THE ETHYL-SULPHURIC ACID REACTION. 387 



ELEC^TROMOTIVE FOIK^E MEASUREMENTS OF THE SYSTEM. 

 H2— Ft— 0.1 M (H('1+KC1)— n^'.Cl,— Hf?. 



N. P^DWARD LOOMIS. 



For several years there has l)eeu si'phI uncertainty in ref^ard to tiie rchi- 

 tive dissociations of . 1 M HCl and . 1 M KCl. It has been shown that tlie 

 conductivity method indicates too high a degree of dissociation for hydro- 

 chloric acid because of increase in the mobility of the hydrogen ion with in- 

 creasing concentration. Most authors in recent years have assumed that at 

 25° both solutions are 86% dissociated in accordance with the recommenda- 

 tion of Lewis and Sargent in 1909^ Since 1912, however, Lewis has ques- 

 tioned his earlier view and the matter has again been left in doulit. 



The Avriter has attempted to secure information upon the relative dis- 

 sociations of 0.1 M HCl and 0.1 M KCl by studying the electromotive force 

 of the system : 



H2— Ft— 0.1 M (HCi+KCD— Hg.Cl,— Hg 

 It is obvious that if 0.1 M HCl is dissociated to the same extent as 0.1 M KCl 

 then the electromotive force of the system 



H2— Ft— 0.1 M HCl " 0.1 M KCl— Hg.Cl2— Hg (2j 



should be the same as that of 



H2— Ft— 0.1 M HCl— HgsCh- Hg 

 since the potential of the calomel electrode is a function only of the chlorine 

 ion. concentration of the surrounding solution. 



Within the limits of experimental error this has been shown to be the 

 ease in an article recently published by the writer.^ The best measure- 

 ments of the electromotive force of the system 



Ho— Ft— 0.1 M HCl " 0.1 M KCl— Hg.Cla- Hg 

 at 25° give a value of 0.3988 or slightly greater depending ui)on the value of 

 the contact potential used in the calculations. The mean of twenty-eight 

 measurements of the system 



H2— Ft— 0.1 M HCl— HgsCU- Hg 

 gave 0.3988+0.0002. It is seen that the electromotive force of the tAvo 

 systems is identical within the limits of our knowledge of the contact potential 

 of 0.1 M HCl— 0.1 M KCl, and consequently 0.1 M HCl and 0.1 M KCl 

 appear to be equally dissociated. 



If 0.1 M HCl and 0.1 M KCl are equally dissociated then according to 

 the isohydric principle it should be possible to mix the solutions in any 

 proportion without changing the degree of dissociation of either. Conse- 

 quently the potential of a calomel electrode surrounded by any mixture of 

 0.1 M HCl and 0.1 M KCl should be the same as that of a 0.1 M KCl calomel 



1 .T. Am. Chem. Soc 31, 363. (1909). 



= The symbol (") indicates that the contact potential has been eUminated. 



' J. Am. Chem. Soc. 38, 2310, (1916). 



