244 THEORY OF COLLOIDAL BEHAVIOR 



low concentrations of salts than to invent names for the two 

 cases. 



We intend to show that where low concentrations of electro- 

 lytes are required for precipitation, the precipitating influence 

 of the salt has the earmarks of the Donnan effect, inasmuch as 

 the effective ion of the salt has the opposite sign of charge to that 

 of the protein particles, while where high concentrations of salts 

 are required no such relation exists; and we conclude from this 

 that where low concentrations of salts suffice for precipitation, 

 the precipitation is due to the diminution of the value (pH inside 

 the colloidal particles minus pH of the outside solution), as shown 

 in the chapter on the charge of colloidal particles, while where high 

 concentrations of salts are required the precipitating influence is 

 due to some other cause, possibly the weakening of the forces of 

 attraction between protein molecules and the molecules of the 

 solvent, e.g., water, through the presence of the salt. This latter 

 conclusion, of course, would imply that the proteins can exist in 

 true crystalloidal solution, the ultimate units being protein 

 molecules or ions. There is no proof against the permissibility 

 of such an assumption in the case of aqueous solutions of crystal- 

 line egg albumin (at the proper temperature, pH, and concentra- 

 tion) or of gelatin. This opinion is shared by S0rensen, who has 

 not hesitated to determine the molecular weight of crystalline 

 egg albumin from the osmotic pressure of its solution. 1 



2. Solutions of gelatin in water require enormous concentra- 

 tions of salts for precipitation, and this process of salting out has 

 no connection with the Donnan equilibrium, since solutions of 

 gelatin are more readily salted out by sulphates than by chlorides 

 regardless of the pH of the protein solution. The same is true 

 for solutions of crystalline egg albumin of pH 4.7 or above; when, 

 however, the pH of the crystalline egg albumin becomes too low 

 (e.g., 2.0 or less), lower concentrations of salts will cause precipita- 

 tion. The reason for this influence of the pH is mentioned at 

 the end of this chapter. 



Eight-tenths per cent solutions of gelatin were prepared at 

 three different pH, namely 4.7 (isoelectric gelatin), 3.8 (gelatin 

 chloride), and 6.4 to 7.0 (Na gelatinate). The purpose was to 



1 S0RENSEN, S. P. L., Studies on proteins; Compt. rend. trav. Lab. Carlsberg, 

 vol. 12, Copenhagen, 1915-17. 



