DEPRESSION OF THE FREEZING-POINT 237 



Protein Salts. — We have hitherto found it convenient in dis- 

 cussing the mode of formation of the salts which the proteins 

 form with bases, only to consider the consequences which arise 

 out of the opening up of a single — COH.N— bond, without 

 involving a consideration of the question whether this —COH.N — 

 bond is supplied by a dicarboxylic acid group (such as glutamic 

 acid), in which case two such bonds must be opened up before 

 actual electrolytic dissociation can occur, or whether it is supplied 

 by a mono-carboxylic acid group (such as glycocoll) in which 

 case dissociation could occur directly one such group was opened 

 up. We have been able to do this because, in the formation of 

 salts with bases, all of the —COH.N— groups which are opened 

 up suffer the introduction of the basic radical and the equiva- 

 lence between the protein salt and the base which gives rise to 

 it is the same as that which would subsist were the salt formed 

 by the opening up of only one —COH.N— group. All of the 

 preceding considerations and theoretical deductions, therefore, 

 remain equally valid whether the —COH.N— groups under 

 consideration in each separate case are derived from a dicarboxylic 

 acid radical or from a mono-carboxylic acid radical. Only, so 

 far as our experimental knowledge goes, when we investigate 

 the freezing-point depression (i.e., the osmotic pressure) of solu- 

 tions of protein salts do we encounter facts which are definitely 

 irreconcilable with the view that the protein salts with bases 

 • are formed through the agency of mono-carboxylic acid radicals 

 and which point unmistakably to the dicarboxylic acid radicals 

 as the active agents in accomplishing these unions. 



It has been shown by Robertson and Burnett (20) that whether 

 the caseinate formed is neutral to litmus and contains 50 X 10"^^ 

 equivalents of base per gram, or neutral to phenolphthalein and 

 containing 80 X 10~^ equivalents of base per gram, the freezing- 

 point depression which is brought about by the dissolved protein salt 

 hears the same proportion to the concentration of the neutralized base. 



The significance of this observation has been alluded to in 

 the latter part of Chap. I. If we prepare a number of solutions 

 all containing the same amount of a base and add to these vary- 

 ing amounts of a polybasic acid R(COOH)„, such that in the 

 first solution only one, in the second two, etc., — COOH groups 

 are neutralized, then, if all these salts are highly dissociated, 

 calling the osmotic pressure of the first 1, that of the second 



