POTENTIALS AT PHASE BOUNDARIES 219 



= mi of the anions is involved. Hence ni + mi = 1. On the 



U2 

 other hand, in the phenol the distribution is — — — = n2 for the 



V2 



cations, and for the anions — ; = m2, and again, n2 + m2 = 1, 



' U2 + V2 



Let us now consider that cross section of the left hand water 

 phase which is in direct contact with the phenol phase. Through 

 this cross section the cations are moving from left to right, and the 

 .anions from right to left. In this cross section ni cations are moving 

 inward from the left and n^ cations outward to the right. If ni > D2 

 then ni — n2 cation will accumulate in this cross section. On the 

 -other hand, in the same cross section m2 anions are moving from 

 the right and mi anions to the left. When ni > n2, then mi must 

 be < m2, and hence m2 — mi anions accumulate in this cross section. 

 Altogether Ui — n2 cations and m2 — mi anions accumulate. But 

 since m2 = 1 — n2 and mi = 1 — ni, therefore, m2 — mi = ni — n2, 

 i.e., the total electrolyte accumulated in this cross section amounts to 

 2 (ni — n2). The same consideration may be applied to the bound- 

 ary layer of the phenol phase, in which we shall find a correspond- 

 ing diminution of the electrolyte in the cross section. The same 

 process occurs in the two boundary layers on the right hand side. 

 Thus we find that the current produces an increase in electrolyte 

 concentration in the water and a decrease in the phenol phase in 

 both boundaries. Because of the spontaneous diffusion imme- 

 diately arising between portions of solutions of unequal concentra- 

 tion, the changes of concentration at the boundary surfaces actually 

 observed do not quite reach the expected values. When, reversely, 

 ni < n2, we obtain an increase of concentration in the phenol and 

 a decrease in the water layer. If the phase boundary potential was 

 equal to zero at the beginning, it can no longer remain after the 

 passage of the current, because of the above changes in concentra- 

 tion at the boundaries. Thus a potential difference arises as a 

 polarization phenomenon at the boundary surfaces. The two 

 oppositely directed potentials of the two boundaries constitute the 

 ►electromotive force of the polarization, which, in turn, must diminish 

 the electric current, just as in the polarization of an electrolyte 

 solution between two platinum electrodes. 



