DIFFUSION POTENTIALS 179 



continuous mechanical process of mixing between the two solutions 

 in such a way that the composition of solution I gradually passes 

 through the intermediate mixture zone into the composition of the 

 solution II. The assumption for the temporary constancy of such 

 a potential is that the two solutions have at least one ion species in 

 common. The Henderson equation is 



(Uj - Vj) - (Uji - Vjj) V{ + Vj' 



^ = 0.0001983 • T . _ , , ,/,, — r^^n-TTT. i«g 



(Uj' + V/) - (u„' + V„') U„' + Vj/ 



in which 



Ui = UiCi + U2C2 + U3C3.... 



Vi = viCi + V2C2 + V3C3 



Ui' = UiWiCi + U2W2C2 + U3W3C3. . . . 



Vi' = ViWiCi + V2W2C2 + V3W3C3 



where c = the concentration (for a negative ion, c). 

 u = the mobiUty of a cation and v of an anion, 

 w = the valence (w for an anion). 

 The subscripts I, II refer to the two solutions, the subscripts 1, 2, 3, 

 etc., to the different ion species. 



This equation was further modified by Gumming. ^ 

 Planck's and Henderson's equations do not yield the same values, 

 except under certain conditions, such as, for instance, when each of 

 the two solutions contains but one salt, in the same concentration, 

 and both having one ion species in common. It is to be expected, 

 therefore, that the diffusion potential should in general represent a 

 somewhat fluctuating value depending on whether Planck's or 

 Henderson's conception of the contact zone is assumed. Indeed, 

 Chanoz''' has shown that the diffusion potential changes with time. 

 Thus when a chain is arranged of the type 



Electrode 11 Solution I | Solution II | Solution I || Electrode 



A B 



we should expect that its E.M.F. would be equal to zero, for here 

 not only the electrode potentials but also the diffusion potentials 



* Gumming, Transact. Faraday Soc. 8, 86 (1912). 



6 Chanoz, Ann. de I'univ. Lyon, Nouv. S6r. 1, 18 (1906), cited from P. 

 Henderson. 



