690 DONNAN EQUILIBRIUM AND MEMBRANE POTENTIALS 



5. The influence of the hydrogen ion concentration on the p.d. of 

 a gelatin chloride solution is similar to that of the hydrogen ion con- 

 centration on the osmotic pressure, swelling, and viscosity of gelatin 

 solutions, and the same is true for the influence of the valency of the 

 anion with which the gelatin is in combination. It is shown that in 

 all these cases the p.d. which exists at equilibrium can be calculated 

 with a fair degree of accuracy from the difference of the pH inside 

 and outside the gelatin solution on the basis of Nernst's logarithmic 

 formula by assuming that the difference in the concentration of 

 hydrogen ions on both sides of the membrane determines the p.d. 



6. The P.D. which exists at the boundary of a gelatin chloride 

 solution and water at the point of equilibrium can also be calculated 

 with a fair degree of accuracy by Nernst's logarithmic formula from 

 the value pCl outside minus pCl inside. This proves that the equa- 

 tion x^ = y { y -\- z) is the correct expression for the Donnan mem- 

 brane equilibrium when solutions of protein-acid salts with mono- 

 valent anion are separated by a collodion membrane from water. 

 In this equation x is the concentration of the H ion (and the mono- 

 valent anion) in the water, y the concentration of the H ion and the 

 monovalent anion of the free acid in the gelatin solution, and z the 

 concentration of the anion in combination with the protein. 



7. The similarity between the variation of p.d. and the variation 

 of the osmotic pressure, swelhng, and viscosity of gelatin, and the 

 fact that the Donnan equilibrium determines the variation in p.d. 

 raise the question whether or not the variations of the osmotic pres- 

 sure, swelUng, and viscosity are also determined by the Donnan 

 equilibrium. 



The measurements referred to in this paper were made by the 

 writer's technical assistants, Mr. M. Kunitz and Mr. N. Wuest, to 

 whom the writer wishes to express his indebtedness. 



