156 



UKEA. 



in petroleum-ether. 1 It possesses a somewhat bitter, cooling taste, 

 resembling saltpetre. 



Preparation, (i) From urine by concentration to a sirupy 

 state, extraction of the residue with absolute alcohol, and concen- 

 tration of the alcoholic extract, by slow spontaneous evaporation 

 in a warm place, until the urea crystallises out. This is then 

 purified by recrystallising from alcohol, decolourising with char- 

 coal if required. Or the urea may be precipitated as nitrate by 

 the addition of pure colourless nitric acid to strongly concentrated 

 urine cooled to 0. The nitrate is then decomposed in water by 

 the addition of barium carbonate, and the urea extracted as before 

 with alcohol, (ii) Synthetically in many ways, of which the 

 most usual and convenient is by mixing equivalent proportions of 

 ammonium sulphate and potassium cyanate; the ammonium 

 cyanate thus formed is evaporated to dryness, whereupon it 

 undergoes a molecular transformation to urea, which is then ex- 

 tracted with alcohol : thus NH 4 . CON = NH 2 . CO . NH 2 . 



It is interesting to note that the above synthesis of urea, obtained 

 in 1828 by Wohler, was the first instance in which a substance 

 ordinarily elaborated by the specific activity of the animal body 

 was artifically prepared. 



Urea readily forms compounds with acids and bases ; of these the 

 following are important as a means of detection and identification. 



Nitrate of urea. (NH 2 ) 2 CO . HN0 3 . 



Obtained by the addition of a slight excess of pure colourless 

 nitric acid to a moderately concentrated solution of urea. The 

 nitrate should separate out rapidly in the form of six-sided or 

 rhombic tables, frequently aggregated in piles, but the successful 

 obtaining of typical crystals requires some attention to the con- 

 centration of the solution. 



FIG. 15. CRYSTALS OF NITRATE OF UREA. (Krukenberg after Kiihne.) 



1 Petroleum-ether consists of the products, with low boiling-points (up to 120), 

 of the distillation of ordinary petroleum. It is also known commercially under the 

 name of ligroin. 



