370 ELECTRICAL CHARGES OF COLLOIDAL PARTICLES 



tried to explain this on the basis of differences in the electrolytic 

 dissociation in the watery and the water-immiscible (oily) phase. 

 Such an explanation cannot be applied to the experiments with 

 protein solutions and yet these latter solutions also show the de- 

 pressing effect of the addition of salt on the p.d. in a most striking 

 way. In this latter case the depressing effect of the salt on the p.d. 

 is due to the Donnan equilibrium and there is no reason why the 

 theory of membrane equilibria should not apply to the p.d. between 

 oily and watery phases since this theory only demands that one ion 

 of the oily phase should be prevented from migrating into the watery 

 phase. Any lipoid ion would fulfill this postulate of the theory. The 

 peculiarities of electrolytic dissociation found by Beutner in non- 

 aqueous solutions must, however, influence the Donnan equilibrium in 

 a secondary way since this equilibrium depends on ionization. 



SUMMARY and CONCLUSIONS. 



1. When a solution of a salt of gelatin or crystalline egg albumin 

 is separated by a collodion membrane from a watery solution (free 

 from protein) a potential difference is set up across the membrane in 

 which the protein is positively charged in the case of protein-acid 

 salts and in which the protein is negatively charged in the case of 

 metal proteinates. The turning point is the isoelectric point of the 

 protein. 



2. Measurements of the pH of the (inside) protein solution and of 

 the outside watery solution show that when equilibrium is established 

 the value pH inside minus pH outside is positive in the case of protein- 

 acid salts and negative in the case of metal proteinates. This is to be 

 expected when the p.d. is caused by the establishment of a Donnan 

 equilibrium, since in that case the pH should be lower outside than 

 inside in the case of a protein-acid salt and should be higher outside 

 than inside in the case of a metal proteinate. 



3. At the isoelectric point where the electrical charge is zero the 

 value of pH inside minus pH outside becomes also zero. 



4. It is shown that a p. d. is established between suspended particles 

 of powdered gelatin and the surrounding watery solution and that the 

 sign of charge of the particles is positive when they contain gelatin- 

 acid salts, while it is negative when the powdered particles con- 

 tain metal gelatinate. At the isoelectric point the charge is zero. 



