96 CHEMICAL STATICS 



acid. Increase in temperature increases the rate at which equi- 

 hbrium is approached but decreases the final amount of acid 

 taken up. 



It is evident that the concentration of acid was not sufficient in 

 any of these solutions to bring about complete neutralization of the 

 casein. When more concentrated solutions were employed, how- 

 ever, a proportion of the casein was dissolved, and the methods of 

 estimation utilized by Van Slyke and Van Slyke could not be 

 employed. From considerations which follow below (Cf. Chap. 

 XII), it appears possible that the compounds of casein with acids 

 which are formed in experiments such as these are in reality soluble, 

 but that while a sufficient proportion of casein remains uncom- 

 bined they are mechanically hindered from passmg out of the 

 casein particles into the solvent (38) (40). Van Slyke and Van 

 Slyke, however, believe that the binding of casein by acids, under 

 these conditions, is an "adsorption" phenomenon (60) (61). 



If one stirs excess of casein in 0.1 iV HCl (or more dilute) rapidly 

 and continuously for over an hour, at room temperature very little 

 or none of the casein passes out into the solvent; the solution 

 remains perfectly clear on filtration and its refractive index is only 

 very slightly changed. If, however, the casein be previously 

 dissolved in dilute NaOH and then precipitated with HCl, the 

 addition of the slightest excess of HCl will then carry it into solu- 

 tion. In other words, wet, freshly precipitated casein dissolves 

 rapidly in dilute acid, but the physical properties of dry, granular 

 casein, hinder the compounds which are formed from passing out 

 of the particles.* Dry serum globulin similarly dissolves with 

 extreme slowness in acids, although, when wet and freshly pre- 

 cipitated, it dissolves in dilute solutions of acids with great readi- 

 ness. 



I have determined the acid-equivalent of casein at "saturation" 

 of the acid with protein in the following manner (41): Weighed 

 amounts of casein were dissolved in measured volumes of alkali of 

 known concentration; the excess of acid over that sufficient to 



* The fact that the casein hydrochloride which is formed is in reahty soluble, 

 although it does not pass out into the solution, explains the dependence of the 

 amount formed upon the dilution of the acid, which was observed by Van 

 Slyke and Van Slyke. Were the casein salt insoluble, as these authors point 

 out, the amount formed should be invariable, or else equivalent to the total 

 amount of acid in the system. 



