177 



of ee. acid used in liberating the casein. Since a quantity of milk has been 

 taken so that each cc. of acid used equals 1 per cent, casein, then each 

 cc. represents 1 per cent, of casein in the sample of milk. For example, it 

 required 9.4 cc. to neutralize 100 cc. of the filtrate, and since it represented 

 12.5 cc. of the acid added to the 200 cc. of the diluted milk, we have 12.5- 

 ^ 9.4 = 3.10 per cent, casein. 



Below are some of Van Slyke's results obtained by this method in com- 

 parison with the official method. 



PERCEXT CASEIX. 



Voh metric Method 



(Van Slyke-Boswortk). Official Method. 



3.00 3.00 



3.40 3.36 



3.30 3.21 



3.20 3.16 



2.90 2.95 



2 . 70 2.60 



The second volumetric method which I wish to consider is that of E. B. 

 Hart, of the University of Wisconsin, published in Research Bulletin, Xo. 

 10, 1910. For speed and accuracy this method offers no advantage over 

 that of Van Slyke's and Bosworth's, just mentioned. However, the method 

 is unique and sound in principle. The fact that free casein has the properties 

 of an acid and can combine with an alkali in a definite proportion, it seems 

 rational that if we dissolve casein in excess of alkali and the uncombined 

 alkali is estimated by titration, using phenolpthalein as an indicator, we 

 are in a position to calculate the casein equivalent per cc. of standard alkali 

 used. This is true, and upon this principle rests Hart's volumetric method. 

 Hart found the casein equivalent for each 1 cc. ^ KOH to be . 10S grams. 

 Therefore, if we titrate the casein obtained from 10.8 grams of milk, we see 

 that each cc. of alkali used must represent 1 per cent, of casein. 



Details of the method. Measure 10.5 cc. or weigh 10.8 grams of milk 

 into a 200 cc. Erlenmeyer flask, add 75 cc. of distilled water at room tem- 

 perature and add to this 1 to 1.5 cc. of a 10 per cent, solution of acetic acid. 

 The flask is given a quick rotary motion, usually 1.5 cc. of acetic acid gives 



5084 — 12 



