158 UREA. 



out in rhombic tables closely resembling those of the nitrate, but they 

 are frequently aggregated into a characteristic prismatic form. As in 

 the case of the nitrate some care is required with respect to the 

 concentration of the respective solutions during its preparation. 



The crystals are less soluble in oxalic acid than in water, 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, dependently upon the concentra- 

 tion and relative amounts of the two solutions, may contain some one of 

 three possible salts, consisting of [(NH a ) 2 C0] 2 . Hg (NO 3 ) 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 (N0 8 ) a . 3 HgO. This is the salt formed in the re- 

 actions on which Liebig's volumetric method for the determination of 

 urea is based. 



The other more important reactions of Urea. 



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

 decomposed, on further raising the temperature it melts at 132-6 01 

 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 a ) + NH 3 . On further heating to a higher temperature (200) it 

 is largely converted into cyanuric acid. When biuret is dissolved in 

 water and treated with caustic soda and dilute sulphate of copper it 

 yields the well-known pink colour employed for the detection of 

 peptones, and hence called the 'biuret reaction.' In the application of 

 the test to urea some caution is requisite while heating the suspected 

 substance to avoid carrying the decomposition beyond the biuret 

 stage. When boiled in aqueous solution with strong sulphuric 

 acid or alkalis it is gradually decomposed under assumption of two 

 molecules of water, into carbonic acid and ammonia ; the same 

 decomposition ensues by simple heating of the aqueous solution in 

 sealed tubes, to 180. This forms the basis for the older 'Bunsen 

 method ' of estimating urea. A similar change (hydration) is produced 

 under the influence of several micro-organisms which are found in urine 

 undergoing alkaline fermentation. Of these the best known is the 



1 Keissert, Ber. d. d. chem. Gesell. Bd. xxm. (1890), S. 2244. 



