POTENTIALS AT PHASE BOUNDARIES 205 



In the case of the Cl~ ions we should have, because of the nega- 

 tive charge of the ion: 



E = - RT In ^^ X ^^ 



C2 Cl 



Both equations must be correct, and this can onl}^ be possible 

 when E = (or, what amounts to the same thing, when the frac- 

 tion following In is equal to 1). 



Only whenever it happens that the concentration of the K-ions 

 in any one of the four parts of the chain is different from that of 

 the Cl-ions, can the E.M.F. be other than zero. Such is the case 

 when a second electrolyte is present, or when the oil phase itself is 

 of an acid or basic character. 



57. The ionic series 



The chains considered above contained at both ends solutions of 

 the same electrolyte in different concentrations, and insofar they 

 are analogous to metal concentration chains. But if we employ 

 different electrolytes at the two ends, then we obtain an analogy to 

 chains containing different metals. As the simplest possible case 

 we shall choose that in which two different electrolytes, but possess- 

 ing an ion in common, are used on the two sides in the same con- 

 centration, as, for example, 



Water 

 NaCl 

 or, 



Water 

 NaCl 



Oil 



Oil 



Water 

 KCl 



Water 



NaSCN 



In the first case the common ion is the anion, in the second the 

 cation. Here the E.M.F. of the chains depends upon the difference 

 in the solubihty of the two electrolytes in the oil phase. If we as- 

 sume that the Cl~ is more soluble than the Na+ ion, then the oil 

 should become negatively charged with respect to the NaCl solu- 

 tion. But if it should happen that on the other side of the oil the 

 difference between the oil-solubility of Cl~ and that of K+ is smaller, 

 then the KCl solution will become less positively charged with re- 

 spect to the oil. Hence the KCl solution becomes the negative 

 pole of this chain. Hence, if in a chain of this order, we should al- 



