MEMBRANE POTENTIALS 



123 



measured in this way was, therefore, the electromotive force 

 of the following cell, 



It is found that in this cell the gelatin solution has a positive 

 charge and the outside solution a negative charge and that the 

 P.D. varies with the pH of the gelatin chloride solution, as indi- 

 cated in Fig. 41. It is also found that the P.D. of gelatin phos- 

 phate solutions is practically identical with the P.D. of gelatin 

 chloride solutions of the same pH and that both are considerably 

 higher (about 50 per cent higher, as we shall see) than the P.D. of 

 gelatin sulphate solutions. We shall also see that the addition 

 of a neutral salt to the gelatin chloride solution depresses the 

 P.D. In other words, electrolytes influence the P.D. between 

 gelatin chloride solution and outside solution in a way similar 

 to that in which they influence the osmotic pressure and the 

 viscosity of the same solution. It becomes, therefore, of con- 

 siderable importance to find out the origin of this P.D. We 

 intend to show that the P.D. is due to the establishment of a 

 Donnan equilibrium between the gelatin chloride solution and the 

 outside aqueous solution (free from gelatin). 



We have already given a brief outline of Donnan 's membrane 

 theory in the first chapter. In our experiment a collodion bag 

 filled with a 1 per cent solution of gelatin chloride is dipped into 

 a beaker containing a solution of HC1 (without gelatin) of origin- 

 ally the same pH as that of the gelatin solution. In this case we 

 have free HC1 inside as well as outside, but in addition we have 

 inside the collodion bag a gelatin chloride solution which ionizes 

 into Cl and a positive gelatin ion. The gelatin ion is unable to 

 diffuse through the collodion membrane but the H ions and Cl 

 ions can diffuse freely through the membrane. Donnan has 

 shown that in this case an equilibrium condition is established 

 in which the product of the concentrations of the H and Cl ions 

 in the outside solution equals the product of the concentrations 

 of the H and Cl ions inside. This equilibrium is expressed by 



