INDIRECT METHOD OF PRECIPITATION 73 



binding-capacity of the vitellin reaches a maximum in the higher 

 concentrations of acid. Assuming that this is the true binding- 

 capacity of vitelhn but that the compound which is formed under- 

 goes hydrolytic dissociation according to the balanced equation : 



Protein hydrochloride + water ^ Protein -|- HCl. 



Erb calculates, in the fourth and fifth columns of the above 

 table, respectively, the "theoretical excess" of HCl (unbound) 

 and the percentage hydrolytic dissociation of the protein salt in 

 each mixture. This assumption is, however, unquestionably 

 incorrect. The acid- or base-combining capacity of the proteins 

 is a function of the concentration of acid or base in their solution 

 and not independent of it, as it is in the case of a strong mono- 

 basic acid which is incapable of undergoing polymerization. This 

 is very readily shown in the case of casein, which is insoluble 

 when uncombined and which would therefore be precipitated if 

 its salts underwent appreciable hydrolytic dissociation. If the 

 alkali-binding capacity of casein were independent of the con- 

 centration of alkali in its solution, therefore, the addition of any 

 quantity of alkali beyond that necessary to carry the casein into 

 solution should be without effect upon the number of equivalents 

 of alkali neutralized by one gram of casein. As one increases 

 the amount of alkali in its solution, however, the alkali-binding 

 capacity of casein, as estimated by the potentiometric method, 

 increases from 11.4 X 10"^ equivalents per gram to 180 X 10~^ 

 equivalents per gram, or 16 times the amount of alkali neces- 

 sary to carry it into solution (40). The alkali-binding capacity 

 of casein is therefore not a constant in the presence of varying 

 amounts of alkali. The same considerations apply, of course, 

 to the varying combining-capacity of serum globulin in the 

 presence of a varying excess of acid or alkali (39) . 



In the case of proteins which are soluble in water when un- 

 combined with acids or with bases the demonstration of the fact 

 that their combining-capacities vary with the quantity of acid 

 or alkah in their solutions is not such a simple matter, but it is 

 not difficult to infer, from data to which reference will be made 

 in succeeding chapters, that such variations in combining-capac- 

 ity are an essential feature of the behavior of these proteins also. 



It is obvious that almost any sufficiently neutral salt of an 

 acid which forms insoluble compounds with the proteins might 



