1256 



UREA. 



but may in other respects be taken as resembling those of the 

 nitrate in respect of their solubilities. 



Of the many salts which urea forms with other bases and 

 salts, those which it yields with mercuric oxide and nitric acid 

 are of most importance. When a solution of mercuric nitrate 

 is added to one of urea a precipitate is formed which, depend- 

 ently upon the concentration and relative amounts of the two 

 solutions, may contain some one of three possible salts, con- 

 sisting of [(NH 2 ) 2 CO] 2 . Hg(NO 3 ) 2 united with 1, 2 or 3 

 molecules of mercuric oxide (HgO). When the solutions are 

 fairly neutral and dilute, the salt with 3 molecules of HgO is 

 formed [(NH 2 ) 2 CO] 2 . Hg(NO 3 ) 2 . 3HgO. This is the salt 

 formed in the reactions on which Liebig's volumetric method 

 for the determination of urea is based. 



FIG. 210. CRYSTALS OF OXALATE OF UREA. (Krukenberg after Kiihne.) 



Urea may be heated dry in a tube to 120 without being 

 decomposed, on further raising the temperature it melts at 

 13-26 and afterwards gives off ammonia, and if heated to 150 

 for some time is converted largely into biuret : 2(NH 2 ) 2 CO 

 = NH 2 . CO . NH . CO(NH 2 ) + NH 3 . On further heating to a 

 higher temperature (200) it is largely converted into cyanuric 

 acid. When biuret is dissolved in water it yields on the addi- 

 tion of caustic soda and dilute sulphate of copper the well- 

 known pink colour employed for the detection of peptones, and 

 hence called the 'biuret reaction.' 



When treated with nitrous acid, e.g. impure yellow nitric 

 acid, it is decomposed finally into carbon dioxide, nitrogen and 

 water: (NH 2 ) 2 CO + 2HNO 2 = CO 2 + 2N 2 + 3H 2 O. A similar 

 decomposition is obtained by the action of sodium hypochlo- 

 rite or hypobromite: (NH 2 ) 2 CO + 3NaBrO = 3NaBr + CO 2 + 

 N 2 + 2H 2 O. Since the volume of nitrogen evolved is constant 



