CASEIN 93 



22.5 X 10~^ equivalents per gram, or exactly double the pro- 

 portion of alkaline earth contained in the insoluble compound. 

 The insoluble compound will, however, dissolve in solutions of 

 NaCl, NH4CI, KCl, etc., probably owing to exchange of bases, 

 with the formation of soluble caseinates of the alkalies (56) (57). 



Magnesium hydrate forms a similar series of compounds with 

 casein, namely a compound insoluble in water but soluble in 5 per 

 cent NaCl solution and containing 11.25 X 10~^ equivalents of 

 base per gram, a soluble compound containing 22.5 X 10~^ 

 equivalents of base per gram, a compound neutral to litmus 

 containing about 56 X 10~^ equivalents of magnesium per gram 

 and a compound neutral to phenolphthalein containing between 

 80 XlO~^ and 90 X 10~^ equivalents of magnesium per gram (62). 



From the phosphorus content of casein (8), we would infer a 

 minimal combining- weight of 4430. The combining- weight corre- 

 sponding to the proportion of NaOH or KOH combined with casein 

 at "saturation" of the base with protein is 8888. This corresponds 

 so closely with twice the minimal combining-weight calculated on 

 the assumption that the casein molecule contains only one atom 

 of phosphorus that we may infer, with Bosworth and Van Slyke, 

 that the molecule of casein contains two atoms of phosphorus. 



The maximum combining-capacity of casein for bases (KOH) 

 has been measured by the author (42), employing the potentio- 

 metric method. In the presence of a considerable excess of KOH, 

 the combining-capacity of casein attains the constant maximum of 

 180 X 10~^ equivalents per gram. In passing from its minimum 

 to its maximum, therefore, the combining-capacity of casein is 

 multiplied sixteen times. 



The compounds of casein with acids have not been so extensively 

 studied as those with bases. Hanunarsten (14) held that there is 

 no true combination between casein and acid, because by pro- 

 longed trituration with water he could remove all of the acid 

 contained in the casein. Our modern conception of balanced 

 reactions deprives this consideration of its weight (59) (36) . Since, 

 however, it has been frequently applied to other protein com- 

 pounds,* it may be worth while to dwell briefly upon the impUca- 

 tions of such a deduction. 



* For example by Hans Trunkel (55): When so much of the published 

 evidence on behalf of the "adsorption theory" is of this unfortunate and 

 inconclusive character, advocates of "adsorption" cannot complain if their 

 hypotheses are viewed with scepticism by physical chemists. 



