New York AGRICULTURAL EXPERIMENT STATION. 335 
tion of its details could be easily utilized in devising a method 
of equal accuracy but of much greater simplicity and rapidity. 
The method which has been thus worked out is, in brief, 
as follows: <A given amount of milk, diluted with water, is 
made neutral to phenolphthalein by addition of a solution of 
sodium hydroxid (caustic soda). The casein is then com- 
pletely precipitated by addition of standardized acetic acid; 
the volume of the mixture is made up to 200 cc. by addition 
of water and then filtered. Into 100 ce. of the filtrate a stand- 
ardized solution of sodium hydroxid is run until neutral to 
phenolphthalein. These solutions are so standardized that 1 
cc. 18 equivalent to 1 per ct. of casein when a definite amount 
of milk is used. Therefore, the number of cubic centimeters 
of standard acid used, divided by 2, less the amount of stand- 
ard alkali used in ihe last titration gives the percentage of 
casein in the milk examined. The operation usually requires 
12 to 15 minutes when apparatus and solutions are at hand 
in convenient forms for ready use; several determinations can 
be carried on simultaneously with much relative economy of 
time. 
This method is based upon the following well-known facts: 
(1) Uncombined casein is insoluble in milk-serum, or water, 
or very dilute acids.© (2) It is acid in properties and com- 
bines with alkalis to form definite chemical compounds, which 
are neutral to phenolphthalein. 
Of the total amount of acid used in the process of precipi- 
tating casein, a portion is taken to set casein free from com- 
bination, thus forming a soluble neutral salt and an insoluble 
compound (free casein) possessing the properties of an acid; 
and, on filtration, this amount of acid, as free casein, is re- 
moved from the mixture. The balance of the acid used in 
the process is accounted for in the filtrate on titration with 
alkali. Therefore, the difference between the total amount of 

SN. Y. Agrl. Expt. Sta. Tech. Bul. No. 3, pp. 105-6; Amer. Chem. Jour. 
38: 409-10. 
