JACQUES LOEB 251 



been melted to bring the volume to 100 cc. The pH of the 1 per cent 

 solution of originally isoelectric gelatin was determined colorimetri- 

 cally. It was found that the pH of the supernatant HCl solution was 

 always considerably smaller than the pH of the gelatin solution 

 (Table I). 



The first row in Table I gives the molecular concentration of the 

 100 cc. of HCl into which the gelatin was originally put. The second 

 row gives the pH of these supernatant HCl solutions after 1 hour, and 

 the third row gives the pH of the gelatin solutions after the supernat- 

 ant HCl solution had been drained off and after the remaining mass 

 of gelatin had been melted and brought to a volume of 100 cc. by add- 

 ing enough distilled water of pH 5.6. It will be noticed: first, that 

 the pH of the supernatant HCl solution after 1 hour is higher than the 

 pH of the original HCl solution owing to the fact that some acid com- 

 bined with the gelatin; and, second, that the pH of the gelatin solu- 

 tion is considerably higher than that of the supernatant solution owing 

 to the Donnan equihbrium, according to which the concentration of 

 free acid outside the gelatin must be greater than inside. 



In order to get the correct difference due to the Donnan equilibrium, 

 solutions of gelatin salts were put into collodion bags and these bags 

 were dipped into beakers containing 350 cc. of solution of the same acid 

 or base as that inside the collodion bag and possessing the same pH as 

 the gelatin solution; the outside solution, of course, was free from gela- 

 tin. In the case of gelatin-acid salts free acid invariably diffused 

 from the gelatin solution into the pure acid solution, e.g. HCl, so that 

 the pH of the latter became smaller and that in the gelatin solution 

 higher. In these experiments the volume of the outside pure HCl 

 solution was 350 cc. and that of the inside 1 per cent gelatin solution 

 only 50 cc. Table II gives the result of one experiment of this kind. 



Thus at equilibrium pH the gelatin solution was 3.8 and the out- 

 side solution 3.2, or inside 3.3, and outside 4.0. This difference is in 

 the same sense and of nearly but not quite the same order of magni- 

 tude as that observed in Table I. 



We may therefore conclude that the real pH of the gelatin solution 

 inside the granules of gelatin was slightly less than that measured by 

 our method. 



