250 ION SERIES AND PROTEINS. II 



The determination of the pH of the gelatin in these experiments 

 requires a short discussion since the pH inside the gelatin is quite 

 different from that in the supernatant liquid, owing to the Donnan 

 equilibrium. Donnan has shown that when a solution of a colloidal 

 salt is separated from the solution of a crystalloidal salt with a common 

 ion by a membrane which is permeable for the crystalloidal but not 

 for the colloidal ions the concentration of the crystalloidal salt is, at 

 the point of equilibrium, always lower on the side of the colloidal 

 solution than on the side of the crystalloidal solution.^ This was 

 invariably the case in our experiments on osmotic pressure reported 

 in the preceding paper. When, for example, a gelatin chloride solu- 

 tion of pH 3.5 was put inside a collodion bag and the latter was dipped 

 into a solution of HCl (without gelatin) also of a pH 3.5, the pH on 

 the two sides of the membrane did not remain the same since some of 

 the free acid was forced from the colloidal solution into the pure 

 acid solution outside the collodion bag, so that the pH of the outside 

 solution fell while that of the inside rose. 



As Procter* has pointed out this Donnan equilibrium must play a 

 role also in the case of the swelling of gelatin since in this case the sur- 

 face of the gelatin granule takes the place of the membrane permeable 

 for the crystalloidal electrolyte but not for the colloid. 



In our experiments 1 gm. of originally isoelectric gelatin was put for 

 1 hour at 20°C. into 100 cc. of acid, e.g. HCl, of different concen- 

 tration varying from m/16 to m/8,192. After an hour equilibrium 

 was reached and the pH of the supernatant fluid was determined. 

 The gelatin was put on a filter (after the volume of the gelatin in the 

 graduate cylinder had been measured) and all the acid was allowed 

 to drain off. A trace of outside acid probably remained on the sur- 

 face of each granule though presumably some of the free acid inside 

 the granule diffused to the surface under the influence of pressure. 

 This error was partly but not completely compensated by adding 

 enough distilled water of pH of about 5.6 to the gelatin after it had 



^ Donnan, F.G., Z. Electrochem., 1911, xvii, 572. Donnan, F. G., and Harris, 

 A. B., /. Chem. Soc, 1911, xcix, 1554. Donnan, F. G., and Garner, W. E., /. 

 Chem.. Soc, 1919, cxv, 1313. 



^Procter, H. R., /. Chem. Soc, 1914, cv, 313. Procter, H. R., and Wilson, 

 J. A., /. Chem. Soc, 1916, cix, 307. 



