562 COLLOIDAL BEHAVIOR OF PROTEINS 



lower, while that of gelatin sulfate was only half as high or not quite 

 half as high as that of gelatin chloride. 



Measurements of the potential differences between the gelatin 

 solution and the outside solution revealed the fact that the curves 

 presenting the p.d. as a function of the hydrogen ion concentration 

 resemble the curves for osmotic pressure. The p.d. curves have 

 a minimum at the isoelectric point, rise steeply with increasing 

 hydrogen ion concentration until a pH of 3.9 is reached, then drop 

 equally steeply again when the pH falls further. Moreover, the max- 

 imum of the P.D. curve for gelatin sulfate is about one-half of that of 

 the maximum of the p.d. curve for gelatin chloride and the p.d. curve 

 for gelatin chloride is about equal to the p.d. curve for gelatin phos- 

 phate, while that of gelatin oxalate is only slightly lower than that 

 of gelatin chloride. 



The next question was whether or not the Nernst formula can ac- 

 count for these differences. The pH of the gelatin solutions and of 

 the outside solutions were measured at the point of equilibrium and 

 it was found that the difference of pH inside minus pH outside mul- 

 tiplied by 58 gave approximately the number of millivolts actually 

 measured. The agreement between the calculated p.d. and the ob- 

 served p.d. was not as perfect as in the case of the salt effect (Table 

 III) but sufficient to leave no doubt that Nernst's theory accounts 

 for these p.d. 



Procter^ has applied the Donnan equilibrium to the theory of swell- 

 ing of gelatin, reaching the conclusion that swelling is an osmotic 

 phenomenon and that the amount of swelling of a gelatin chloride 

 solution is determined by the concentration of the free ions inside the 

 gel minus the concentration of the free ions in the outside solution. 

 He did not measure the pH. By filling this gap the writer was able 

 to satisfy himself that the depressing influence of salts upon the 

 swelling of gelatin is due to a diminution of the difference of pH inside 



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

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



