THE STABILITY OF PROTEIN SOLUTIONS 261 



salts with a cation of higher valency a concentration of almost 

 M/500. 



We, therefore, find that for the flocculation of Na gelatinate, 

 originally of pH 10.0, salts with bivalent cation are about 20 

 times as efficient as salts with monovalent cation. This is 

 roughly in harmony with the relative influence of these ions on 

 the osmotic pressure of solutions of Na gelatinate, where the 

 efficiency of a M/ 16 solution of NaCl is equaled by that of a M/512 

 solution of CaCl 2 (Fig. 37). 



Schulze, Linder and Picton, and Hardy found that the precipi- 

 tating ion has the opposite sign of charge to that of the colloidal 

 particle and that the precipitating efficiency of the ion increases 

 with its valency. These experiments suggest that the rule of 

 Schulze, Linder and Picton, and Hardy is only a consequence 

 of the Donnan equilibrium. 



6. If the osmotic pressure set up between coalescing protein 

 ions is able to prevent the formation of new micellae and thus to 

 contribute towards the stabilization of a solution of gelatin in a 

 solution of much alcohol and little water, we can predict another 

 result which will become clear from Fig. 77. This figure repre- 

 sents the influence of different concentrations of NaCl on the 

 osmotic pressure of 1 per cent solutions of originally isoelectric 

 gelatin brought to pH 1.8, 4.1, and 3.1 by the addition of different 

 quantities of HC1. It is obvious from the curves that it requires 

 a higher concentration of NaCl to bring the osmotic pressure of 

 the gelatin chloride solution to the same low value, e.g., 125 mm., 

 when the pH is 3.1 than when it is 4.1. At pH 3.1 the concen- 

 tration of NaCl must be between M/64 and M/128 and at pH 

 4.1 the concentration can be less than M/512. At pH 1.8 no 

 addition of salt is required since the osmotic pressure is already 

 below 125 mm. If it be true that the difference of osmotic 

 pressure between the inside of the nascent micellae and the sur- 

 rounding solution is one of the forces guaranteeing the stability 

 of the solution of gelatin in an alcohol-water mixture when the 

 critical limit of alcohol is exceeded, it is obvious that the con- 

 centration of salt required for flocculation should vary with the 

 original pH of the gelatin solution in the way characteristic for 

 the Donnan equilibrium, namely that near the isoelectric point 

 little or no salt should be required for precipitation, that with 



