CASEIN 89 



earths can be precipitated without alteration of their composition 

 by the addition of alcohol to their solutions in water.* By direct 

 analysis of this precipitate Van Slyke and Hart (59) have shown 

 that the calcium salt which is formed in this way contains 2.4 per 

 cent of calcium oxide or from 80 X 10~^ to 90 X 10~^ equivalents of 

 calcium per gram (7). This salt, in solution, is neutral to phenol- 

 phthalein, and the alcohol precipitate from a lime-water solution of 

 casein which has been neutralized to phenolphthalein contains ex- 

 actly the same percentage of CaO (unneutralized by mineral acidsj 

 as that from the solution of casein in a carbonate-solution. Similar 

 results had previously been obtained by Soldner (51) (52), de Jager 

 (18) and Laquer and Sackur (22), who, however, employed volu- 

 metric methods of measurement. Other figures for the percentage 

 of CaO in this compound which have been obtained by various 

 observers are: Courant (12), 2.91 and Timpe (54), 2.62. These 

 figures are not very divergent, and since the great majority of 

 observers agree in placing the percentage at 2.4, we may assume 

 that this result is not far from correct. The percentage of casein 

 in alkaline solutions can be fairly accurately determined by titra- 

 tion to neutrality to phenolphthalein, upon the assumption that in 

 solutions of this alkalinity (OH concentration) casein is combined 

 with 2.4 per cent of CaO or with an equivalent quantity of other 

 (strong) bases. 



If, however, a lime-water solution of casein be neutralized to 

 litmus instead of to phenolphthalein, by the addition of mineral 

 acid or of casein, the alcoholic precipitate now contains only 1.5 

 per cent of CaO (Van Slyke and Hart (59)). The same estimate of 

 the combining-capacity of casein at absolute neutrality (i.e., in 

 solutions neutral to litmus) has been made by Soldner (51) (52) 

 and by myself (42) employing the potentiometric method. The 

 casein from goats' milk combines with bases in the same propor- 

 tions as the casein from cows' milk (9). The compound of casein 

 with 2.4 per cent of CaO, or an equivalent quantity of any other 

 strong base, is termed by Soldner and by Van Slyke and Hart the 

 "basic" caseinate, by Courant the " tricaseinate " of the base. 

 That with 1.5 per cent of CaO, or an equivalent quantity of other 

 strong bases is termed by Soldner and by Van Slyke and Hart the 



* Cf. Chap. II. The "basic" salt of potassium (i.e., neutral to phenol- 

 phthalein) is soluble in 98.6 per cent alcohol, but it may be precipitated from 

 this solution by the addition of ether (45). 



