222 HYDROGEN ION CONCENTRATION 



For the sake of preserving the continuity of argument developed 

 in the preceding chapter let us choose an interpretation of this 

 phenomenon somewhat different from Donnan's. 



Let us imagine two electrolyte solutions separated by a membrane 

 which is impermeable to one of the ionic species present. Let, for 

 instance, the membrane be one of collodion and let one of the solu- 

 tions be a KCl solution and the other of the chloride of a cation 

 which cation cannot diffuse through the membrane. A process of 

 diffusion ensues leading to a certain state of equilibrium. But the 

 above non-penetrating cation cannot take part in this diffusion. If 

 the system is initially 



Colloid-chloride 



(dissociated into 



colloid-cation+ + Cl~) 



Membrane 



KCl 



(dissociated into 

 K+ + C1-) 



diffusion will at once occur through the membrane. ^ K-ions will 

 diffuse from right to left, Cl-ions will diffuse, according to the con- 

 ditions, from right to left, or reversely. But there is no possibility 

 that the concentrations of K+ become the same in both solutioQS, 

 nor could that happen for the Cl-ions. For, if this were the case, 

 then the solution on the left would contain, because of the presence of 

 the colloid-cations, more cations than anions, which state would 

 violate the law of electroneutrality. It is more likely that the colloid- 

 cations will hold back a part of the Cl-ions. For in the state of 

 equilibrium the total sum of cations in the solution on the left (col- 

 loid cations + K+) must be equal to its content of anions (Cl~), 

 and, similarly, in the solution on the right side, the concentration of 

 the K-ions must equal that of its Cl-ions. The inequahty of the con- 

 centrations of the oppositely charged ions can be but immeasurably 

 small, and it effects a potential difference on both sides of the mem- 

 brane. In the first place then, we find an anal3i:ically demonstrable 

 difference between the Cl-ion concentrations in the solutions, and, 

 secondly, we also find a potential difference. These two phenomena 

 are definitely related to each other, and this relation can be calculated. 

 Thus, if after the establishment of equilibrium, we should immerse 

 in each of the two solutions, a reversible Cl~ electrode (i.e., a gas 



^ The term "colloid" in the above scheme does not in this case signify that 

 the substance is not in molecularly dispersed solution, but it only signifies 

 that it does not diffuse through the membrane. 



