassic permanganate, iodine, potassic bichromate, and red potassic prussiate. The reducing agents are sulphurous acid, sodic hyposulphite,* sodic thiosulphate, oxalic acid, ferrous oxide, arsenious anhydride, stannous chloride, yellow potassic prussiate, and zinc or magnesium. With this variety of materials a great many combinations may be arranged so as to make this system of analysis very comprehensive ; * Schiitzenberger's preparation is here meant. 106 VOLUMETRIC ANALYSIS. 30. but the following are given as sufficient for almost all purposes, and as being susceptible of the greatest amount of purity and stability of material, with exceedingly accurate results : 1. Permanganate and ferrous salts (with the rose colour as indicator); permanganate and oxalic acid (with the rose colour as indicator). 2. Potassic bichromate and ferrous salts (with cessation of blue colour when brought in contact with red potassic prussiate, as indicator). 3. Iodine and sodic thiosulphate (with starch as indicator) ; iodine and sodic arsenite (with starch as indicator.) PREPARATION OF STANDARD SOLUTIONS. PERMANGANIC ACID AND FERROUS OXIDE. 1. Potassic Permanganate. Mn 2 K 2 O s = 315-6. Decinormal Solution = 3-156 gin. per liter. 30. THE solution of this salt is best prepared for analysis by dissolving the pure crystals in fresh distilled water, and should be of such a strength that 17 '85 c.c. will oxidize 1 decigram of iron. The solution is then decinormal. If the salt can be had perfectly pure and dry, 3 '156 gm. dissolved in a liter of water at 16 C. will give an exactly decinormal solution ; but, nevertheless, it is always well to verify it as described below. If well kept, it will retain its strength for several weeks, but should from time to time be verified by titration in one of the following ways : 2. Titration of Permanganate. (a) With Metallic Iron. The purest iron to be obtained is thiii annealed binding-wire free from rust, generally known as flower wire. About O'l gm. of wire is dissolved in dilute pure sulphuric acid, by the aid of heat, in a small flask closed with a cork, through which a fine glass tube is passed, so that the hydrogen which is evolved escapes under slight pressure, thus preventing the access of air; or the apparatus shown in 59 may be used. When the iron is all dissolved the flask may be two-thirds filled with cold, recently boiled, distilled water, and the titration with permanganate commenced and concluded as in the case of the double sulphate. The decomposition which ensues from titrating ferrous oxide by permanganic acid may be represented as follows : lOFeO and Mn 2 7 = 2MnO and 5Fe 2 3 ' (b) With Ammonio-ferrous Sulphate. In order to ascertain the strength of * the permanganate, it may be titrated with a weighed quantity of this substance instead of metallic iron. 30. VERIFICATION OF PERMANGANATE. 107 This salt is a convenient one for titrating the permanganate, as it saves the time and trouble of dissolving the iron, and when perfectly pure, it can be depended on without risk. To prepare it, 139 parts of the purest crystals of ferrous sulphate, and 66 parts of pure crystallized ammonic sulphate are separately dissolved in the least possible quantity of distilled water of about 40 C. (if the solutions are not perfectly clear they must be filtered) ; mix them at the same temperature in a porcelain dish, adding a few drops of pure sulphuric acid, and stir till cold. During the stirring the double salt will fall in a finely granulated form. Set aside for a few hours, then pour off the supernatant liquor, and empty the salt into a clean funnel with a little cotton wool stuffed into the neck, so that the mother-liquor may drain away ; the salt may then be quickly and repeatedly pressed between fresh sheets of clean filtering paper. Lastly, place in a current of air to dry thoroughly, so that the small grains adhere no longer to each other, or to the paper in which they are contained, then preserve in a stoppered bottle for use. The formula of the salt is Fe (^ T H 4 ) 2 (SO 4 ) 2 , 6H 2 = 392. Consequently it contains exactly one-seventh of its weight of iron ; therefore 0*7 gm. represents O'l gm. Ee, and this is a convenient quantity to weigh for the purpose of titrating the permanganate. 0'7 gm. being brought into dilute solution in a flask or beaker, and 5 or 6 c.c. of dilute sulphuric acid (1 to 5) added (the titration of permanganate, or any other substance by it, should always take place in the presence of free acid, and preferably sulphuric), the permanganate is delivered from a burette with glass tap divided in i- or T V c.c., until a point occurs when the rose colour no longer disappears on shaking. A drop or two of the permanganate in excess suffices to produce this effect. The titration is now ended, and should the quantity not be strictly correct, the number of c.c. used may be marked upon the bottle as the quantity for O'l gm. Pe, or the factor found, which is necessary to reduce it to decinormal strength, or if too strong, it may be diluted to that strength at once. (c) With Oxalic Acid. This is perhaps the least recommendable of the methods of titrating permanganate, owing to the difficulty of preparing oxalic acid of definite hydration. 0'63 gm. of the pure acid is to be weighed, or 10 c.c. of normal solution measured with a pipette, brought into a flask with dilute sulphuric acid, as in the case of the iron salt, and considerably diluted with water, then warmed to about 60 C., and the permanganate added from the burette. The colour disappears slowly at first, but after- wards more rapidly, becoming first brown, then yellow, and so on to colourless. More care must be exercised in this case than in the titration with iron, as the action is less decisive and rapid. (d) With Lead pxalate. S t o 1 b a prefers this salt to oxalic acid, for the reasons that it contains no water, is not liable to absorb any from exposure, and has a high molecular weight, 1 part of the salt representing 0'42799 of oxalic acid, or 63 oxalic acid = 147'2 lead oxalate. The method of titration is similar to that with oxalic acid, using dilute sulphuric acid, and warming the mixture to insure the complete decomposi- tion of the salt into lead sulphate and free oxalic acid. Sodic oxalate is also anhydrous and equally serviceable. The lead oxalate is prepared by precipitating pure lead acetate with oxalic acid in excess, and washing the precipitate by decantation with warm water till all free acid is removed ; the precipitate is then dried at 120 C., and preserved for use. Some operators prefer to use ammonic oxalate in place of oxalic acid or lead oxalate, as being a substance of definite hydration, and easily obtained in a pure state. Its formula is (NH 4 ) 2 C 2 O 4 , H 2 O=142= 2Fe. The titration is carried on precisely as in the case of oxalic acid. 108 VOLUMETRIC ANALYSIS. 30. (e) "With Hydrogen Peroxide in the Nitrometer. 111 a paper 011 this subject by Lunge (/. S. C. I. ix. 21), it is shown by very carefully conducted experiments with purest materials and verified apparatus that exceedingly accurate results may be obtained by the modified nitrometer with patent tap (illustrated at the end of Part VII.). Lunge's experiments were made on a semi-normal solution of permanganate (1 c.c. = 0*004 gin. 0), but whether equally exact results would be obtained with ^ permanganate I cannot say, not having tried it ; but of course an approximately semi-normal solution may be made and reduced to either f or ~ strength, if desired, by dilution with fresh distilled water. The exact method of using this new gas volumeter will be described under the head of Nitrometer in Part VII. ; but so far as permanganate is concerned it w r as found that convenient quantities of substances to use were 10 c.c. of f permanganate, 15 c.c. of ordinary 10 volume H 2 2 , and 30 c.c. of sulphuric acid 1 : 5. The nitrometer having been charged with water, the mixture was shaken up and allowed to stand ten minutes, shaken again and read off after five minutes. The volume of oxygen so obtained was corrected for temperature and pressure, then calculated into weight. The results of three experiments using the quantities mentioned above were as follows : 1. Corrected volume of O 55'92 c.c. =0*004007 gm. 2. 55*82 c.c. = 0-004000 3. 55'82 c.c. = 0*004000 Average 0'004002 gm. of oxygen per c.c. of solution. Three experiments with the same permanganate solution gave, when iron wire was used, an average of 0-00399 gm., and with oxalic acid '003997 gm. of oxygen respectively per c.c. As Lunge says: "We cannot but infer that standardizing a solution of permanganate with hydrogen peroxide in the nitrometer when observing the prescribed precautions is one of the most accurate known methods for this purpose, and withal possesses the great advantage that it is carried out within an extremely short time, without requiring a fundamental substance of accurately known composition." Many other substances have been proposed for standardizing permanganate, such as potassic ferrocyanide, thiocyanate, vanadic oxide, etc., but they are all inferior in value to those above named. 3. Precautions in Titrating- with Permanganate. It must be borne in mind that free acid is always necessary in titrating a substance with permanganate, in order to keep the resulting manganous oxide in solution. Sulphuric acid, in a dilute form, has no prejudicial effect on the pure permanganate, even at a high temperature. With hydrochloric acid the solution to be titrated must be very dilute arid of low temperature, otherwise chlorine will be liberated and the analysis spoiled. This acid acts 31. FACTORS FOE PERMANGANATE. 109 as a reducing thus ig agent on permanganate in concentrated solution, Mii 2 T + 14HC1 = 7H 2 + 10C1 + 2MnCR The irregularities due to this reaction may be entirely obviated by the addition of a few grains of manganous or magnesic sulphate before titration. Organic matter of any kind decomposes the permanganate, and the solution therefore cannot be filtered through paper, nor can it be used in Mohr's burette, because it is decomposed by the india-rubber tube. It may, however, be filtered through gun cotton or glass wool. TITRATION OF FERRIC SALTS BY PERMANGANATE. 31. ALL ferric compounds requiring to be estimated by permanganate must, of course, be reduced to the ferrous state. This is best accomplished by metallic zinc or magnesium in sulphuric acid solution. Hydrochloric may also be used with the precautions mentioned. The reduction occurs on simply adding to the warm diluted solution small pieces of zinc (free from iron, or at least with a known quantity present) or coarsely powdered magnesium until colourless ; or until a drop of the solution brought in contact with a drop of potassic sulphocyanide produces no red colour. All the zinc or magnesium must be dissolved previous to the titration. The reduction may be hastened considerably as shown in 59.3. When the reduction is complete, no time should be lost in titrating the solution. CALCULATION OF ANALYSES MADE WITH PERMANGANATE SOLUTION. 32. THE calculation of analyses with permanganate, if the solution is not strictly decinormal, can be made by ascertaining its factor, reducing the number of c.c. used for it to decinormal strength, and multiplying the number of c.c. thus found by ^-Q^Q-g- of the equivalent weight of the substance sought ; for instance Suppose that 15 c.c. of permanganate solution have been found to equal O'l gm. iron; it is required to reduce the 15 c.c. to decinormal strength, which would require 1000 c.c. of per- manganate to every 5 '6 gm. iron, therefore 5 '6 : 1000 : : O'l : x = 17-85 c.c. ; 17-85 x 0-0056 = 0-09996 gm. iron, which is as near to O'l gin. as can be required. Or the factor necessary to reduce the number of c.c. used may be found as follows: O'l : 15 : : 5'6 : 5c = 84 c.c., therefore -~-;- = l'19. Consequently 1*19 is the factor by which to reduce the number of c.c. of that special permanganate used in any analysis to the decinormal strength, from whence the weight of substance sought may be found in the usual way. 110 VOLUMETRIC ANALYSIS. 32. Another plan is to find the quantity of iron or oxalic acid repre- sented by the permanganate used in any given analysis, and this being done the following simple equation* gives the required result : Fe (56) eq. weight of . the weight the weight of or : the substance : : of Fe or : substance (63) sought found sought In other words, if the equivalent weight of the substance analyzed be divided by 56 or 63 (the respective equivalent weights of iron or oxalic acid), a factor is obtained by which to multiply the weight of iron or oxalic acid, equal to the permanganate used, and the product is the weight of the substance analyzed. For example : sulphuretted hydrogen is the substance sought, the eq. weight of H 2 S corresponding to 2 eq. Fe is 17 ; let this number therefore be divided by 56, gg = 0'3036, therefore, if the quantity of iron represented by the permanganate used in an estimation of H 2 S be multiplied by 0'3036, the product will be the weight of the sulphuretted hydrogen sought. Again : in the case of manganic peroxide whose equivalent weight is 43*4. 43-4 ._ The weight of iron therefore found by permanganate in any analysis multiplied by the factor 0'775 will give the amount of manganic peroxide, MnO 2 . Again: if m gm. iron = It c.c. permanganate, then 1 c.c. permanganate = -7; gm. metallic iron. The equivalents here given are on the hydrogen scale,, in accordance with the normal system of solutions adopted ; and thus it is seen that two equivalents of iron are converted from the ferrous to the ferric state by the same quantity of oxygen as suffices to oxidize one equivalent of oxalic acid, sulphuretted hydrogen, or manganic peroxide. 1 c.c. decinormal permanganate is equivalent to 0*0056 gm. Fe estimated in the ferrous state 0-0072 , FeO 0-008 Fe 2 3 0-003733 Fe ,' froinFeS 0-0059 Sn 5 , SnCl 2 0-00295 Sn 3 , SnS 2 0-00315 Cu , CuS 0-00274 Mn \ , MnS 0-00315 Cu , Cu+Fe 2 Cl 6 0-0063 Cu t CuO+Fe 0-0017 H 2 S ) ?> 0-0008 O 0-OOF3 0-002 Ca from CaC 2 O 4 0-0120 Ur UrO, etc., etc. 33. BICHROMATE ANALYSES. Ill When possible the necessary factors will be given in the tables preceding any leading substance. CHROMIC ACID AND FERROUS OXIDE. 33. POTASSIC bichromate, which appears to have been first proposed by Penny, possesses the advantage over permanganate, that it is absolutely permanent in solution, may easily be obtained in a pure state, and its solution may be used in Mohr's burette without undergoing the change peculiar to permanganate : on the other hand, the end of the reaction in the estimation of iron can only be known by an external indicator ; that is to say, a drop of the mixture is brought in contact with a drop of solution of red potassic prussiate (freshly prepared) upon a white slab or plate. While the ferrous oxide is in tolerable excess, a rich blue colour occurs at the point of contact between the drops ; but as this excess continues to lessen by the addition of the bichromate, the blue becomes somewhat turbid, having first a green, then a grey, and lastly a brown shade. When the greenish-blue tint has all disappeared, the process is finished. This series of changes in the colour admits of tolerably sure reading of the burette, after some little practice is obtained. The reaction between chromic acid and ferrous oxide may be represented by the formula : 2Cr0 3 + 6FeO = Cr 2 3 + 3Fe 2 3 . The decomposition takes place immediately, and at ordinary temperatures, in the presence of free sulphuric or hydrochloric acid. Nitric acid is of course inadmissible. The reduction of ferric compounds to the ferrous state may be accomplished by zinc,* magnesium, sodic sulphite, ammonic bisulphite, or sulphurous acid; or, instead of these, stannous chloride may be used, which acts very rapidly as a reducing agent upon ferric oxide, the yellow colour of the solution disappearing .almost immediately. In the analysis of iron ores, reduction by the latter is very serviceable ; the greatest care, however, is necessary that the stannous chloride is not present in excess, as this would consume the bichromate solution equally with the ferrous oxide, and so lead to false results. The discharge of the yellow colour of the iron solution may with care be made a very sure indicator of the exact point of reduction. The concentrated hydrochloric solution of iron is heated to gentle boiling, and the moderately dilute tin solution added with a pipette, waiting a moment for each addition till the last traces of colour have disappeared ; the solution is then poured into a beaker, diluted * When zinc is used, the zinc ferricyanide somewhat obsct-.res the critical point in testing with the indicator. 112 VOLUMETRIC ANALYSIS. 33. with boiled and cooled water, and titrated with the bichromate as usual. An extra security is obtained by adding a few drops of potassic sulphocyanide to the solution, the disappearance of the blood-red colour indicating that no more ferric oxide is present. In order to obviate the inaccuracy which would be produced by an excess of tin in the state of protosalt, Mohr recommends that chlorine water should be added by drops to the mixture, until a rod moistened with it and brought in contact with blue iodide of starch paper no longer removes the colour ; the excess of stannous chloride is then all converted into stannic chloride, and the titration with bichromate may proceed as usual. It is absolutely necessary that the solution of potassic ferri- cyanide used as the indicator with bichromate should be free from ferrocyanide ; and as a solution when exposed to air for a short time becomes in some measure converted into the latter, it is necessary to use a freshly prepared liquid. 1. Preparation of the Decinormal Solution of Bichromate. 4 - 9 13 gin. per liter. The reaction which takes place between potassic bichromate and ferrous oxide is as follows : 6FeO + Cr 2 K 2 7 = 3Fe 2 3 + Cr 2 3 + K 2 It is therefore necessary that J eq. in grams should be used for the liter as a normal solution, and -f^ for the decinormal ; and as it is preferable on many accounts to use a dilute solution, the latter is the more convenient for general purposes. According to the latest and most reliable researches, the equivalent number of chromium is 52 '4, and consequently that of potassic bichromate is 294*8 ; if, therefore, ^ of this latter number = 4*9 14 gm. be dissolved in a liter of water, the decinormal solution is obtained. On the grain system, 49*14 grains to 10,000 grains of water will give the same solution.* 1 c.c. of this solution is capable of yielding up yo-J^Q- eq. in grams of oxygen, and is therefore equivalent to the yoihnj e( l' ^ any substance which takes up 1 equivalent of oxygen. 2. Solution of Stannous Chloride. About 10 gin. of pure tin in thin pieces are put into a large platinum capsule, about 200 c.c. strong hydrochloric acid poured over it, and heated till it is dissolved ; or it may be dissolved in a porcelain capsule or glass flask, adding pieces of platinum foil to excite a galvanic current. The solution so obtained is diluted to * Pin-e bichromate powdered and dried in the air bath should be used, and not the fused salt. 34 IODOMETRIC ANALYSES. 113 al)out a liter with distilled water, and preserved in the bottle (fig. 20) to which the air can only gain access through a strongly alkaline solution of pyrogallic acid. "When kept in this manner, the strength will not alter materially in a month. If not so preserved, the solution varies considerably from day to day, and therefore should always be titrated before use as described in 60.1. Examples of Iron Titration : 0'7 gm. of pure and dry ammonio-ferrous sulphate = 0'1 gm. iron, was dissolved in water, and titrated with decinormal bichromate, of which 17*85 c.c. were required; this multiplied by 0'0392 gave 0'699 gm. instead of 0'7 gm. 0'56 gm. of iron wire required 998 c.c. =0*5588 gm. ; as it is impossible to obtain iron wire perfectly pure, the loss is undoubtedly owing to the impurities. If the bichromate solution should from any accidental cause be found not strictly of decinormal strength, the factor necessary for converting it must be found as previously described. IODINE AND SODIC THIOSTJLPHATE. 34. THE principle of this now beautiful and exact method of analysis was first discovered by Dupasquier, who used a solution of sulphurous acid instead of sodic thiosulphate. Buns en im- proved his method considerably by ascertaining the sources of failure to which it was liable, which consisted in the use of a too concentrated solution of sulphurous acid. The reaction between iodine and very dilute sulphurous acid may be represented by the formula SO 2 + 1 2 + 2H 2 = 2HI + H 2 SO. If the sulphurous acid is more concentrated, i.e. above 0*04 per cent., in a short time the action is reversed, the irregularity of decomposition varying with the quantity of water present, and the rapidity with which the iodine is added.* Sulphurous acid, however, very rapidly changes by keeping even in the most careful manner, and cannot therefore be used for a standard solution. The substitution of sodic thiosulphate is a great advantage, inasmuch as the salt is easily obtained in a pure state, and may be directly weighed for the standard solution. The reaction is as follows : 2Na 2 S 2 3 + 21 = 2XaI + Xa 2 S 4 6 , the result being that thiosulphuric acid takes oxygen from the water, with the production of tetrathionic and hydriodic acids in combination with soda. In order to ascertain the end of the reaction in analysis by this method an indicator is necessary, and the most delicate and sensitive * This irregularity is now obviated "by the method of Giles and Shearer ( 72), in which solutions of SQ2 or sulphites of any strength may be accurately titrated with iodine, by adding the latter to the former in excess, and when the reaction is complete titrating the excess of iodine with thiosiUphate. 1 114 VOLUMETRIC ANALYSIS. 34 for the purpose is starch, which produces with the slightest trace of free iodine in cold solution the well-known blue iodide of starch. Hydriodic or mineral acids and iodides have no influence upon the colour. Caustic alkalies destroy it. The principle of this method, namely, the use of iodine as an indirect oxidizing body by its action upon the elements of water, forming hydriodic acid with the hydrogen, and liberating the oxygen in an active state, can be applied to the determination of a great variety of substances with extreme accuracy. Bodies which take up oxygen, and decolorize the iodine solution, such as sulphurous acid, sulphites, sulphuretted hydrogen, alkaline thiosulphites and arsenites, stannous chlorides, etc., are brought into dilute solution, starch added, and the iodine delivered in with constant shaking or stirring until a point occurs at which a final drop of iodine colours the whole blue a sign that the substance can take up no more iodine, and that the drop in excess- has shown its characteristic effect upon the starch. Free chlorine, or its active compounds, cannot, however, be titrated with thiosulphate directly, owing to the fact that, instead of tetrathionic acid being produced as with iodine, sulphuric acid occurs, as may be readily seen by testing with baric chloride. In such cases, therefore, the chlorine must be evolved from its compound and passed into an excess of solution of pure potassic iodide, where it at once liberates its equivalent of iodine, which can then, of course, be estimated with thiosulphate. All bodies which contain available oxygen, and which evolve chlorine when boiled with- strong hydrochloric acid, such as the chromates, manganates, and all metallic peroxides, can be readily and most accurately estimated by this method. 1. Preparation of the Decinormal Solution of Iodine. 1 = 126-5 ; 12-65 gin. per liter. Chemically pure iodine may be obtained by intimately mixing dry commercial iodine with about one-fourth of its weight of potassic iodide, and gently heating the mixture between two large watch- glasses or porcelain capsules ; the lower one being placed upon a heated iron plate, the iodine sublimes in brilliant plates, which are absolutely pure. The watch-glass or capsule containing the iodine is placed under the exsiccator to cool, and also to deprive it of any traces of watery vapour; then 12 -65 gm. are accurately weighed, and together with about 18 gm. of pure potassic iodide (free from iodate)* dissolved * Morse and Burton (Amer. CTiem. Jour., 1888) state that potassic iodide may be completely freed from iodate by boiling a solution of it with zinc amalgam, prepared by snaking zinc dust in good proportion with mercury in presence of tartaric acid, and washing with water. The iodate is completely reduced with formation of zinc hydroxide. The pure solution of iodide is filtered for use through a paper filter saturated with hot water. 34 IODOMETEIC ANALYSES. 115 in about 250 c.c. of water, and diluted to exactly one liter. The same solution may be obtained by dissolving 126 '5 grains of iodine, and 180 of potassic iodide, in 10,000 grains of water; in either case the solution is strictly decinormal. The flask must not be heated in order to promote solution, and care must be taken that iodine vapours are not lost in the operation. The solution is best preserved in stoppered bottles, kept in the dark, and which should be completely filled ; but under any circumstances it does not hold its strength well for any length of time, and consequently should be titrated before use in analysis. Several substances have been suggested for the verification of iodine solution, but the preference should be given to ^ arsenite of soda, described in 36. There is no difficulty in procuring pure arsenious acid, and the alkaline solution is practically un- changeable if kept in well-closed bottles. 20 c.c. of ^ alkaline arsenite should be put into a beaker, diluted, a little solution of ammonic carbonate added, together with starch, and the iodine added until the blue colour is produced. An alkalimetric method is proposed by Kalmann, in which the iodine solution is diluted with water, and H 2 S passed into the liquid till colourless; the hydriodic acid so produced is then titrated with ~$ alkali and methyl orange. 2. Decinormal Sodic Thiosulphate. Na 2 S 2 3 ,5H 2 - 248 = 24'8 gm. per liter. As it is not difficult either to manufacture or procure pure sodic thiosulphate, this quantity, powdered and dried between blotting- paper, may be weighed directly, and dissolved in a liter of distilled water, and then titrated with the iodine solution and a little starch indicator : or the solution may be checked with ~ bichromate as recommended by Mohr, by digesting a measured volume of the bichromate with an excess of potassic iodide, and hydro- chloric acid, in the stoppered flask (fig. 31) or similar well-closed vessel. When the mixture has cooled, the liberated iodine is measured by the thiosulphate, and its power ascertained. If impure thiosulphate should have been used, or the sample was not entirely free from accidental moisture, it will be necessary to find a factor by which to reduce it to decinormal strength, as described for previous solutions ; or the amount of impurity being known, a fresh quantity may be prepared of proper strength. It is advisable to preserve the solution in the dark. After a time all solutions of' thiosulphate undergo a slight amount of oxidation, and sulphur deposits upon the bottle; it is therefore always better to examine it previous to use. Mohr states that the tendency to deposit sulphur in the solution may be entirely i 2 116 VOLUMETRIC ANALYSIS. 34. obviated by adding to the $ solution sesquicarbonate of ammonia, in the proportion of 2 grams to the liter.* Har court and Esson (Phil. Trans. [5] clvi. 205) state that a little caustic soda greatly increases the stability. Beside the decinormal iodine and thiosulphate, it is convenient in some cases to use centinormal solutions, which can readily be prepared by diluting 100 c.c. of each decinormal solution to 1 liter. In using the iodine solution Mohr's burette may be employed, "but care must be taken that the solution is not left in it for any length of time, as decomposition slowly takes place, and the tube becomes hard ; the tap burette is on this account preferable. 3. Starch Indicator. One part of clean arrowroot, or potato starch, is first mixed smoothly with cold water into a thin paste, then gradually poured into about 150 or 200 times its weight of boiling water, the boiling continued for a minute, then allowed to stand and settle ; the clear solution only is to be used as the indicator, of which a few drops only are necessary, f The solution may be preserved for some time by adding to it about one per cent, of salicylic acid, or saturating it with common salt; this, however, is not so sensitive as the freshly prepared solution. Starch Paste. A very convenient form of soluble starch is made by mixing 6 gm. starch with 100 gm. pure glycerine, heating for an hour to 100 C., pouring into about double its volume of water, then adding sufficient strong alcohol to precipitate the soluble starch, which is filtered off and preserved in a moist pasty state. When required, a minute quantity is taken with a glass rod. Concentrated Solution of Starch. This will keep almost any length of time. Made by rubbing about 5 gm. starch to a smooth emulsion, with about 50 c.c. water. Then add 25 c.c. of 50 per cent, solution of caustic potash and shake well, dilute with half a liter of water, boil, and allow to settle. This indicator answers very well in cases where the alkali is of no consequence, but is not available for the delicate acidimetric method by iodic acid unless the alkali is exactly corrected. It answers well, however, with the addition of 2 gm. of potassic iodide as a reagent for nitrites, and keeps perfectly though exposed to light. * This has been proved by my experiments not to be correct, and Topf (Z. a. C. xxvi. 137 217) in a long series of experiments in iodometry comes to the same conclusion. Bicarbonate of soda or potash have a better effect, but even these are open to objection. It is undoubtedly better to make fresh solutions from pure thiosulphate at intervals of a month or so. t In iodometric analyses it is always advisable in titrating the free iodine with thio- sulphate or arsenious solution to delay adding the starch until the iodine colour is nearly removed : a much more delicate ending may be obtained and with very little starch. 35. IODOMETPJC ANALYSES. 117 ANALYSIS OF SUBSTANCES BY DISTILLATION WITH HYDROCHLORIC ACID. 35. THERE are a great variety of substances containing oxygen, which when boiled with hydrochloric acid yield chlorine, equivalent Fig. 29. to the whole or a part only of the oxygen they contain according to circumstances. Upon this fact are based the variety of analyses which may be accomplished by means of iodine and sodic thio- sulphate, or arsenite ; the chlorine so evolved, however, is not itself estimated, but is conveyed by means of a suitable apparatus into a solution of potassic iodide, thereby liberating an equivalent quantity of iodine. This latter body is then estimated by thiosulphate ; the quantity so found is, therefore, a measure of the oxygen existing in the original substance, and consequently a measure of the substance itself. Analyses of this class may be made the most exact in the whole range of volumetric analysis, far outstripping any process by weight. The apparatus used for distilling the substances, and conveying the liberated chlorine into the alkaline iodide, may possess a variety of forms, the most serviceable, however, being the three kinds devised respectively by Bunsen, Fresenius, and Mohr. Buns en's arrangement consists of an inverted retort, into the neck of which the tube from the small distilling flask is passed. 118 VOLUMETEIC ANALYSIS. 35. A drawing of the entire apparatus may be seen in most treatises on chemical analysis. Owing to the great solubility of HC1 in the form of gas, the apparatus must be so constructed that when all Cl is liberated and HC1 begins to distil, the liquid may not rush back to the flask owing to condensation. Pig. 30. The best preventive of this regurgitation is, however, suggested by Fresenius, and applicable to each kind of apparatus; namely, the addition of a few pieces of pure magnesite. This substance dissolves but slowly in the hydrochloric acid, and so keeps up a constant flow of CO 2 , the pressure of which is sufficient to prevent the return of the liquid. The apparatus contrived by Fresenius is shown in fig. 29, and is exceedingly useful as an absorption apparatus for general purposes. Mohr's apparatus is shown in fig. 30 and is, on account of its simplicity of construction, very easy to use. The distilling flask is of about 2 oz. capacity, and is fitted with a 35. IODOMETRIC ANALYSES. 119 cork soaked to saturation in melted paraffin ; through the cork the delivery tube containing one bulb passes, and is again passed through a common, cork, fitted loosely in a stout tube about 12 or 13 inches long and 1 inch wide, closed at one end like a test tube. This tube, containing the alkaline iodide, is placed in an hydrometer glass, about 12 inches high, and surrounded by cold water; the delivery tube is drawn out to a fine point, and reaches nearly to the bottom of the condenser. No support or clamp is necessary, as the hydrometer glass keeps everything in position. The substance to be distilled is put into the flask and covered with strong hydrochloric acid, the magnesite added, the condenser supplied with a sufficient quantity of iodide solution, and the apparatus put together tightly. Either an argand or common spirit lamp, or gas, may be used for heating the flask, but the flame must be manageable, so that the boiling can be regulated at will. In the case of the common spirit lamp it may be held in the hand, and applied or withdrawn according to the necessities of the case : the argand spirit or gas lamp can, of course, be regulated by the usual arrangements for the purpose. If the iodine liberated by the chlorine evolved should be more than will remain in solution, the cork of the condensing tube must be lifted, and more solution added. When the operation is judged to be at an end, the apparatus is disconnected, and the delivery tube washed out into the iodide solution, which is then emptied into a beaker or flask and preserved for analysis, a little fresh iodide solution is put into the condenser, the apparatus again put together, and a second distillation commenced, and continued for a minute or so, to collect avery trace of free chlorine present. This second operation is only necessary as a safeguard in case the first should not have been complete. The solutions are then mixed together and titrated in the manner previously described. In all cases the solution must be cooled before adding the thiosulphate, otherwise sulphuric acid might be formed. Instead of the large test tube, some operators use a (J tube to contain the potassic iodide, having a bulb in each limb, but the latter is not necessary if magnesite is used. The solution of potassic iodide may conveniently be made of such a strength that -^ eq. or 33*2 gm. are contained in the liter. 1 c.c. will then be sufficient to absorb the quantity of free iodine, repre- senting 1 per cent, of oxygen in the substance analyzed, supposing it to be weighed in the metric system. In examining peroxide of manganese, for instance, 0'436 gm. or 4*36 grn. would be used, and supposing the percentage of peroxide to be about sixty, 60 c.c. or dm. of iodide solution would be sufficient to absorb the chlorine and keep in solution the iodine liberated by the process ; it is advisable, however, to have an excess of iodide, and, therefore, in. this case, about 70 c.c. or dm. should be used. A solution of 120 VOLUMETRIC ANALYSIS. indefinite strength will answer as well, so long as enough is used to absorb all the iodine. It may sometimes happen that not enough iodide is present to keep all the liberated iodine in solution, in which case it will separate out in the solid form ; more iodide, however, may be added to dissolve the iodine, and the titratioii can then be made as usual. The process of distillation above described may be avoided in many cases. There are a great number of substances which, by mere digestion with hydrochloric acid and potassic iodide at an elevated temperature, undergo decomposi- a ~ tion quite as completely as by distillation. <^J|L of the funnel is first tied over with a piece of fine cotton cloth, then two thicknesses of Swedish filter " paper, and again with a piece of cotton cloth, the whole being securely tied with waxed thread. In precipitating SO 3 by baric chloride, there occurs a point similar to the so-called neutral point in silver assay, when in one and the same solution both barium and sulphuric acid after a minute or two produce a cloudiness. Owing to this circumstance, the barium solution must not be reckoned exactly by its amount of Bad 2 , but by its Avorking effect; that is to say, the process must be considered ended when the addition of a drop or two of barium solution gives no cloudiness after the lapse of two minutes. The Analysis: The solution containing the SO 3 being prepared, and preferably in HC1, the vessel A is filled with warm distilled water, and the pinch-cock opened so as to fill the filter to the bend C ; the cock is then opened and shut a few times so as to bring the water further down into the tube, but not to fill it entirely; the water is then emptied out of A, and about 400 c.c. of boiled distilled water poured in together with the SO 3 314 VOLUMETRIC ANALYSIS. 74. solution, then, if necessary, a small quantity of HC1 added, and the baric chloride added in moderate quantity from a burette. After mixing well, and waiting a few minutes, a portion is drawn off into a small beaker, and poured back without loss into A ; a small quantity is then drawn off into a test tube, and two drops of baric chloride added. So long as a precipitate occurs, the liquid is returned to A, and more barium added until a test is taken which shows no distinct cloudiness ; the few drops added to produce this effect are deducted. If a distinct excess has been used, the analysis must be corrected with a solution of SO 3 corresponding in strength to the barium solution. A simpler and even more serviceable arrangement of apparatus on the above plan may be made, by using as the boiling and precipitating vessel an ordinary beaker standing on wire gauze or a hot plate. The filter is made by taking a small thistle funnel, tied over as described, with about two inches of its tube, over which is tightly slipped about four or five inches of elastic tubing, terminating with a short piece of glass tube drawn out to a small orifice like a pipette ; a small pinch-cock is placed across the elastic tube just above the pipette end, so that when hung over the edge of the beaker with the funnel below the surface of the liquid, the apparatus will act as a syphon. It may readily be filled with warm distilled water by gentle suction, then transferred to the liquid under titration. By its means much smaller and more concentrated liquids may be used for the analysis, and consequently a more distinct evidence of the reaction obtained. SULPHURETTED HYDROGEN. IPS = 34. 1 c.c. YQ- arsenious solution = 0'00255 gin. H 2 S. 1. By Arsenious Acid (Mohr). 74. THIS residual process is far preferable to the direct titration of sulphuretted hydrogen by iodine. The principle is based on the fact, that when H 2 S is brought into contact with an excess of arsenious acid in hydrochloric acid solution, arsenic sulphide is formed ; 1 eq. of arsenious acid and 3 eq. of sulphuretted hydrogen produce 1 eq. of arsenic sulphide and 3 eq. of water, As 2 3 + 3H 2 S = As 2 S 3 + 3H 2 0. The excess of arsenious acid used is found by --$ iodine and starch, as in 36. In estimating the strength of sulphuretted hydrogen water, the following plan may be pursued. A measured quantity, say 10 c.c., of /^ arsenious solution is put into a 300 c.c. flask, and 20 c.c. of sulphuretted hydrogen water added, well mixed, and sufficient HC1 added to produce a distinct acid reaction ; this produces a precipitate of arsenic sulphide, and the liquid itself is colourless. The whole is then diluted to 300 c.c., filtered through a dry filter into a dry vessel, 100 c.c. of the filtrate taken out and neutralized with sodic bicarbonate, then 74 SULPHURETTED HYDKOGEN. titrated with ^ iodine and starch. The quantity of arsenious acid so found is deducted from the original 10 c.c., and the remainder multiplied by the requisite factor for H 2 S. The estimation of H 2 S contained in coal gas, may by this method be made very accurately by leading the gas very slowly through the arsenious solution, or still better, through a dilute solution of caustic alkali, then adding arsenious solution, and titrating as before described. The apparatus devised by Mohr for this purpose is arranged as follows : The gas from a common burner is led by means of a vulcanized tube into two successive small wash bottles, containing the alkaline solution; from the last of these it is led into a large Woulff 's bottle filled with water. The bottle has two necks, and a tap at the bottom ; one of the necks contains the cork through which the tube carrying the gas is passed; the other, a cork through which a good-sized funnel with a tube reaching to the bottom of the bottle is passed. When the gas begins to bubble through the flask, the tap is opened so as to allow the water to drop rapidly ; if the pressure of gas is strong, the funnel tube acts as a safety valve, and allows the water to rise up into the cup of the funnel. When a sufficient quantity of gas has passed into the bottle, say six or eight pints, the water which has issued from the tap into some convenient vessel is measured into cubic inches or liters, and gives the quantity of gas which has displaced it. In order to insure accurate measurement, all parts of the apparatus must be tight. The flasks are then separated, and into the second 5 c.c. of arsenious solution placed, and acidified slightly with HC1. If any traces of a precipitate occur it is set aside for titration with the contents of the first flask/into which 10 c.c. or so of arsenious solution are put, acidified as before, both mixed together, diluted to a given measure, filtered, and a measured quantity titrated as before described. This method does not answer for very crude gas containing large quantities of H 2 S unless the absorbing surface is largely increased. 2. By Permanganate (Mohr). If a solution of H 2 S is added to a dilute solution of ferric sulphate, the ferric salt is reduced to the ferrous state, and free sulphur separates. The ferrous salt so produced may be measured accurately by permanganate without removing the separated sulphur. Ferric sulphate, free from ferrous compounds, in sulphuric acid solution, is placed in a stoppered flask, and the solution of H 2 S added to it with a pipette ; the mixture is allowed to stand half an hour or so, then diluted considerably, and per- manganate added until the rose colour appears. 56Fe = 17 H 2 S or each c.c. of T ^- permanganate represents 0*0017 gm. of H 2 S. The process is considerably hastened by placing the stoppered flask containing the acid ferric liquid into hot water previous to the addition of H 2 S, and excluding air as much as possible. 316 VOLUMETRIC ANALYSIS. 75. 3. By Iodine. Sulphuretted hydrogen in mineral waters may be accurately estimated by iodine in the following manner : 10 c.c. or any other necessary volume of Y^ iodine solution are measured into a 500 c.c. flask, and the water to be examined added until the colour disappears. 5 c.c. of starch liquor are then added, and T ^ iodine until the blue colour appears ; the flask is then filled to the mark with pure distilled water. The respective volumes of iodine and starch solution, together with the added water, deducted from the 500 c.c., will show the volume of water actually titrated by the iodine. A correction should be made for the excess of iodine necessary to produce the blue colour. Fresenius examined the sulphur water of the Grindbrimnen, in Frankfurt a. M. (Z. a. C. xiv. 321), both volumetrically and by weight for IPS with very concordant results. 361*44 gm. of water (correction for blue colour being allowed) required 20*14 c.c. of iodine, 20*52 c.c. of which contained 0*02527 of free iodine = H 2 S 0*009194 gin. per million. 444*65 gin. of the same water required, under the same conditions, 25*05 c.c. of the same iodine solution = IPS 0*009244 gm. per million. By weight the IPS was found to be 0*009377 gm. per million. TANNIC ACID. 75. THE estimation of tannin in the materials used for tanning is by no means of the most satisfactory character. Many methods have been proposed, and given up as practically useless. In the previous editions of this book Lowenthal's method (originated by Est court) as then perfected was given; but it is still somewhat deficient in accuracy or constancy of results, although much ingenuity and intelligence have been expended on it. One difficulty is still unsurmoimted, and that is, the preparation of a pure tannic acid to serve as standard. The various tannins in existence are still very imperfectly understood,* but so far as the comparative analysis of tanning materials among themselves is concerned, the method in question is theoretically the best. The principle of the method depends on the oxidation of the tannic acid, together with other glucosides and easily oxidizable substances, by permanganate, regulated by the presence of soluble indigo-carmine, which also acts as an indicator to the end of the reaction. The total amount of such substances being found and *Von Schroder, whose suggestions have been adopted by the German Association of Tanners, selects a commercial pure tannic acid for use as a standard by dissolving 2 gm. in a liter of water. 10 c.c. of this is titrated with permanganate as described. 50 c.c. are then digested twenty hours with 3 gm. moistened hide powder. 10 c.c. of the filtrate from this is then titrated, and if the permanganate consumed amounts to less than 10 per c^nt. of the total consumed by the tannin, it is suitable for a standard. 1000 parts being considered equivalent in reducing power to 1048 parts of tannin precipitable by hide, according to Hammer's experiments, therefore Von Schroder, aiter titrating as described, calculates the dry matter, and multiplies by the round number 1'05 to obtain the value in actual tannin precipitable by hide. 75. TANNIC ACID. SI 7 expressed by a known volume of permanganate, the actual available tannin is then removed by gelatine, and the second titration is made upon the solution so obtained in order to find the amount of oxidizable matters other than tannin. The volume of permanganate so used, deducted from the volume used originally, shows the amount of tannin actually available for tanning purposes expressed in terms of permanganate. It will be at once seen that this method is essentially a practical one, because it is only the particular tannin capable of combining with organic tissue which is estimated. It has been critically examined with approbation by good authorities, among whom may be mentioned, Procter (C. N. xxxvi. 59 ; ibid, xxxvii. 256), Kathreiner (Z. a. C. xviii. 112), (Dingler's Polyt. Jour. cxxvii. 481), and Hewitt (Tanner's Jour., May, 1877, 93). My own experiments have shown that for all materials containing tannin, even catechu, it is the best process yet discovered, but requires patient practice to ensure concordant results. Lowenthal's description of the method is given in Z. a. C. xvi. 33. The extraction of the tannic acid from the raw material is best performed by boiling it in a large flask with about a liter of distilled water for half an hour, then straining, and diluting when cold to 1 liter. Portions are filtered if necessary. Concentrated extracts are dissolved before titration by adding them to boiling water, then cooling and diluting to the measure. In the case of strong materials such as sumach or valonia 10 gm., or oak-bark 20 gm., are used. The quantity of these extracts to be used for titration must be regulated to some extent by the amount of permanganate required to oxidize the tannic and gallic acids present. Practice and experience will enable the operator to judge of the proper propor- tions to use in dealing with the various materials, bearing in mind that volumetric processes are largely dependent upon identity of conditions for securing concordant results. Procter, who is probably one of the best authorities on this subject, has modified to some extent the details of this process (/. S. C. I. iii. 82, and ibid. v. 79), and these modifications are embodied here. Standard Solutions and Re-agents, Standard Potassic permanganate. Kathreiner recommends that this solution should contain not more than 1*333 gm. of the pure salt per liter (better only about 1 gm.) ; therefore, if the operator is accustomed to use the decinormal solution, a very convenient strength is made by diluting one volume of it with two of water, thus obtaining a solution of -^ strength ( = 1 '052 gm. per liter). This standard is the more advisable because it enables the 318 VOLUMETRIC ANALYSIS. 75. operator to calculate its value into oxalic acid, and so arrive at the theoretical standards adopted by Neubauer and Oser; namely, that 0*063 gm. of oxalic acid represents 0*04157 gm. of gallo-tannic acid (gall-nut tannin), or 0*062355 gm. of querci-tannic acid (oak bark tannin). These coefficients for calculation are now largely adopted, and are certainly preferable to standardizing the perman- ganate upon any specimen of so-called pure tannin. 30 c.c. of ^ permanganate will therefore represent 0*063 gm. of oxalic acid or the weights of tannin above mentioned. Solution of Indigo Carmine. This should be a clear solution of about 5 gin. to the liter with about 50 c.c. of pure H 2 S0 4 . Solution of Gelatine. This solution is used to precipitate the available tannin in any given solution after its total oxidizable matters have been determined by the indigo and permanganate. It should be made fresh for each series of titrations, by dissolving 2 gm. of kelson's gelatine in 100 c.c. of water and filtering. Dilute Sulphuric Acid. 1*10. Processes of Titration: The first thing to be done is to ascertain the relationship between the permanganate and indigo solutions (it is assumed that the permanganate is correct as regards its relation to oxalic acid), and therefore 10 or 20 c.c. of the indigo are measured into a white porcelain basin, and diluted to f of a liter with distilled water, or good ordinary water free from organic matter or other substances capable of reducing permanganate. 10 c.c. of the dilute acid are measured in, and the permanganate delivered in with a hand-pipette in drops, with constant stirring, until the colour is just discharged, leaving a clear faint yellow tint, with just a shade of pink at the rim. This experiment will act as a guide to the final adjustment of the indigo with an accurate 30 c.c. burette in T V, which should be of such dilution that about 20 c.c. correspond to about 15 c.c. of permanganate. Titration of the Tanning Material : It is very important, in order to avoid uncertainty in the end-point of the reaction, that only so much material shall be used as shall consume about 7 or 8 c.c. of permanganate of -j^j- strength above that point which is required for the indigo. Procter and Kathreiner both insist upon these proportions, and the general method adopted by them is to add 20 c.c. of indigo with 10 c.c. of dilute acid to about of a liter of water, in a porcelain dish, followed by 5 c.c. of tannin solution. The permanganate is then delivered in very slowly, with constant stirring, until a faint rose colour appears round the edges of the liquid. The time allowed for the titration is also very important. Von Schroder, representing the Association of German Tanners, prefers to add the permanganate 1 c.c. at a time with vigorous stirring, until the colour of the liquid indicates that a few drops only are required to end the titration. Procter, on the other hand, prefers the rapid drop method for the commencement, and until near the end. He also finds that the method of stirring influences the result in no very slight degree. Whatever plan the 75. TANNIC ACID. 319 operator adopts, it is advisable to keep consistently to it in order that the results may be comparatively the same. It must be remembered that neither by this nor any other method is it possible to accurately estimate the tannin, but only as a means of comparing two samples of the same material. Precipitation of the Tannin, and subsequent Titration of Substances other than Tannin. Procter's procedure is to take 50 c.c. of the tannin infusion (5 c.c. of which has been titrated), and add to it 28'6 c.c. of gelatine solution in a flask holding about 150 c.c. The mixture is well shaken, then saturated with clean table salt, and 10 c.c. of the dilute acid added, together with a teaspoonful of kaolin : the whole is vigorously shaken, then filtered, and made up to exactly 100 c.c. 10 c.c. of this liquid, representing 5 c.c. of the tannin decoction, is then titrated in precisely the same manner as before. The calculation of percentage is then made as follows : Let the first titration (two of which should be made for security) be called a ; the second, also in duplicate, I. If further, c be the quantity of permanganate required to oxidize 10 c.c. of ^ oxalic acid, and 10 gm. of substance have been employed for 1 liter of decoction, then c : (a b) : : 6*3 : x, where x is the percentage of tannin expressed in terms of oxalic acid. Hunt, who is also an undoubted authority on tannin estimation, differs from Procter on the question of saturating the liquid for final titration with salt (/. C. S. I. iv. 263), 011 the ground that, in the case of material containing much gallic acid, some of it is precipitated with the tannin, thus leading to higher results. This he has proved by experiment, and therefore prefers to act as follows : 50 c.c. of the tannin solution are run into a small dry flask, to this 25 c.c. of the fresh filtered gelatine solution are added, and the flask shaken. 25 c.c. of a saturated solution of salt, containing 50 c.c. of strong H 2 SO 4 per liter are now added, and about a teaspoonful of kaolin or baric sulphate. The flask is thoroughly shaken for a few minutes, after which a clear bright filtrate may be obtained. For materials containing over 45 per cent, tannin, it is advisable to take 25 c.c. instead of 50, and to use 50 c.c. of salt, the amount of gelatine solution being the same. The same authority also states that, for gambier and its allies, the method of titration as above described does not give accurate results, inasmuch as the gelatine and salt do not remove all the substances of tanning value from the liquid. In such case it is necessary to digest the liquid for at least twelve hours with pure dry skin shavings in powder. The mixture is then filtered and titrated in the usual way. It is impossible to give here the opinions held by various authorities on this subject, therefore the reader who desires fuller information should consult the papers to which reference has been made. The following table given by Hunt is however appended, as the result of careful working, and as a guide to the nature of various tanning materials : The " total extract " in the table was determined by evaporating 320 VOLUMETRIC ANALYSIS. 75. a portion of the tannin solution to clryiiess in a small porcelain basin and drying the residue at 110 C. The "insoluble matter" was also dried at 110 C. NAME OF MATERIAL. Total matters oxidized by Perman- ganate, as Oxalic Ac. Tannin, as Oxalic Ac. (Procter) Tannin, as Oxalic Ac. (Hunt) Total Extract. Insoluble. per cent. per cent. per cent. per cent. per cent. English Oak Bark . . . 15-70 13-54 11-97 18-38 66-15 CanadiauHemlock Bark 9-03 7-46 7-08 13-96 75-25 Larch Bark . . 8'20 7"17 6'15 20'64 60" 80 Mangrove Bark 31-35 2971 28-48 26-60 49-70 Alder Bark 8-27 6-15 5-73 19-36 68-00 Blue Gum Bark 10-18 8-91 8-91 11-76 74-65 Valonia 37-41 35-24 30-50 38-50 46-05 Myrabolans 48-23 38-43 38-00 42-80 Sumach 42-53 34-30 31-46 44-10 47-77 Betel Nut 15.91 13-87 13-79 17-94 67-00 Turkish Blue Galls ... 73-38 65-83 59-96 48*40 36-35 Aleppo Galls 98-85 87-82 83-05 68-80 14-32 Wild Galls 26"21 18'75 16'56 31-70 54'17 Divi-Divi 66-98 62-62 61-22 54-38 29-90 Balsamocarpon (poor and old sample) . . . 50-49 37-76 32-88 57-14 28-25 Pomegranate Kind . . . 27-58 24-18 23-12 41*00 49-50 Tormentil Root 22-27 20-98 20-68 19-70 67-95 Rhatany Eoot 22-27 20-15 19-30 18-80 66-00 Pure Indian Tea 23-06 18*65 17-40 34-46 53-40 Pure China Tea 18-03 1421 14-09 24-50 62-60 Cutch 57-65 51-95 44-24 61-60 4'75 Gum Kino 66-39 59-55 51-55 79-30 i-oo Hemlock Extract 35-16 33-17 30-98 48-78 Oak wood Extract . . . 33'49 26-90 23-86 37-78 Chestnut Extract . . . 39-77 32-63 28-88 50-28 Quebracho Extract ... 48-22 44-45 40-84 49-00 " Pure Tannin " 135-76 122-44 121-93 Tan Liquor, sp. gr. 1/030 Spent Tan Liquor, sp. 4-84 3-14 2-10 6-01 gr. T0165 1-40 0-37 0'25 3-10 Absorbed by Dry Pure Skin. Gambier, Cube 70-12 51-07 74-40 5-31 Sarawak ... 63-13 47-09 70-70 3-67 Bale 56-00 43-70 63-54 1-40 Other Methods of Estimating- Tannin. Direct Precipitation by Gelatine. The difficulty existing with this method is that of getting the precipitate to settle, so that it may be clearly seen when enough gelatine has been added. Tolerably good results may sometimes be obtained by using a 76. TIN. 321 strong solution of sal ammoniac or chrome alum as an adjunct. The best aid is probably barium sulphate, 2 or 3 gm. of which should be added to each portion of liquid used for titration. The Standard Solution of Gelatine should contain 1'33 gm. of dry gelatine per liter, in which is also mixed 100 c.c. of cherry laurel water to preserve it. 45 c.c. = 0*05 gm. tannin (Carles). This method is adapted only for rough technical purposes, as also the following. Direct Precipitation by Antimony. This method is still in favour with some operators ; but, like the gelatine process, is beset with the difficulty of getting the precipitate to settle. The Standard Antimony Solution is made by dissolving 2*611 gm. of crystals of emetic tartar dried at 100 C. in a liter. 1 c.c. = 0*005 gm. tannin. 50 c.c. of the tannin solution may be taken for titration, to which is added 1 or 2 gm. of sal ammoniac, and the antimonial solution run in until no further cloudiness is produced. In both the above methods the final tests must either be made by repeatedly filtering small portions to ascertain whether the precipitation is complete, or by bringing drops of each liquid together on black glass or a small mirror. TIN. Sn=118. Metallic iron x 1*0536 = Tin. Double iron salt x 0*1505= ,, Factor for ^ iodine or permanganate solution 0*0059 76. THE method, originally devised by Streng, for the direct estimation of tin by potassic bichromate, or other oxidizing agents in acid solution, has been found most unsatisfactory, from the fact that variable quantities of water or acid seriously interfere with the accuracy of the results. The cause is not fully under- stood, but that it is owing partly to the oxygen mechanically contained in the water reacting on the very sensitive stannous chloride there can be very little doubt, as the variations are considerably lessened by the use of water recently boiled and cooled in closed vessels. These difficulties are set aside by the processes of Lenssen, Lbwenthal, Stromeyer, and others, now to be described, and which are found fairly satisfactory. 1. Direct Titration by Iodine in Alkaline Solution (Lenssen). Metallic tin or its protosalt, if not already in solution, is dissolved in hydrochloric acid, and a tolerable quantity of Kochelle 322 VOLUMETRIC ANALYSIS. 76. salt added, together with sodic bicarbonate in excess. If enough tartrate be present, the solution will be clear; starch is then added, and the mixture titrated with ~ iodine. Metallic tin is best dissolved in HC1 by placing a platinum crucible or cover in contact with it, so as to form a galvanic circuit. Benas (Ghem. Centr-blatt. li. 957) points out that the chief error in the estimation as above arises from oxygen dissolved in. the liquid, or absorbed during the operation. In order to obtain constant results, it is necessary to dissolve the tin compound in HC1, dilute with oxygen-free water, and add at once excess of standard iodine, which excess is found by residual titration with standard thiosulphate. 2. Indirect Titration by Ferric Chloride and Permanganate (Lowenthal, Stromeyer, etc.). This method owes its value to the fact, that when stannous chloride is brought into contact with ferric or cupric chloride, it acts as a reducing agent, in the most exact manner, upon these compounds, stannic chloride being formed, together with a pro- portionate quantity of ferrous or cuprous salt, as the case may be. If either of the latter be then titrated with permanganate, the original quantity of tin may be found, the reaction being, in the case of iron, SnCl 2 + Fe 2 Cl 6 = SnCl 4 + 2FeCl 2 . 56 iron = 59 tin. If decinormal permanganate, or the factor necessary to convert it to that strength, be used, the calculation by means of iron is not necessary. The Analysis : The solution of stannous chloride, or other protosalt of tin in HC1, or the granulated metal, is mixed with pure ferric chloride, which, if tolerably concentrated, dissolves metallic tin readily, and without evolution of hydrogen, then diluted with distilled water, and titrated with perman- ganate as usual. To obtain the most exact results, it is necessary to make an experiment with the same permanganate upon a like quantity of water, to which ferric chloride is added; the quantity required to produce the same rose colour is deducted from the total permanganate, and the remainder calculated as tin. Stannic salts, also tin compounds containing iron, are dissolved in water, HC1 added, and a plate of clean zinc introduced for ten or twelve hours ; the tin so precipitated is carefully collected and washed, then dissolved in HC1, and titrated as above ; or the finely divided metal may at once be mixed with an excess of ferric chloride, a little HC1 added, and when solution is complete, titrated with permanganate. 4 eq. of iron (=224) occurring in the form of ferrous chloride represent 1 eq. (=118) of tin. Tin may also be precipitated from slightly acid peroxide solution as sulphide by H 2 S, the sulphide well washed, and mixed with ferric chloride, the mixture gently warmed, the sulphur filtered off, and the filtrate then titrated with permanganate as above. 4 eq. of iron = 1 eq. of tin. 77. ZINC. 323 Tin Ore. In the case of analysis of cassiterite, Arnold (C. N. xxxvi. 238) recommends that 1 gm. of the very finely powdered mineral be heated to low redness for two hours in a porcelain boat in a glass tube with a brisk current of dry and pure hydrogen gas, by which means the metal is reduced to the metallic state. It is then dissolved in acid ferric chloride, and titrated with perman- ganate or bichromate in the usual way. URANIUM. Ur = 240. 77. THE estimation of uranium may be conducted with great accuracy by permanganate, in precisely the same way as ferrous salts ( 59). The metal must be in solution either as acetate, sulphate, or chloride, but not nitrate. In the latter case it is necessary to evaporate to dryness with excess of sulphuric or hydrochloric acid, or to precipitate with alkali, wash and redissolve in acetic acid. The reduction to the uranous state is made with zinc, but as the end of reduction cannot, like iron, be known by the colour, it is necessary to continue the action for a certain time ; in the case of small quantities a quarter, larger half an hour, at a temperature of 50 to 60 C., and in the presence of excess of sulphuric acid ; all the zinc must be dissolved before titration. The solution is then freely diluted with boiled water, sulphuric acid added if necessary, and then permanganate until the rose colour is faintly permanent. The ending is distinct if the solution be well diluted, and the reaction is precisely the same as in the case of ferrous salts ; namely, 2 eq. of uranium existing in the uranous state require 1 eq. of oxygen to convert them to the uranic state ; hence 56 Fe = 120 Ur, consequently the strength of any permanganate solution in relation to iron being known, it is easy to find the amount of uranium. ZINC. Zn=65. 1 c.c. $ solution = 0-00325 gm. Zinc. Metallic iron x 0-5809 = Zinc. x 0-724 = Zinc oxide. Double iron salt x 0*08298 = Zinc. x 0-1034 = Zinc oxide. 1. Indirect Method (Mann). 78. THIS process gives exceedingly good results, and consists in precipitating the zinc as hydrated sulphide, decomposing the Y 2 324 VOLUMETKIC ANALYSIS. 78. sulphide with moist silver chloride, then estimating the zinc chloride so formed with animonic sulphocyanate as in Volhard's method ( 39). The requisite materials are Silver chloride. Well washed and preserved from the light under water. Standard Silver nitrate. 33'18 gm. of pure silver, dissolved in nitric acid and made up to 1 liter, or 52*3 gm. silver nitrate per liter. If made direct from silver, the solution must be well boiled to dissipate nitrous acid. 1 c.c. =0'01 gm. of* zinc. Ammonic sulphocyanate. Of such strength that exactly 3 c.c. suffice to precipitate 1 c.c. of the silver solution. Ferric Indicator and Pure Xitric Acid (see 39.3 and 4). The Analysis : 0'5 to 1 gm. of the zinc ore is dissolved in nitric acid. Heavy metals are removed by H 2 S, iron and alumina by double precipitation with ammonia. The united filtrates are acidified with acetic acid, and H 2 S passed into the liquid until all zinc is precipitated as sulphide. Excess of H 2 S is removed by rapid boiling, so that a drop or two of the filtered liquid gives no further stain on lead paper. The precipitate is then allowed to settle, decanted while hot, the precipitate brought on a filter with a little hot water, and without further washing, the filter with its contents is transferred to a small beaker, 30 50 c.c. of hot water added, well stirred, and so much moist silver chloride added as i? judged necessary to decompose the sulphide, leaving an excess of silver. The mixture is now boiled till it shows signs of settling clear ; 5 or 6 drops of dilute sulphuric acid (1:5) are added to the hot mixture, and in a few minutes the whole of the zinc sulphide will be converted into zinc chloride. The free sulphur and excess of silver chloride are now filtered off, washed, and the chloride in the mixed filtrate and washings estimated as follows : To the cool liquid, measuring 200 or 300 c.c., are added 5 c.c. of ferric indicator, and so much pure nitric acid as is necessary to remove the yellow colour of the iron. A measured excess of the standard silver solution is then delivered in with the pipette, and without filtering off the silver chloride, or much agitation, so as to clot the precipitate, the sulphocyanate is cautiously added, with a gentle movement after each addition, until a permanent light brown colour appears. The volume of silver solution represented by the sulphocyanate being deducted from that originally used, will give the volume to be calculated to zinc, each c.c. being equal to O'Ol gm. Zn. 2. Precipitation as Sulphide and subsequent titration with Ferric Salts and Permanganate (Schwarz). The principle of this method is based on the fact, that when zinc sulphide is mixed with ferric chloride and hydrochloric acid, or better still, with ferric sulphate and sulphuric acid, ferrous or zinc chloride, or sulphates respectively, and free sulphur are produced. If the ferrous salt so produced is estimated with permanganate or 78. ZINC. 325 bichromate, the proportional quantity of zinc present is ascertained. 2 eq. Fe represent 1 eq. Zn. Preparation of the Ammoniacal Zinc Solution. In the case of rich ores 1 gm., and poorer qualities 2 gm., of the finely powdered material are placed into a small wide-mouthed flask, and treated with HC1, to which a little nitric acid is added, the mixture is warmed to promote solution, and when this has occurred the excess of acid is evaporated by continued heat. If lead is present, a few drops of concentrated sulphuric acid are added previous to complete dryness, in order to render the lead insoluble; the residue is then extracted with water and filtered. Should metals of the fifth or sixth group be present, they must be removed by H 2 S previous to the following treatment. The solution will contain iron, and in some cases manganese. If the iron is not already fully oxidized, the solution must be boiled with nitric acid ; if only traces of manganese are present, a few drops of bromized HC1 should be added. When cold, the solution may be further diluted if necessary, and then super-saturated with ammonia to precipitate the iron ; if the proportion of this metal is small, it will suffice to filter off and wash the oxide with ammoniacal warm water, till the washings give no precipitate of zinc on adding ammonic sulphide. Owing to the fact that this iron precipitate tenaciously holds about a fifth of its weight of zinc, it will be necessary when the proportion is large to redissolve the partly washed precipitate in HC1, and reprecipitate (best as basic acetate) ; the filtrate from this second precipitate is added to the original zinc filtrate, and the whole made up to a liter. The Analysis : The ammoniacal zinc solution (prepared as described above) is heated, and the zinc precipitated in a tall beaker, with a slight excess of sodic or ammonic sulphide, then covered closely with a glass plate, and set aside in a warm place for a few hours. The clear liquid is removed by a syphon, and hot water containing some ammonia again poured over the precipitate, allowed to settle, and again removed, and the washing by decantation repeated three or four times ; finally, the precipitate is brought upon a tolerably large and porous filter, and well washed with warm water containing ammonia, till the washings no longer discolour an alkaline lead solution. The filter pump may be used here with great advantage. The filter with its contents is then pushed through the funnel into a large flask containing a sufficient quantity of ferric sulphate mixed with sulphuric acid, immediately well stopped or corked, gently shaken, and put into a warm place ; after some time it should be again well shaken, and set aside quietly for about ten minutes. After the action is all over the mixture should possess a yellow colour from the presence of undecomposed ferric salt ; when the cork or stopper is lifted there should be no odour of H 2 S. The flask is then nearly filled with cold distilled water, if necessary some dilute sulphuric acid added, and the contents of the flask titrated with permanganate or bichromate as usual. The free sulphur and filter will have no reducing effect upon the permanganate if the solution be cool and very dilute. 3. Precipitation by Standard Sodic Sulphide, -with Alkaline Lead Solution as Indicator (applicable to most Zinc Ores and Products). The Ammoniacal Solution of Zinc is prepared just as previously described in Schwarz's method. Standard Sodic sulphide. A portion of caustic soda solution is 326 VOLUMETRIC ANALYSIS. 78. saturated with H 2 S, sufficient soda added to remove the odour of the free gas, and the whole diluted to a convenient strength for titrating. Standard Zinc Solution. 44'12 gm. of pure zinc sulphate is dissolved to the liter. 1 c.c. will then contain O'Ol gm. of metallic zinc, and upon this solution, or one prepared from pure metallic zinc of the same strength, the sulphide solution must be titrated. Alkaline Lead Indicator. Is made by heating together acetate of lead, tartaric acid, and caustic soda solution in excess, until a clear solution is produced. It is preferable to mix the tartaric acid and soda solution first, so as to produce sodic tartrate ; or if the latter salt is at hand, it may be used instead of tartaric acid. Some operators use sodic nitroprusside instead of lead. The Analysis : 50 c.c. of zinc solution ( 0'5 gm. Zn) are put into a beaker, a mixture of solutions of ammonia and ammonic carbonate (3 of the former to about 1 of the latter) added in sufficient quantity to redissolve the precipitate which first forms. A few drops of the lead solution are then, by means of a glass rod, placed at some distance from each other, on filtering paper, laid upon a slab or plate. The solution of sodic sulphide contained in an ordinary Mohr's burette is then suffered to flow into the zinc solution until, on bringing a drop from the mixture and placing it upon the filtering paper, so that it may expand and run into the drop of lead solution, a black line occurs at the point of contact ; the reaction is very delicate. At first it will be difficult, probably, to hit the exact point, but a second trial with 25 or 50 c.c. of zinc solution will enable the operator to be certain of the corresponding strength of the sulphide solution. As this latter is always undergoing a slight change, it is necessary to titrate occasionally. Direct titration with pure zinc solution gave 99'6 and 100'2, instead of 100. Groll recommends the use of protochloride of nickel as indicator, instead of sodic nitroprusside or lead. The drops are allowed to flow together on a porcelain plate ; while the point of contact shows a blue or green colour the zinc is not all precipitated by the sodic sulphide, therefore the latter must be added until a greyish black colour appears at contact. Another indicator is paper soaked in a nearly neutral dilute solution of cobaltous chloride, which when dry and cold is colour- less. When touched with a drop of liquid containing sodic sulphide it turns to a green tint, rapidly becoming brown when warmed. 4. Precipitation as Sulphide -with Ferric Indicator (Schaffner). Schaffner's modification of this process, and which is used constantly at the laboratory of the Vieille Montagne and the Rhenish Zinc Works, is conducted as follows : For ores containing over 35 per cent, zinc, 0'5 gm. is taken ; for poorer ones, 1 gm. to 2 gm. Silicates, carbonates, or oxides, are treated with hydro- 78. ZINC. 327 chloric acid, adding a small proportion of nitric acid at boiling heat to peroxidize the iron. Sulphur ores are treated with aqua rer/ia, evaporated to dryness, and the zinc afterwards extracted by hydro- chloric acid; the final ammoniacal solution is then prepared as described on page 325. The Analysis : The titration is made with a solution of sodic sulphide, 1 c.c. of which should equal about O'Ol gin. Zn. The Vieille Montague laboratory uses ferric chloride as an indicator, according to Schaffner's method. For this purpose a single drop or some few drops of this chloride are let fall into the ammoniacal solution of zinc. The iron which has been added is at once converted into red flakes of hydrated ferric oxide, which float at the bottom of the flask. If sodic sulphide be dropped from a burette into the solution of zinc, a white precipitate of zinc sulphide is at once thrown down, and the change in the colour of the flakes of iron from red to black shows the moment when all the zinc is sulphuretted, and the titration is ended. It is advisable to keep the solution for titration at from 40 to 60 C. Titration carried out under exactly equal conditions, with a known and carefully weighed proportion of zinc, gives comparative data for calculation, and thus for the determination of the contents of any zinc solution by means of a simple equation. If, for example, 30'45 c.c. of sodic sulphide have been used to precipitate 0'25 gm. of zinc, 1 c.c. of it will precipitate 8'21 m.gm. of zinc (30'45 : 0'25=1 : x, and therefore tf=0'00821). Practice has pointed out the following concentrations and pro- portions as best suited for the successful execution of these titrations. The sodic sulphide solution which is used for titration must be such that 1 c.c. precipitates 8 to 10 m.gm. of zinc. The solution which contains the zinc which has to be estimated will comply with the best conditions, if its volume varies between the limits of 175 and 225 c.c. It is advisable that all solutions of zinc should be pretty equally saturated with ammonia, so that the ferric chloride added as an indicator may be precipitated in the same manner in all samples, and so that the flakes of iron may be always clean and clear. Freshly prepared ferric chloride gives the most delicate results. The essential point of the volumetric process practised at the Vieille Montagne is the perfect uniformity of working adopted in the assays with reference to the volume of the solutions and reagents used and the colour of the indicator. In titrating, the same quantities of ferric chloride, hydrochloric acid and ammonia are steadily used. Work is done always at one tem- perature and in the same time, particularly at the end of the operation, when the iron begins to take on that characteristic colour which the flakes take at the edges points which should not be overlooked. As a further precaution, the titrating apparatus is provided in duplicate, two assays being always made. It permits the execution of several titrations without the necessity of a too frequent renewal of sodic sulphide, which is stored in a yellow flask of large capacity supplying two MolirV. burettes, under which the beakers can be placed and warmed. A mirror 328 VOLUMETRIC ANALYSIS. 78. shows by reflection the iron flakes which settle down after shaking the liquid. Lunge uses for the indicator paper soaked with basic ferric chloride, strips of which are so suspended during titration that about half the strip dips into the liquid. Too much stress cannot be laid upon the necessity of standard- izing the sodic sulphide under the same conditions as to volume of fluid, proportions of NH 3 and HC1, and colour of the indicator, as will actually occur in the analysis. 5. Estimation as Ferrocyanide. In Acetic Acid Solution (Galetti). When ores containing zinc and iron are dissolved in acid, and the iron precipitated with ammonia, the ferric oxide invariably carries down with it a portion of zinc, and it is only by repeated precipitation that the complete separation can be made. In this process the zinc is converted into soluble acetate, and titrated by a standard solution of potassic ferrocyanide in the presence of insoluble ferric acetate. The Standard Solution of Potassic ferrocyanide, as used by Galetti, contains 41 '250 gm. per liter. 1 c.c. = 0*01 gm. Zn, but its actual working power must be fixed by experiment. Standard Zinc Solution, 10 gm. of pure metallic zinc per liter dissolved in hydrochloric acid. The process is available in the presence of moderate quantities of iron and lead, but copper, manganese, nickel, and cobalt must be absent. The adjustment of the ferrocyanide solution (which should be freshly prepared at short intervals) must be made in precisely the same way, and with the same volume of liquid as the actual analysis of ores, and is best done as follows : 25 c.c. of zinc solution are measured into a beaker, 15 c.c. of liquid ammonia of sp. gr. 0'900 added to render the solution alkaline, then very cautiously acidified with acetic acid, and 50 c.c. of acid ammonic acetate (made by adding together 20 c.c. of ammonia of sp. gr. 0'900, 15 c.c. of concentrated acetic acid and 65 c.c. of distilled water), which is poured into the mixture, then diluted to 250 c.c., and warmed to about 50 C. The titration is then made with the ferrocyanide solution by adding it from a burette until the whole of the zinc is precipitated. Galetti judges the ending of the process from the first change of colour from white to ash grey, which occurs when the ferrocyanide is in excess ; but I prefer to ascertain the ending by taking drops from the solution, and bringing them in contact with solution of uranic acetate on a white plate until a faint brown colour appears. The ferrocyanide solution should be of such strength that measure for measure it agrees with the standard zinc solution. In the present case 25 c.c. would be required. In examining ores of zinc, such as calamine and blende, Galetti takes 0'5 gm. for the analysis, and makes the solution up to 500 c.c. Calamine is at once treated with HC1 in sufficient quantity to bring it into solution. 78. ZINC. 329 Blende is treated with aqua regia, and evaporated with excess of HC1 to remove nitric acid. The solutions of zinc so obtained invariably contain iron, which together with the zinc is kept in solution by the HC1, but to insure the peroxidation of the iron, it is always advisable to add a little potassic chlorate at a boiling heat during the extraction of the ore. The hydrochloric solution is then diluted to about 100 c.c., 30 c.c. of ammonia added, heated to boiling, exactly neutralized with acetic acid, 100 c.c. of the acid ammonic acetate poured in, and diluted to about 500 c.c. The mixture as prepared will contain all the zinc in solution, and the iron will be pre- cipitated as acetate. The titration may at once be proceeded with at a temperature of about 50 to 60 C. by adding the ferrocyanide until the necessary reaction with uranium is obtained. As before mentioned, Galetti takes the change of colour as the ending of the process, and when iron is present this is quite distinguishable, but it requires considerable practice to rely upon, and it is therefore safer to use the uranium indicator. When using the uranium, however, I prefer to dilute the zinc solution less, both in the adjustment of the standard ferrocyanide and the analysis of ores. The dilution is necessary with Galetti's method of ending the process, but half the volume of liquid, or even less, is better with the external indicator. In Hydrochloric Acid Solution (Fahlberg and Maxwell Lyte). This method is not available in the presence of iron, copper, nickel, cobalt, or manganese. The Standard Solution of Ferrocyanide. 1 c.c. = O01 gm. of zinc. Lyte finds that this is obtained by dissolving 43*2 gm. of pure potassic ferrocyanide and diluting to 1 liter. This corresponds volume for volume with a solution of 10 gm. of pure zinc in excess of hydrochloric acid diluted to 1 liter. My experiments confirm this, but each operator is advised to adjust his solutions by experiment, always using the same quantities of reagents and volume of liquid. The end of the reaction between the zinc and ferrocyanide is found by uranium. The Analysis : If a solution of zinc freely acidified with HC1 is heated to nearly boiling point, two or three drops of uranic solution added, and the ferrocyanide delivered into the mixture from a burette, white zinc ferro- cyanide immediately precipitates, and as the drops of ferrocyanide fall into the mixture, a brown spot of uranic ferrocyanide appears, but disappears again on stirring so long as free zinc exists in solution. The moment all the zinc is converted into ferrocyanide, the addition of test solution tinges the whole liquid brown. This addition of uranium to the liquid may be used as a guide to the final testing on a porcelain plate, since as the pre- cipitation approaches completion, the tinge of brown disappears more slowly. The actual ending, however, is always ascertained by spreading a drop or two of the liquid upon the plate, bringing into contact with it a glass rod moistened with uranic solution ; when the same shade of colour is produced as occurred in the original titration of the ferrocyanide solution, the process is ended. Lyte gives the following method of treating a blende containing lead, copper, and iron (C. N. xxi. 222) : 2 gm. of finely powdered ore were boiled with strong HC1 and a little KC1O 3 , the insoluble matter again treated in like manner, the solutions mixed and evaporated somewhat, washed into a beaker, cooled, and moist 330 VOLUMETEIC ANALYSIS. 78. baric carbonate added to precipitate iron, allowed to stand a few hours, then filtered into a 200 c.c. flask containing 10 c.c. of strong HC1, and washed until the exact measure was obtained. 20 c.c. (=0'2 gm.) of blende were measured into a small beaker, diluted with the same quantity of water, 3 drops of uranic solution added, and the ferrocyanide delivered in from a burette. When 70 c.c. were added the brown tinge disappeared slowly ; the testing on a white plate was then resorted to, and the ferrocyanide added drop by drop, until the proper effect occurred at 73 c.c. As a slight excess of ferrocyanide was necessary to produce the brown colour, 0*2 c.c. was deducted, leaving 72'8 c.c. as the quantity necessary to precipitate all the zinc. The 0'2 gm. of blende therefore contained 0'0728 gm. of Zn or 36'4 per cent. The sample in question contained about 2 '7 per cent, of copper, but this was precipitated with the iron by the baric carbonate ; had it contained a larger quantity, the process would not have been available unless the copper was removed by other means. Fahlberg.(Z. a. C. 1874, 379) estimates the zinc in hydro- chloric solution with ferrocyanide and uranic indicator, but recommends the addition of ammonic chloride to the extent of about five times the weight of zinc present. With the well- known retarding effect of ammonia salts on the uranium reaction, the operator must be careful to carry out his analysis precisely in the same way as the original titration. Ores containing galena or copper are treated with aqua regia, then boiled with excess of HC1, the heavy metals precipitated with H 2 S and filtered off, the iron peroxidized with HNO 3 or KC1O 3 , cooled, precipitated with ammonia, dissolved and re-precipitated twice, to remove all zinc. The ammoniacal solutions are then mixed, neutralized with HC1, 10 or 15 c.c. of concentrated HC1 added, then titrated with ferrocyanide. Fah Ib erg stated in his original paper that the process yields good results with ores containing lead, copper, manganese, and iron ; but this is certainly not true as to manganese, and he has since acknowledged the fact. He states, however, that if moist brown lead dioxide is shaken up with the sulphuric solution of Zn, the whole of the Fe and Mn are precipitated. The liquid is filtered off, precipitate washed, then nitrate and washings titrated with ferrocyanide. M ah on (Amer. Chem. Journ. iv. 53) uses the ferrocyanide method much in the same way as above described, but finds that Mn must be absent to ensure good results. In the presence of Mn he separates the Zn from a strong acetic solution with IPS. The sulphide is then dissolved in HC1 and titrated as before. Technical process for Ores containing- Iron. Voigt (Zeit. ang. Chem. 1889, 307, 308). The solution of the substance in hydrochloric acid is oxidized with nitric acid and diluted to about 100 c.c. Sufficient potassic tartrate to keep the iron in solution is added, and then ammonia to feeble alkalinity, and the liquid is further diluted to about 250 c.c. Standard solution of potassic ferrocyanide is then run in, until a drop of the mixture brought in contact -with strong acetic acid develops a permanent blue. The ferrocyanide is of suitable strength if 1 c.c. is equal to O'Ol gm. of zinc. About 46 gm. of the salt is dissolved to a liter, and the solution is standard- ized against one of zinc made by dissolving 12'461 gm. of zinc oxide in 78. ZINC. 331 hydrochloric acid and diluting to a liter ; 10 c.c. of this solution is mixed with 5 gm. of potassic tartrate, a few drops of ferric chloride, ammonia, and water to 250 c.c., and should require 10 c.c. of the ferrocyanide. An essential condition is that the excess of ammonia should he as small as possible. Incorrect results are obtained when much manganese is present ; lead is not injurious. 6. Estimation of Zinc as Oxalate. This method is based on the fact that all the metals of the magnesia group are precipitated in the absence of alkaline salts by oxalic acid, with the addition of alcohol. The cases are very few in which such, a method can be made available, but the process as described by W. G. Leison (Sill im aw' 8 Journ. Sept. 1870) is -here given. The zinc compound is obtained, preferably as sulphate, in neutral solution, and strong solution of oxalic acid and a tolerable quantity of strong alcohol are added. Zinc oxalate quickly separates in a fine crystalline powder, which when washed by alcohol from excess of oxalic acid and dried, can be dissolved in hot dilute sulphuric acid, and titrated with permanganate ; the amount of zinc is calculated from the weight of oxalic acid so found. If the zinc oxalate be washed on a paper filter, it cannot be separated from the paper without contamination with fibres of that material, which would of course affect to some extent the permanganate solution. Hence it is advisable to filter through very clean sand, best done by a special funnel ground conical at the throat ; into this is dropped a pear-shaped stopper with a long stem, the pear-shaped stopper fitting the funnel throat tightly enough to prevent sand but not liquids from passing; a layer of sand being placed upon the globular end of the stopper and packed closely, the liquid containing the oxalate is brought upon it and so washed ; finally the stopper is lifted, the sand and oxalate washed through with dilute acid into a clean flask, and the titration completed. 7. Zinc Dust. The value of this substance depends upon the amount of metallic zinc contained in it ; but as it generally contains a large proportion of zinc oxide, the foregoing methods are not available for its valuation. The volume of hydrogen yielded by it on treatment with acids appears to me to be the most accurate, as suggested by Fresenius or by Barnes (/. S. C. I. v. 145). This may very well be done in the nitrometer with decomposing flask, and comparing the volume of gas yielded by pure zinc and the sample of dust under examination. Weil decomposes a known volume of standard solution of copper by digesting 0*4 gm. of the zinc dust in a platinum capsule, with 50 c.c. of copper solution containing 0'5 gm. Cu. The zinc precipitates metallic copper equivalent for equivalent. After removing the zinc refuse and metallic copper by filtration and washing, an aliquot portion of the filtrate is titrated with standard tin solution for the excess of copper as described in 54.6. The 332 VOLUMETRIC ANALYSIS. 79. amount of Cu precipitated, when multiplied by the factor 1*0236, will give the Zn in the 0*4 gm. of dust. Many other methods have been proposed for the valuation of this substance. The best is that of Klemp (Z. a. C. xxix. 253), which consists in treating the dust with an excess of caustic potash and potassic iodate : the latter is reduced in definite pro- portion by the metallic zinc to potassic iodide, and the latter estimated by distillation in the iodometric apparatus, fig. 29, 30, or 34. The solutions of potash and iodate must be somewhat concentrated, and the mixture with the zinc dust must be intimate, which may be best secured by shaking the whole together in a well-stoppered 200 c.c. flask with glass beads. A 5 per cent, solution of iodate should be used, and the potash solution should be about 40 per cent. For 1 gm. of the dust, 30 c.c. of the iodate and so much of the potash solution and water should be used as to measure 130 c.c. The weighed substance, together with the beads, being already in the flask, the solutions are added, the stopper greased with vaseline, tied down and shaken for five minutes, then heated on the water bath, with occasional shaking, for one hour. The flask is then cooled and the contents diluted to 250 or 500 c.c., and 50 or 100 c.c. placed in the distilling flask, acidified with sulphuric acid, and the iodine so set free distilled into solution of potassic iodide, and titrated with thiosulphate in the usual way. Each 0'2 gm. of iodine so found = 0-25644 gm. Zn. 8. Zinc Oxide and Carbonate. Benedikt and Cantor (Zeit. angew. Cliem. 1888, 236, 237) shew that zinc oxide and carbonate can be accurately titrated with standard acid and alkali, using methyl orange as indicator, and other zinc salts, using phenolphthalein. The oxide or carbonate is dissolved in excess of acid, and the excess titrated back by soda solution. Zinc salts are dissolved in water (50 c.c. to O'l gm. ZnO), phenolphthalein is added, and then standard soda solution to intense red colour. A few more c.c. of soda are then added, the mixture is boiled for some minutes, and the excess of soda titrated. If either free acid or zinc oxide is present in the zinc salt, it is neutralized in presence of methyl orange by alkali or acid, as the case may be. % VANADIUM. V=51'2. 79. VANADIUM salts, or the oxides of this element, may be very satisfactorily titrated by reduction with a standard ferrous solution; thus 2FeO + V0 5 =Fe 2 3 + VO 4 . 79. VANADIUM. 333 1 gm. of Fe represents 1*630357 gm. of vanadic pentoxide. Lindemann (Z. a. C. xviii. 99) recommends the use of a solution of ferrous ammonio-sulphate (double iron salt) standardized by _N_. potassic bichromate. Of course it is necessary that the vanadium compound should be in the highest state of oxidation, preferably in pure sulphuric acid solution. The blue colour of the tetroxide in the dilute liquid has* no misleading effect in testing with ferridcyanide. With hydrochloric acid great care must be taken to insure absence of free Cl or other impurities. The end-point in the case of this acid is different from that with sulphuric acid, owing to the colour of the ferric chloride, the mixture becoming clear green. The accuracy of the reaction is not interfered with by ferric or chromic salts, alumina, fixed alkalies, or salts of ammonia. Vanadic solutions being exceedingly sensitive to the action of reducing agents, great care must be exercised to exclude dust or other carbonaceous matters, alcohol, etc. 334 VOLUMETRIC ANALYSIS. 80. APPENDIX TO PAKT Y. BORIC ACID AND EQUATES. Boric anhydride B 2 3 =70. 80. THE soda in borax may, according to Thompson, be very accurately estimated by titrating the salt with standard H 2 S0 4 and methyl orange or lacmoid paper. Litmus and phenacetolin give very doubtful end-reactions : phenolphthalein is utterly useless. Example : 1*683 gm. sodic pyroborate in 50 c.c. of water required in one case 16*7 c.c. normal acid, and in a second 16'65 c.c. The mean of the two represents 517 gm. Na 2 O. Theory requires 0*516 gm. Since borates are now much used as preservative agents, a method which may be relied upon for technical purposes will be of considerable service, and such is described by E. F. Smith (Amer. Cliem. Journ. 1882). The process depends on the fact, that if a solution of manganese sulphate is added to one of borax, then alcohol in equal volume, a white flocculent precipitate of MnB 4 7 separates, which is insoluble in the alcoholic liquid. The excess of manganese sulphate is then determined after expulsion of the alcohol by permanganate, according to Guyard's or Volhard's method (64. 2). Example: 10 c.c. of borax solution containing O'l gm. ; to this were added 10 c.c. of solution of manganese sulphate containing 0*06 gm. MnSO 4 and 20 c.c. of strong alcohol. The mixture was well stirred, covered up, and allowed to stand for half an hour ; then filtered (best with suction pump) and well washed with alcohol. Filtrate and washings then evaporated to dry ness in a platinum or porcelain basin. The residual Mn was then dissolved in water, some strong solution of zina sulphate added, heated near to boiling, and permanganate delivered in from a burette until a permanent pink colour was produced. The strength of the permanganate was such that 18*5 c.c.=10 c.c. of MnSO 4 solution by the same method of titration. The amount of MnSO 4 so found deducted from that originally added gave the amount combined with the boracic acid. In this instance 6'4 c.c. of permanganate were required=0'0207 gm., showing 0*0393 gm. to be combined=36'44 per cent. B 2 3 . Theory requires 36'6 per cent. A mean of eighteen determinations gave 36*5 per cent, of B 2 3 in the sample of pure borax. The method is therefore very fairly accurate for soluble borates. In estimating the boracic acid in insoluble borates, as tourmaline, the following course was pursued : The finely pulverized substance was fused with a weighed quantity of pure sodic carbonate, the fused mass exhausted with water, and to the filtrate 80. BORATES. 335 containing all the sodic borate, together with some silicate and aluminate, was added an amount of pure ammonic sulphate molecularly equivalent to the sodic carbonate. The solution was then digested until all the ammonia was expelled and the volume of the liquid largely reduced. Any silica or alumina which had separated was now filtered off, and the precipitate thoroughly washed with hot water. The solution, again reduced in volume and containing only the borate and sulphates of sodium and ammonium, was mixed with a definite amount of a manganese sulphate solution (strength previously determined), alcohol added, and after standing one half-hour the borate was removed by filtration, the filtrate evaporated to dryness, and the residue carefully ignited to expel the ammonium salt. The manganese sulphate left was dissolved in water, and the titration carried out as before described. In a specimen of tourmaline from New York the boracic acid found by this method was 9 '7 per cent. Another portion of the same material with Marignac's method yielded 10 per cent. B 2 3 . In another tourmaline (locality unknown) two determinations by this method gave 6*55 and 6*32 per cent. B 2 3 , while with Marignac's method the amount obtained was 6 '8 per cent. B 2 3 . In some instances upon evaporating the alcoholic solution preparatory to determining the excess of manganese sulphate, brownish flocks separated. These were always dissolved in a little sulphuric acid and then evaporated to dryness. It is perhaps hardly necessary to say that some delicacy of manipulation is necessary in carrying out this process, especially if very small quantities are dealt with. A rapid and fairly accurate estimation of free boric acid may be made as suggested by Will (Arch. Pkarm. [3], 25, 11011113), by titration with a standard baryta solution, which is added to the solution of the acid until the turbidity appearing at first is completely and exactly removed. The amount of baric hydroxide used is exactly double the equivalent of the boric acid present, according to the equation 4H 3 BO 3 -j-Ba (HO) 2 =BaB 4 O 7 -f 7H 2 O. Schwarz has recently shown that the boric acid set free from borax by nitric acid does not affect Congo-red, whilst the slightest excess of nitric acid produces a blue-violet tint, and on this has based an obvious method for estimating the amount of acid in the borax. The boric acid thus set free may also be determined by titration with baryta water as above, or ethyl or methyl orange may be substituted for the Congo-red, and hydro- chloric acid may be used to decompose the borax. A mixture of free boric acid and borax solution may be dealt with by a combination of the two methods, that is, by first titrating with standard acid, and then titrating the total boric acid with baryta water. Instead of first setting free the acid, a solution of pure borax can be titrated directly by baryta solution, but only half the amount of standard solution is now required, since the equivalent amount of sodic hydroxide set free replaces the other half of baryta. In the presence of borax, chlorides can be estimated directly by means of silver nitrate, using potassic chromate as indicator ; free boric acid interferes with the reaction in this case ; the free acid is first determined by means of baryta solution ; then normal soda solution equivalent to at least half the baryta solution employed is added, and then dilute sulphuric acid until neutral ; the chlorides can now be directly estimated. The addition of soda solution may be omitted when estimating chlorides in the presence of little free boric acid and much borax, just as in the presence of borax only. 336 VOLUMETRIC ANALYSIS. 81. Of course, the chlorides and boric acid can be determined in separate portions, and can be directly titrated. The chlorides after neutralization by means of soda solution. Boric acid in presence of sulphates is estimated by the aid of phenacetolin as indicator. To the solution of boric acid and sulphates, two drops of an alcoholic solution of the indicator are added, and then standard baryta solution until a faint yellow tint appears ; normal hydrochloric acid is now added until a faint rose tint becomes a decided rose colour, the corresponding amount of baryta solution is deducted from the original amount taken, and the remainder indicates the boric acid. A further addition of hydrochloric acid until the colour changes from rose to yellow with the last drop, serves as a control estimation ; the acid thus taken being equivalent to the baryta combined with the boric acid. Halving the baryta and calculating the equivalent boric acid, this should agree with the amount first formed. Sulphates of the alkalies and of the alkaline earths, excepting magnesia, may be present. A little practice is required to get the rose colour accurately, and it is found better to use normal acid rather than a weaker one. For the estimation of borax in the presence of sulphates, Schwarz's method may be directly applied. OILS AND FATS. 81. THE examination of fatty matters by titration of their soluble or volatile and total fatty acids has of late assumed very considerable importance, in view of furnishing results which aid in determining the amount of adulteration to which they are subject. It has been found especially serviceable in the case of butter, and two methods are in vogue, both of which give good technical results. The same methods are more or less available for the examination of fats other than butter; and further experi- ments by various operators have rendered the methods of value for differentiating various fatty bodies. The titration methods were originated by Koettstorfer (Z. a. C. xix. 199) and Eeichert (Zi. a. C. xviii. 68), both being based on suggestions originating with the veteran chemist Chevreul, and by Hehner and Angell in their well-known treatise on Butter Analysis. The same may be said to a great extent of another novel and interesting method of examining the nature and composition of various fats, by the power they possess of absorbing bromine or iodine. This method, as regards bromine, has been worked out with considerable diligence and ability by Mills and Siiodgrass (/. S. C. I. ii. 435 and ibid iii. 366), also by Allen (ibid v. 68, also in his well-known treatise on Organic Analysis). The iodine method of Hubl is described in J. S. C. I. iii. 641. These various methods have been most voluminously discussed in their chemical and practical aspects, so that it must suffice here to give shortly the methods of analysis. It is only perhaps necessary to say that Hubl's iodine method is now generally adopted in preference to the absorption by bromine. 81. OILS AND FATS. 337 Butter. Reichert's Method. This consists in saponifying the fat to be examined by an alkali, separating the fixed acids by neutralizing the alkali, and distilling off the volatile acids (chiefly butyric) for titration with standard acid. In this and Koettstorfer's method, where also alcoholic solution of caustic alkali is used, it is essential to avoid absorption of CO 2 by long exposure. The Analysis : 2'5 gm. of the sample, aftar melting on the water bath, allowing the water to settle and filtering through a dry filter, are weighed into a flask holding about 160 c.c., and to it is added a sufficiency of alcoholic solution of potash to saponify the fat on heating in the water bath. 25 c.c. of solution of about | strength is generally sufficient. Close the flask with a cork fitted with a long vertical tube, and continue the heating in the water bath until perfect solution is obtained, then transfer the liquid to a porcelain or glass basin, and heat in the bath till the alcohol is completely evaporated. The residue is then dissolved in about 50 c.c. of water, transferred back to the flask, dilute H 2 SO 4 added in moderate excess, then diluted with water to about 75 c.c. Some small pieces of pumice strung together with intervening small spirals of thin platinum wire are then placed in the liquid, the flask connected with a small condenser, and the liquid distilled on a small sand bath until at least two-thirds of its volume have been collected. The distillate is then filtered if necessary, and carefully titrated with $ soda or potash, using phenolphthalein as indicator. The distillate from 2 '5 gm. of a genuine butter requires, according to all experience, not less than 12 '5 c.c. of ^ alkali, when the method is conducted as above described. Higher figures may doubtless be obtained by carrying the distillation still further. Keichert himself obtained an average of 14 c.c. in the case of pure butters ; but his paper describes the details of the process so imperfectly, that a doubt is left as to how far the distillation was carried. If the number of c.c. of acid used is to be expressed in terms of butyric acid the factor 0*352 may be used (Allen), and in the case of requiring 12 '5 c.c. of acid with 2 '5 gm. of butter the resulting figure is 4*41. The results of many operators reduced in this way show from 4*41 to 4 '96, but none of them fell below the former number, which may therefore be taken as the inferior limit.* Butter from goats' and ewes' milk gave practically the same results, while butterine or oleomargarine gave in no case higher than 0'5, and generally much less. Cocoanut oil, among the possible adulterants of butter, gave the highest figure, viz., 1-15 (Moore); but this is so far below the figure for butter, that even a very large admixture could at once be detected. It may be remarked that the exact weighing of 2*5 gm. of butter into a flask, especially if not solidified, is a difficult operation; it is * Instead of expressing the result in terms of butyric acid (eq. = 88), All en advises the statement of the weight of KHO neutralized by the distillate from 100 gm. of fat. This is obtained by multiplying the volume of ^ solution neutralized by the distillate from 2'5 gm. of fat by 0-2241. This gives the percentage by weight of KHO required by the fat. Z 338 VOLUMETRIC ANALYSIS. 81. therefore advisable to operate with about this quantity, noting the exact weight,, and arriving at a comparative result by calculation. The method above described is sufficiently accurate in the majority of cases, but Wo liny has shown, by a long and careful series of experiments (Bied. Centr. 1887, 699, also Analyst, xii. 203), that there are sources of error which must be removed before absolutely accurate results can be secured. The chief of these he ascribes to carbonic acid, which may at any time be present in the alcoholic potash. He therefore uses a 50 per cent, aqueous solution of soda in place of the potash, and prefers baryta solution for the final titration. In order to avoid exposure to the air during the saponification, and also loss of volatile acids, Wollny devised the following arrangement. A condenser slanting upward at an angle of 45 is fixed near the water bath upon which the saponification is to take place. The flask is connected with the condenser by means of a "|~-piece and india-rubber tubes, so that the leg of the T-piece can be directed upward or downward as desired. During saponifi- cation, which should take half an hour on the boiling water bath, the leg of the "[~~pi ece * s directed upward, being closed with a short piece of india-rubber and glass rod. The alcohol in this manner runs back into the flask. After that time the "]~~pi ece is turned downward and opened : the alcohol can thus be collected in a flask standing beneath. After twenty minutes, when distillation is complete, the ~]~"pi ece i g again turned upwards, arid through it 100 c.c. of boiling water are run into the flask by means of a pipette being tightly joined to the short piece of india-rubber. The "|""pi ece is closed again until the soap is completely dis- solved in the water, solution being assisted by gently shaking the flask. The Analysis : 5 gm. of the clear butter fat are accurately weighed into a 300 c.c. flask (round form, length of neck 7 or 8 centimeters, width of neck 2 centimeters), 2 c.c. of 50 per cent, soda solution (which must be preserved so that carbonic acid cannot be absorbed), and 10 c.c. of 96 per cent, alcohol, are added, and the mixture is heated under a reflux condenser for fifteen minutes in a boiling water bath. The alcohol is then distilled off whilst the flask is heated for at least half an hour ; 100 c.c. of boiling water are added under due precautions, and the flask heated until the soap is completely dissolved. 40 c.c. of sulphuric acid (25 c.c. H 2 SO 4 in 1 liter) and two pieces of pumice of the size of a pea are added, and the flask is at once connected with a condenser by means of a glass tube, 7 centimeters wide, and having, at a distance of 1 centimeter above the cork, a bulb of a diameter of 22-5 centimeters. The tube is bent immediately above the bulb upward in an oblique angle, in which direction it extends for 5 centimeters, and is then again bent downward, also in oblique angle, and then connected with a condenser by means of an india-rubber tube. The flask is then heated by means of a very small flame, until the insoluble fatty acids are completely fused ; 110 c.c. are then distilled off into a graduated flask, the distillation lasting thirty minutes ; the distillate is mixed, and 100 c.c. filtered oft'. This is transferred into a beaker, 1 c.c. of phenolphthalein solution. 81. OILS AND FATS. 339 (5 gm. in 1 liter 50 per cent, alcohol) added and titrated with decinormal baryta solution. To the volume of baryta used one-tenth is added, and the figure obtained by blank experiment is subtracted; the latter should not amount to more than 0'33 c.c. Koettstorfer's Method. This operation estimates the saponi- fying equivalent of any fatty substance, but is allowed on all hands to be less satisfactory in discriminating mixtures of other fats with butter, although extremely useful. In this method the whole of the acids existing in the fat are estimated. The solutions required are the following : Standard Hydrochloric Acid. Semi-normal strength, i.e., 18'185 gm. per liter. Standard Solution of Caustic Potash in Alcohol. Methylated spirit, previously digested with permanganate, dehydrated with dry potassic carbonate, then distilled, rejecting the first portions, may be used in place of pure alcohol. In any case the strength should not be less than 90 per cent., and the solution should be freshly made to avoid any deep colouration likely to interfere with the indicator. As it rapidly changes in strength, it is not possible to rely upon its being semi-normal, but it should be roughly adjusted at about that strength with absolutely accurate hydrochloric acid, and a blank experiment made side by side with each titration of fat. The excess of potash used in the fat titration is thus expressed in terms of - acid, and to arrive at the percentage of potash each c.c. is multiplied by 0*02805. The saponification equivalent of the fat or oil is found by dividing the weight in milligrams of the sample by the number of c.c. of normal (not *) acid corresponding to the alkali neutralized by the oil. If the percentage of potash is known, the saponifying equivalent may be found by dividing this percentage into 5610, or if 2s"aHO is the alkali used, into 4000. The Analysis: Prom 2 to 2' 5 gm. of the fat, previously purified by melting and filtration, are carefully weighed into a flask fitted with vertical tube. 25 c.c. of standard potash are then added, the mixture heated on the water bath to gentle boiling, with occasional agitation, until a perfectly clear solution is obtained . Koettstorfer recommends heating for fifteen minutes ; but in the case of butters this is generally more than sufficient ; with other fats twenty minutes to half an hour may be required. At the end of the saponification the flasks are removed from the bath, a definite and not too small a quantity of phenolphthalein added, and the titration carried out with as little exposure to the air as is possible. The method of calculation adopted by Koettstorfer is to ascertain the number of milligrams of KHO required to saturate the acids contained in 1 gm. of fat, or, in other words, parts per 1000. He found that, operating in this way, pure butters required from 221 '5 to 232 '4 m.gm. of KHO for 1 gm., whereas the fats usually mixed with butter, such as beef, mutton, and pork z 2 340 VOLUMETRIC ANALYSIS. 81. fat, required a maximum of 197 m.gm. for 1 gm., and other oils and fats much less. Practically this means that the amount of KHO required for genuine butters ranges from 23 '24 to 22 '15 per cent., the latter being the inferior limit. If caustic soda is used instead of potash, other numbers must of course be used. My experience, and, I believe, also that of others, shows that the method cannot be depended upon in the case of old re-melted butters, although perfectly genuine. The following list shows the parts of KHO required per 1000 of fat ; the first four being calculated from their known equivalents, the rest obtained experimentally by Koettstorfer, Allen, Stoddart, or Archbutt: Tripalmitin Tristearin Triolein Tributyrin Cocoanut Oil Dripping Lard . Horse Pat Lard Oil Olive Oil Niger Oil 208-8 Linseed 189-1 Cotton Seed 190-4 Whale . 557-3 Seal . 270-0 Colza and E pe 197-0 Cod Oil 195-6 Pilchard 199-4 Castor . 191196 191196 Sperm . Shark . 189191 189195 191196 190191 191196 175179 182187 186187 176178 130134 84-5 A further application of this method may be made in estimating separately the amounts of alkali required for saturating the free fatty acids and saponifying the neutral glycerides or other ethers of any given sample of fat, oil, or wax (see Allen, Organic Analysis ii. 45, 76). Titration of Miscellaneous Oils and Fats with Bromine or Iodine. The best method of carrying out this examination as regards bromine, appears to be that of Mills and Snod grass, to which reference has previously been made. The idea of using bromine is by no means new. Cailletet in 1857 adopted such a method; but the difficulty then, and up to the time when the task was undertaken by the operators mentioned, was the accurate measure- ment of the excess of bromine used, and the adaptation of such a solvent for both the fats and the bromine as would exclude the presence of water, and the tendency to form substitution products of variable and unknown character in preference to merely additive products. Our knowledge of the exact composition of the great family of fats and oils is at present limited, and it is not possible to make this reaction possess any strict chemical valency ; but experiment has shown that there are certain well-defined fats which absorb 81. OILS AND FATS. 341 within a very narrow limit the same amount of the halogen under the same conditions, and hence the method may be made highly suggestive as to mixtures of various fats whose absorption powers have been observed. In the first instance the common solvent used for the fat and the bromine was carbon disulphide ; but although very good results Avere obtained, compared -with solvents previously tried by other operators, there were the drawbacks of its offensive smell, and the solutions of bromine in it did not possess much stability. Finally, Dr. Mills adopted carbon tetrachloride as the medium with the happiest effects; and it was found that the bromide solution could be preserved for at least three months without diminution of standard. On the other hand, by using this medium, there is the necessity of working with greater delicacy, since the presence of the merest trace of water has more effect in producing substitution compounds than in the case of the disulphide. The accurate estimation of the excess of bromine, after the absorp- tion is complete, is necessarily a matter of great importance ; and this can be done either by comparison of colour with bromine solu- tion of known strength (the least effective method) $ or" by titratioii with thiosulphate, using starch and potassic iodide as the indicator, which is better. But, best of all, the operators after long research found that by using (3 naphthol (a substance which is readily and cheaply obtainable, and which forms in the presence of carbon tetrachloride a mono-bromo derivative) they could construct a solution of corresponding strength to the standard bromine, and thus titrate back in the same way as is commonly practised in alkalimetry. Very fair results were obtained colorimetrically by adopting the device of interposing a stratum of chromate solution (page 128), so as to neutralize the yellow colour produced with some of the fish oils, and which tended to mask the red colour of the bromine. Experiments showed that, using a bromine solution having a mean standard of 0*00644 gm. per c.c., the average probable error per cent, in a single result, when adopting the colour method or the thiosulphate and iodine was 0'62, whereas with /3 naphthol it was reduced to 0*46. But it is hardly necessary to say that, using such a small portion of material as is absolutely necessary in order to avoid secondary results, considerable care and practice are required. The sample of oil or fat must be dried as completely as possible, by heating and subsequent filtering through dry scraps of bibulous paper, or through dry double filters, before being weighed. The Analysis: O'l to 0*2 gm. of the fat is dissolved in 50 c.c. of the tetrachloride and standard bromine added, until at the end of 15 minutes there is a permanent red colour. If the colorimetric method is used, 50 c.c. of tetrachloride is tinted with standard bromine to correspond. If the iodine re-action, the solution of brominated material is added to potassic iodide and starch, and y^- sodic thiosulphate delivered in from a burette till 342 VOLUMETRIC ANALYSIS. 81. the colour is discharged. If, on the other hand, the standard naphthol solution is used, it is also cautiously added from a burette until the colour is removed. It is imperative that the operations in all cases be carried on out of direct sunlight. If the operator is unable to use carbon tetrachloride, the disulphide may be used ; but the solution of bromine in this medium is less stable, and must be checked more frequently. Somewhat larger portions of oil or fat may however be used for the analysis. It may be of service to give some few of the results obtained by Mills and Snodgrass. Absorption per cent. OILS. FATS. WAXES. Almond (from Beef - - 35-01 Beeswax - O'OO bitter fruit) 26'27 Butter (fresh) - 27'93 Carnauba - 33'50 Do. (from sweet) 5374 Do. (commercial) 25'0 Japan (1) - 2'33 Cod - - 83-00 Butterine Scotch 36'32 Do. (2) - 1-53 Nut - - 30-24 Do. (Trench) - 39'7l Myrtle - - 6'34 Ling Liver - 82 '44 Cocoanut - 5*70 Mustard - 46'15 Vaseline - 5'55 Neatsfoot - 38'33 Stearic Acid - O'OO Olive - - 60-61 Lard - - 37'29 Palm - - 35-00 Seal - - 57-34 Whale - -30-92 Linseed - - 76'09 Mineral Oil - 30'31 Shale Oil ) according to > 22 to 12 sp. gr. J Aniline - - 169'8 Turpentine (dry) 236'0 The same operators determined the percentage absorption by pure anhydrous turpentine, aniline and olive oil purified by filtration after long standing at low temperature. The calculated values are based on the known ratios C io H i6 : Br 2 and (C 3 H 5 ) (C 18 H 33 2 ) 3 : Br 6 . The mean of three estimations each in turpentine and aniline were 236 '0 and 169 '8 per cent., five estimations in olive oil (triolein) 54 per cent. The percentage by calculation is respectively 235-3, 172, and 54 '3. The Iodine Method. This has been worked out by Baron Hubl and others, but is not nearly so expeditious as the method just described ; though, as before stated, it has to a large extent replaced 81. OILS AND FATS. 343 it, owing mainly to the fact that less trouble is required, and the reactions involved are less delicate while equally accurate. The Standard Iodine Solution. This is made by dissolving respectively 25 gm. of iodine and 30 gm. of mercuric chloride in separate portions of strongest alcohol, say 500 c.c., then mixing the two liquids, and allowing to stand for some hours before taking the standard with thiosulphate and starch. This solution must always be standardized before use. The Analysis : 0'2 to 0'5 gm. of the fat or oil is dissolved in 10 c.c. of purest chloroform in a well stoppered flask, and 20 c.c. of the iodine solution added. The amount must finally be regulated, so that after not less than two hours' digestion the mixture possesses a dark brown tint ; under any circumstances it is necessary to have a considerable excess of iodine (at least double the amount absorbed ought to be present), and the digestion should be from six to eight hours. Some KI solution is then added, the whole diluted with 150 c.c. of water, and & thiosulphate delivered in till the colour is nearly discharged. Starch is then added, and the titration finished in the usual way. The numbers obtained by Hubl are given in J. S. C. I. iii. 642. A blank experiment should in every case be made side by side with the sample, using the same proportions of chloroform and iodine solution. Allen states that, in both these methods of titration, the amount of halogen taken up may be considered as a measure of the unsaturated fatty acids (or their glycerides) present. Thus, the acids of the acetic or stearic series exhibit no tendency to combine with bromine or iodine under the conditions of the experiments, while the acids of the acrylic or oleic series assimilate two, and the acids of the linoleic series four atoms of the halogen. "We are indebted to R. T. Thompson and H. Ballaiityne (/. S. C. I. ix. 588) for a very careful revision of the constants required in the analysis of Oils and Fats, the results of which are given in the following table. The lards operated upon were rendered by themselves and are therefore genuine. The fact is brought out that for each 0*1 increase in specific gravity, there is an increase of 1*3 per cent, iodine absorption, and beef fat seems to follow the same rule. Cotton seed oil shows only about half that proportion. In using the iodine absorption method these operators found that some oils required fully eight hours for complete absorption, and they recommend, as a rule, to start the digestion in the evening and titrate the solutions on the following morning. 344 VOLUMETRIC ANALYSIS. 81. Table of Constants in the Analysis of Oils. Nature of Oil or Fat. Sp. Gr. at 15-5 C. Sp. Gr. at 99 C. Iodine Absorptn. KOH Neutrlizd. Free Acid. per cent. per cent. per cent. Olive (Gioja) 915-6 79-0 19-07 9'42 Olive (Gioja) after re- moval of free acid . . . 915-2 . 79-0 19-07 None. Olive 914-8 83-2 18-93 3-86 Olive 914-7 80-0 23-78 Olive 916-8 83-1 19-00 5-19 Olive ... 916-0 81-6 19-83 Olive (for dyeing) 915'4 78'9 19'00 9-67 Olive 914-5 86-4 18-90 11-28 Olive (for cooking) 915-1 83-1 19-20 4-15 Olive (for cooking) 916-2 81-2 19-21 Not done Lard (from omentum) ... 859-8 52-1 . Lard (from leg) . . . 860'5 61'3 Lard (from ribs) 860-6 62-5 Beef fat (from suet) . . . 857-2 34-0 Beef fat (oleomargarine) 858-2 46-2 Pat from marrow of ox... 858-5 45-1 19-70 Fat from bone of ox ... 859-2 47-0 19-77 Cotton seed 923-6 868-4 110-1 Cotton seed 922-5 106-8 19-35 0-27 Linseed (Baltic) 934-5 187-7 19-28 Linseed (East India) . . . 931-5 178-8 19-28 Linseed (River Plate) ... 932-5 . . 175-5 19-07 . Linseed 932-5 173-5 19-00 0-76 Linseed 931-2 168-0 19-00 Rape 916-8 105-6 17-53 2-43 Rape 913-1 110-7 17-33 . Rape 914-5 104-1 17-06 2-53 Eape 915-0 104-5 17-19 3-10 Eape 914-1 100-5 17-39 Castor (commercial) 967-9 83-6 18-02 2-16 Castor (commercial) 965-3 17*86 Castor (medicinal) 963-7 17-71 Arachis (commercial) ... 920-9 98-7 19-21 6-20 Arachis (French refined) 917-1 98-4 18-93 0-62 Lard oil (prime) 917-0 76-2 Southern sperm 880*8 81'3 13'25 Arctic sperm (bottle-nose) 879-9 82-1 13-04 Whale (crude Norwegian) 920-8 109-2 Whale (pale) 919-3 110-1 Seal (Norwegian) 925-8 152-1 - - Seal (cold drawn, pale) . . . 926-1 145-8 19-28 Seal (steamed, pale) 924-4 142-2 18-93 Seal (tinged) 925-7 152-4 Seal (boiled) ... 923-7 142-8 Menhaden 931-1 160-0 18-93 Newfoundland cod 924-9 . 160-0 Scotch cod 925-0 158-7 . Cod liver (medicinal) . . . 926-5 166-6 18-51 0-36 Mineral 873-6 12-8 Mineral 886-0 26-1 Rosin 986-0 67-9 82. GLYCERIN. 345 GLYCERIN (GLYCEROL). C 3 H 8 3 = 92. 82. UP to a very recent time no satisfactory method of determining glycerin had been devised, but the problem has now been solved in a tolerably satisfactory manner. The permanganate method appears to have been originally suggested by Wanklyn, improved by him and Fox, and further elaborated by Benedikt and Zsigmondy (Cliem. Zeit. ix. 975). It depends on the saponification of the fat, and oxidation of the resultant glycerin by permanganate in alkaline solution, with formation of oxalic acid, carbon dioxide, and water, thus C 3 H s 3 + 30 2 = C 2 H 2 O + CO 2 + 3H 2 0. Aqueous solutions of glycerin may of course be submitted to the method very easily. The -excess of permanganate is destroyed by a sulphite, the liquid filtered from the manganese precipitate, the oxalic acid then precipitated by a soluble calcium salt in acetic solution, and the precipitated calcic oxalate, after ignition to convert it into carbonate, titrated with standard acid in the usual way, or the oxalic precipitate titrated with permanganate. The oxalic solution may be titrated direct after addition of H 2 S0 4 with permanganate ; but Allen and Belcher have found this method faulty, probably from the formation of a dithionate, due to the sulphite. On the other hand, they have obtained very satisfactory results by the alkalimetric or the permanganate titration, on known weights of pure oxalic acid and glycerin. These operators have also shown that, in the case of dealing with fats, where it has been recommended by Wanklyn and Fox to use ordinary alcohol as the solvent, and by Benedikt methyl alcohol, both these media, especially ethylic alcohol, produce in themselves a variable quantity of oxalic acid when treated with alkaline permanganate, and hence vitiate the process. Again, if it be attempted to avoid this by boiling off the alcohols, there is a danger of losing glycerin.* Allen's method with oils and fats is as follows : 10 gm. of the fat or oil are placed in a strong small bottle, together with 4 gm. of pure KHO dissolved in 25 c.c. of water. A solid rubber stopper is then used to close the bottle, and tied down firmly with wire. It is then placed in boiling water, or in a water oven, and heated, with occasional shaking, from 6 to 10 hours, or until the contents are homogeneous, and all oily globules have disappeared. When saponification is complete, the bottle is emptied into a beaker and diluted with hot water which should give a clear * In dealing with waxes or similar "bodies including sperm oil, potash, dissolved in methyl alcohol must be used for the saponification, as it is almost impossible to do it with aqueous potash. 346 VOLUMETRIC ANALYSIS. 82. solution, the fatty acids are then separated by dilute acid, filtered, and the nitrate made up to a given volume. This solution, which will usually contain from 0'2 to 0'5 of glycerol, according to its origin, is transferred to a porcelain basin and diluted with cold water to about 400 c.c. From 10 to 12 gm. of caustic potash should next be added, and then a saturated aqueous solution of potassic permanganate until the liquid is no longer green but blue or blackish. An excess does no harm. The liquid is then heated and boiled for about an hour, when a strong- solution of sodic sulphite should be added to the boiling liquid until all violet or green colour is destroyed. The liquid containing the precipitated oxide of manganese is then poured into a 500 c.c. flask, and hot water added to 15 c.c. above the mark, the excess being an allowance for the volume of the precipitate and for the increased measure of the hot liquid. The solution is then passed through a dry filter, and, when cool, 400 c.c. of the filtrate should be measured off, acidified with acetic acid, and precipitated with calcic chloride. The solution is kept warm for three hours, or until the deposition of the calcic oxalate is complete, and is then filtered, the precipitate being washed with hot water. The precipitate consists mainly of calcic oxalate, but is liable to be contaminated more or less with calcic sulphate, silicate, and other impurities, and hence should not be directly weighed. It may be ignited, and the amount of oxalate previously present deduced from the volume of normal acid neutralized by the residual calcic carbonate, but a preferable plan is to titrate the oxalate by standard permanganate. Tor this purpose, the filter should be pierced and the precipitate rinsed into a porcelain basin. The neck of the funnel is then plugged, and the filter filled with dilute sulphuric acid. After standing for five or ten minutes this is allowed to run into the basin and the filter washed with water. Acid is added to the contents of the basin in quantity sufficient to bring the total amount used to 10 c.c. of concentrated acid, the liquid diluted to about 200 c.c., brought to a temperature of about 60 C., and decinormal permanganate added gradually till a distinct pink colouration remains after stirring. Each c.c. of permanganate used corresponds to 0'0045 gm. of anhydrous oxalic acid, or to 0'004fc5 gm. of glycerin. Operating in the way described, the volume of permanganate solution required will generally range between 70 and 100 c.c. Otto Hehner has experimented largely on the estimation of glycerol in soap leys and crude glycerins, the results of which are given in J. S. C. I. viii. 4. The volumetric methods recommended in preference to the permanganate are the oxidation with potassic bichromate or the conversion of the glycerol into triacetin. The Bichromate Method. One part of glycerol is completely converted into carbonic acid by 7 '486 parts of bichromate in the presence of sulphuric acid. The solutions required are : Standard Potassic bichromate. 74 '86 gm. of pure potassic bichromate is dissolved in water. 150 c.c. of concentrated sulphuric acid added, and when cold diluted to a liter. 1 c.c. = '01 gm. glycerol. A weaker solution is also made by diluting 100 c.c. of the strong solution to a liter. These solutions should be controlled by a ferrous solution of known strength, if there is any doubt about the purity of the bichromate. 82. GLYCERIN. 347 Solution of double Iron salt. 240 gm. of ferrous ammonium sulphate is dissolved with 50 c.c. of concentrated sulphuric acid to a liter, and its relation to the standard bichromate must be accurately found from time to time by titration with the latter, using the ferricyanide indicator ( 33, p. 111). The Analysis : With concentrated or tolerably pure samples of glycerin it is only necessary to take a small weighed portion, say 0'2 gm. or so, dilute moderately, add 10 or 15 c.c. of concentrated sulphuric acid and 30 or 40 c.c. of the stronger bichromate, place the beaker covered with a watch glass in a water bath and digest for two hours ; the excess of bichromate is then found by titration with the standard iron solution. The weaker bichromate is useful in completing the titration where accuracy is required. As the stronger bichromate and the iron solution are both concentrated, they must be used at a temperature as near 16 C. as possible. In the case of crude glycerin it must be purified from chlorine or aldehyde compounds as follows : About 1'5 gm. of the diluted sample is placed in a 100 c.c. flask, some moist silver oxide added, and allowed to stand 10 minutes. Basic lead acetate is then added in slight excess, the measure made up to 100 c.c., filtered through a dry filter, and 25 c.c. or so digested with excess of bichromate, and titrated as before described. The Acetin Method. This method is due to Benedikt and Cantor (Monatslieft ix. 521), and recommends itself by its simplicity and rapidity as compared with other methods. Hehner has pointed out the precautions necessary to insure accuracy as follows : The Analysis : About 1*5 gm. of the crude glycerin is placed in a round- bottomed flask, together with 7 gm. of acetic anhydride and 3 gm. of perfectly anhydrous sodic acetate ; an upright condenser is attached to the flask, and the contents are heated to gentle boiling for one hour and a half. After cooling, 50 c.c. of water are added, and the mixture heated until all triacetin has dissolved. The solution is then filtered into a large flask, the residue or filter well washed, the liquid cooled, some phenolphthalein added, and the acidity exactly neutralized by a dilute solution of caustic soda. The triacetin is then saponified by adding 25 c.c. of an approximately 10 per cent, solution of pure caustic soda standardized on normal sulphuric or hydrochloric acid, and boiling for 10 minutes, taking care to attach a reflux condenser to the flask. The excess of alkali is then titrated back with normal acid, each c.c. of which represents 0'0306T gm. of glycerin. It is essential that the processes of analysis should be rapid and continuous, and especially that the free acetic acid in the first process be neutralized very cautiously, and with constant agitation to avoid the local action of alkali. Weak soap lyes should be concentrated to 50 per cent, of glycerin if estimated by the acetin method ; if not the bichromate method must be used. For fats and soaps about 3 gm. should be saponified with alcoholic potash, diluted with 200 c.c. of water, the fatty acids separated and filtered off. The filtrate and washings are then rapidly boiled to one-half and titrated with bichromate. 3-48 VOLUMETRIC ANALYSIS. 83. PHENOL (CABBOLIC ACID). 83. THE only method claiming accuracy for the estimation of this substance volumetrically was originated by Koppeschaar (Z. a. C. xvi. 233), and consists in precipitating the phenol from its aqueous or dilute alcoholic solution with bromine water in the form of tribromphenol. The strength of the bromine water was established by Koppeschaar, by titration with thiosulphate and potassic iodide with starch. Allen modifies the process as follows : A certain weight of the sample is dissolved in water; as much as corresponds to O'l gm. of phenol is taken out and put into a stoppered bottle holding 250 c.c. Further, to 7 c,c. of normal soda solution ( = 0'04 gin. NaOH per c.c.) bromine is gradually added till a yellow colour appears and remains ; the liquid is then boiled till it has become colourless again. It now contains 5 molecules of sodic bromide and 1 of sodic bromate. When completely cooled, it is put into the phenol solution, after which 5 c.c. con- centrated hydrochloric acid is at once added, and the bottle stoppered and shaken for some time. The reactions are : II. C 6 H 6 0+6Br=C 6 H 3 Br 3 0+3HBr. The bromine set free in the first, and not fixed by phenol in the second reaction, must be still free, and is estimated by adding potassic iodide and titrating the iodine liberated, by YTT thiosulphate : III. 2KI+Br 2 =2KBr+2l. IV. I 2 +2Na 2 S 2 3 =Na 2 S 4 O 6 +2NaI. For this purpose the bottle is allowed to stand for 15 or 20 minutes; a solution of about 1*25 gm. potassic iodide (free from iodate) is added, the bottle is stoppered, shaken up, and allowed to rest. Its contents are now poured into a beaker ; the bottle is rinsed out, a little starch solution is added, and thiosulphate is run in from a burette till the blue colour is gone. (It will be best not to add the starch till the colour of the liquid has diminished to light yellow.) The calculation is made as follows : 7 c.c. of normal soda solution neutralize 0'56 gm. of bromine, all of which is liberated by HC1. O'l gm. phenol would require 0'4068 and leave a surplus of 01532 gm. ; the latter would liberate enough iodine to saturate 19'5 c.c. of T N T thiosulphate. Every c.c. of thiosulphate used over and above this indicates 0'00197 gm. impurities in O'l gm. of the sample that is, 1*27 per cent. If a number of estimations have to be made at one time, it would seem decidedly preferable to adopt Koppeschaar's original method, rather than to prepare special bromine solution as above. For the estimation of phenol in raw products, Tb'th (Z. a. C. xxv. 160) modifies the bromine process as follows : 20 c.c. of the impure carbolic acid are placed in a beaker with 20 c.c. of caustic potash solution of T3 sp. gr., well shaken, and allowed to stand for half an hour, then diluted to about k liter with water. By this treatment the foreign impurities are set free, and may mostly be removed by filtration ; 84. CAftBON BISULPHIDE. 349 the filter is washed with warm water until all alkali is removed. The filtrate and washings are acidulated slightly with HC1, and diluted to 3 liters. 50 c.c. are then mixed with 150 c.c. of standard bromine solution, and then 5 c.c. concentrated HC1. After twenty minutes, with frequent shaking, 10 c.c. of iodide solution are added, mixed, and allowed to rest three to five minutes, then starch, and the titration with thiosulphate carried out as usual. Example : 20 c.c. raw carbolic oil were treated as above described. 50 c.c. of the solution, with 150 c.c. bromine solution (made by dissolving 2'04 gm. sodic bromate and 6'959 gm. sodic bromide to the liter), then 5 c.c. of HC1, required lY'8 c.c. of thiosulphate for titration. The 150 c.c. bromide =0'237 gm. Br. The 17'8 c.c. thiosulphate required for residual titration = 0'052 gm. Br, leaving 0*185 gm. Br for combination with the phenol. According to the equation One niol. phenol =3 mol. Br, hence the percentage of phenol was 10'86. Kleinert (Z. a. C. xxxiii. 1) suggests, and his experiments appear to prove, that in titrating acid creosote oil by Koppeschaar' s method for phenol, a serious error occurs in virtue of such oil containing substances of higher boiling point than phenol, which are soluble in water, and behave with bromine in the same manner as true phenol. CARBON BISULPHIDE AND THIOCARBONATES. 84. FOR the purpose of estimating carbon disulphide in the air of soils, gases, or in thiocarbonates, Gas tine has devised the following process (Compt. Rend, xcviii. 1588) : The gas or vapour to be tested is carefully dried, and then passed through a concentrated solution of recently fused potassic hydroxide in absolute alcohol. The presence of even traces of water seriously diminishes the delicacy of the reaction. The alcoholic solution is afterwards neutralized with acetic acid, diluted with water, and tested for xanthic acid by adding copper sulphate. In order to determine the distribution of carbon bisulphide introduced into the soil, 250 c.c. of the air in the soil is drawn by means of an aspirator through sulphuric acid, and then through bulbs containing the alcoholic potash. For quantitative determinations, a larger quantity of air must be used, and the xanthic acid formed is estimated by means of the reaction 2C 3 H 6 OS 2 +I 2 =2C 3 H 5 OS 2 +2HI. The alkaline solution is slightly acidified with acetic acid, mixed with excess of sodic bicarbonate, and titrated in the usual way with a solution of iodine containing T68 gm. per liter, 1 c.c. of which is equivalent to 1 m.gm. of carbon bisulphide. To apply this method to thiocarbonates, about 1 gm. of the substance, together with about 10 c.c. of water, is introduced into a small flask and decomposed by a solution of zinc or copper sulphate, the flask being heated on a water bath, and the evolved carbon bisulphide passed, first through sulphuric acid, and then into alcoholic potash. In the case of gaseous mixtures of carbon bisulphide, nitrogen, hydrogen sulphide, carbonic anhydride, carbonic oxide, and water-vapour, the gas is passed through a strong aqueous solution of potash, then into sulphuric acid, and finally into 350 VOLUMETRIC ANALYSIS. 85. alcoholic potash. The thiocarbonate formed in the first flask is decomposed by treatment with copper or zinc sulphate as above, and the xanthic acid obtained is added to that formed in the third flask, and the whole titrated with iodine. Another method available for technical purposes, such as the comparative estimation of CS 2 in coal gas, or in comparing samples of thioearbonates, is as follows : The liquid, or other substance, containing the disulphide, is added to strong alcoholic potash or gas containing the CS 2 , is passed slowly through the alkaline absorbent. The disulphide unites with the potassic ethylate to form potassic xanthate. The liquid is neutralized with acetic acid and the xanthate is then estimated by titrating with a standard solution of cupric sulphate (12*47 gm. per liter), until an excess of copper is found by potassic ferrocyanide used as an external indicator. Each c.c. of copper solution represents 0'0076 gm. CS 2 . MOLYBDENUM AND TUNGSTEN. Mo = 95-8. W=184. 85. VOLUMETRIC methods are probably of very limited use in the case of these substances, but cases may arise in which their estimation is more simply accomplished by volumetric than by gravimetric means. Von d. Pf or d ten (0. N.I. 18) gives the following processes : The solution of the salt is mixed, for molybdenum, with 5060 c.c., and for tungsten with 70 80 c.c. of HC1 of 27 per cent. There are then added for molybdenum, 8 10 gm., and for tungsten 14 15 gm. zinc in the form of rods, and in as large pieces as possible. The solution may contain 03 gm. molybdic oxide, or O'l gm. of tungstic oxide ; in the latter case, the solution is previously heated on the water bath, and the HC1 and zinc are then added ; the deposition of tungstic oxide in a solid state is thus avoided. Towards the end of the reduction, a little heat may sometimes be applied to the molybdenum solution also with advantage. When the molybdenum solution has become yellow, and that of tungsten red, the flask is cooled in the case of tungsten with especial care. The remainder of the procedure is different. The molybdenum solution is poured into a porcelain capsule containing 40 c.c. dilute sulphuric acid, and 20 c.c. of manganous sulphate solution free from ferrous salt, and containing 200 gm. per liter. An equal volume of water is added, and a dilute solution of standard permanganate is run in. The results are accurate : 1 c.c. KMnO 4 = 0-000752185 O=0'0045131lMoO 3 . The reduced tungsten solution is rinsed quickly into a capsule in which there is an excess of permanganate, 70100 c.c. dilute sulphuric acid, 40 c.c. manganous sulphate solution, but otherwise no water. Not until the flask has been rinsed out is the liquid diluted to 1 liter. In presence of such large quantities of HC1 the manganous sulphate exerts its power of transferring oxygen only in concentrated solutions. Quick working is essential. An excess of ferrous sulphate is now run in, and' the solution is finally titrated with permanganate. 85. MOLYBDENUM AND LEAD. 351 Schindler's Method for Molybdenum and Lead (Z. a. C. xxvii. 137). This process has already been referred to under Phosphoric acid ( 69). It was originally designed for the estimation of molybdenum and incidentally applied to lead. It is based on the fact that lead acetate, when added to a hot solution of ammonic molybdate, gives a precipitate of PbMoO 4 , which is insoluble in acetic acid. Any excess of the molybdate solution shows a colour varying from yellow to blood red with a freshly prepared solution of tannin (1 300) as indicator, according to the amount present. The method requires the following solutions : Standard Lead acetate. 50 gm. of acetate is dissolved in a liter of water with 10 c.c. of acetic acid; it is then to be standardized with a known weight of pure ammonic molybdate. Standard Ammonic molybdate. This may be made with the ordinary commercial salt by dissolving about 20 gm. in 700 800 c.c. of water, adding a little ammonia to render it clear. It is standardized by the following method, so that 1 c.c. shall equal 1 c.c. of lead solution. The Analysis : To 50 c.c. of the molybdenum solution faintly acidified with acetic acid, about 300 c.c. of boiling water is added, and the standard lead solution run in until the whole of the molybdic acid is precipitated, and a slight excess of lead occurs, which may be known by removing a drop from the clear liquid, and bringing it in contact with a drop of the tannin indicator on a porcelain tile; if an excess of lead has been used no colour occurs. Standard molybdic solution is then cautiously added, until by similar testing a distinct orange colour appears. The volume of the latter is deducted from the volume of lead solution originally used and the remainder calculated to molybdic acid, 207Pb=144MoO 3 . The method is capable of giving very good results, and is applicable to the analysis of lead compounds, such as white lead, by dissolving in nitric acid, neutralizing with ammonia, then acidifying with acetic acid before titrating. The accuracy of the method depends on a correct molybdic standard ; the pure ammonium salt has the formula (NH 4 ) 6 Mo 7 24 + 4H 2 0, and should when ignited in a platinum crucible to a constant weight give 81 '55 per cent. MoO 3 . 352 VOLUMETFJC ANALYSIS. 8 86. PAET VI. SPECIAL APPLICATIONS OF THE VOLUMETRIC SYSTEM TO THE ANALYSIS OF URINE, POTAELE WATERS, SEWAGE, ETC. ANALYSIS OF URINE. 86. THE complete and accurate determination of the normal and abnormal constituents of urine presents more than ordinary difficulty to even experienced chemists, and is a hopeless task in the hands of any other than such. Fortunately, however, the most important matters, such as urea, sugar, phosphates, sulphates, and chlorides, can all be determined volumetrically with accuracy by ordinary operators, or by medical men who cannot devote much time to practical chemistry. The researches of Liebig, Neubauer, Bence Jones, Vogel, Beale, Hassall, Pavy, and others, during the last few years, have resulted in a truer knowledge of this important secretion; and to the two first mentioned chemists we are mainly indebted for the simplest and most accurate methods of estimating its constituents. With the relation which the proportion of these constituents bear to health or disease the present treatise has nothing to do, its aim being simply to point out the readiest and most useful methods of determining them quantitatively. Their pathological importance is very fully treated by some of the authorities just mentioned, among the works of which Neubauer and Vo gel's Analyse des Hams, Beale 's Urine, Urinary Deposits, and Calculi, and Menu's Traite de Cliimie Medicate, are most prominent and exhaustive ; and we now have the collected experience of all the best authorities in the world in The Pathological Handbook of Drs. Lander Brunton, Klein, Foster, and Burdon Sanderson (Churchill). The gram system of weights and measures will be adopted throughout this section, while those who desire to use the grain system will have no difficulty in working, when once the simple relation between them is understood* (see 9, p. 22). The question of weights and measures is, however, of very little consequence, if the analyst considers that he is dealing with relative parts or pro- * In a word, wherever c.c. occurs, dm. may be substituted; and in case of usin.? grains for grams, move the decimal point one place to the right ; thus 7'0 grams would be changed to 70 grains. Of course it is understood that where grams are taken c.c. must be measured, and with grains dm., the standard solution being the same for both systems. 86. URINE. 353 portions only ; and as urine is generally described as containing so many parts of urea, chlorides, or phosphates, per 1000, the absolute weight may be left out of the question. The grain system is more readily calculated into English ounces and pints, and therefore is generally more familiar to the medical profession of this country. One thing, however, is necessary as a preliminary to the exami- nation of urine, and which has not generally been sufficiently considered ; that is to say, the relation between the quantity of secretion passed in a given time, and the amount of solid matters found in it by analysis. In a medical point of view it is a mere waste of time, generally speaking, to estimate the constituents in half-a-pint or so of urine passed at any particular hour of the day or night, without ascertaining the relation which that quantity, with its constituents, bears to the whole quantity passed during, say, 24 hours ; and this is the more necessary, as the amount of fluid secreted varies very considerably in healthy persons ; besides this, the analyst should register the colour, peculiarity of smell (if any), consistence, presence or absence of a deposit (if the former, it should be collected for separate analysis, filtered urine only being used in such cases for examination), and lastly its reaction to litmus should be observed. 1. Specific Gravity. This may be taken by measuring 10 c.c. with an accurate pipette into a tared beaker or flask. The observed weight say is 10*265 gm. ; therefore 1026*5 will be the specific gravity, water being 1000. Where an accurate balance, pipette, or weights are not at hand, a good urinometer may be used. These instruments are now to be had with enclosed thermometer and of accurate graduation. 2. Estimation of Chlorides (calculated as Sodic Chloride). This may be done in several ways, and I have placed the methods in the order in which I consider they ought to be ranked as regards accuracy. Liebig's method is by far the simplest, but the end-point is generally so obscure that the liability to error is very great. Mohr's method I have modified by the use of ammonic in place of potassic nitrate, owing to the solvent effect which the latter has been found to produce on silver chromate. By ignition the ammonia salt is destroyed. (a) By Silver Nitrate (Mohr). 10 c.c. of the urine are measured into a thin porcelain capsule, and 1 gm. of pure ammonic nitrate in powder added ; the whole is then evaporated to dryness, and gradually heated over a small spirit lamp to low redness till all vapours are dissipated and the residue becomes white; it is then dissolved in a small quantity of water, and the carbonates A A 354 VOLUMETRIC ANALYSIS. 86. produced by the combustion of the organic matter neutralized by dilute acetic acid ; a few grains of pure calcic carbonate to remove all free acid are then added, and one 'or two drops of solution of potassic chromate. The mixture is then titrated with -^ silver, as in 37.2 (&). Each c.c. of silver solution represents 0'005837 gm. of salt, consequently if 12*5 c.c. have been used, the weight of salt in the 10 c.c. of urine is 0*07296 gm., and as 10 c.c. only were taken, the weight multiplied by 10, or what amounts to the same thing, the decimal point moved one place to the right, gives 7 '296 gm. of salt for 1000 of urine. If 5*9 c.c. of the urine are taken for titration, the number of c.c. of ^V silver used will represent the number of parts of salt in 1000 parts of urine. Pibram (Vierteljdhrsh, f. prad. Heilk, cvi. 101) obviates the necessity for evaporating the urine with a nitrate previous to titration by heating the urine with permanganate. 10 c.c. of urine are mixed with about 5 c.c. of ^ permanganate, and 40 c.c. of water, then brought nearly to boiling; by this means a brown flocculent precipitate is produced, consisting of organic matter and manganous salt, which is filtered away, leaving the clear liquor colourless, so that an excess of permanganate shows the rose tint at once. Enough permanganate must be used to give this tint, which is then removed by a drop or two of dilute oxalic acid solution, a little calcic carbonate added, and the fluid titrated with ~Q silver solution and chromate as before described. This method gives very good results with normal urines. (b) By Volhard's Method. This is a direct estimation of Cl by excess of silver and the excess found by ammonic sulphocyanate ( 39), which gives very good results in the absence of much organic matter, and is carried out as follows : 10 c.c. of urine are placed in a 100 c.c. flask and diluted to about 60 c.c. 2 c.c. of pure nitric acid and 15 c.c. of standard silver solution (1 c.c. = 0'01 gm. NaCl) are then added ; the closed flask is well shaken, and the measure made up to 100 c.c. with distilled water. The mixture is then passed through a dry filter, and about 70 or 80 c.c. of the clear fluid titrated with standard ammonic sulphocyanate for the excess of silver, using the ferric indicator described on page 127. The relative strength of the silver and sulphocyanate being known, the measure of the former required to combine with the chlorine in the 7 or 8 c.c. of urine is found and calculated into NaCl. Arnold (F finger's Arcliiv. xxxv. 541) carries out this process as follows : 10 c.c. of urine are mixed with 10 to 20 drops of nitric acid sp. gr. 1*2, 2 c.c. of ferric indicator, and 10 to 15 drops of solution of permanganate to oxidize organic matter. The liquid is then titrated according toVolhard's directions. 86. URINE. 355 Another variation of Molir's method is proposed by Zuelzer (Bericlite, xviii. 320). 10 c.c. of urine are acidulated with nitric acid, and precipitated with silver nitrate; the silver chloride is filtered off and washed, then dissolved in ammonia : from this solution the silver is precipitated with fresh NH 4 S. The excess of sulphide is then removed by cadmic nitrate, the liquid diluted to a definite volume, and an aliquot part filtered off, acidulated with nitric acid, neutralized with calcic carbonate, then titrated with T ^ silver and chromate. This method is cumbrous, and, if the reagents are not extremely pure, liable to great inaccuracy. (c) By Mercuric Nitrate (Liebig). The principle of this method is as follows : If a solution of mercuric nitrate, free from any excess of acid, is added to a solution of urea, a white gelatinous precipitate is produced, containing urea and mercuric oxide in the proportions of 1 eq. of the former to 4 eq. of the latter (4HgO + Ur). When sodic chloride, however, is present in the solution, this precipitate does not occur until all the sodic chloride is converted by double decomposition into mercuric chloride (sublimate) and sodic nitrate, the solution remaining clear ; if the exact point be overstepped, the excess of mercury immediately produces the pre- cipitate above described, so that the urea present acts as an indicator of the end of the process. It is therefore possible to ascertain the proportion of chlorides in any given sample of urine by this method, if the strength of the mercurial solution is known, since 1 eq. of mercuric oxide converts 1 eq. of sodic chloride into 1 eq. each of corrosive sublimate and sodic nitrate. Standard Solution of Mercuric nitrate. It is of great im- portance that the solution be pure, for if the mercury from which it is made be contaminated with traces of other metals, such as bismuth, silver, or lead, they will produce a cloudiness in the liquid while under titration, which may possibly obscure the exact ending of the reaction; therefore 18 '42 gm. of the purest precipi- tated mercuric oxide are put into a beaker, with a sufficiency of pure nitric acid of about 1*20 spec. grav. to dissolve it by the aid of a gentle heat ; the clear solution so obtained is evaporated on the water bath to remove any excess of free acid. When the liquid is dense and sirupy in consistence, it may be transferred to the graduated cylinder or flask and diluted to a liter. 1 c.c. of the solution so prepared is equal to 0*01 gm. of sodic chloride, or 0-006059 gm. of chlorine. If pure mercuric oxide is not at hand, the solution is best made by weighing 25 gm. of mercuric chloride, which is dissolved in about a liter of water and the oxide precipitated with a slight excess of caustic potash or soda. The precipitate of yellow oxide is allowed to settle clear and the liquor decanted. It is repeatedly washed in this manner with warm distilled water until the washings show no amount of alkali or alkaline chloride; the precipitate is then dissolved in the smallest quantity of pure nitric acid, and diluted to about 950 c.c. If any great excess of nitric acid is present, it may be cautiously neutralized by pure sodic hydrate or carbonate. A A 2 356 VOLUMETRIC ANALYSIS. 86. Verification of the Mercuric Solution. This is carried out by the help of the following solutions : Pure Sodic chloride. 20 gm. per liter. Solution of Urea. 4 gm. of pure urea in 100 c.c. Solution of pure Sodic sulphate. Saturated at ordinary tem- peratures. This is used to regulate the action of the free acid which is liberated in the reaction. In the case of natural urine it is not necessary. Process of Titration: 10 c.c. of the standard sodic chloride ( = 0'2 gm. NaCl) are placed in a small beaker, together with 3 c.c. of the urea solution, and 5 c.c. of sodic sulphate. The mercuric solution is then delivered in from the burette, with constant stirring, until a decided permanent white pre- cipitate is seen to form. A mere opalescence may occur even at the beginning, arising from slight impurities in the mercury, but this may be disregarded. If the mercuric solution has been made from weighed pure oxide, exactly 20 c.c. should be required ; if, on the contrary, it has been made from the fresh unweighed oxide, somewhat less than 20 c.c. should be required. Say that 18'5 c.c. have been found to give the necessary reaction, then the solution must be diluted with distilled water in the proportion of 1'5 c.c. to every 18'5, or 925 c.c. made up to a liter. It may happen that the solution, when made from weighed dry mercuric oxide, is not correct, owing to the difficulty of obtaining perfectly pure material ; in such case a factor must be used to bring the volume used to the correct standard. (cC) Baryta Solution for removing- Phosphoric and Sulphuric Acids. Before urine can be submitted to titration by the mercurial solution, it is necessary to remove the phosphoric acid, and the proper agent for this purpose is a mixture composed of 1 volume of cold saturated solution of pure baric nitrate and 2 volumes ditto baric hydrate ; the same agent is used previous to the estimation of urea, and may be simply designated Baryta Solution. The Analysis : 40 c.c. of the clear urine are mixed with 20 c.c. of baryta solution, and the thick mixture poured upon a small dry filter; when sufficient clear liquid has passed through, 15 c.c. (=10 c.c. of urine) are taken with a pipette and just neutralized, if necessary, with a drop or two of nitric acid. If not alkaline, the probability is that sufficient baryta solution has not been added to precipitate all the phosphoric and sulphuric acids. This may be known by adding a drop or so of the baryta solution to the filtrate ; if any precipitate is produced, it will be necessary to mix a fresh quantity of urine with three-fourths or an equal quantity of baryta, in which case 17i or 20 c.c. must be taken to represent 10 c.c. of urine; the excess in either case of baryta must be cautiously neutralized with nitric acid. The vessel containing the fluid is then brought under a Mohr's burette containing the mercurial solution, and small portions delivered in with stirring, until a distinct permanent precipitate is produced. The volume of solution used is then read off and calculated for 1000 parts of urine. Example: 15 c.c. of the liquid prepared with a sample of urine, as described above ( = 10 c.c. of urine), required 6'2 c.c. of mercurial solution: the quantity of salt present was therefore 0'062 gm., or 6'2 parts in 1000 parts of urine. 86. URINE. 357 3. Estimation of TJrea (Lie big;). The combination between urea and mercuric oxide in neutral or alkaline solutions has been alluded to in the foregoing article on chlorides ; it will therefore probably be only necessary to say that the determination of urea in urine is based on that reaction ; and as the precipitate so produced is insoluble in water or weak alkaline solutions, it is only necessary to prepare a standard solution of mercury of convenient strength, and to find an indicator by which to detect the point when all the urea has entered into combination with the mercury, and the latter slightly predominates. This indicator is sodic carbonate. Liebig's instructions are, that when in the course of adding the mercurial solution from the burette to the urine, a drop of the mixture is taken from time to time and brought in contact with a few drops of solution of sodic carbonate on a glass plate or in a watch-glass, no change of colour is produced at the point of contact until the free urea is all removed ; when this is the case, and the mercury is slightly in excess, a yellow colour is produced, owing to the formation of hydrated mercuric oxide. The compound of urea and mercury consists, according to Liebig's analysis, of 1 eq. of the former to 4 eq. of the latter ; that is to say, if the nitric acid set free by the mixture is neutralized from time to time with sodic carbonate or other suitable alkali. If this be not done, the precipitate first formed alters in character, and eventually consists only of 3 eq. of mercury with 1 of urea. In order to produce the yellow colour with sodic carbonate, there must be an excess of mercurial solution. Theoretically, 100 parts of urea should require 720 parts of mercuric oxide; but practically, 772 parts of the latter are necessary to remove all the urea, and at the same time show the yellow colour with alkali; consequently the solution of mercuric nitrate must be of empirical strength, in order to give accurate results. Preparation of the Mercuric Solution. 77*2 gill, of red mercuric oxide, or 71 '5 gm. of the metal itself, are treated with nitric acid, as described in the previous article on chlorides, and in either case diluted to 1 liter : 1 c.c. of the solution is then equal to O'Ol gm. of urea. (The extreme care required to remove traces of foreign metals from the mercury is not so necessary here as in the foregoing instance, but no large amount of free acid must be present.) Dragendorff prefers to use mercuric chloride in the preparation of the standard solution, by weighing 9 6 '855 gm. of the pure salt, which is dissolved in water, then precipitated with dilute caustic soda, the precipitate well washed by decantation until free from chlorine, then dissolved in a slight excess of nitric acid, and the solution diluted to 1 liter. 358 VOLUMETRIC ANALYSIS. 86. The Analysis : Two volumes of the urine are mixed with one of baryta solution as before described in the case of chlorides (reserving the precipitate for the determination of phosphoric acid, if necessary), and 15 c.c. ( = 10 c.c. of urine) taken in a small beaker for titration; it is brought under the burette containing the mercurial solution (without neutralizing the excess of baryta, as in the case of chlorides), and the solution added in small quantities' so long as a distinct precipitate is seen to form. A plate of glass laid over dark paper is previously sprinkled with a few drops of solution of sodic carbonate, and a drop of the mixture must be brought from time to time, by means of a small glass rod, in contact with the soda. So long as the colour remains white, free urea is present in the mixture ; when the yellow colour is distinctly apparent, the addition of mercury is discontinued, and the quantity used calculated for the amount of urea. _ It is always advisable to repeat the analysis, taking the first titration as a guide for a more accurate estimation by the second. Example : 15 c.c. of urine deprived of phosphates ( = 10 c.c. of the original urine) were titrated as described, and required 17'6 c.c. of mercurial solution; consequently there was 0'176 gm. of urea present in the 10 c.c., or 17'6 parts in the 1000 of urine. The experiments of Rautenberg (Ann. d. Cliem. u. Pliarm. cxxxiii. 55) and Pfliiger (Z. a. G. xix. 375) show, however, that the method, as devised by Liebig, is open to serious errors, due to the uncertainty in the point of neutralization. Pfl tiger's researches are very complete, and lead to the follow- ing modification of the process. A solution of pure urea is prepared containing 2 gm. in 100 c.c. 10 c.c. of this solution is placed in a beaker, and 20 c.c. of the mercury solution ran into it in a continuous stream ; the mixture is then immediately brought under a burette containing normal sodic carbonate, and this solution is added with constant agitation until a permanent yellow colour appears. The volume of soda solution so used is noted as that which is necessary to neutralize the acidity produced by 20 c.c. of the mercury solution in the presence of urea. Pfliiger found that by titrating 10 c.c. of the urea solution by small additions of the mercury, and occasional neutralization, the end of the reaction occurred generally at from 17 '2 to 17 '8 c.c. of mercury ; but when he ran in boldly 19 '7 c.c. of mercury, followed immediately by normal sodic carbonate to near neutrality, then alternately a drop or two of first mercury, then soda, the exact point was reached at 20 c.c. of mercury; and when 10 c.c. of the mercury solution which gave this reaction were analyzed as sulphide- by weight, a mean of several determinations gave 0'7726 gm. of HgO, which agrees very closely with Liebig's number. In the case of titrating urine, the following method is adopted: A plate of colourless glass is laid upon black cloth, and some drops of a thick mixture of sodic bicarbonate (free from carbonate) and water placed upon it at convenient distances. The mercury solution is added to the urine in such volume as is judged appropriate, and from time to time a drop of the white mixture is placed beside the bicarbonate so as to touch, but not mix completely. At first the urine mixture remains snow-white, but with further additions of mercury a point at last occurs when the white gives 86. URINE. 359 place to yellow. When the colour has developed itself, both drops are rubbed quickly together with a glass rod : the colour should disappear. Further addition of mercury is made cautiously until a faint yellow is permanent. Now is the time to neutralize by the addition of the normal soda to near the volume which has been found necessary to completely neutralize a given volume of mercury solution. If the time has not been too long in reaching this point, it will be found that a few tenths of a c.c. will suffice to complete the reaction. If, however, much time has been consumed, it may occur that, notwithstanding the mixture is distinctly acid, the addition of soda produces a more or less yellow colour : in this case, nothing is left but to go over the analysis again, taking the first trial as a guide for the quantities of mercury and soda solutions, which should be delivered in one after the other as speedily as possible until the exact end is reached. It is absolutely necessary, with this modified process, to render the urine perfectly neutral, after it is freed from phosphates and sulphates by baryta solution. Corrections and Modifications (Liebig). In certain cases the results obtained by the above methods are not strictly correct, owing to the variable state of dilution of the liquid, or the presence of matters which affect the mercury solution. The errors are, however, generally so slight as not to need correction. Without entering into a full description of their origin, I shall simply record the facts, and give the modifications necessary to be made where thought desirable. The Urine contains more than 2 per cent, of Urea, i.e., more than 20 parts per 1000. This quantity of urea would necessitate 20 c.c. of mercurial solution for 10 c.c. of urine. All that is necessary to be done when the first titration has shown that over 2 per cent, is present, is to add half as much water to the urine in the second titration as has been needed of the mercurial solution above 20 c.c. Suppose that 28 c.c. have been used at first, the excess is 8 c.c., therefore 4 c.c. of water are added to the fluid before the second experiment is made. The Urine contains less than 2 per cent, of Urea. In this case, for every 4 c.c. of mercurial solution less than 20, O'l c.c. must be deducted, before calculating the quantity of urea ; so that if 16 c.c. have been required to produce the yellow colour with 10 c.c. urine, 15'9 is to be considered the correct quantity. The Urine contains more than 1 per cent, of Sodic Chloride, i.e., more than 10 parts per 1000. In this case 2 c.c. must be deducted from the quantity of mercurial solution actually required to produce the yellow colour, with 10 c.c. of urine. The Urine contains Albumen. In this case 50 c.c. of the urine are boiled with 2 drops of strong acetic acid to coagulate the albumen, the precipitate allowed to settle thoroughly, and 30 c.c. of the clear liquid mixed with 15 c.c. of baryta solution, filtered, and titrated for both chlorides and urea, as previously described. The Urine contains Ammonic Carbonate. The presence of this substance is brought about by the decomposition of urea, and it may sometimes be of interest to know the quantity thus produced, so as to calculate it into urea. As its presence interferes with the correct estimation of urea direct, by mercurial solution, a portion of the urine is precipitated with baryta as usual, and a quantity, representing 10 c.c. of urine, evaporated to dryness in the water bath to expel the ammonia, the residue then dissolved in a little 360 YOLUMETEIC ANALYSIS. 86. water, and the urea estimated in the ordinary way. On the other hand, 50 or 100 c.c. of the urine, not precipitated with baryta, are titrated with normal sulphuric acid and litmus paper, each c.c. of acid representing 0'017 gm. of ammonia, or 0'03 gm. of urea. Pfliiger's correction for concentration of the urea differs from Liebig's, his rule being as follows : Given the volume of urea solution + the volume of NaCO 3 required + the volume of any other fluid free from urea which may he added, and call this V 1 ; the volume of mercury solution is V 2 ; the correction, C, is then C= (V 1 V 2 ) x 0-08. This formula holds good for cases where the total mixture is less than three times the volume of mercury used. With more concentrated solutions this formula gives results too high. Pf eif f er (Zeit. f. Biol. xx. 540) has made a careful comparison of Liebig's (as modified by Pf lliger) and Kautenberg's methods of estimating urea. The essential difference of Rautenberg's method consists in maintaining the urea solution neutral throughout by successive additions of calcic carbonate ; under these conditions, the composition of the precipitate differs from that formed when the titration is made according to Pfliiger's process, a fact which accounts for the diminished consumption of mercuric nitrate in the former method. The general conclusions from his observations may be summarized as follows: (1) In estimating the correction for sodic chloride, the amount of free acid should be as small as possible, and 0*1 c.c. should be subtracted from every c.c. of mercuric nitrate used, but in human urine it is preferable to precipitate the chlorine with silver nitrate, as a slight excess of the latter does not influence the result. (2) The coefficient for dilution should be determined afresh for every new standard solution. 4. Estimation of tJrea by its conversion into Nitrogen Gas. If a solution of urea is mingled with, an alkaline solution of hypochlorite or hypobromite, the urea is rapidly decomposed and nitrogen evolved, which can be collected and measured in any of the usual forms of gas apparatus described in the section on analysis of gases. Test experiments with pure urea have shown, that the whole of the nitrogen contained in it is eliminated in this process, with the exception of a constant deficit of 8 per cent. In the case of urine there are other nitrogenous constituents present, such as uric acid, hippuric acid, and creatinine, which render up a small proportion of their nitrogen in the process, but the quantity so obtained is insignificant, and may be disregarded. Consequently, for all medical purposes, this method of estimating urea in urine is sufficiently exact. 86. URINE. 361 In the case of diabetic urines, however, Mehu and others have pointed out that this deficiency is diminished, and if, in addition to the glucose present, cane sugar be also added, it will almost entirely disappear. Mehu therefore recommends that in the analysis of saccharine urines cane sugar be added to ten times the amount of urea present, when the difference between the actual and theoretical yield of nitrogen will not exceed 1 per cent (Bull. Soc. Chim. [2] xxxiii. 410). Kussell and West (/. C. S. [2] xii. 749) have described a very convenient apparatus for working the process, and which gives very good results in a short space of time. This method has given rise to endless forms of apparatus devised by various operators, including Mehu, Yvon, Dupre, Apjohn, Maxwell Simpson, O'Keefe, etc., etc. : the principles of construction are all, however, the same. Those who may wish to construct simple forms of apparatus from ordinary laboratory appliances, will do well to refer to the arrangements of Dupre (/. C. S. 1877, 534) or Maxwell Simpson (ibid. 538). The apparatus devised by Russell and West is shown in fig. 45, and may be described as follows : The tube for decomposing the urine is about 9 inches long, and about half an inch inside diameter. At 2 inches from its closed end it is narrowed, and an elongated bulb is blown, leaving the orifice at its neck of an inch in diameter; the bulb should hold about 12 c.c. The mouth of this tube is fixed into the bottom of a tin tray about If inch deep, which acts as a pneumatic trough ; the tray is supported on legs long enough to allow of a small spirit lamp being held under the bulb tube. The measuring tube for collecting the nitrogen is graduated into cubic centimeters, and of such size as to fit over the mouth of the decom- posing tube ; one holding about 40 c.c. is a convenient size. Eussell and West have fixed by experiment the propor- tions, so as to obviate the necessity for correction of pressure and temperature, namely, 37 "1 c.c. = 0'1 gm. of urea, since they found that 5 c.c. of a 2 per cent, solution of urea constantly gave 37'1 c.c. of nitrogen at ordinary temperatures and pressures. The entire apparatus can be purchased of most operative chemists for a moderate sum. Hypobromite Solution. This is best prepared by dissolving 100 gm. of caustic soda in 250 c.c. of water and adding 25 c.c. of Tig. 45. 362 VOLUMETEIC ANALYSIS. 86. bromine ; this mixture gives a rapid and complete decomposition of the urea. It is always best prepared in small quantities as required. Strong solution of sodic or calcic hypochlorite answers equally well, and possesses the advantage of keeping better. The Analysis : 5 c.c. of the urine are measured into the bulb-tube, fixed in its proper position, and the sides of the tube washed down with distilled water so that the bulb is filled up to its constriction. A glass rod, having a thin band of india-rubber on its end, is then passed down into the tube so as to plug up the narrow opening of the bulb. The hypobromite solution is then poured into the upper part of the tube until it is full, and the trough is afterwards half filled with water. The graduated tube is filled with water, the thumb placed on the open end, and the tube is inverted in the trough. The glass rod is then pulled out, and the graduated tube slipped over the mouth of the bulb-tube. The reaction commences immediately, and a torrent of gas rises into the measuring tube. To prevent any of the gas being forced out by the reaction, the upper part of the bulb-tube is slightly narrowed, so that the gas is directed to the centre of the graduated tube. With the strength of hypobromite solution above described, the reaction is complete in the cold in about ten or fifteen minutes ; but in order to expedite it, the bulb is slightly warmed. This causes the mixing to take place more rapidly, and the reaction is then complete in five minutes. The reaction will be rapid and complete only when there is considerable excess of the hypobromite present. After the reaction the liquid should still have the characteristic colour of the hypobromite solution. The amount of constriction in the tube is by no means a matter of indifference, as the rapidity with which the reaction takes place depends upon it. If the liquids mix too quickly, the evolution of the gas is so rapid that loss may occur. On the other hand, if the tube is too much constricted, the reaction takes place too slowly. The simplest means of supporting the measuring tube is to have the bulb-tube corked into a well, which projects from the bottom of the trough about one inch downwards. The graduated tube stands over the bulb-tube, and rests upon the cork in the bottom of the well. It is convenient to have, at the other end of the trough, another well, which will form a support for the measuring tube when not in use. To avoid all calculations, the measuring tube is graduated so that the amount of gas read off expresses at once what may be called the percentage amount of urea in the urine experimented upon ; i.e. the number of grams in 100 c.c., 5 c.c. being the quantity of urine taken in each case. The gas collected is nitrogen saturated with aqueous vapour, and the bulk will obviously be more or less affected by temperature and pressure. Alterations of the barometer produce so small an alteration in the volume of the gas, that it may be generally neglected ; e.g. if there are 30 c.c. of nitrogen, the quantity preferred, an alteration of one inch in the height of barometer would produce an error in the amount of urea of about 0'003 ; but for more exact experiments, the correction for pressure should be introduced. In the wards of hospitals, and in rooms where the experiments 86. TJKINE. 363 are most likely to be made, the temperature will not vary much from 65 F., and a fortunate compensation of errors occurs with this form of apparatus under these circumstances. The tension of the aqueous vapour, together with the expansion of the gas at this temperature, almost exactly counterbalances the loss of nitrogen in the reaction. The authors found from experience that 5 c.c. of urine is the most advantageous quantity to employ, as it usually evolves a con- venient bulk of gas to experiment with, i.e. about 30 c.c. They have shown that 5 c.c. of a standard solution containing 2 per cent, of urea evolve 37*1 c.c. of nitrogen, and have consequently taken this as the basis of the graduation of the measuring tube. This bulk of gas is read off at once as 2 per cent, of urea, and in the same way the other graduations on the tube represent percentage amounts of urea. If the urine experimented with is very rich in urea, so that the 5 c.c. evolve a much larger volume of gas than 30 c.c., then it is best at once to dilute the urine with its own bulk of water ; take 5 c.c. of this diluted urine, and multiply the volume of gas obtained by two. If the urine contains much albumen, this interferes with the process so far that it takes a long time for the bubbles of gas to subside, before the volume of gas obtained can be accurately read off. It is therefore better in such cases to remove as much as possible of the albumen by heating the urine with two or three drops of acetic acid, filtering, and then using the filtrate in the usual manner. The hypobromite method of estimating urea has given rise to much discussion during the last few years, and among other contributions, there occurs one from Dr. Wormley, which negatives the idea that there is necessarily a loss of nitrogen by this process (C. N. xlv. 27). This operator gives the result of his experiments as follows : For the purpose of examining the accuracy of this process for urea, without the presence of cane sugar or glucose, the form of apparatus, at least in principle, advised by Apjohn (C. N. xxxi. 37) was employed. This consists of a wide-mouthed bottle in which is placed the reagent, and also a small test-tube, for containing the urea solution, of about 10 c.c. capacity and of such length as to stand inclined in the bottle. The mouth of the bottle is closed with a rubber stopper carrying a glass tube, by which it is connected by rubber tubing to a graduated burette divided into 0*1 c.c. and suspended in a long cylinder of water from an adjustable arm. The urea solution is placed in the small tube within the charged bottle, the apparatus closed, and when there is no longer any change in the height of the column of liquid within the graduated tube, this is so adjusted that the surface of the contained liquid exactly coincides with that in the cylinder. This point, the temperature, 364 VOLUMETRIC ANALYSIS. 86. and in exact experiments the barometric pressure, being noted, the urea solution is mixed with the reagent by inclining the bottle and gently shaking the mixture. As the evolved nitrogen collects in the burette, the latter is gradually raised to relieve the contained gas from the increased pressure. When the evolution of gas has entirely ceased and there is no longer any change in the volume of gas, the tube is finally adjusted and the exact volume noted. The hypobromite employed was the same as described p. 361. In applying the reagent, it was diluted with a volume and a half of pure water. With this arrangement a series of experiments was performed employing 1 c.c. of a standard solution of pure urea varying in strength from 1 to 6 per cent., variously diluted, and added to varying quantities of the reagent. These experiments gave different results, in some only about 90 per cent., and even less, of the nitrogen being evolved, while in others a larger proportion was obtained, and in still others the icliole of the nitrogen was set free. It was finally observed that under certain conditions the whole of the nitrogen is uniformly eliminated. These conditions are : 1. The reagent should be freshly prepared. 2. The urea solution should be wholly added to the reagent, none of the latter being allowed to mix with the urea solution in the containing tube. 3. The amount of urea operated upon should not exceed one part to about twelve hundred parts of the diluted reagent. Moreover, the diluted urea solution should be added in small portions at a time to the reagent, thoroughly mixed, and the effervescence allowed to cease before any further addition of urea. So, also, it would appear, at least when comparatively large quantities of urea are present, that the surrounding temperature should not be less than about 20 C. (68 R). In the practical application of the test, if a 2 per cent, solution of urea is under examination, 1 c.c. of the solution, diluted with from 5 to 10 c.c. water, is placed in the containing tube, and the mixing bottle charged with 10 c.c. of the reagent diluted with 15 c.c. of water; whereas, for 1 c.c. of a 4 per cent, solution of urea, similarly diluted, not less than about 50 c.c. of the diluted reagent should be employed. In a final series of experiments, in which the above conditions were observed, the temperature being noted to ^ of a degree, and the results reduced to the standard temperature and pressure, the following average results were obtained : Urea employed. Nitrogen evolved = 10 milligrams 9*98 rn.gm. urea. 20 20-07 30 29-95 40 39-88 86. URINE. In these experiments it was assumed that 1 gm. of urea contains 372 c.c. of nitrogen, measured at C. and 760 m.m. barometric pressure; or, that each c.c. of nitrogen evolved, measured under the conditions stated, represented 0'002688 gm. urea. During these investigations it was observed, in cases in which the whole of the nitrogen was not evolved, that so long as the conditions remained the same, the relative proportion of the nitrogen eliminated was pretty uniform. Hence, if the volume of nitrogen evolved from a known quantity of urea under certain conditions, or by a given form of apparatus, be determined, the result may be taken as the basis for the determination of the urea in the urine with sufficient accuracy for clinical purposes. Hamburger (Zeit. f. Biol. xx. 286) refers to Pfliiger's modification of Liebig's method, which although an improvement leaves much to be desired; his own method is founded on Quinquand's (Monit. Scien. 1882, 2), in which the de- composition of urea by sodic hypobromite is supposed to take place thus : 3jSTaBr + 2H 2 + CO 2 + W. This reaction requires the proportion of bromine, sodic hydrate, and water to be exactly balanced or incorrect results will be obtained. The author claims for his method that it will yield correct results, no matter in w r hat proportions these reagents are present. It consists essentially in adding an excess of an alkaline solution of sodic hypobromite of known strength to the liquid containing urea, then destroying the excess of hypobromite with an excess of standard sodic arsenite (=19 '8 gm. As 2 3 per liter), and finally determining the amount of arsenite remaining unoxidized, by titration with standard iodine solution, the amount of urea then being readily calculated from the amount of sodic arsenite remaining unoxidized. The author's experiments as to the accuracy of the method, show that a certain quantity of urea always requires the same amount of hypobromite, and that the dilution of the solution of urea has no effect on the quantity of hypobromite employed. To decide on the applicability of the method to natural urine, great pains were taken, the urea being determined as described, the effect of its dilution with water studied, pure urea added, and the whole estimated, and lastly sodic hypobromite of various degrees of concentration employed ; the results of the experiments are given very fully and tabulated. On the whole, they are very satisfactory, the differences falling well within the limits of errors of observation and manipulation : the method is therefore considered applicable to the determination of urea in urine. 366 VOLUMETRIC ANALYSIS. 86. 5. Estimation of Phosphoric Acid (see also 69). The principle of this method is fully described at page 273. The following solutions are required : (1) Standard Uranic acetate or nitrate. 1 c.c. = 0*005 gm. P 2 5 (see p. 274). (2) Standard Phosphoric acid (see p. 275). (3) Solution of Sodic acetate (see p. 274). (4) Solution of Potassic ferrocyanide. About 1 part to 20 of water, freshly prepared, or some of the finely powdered salt. The Analysis : 50 c.c. of the clear urine are measured into a small beaker, together with 5 c.c. of the solution of sodic acetate (if uranic nitrate is used). The mixture is then warmed in the water bath, or otherwise, and the uranium solution delivered in from the burette, with constant stirring, as long as a precipitate is seen to occur. A small portion of the mixture is then removed with a glass rod and tested as described (p. 275) ; so long as no brown colour is produced, the addition of uranium may be continued ; when the faintest indication of this reaction is seen, the process must be stopped, and the amount of colour observed. If it coincides with the original testing of the uranium solution with a similar quantity of fluid, the result is satisfactory, and the quantity of solution used may be calculated for the total phosphoric acid contained in the 50 c.c. of urine; if the uranium has been used accidentally in too great quantity, 10 or 20 c.c. of the same urine may be added, and the testing concluded more cautiously. Suppose, for example, that the solution has been added in the right proportion, and 19'2 c.c. used, the 50 c.c. will have contained 0'096 gm. phosphoric acid ( = T92 per 100). With care and some little practice the results are very satisfactory. Earthy Phosphates. The above determination gives the total amount of phosphoric acid, but it may sometimes be of interest to know how much of it is combined with lime and magnesia. To this end 100 or 200 c.c. of the urine are measured into a beaker, and rendered freely alkaline with ammonia ; the vessel is then set aside for ten or twelve hours, for the precipitate of earthy phosphates to settle : the clear fluid is then decanted through a filter, the precipitate brought upon it and washed with ammoniacal water; a hole is then made in the filter and the precipitate washed through; the paper moistened with a little acetic acid, and washed into the vessel containing the precipitate, which latter is dissolved in acetic acid, some sodic acetate added, and the mixture diluted to about 50 c.c. and titrated as before described ; the quantity of phosphoric acid so found is deducted from the total previously estimated, and the remainder gives the quantity existing in combination with alkalies. 6. Estimation of the Sulphuric Acid. Standard Baric chloride. A quantity of crystallized baric chloride is to be powdered, and dried between folds of blotting- paper. Of this, 30-5 gm. are dissolved in distilled water, and the liquid made up to a liter. 1 c.c. = 0'01 gm. of SO 3 . Solution of Sodic sulphate. 1 part to 10 of water. 86. URINE. 367 The Analysis : 100 c.c. of the urine are poured into a beaker, a little hydrochloric acid added, and the whole placed on a small sand bath, to which heat is applied. When the solution boils, the baric chloride is allowed to flow in very gradually as long as the precipitate is seen distinctly to increase. The heat is removed, and the vessel allowed to stand still, so that the precipitate may subside. Another drop or two is then added, and so on, until the whole of the SO 3 is precipitated. Much time, however, is saved by using Be ale's filter, represented in fig. 19. A little of the fluid is thus filtered clear, poured into a test-tube, and tested with a drop from the burette ; this is afterwards returned to the beaker, and more of the test solution added, if necessary. The operation is repeated until the precipita- tion is complete. In order to be sure that too much of the baryta solution has not been added, a drop of the clear fluid is added to the solution of sodic sulphate placed in a test-tube or upon a small mirror (see 73.3). If no precipitate occurs, more baryta must be added ; if a slight cloudiness takes place, the analysis is finished; but if much precipitate is produced, too large a quantity of the test has been used, and the analysis must be repeated. For instance, suppose that 18 '5 c.c. have been added, and there is still a slight cloudiness produced which no longer increases after the addition of another J- c.c., we know that between 18 \ and 19 c.c. of solution have been required to precipitate the whole of the sulphuric acid present, and that accordingly the 100 c.c. of urine contain between 0*185 and 0*19 gm. of SO 3 . 7. Estimation of Sugar. Feh ling's original method is precisely the same as described in 71, but if the urine has decomposed so as to contain ammonia or ammonic carbonate, this method must be discarded and Pavy's or Knapp's solution used. The Analysis : 10 c.c. of the clear urine are diluted by means of a measuring flask to 200 c.c. with water, and a large burette filled with the fluid; 10 c.c. of the copper solution (=0'05 gm. of sugar) are then measured into a white porcelain capsule, 40 c.c. of distilled water added, the vessel arranged over a spirit or gas lamp under the burette, and brought to boiling ; the diluted urine is then delivered in cautiously from the burette until the bluish colour has nearly disappeared. The addition of the urine must then be continued more carefully, allowing the red precipitate to subside after each addition by removing the heat, when by gently sloping the capsule, the clear liquid allows the white sides of the capsule to be seen, so that the faintest shade of blue would be at once perceptible. When the colour is all removed, the burette is read off, and the quantity of sugar in the urine calculated as follows : Suppose that 40 c.c. of the diluted urine have been required to reduce the 10 c.c. of copper solution, that quantity will have contained 0'05 gm. of sugar ; but, the urine being diluted 20 times, the 40 c.c. represent only 2 c.c. of the original urine ; therefore 2 c.c. of it contain 0'05 gm. of sugar, or 25 parts per 1000. The Pavy-Fehling solution is much more generally adapted to urine, and is prepared and used precisely as described in 71. The Analysis: 10 c.c. of clear urine are diluted as just described, and delivered cautiously from the burette into 50 or 100 c.c. of the Pavy- 368 VOLUMETEIC ANALYSIS. 86. Fehling liquid (previously heated to boiling) until the colour is discharged. The calculation is the same as before. 100 c.c. of Pavy-Fehling solution =0*05 gin. glucose. The ammoniacal fumes are best absorbed by leading an elastic tube from the reduction flask into a beaker of water ; the end of the tube should be plugged with a piece of solid glass rod, and a transverse slit made in the elastic tube just above the plug. This valve allows the vapours to escape, but prevents the return of the liquid in case of a vacuum. Knapp's solution, which is equally applicable to urine, is described on p. 296, and possesses the advantage over the Pavy solution that no objectionable fumes are given off. The Analysis: 10, 20, or 40 c.c. of Knapp's solution is diluted with four volumes of water, and heated to boiling. The diluted urine containing from half to one per cent, of sugar is then gradually added until all the mercury is precipitated, as shown by the method of testing given on p. 296. Each 10 c.c. of solution=0'025 gm. of sugar. 8. Estimation of Uric Acid. The determination of uric acid in urine is not often considered of much consequence ; there are, however, circumstances under which it is desirable, especially in urinary deposits. As the quantity present in urine is very small, it is necessary to take, say, from 200 to 500 c.c. for the estimation. The urine being measured into a beaker, from 5 to 8 c.c. of pure hydro- chloric acid are added, the whole well mixed, covered with a glass plate, and set aside in a cellar for 24 or 30 hours ;* at the end of that time the uric acid will be precipitated in small crystals upon the bottom and sides of the beaker. The supernatant liquid is decanted, washed once with cold distilled water, then dissolved in a small quantity of pure solution of potash diluted to 150 c.c. or so with distilled water, acidified strongly with sulphuric acid, and titrated precisely as oxalic acid ( 30.2 c), with ^V permanganate, each c.c. of which is equal to 0'0075 gm. of uric acid. This method is 'not absolutely correct, owing to the fact that with the uric acid there is always precipitated a certain amount of colouring matter of the urine, which destroys the per- manganate equally with the uric acid. The method by weighing is, however, open to the same objection, beside being very troublesome, so that no advantage is gained by the latter plan. Has sail states that the normal quantity of uric acid in urine has hitherto been considerably under-estimated, and that if the urine is concentrated by evaporation before precipitating with hydrochloric acid, a much larger quantity will be obtained (Lancet, Feb. 1865). Haycraft (Brit. Med. Journ. 1885, 1100) has devised a method of estimating uric acid, which although it cannot be said to be absolutely accurate, is capable of giving fairly good results in the hands of a careful operator. The method is based on the fact that uric acid combines with silver as silver urate, which is practically insoluble in water, ammonia, or acetic acid, but perfectly *If 200 c.c. of urine are violently agitated for five minutes with 5 c.c, of fuming HC1, the separation will be complete in an hour. 86. URINE.. 369 soluble in nitric acid. The chief drawback to the method is the peculiar nature of the precipitate of silver urate, which is slimy and difficult to wash ; this, however, is overcome by collecting the precipitate on an asbestos filter attached to a filter pump. The filter is easily made by half filling a small funnel with broken glass, upon which small asbestos fibres suspended in water are poured to the depth of J inch and evenly distributed. Such a filter may be used repeatedly for the same operation. The estimation of the uric acid depends upon the titration of the silver with which it is combined, by Volhard's method ( 39). The necessary solutions are T J^ Ammonic thiocyanate. Standardized by a silver solution of known strength. 1 c.c. = 0*00168 gm. uric acid. Ammoniacal Silver solution. 5 gm. silver nitrate in about 100 c.c. of water, precipitated and re-dissolved in ammonia to a clear solution. Ferric indicator and pure Nitric acid. The same as described in 39. The Analysis: 25 c.c. of urine arc placed in a small "beaker, together with about 1 gm. of sodic bicarbonate and 2 or 3 drops of strong ammonia. This precipitates ammonio-magnesic phosphate and prevents reduction of silver. 1 2 c.c. of ammoniacal silver solution are then added, which at once precipitates the silver as urate. The mixture is now placed on the filter, and washed until the washings show no trace of silver by testing with salt. The precipitate is then dissolved in a few c.c. of nitric acid, washed into a flask, and the titration carried out precisely as described in 39. The number of c.c. of thiocyanate used multiplied by 0'00168 gives the uric acid. If the urine to be examined contains albumen, it must be first removed by acidifying slightly with acetic acid, heating, and filtering.* 9. Estimation of lame and Magnesia. 100 c.c. of the urine are precipitated with ammonia, the precipitate re-dissolved in acetic acid, and sufficient ammonic oxalate added to precipitate all the lime present as oxalate. The precipitate is allowed to settle in a warm place, then the clear liquid passed through a small filter, the precipitate brought upon it, washed with hot water, the filtrate and washings set aside, then the precipitate, together with the filter, pushed through the funnel into a flask, some sulphuric acid added, the liquid freely diluted, and titrated with permanganate, precisely as in 48 ; each c.c. of ^j- permanganate required represents 0'0028 gm. of CaO. * It has been suggested to shorten this method by using an exactly known amount of silver which shall be in excess, diluting to a definite measure, then filtering off an aliquot portion through an ordinary filter, and estimating the excess, thus finding the amount combined as urate. This avoids the tedious filtration and washing of the precipitate, and gives results agreeing with the original method with pure solutions of uric acid. With urine both methods are variable as compared with other methods, due probably, to obscure causes. Much discussion has arisen among physiological chemists as to the various methods of estimating uric acid, and a convenient exact method is still much wanted. B B 370 VOLUMETRIC ANALYSIS. 86. Instead of the above method the following may be adopted : The precipitate of calcic oxalate, after being washed, is dried and, together with the filter, ignited in a platinum or porcelain crucible, by which means it is converted into a mixture of calcic oxide and carbonate. It is then transferred to a flask by the aid of the washing bottle, and an excess of ^5- nitric acid delivered in with a pipette. The amount of acid, over and above what is required to saturate the lime, is found by T ^ caustic alkali, each c.c. of acid being equal to 0'0028 gm. of CaO. In examining urinary sediment or calculi for calcic oxalate, it is first treated with caustic potash to remove uric acid and organic matter, then dissolved in sulphuric acid, freely diluted, and titrated with permanganate ; each c.c. of j solution represents 0'0054 gm. of calcic oxalate. Magnesia. The nitrate and washings from the precipitate of calcic oxalate are evaporated 011 the water bath to a small bulk, then made alkaline with ammonia, sodic phosphate added, and set aside for 8 or 10 hours in a slightly warm place, that the magnesia may separate as ammonio-magnesic phosphate. The supernatant liquid is then passed through a small filter, the precipitate brought upon it, washed with ammoniacal water in the cold, and dissolved in acetic acid, then titrated with uranium solution, as in 69 ; each c.c. of solution required represents OO02815 gm. of magnesia. 10. Ammonia. The only method hitherto applied to the determination of am- monia in urine is that of Schlbsing, which consists in placing a measured quantity of the urine, to which milk of lime is previously added, under an air-tight bell-glass, together with an open vessel containing a measured quantity of titrated acid. In the course of from 24 to 36 hours all the ammonia will have passed out of the urine into the acid, which is then titrated with standard alkali to find the amount of ammonia absorbed. One great objection to this method is the length of time required, since no heating must be allowed, urea being decomposed into free ammonia, when heated with alkali. There is also the uncertainty as to the completion of the process ; and if the vessel be opened before the absorption is perfect, the analysis is spoiled. The following plan is recommended as in most cases suitable : When a solution containing salts of ammonia is mixed with a measured quantity of free fixed alkali of known strength, and boiled until ammoniacal gas ceases to be evolved, it is found that the resulting liquid has lost so much of the free alkali as corresponds to the ammonia evolved ( 18) ; that is to say, the acid which existed in combination with the ammonia in the original liquid has simply changed places, taking so much of the fixed alkali (potash or soda) 86. UKINE. 371 as is equivalent to the ammonia it has left to go free. In the .case of urine being treated in this way, the urea will also be decomposed into free ammonia, but happily in such a way as not to interfere with the estimation of the original amount of ammoniacal salts. The decomposition is such that, while free ammonia is evolved from the splitting up of the urea, carbonate of fixed alkali (say potash) is formed in the boiling liquid, and as this reacts equally as alkaline as though it were free potash, it does not interfere in the slightest degree with the estimation of the original ammonia. The following is the best method of procedure : 100 c.c. of the urine are exactly neutralized with $ soda or potash, as for the estimation of free acid; it is then put into a flask capable of holding five or six times the quantity, 10 c.c. of normal alkali added, and the whole brought to boiling, taking care that the bladders of froth which at first form do not boil over. After a few minutes these subside, and the boiling proceeds quietly. When all ammoniacal fumes are dissipated, the lamp is removed, and the flask allowed to cool slightly ; the contents then emptied into a tall beaker, and normal nitric acid delivered in from the burette with constant stirring, until a fine glass rod or small feather dipped in the mixture and brought in contact with violet-coloured litmus paper produces neither a blue nor a red spot. The number of c.c. of normal acid are deducted from the 10 c.c. of alkali, and the rest calculated as ammonia. 1 c.c. of alkali= 0'017 gm. of ammonia. Example : 100 c.c. of urine were taken, and required 7 c.c. of ^ alkali to saturate its free acid ; 10 c.c. of normal alkali were then added, and the mixture boiled until a piece of moistened red litmus paper was not turned blue when held in the steam; 4'5 c.c. of normal acid were afterwards required to saturate the free alkali ; the quantity of ammonia was therefore equal to 5'5 c.c., which, multiplied by 0'017, gave 0'0935 gm. in 1000 of urine. It must be borne in mind, that the plan just described is not applicable to urine which has already suffered decomposition by age or other circumstances so as to contain carbonate of ammonia ; in this case it would be preferable to adopt Schlosing's method; or where no other free alkali is present, direct titration with normal acid may be adopted. 11. Estimation of Free Acid. The acidity of urine is doubtless owing to variable substances, among the most prominent of which appear to be acid sodic phos- phate and lactic acid. Other free organic acids are probably in many cases present. Under these circumstances, the degree of acidity cannot be placed to the account of any particular body ; neverthe- less, it is frequently desirable to ascertain its amount, which is best done as follows : 100 c.c. of urine are measured into a beaker, and ^ alkali delivered in from a small burette, until a thin glass rod or feather, moistened with the mixture and streaked across some well-prepared violet litmus paper, produces no change of colour ; the degree of acidity is then registered as being equal to the quantity of T ^- alkali used. B B 2 372 VOLUMETRIC ANALYSIS. 86. 12. Estimation of Albximen. (a) By Weight : 100 c.c. of the clear urine (or less than that quantity if much albumen is present, the 100 c.c. being made up with water) are introduced into a good-sized beaker, and heated in the water bath for half an hour. If the urine is sufficiently acid, the albumen will be separated in flocks. Should this not be the case at the end of the half-hour's heating., and the fluid merely appears turbid, one or two drops (not more, unless the urine is alkaline) of acetic acid are added, and the heating continued until the albumen separates in flocks; the beaker is then put aside till the precipitate has settled, and the clear liquid passed through a small filter (previously dried at 212, then cooled between two watch-glasses held together with a spring clip, and weighed) ; the precipitate is then washed with a little hot water, and brought upon the filter without loss, the beaker washed out with hot distilled water, and the last traces of precipitate loosened from the sides with a feather. The filter, with its contents, is then repeatedly washed with hot water, until a drop of the filtrate evaporated on a piece of glass leaves no residue. The funnel containing the filter is then put into a warm place to dry gradually; lastly, the filter removed into one of the watch-glasses and dried thoroughly in the air bath at 110 C., or 220 Fahr. ; another watch-glass is then covered over that containing the filter, the spring clip passed over to hold them together, the whole cooled under the exsiccator and weighed. The weight of the glasses, filter, and clip, deducted from the total, gives the weight of albumen in 100 c.c. of urine. (J) By Measure: In order to avoid the tedious process of estimating the albumen as just described, Bode ker devised a method of titration which gives approximate results when the quantity of albumen is not too small, say not less than 2 per cent. The principle is based on the fact that, potassic ferrocyanide completely precipitates albumen from an acetic acid solution in the atomic proportions of 211 ferrocyanide to 1612 albumen. Standard Solution of Ferrocyanide. 1'309 gm. of the pure salt in a liter of distilled water. 1 c.c. of the solution precipitates O'Ol gm. of albumen. It must be freshly prepared. The Analysis : 50 c.c. of the clear filtered urine are mixed with 50 c.c. of ordinary commercial acetic acid, and the fluid put into a burette. Five or six small filters are then chosen, of close texture, and put into as many funnels, then moistened with a few drops of acetic acid, and filled up with boiling water ; by this means the subsequent clear filtration of the mixture is considerably facilitated. 10 c.c. of the ferrocyanide solution are then measured into a beaker, and 10 c.c. of the urinary fluid from the burette added, well shaken, and poured upon filter No. 1. If the fluid which passes through is bright and clear with yellowish colour, the ferrocyanide will be in excess, and a drop of the urine added to it will produce a cloudiness. On the other hand, if not enough ferrocyanide has been added, the filtrate will be turbid, and pass through very slowly ; in this case, frequently both the ferrocyanide and the urine will produce a turbidity when added. In testing the filtrate for excess of ferrocyanide, care must be taken not to add too much of the urine, lest the precipitate of hydroferrocyanide of albumen should dissolve in the excess of albumen. According to the results obtained from the first filter, a second trial is made, increasing the quantity of urine or ferrocyanide half or as much again, and so on until it is found that the solution first shown to be in excess is reversed. A trial of the mean between this quantity and the previous one will bring- the estimation closer, so that a final test may be decisive. Example: 50 c.c. of urine passed by a patient suffering from B right's I 87. URINE. 373 disease were mixed with the like quantity of acetic acid, and tested as follows : In filtrate Urine. Ferrocyanide. Urine Ferrocyanide gave 1. 10 c.c. 10 c.c. prec. 2. 10 20 prec. 3. 10 15 prec. 4. 10 17'5,, faint prec. 5. 10 18 Therefore the 10 c.c. of diluted urine ( = 5 c.c. of the original secretion) contained 0*18 gm. albumen, or 36 parts per 1000. 13. Estimation of Soda and Potash. 50 c.c. of urine are mixed with the same quantity of baryta solution, allowed to stand a short time, and filtered; then 80 c.c. ( = 40 c,c. urine) measured into a platinum dish and evaporated to dryness in the water bath ; the residue is then ignited to destroy all organic matter, and when cold dissolved in a small quantity of hot water, ammonic carbonate added so long as a precipitate occurs, filtered through a small filter, the precipitate washed, the filtrate acidified with hydrochloric acid and evaporated to dryness, then cautiously heated to expel all ammoniacal salts. The residue is then treated with a little water and a few drops each of ammonia and ammonic carbonate, filtered, the filter thoroughly washed, the filtrate and washings received into a tared platinum dish, then evaporated to dryness, ignited, cooled, and weighed. By this means the total amount of mixed sodic and potassic chlorides is obtained. The proportion of each is found by titrating for the chlorine as in 38, and calculating as directed on page 126. 14. Estimation of Total Nitrogen. This can now be easily accomplished by KjeldahPs method ( 18.5) and is especially serviceable, since it has been found that the results of the titration method for urea by Liebig's process, either in its original way or by subsequent modifications, cannot give the true data for calculating the nitrogen in any given specimen of urine. The Analysis : 5 c.c. of urine of average concentration are measured into a flask holding about 300 c.c., together with 20 c.c. of sulphuric acid, then heated to boiling, and the heat continued until all vapour and gases are given off and the fluid possesses a clear yellow tint. 25 to 30 minutes generally suffices. The flask is then suffered to cool, the liquid diluted, and distilled with caustic soda and zinc as described on page 79. ANALYSIS OF NATURAL WATERS AND SEWAGE. 87. THE analysis of natural waters and sewage has for a long period received the attention of chemists, but until lately no methods of examination have been produced which could be said to satisfy the demands of those who have been interested in the subject from various points of view. The researches of Clark, 374 VOLUMETRIC ANALYSIS. 87. Frankland, Miller, Wanklyn, Tidy, Bischof, Warington, and others, have, however, now brought the whole subject into a more satisfactory form, so that it may fairly be said that, as regards accuracy of chemical processes, or interpretation of results from a sanitary point of view, very little addition is required. Consider- able space will be devoted to the matter here ; and as most of the processes are now volumetric, and admit of ready and accurate results, the general subject naturally falls within the scope of this work. Considerable pains have been taken to render the treatment of the matter practical and trustworthy. Since the various processes necessitate the use of peculiar materials and apparatus, the preparation and arrangement of these will be described at some length previous to the introduction of the general subject. THE PREPARATION OF REAGENTS. A. Reagents required for the Estimation of Nitrogen present as Ammonia. (a) Nessler's Solution. Dissolve 6 2 '5 gm. of potassic iodide in about 250 c.c. of distilled water, set aside a few c.c., and add gradually to the larger part a cold saturated solution of corrosive sublimate until the mercuric iodide precipitated ceases to be redissolved on stirring. When a permanent precipitate is obtained, restore the reserved potassic iodide so as to redissolve it, and continue adding corrosive sublimate very gradually until a slight precipitate remains undissolved. (The small quantity of potassic iodide is set aside merely to enable the mixture to be made rapidly without danger of adding an excess of corrosive sublimate.) Next dissolve 150 gm. of solid potassic hydrate (that usually sold in sticks or cakes) in 150 c.c. of distilled w r ater, allow the solution to cool, add it gradually to the above solution, and make up with distilled water to one liter. On standing, a brown precipitate is deposited, and the solution becomes clear, and of a pale greenish-yellow colour. It is ready for use as soon as it is perfectly clear, and should be decanted into a smaller bottle as required. (/3) Standard Solution of Ammonic chloride. Dissolve 1-9107 gm. of pure dry ammonic chloride in a liter of distilled water ; of this take 100 c.c., and make up to a liter with distilled water. The latter solution will contain ammonic chloride corresponding to 0-00005 gm. of nitrogen in each c.c. In use it should be measured from a narrow burette of 10 c.c. capacity divided into tenths. [If it is desired to estimate "ammonia" rather than "nitrogen as ammonia," take 1'5735 gm. of ammonic chloride instead of 1'9107 gm. 1 c.c. will then correspond to O'OOOOS gm. of ammonia (NH 3 ).] (y) Sodic carbonate. Heat anhydrous sodic carbonate to 87. NATURAL WATERS AND SEWAGE. S75 redness in a platinum crucible for about an hour, taking care not to fuse it. Whilst still warm rub it in a clean mortar so as to break any lumps which may have been formed, and transfer to a clean dry wide-mouthed stoppered bottle. (3) "Water free from Ammonia. If, when 1 c.c. of Kessler's solution (A. a) is added to 100 c.c. of distilled water in a glass cylinder, standing on a white surface (see Estimation of Ammonia), no trace of a yellow tint is visible after five minutes, the water is sufficiently pure for use. As, however, this is rarely the case, the following process must usually be adopted. Distil from a large glass retort (or better, from a copper or tin vessel holding 15 20 liters) ordinary distilled water which has been rendered distinctly alkaline by addition of sodic carbonate. A glass Liebig's condenser, or a clean tin worm should be used to condense the vapour ; it should be connected to the still by a short india-rubber joint. Test the distillate from time to time with Nessler's solution, as above described, and when free from ammonia collect the remainder for use. The distillation must not be carried to dryness. Ordinary water may be used instead of distilled water, but it occasionally continues for some time to give off traces of ammonia by the slow decomposition of the organic matter present in it. B. Reagents required for the Estimation of Organic Carbon and Nitrogen. (a) Water free from Ammonia and Organic Matter. Distilled water, to which 1 gm. of potassic hydrate and 0'2 gm. of potassic permanganate per liter have been added, is boiled gently for about twenty-four hours in a similar vessel to that used in preparing water free from ammonia (A. ), an inverted condenser being so arranged as to return the condensed water. At the end of that time the condenser is adjusted in the usual way, and the water carefully distilled, the distillate being tested at intervals for ammonia, as in preparing A. & When ammonia is no longer found the remainder of the distillate may be collected, taking care to stop short of dryness. The neck of the retort or still should point slightly upwards, so that the joint which connects it with the condenser is the highest point. Any particles carried up mechanically will then run back to the still, and not contaminate the distillate. The water thus obtained should then be rendered slightly acid with sulphuric acid, and re-distilled from a clean vessel for use, again stopping short of dryness. (/3) Solution of Sulphurous acid. Sulphurous anhydride is prepared by the action of pure sulphuric acid upon cuttings of clean metallic copper which have been digested in the cold with concentrated sulphuric acid for twenty-four hours, and then washed 376 '- VOLUMETRIC ANALYSIS. 87. with water. The gas is made to bubble through water to remove mechanical impurities, and then conducted into water free from ammonia and organic matter (B. a) until a saturated solution is obtained. (y) Solution of Hydric sodic sulphite. Sulphurous anhydride, prepared and washed as above, is passed into a solution of sodic carbonate made by dissolving ignited sodic carbonate (A. y) in water free from ammonia and organic matter (B. a). The gas is passed until carbonic anhydride ceases to be evolved. () Solution of Ferrous chloride. Pure crystallized ferrous sulphate is dissolved in water, precipitated by sodic hydrate, the precipitate well washed (using pure water B. a for the last washings), and dissolved in the smallest possible quantity of pure hydrochloric acid. Two or three drops must not contain an appreciable quantity of ammonia. It is convenient to keep the solution in a bottle with a ground glass cap instead of a stopper, so that a small dropping tube may be kept in it always ready for use. () Cupric oxide. Prepared by heating to redness with free access of air, on the hearth of a reverberatory furnace, or in a muffle, copper wire cut into short pieces, or copper sheets cut into strips. That which has been made by calcining the nitrate cannot be used, as it appears to be impossible to expel the last traces of nitrogen. After use, the oxide should be extracted by breaking the combustion tube, rejecting the portion which was mixed with the substance examined. As soon as a sufficient quantity has been recovered, it should be recalcined. This is most conveniently done in an iron tube about 30 m.m. in internal diameter, and about the same length as the combustion furnace. One end should be closed with a cork, the cupric oxide poured in, the tube placed in the combustion furnace (which is tilted at an angle of about 15, so as to produce a current of air), the cork removed, and the tube kept at a red heat for about two hours. In a Hofmann.'s gas furnace, with five rows of burners, two such tubes may be heated at the same time if long clay burners are placed in the outer rows, and short ones in the three inner rows. If the furnace has but three rows of burners, a rather smaller iron tube must be used. When cold, the oxide can easily be extracted, if the heat has not been excessive, by means of a stout iron wire, and should be kept in a clean dry stoppered bottle. Each parcel thus calcined should invariably be assayed by filling with it a combustion tube of the usual size, and treating it in every respect as an ordinary combustion. It should yield only a very minute bubble of gas, which should be almost wholly absorbed by potassic hydrate. (The quantity of CO 2 found should not correspond to more than 0*00005 gm. of C, otherwise the oxide must be recalcined). The finer portions of the 87. NATURAL WATERS AND SEWAGE. 377 oxide should, after calcining, be sifted out by means of a sieve of clean copper gauze, and reserved for use as described hereafter. New cupric oxide as obtained from the reverberatory furnace should be assayed, and if not sufficiently pure, as is most likely the case, calcined as above described, and assayed again. () Metallic Copper. Fine copper gauze is cut into strips about 80 m.m. wide, and rolled up as tightly as possible on a copper wire so as to form a compact cylinder 80 m.m. long. This is next covered with a tight case of moderately thin sheet copper, the edges of which meet without overlapping. The length of the strip of gauze, and the consequent diameter of the cylinder, must be regulated so that it will fit easily, but not too loosely in the combustion tubes. A sufficient number of these cylinders being prepared, a piece of combustion tube is filled with them, and they are heated to redness in the furnace, a current of atmospheric air being passed through them for a few minutes in order to burn off organic impurity, and coat the copper gauze superficially with oxide. A current of hydrogen, dried by passing through strong sulphuric acid, is then substituted for the air, and a red heat maintained until hydrogen issues freely from the end of the tube. It is then allowed to cool, the current of hydrogen being continued, and when cold the copper cylinders are removed, and kept in a stoppered bottle. After being used several times they must be heated in a stream of hydrogen as before, and are then again ready for use. The heating in air need not be repeated. (?/) Solution of Potassic bichromate. This is used as a test for and to absorb sulphurous anhydride which may be present in the gas obtained by combustion of the water residue. It should be saturated, and does not require any special attention. The yellow neutral chromate may also be used, but must be rendered slightly acid, lest it should absorb carbonic as well as sulphurous anhydride. (0) Solution of Potassic hydrate. A cold saturated solution, made by dissolving solid potassic hydrate in distilled water. (t) Solution of Pyrogallic acid. A cold saturated solution, made by dissolving in distilled water solid pyrogallic acid obtained by sublimation. (ic) Solution of Cuprous chloride. A saturated solution of cupric chloride is rendered strongly acid with hydrochloric acid, a quantity of metallic copper introduced in the form of wire or turnings, and the whole allowed to stand in a closely stoppered bottle until the solution becomes colourless. (X) Oxygen. Blow a bulb of about 30 c.c. capacity at the end of a piece of combustion tube, and draw out the tube so that its internal diameter for a length of about 300 m.m. is about 3 m.m. 378 VOLUMETRIC ANALYSIS. 87. This is done in order that the capacity of the apparatus apart from the bulb may be as small as possible. Cut the tube at the wide part about 10 m.m. from the point at which the narrow tube commences, thus leaving a small funnel-shaped mouth. Then introduce, a little at a time, dried, coarsely powdered, potassic chlorate until the bulb is full. Cut off the funnel, and, at a distance of 100 m.m. from the bulb, bend the tube at an angle of 45, and at 10 m.m. from the end bend it at right angles in. the opposite direction. It then forms a retort and delivery tube in one piece, and must be adjusted in a mercury trough in the usual manner, taking care that the end does not dip deeper than about 20 m.m. below the surface, as otherwise the pressure of so great a column of mercury might destroy the bulb when softened by heat. On gently heating, the potassic chlorate fuses and evolves oxygen. The escaping gas is collected in test tubes about 150 m.m. long and 20 m.m. in diameter, rejecting the first 60 or 80 c.c., which contain the nitrogen of the air originally in the bulb retort. Five or more of these tubes, according to the quantity of oxygen required, are collected and removed from the mercury trough, in. very small beakers, the mercury in which should be about 10 m.m. above the end of the test tube. Oxygen may be kept in this way for any desired length of time, care being taken, if the temperature falls considerably, that there is sufficient mercury in the beaker to keep the mouth of the test tube covered. About 10 c.c. of the gas in the first tube collected is transferred by decantation in a mercury trough to another tube, and treated with potassic hydrate and pyrogallic acid, when, if after a few minutes it is absorbed, with the exception of a very small bubble, the gas in that and the remaining tubes may be considered pure. If not, the first tube is rejected, and the second tested in the same way, and so on. (jj) Hydric metaphosphate. The glacial hydric metaphosphate, tisually sold in sticks, is generally free from ammonia, or very nearly so. A solution should be made containing about 100 gm. in a liter. It should be so far free from ammonia as that 10 c.c. do not contain an appreciable quantity. (v) Calcic phosphate. Prepared by precipitating common disodic phosphate with calcic chloride, washing the precipitate with water by decantation, drying, and heating to redness for an hour. C. Reagents required for the Estimation of Nitrogen present as Nitrates and Nitrites (drum's process). (a) Concentrated Sulphuric acid. The ordinary colourless acid is usually free from nitrates and nitrites. It should be tested before use by the method described hereafter for the estimation of nitrogen as nitrates ( 83.6). 87. NATURAL WATERS AND SEWAGE. 379 (/3) Potassic permanganate. Dissolve about 10 gm. of crys- tallized potassic permanganate in a liter of distilled water. (y) Sodic carbonate. Dissolve about 10 gm. of dry, or an equivalent quantity of crystallized sodic carbonate free from nitrates, in a liter of distilled water. Por the Estimation of Nitrogen as Nitrates and Nitrites in "Waters containing a very large quantity of Soluble Matter, but little Organic Nitrogen. (2) Metallic Aluminium. As thin foil. (e) Solution of Sodic hydrate. Dissolve 100 gm. of solid sodic hydrate in a liter of distilled water ; when cold, put it in a tall glass cylinder, and introduce about 100 sq. cm. of aluminium foil, which must be kept at the bottom of the solution by means of a glass rod. When the aluminium is dissolved, boil the solution briskly in a porcelain basin until about one-third of its volume has been evaporated, allow to cool, and make up to its original volume with water free from ammonia. The absence of nitrates is thus ensured. () Broken Pumice. Clean pumice is broken in pieces of the size of small peas, sifted free from dust, heated to redness for about an hour, and kept in a closely stoppered bottle. (77) Hydrochloric acid free from Ammonia. If the ordinary pure acid is not free from ammonia, it should be rectified from sulphuric acid. As only two or three drops are used in each experiment, it will be sufficient if that quantity does not contain an appreciable proportion of ammonia. Por the Estimation of Nitrogen as Nitrates and Nitrites by the Indigo Process. The necessary solutions for this method have already been fully described 011 page 240. Por the Estimation of Nitrites by G-riess's Process. (6) Meta-phenylene-diamine. A half per cent, solution of the base in very dilute sulphuric or hydrochloric acid. The base alone is not permanent. If too highly coloured, it may be bleached by pure animal charcoal. (t) Dilute Sulphuric acid. One volume of acid to two of water. (K) Standard Potassic or Sodic nitrite. Dissolve 0*406 gm. of pure silver nitrite in boiling distilled water, and add pure 380 VOLUMETRIC ANALYSIS. . 87. potassic or sodic chloride till no further precipitate of silver chloride occurs. Make up to a liter ; let the silver chloride settle, and dilute 100 c.c. of the clear liquid to a liter. It should be kept in small stoppered bottles completely filled, and in the dark. 1 c.c. = 0-01 m.gm. The colour produced by the reaction of nitrous acid 011 meta- phenylene-diamiiie is triamidoazo-benzene, or "Bismarck brown." D. Reagents required for the Estimation of Chlorine present as Chloride. (a) Standard Solution of Silver nitrate. Dissolve 2 -3944 gm. of pure recrystallized silver nitrate in distilled water, and make up to a liter. In use it is convenient to measure it from a burette which holds 10 c.c. and is divided into tenths. (/3) Solution of Potassic chr ornate. A strong solution of pure neutral potassic chromate free from chlorine. It is most con- veniently kept in a bottle similar to that used for the solution of ferrous chloride (B. 2). E. Reagents required for determination of Hardness. (a) Standard Solution of Calcic chloride. Dissolve in dilute hydric chloride, in a platinum dish, 0*2 gm. of pure crystallized calcite, adding the acid gradually, and having the dish covered with a glass plate, to prevent loss by spirting. When all is dissolved, evaporate to dryness on a water bath, add a little distilled water, and again evaporate to dryness. Repeat the evaporation several times to ensure complete expulsion of hydric chloride. Lastly, dissolve the calcic chloride in distilled water, and make up to one liter. (/3) Standard Solution of Potassic soap. Rub together in a mortar 150 parts of lead plaster (Emplast. Plumbi of the druggists) and 40 parts of dry potassic carbonate. When they are fairly mixed, add a little methylated spirit, and continue triturating until an uniform creamy mixture is obtained. Allow to stand for some hours, then throw on to a filter, and wash several times with methylated spirit. The strong solution of soap thus obtained must be diluted with a mixture of one volume of distilled water and two volumes of methylated spirit (considering the soap solution as spirit), until exactly 14 '2 5 c.c. are required to form a permanent lather with 50 c.c. of the standard calcic chloride (E. a), the experiment being performed precisely as in determining the hard- ness of a water. A preliminary assay should be made with a small quantity of the strong soap solution to ascertain its strength. After making the solution approximately of the right strength, 88. NATUKAL WATEIiS AND SEWAGE. 381 allow it to stand twenty four hours ; and then, if necessary, filter it, and afterwards adjust its strength accurately. It is better to make the solution a little too strong at first, and dilute it to the exact strength required, as it is easier to add alcohol accurately than strong soap solution. THE ANALYTICAL PROCESSES. 88. To form, for sanitary purposes, an opinion of the character of a natural water or sewage, it will in most cases suffice to determine the nitrogen as ammonia, organic carbon, organic nitrogen, total solid matter, nitrogen as nitrates and nitrites, suspended matter, chldrine, and hardness ; and in the following pages the estimation of these will be considered in detail, and then, more briefly, that of other impurities. The method of estimating nitrogen as ammonia is substantially that described by the late W. A. Miller (/. C. S. [2] iii. 125), and that for estimating organic carbon and nitrogen was devised by Frankland and Armstrong, and described by them in the same journal ([2] vi. 77 et seq.). 1. Collection of Samples. The points to be considered under this head are, the vessel to be used, the quantity of water required, and the method of ensuring a truly representative sample. Stoneware bottles should be avoided, as they are apt to affect the hardness of the water, and are more difficult to clean than glass. Stoppered glass bottles should be used if possible ; those known as " Winchester Quarts," which hold about two and a half liters each, are very convenient and easy to procure. One of these will contain sufficient for the general analysis of sewage and largely polluted rivers, two for well waters and ordinary rivers and streams, and three for lakes and mountain springs. If a more detailed analysis is required, of course a larger quantity must be taken. If corks must be used, they should be neic, and well washed with the water at the time of collection. In collecting from a well, river, or tank, plunge the bottle itself, if possible, below the surface ; but if an intermediate vessel must be used, see that it is thoroughly clean and well rinsed with the water. Avoid the surface water and also any deposit at the bottom. If the sample is taken from a pump or tap, take care to let the water which has been standing in the pump or pipe run off before collecting, then allow the stream to flow directly into the bottle. If it is to represent a town water-supply, take it from the service pipe communicating directly with the street main, and not from a cistern. In every case, first fill the bottle completely with the water, thus expelling all gases and vapours, empty it again, rinse once or 382 VOLUMETRIC ANALYSIS. 88. twice carefully with the water, and then fill it nearly to the stopper, and tie down tightly. At the time of collection note the source of the sample, whether from a deep or shallow well, a river or spring, and also its local name so that it may be clearly identified. If it is from a well, ascertain the nature of the soil, subsoil, and water-bearing stratum; the depth and diameter of the well, its distance from neighbouring cesspools, drains, or other sources of pollution ; whether it passes through an impervious stratum before entering the water-bearing stratum, and if so, whether the sides of the well above this are, or are not, water-tight. If the sample is from a river, ascertain the distance from the source to the point of collection; whether any pollution takes place above that point, and the geological nature of the district through which it flows. If it is from a spring, take note of the stratum from which it issues. 2. Preliminary Observations. In order to ensure uniformity, the bottle should invariably 'be well shaken before taking out a portion of the sample for any purpose. The colour should IDC observed as seen in a tall, narrow cylinder standing upon a white surface. It is well to compare it with distilled water in a similar vessel. The taste and odour are most easily detected when the water is heated to 30 35 C. Before commencing the quantitative analysis, it is necessary to decide whether the water shall be filtered or not before analysis. This must depend on the purpose for which the examination is undertaken. As a general rule, if the suspended matter is to be determined, the water should be filtered before the estimation of organic carbon and nitrogen, nitrogen as ammonia, and total solid residue; if otherwise, it should merely be shaken up. If the suspended matter is not determined, the appearance of the water, as whether it is clear or turbid, should be noted. This is conveniently done when measuring out the quantity to be used for the estimation of organic carbon and nitrogen. If the measuring flask be held between the eye and a good source of light, but with an opaque object, such as a window bar, in the line drawn from the eye through the centre of the flask, any suspended particles will be seen well illuminated on a dark ground. Water derived from a newly sunk well, or which has been rendered turbid by the introduction of innocuous mineral matter from some temporary and exceptional cause should be filtered, but the suspended matter in most such cases need not be determined. The introduction of organic matter of any kind would almost always render thr sample useless. 3. Estimation of Nitrogen as Ammonia. Place about 50 C.C. of the water in a glass cylinder about 150 m.m. high, and of about 88. NATURAL WATERS AND SEWAGE. 383 70 c.c. capacity, standing upon a white glazed tile or white paper. Add about 1 c.c. of Nessler's solution (A. a), stir with a clean glass rod, and allow to stand for a minute or so. If the colour then seen does not exceed in intensity that produced when O'l c.c. of the standard ammonic chloride (A. /3) is added to 50 c.c. of water free from ammonia (A. ), and treated in the same way, half a liter of the water should be used for the estimation. If the colour be darker, a proportionately smaller quantity should be taken ; but it is not convenient to use less than 20 or 25 c.c. If it has been decided that the water should be filtered before analysis, care must be taken, should it contain only a small quantity of ammonia, that the filter paper is free from ammonia. If it is not, it must be steeped in water free from ammonia for a day or so, and when used, the first portion of the filtrate rejected. Washinr/ with water, even if many times repeated, is generally ineffectual. When a large quantity of ammonia is present, as in highly polluted water and sewage, any ammonia in the filter paper may be neglected. A moderate quantity of suspended matter may also generally be neglected with safety, even if the water is to be filtered in estimating organic carbon and nitrogen and total solid matter. The water, filtered or unfiltered as the case may be, should be carefully measured and introduced into a capacious retort, connected by an india-rubber joint with a Liebig's condenser, the volume being if necessary, made up to about 400 c.c. with water free from ammonia. Add about 1 gm. of sodic carbonate (A. y), and distil rapidly, applying the lamp flame directly to the retort, and collect the distillate in a small glass cylinder, such as is described above. When about 50 c.c. have distilled into the first cylinder, put it aside and collect a second 50 c.c., and as soon as that is over remove the lamp, and add to the second distillate about 1 c.c. of Nessler's solution, stir with a clean glass rod, and allow to stand on a white tile or sheet of paper for five minutes. To estimate the ammonia present, measure into a similar cylinder as much of the standard ammonic chloride solution as you judge by the colour to be present in the distillate ; make it up with water free from ammonia to the same volume, and treat with Nessler's solution in precisely the same way. If, on standing, the intensity of colour in the two cylinders is equal, the quantity of ammonia is also equal, and this is known in the trial cylinder. If it is not equal, another trial must be made with a greater or less quantity of ammonic chloride. The ammonic chloride must not be added after the Nessler's solution, or a turbidity will be produced which entirely prevents accurate comparison. If the ammonia in the second distillate does not exceed that in 0*2 c.c. of the standard ammonic chloride, the distillation need not be proceeded with any farther, but if otherwise, successive quantities must be distilled and tested until ammonia ceases to be found. If the ammonia in the second distillate corresponds to 0*4 c.c. or less of the ammonic chloride, that in the 384 VOLUMETRIC ANALYSIS. 88. first may be estimated in the same way ; but if the second contains a greater quantity of ammonia, the first must be measured, and an aliquot part taken and diluted to about 50 c.c. with water free from ammonia, as it is likely to contain so much ammonia as to give a colour too intense to admit of easy comparison. A colour produced by more than 2 c.c. of ammonic chloride cannot be conveniently employed.' 55 ' When, as in the case of sewage, a large quantity of ammonia is known to be present, it saves trouble to distil about 100 c.c. at first, and at once take an aliquot part of that, as above described. If the liquid spirts in distilling, arrange the retort so that the joint between the retort and condenser is the highest point ; the distillation will, proceed rather more slowly, but anything carried up mechanically will be returned to the retort. When the ammonia has been estimated in all the distillates, add together the corresponding volumes of ammonic chloride solution ; then, if 500 c.c. have been employed for the experiment, the number of c.c. of ammonic chloride used divided by 100 will give the quantity of nitrogen as ammonia in 100,000 parts of the water; if less than that, say y c.c. have been used, multiply the volume of ammonic chloride by 5 and divide by y. Before commencing this operation, ascertain that the retort and condenser are free from ammonia by distilling a little common water or distilled water with sodic carbonate until the distillate is free from ammonia. Remove the residue then, and after each estimation, by means of a glass syphon, without disconnecting the retort. If a small quantity of water is to be distilled, the residue or part of it from a previous experiment may be left in the retort, instead of adding water free from ammonia, care being taken that the previous distillation was continued until ammonia ceased to be evolved. When urea is present the evolution of ammonia is long continued, owing to the decomposition of the urea. In such cases, collect the distillate in smaller quantities, and as soon as the first rapid diminution in the amount of ammonia has ceased, neglect the remainder, as this would be due almost wholly to decomposition of the urea. 4. Estimation of Organic Carbon and Nitrogen. This should be commenced as soon as the nitrogen as ammonia has been determined. If that is less than 0*05 part per 100,000, a liter should be used; if more than 0'05, and less than 0'2, half a liter ; if more than 0'2 and less than 1*0, a quarter of a liter; if more than 1*0, a hundred c.c. or less. These quantities are given as a guide in dealing with ordinary waters and sewage, but subject to variation in exceptional cases. A quantity which is too large should be * In order to insure absolute accuracy in Nesslerizing it is necessary that the distillate should be of the same temperature as the standard liquid made by mixing the ammonic chloride with distilled water. Hazen and Clark (Amcr. Chem. Jour. xii. 425) found that the water Nesslerized from a metal condenser, immediately after collection, gave a lower figure than when the two liquids were allowed to assume the same temperature. 88. NATURAL WATERS AND SEAVAGE. 385 avoided as entailing needless trouble in evaporation, and an inconveniently bulky residue and resulting gas. If it is to be filtered before analysis, the same precaution as to filter paper must be taken as for estimation of nitrogen as ammonia, the same filter being generally used. Having measured the quantity to be used, add to it in a capacious flask 15 c.c. of the solution of sulphurous acid (B. /3), and boil briskly for a few seconds, in order to decompose the carbonates present. Evaporate to dryness in a hemispherical glass dish, about a decimeter in diameter, and preferably without a lip, supported in a copper dish with a flange (fig. 46 d e). The flange has a diameter of about 14 centimeters, is sloped slightly towards the centre, and has a rim of about 5 m.m. turned up on its edge, except at one point, where a small lip is provided. The concave portion is made to fit the contour of the outside of the glass dishes, and is of such a depth as to allow the edge of the dish to rise about 15 m.m. above the flange. The diameter of the concavity at / is about 90 m.m., and the depth at g about 30 m.m. A thin glass shade, such as is used to protect statuettes, about 30 centimeters high, stands on the flange of the copper dish, its diameter being such as to fit without difficulty on the flange, and leave a sufficient space between its interior surface and the edge of the glass dish. The copper dish is supported on a steam or water bath, and the water as it evaporates is condensed on the interior of the glass shade, runs down into the copper dish, filling the space between it and the glass dish, and then passes off by the lip at the edge of the flange, a piece of tape held by the edge of the glass shade, and hanging over the lip, guiding it into a vessel placed to receive it. We are indebted to Bischof for an improved apparatus for evaporation, which by keeping the dish always full by a self-acting contrivance, permits the operation to proceed without attention during the night, and thus greatly reduces the time required. This form of apparatus is shown in fig. 46. The glass dish d is supported by a copper dish e as described above, and resting on the latter is a stout copper ring li which is slightly conical, being 115 m.m. in diameter at the top and 130 at the bottom. At the top is a narrow flange of about 10 m.m. with a vertical rim of about 5 m.m. The diameter across this flange is the same as the diameter of the dish e, so that the glass shade i will fit securely either on li or e. The height of the conical ring is about 80 m.m. The automatic supply is accomplished on the well-known prin- ciple of the bird fountain, by means of a delivery tube l>, the upper end of which is enlarged to receive the neck of the flask a con- taining the water to be evaporated, the joint being carefully ground so as to be water-tight. The upper vertical part of b, including this enlargement, is about 80 m.m. in length, and the sloping part about 260 m.m. with a diameter of 13 m.m. The lower end which goes into the dish is again vertical for about 85 m.m., and c c S86 VOLUMETRIC ANALYSIS. carries a side tube c of about 3 m.m. internal diameter, by which air enters the delivery tube whenever the level of the water in the dish falls below the point at which the side tube joins the delivery tube. The distance from this point to the end of the tube which rests on the bottom of the dish at #, and is there somewhat con- stricted, is about 30 m.m. The side tube c should not be attached on the side next the flask, as if so the inclined part of I) passes over its mouth and renders it very difficult to clean. Mills Fig. 47. Tig. 4G. prevents circulation of liquid in the sloping part of the tube by bending it into a slightly undulating form, so that permanent bubbles of air are caught and detained at two points in it. The flask a should hold about 1200 c.c. and have a rather narrow neck about 20 m.m. and a flat bottom. A small slot is cut in the upper edge of the copper ring h to accommodate the delivery tube, as shown in fig. 47. Its size and shape should be such that the tube does not touch the edge of the glass shade i t lest water 88. NATURAL WATERS AND SEWAGE. 387 running down the inner surface of the shade should find its way down the outside of the delivery tube into the dish. This being avoided, the opening should be as closely adjusted to the size of the delivery tube as can be. The copper dish e should rest on a steam or water bath, so that only the spherical part is exposed to the heal After the addition of the 15 c.c. of sulphuric acid, the water may either be boiled in the flask a, or in another more capacious one, and then transferred to a. It should be allowed to cool before the delivery tube is adjusted, otherwise the joint between the two is liable to become loose by expansion of the cold socket of the delivery tube, after being placed over the hot neck of the flask. The glass dish having been placed on the copper dish e, the conical ring li is fitted on, and the flask with the delivery tube attached inverted, as shown in. fig. 46, , b. This should not be done too hurriedly, and with a little care there is no risk of loss. The flask is supported either by a large wooden filtering stand, the ring of which has had a slot cut in it to allow the neck of the flask to pass, or by a clamp applied to the upper end of the delivery tube where the neck of the flask fits in. The delivery tube having been placed in the slot made to receive it, the glass shade is fitted on, and the evaporation allowed to proceed. When all the water has passed from the flask into the dish, the flask and delivery tube, and the conical ring h may be removed, and the glass shade placed directly on the dish e until the evaporation is complete. If the water is expected to contain a large quantity of nitrates, two or three drops of chloride of iron (B. ) should be added to the first dishful; and if it contains little or no carbonate, one or two c.c. of hydric sodic sulphite (B. y). The former facilitates the destruc- tion of nitrates and nitrites, and the latter furnishes base for the sulphuric acid produced by oxidation of the sulphurous acid, and which would, if free, decompose the organic matter when concen- trated by evaporation. An estimate of the quantity of carbonate present, sufficiently accurate for this purpose, may generally be made by observing the quantity of precipitate thrown down on addition of sodic carbonate in the determination of nitrogen as ammonia. With sewages and very impure waters (containing upwards of 0*1 part of nitrogen as ammonia per 100,000 for example) such great precaution is hardly necessary, and the quantity to evaporate being small, the evaporation may be conducted in a glass dish placed directly over a steam bath, and covered with a drum or disc of filter paper made by stretching the paper by means of two hoops of light split cane, one thrust into the other, the paper being between them, in the Avay often employed in making dialysers. This protects the contents of the dish from dust, and also, to a great extent, from ammonia which may be in the atmosphere, and which would impair c c 2 388 VOLUMETRIC ANALYSIS. 88. the accuracy of the results. As a glass dish would be in some danger of breaking by the introduction of cold water, the flask containing the water being evaporated in this or in the first described manner, must be kept on a hot plate or sand bath at a temperature of about 60 or 70 C., and should be covered with a watch-glass. This precaution is not necessary when Bischof's apparatus is used. If, at any time, the water in the flask ceases to smell strongly of sulphurous acid, more should be added. The preliminary boiling may be omitted when less than 250 c.c. is used. When the nitrogen as nitrates and nitrites exceeds 0'5 part, the dish, after the evaporation has been carried to dryness, should be filled with distilled water containing ten per cent, of saturated sulphurous acid solution, and the evaporation again carried to dryness. If it exceeds I'O part, a quarter of a liter of this solution should be evaporated on the residue; if 2*0 parts, half a liter; and if 5 parts, a liter. If less than a liter has been evaporated, a proportionally smaller volume of this solution may be used. The estimation of nitrogen as nitrates and nitrites will usually be accomplished before this stage of the evaporation is reached. M. W. Williams proposes to avoid the use of sulphurous acid, with its acknowledged disadvantages and defects, by removing the nitric and nitrous acids with the zinc-copper couple and converting them into ammonia. If the amount is large, it is best distilled from a retort into weak acid; if small, into an empty Messier tube. The amount so found is calculated into nitrogen as nitrates and nitrites, if the latter are found in the water. The residue, when free from ammonia is further concentrated, the separated carbonates re-dissolved in phosphoric or sulphurous acid, in just sufficient quantity, then transferred to a glass basin for evaporation to dryness as usual ready for combustion (/. C. S. 1881, 144). In the case of sewage, however, it is advisable to employ hydric metaphosphate in the place of sulphurous acid, as the ammonic phosphate is even less volatile than the sulphite. This can only be employed for sewage and similar liquids, which are free from nitrates and nitrites. To the measured quantity of liquid to be evaporated add, in the glass dish, 10 c.c. of the hydric metaphos- phate (B. fj,), and, in order to render the residue more convenient to detach from the dish, about half a gram of calcic phosphate (B. v), and proceed as usual. No chloride of iron, sulphurous acid, or sodic sulphite is required; nor is it necessary to boil before commencing the evaporation. The next operation is the combustion of the residue. The combustion tube should be of hard, difficultly fusible glass, with an internal diameter of about 10 m.m. Cut it in lengths of about 430 m.m., and heat one end of each in the blowpipe flame to round the edge. Wash well 'with water, brushing the interior carefully a tube brush introduced at the end whose edge has been 88. NATURAL WATERS AND SEWAGE. 389 rounded, rinse with distilled water, and dry in an oven. When dry, draw off and close, at the . blowpipe, the end whose edge has been left sharp. The tube is then ready for use. Pour on to the perfectly dry residue in the glass dish, standing on a sheet of white glazed paper, a little of the fine cupric oxide (B. ), and with the aid of a small elastic steel spatula (about 100 in.m. long and 15 m.m. wide) carefully detach the residue from the glass and rub it down with the cupric oxide. The spatula readily accommodates itself to the curvature of the dish, and effectually scrapes its surface. When the contents of the dish are fairly mixed, fill about 30 m.m. of the length of the combustion tube with granulated cupric oxide (B. c), and transfer the mixture in the dish to the tube. This is done in the usual way by a scooping motion of the end of the tube in the dish, the last portions being transferred by the help of a bent card or a piece of clean and smooth platinum foil. Rinse the dish twice with a little fine cupric oxide, rubbing it well round each time with the spatula, and transfer to the tube as before. Any particles scattered on the paper are also to be put in. Fill up to a distance of 270 m.m. from the closed end with granular cupric oxide, put in a cylinder of metallic copper (B. ), and then again 20 m.m. of granular cupric oxide. This last is to oxidize any traces of carbonic oxide which might be formed from carbonic anhydride by the reducing action of iron or other impurity in the metallic copper. Now draw out the end of the tube so as to form a neck about 100 m.m. long and 4 m.m. in diameter, fuse the end of this to avoid injury to the india-rubber connector, and bend it at right angles. It is now ready to be placed in the combustion furnace and attached to the Sprengel pump. The most convenient form of this instrument for the purpose is shown in fig. 48. The glass funnel a is kept supplied with mercury, and is connected by a caoutchouc joint with a long narrow glass tube which passes down nearly to the bottom of a wider tube d, 900 iii.m. long, and 10 m.m. in internal diameter. The upper end of d is cemented into the throat of a glass funnel e from which the neck has been removed. A screw clamp ?; regulates the flow of mercury down the narrow tube. A piece of ordinary glass tube / g, about 6 m.m. in diameter and 600 m.m. in length, is attached at cj to a tube