POTENTIALS AT PHASE BOUNDARIES 191 



Let US take a special case in which the two ion species are distrib- 

 uted between the phases I and II according to their true distribu- 

 tion coefficients. (This is only possible when both the anion and 

 cation of the electrolyte used have the same distribution coefficient 

 a). Then each of the ions is in concentration ac in phase I and in 

 concentration c in phase II. Now, if we dip reversible metal elec- 

 trodes into the two solutions, we obtain as the electrode potential 



Ca • C 



on the left, tti = RT In—, and on the right 7r2 = RT In- , where 



A ij 



A and B denote the electrolytic solution tensions on the left and 

 on the right side respectively. Since in this case a boundary poten- 

 tial cannot develop, and hence the chain cannot yield a current, it 

 follows that TTi — 7r2 = 0, i.e. : 



RT In — = RT In - 

 A B 



or 



a _ J. 

 A ~ B 



Stated in words this means: Every ion-species tends to so dis- 

 tribute itself between two phases that the ratio of its concentrations 

 (or better, its active masses) in these phases should be the same as 

 that of the electrolytic solution tensions of similar electrodes in the 

 same phases. The attainment of such an equihbrium, as was said 

 above, is only then possible, when the distribution coefficients of 

 the anion and cation are equal. In general, however, this kind of 

 distribution is prevented by the resulting potential difference. Then 

 from the joint action of the two partial forces, the tendency towards 

 distribution and the electrostatic force, a kind of equilibrium results, 

 which is different from the one to be expected on the basis of the 

 true distribution coefficients. 



54. Phase boundary chains 



The same happens with a single phase potential as with a single 

 electrode potential: it does not generate any electric current. Only 

 a combination of two different metal electrode potentials or of two 



