NOTES UPON T H E POTENTIALS O F CALOMEL AND HYDROGEN ELECTRODES BY N. EDWARD LOOMIS
Because of recent references in chemical literature t o two articles published a few years ago by Loomis and Acreel it has seemed advisable to recalculate certain of their results in the light of subsequent work. Loomis and Acree adopted the value 0.339 volt for the potential of the electrode 0 . I KCI-HgCI-Hg at 25 O C. Subsequently, Auerbach? reviewed the literature in this field and decided that 0.337 volt was the most probable value for the potential of this electrode. In a recent article Lewis and Randall3 conclude from a careful review of previous work and from unpublished results of their own that the electromotive force of the combination H2-0.1 -qTHCI-0.1 KCI-HgCI-Hg is 0.3990 after correction has been made for the contact potential of the system. If we assume with Lewis that solutions of hydrochloric acid are dissociated to the same extent as solutions of potassium chloride at the same dilution, the degree of dissociation of 0. I A- HC1 is 86 percent and ,the potential of the decinormal hydrochloric acid hydrogen electrode is 0.0629. From this we readily calculate the potential of the decinormal calomel electrode to be 0.3990 - 0.0629 = 0.3361 volt This result is given added weight by the fact that unpublished experimental work by Sebastian in Lewis’ laboratory upon the electromotive force of the system Hz-0. I AT HC1-HgCI-Hg yields the same figure, 0.3990 volt. This is just the result which we should expect if decinormal hydrochloric acid is -
~~
1
2 3
Am. Chem. J o u r , 46, jSj, 621 (1911). Zeit. Elektrochemie, 18, 13 (1912). Jour. Am. Chem. SOC.,36, 1974 (1914).
' P o t e d a l s o j Calomel and HydrogeR Electrodes
66 I
dissociated to the same extent as decinormal potassium chloride. On the other hand both Loomis and Acree and Myers and Acreel obtained a voltage of 0.4000 for the above system, indicating a value of 0.3371 for the potential of the calomel electrode. It is seen that all of the recent work points to a lower value for the potential of the decinormal calomel electrode than that which Loomis and Acree adopted for their calculations. This value probably lies between 0.336 and 0.337 volt. It should be emphasized that before the electromotive force method can be applied to the study of chemical problems with any great degree of accuracy it is absolutely necessary that the potential of the calomel electrode or that of the decinormal hydrochloric acid hydrogen electrode be known more exactly than it is a t present and that an accurate method for measuring or calculating contact potentials be developed. I n the subsequent part of this article the hydrogen electrode experiments of Loomis and Acree are recalculated by using 0.336 as the potential of the decinormal calomel electrode. In the study of the hydrolysis of aniline hydrochloride H'conc. X 100 was found to be 2 . 0 7 and 2.93 for the ratio Total salt sixteenth and thirty-second normal solutions respectively. When 0.336 volt is used for the potential of the calomel electrode these values become 1.84 and 2.60 respectively. Assuming again that hydrochloric acid is dissociated to the same extent as corresponding solutions of potassium chloride we have the following degrees of ionization:' S i 1 6 .!- HC1 = 89.7Yc. S , 3 2 S H C 1 = 92.6%. 1.84 0.897 = 2 . 0 5 7 ~for the degree of hydrolysis of 16 aniline hydrochloride. 2.60'0.926 = 2.815 for the degree of hydrolysis of .!-3,'2 aniline hydrochloride. From these values we may calculate the ratio of K,,Kb.3 For V = 16, ~
Am. Chem. Jour., 50, 396 (1913). These dissociation figures zre calculated from the conductivity d a t a of Jones and \Vest at 25' C. See Landolt and Bornstein, p. 1 1 1 2 . See p. 629 of original article.
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iV. Edward Loomis
Xverage 1-alue of K,, Ki, = 2.j7 X IO-’ Tizard’s value’ = 2.42 x 1 0 - 3 Bredig’s value? = 2.4 x 10-3 Average value u-ith 0.339 = 3.37 x 1 0 - 5 Experiments were also carried out upon the degree of ionization of 0.25 *Iacetic 7 acid. Assuming that saturated potassium chloride solution entirely annuls the contact potential the dissociation is found to be 0.99 percent when 0.339 is used for the potential of the calomel cell and 0.88 percent when 0.336 is used. The latter figure agrees much better with 0.89 percent calculated from the conductivity data of White and Jones.3 There would also be a change in the degree of dissociation of acetic acid in the presence of neutral salts but since the relative values would change but slightly from those given in the original article these have not been recalculated. The experiments with both aniline hydrochloride and acetic acid show that the electromotive force method gil-es results in closer concordance with other methods when 0.336 is adopted for the potential of the decinormal calomel electrode than when a higher value is used. U-hile speaking of the experiments upon acetic acid opportunity will be taken to answer a rather misleading comment recently made by IZlcBain and Coleman. They say, P. I 52 j t Loomis and Acree have applied the electromotive force method to the case of acetic acid with and without the addition of potassium chloride. They obtain diametrically opposite results in each concentration of salt, depending upon whether they assumed that ammonium nitrate or potassium chloride (‘
‘Jour. Chem. SOC.,98, 2492 ( 1 9 1 0 ) . Zeit. phys. Chem., 13, 289 (18941. A \ n ~Chem. , Jour., 44, I j9 i 1910). Jour. Chem. SOC., 105, 1 5 1 7 (1914).
Poteiztials o j Caloivel aiid Hydrogeu Electrodes
663
in saturated solution, had removed the diffusion potential ; thus 2.06 S KC1 either lowers the dissociation of acetic acid to j4.5 percent or increases it to 176 percent." The actual figures were 0.797 percent for the degree of dissociation when ammonium nitrate was used and I .30 percent when potassium chloride was used to annul the contact potential. I n the first case the ionization appears to be repressed to 73.2 percent of its original value; in the second case increased to 132.7 percent of its original value. McBain and Coleman neglected to state or, perhaps, failed to realize that our experiments were carried out for the express purpose of determining the relative value of potassium chloride, ammonium nitrate, calcium acetate, potassium iodide, and potassium bromide for eliminating contact potential ; we showed conclusively that potassium chloride is the most effective and that consequently much more weight is to be gii-en to that series of measurements in which potassium chloride is used.' The author also wishes to call attention to an error which appeared in the original article of Loomis and Acree. In the original dissertation corrections for barometric pressure were omitted because such corrections were in most cases within the limits of experimental error. Before the publication of the work, howel-er, it was decided to include these corrections and the published calculations were made by 1Iyers and Xcree. Because their work covering a narrow range gave 1 This question oi "salt the efiect o i salts on the activity of ions in electromotil-e iorce me onductivities, and reaction velocities rtation, P. jz: and in many papers was diwuised hy Loomis anti b3- Xcree ( A i mChem. , Jour., 44, I j 9 and later) arid is the su1,ject oi a n u m l ~ e r of in\-estigationq in ;\crec'i laboratory. Our ideas should not ha\-? heen mi.understood by 11cBain and Coleman and ha\-e recently been idl>- confirmed by the work of Poma tZeit. phys. Chem., 88, 6 7 1 ) . It should alto be pointed out in discussing Len.is' paper 1 . w especially Jour. -Am. Chem. Soc., 36, 19;s) t h a t .lcrec (-1m. Chem. Jour., 4 1 , 475: 48, 369, etc.) has already d i s c u . 4 the relation of the work of Lewis and ol Lorenz and Bohi on the potential of the hydrogen electrode in solutions of potassium hl-droside and hydrochloric acid t o the ionization of water, and its bearing on the question whether water c a u ~ c ~ hydrolytic metatheses through its ions or its molecule?, or both. (Signed) S.F. XCREE.
N.Edward Loomis
664
more consistent results and because of other facts connected with the hydrogen electrode which they will publish later, they used the equation R.T e =
-F-log p
for their calculations instead of the thermodynamic equation R.T e = -log p 2F in which e represents the desired correction, P the barometric pressure in atmospheres, and R , T and F have their usual significance. The work of Czepinski, Haber, and Lewis3 over a much wider range has sufficientlyproved the correctness of the second equation.4 As a result all barometric corrections appearing in the papers of Loomis and Acree and of Myers and Acree are probably just twice too great. This makes a maximum error in some individual cases of 0.0001 volt but it should be noted that because of the nearly normal average barometric pressure in Baltimore the average value for the comparison of the hydrogen and calomel electrodes (see P. 610 of the original article) is not affected by this change. ___
Departmeizt of Chemastq Purdue Cnauerszty
4
- - __ Zeit anorg Chem , j o , I ( 1 9 0 2 ) Ibid , 51, 289 (1906) Jour Am Chem SOC,33, 3 0 j J I g I I ) , 36, 197-1. (191-1.) See also n’alpole Jour Chem SOC 105,2 5 2 3 k1914.1
