New York Agricultural Experiment Station. 297 



molecule of casein yielding two molecules of paracasein, and that 

 this splitting of casein is not accompanied by a cleavage of any of the 

 elements contained in the original casein molecule. 



I. THE CAUSE OF ACIDITY OF FRESH MILK OF COWS AND 

 A METHOD FOR THE DETERMINATION OF ACIDITY. 



LUCIUS L. VAN SLYKE and ALFRED W. BOSWORTH. 

 INTRODUCTION. 



The usual method employed in determining the acidity of milk 

 is to add a few drops of a solution of phenolphthalein as indicator 

 to 100 c.c. of milk and then titrate with ^ NaOH. By the use 

 of this method it is found that 100 c.c. of milk, when strictly fresh, 

 will require the addition of 15 to 20 c.c. of the alkali in order to 

 produce a faint but permanent pink coloration. 



The acidity of fresh milk has been commonly attributed to the 

 presence of acid phosphates and casein, and we will now consider 

 the relation of these constituents to milk acidity. 



That the acidity of milk is due to the presence of acid phosphates 

 (MH 2 P0 4 ) is indicated by the fact that milk is strongly alkaline 

 to methyl orange. Further, it is well known that phosphates can 

 not be titrated with any degree of accuracy in the presence of 

 calcium salts, due to the fact that some of the insoluble dical- 

 cium phosphate (CaHP0 4 ), which is formed during the titration, 

 hydrolyzes, changing into calcium hydroxide and phosphoric acid, 

 and then the calcium hydroxide unites with more dicalcium phos- 

 phate, forming tricalcium phosphate (Ca 3 P 2 08). 2 These facts may 

 be represented by the following equations: 



(1) CaHP0 4 + 2 H,0 ^ Ca(OH) 2 + H3PO4 



(2) 2 CaHPO, + Ca(OH) 2 -» Ca 3 P 2 3 + 2 H 2 0. 



That tricalcium phosphate is formed during the titration of any 

 solution containing phosphoric acid and calcium salts is easily 

 demonstrated by an anlysis of the precipitate always appearing; 

 this precipitate is tricalcium phosphate, which is characterized 

 by its appearance, varying from a flocculent to a gelatinous con- 

 dition according to the concentration of the calcium and phos- 

 phates in the solution. 



Dibasic phosphates are neutral to phenolphthalein and mono- 

 phosphates are acid to this indicator; phosphoric acid, therefore, 

 acts as a diabasic acid to phenolphthalein. In the reaction repre- 

 sented above, we have, in place of the original molecule of neutral 

 dicalcium phosphate, one molecule of free phosphoric acid, whereby 



2 Cameron and Hurst. Journ. Amer. Chem. Soc, 26:905. 1904. 



