POTENTIALS AT PHASE BOUNDARIES 217 



Thus there is also formed K2SO4, and it is present on both sides 

 of the precipitation membrane. Although the membrane is con- 

 sidered impermeable for salts, yet it is certainly not so in the abso- 

 lute sense. The fact that the membrane becomes thicker in the 

 course of time shows that the salts do penetrate to a certain extent. 

 As far as the potassium ferrocyanide solution is concerned the dif- 

 fusion of the K2SO4 is immaterial. In the CUSO4 solution, on the 

 other hand, it is only then unimportant, when it already contains 

 a previously added large amount of a potassium salt, otherwise 

 it is just the trace of K2SO4 which, on diffusing through, affects the 

 E.M.F. of the chain appreciably. Thus our chain may be schema- 

 tized as: 



Solution 

 of K"*" ions 



Ci 



Membrane 



Solution 

 of K"*" ions 



C2 



In order to explain the concentration effect it must be assumed 

 that the membrane is a phase which also contains K-ions, but in 

 an almost constant concentration, just as in the case of the acid- 

 containing oil phase. We may assume for this purpose that the 

 membrane does not consist of entirely pure Cu2Fe(CN)6. These 

 amorphous substances "carry along with them on precipitation a 

 small portion of the soluble salts," or, there is formed some sort of 

 combination of Cu2Fe(CN)6 with K2SO4. It can also be shown 

 analytical!}' that the Cu2Fe(CN)e precipitate always contains alkali 

 metal salts. It is uncertain whether, in this case, adsorption, solid 

 solution or chemical combination is involved. But the very fact 

 that the membrane holds K-salts in a firm way is sufficient in itself 

 for our purposes. In the case of the acid-containing oil it was the 

 affinity of the acid for the cations (or bases) which produced the 

 almost constant concentration of cations in the portions of the oil 

 phase in contact with the aqueous electrolyte solutions. In this 

 case of precipitation membranes it is an affinity of the amorphous 

 membrane substance for the salt which produces a similar effect. 

 It is therefore possible to explain membrane potentials on the same 

 basis as "oil-chain-potentials." Whether this explanation is the 

 only correct one the future will show, but it does possess the advan- 

 tage in that it can be easily correlated with other conceptions in a 

 readily demonstrable way. We shall see in the next chapter that 



