THE OVERVOLTAGE OF CHLORINE BY GRACE GREENWOOD SPENCER THESIS FOR THE DEGREE OF BACHELOR OF SCIENCE CHEMICAL ENGINEERING COLLEGE OF LIBERAL ARTS AND SCIENCES UNIVERSITY OF ILLINOIS 1922 ) 922 -5p 3 UNIVERSITY OF ILLINOIS Al§'Y._lAj 192.^ •_ THIS IS TO CERTIFY THAT THE THESIS PREPARED UNDER MY SUPERVISION BY Grace Greenwood Spencer ENTITLED j-Le Cvervcl'tap e 1 IS APPROVED BY ME AS FULFILLING THIS PART OF THE REQUIREMENTS FOR THE DEGREE OF Bachelor of Science in Chemical Engineeri ng . ACTING HEAD OF DEPARTMENT OF --CHEMISTRY 500188 Digitized by the Internet Archive in 2016 https://archive.org/details/overvoltageofchlOOspen Acknowledgement The completion of this research ha3 been made possible by the unfailing kindness and helpful suggestions of Doctor J. H. Reedy, to whom the writer wishes to egress her sincere gratitude and appreciation for hi3 assistance in both the experimental work and in the writing of this thesis. Table of Contant3. Pag I. Introduction 1 A. Statement of the Problem 1 B. Methods of Measurement .1 C. Definition of Overvoltage 3 D. Review of the Theories of Overvoltage ...4 E. Methods for Determining Overvoltage .....5 II. Experimental 7 A. Apparatus 7 B. Procedure 8 C. Results with 1. Smooth Platinum . 9 2. Platinized Platinum 9 3. Copper . 10 4 . Carbon 10 5. Gold .11 III. Discussion of Results 12 A. Effect of Concentration ..12 B. Effect of Surface 12 C. Effect of Time 12 D. Effect of Current Density ,...13 E. Mechanism of the Reaction 14 F. Comparison with Newbery's Results 16 G. Comparison with the Overvoltage of Hydrogen and Oxygen 17 IV. Summary ...17 A Study of the Overvoltage of Chlorine . I • Introduction . The purpose of this investigation w as to determine the factors which influence the overvoltage of chlorine and how much each factor influences this overvoltage. In spite of the fact that thi3 phenomena is well known and that the knowledge that the overvoltage of hydrogen and of oxygen have many useful applications, no exact agreement has been reached as to the best method of measuring it. Two radically different methods have been used, namely: 1. The commutator method, in which the current is periodically interrupted by means of a commutator devise while the readings are being made. 2. The direct methou, in which the exciting current remains flowing while the potentiometer readings are taken. These two methods of measurement have been the subject of much discussion, especially, between Newbery (l) and the American chemists. The commutator method is used by Newbery almost exclusively, while the direct method has been used by the (1). J. Chem. Soc., 165, 2420 (1314); 109, 1051 (1916); 119, 477 (1921). J. Am. Chem. Soc., 42, 2007 (1920). 2 American chemists. Until recently no attempt has been made to compare these two methods. Tartar and Keyes (l) recently have published an article, in which they compared the results obtained by determining overvoltage by the two methods. As a result of the discussion of these methods, there has been some controversy over what is meant by the term overvoltage. According to Newbery (2) "the potential of an electrode at which molecular hydrogen is being formed from hydrogen ions depends not only upon the overvoltage but also upon the applied electromotive force, the resistance of the electrode, and especially upon the transfer resistance. "Overvoltage is of necessity an active voltage or electromotive force, capable of doing work after all external sources of electromotive force have been removed. Transfer resistance is a purely passive or frictional force which entirely (Ceases when the external electromotive force is interrupted. These forces work in the same direction during the passage of the current; but directly this current is removed, the overvoltage alone exists and may then be measured. Although overvoltage and transfer resistance occur together in this way, they are mutually independent and frequently vary in opposite directions." According to Mac Innes, on the other hand, "Newbery* s definition of the term overvoltage should be reserve d for potentials determined by a commutator devise, which periodically opens the exciting current at the electrode under examination (1). J. Am. Chem. Soc., 44, 557 (1922). (2^. J. Am. Chem. Soc., 42, 2007 (1920). 3 and closes the potentiometer circuit connecting the electrode and a reference electrode. In the work of practically every other person in this field, the exciting current remains flowing while the potentiometer measurements are made . " (l) The chief objection to the commutator method is that the values when obtained are net practical, as they are not obtained under conditions which will exist when overvoltage measurements are of value. In fact, the value obtained when the exciting current is flowing is undoubtedly the "working" potential and the only one which is of practical value. In spite of the controversy which has arisen over the method of measuring overvoltage, it is now generally agreed that cyvervoltage is the electromotive force which acts counter to the applied electromot i ve. force during electrolysis. In this investigation the overvoltage of chlorine is the excess of jsctential that exists between the anode at which molecules of chlorine gas are being formed from the solution containing chlorine ions over that of an anode whose metal surface is saturated with chlorine gas. Or, in other words, chlorine overvoltage is the difference of potential between a standard electrode and an electrode in the same solution at which molecules of chlorine gas are being formed from a solution of hydrochloric acid. This may be better illustrated by the diagram in figure 1. "The overvoltage, therefore, represents the excess energy required to form a substance over that given by the re-solution (1). J. Am. Chem. Soc., 42, 2233 (1920). cpt hod £ anode u: 3 is formed. This as a solid solution wdiuld generate a high back electromotive force , and therefore give a high oxygen overvoltage at a platinum anode. In the case of other metals -where the higher oxide is unstable, it would not accumulate in appreciable quantities and hence the overvoltage wouio be low. Hydrogen overvoltage would be explained by the assumption of the formation of a solid solution of the hydride. This theory is probably true for a large number of cases but the formation of these hydrides is probably a consequence cd and net an explanation of overvoltage. Also this theory does not explain the cvervoitageof the metals. Aside from the two distinct methods of measurement of the overvoltage, there ere two generally accepted methods for determining this overvoltage. Either method of measurement may be employed depending upon what the experimenter understands by the term overvoltage. These two generally accepted methods are: 1. The bubble formation method, that is , the potential as opposed to the rate of formation of the bubbles if gas. 2. The current density method, that is, the potent!?. 1 as opposed to the density of the exciting current. The current density method is the one used in this particular investigation. It was assumed in this method of measurement that 1. Neither the size or the shape of the containing vessel nor the distance between the ancae and the cathode should exert any influence. 2. The value found must represent an electromotive force and not a resistance; this involves the elimination of c . * 7 appreciable potential drop due to the resistance of an electrode or to any resistance of whatever nature at the surface of the electrode (l). I I . Experimental . The purpose of this investigation was to determine the effects of the anodes of copper, of gold, of smooth platinum, of carbon, and of platinized platinum on the overvoltage of chlorine from solutions of normal, tenth normal, hundredth normal, and in some cases thousandth normal hydrochloric acid ; and to compare these results as far as possible with those obtained by Newbery in which he used the commutator method (2). Current densities varying from zero to ten milliamperes were used in nearly every case. Nothing higher than ten milliamperes is recorded in the tables because the bend in the curves was in each case found to occur at very low current densities, nothing higher than four milliamperes. The apparatus consisted of a Leeds and Ncrthrup potentiometer, a D'arsonval galvanometer of the Leeds and Northrup type, a lead storage cell, an Edison cell, a resistance bcx, a slide wire resistance, a small galvanometer standardized to measure fractions of a rcill.iarope.re , a mill iarome ter, a standard calomel electrode, and an electrolytic cell. The electrolytic ceil consisted of a stationary bright platinum cathode and a stationary anode. The acid contained in the cell (1) . J. Am. Cherc. Soc., 44, 557 (1922), (2) . J. Chem. Soc., 113, 477 (1921). 8 /\ mechanically stirred by means of a winged glass stirrer. The arrangement of the apparatus consisted of an electrolytic and /potentiometer circuit. In the electrolytic circuit an Edison cell was used to generate the exciting current, the density of which was varied by means of the slide wire resistance. The small galvanometer was used fdr measuring the value of thfe exciting was current until one half of a milliampere reached, all higher values are measured on the milliamme ter . An intermediate vessel of saturated ammonium nitrate was used to eliminate as far a3 possible the liquid-liquid potential. In the potentiometer circuit a standard cadmium cell was used in order to regulate the exciting current generated by the lead storage cell. The arrangement of the apparatus i3 shown in figure 2. By this arrangement the change in the overvoltage as the density of the exciting current was increased can be measured directly. Care was exercised so that the readings in each case were taken as rapidly and a3 uniform as possible in order to have a uniform basis for comparison of the results. The change in current density was a gradual advance and in no case was it changed from a higher to a lower value to take any reading. The results obtained are listed in the following tables. Table 1 shows the change of the overvoltage of chlorine using a smooth platinum anode from concentrations of hydrochloric acid equal to 0.900 normal, 0.096 normal, and 0.014 normal acid with a change in current density from zero to ten milliamperes . In a similar manner table 2 shows the change on platinized platinum, table 3 on copper, table 4 on carbon, and table 5 on gold. . • CM I 0 ? 9 Table 1 Smooth Platinum. 0.900 N Hcl current overvoltage m.a. volte. 0.1 0.8278 0.2 0.8385 0.3 0. 8453 0.4 0.851? 0.5 0.8556 0.6 0.8606 1.0 0.8723 2.0 0.8850 3.0 0.8939 4.0 0.9000 5.0 0.9064 6.0 0.9123 7.0 0.9156 8.0 0.9190 9.0 J . 9 <329 10.0 0.9258 0.900 N HC1 current overvoltage m.a. volta . 0.1 0.9250 0.2 0.9386 0.3 0.9449 0.4 0.9498 0.5 0.9546 0.6 0.9569 1.0 0.9665 2.0 0.9752 3.0 0.9822 4.0 0.9872 5.0 0.9912 6.0 0.994? 7.0 0.9969 8.0 1.0000 9.0 1.0030 10.0 1.0055 0.096 N HC1 current overvoltage m.a. volta . 0.1 0.9517 0,2 0.9624 0.3 0.9738 0.4 0.9803 0.5 0.. 9865 0.6 0.-9942 1.0 1.0098 2.0 1.0318 3.0 1.0452 4.0 1.0604 5.0 1.0740 6.0 1.0850 7.0 1.0915 8.0 1 . 0945 9.0 1.1036 10.0 1.1105 Table 2 Platinized 0 . 096 N Hrjl current overvoltage m.a. vclta. 0,1 0.9919 0.2 1.0036 0.3 1.0117 0.4 1.0134 0.5 1.0168 co o 1,0203 1.0 1.0336 2.0 1.047? 3.0 1.0580 4.0 1.0640 5.0 1.0703 6,0 1.0765 7.0 1. :847 8.0 1.0896 9.0 1.0944 10.0 1.1020 0.014 N HC1 current overvoltage m.a. vclta . 0,1 1.1346 0.2 1.2085 0.3 1 . 2553 0.4 1.2793 0.5 1.2963 0.6 1.3094 1.0 1.3518 2.0 1.3S32 3.0 1.4048 4.0 1.4185 5.0 1.4298 6.0 1.4422 7.0 1.4513 8.0 1.4590 9.0 1.4693 10.0 1.4760 tinum. 0.014 N HOI current overvoltage m.a. volta . 0.1 1,0335 0.2 1,0558 0.3 1 . 0670 0,4 1.0708 0.5 1,0770 0.6 1.0822 1.0 1.0946 2,0 1.1002 3.0 1.1094 4.0 1.1150 5.0 1.1195 6.0 1.1287 7.0 1.1341 8.0 1.1480 9.0 1.1504 10.0 1.1556 Volfs / MiUiamperes 10 Table 3 Copper 0.935 N HC1 0.107 N HC1 0.013 N HC1 current overvoltage current overvoltage current overvoltage m.a. volt 9 . m.a . volts . ~ rn. a. volts'^ 0.11 -0.4146 0.11 -0.2749 0.10 -0.1154 0.18 0.4056 0.18 0.2607 0,20 0. 1094 0.28 0.3932 0.30 0.2459 0.30 0.1016 0.46 0.3810 0.39 0.2400 0.44 0.0937 1.00 0.3521 1.00 0.3284 0.60 0.0833 2.00 0.3336 2.00 0.3100 1.00 0,0459 3.30 0.3207 3.00 0.1960 2.00 0.0071 4.00 0.3149 4.00 0.1806 3.00 + 0 . 0523 5.00 0.3106 5.00 0.1707 4.00 0.0879 6.00 0.3036 6.00 0.1612 5 . 00 0.1233 7.00 0.3000 7 . 00 0.1517 6.00 0.1641 8.50 0.2S37 8.00 0.0775 7.00 0.2004 10.00 0.2799 9.00 10.00 0. 0667 0.0526 8.00 9.00 10.00 0.2405 0.2820 0.3239 Table 4 Ga rbon 0.914 N HC1 0.102 N HC1 0.010 N H01 current overvoltage current overvoltage current overvoltage m.a. volts . m. a. volts . m.a. volts . 0.05 0.3452 0.10 0.6826 0.08 0.7605 0.18 0.3642 0.18 0.6929 0.20 0.7660 0 . 26 0.3817 0.28 0.6953 0.30 0.7776 0.40 0.4569 0.43 0.7072 0. 40 0.7856 0.48 0. 4832 1.00 0.7703 0.56 0.8242 1.00 0.5332 1.50 0.8473 1.00 0.9306 2.00 0.6615 2.40 0.9126 2.00 1.0334 3.00 0.7624 3.50 1.0091 3.00 1.1308 4.00 0.8223 5.00 1.0625 4,00 1.1846 5.00 0. 8690 6.00 1 . 0903 5. 00 1.2372 6 . 00 0.8986 7.00 1.1254 6. 00 1.2841 7.00 0.9267 8.00 1.1452 7.00 1.3307 8.00 0.9428 9.00 1.1745 8.00 1.3704 9.00 0.9724 10.00 1.1975 9.00 1.4049 10.00 0,9942 10.00 1.4535 11 Table 5 Gold 0.935 N HG1 0.107 IT HC1 0 . 013 N HC1 current ovsrvol tage current overvoltage current overvoltag m . a . volts . ra. a. volts. m. a. vcltsT 0.00 0,4377 0.12 0.6845 0.03 0.8331 0 . 03 0.4554 0.32 0.7253 0,09 0.8549 0.06 0.4745 0,42 0.7377 0,23 0.8748 0,09 0.4835 0.52 0.7493 0.54 0.8956 0.10 0,5228 1.00 0.7810 1.00 0.9223 0.26 0.5588 2.00 0.8052 2.00 0.9421 0.38 0.5880 3.00 0.8177 3.00 0.9530 0.60 0.6096 4.00 0.82.57 4.00 0.9605 1.00 0.6348 5.00 0. 8353 5.00 0.9698 2 . 00 0.6842 6. 00 0.8405 6.00 0,9601 3.00 0.7131 7.00 C . 8456 7.00 0,9899 4.00 0.7240 8,00 0.8500 8.00 0.9956 5,00 0.7324 9.00 0.8549 9.00 1.0036 6.00 0.7381 10,00 0 . 8605 1 0,00 1.0112 7.00 0,7421 8,00 0.7493 9.00 '0. 7525 10.00 0.7567 0.0012 N HOI current overvoltage re. a. volts . 0.02 0.8484 0.03 0.9156 0.04 0183 0.06 1,2265 0*07 1.4093 0.14 1,4650 0.24 1.4892 0.34 1.5081 0.44 1,5223 0.60 1,5396 1,00 1.5781 2.00 1,6238 3.00 1,6625 4.00 1.6886 5.00 1.7073 miiamperes 12 III. Discussion of_ Resul ts . The results of this experiment are similar to those commonly known for hydrogen and oxygen, namely, that the overvoltage varies with the concentration of the acid used and with the electrode surface. A decrease in con centration of the anion or an increase in concentration of the cation of the anode metal increases .the overvoltage .. This was found to be true for every electrons surface used, but in some cases the variation was greater than in others. In regard to the electrode surface the values varied from cooper with a negative value of about 0.42 volts in normal HC1 and with a current density of 0.1 m.a. per square decimeter, through carbon with a positive value of 0.35 volts, gold with a positive value cf 0.53 volts, smooth platinum with a positive value cf 0.83 volts, to platinized platinum with a positive value of 0.S3 volts. In spite of the fact that both Newbery (l) and Tartar and Keyes (2) consider that the time element is a very important item in the determination of the overvoltage, for values as low as those used in this experiment and where the time involved in taking the readings is as short as it was in these cases it is not necessary to consider it. Several control tests were run in order to determine the effect of time 6n the overvoltage under the conditions involved. Although the time effect is great enough to prevent complete agreement in the values, yet the variation was not great enough to appreciably effect the results. Since the effect of time on the overvoltage is the same as the effect of a (1) . J. Chem. Soc., 119, (1921). (2) . J, Am. Chem. Soc., 44, 557 (1922). 13 decrease in concentration of the aniens or an increase in concentration of the cations of the anode metal, it seems highly probable that this increase in potential which has been attributed to the time involved is very likely due to the fact that chlorine is being given off from the solution and hence lowering the concentration of the aniens particularly around the anode. Several facts point to this conclusion, first, that the potential increases in every case with a decrease iij concentration of the anion or with an increase in concentration of the cation of the anode metal, second, that the greater the current density the more rapidly is the potential increased in a given time 0 With the exception of the results obtained with the carbon anode, the bending point in the curve or, in other words, the point of maximum overvoltage occurs before a current density of one riilliampere per square decimeter is reached. In the case of the smooth platinum, the bend occurs at about four tenths of a milliampere when normal hydrochloric acid is used, and at eight tenths when hundredth normal acid is used. With the platinised platinum the bend occurs at about the seme current density in each of the varies concentrations used, this value in each case was between two and three tenths of a milliampere. With the gold the point of maximum overvoltage occurs at about four tenths of a milliampere for the normal acid, at about five tenths with the tenth normal, at about seven tenths with the hundredth and with the thousandth normal. When the copper anode is used the bend comes at four tenths of a milliampere in each case. Due to the fact that copper is very soluble in hydrochloric acid when an electric current is being passed through the ceil, it was very difficult to 14 secure checks on the results obtained. Those which are tabulated although they are the best that could be obtained at the time are not very reliable as very many different kinds of results were obtained during the experimentation. The ones given were selected because they seemed to be the nearest to the average and because the curves which represent them were most nearly in agreement Wj^th the results obtained with the other anodes. The copper anode was a copper wire sealed into a piece of glass tubing with sealing wax. In the case of the carbon anode used, there seemed to be quite a divergency in the results obtained. This variation in the values, unlike that of copper, tended to cause an irregular curve rather than causing several different kinds of curves as wasffound in the case of the copper. But by plotting the best results obtained it was found that the bend in the curve occurred at about two and five tenths railliamperes for the normal acid, and at about three miliiamperes for the tenth and the hundredth normal acids. Another point which was particularly noticeable was that after the point of flexure was reached the more rapidly that the readings were taken the more nearly the curve approached the perpendicular, indicating that if it were possible to take ail the readings under absolutely the same conditions of concentration that after the flexure point was reache'd the variation of the overvoltage with the current density is equal to zero. The point of flexure or the point of maximum overvoltage seems to be due to the fact that at that point chlorine gas is given off, probably according to the equation 2 Cl' + 2© = Clg 15 In the case of the copper anode which in some cases showed a double flexure, the second point of inflection is probably due to the going into solution of the copper ions. For example, in the case of the copper anode in the tenth normal acid solution in the interval between 7.8 milliamperes and 8,0 milliarcperes the copper plated very rapidly upon the cathode. This showed that the copper dissolved from the anode according to the equation Cu 4 2 © - Cu" (2) and then after forming the copper ions in solution these ions again give up the positive charges and plate onto the cathode according to the equation Cu* 4 2© s Cu (3) In every case where this plating occired it did not start until a given current density was reached and then it plated on very rapidly The point of plating was also the point of inflection in the curve. This would indicate that a certain amount of energy had to be stored up before the reaction (3) could start, but after it had once started it would progress very rapidly. This explains the occurrence of the point of flexure in the curve, but the best explanations of the actual occurrence of overvoltage are probably those given by Bennet and Thompson (l) and Bancroft (2))and Rideal (3). Bennet and Thompson conclude that any chemical reaction, consisting of more than one step, in generating electricity can not be strictly reversible, but requires more electrical eitergy to reform the substance than is given by the reverse reaction. This (1) , J. Phya. Chem. , 20, 236 (1316). (2) . J. Phys. Chem., 20, 396 (1916). (3) . J. Am. Chem. Soc., 42, 94 (1920). 16 irreversibility gives rise to overvoltage, since the quantity factor is constant. The theory is that the excess of back electromotive force of the system during electrolysis over the reversible electromotive force of the system consisting of the final product, is due to the accumulation during such electrolysis of unstable intermediate products above the equilibrium concentration. These products are unquestionably active hydrogen, K^* active oxygen, Op, etc. These products have bean shown to be more reactive than the final products, and are sufficently active to explain the overvoltages found experimentally, Rideal explains overvoltage on the ground of the adsorption theory, namely, that it is the measure of the energy required for the desorption of the hydrogen from the metallic surface. He also considers that a monatomic gas is formed, which influences the overvoltage. In comparison with the results obtained by Newbery (l) with the commutator method, it may be said that the results obtained by t the direct method are in every case higher in value than the ones obtained by the commutator method. The curves are not very much alike either as to general shape or as to the point of flexure. In ties first place the point of flexure obtained by Newbery was at a much wider range of current densities than those used in this experimentation, also he aid not tabulate his results for readings taken as frequently as those obtained in this work. In almost every case the point of flexure found in this investigation could not be found from the readings obtained by Newbery as his first three readings were taken at two, four, and six milliamperea . fl). J. Chem. Soc., 119, 477(1921). . . . . ' . , . . . ' , . 17 Another fact which was pointed out by Tartar and Keyes in their work in comparing the results obtained by the direct and by the commut&tbr methods of measurement of overvoltage, is the fact that the point of inflection in the curve is approached gradually when measurements are made by the direct method and that it occurs abruptly when the measurements are made by the commutator method. This abrupt change would indicate that the potential was gradually increased until a critical point was reached where the chlcrina? wae suddenly evolved, which is not the generally accepted opinion. One radical difference exists between the overvoltage of chlorine and the overvoltage of hydrogen and oxygen. This difference is that in the case of both oxygen and hydrogen the overvoltage increases with the polish of the electrode surface used, for example, the overvoltage of both hydrogen and oxygen on smooth platinum is much higher than that on platinized platinum. On the other hand, the overvoltage of chlorine is higher on platinized platinum than on the smooth platinum. No satisfactory explanation for this divergency can be given at this time. IV . Summary . 1. In every case, a decrease in concentration of the anion or an increase in concentrat ion of the cation of the anode metal increases the overvoltage. 2. The overvoltage varies with the electrode surface used, but unlike the overvoltage of hydrogen and oxygen it doe3 not always increase with the polish. 3„ The effect which has been commonly attributed to the time . . . . . 18 is here regarded to be due to the change in concentration close to the anode. 4. The overvoltage increases with an increase in current deneit density very rapidly until the point of flexure is reached, but after that only very slightly if at all, a limiting value is therefore indicated. 5. The overvoltage is due to the formation cf the molecules of gas from the ions in the solution. This formation is not accomplished in one step but involves several electrochemical reactions and is therefore an irreversible process. 6. The results obtained did not agree with those obtained by Ne vbery either in the absolute value or in the general shape of the curves obtained. . - 19 V. Bibliography . 1. Bennet & Thompson: Trans. Far, Soc., 29, 269 (1916). 2. Richards: Trans. Far. 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