METHODS OF TESTING MILK FOR CASEIN 19! 



(9) Measuring-cylinders, accurately graduated and 

 holding 1000 cc. 



Solution. Three solutions are used: (i) Sodium 

 hydroxide, (2) acetic acid and (3) phenolphthalein. 



(1) Sodium hydroxide (caustic soda). This solu- 

 tion is most conveniently prepared by purchasing from 

 a reliable chemical-supply house the standard normal 

 solution and then diluting exactly 100 cc. of this to 

 1260 cc. (or 79.5 cc. to 1000 cc.), using pure distilled 

 water, if possible, or else as pure rain water as is ob- 

 tainable. Alkali solutions must be kept in tightly- 

 stopperel bottles (p. 138). "Alkaline tablets" (p. 142) 

 cannot be used for the casein test. 



(2) Acetic acid. This solution is so made that i cc. 

 will neutralize i cc. of the alkali solution. The simplest 

 method of preparation is to purchase a standard nor- 

 mal solution and dilute 100 cc. to 1260 cc. (or 79.5 cc. 

 to 1000 cc.). Add a small amount of dry mercuric 

 chloride (corrosive sublimate) to prevent fermentation, 

 and keep the solution in tightly-stoppered bottles. 



(3) Phenolphthalein. Dissolve i gram of the dry 

 powder in 100 cc. of 50 per cent, alcohol, adding one 

 or more drops of dilute alkali until the solution is very 

 slightly pinkish in color. 



Performing the test. For convenience, the pro- 

 cess can be divided into six stages or operations: (i) 

 Measuring and diluting sample of milk, (2) neutraliz- 

 ing the milk, (3) precipitation of casein, (4) filtration 

 of casein, (5) titration with alkali, (6) calculation of 

 results. 



(i) Measuring and diluting sample of milk. The 

 milk to be tested is well mixed and a 17.6 cc. pipette 



