COMBINING CAPACITY 201 



has reacted with casein to dissolve it, we may suppose that one 



N.HOC group (or two such groups, if dicarboxylic radicals 

 are involved) in each molecule of casein has been opened up, 

 in accordance with the equation: jj 



^ ' ++ 

 -N.HOC- + KOH = - ^N + KOC- 



I 

 OH 



From the Guldberg and Waage mass law we know that if this 

 equation represents the true nature of the reaction which occurs, 

 then the mass of caseinate formed must be dependent (the tem- 

 perature and other conditions of the reaction being constant) 

 only upon the reacting masses of free casein and KOH and not 

 upon the total dilution of the system. At the completion of the 

 reaction these masses are both very small, since free casein is 

 inappreciably soluble in water and the solution of this salt of 

 casein is acid in reaction (Cf. Chap. V). Upon further addition 

 of alkali, however, another N.HOC group (or pair of groups) 

 is opened up and when another 11.4 X 10~ 5 equivalents of alkali 

 have been added per gram of casein we have now four ions of 

 caseinate instead of two. Whether these ions are derived from 

 one, or from two, molecules of caseinate these data do not enable 

 us to decide. As the tables show, the solution of this salt is also 

 acid to litmus. Upon still further addition of alkali yet another 



N.HOC group (or pair of groups) in each casein molecule 

 is opened up, and when 22.8 X 10~ 5 more equivalents of KOH 

 (= 45.6 X 10~ 5 per gram in all) have been added all of the ions 

 of caseinate have again been split in half. The solution of this 

 salt is, as we shall see, almost exactly neutral to litmus. Again 

 doubling the alkali-content the ions will be split again, yielding 

 a salt containing 91.2 X 10~ 5 equivalents of KOH per gram, the 

 solution of which, as the above tables reveal, is alkaline to phenol- 

 phthalein (62) and which, therefore, presumably, requires the 

 presence of a slight excess of KOH to maintain its stability, i.e., 

 to push the equilibrium in the above reaction sufficiently over 

 to the right. We may suppose that this type of reaction is re- 

 peated until the last N.HOC group that is dissociable by 

 alkali has been opened up, when the combining capacity of the 

 casein reaches, as we have seen, a constant maximal value. This 

 maximum value of the combining capacity must necessarily, if 



