154 



THEORY OF COLLOIDAL BEHAVIOR 



It can be shown that the P.D. between the micellae and the 

 surrounding solution is influenced in the same way by pH, 

 valency, and salts as is the P.D. between a protein solution and a 

 watery solution separated from each other by a collodion mem- 

 brane. It was then necessary to ascertain whether this P.D. 

 was due to Donnan equilibrium, and for this purpose the pH 



Fia. 42. Method of measuring P.D. between gel and surrounding solution. 



inside of the (melted) gelatin micellae and the pH of the outside 

 solution were measured with the hydrogen electrode and the 

 potentiometer. It was found that the value 58 (pH inside the 

 micellae minus pH outside) agreed as closely with the P.D. 

 observed with the Compton electrometer as the accuracy of the 

 measurements permitted. This makes it highly probable that 

 the electrical charge of the micellae is determined by the Donnan 

 equilibrium. We will again call the value 58 (pH inside micellae 

 minus pH outside) the calculated P.D. though it was in reality 

 also observed. The accuracy of the P.D. measurements is not as 

 great as in the case of the experiments of the preceding chapter, 

 for reasons which we have not yet been able to ascertain. 



