JACQUES LOEB 355 



At the isoelectric point of gelatin the value of pH inside minus pH out- 

 side is zero, (2) When the collodion bag contains a solution of 

 gelatin-acid salt, the value pH inside minus pH outside is positive. 

 (3) The value pH inside minus pH outside increases at first when 

 acid is added to isoelectric gelatin with the increase in acid, but soon 

 reaches a maximum and diminishes again upon the addition of further 

 acid. It is shown that the p.d. observed with the Compton electrome- 

 ter between the solution and the water varies in exactly the same 

 way, (4) The addition of a neutral salt to a solution of gelatin 

 chloride at the pH where the observed p.d. is about a maximum 

 diminishes the value of pH inside minus pH outside in the same way 

 as it diminishes the observed p.d. (5) The main fact was that the 

 value 58 (pH inside minus pH outside) agreed quantitatively with 

 the observed p.d. 



These facts show that the p.d. between a gelatin chloride solution 

 and a watery solution (separated by a collodion membrane) is caused 

 exclusively by a difference in the concentration of diffusible ions inside 

 and outside the gelatin solution. If there were a second source for the 

 P.D., the P.D. obtained from the value 58 (pH inside minus pH 

 outside) would be always smaller than the p.d. observed with the aid 

 of a Compton electrometer. The reader will therefore see that the 

 quantitative agreement between the values of 58 (pH inside minus 

 pH outside) with the observed p.d. between the gelatin chloride 

 solution and the outside solution is the essential proof that only the 

 Donnan equihbrium is responsible for the difference of potential 

 between the gelatin chloride solution and an outside watery 

 solution. 



This paper intends to fill out several gaps left in the preceding 

 pubHcation. Thus it was not proven that on the basis of the Donnan 

 equilibrium the gelatin must have a negative charge on the alkali 

 side of the isoelectric point. When we put a solution of Na gelatinate 

 into a collodion bag and dip the bag into water, the Donnan equilib- 

 rium demands that NaOH be expelled from the solution of Na 

 gelatinate through the collodion membrane into the outside solution, 

 and that when equilibrium is established between the solutions 

 of Na gelatinate and water the concentration of NaOH must be 

 greater in the outside watery solution than in the solution of Na 



