60 DAIRY LABORATORY GUIDE 



volume within twenty-four hours. The supernatant 

 fluid is perfectly clear and gives no trace of proteid 

 by any tests. The solution which seems to be the 

 optimum is composed as follows : 



Phosphotungstic acid, 25 grams; distilled water, 

 125 c.c. 



After thorough solution is obtained there is added : 



Hydrochloric acid (cone), 25 c.c, diluted with 

 distilled water, 100 c.c. 



This yields 250 c.c. of a 10 per cent solution of 

 phosphotxmgstic acid in about 3 per cent HCl. The 

 solution is quite stable if "kept in a dark bottle, and 

 gives satisfactory results after months of standing. 

 It is desirable that the components be mixed as 

 indicated, i.e., the well-diluted HCl added after 

 solution of the phosphotungstic acid in order to avoid 

 precipitation. 



The diluted milk is poured into the tube to the 

 mark U, being careful to read from the bottom of 

 the meniscus. The phosphotungstic acid solution is 

 added to the mark R, the tube corked and slowly 

 inverted twelve times to secure thorough mixing; 

 care being had to avoid shaking roughly and thus 

 mixing air in the fluid. The tube is then placed in 

 a rack for twenty four-hours and the percentage read 

 off at the level of the top of the precipitate. Fractions 

 of per cent between the graduations are readily 

 judged by the eye. At dilutions of 1 part in 10, 

 percentage of proteid is read directly from the scale, 

 while if the dilution be 1 in 20, we multiply the 

 reading by two; if 1 in 5, we divide by two. 



