8 PHYSIOLOGICAL AND PATHOLOGICAL CHEMISTRY. 



part of the filtrate is made alkaline with ammonia, acidified 

 with acetic acid and then divided into two parts; to one part, 

 in order to prove the presence of calcium, add ammonium 

 oxalate (white precipitate of calcium oxalate); to the other 

 part add uranyl nitrate (yellowish-white precipitate of uranyl 

 phosphate). The phosphoric acid may also be tested for 

 directly in the hydrochloric acid solution by means of ammo- 

 nium molybdate. Add a few drops of the hydrochloric acid 

 solution to some cubic centimeters of the ammonium molyb- 

 date solution: yellow precipitate. 



3. Casein (C). 



In order to purify the casein and especially to free it from 

 the fat, put it into a dish with 250 cc. of water and add very 

 dilute caustic soda solution (1 : 10) with constant stirring and 

 quite slowly. The mixture must at no time have a strong 

 alkaline reaction. When the greater part of the casein has 

 dissolved, filter. The filtrate is usually somewhat cloudy. 

 If necessary it may be again filtered; absolute clearness of the 

 filtrate is, however, only attained with difficulty. The solu- 

 tion is precipitated by acidifying cautiously with acetic acid. 

 The casein which separates is first washed by decantation r 

 which may usually be done without any essential loss, then 

 filtered and washed. 



This is used in the following reactions: 



1. Since casein is essentially a proteid, it gives all the 

 reactions of the coagulated and insoluble proteids described 

 under albumin. 



2. A portion is shaken with water and a few drops of 

 sodium carbonate solution ; it dissolves therein clear or almost 

 clear. If the solution is quite cloudy (fat, calcium phosphate), 

 then the casein must be redissolved in water containing 

 sodium hydroxide and be precipitated once more with acetic 

 acid. 



