JACQUES LOEB 713 



unequal distribution of the crystalloidal ions on the opposite sides of 

 the membrane and not on variations in so called colloidal properties 

 of proteins. The quantity which changes with the pH and the 

 valency of the anion of a protein-acid salt is on this assumption not 

 the degree of hydratation or dispersion of the protein particles but 

 the value pH inside minus pH outside, as stated more fully in the 

 two preceding papers.^ 



SUMMARY. 



1. It had been shown in previous publications that the osmotic 

 pressure of a 1 per cent solution of a protein-acid salt varies in a char- 

 acteristic way with the hydrogen ion concentration of the solution, 

 the osmotic pressure having a minimum at the isoelectric point, ris- 

 ing steeply with a decrease in pH until a maximum is reached at pH 

 of 3.4 or 3.5 (in the case of gelatin and crystalline egg albumin), this 

 maximum being followed by a steep drop in the osmotic pressure 

 with a further decrease in the pH of the gelatin or albumin solution. 

 In this paper it is shown that (aside from two minor discrepancies) 

 we can calculate this effect of the pH on the osmotic pressure of a 

 protein-acid salt by assuming that the pH effect is due to that un- 

 equal distribution of crystalloidal ions (in particular free acid) on 

 both sides of the membrane which Donnan's theory of membrane 

 equilibrium demands. 



2. It had been shown in preceding papers that only the valency 

 but not the nature of the ion (aside from its valency) with which a 

 protein is in combination has any effect upon the osmotic pressure of 

 the solution of the protein ; and that the osmotic pressure of a gelatin- 

 acid salt with a monovalent anion {e.g. CI, NO3, acetate, H2PO4, 

 HC2O4, etc.) is about twice or perhaps a trifle more than twice as high 

 as the osmotic pressure of gelatin sulfate where the anion is bivalent; 

 assuming that the pH and gelatin concentrations of all the solutions 

 are the same. 



It is shown in this paper that we can calculate with a fair degree of 

 accuracy this valency effect on the assumption that it is due to the 

 influence of the valency of the anion of a gelatin-acid salt on that 

 relative distribution of the free acid on both sides of the membrane 

 which Donnan's theory of membrane equilibrium demands. 



