222 Testing Milk and Its Products. 



and allowed to settle) ; the contents of the Jena flasks 

 are now heated to boiling, and the distillation is contin- 

 ued for forty minutes to an hour, until all ammonia 

 has been distilled over. 



The excess of acid in the Erlenmeyer receiving-flask 

 is then accurately titrated back by means of a tenth- 

 normal standard ammonia-solution, using a cochineal- 

 solution 1 as an indicator. From the amount of acid 

 used, the per cent, of nitrogen is obtained; and from it, 

 the per cent, of casein and albumen in the milk by mul- 

 tiplying by 6.25. 2 The amount of nitrogen contained in 

 the chemicals used is determined by blank experiments 

 and deducted from the nitrogen obtained as described. 



Example: The weight of 5 cc. of milk (as obtained in deter- 

 mining the water in the milk) was 5.1465 grams. 5 cc. of stand 

 ard HC1 are added to the receiver, and 1.55 cc. of - - alkali- 

 solution are used in titrating back the excess of acid. 1.55 cc. 



of -^- alkali=^^=.31 cc. acid-solution; the ammonia dis- 



10 5 



tilled over therefore neutralized 5.00 .31=4.69 cc. acid. By 



blank trials it was found that the reagents used furnished an 

 equivalent of .02 cc. acid in the distillate; this quantity sub- 

 tracted from the acid-equivalent of the nitrogen of the milk 

 leaves 4.67 cc. 1 cc. semi-normal HCl-solution corresponds to 

 7 milligrams or .007 gram of nitrogen; 4.67 cc. -^-HCl therefore 

 represents .03269 gram of nitrogen. The quantity of nitrogen 

 was obtained from the 5.1465 grams of milk measured out; the 

 milk therefore contains i^^^-=.635 per cent, of nitrogen, and 

 .635X6.25=3.97 per cent, of casein and albumeto. 



256. Casein and albumen may be determined sepa- 

 rately by Van Slyke's method; 3 10 grams of milk are 



1 Sutton, Volumetric Analysis, 4th edition, p. ttl. 



2 The factor 6.30 or 6.37 is more correct for the albuminoids of milk, 

 but has not yet been generally adopted (p. 15, foot note>. 



8 Bulletin 107, p. 117, Bur. of Ohem., U. S. Dept. of Agriculture. 



