UREA, 581 



when heated with concentrated sulphuric acid, become oxidised, and all the 

 nitrogen (except such as may be originally present in combination with 

 oxygen) is converted into ammonia. The resulting ammonia is liberated 

 by the addition of caustic alkali, and distilled into a measured quantity of 

 standard acid ; its amount being finally determined by titration. Kjeldahl's 

 method gives admirable results with urine, and may be applied to 5 c.c. of 

 the fluid. 



(b) Urea CO(NH 2 ) 2 . The presence of urea in the urine was first 

 demonstrated by Eouelle in 1773. It is the chief end-product of nitro- 

 genous metabolism in all mammals, in amphibia, and in fishes. In 1828 

 it was obtained artifically by Wohler, by heating the isomeric substance 

 ammonium cyanate (NH 4 .CNO). 



The chemical constitution of urea is that of an amide of carbonic 

 acid (carbamide). 



Properties. Urea crystallises in colourless needles or rhombic prisms, 

 containing no water of crystallisation, and melting at about 130 C. It 

 is freely soluble in alcohol, and still more so in water ; in pure ether or 

 chloroform it is insoluble. Urea, like other amides of acids, is neutral 

 to litmus ; but, owing to the presence of two ammonia residues in its 

 molecule, it exhibits weak basic properties, and forms loose molecular 

 compounds, analogous to salts, two of which are of practical im- 

 portance. 



Urea nitrate = CO(NH 2 ) 2 .N0 2 .OH. This compound crystallises out 

 when excess of pure nitric acid is added to a not too weak solution of 

 urea ; excess of the acid assists its separation, as it is less soluble in 

 nitric acid than in water ; crystallisation is accelerated by shaking and 

 cooling the mixture. The fundamental form of the crystals is a rhombic 

 table, of which the more acute angles measure 82 ; but, by truncation 

 of the angles, six-sided tablets are commonly formed, and these are apt 

 to adhere together and overlap like tiles on a roof (Fig. 49). When 

 rapidly heated, the crystals deflagrate. At 140 they decompose into 

 nitrous oxide, carbon dioxide, and ammonium nitrate. 



Urea oxalate = CO(NH 2 ) 2 .(COOH) 2 is formed in an analogous 

 manner by mixing solutions of urea and oxalic acid ; like the nitrate, 

 this salt is less soluble in excess of the acid. Its crystals belong funda- 

 mentally to the same type as those of the preceding compound, but are 

 apt to appear as thick short rhombic prisms (Fig. 49). 



A crystalline combination of urea with phosphoric acid is also known, and 

 others with various organic and inorganic acids. 



Crystalline compounds are also formed with certain neutral salts; that 

 with sodium chloride is said occasionally to form when urine is concentrated 

 on the water bath. The compound with palladium chloride is very insoluble. 

 A molecular combination with basic mercury nitrate is quite insoluble in water, 

 and is of historic interest, as its formation is the basis of the classical method 

 of urea estimation suggested by Liebig in 1853 (vide infra). 



When fused and gently heated after fusion, urea yields biuret and 

 cyanuric acid. Two reactions occur as follows : 



2NH 2 .CO.NH 2 - NH 2 .CO.NH.CO.NH 2 + H 3 N ; and 

 (biuret) 



3NH 2 .CO.NH 2 - C 3 N 3 (OH) 3 + 3(H 3 N) 



(cyanuric acid) 



