156 



UREA. 



Urea is very soluble in cold water, distinctly less soluble in cold 

 alcohol, readily so in hot; it is insoluble in anhydrous ether and in 



FIG. 14. UREA CRYSTALS SEPARATED BY SLOW EVAPORATION FROM 

 AQUEOUS SOLUTION. (After Funke.) 



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

 sembling saltpetre. 



Preparation, (i) From urine by concentration to a syrupy state, 

 extraction of the residue with absolute alcohol and concentration of the 

 alcoholic extract, by slow spontaneous evaporation in a warm place, 

 until the urea crystallises out. This is then purified by recrystallisiiig 

 from alcohol, decolourising with charcoal 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 pro- 

 portions 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 extracted 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 

 artificially prepared. 



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

 following are important as a means of detection and identification. 



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. 



