196 



HYDROGEN ION CONCENTRATION 



state of equilibrium with respect to its adjacent aqueous phase. 

 Actually, the ionic distribution may be represented as: 



Water 



Na+ (ci) 



CI- (ci) 



Salicylate 

 H+ 



in traces 



Oil 



Salicylate" 



Na+ Na+ 

 (c'l) (c'2) 



Water 



Na+ (C2) 

 CI- (C2) 



Salicylate" 



H+ 



in traces 



If we neglect the ions present in traces only the arrangement 

 becomes: 



Water Oil Water 



Na+ (ci) Salicylate- Na+ (C2) 



CI- (ci) Na+ Na+ CI" (ca) 



(c'l) (c'2) 



Hence, the ion common to all three phases is the Na-ion only. 



We can, therefore, trace the formation of the potential differences 

 at the phase boundaries to the tendency of Na-ions present in the 

 oil, in a state of electrostatically evoked equilibrium, to approach 

 the true equilibrium, such as would ensue in the absence of electro- 

 static opposition, by repassage into the aqueous phase. The free 

 Na-ions upon reaching the water phase render it positive with re- 

 spect to the oil, and that side of the water phase is the more positive 

 which is adjacent to the part of the oil phase containing the higher 

 concentration of Na-ions. If, however, the common ion is a nega- 

 tive ion, as in the chain: 



then the same considerations apply to the Cl-ion, and the side which 

 was the more positive in the former chain becomes in this chain 

 the more negative. This is the manner in which the direction of 

 the current is determined in these chains. 



56. The concentration effect 



When two aqueous solutions containing an electrolyte in different 

 concentrations, Ci and C2, are separated by an oil phase, then, 

 according to our experience, the E.M.F., of such a chain may He 



