190 MILK 



The accuracy of the method is shown by the following table: 



COMPARISON OF RESULTS OF OFFICIAL METHOD AND HART'S VOLUMETRIC METHOD 



The volumetric method of Van Slyke and Bosworth is given by 

 Van Slyke as follows: "Into a 200-c.c. flask measure 17.5 c.c. (18 

 grams) of milk, add about 80 c.c. of water, and 1 c.c. of phenol- 

 phthalein solution; after which run in a solution of sodium hy- 

 droxid until the mixture is neutral. Standardized acetic acid is then 

 added until the casein is completely precipitated, the voliune of the 

 mixture is made up to 200 c.c. by addition of water, and the whole 

 is filtered. Into 100 c.c. of the clear filtrate standardized sodium 

 hydroxid solution is run until neutral. The solutions are so 

 standardized that 1 c.c. is equivalent to 1 per cent, of casein in the 

 milk examined. Therefore, the nuinber of cubic centimeters of 

 standard acid used divided by 2, less the amount of standard 

 alkali used in the final titration, gives the percentage of casein 

 in milk. The operation usually requires twelve to fifteen minutes 

 when apparatus and solutions are at hand in convenient form for 

 ready use; several determinations can be carried on at the same 

 time to advantage. 



"The reagents needed in this test are: Sodium hydroxid solu- 

 tion: 10 c.c. standard normal solution are diluted to 1260 c.c. 

 with distilled water. Alkali tablets cannot be used for the test. 



"Acetic acid: The acetic acid solution is diluted so that 1 c.c. 

 will neutralize 1 c.c. of the sodium hydrate solution." 



The apparatus used for the test of Van Slyke and Bosworth 

 are shown in Fig. 68. 



The milk to be tested should not be more than twenty-four 

 hours old and must have been kept cold. Sour milk cannot be 

 used. If the milk cannot be tested fresh, mercuric chlorid must 

 be added in the proportion of 1 : 1000 to 1500. Commercial 

 mercuric chlorid tablets containing coloring-matter cannot be 

 used. 



