CONTRIBUTIONS TO THE STUDY OF STRONG ELECTROLYTES. I. THE ELIMINATION OF POTENTIAL DUE TO LIQUID CONTACT. 11. THE POTENTIALS OF SILVER NITRATE SOLUTIONS. By ALEXANDER CHARLES CUMMING, D.Sc. (1851 Exhibition Research Scholar).‘k ( A Paper read before the Faraday Society on Tuesday, December 11, 1906, DR. T. M. LOWRY in the Chair.) I. THE ELIMINATION OF POTENTIAL DUE TO LIQUID CONTACT. It has been a constant source of difficulty in electromotive force measure- ments that, in addition to the electromotive forces between the electrodes and the solutions, there is another source of potential at the surface of contact of the two solutions one wishes to compare. Any actual measurement is the sum of these two, and to obtain one separately is not always easy. Planck (Wied. Ann. 40, p.561, 1890) has shown how the E.M.F. due to the contact of two dissimilar dilute solutions may be calculated if the composition and ionic con- centrations are approximately known. In practical work the data required for Planck‘s formula are often unavailable, and in any case the calculation is a most laborious one. Tower (Zeit. physik. Chem. 20, p. 198) showed that if two solutions were connected by a potassium chloride solution, the (‘ diffusion” E.M.F. was materially lowered, and, following a suggestion in Luther Ostwald’s book, it has become customary to use a N or a saturated potassium chloride solution, and to assume that the diffusion E.M.F. is thereby eliminated, so that the observed E.M.F. is solely from the electrode potentials. It may be readily proved that this is not quite accurate.If it is permissible to add a neutral salt to both solutions the diffusion potential is eliminated (Sackur, Zeit. physik. Chem. 38, p. 129), but such addition of salt naturally alters the ionisation cf the salts already there, and this cannot always be allowed for. N. Bjerrum (Zeit. physik. Chem. 53, p. 428) has recently investigated the effect of joining solutions with different concentrations of H ions by various strengths of potassium chloride solutions. He found that a saturated potassium chloride solution did not remove all the diffusion potential, but deduced an empirical rule that the amount of diffusion potential still un- removed was equal to the difference in the E.M.F. with saturated and with half-saturated potassium chloride as the connecting solutions.The reason for taking this particular correction is not clear, but for solutions containing H ions it appears to be satisfactory over the range of concentrations investigated by Bjerrum. In the course of another research this question of the elimination of diffusion potential was of importance, and a systematic attempt was made to * Communicated by N. T. M. WILSMORE, M.Sc. N 2’3214 CONTRIBUTIONS TO THE STUDY find the best method. The various methods were tried for two extreme cases, in one of which the anion was very much faster than the cation, and in the other the cation velocity was much the greater. In one set the system measured was the following :- Hg . Hg,Cl, I! HC1= C, X = C, HCl= C, Hg,CI, . Hg where C, = N, C, = - and X was any desired solution of known concentra- tion C,.The special case where X was onitted and the two solutions directly connected may be first considered. For the simple case of two solutions of the same substance, but of different concentrations, the electrode E.M.F. is giverl by the equation (Nernst, Zed. fhysik. Chem. 4, p. 129) :- (I II I1 N 10' where C, and C, are the ionic concentrations. The value of- RT - - log'' at 25' = 0'0591 Volt. F ' log, - The ratio 5 may be obtained from the conductivities, and the calculated electrode E.M.F. = 0.0563 volt. This calculation is made on the assumption that the influence of the calomel is the same in both cases, so that it may be neglected. This may not be quite correct, but, for present purposes, a small error from this source is of little importance.The diffusion E.M.F. between two solutions of the same substance, b u t of different concentrations, may be calculated from the formula- c, U - v RT c r t x v F C, e d = - - , - . log 2 where u and v are the ionic velocities. The values given by Ostwald for H and C1 are 347 and 75 respectively, so that- ed = 0.0381 volt. The mean experimental value from a number of readings with the two cells in direct connection was 0.0950 volt. Subtracting the calculated diffusion E.M.F. we have for the system- Hg I/ HgzC1, /I HC1= I HCI = . I Hg,Cl I/ il a ll Hg e = 0.0950 - 0.0381 = 0.0569 volt, as against the calculated value of 0.0563 volt. As a standard value against which to compare the later results, the E.M.F. was taken as 0.057 volt to within a millivolt. The results when the two cells were joined by various alt solutions may now be given.Since chlorides cannot always be used nitrates were tried CONNECTING SOLUTION. E.M.F. . . . . . . . . . . . . . . . Direct connection 0.0950 N Potassium nitrate . . . . . . . . . . . . . . . 0.0760 3 N ,, ,, (saturated) . . . . . . . . . 0.0683 With the saturated solution there was still a large amount, 0.011 volt, of diffusion potential not removed. Ammonium nitrate gave similar values with like concentrations, but in this case much stronger solutions could be used.OF STRONG ELECTROLYTES 21 5 CONNECTING SOLUTION. Direct connection . . . . . . . . . . . . . . . Ammonium nitrate . . . . . . . . . . . . 4 N 1 N 9 , 5 N 9 ) I O N ,, Saturated ammonium nitrate .. . . . . . . . )) 2 ) . . . . . . . . . . . . - 2 )) ,) )) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Correct value = 0.057. E.M.F. 0*0950 0.0848 0.0800 0.061 7 0.0566 0.0565 0'0740 To prove that the diffusion E.M.F. depends only on the strength of the solutions at the surface of contact, the following system was measured- The E.M.F. found was the same as that of the system- Though there is little experimental evidence on the subject, the principle underlying the use of potassium chloride as a connecting solution is that a salt which is to be of use for this purpose, must have an anion and a cation, which possess, as nearly as possible, equal velocities. That this is really a requisite condition may be readily shown, e.g., by the use of sodium nitrate or lithium chloride solutions.CONNECTING SOLUTION. E.M.F. Direct connection . . . . . . . . . . . . . . . 0.0950 N - lithium chloride . . . . . . . . . . . . . . . 0.0880 ), . . . . . . . . . . . . . . . 0-0682 N 1 ) ) , . . . . . . . . . . . . . . . 0.0464 4 N )) 8 N ,) 8 N Sodium nitrate . . . . . . . . . . . . . . . 0.0570 Saturated sodium nitrate . . . . . . . . . . . . 0.043 2 I 0 . . . . . . . . . . . . . . . . 2 N ,, , O"574 ,) . . . . . . . . . . . . . . . 0.0330 There is no sign in this case of a constant value being obtained with in- crease in the concentration, and the agreement of two of the above results with the correct value, 0.057 volt, is evidently fortuitous. A similar set of experiments was next carried out with solutions of a different type, in which the diffusion E.M.F.was in the reverse direction to the above. The two cells were N and -- lithium chloride solutions, each with a Hg 11 Hg,Cl, electrode. When the cells were directly connected the observed E.M.F. was 00361 volt. The diffusion potential calculated from the conductivities =0*0169 volt, and adding this to the observed value we have for the electrode E.M.F., e = 0.0530 volt. When the E.M.F. is calculated in the same manner as in the previous N I 02 16 CONTRIBUTIONS TO T H E STUDY E.M.F. BETWEEN N. AND Y f o HYDROCMORIC AciD. HYDROCHLORIC AciD -3------ ----- D I e 3 4 5 6 7 ti 7 IO II 1 2 1 3 rq STRENGTH OF CONNECTING SOLUTION. experiment, with the conductivities as a measure of C, and C,, the value.calculated is 0'0522 volt. The results with various connecting solutions may now be given. CONNECTING SOLUTION. E.M.F. Direct connection . . . . . . . . . . . . . . . 0.0361 N ammonium nitrate . . . . . . . . . . . . 0.0480 0.0532 5 N ?7 I O N ?, ,, 0.0543 Saturated potassium nitrate . . . . . . . . . . . . 0'0502 Saturated potassium chloride . . . . . . . . . 0.0515 J , . . . . . . . . . . . . . . . . . . . . . . . . Strong solutions of ammonium nitrate proved so satisfactory for both HC1 and Li Cl solutions, that it seems probable that a strong ammonium nitrate solution will be found generally useful for the elimination of diffusion potential, particularly when the electrode solutions are concentrated, as the connecting solution should always be much more concentrated than the solutions to be measured.The comparatively great solubility of ammonium nitrate makes it, there-OF STRONG ELECTROLYTES 217 fore, preferable to potassium chloride when the electrode solutions are strong. The following observation is not directly connected with the above experiments, but is of some interest in itself. N When the N and - HCI cells were connected by means of a hydrochloric acid solution of some other concentration, it was found that the E.M.F. remained the same, though the strength of the connecting solution was varied from - up to the most concentrated acid. I have not been able to find any reference to a similar observation, but it may be readily shown to follow from Nernst’s theory. The E.M.F. between two solutions of the same substance, but of different ionic concentrations, C, and C,, is given by the formula- I 0 N I00 If, in the case we are considering, C, and C, be the concentrations in the two cells, and C, that of the connecting solution, the diffusion potential between one cell and the connecting solution is- ?2.PT . log c, u + v F c4 and between the other cell and the connecting solution, The resultant diffusion potential is the difference between these- or the same as with direct contact, whatever the value of C, may be. This, of course, applies to any two cells joined by a solution of the same substance. In addition to the experiments with the hydrochloric acid cells, it was proved to be true for the pair of lithium chloride cells used in the above experiments.With direct connection, or when joined by a lithium chloride N solution of any concentration from - up to SN, the observed E.M.F. remained the same. 1 0 11. THE POTENTIALS OF SILVER NITRATE SOLUTIONS. It is well known that the Ostwald dilution law, which holds so well for weak electrolytes in aqueous solutions, finds an apparent exception in the case of strong electrolytes ; in other words, the alteration in the proportion of ionised to unionised substance with dilution is not in accord with the law of mass action, if we take the conductivity as a measure of the amount ionised. This has been a fruitful source of experiment and discussion, and in the following research an attempt was made to obtain some fresh information on one of the disputed points. While many accept the conductivity as a measure of the ionisation, others, and notably Jahn, believe that in strong solutions the conductivity yields too high an estimate, i.e., that the degree of dissociation is less than L.PCc Jahn (Zeit. physik. Chew. 33, p. 370) has measured the ionic concentrations2 18 CONTRIBUTIONS TO THE STUDY of potassium chloride and other salts by means of electromotive force measure- ments, and according to his results the substances examined obey the dilution law in very dilute solution. The following experiments with silver nitrate were undertaken in the hope that further information in this direction might be obtained. Silver nitrate is readily obtained pure ; it can be measured with pure silver rods as electrodes, and thus eliminate any possibility of error from the use of a depolariser, such as calomel, which must, even if only to a small extent, go into solution ; and finally it possesses the advantage that its two monad ions have almost identical velocities, so that the correction for diffusion potential is very small.ExferimentaZ.-All measurements were made in a thermostat at 25O. The E.M.F. was determined by the Poggendorff compensation method with a DArsonval galvanometer, or, in the case of solutions below -, a Lippman electrometer as zero instrument. One set was made by electrically coating platinum wires with silver from a potassium silver cyanide solution. Another set was made in the same manner with a solution of silver chloride in strong hydrochloric acid, while the third set were pure silver wires obtained from Herzus.At first these wires were sealed into tubes with fusible glass, and the junction covered with a layer of asphalt, and then with paraffin. It was found, however, that the contact between the silver and the cover was never perfect, but always introduced errors, while plain uncovered silver rods answered the purposc excellently. These rods were always cleaned before use with ammonia, dilute nitric acid, and then water, and were found to give no difference of potential in a - silver nitrate solution. At least two electrodes were used in each cell, and as it was found that the E.M.F. was the same with all three kinds of electrodes, the silver rods were generally employed, as they were the most convenient. ?!- Silver Nitrate.-With the solutions in direct connection with one N I00 The silver electrodes were prepared in three different ways. N I0 I 0 I00 another a number of experiments were made.N Example: Two silver rods (AI and A2) served as electrodes in the - All four rods had I 0 N solution, and two others (BI and B2) in the - solution. I 0 0 N been cleaned just before use and tested against one another in - silver nitrate. I0 AI against BI =0-0588 volt. A2 ,, BI =o'o5y1 ,, AI ,, B2=0*0;91 ,, A2 ,, R2=0'0590 ,, Mean = 0'0590 ,, Three other similar experiments gave the following mean values :- 0.0591, 0.0589, 0-0587 volt. In some other experiments, in which the number of electrodes or some small detail in the method of measurement was altered, the results were as follows :- 0.0591, 0'0592, 0.0588, 0.0591, 0'0590, 0.0589.OF STRONG ELECTROLYTES Platinum points plated with silver from a solution of silver chloride in strong hydrochloric acid were next used as electrodes. E.M.F.= o*osgo 0'05go volt. ln each cell two electrodes were next tried, one (AI) a silver rod and the other (Az) a platinum point plated with silver from a potassium silver cyanide solution. In the other cell were a similar pair of electrodes, BI and Bz respectively. AI against BI =0*0593 volt AI ,, B2=0'0590 ,, A2 ,, BI =0-0590 ,, A2 ,, Bz =0'0591 ,, N N The mean value for all the measurements of - against - silver nitrate was 0-05cp volt. When the diffusion E.M.F. was eliminated by connecting the two cells by a I O N ammonium nitrate solution, the mean value for the E.M.F.was 0.0556 volt. SimiIarly with saturated potassium nitrate as the connecting solution the mean E.M.F. was 0.0555 volt. Nernst (Zeit. fhysik. Chem. 4, p. 129) has given the formula for the diffusion potential between two strengths of the same solution as- I 0 I 0 0 From Ostwald's figures u at 2 5 O = 71-0, and v = 63.0. The conductivities of Lob and Nernst (Zeit. physik. Chem. 2, p. 948) may be taken as a measure of C, and C,, so that- 8 x 0'0591 x log,o- 1'003 ed= - I34 0.1217 =0'0033 Volt. Even if it be not quite correct to take the conductivities as a measure of C, and C,, the ionic concentrations, it must be near enough to the truth for the calculation of this small correction. N N We have now the E.M.F. of - . - silver nitrate by three methods, the difference being due to the different methods for the elimination of the diffusion potential.I 0 I 0 0 ELIMINATION OF DIFFUSION POTENTIAL. E.M.F. By calculation . . . . . . . . . . . . . . . . . . 0.0556 By ammonium nitrate . . . . . . . . . . . . 0.0556 By saturated potassium nitrate . . . . . . . . . 0.0555 Nernst in his fundamental paper gave the formula for the E.M.F. between two solutions of the same kind as- and suggested the conductivities as a measure of C, and C,. If we calculate the value of e from the conductivities of silver nitrate in - I 0 and - I 0 0 solution, we obtain e = 0.0557 volt. N N220 CONTRIBUTIONS TO THE STUDY The close agreement between this value and that found experimentally proves that the ionic concentrations as measured by the conductivity and by E.M.F.measurements are the same, or at least in the same ratio. Special attention must be directed to this, as it accords with the view of Ostwald and Arrhenius that the conductivity yields a true measure of the ionic concentra- tion, in contradistinction to the results of Jahn (Zoc. cit.). Some other strengths of solution were also measured, but not in so much detail as the pair given above. N - against -!!- Silver Nitrate.-The electrodes used were silver rods. I00 1 ,- The potential in all cases where a solution below -was used was badly defined and hard to measure. Six experiments, in each case with two electrodes in each cell, were made with the solutions directly connected. The mean values were as follows :- N I00 0'0612 0'0620 0.0618 0.06 I5 0'0622 0.0618 mean = om618 volt. Calculated diffusion potential = 0.0038 volt, so minus dffusion potential Six similar separate experiments, but with elimination of diffusion potential E.M.F. = 0.580 volt. by 10 N ammonium nitrate solution- 0'0577 0'0579 0'0577 0.0591 0.0578 0'0579 mean = 0.0579 volt. The E.M.F. calculated from Nernst's equation with the conductivities as a measure of C, and C, = 0.0580 volt. N against - Silver Nitrate.- I 0 Mean E.M.F. with direct connection = 0.0312 volt. Calculated diffusion potential = o*oozz e = 0*0290 With elimination of diffusion potential- (a) by ammonium nitrate = 0.0286 ( b ) by saturated potassium nitrate = 0*0290. The E.M.F. calculated from Nernst's equation and the conductivities is 0.0292 volt. Conclusion.-The deduction from these experiments is that for silver nitrate the electromotive forces of its solutions are such as to support the belief that the conductivity gives a true measure of the ionic concentration. A. A. Noyes (Technological Quarterly, 1904, 17, p. 293), from a review of the experimental data on the subject, regards this as generally true for strong electrolytes, and it may be pointed out that the experiments in the first part of this Paper with hydrochloric acid and lithium chloride cells also support this view. It is with great pleasure that I take this opportunity to thank Professor Abegg for his kind assistance during the progress of this research. UNIVERSITY OF BRESLAU.