210 GUGGENHEIM art. e 



tion of the cell in order to avoid a discussion of diffusion poten- 

 tials. A workable cell would be the following : 



Cu 



Solution Si containing 

 Cw++ and large excess 

 of other ions 



Solution ^2 containing 

 Zn+"'' and large excess 

 of other ions 



Zn 



Cu. 



The diffusion potential between the two solutions Si and S2 

 could be made negligible by making the composition of the two 

 solutions substantially the same apart from the Cm++ ions in the 

 one and Z7i++ ions in the other, the concentration of these 

 being in both cases small compared with the concentrations 

 of the other cations. 



In the metallic phases we have the purely chemical, homoge- 

 neous equihbrium conditions 



[/icu-P + 2[Ms,-P = Me:, (109.1) 



[y.zn^f' +2[M^,-f" = Mf:, (109.2) 



where ^^'^ and ^f^' are independent of the electric states of 

 the respective phases. The contact equilibrium conditions are 



Ucu++]; = Ucu«-]"> (110.1) 



Uzn-]^" = Uzn-l^ (110.2) 



wr'^ = UEi-f"- (110.3) 



Combining (107), (109), (110) we obtain for the electromotive 

 force E 



2FE = [ncu+A" — [^J■cu^]" 



= I'cl - 4l + [/^^"-l^ - ^^cu*^^^^ (111) 



or, in terms of activity coefficients, 



2f <-/i- 



E = E' + ^\og '^■^, (112) 



where E° is independent of the composition of the solution, the 

 values of the mol fractions N^ and activity coefficients f^ being 

 those in the solution. 



