URINE AND UREA 119 



until urea nitrate separates on cooling. The precipitated 

 nitrate is filtered off, dissolved in water, and i gram of 

 potassium permanganate added in order to destroy the 

 colouring matter. Barium carbonate is now added to 

 neutralise the excess of acid and decompose the urea 

 nitrate : 



2(CON2H,. HNO3) + BaCOg 



= 2CON2H4+Ba(N03)2+H204-C02. 



The mixture is now evaporated to complete dryness 

 on the water-bath and the residue extracted with 

 alcohol, which removes only the urea. On concentrat- 

 ing the alcoholic solution the urea separates in the form 

 of rhombic prisms, which readily dissolve in water. 



102. Experiments with Urea. 



(a) A small quantity of urea is strongly heated in 

 a porcelain or platinum dish, whereby decomposition 

 takes place, ammonia being liberated and biuret 

 formed : 



2CO(NH2)2 = NH2.CO.NH.CO.NH2+NH3. 



Biuret. 



The residue is dissolved in dilute caustic soda and 

 treated with a few drops of copper-sulphate solution ; a 

 beautiful violet-red coloration is produced. This is the 

 " biuret reaction " frequently used as a characteristic 

 test for the proteins (see 9, c, p. 15). 



{b) A small quantity of urea is heated with caustic- 

 soda solution. It decomposes with the formation of 

 ammonia and carbon dioxide : 



CO(NH2)2+ H2O = CO2 + 2NH3. 



A similar reaction proceeds in the manure heap, on 

 exposing urine to the air and in the soil, under the 



