UREA or CARBAMIDE, a colourless crystalline substance known for about 150 years as a constituent of urine. It results from the oxidation of certain nitrogenous materials—proteins (q.v.)--in the animal organism and is not only present in the urine of higher animals (mammals) but also in their blood, bile, milk, perspiration and other body fluids. Moreover, it has been recog nised in many lower forms of animal life—coelenterates, echin oderms, insects, worms, crustaceans, molluscs and fish, and has been identified in certain plants and in fungi and moulds, in the leaves of endive, potato, spinach, carrot and turnip, and in the seedlings of cereals and leguminates. Human urine contains 2 to 5% of urea which is excreted by an adult man to the extent of about 3o grams (one ounce) daily.
In 1773 H. M. Rouelle obtained, by alcoholic extraction of the solid residue from evaporated urine, an alkaline nitrogenous sub stance which on bacterial fermentation yielded carbonic acid and ammonia. Subsequently A. F. Fourcroy and Vauquelin (1798) prepared urea nitrate from urine and the isolation of pure urea from this liquid was first accomplished by M. Proust in 1821. F. \Willer in 1828 announced his famous discovery of the artificial formation of urea from ammonia and cyanic acid. Potassium cyanate, obtained by fusing potassium ferrocyanide with red lead or manganese dioxide, was mixed with ammonium sulphate, and the solution evaporated to dryness, when an alcoholic extract of the residue yielded urea.
The isolation of urea from urine is most conveniently effected through its nitrate or oxalate, which are two of its most charac teristic salts. Urea crystallises in long rhombic needles or prisms having a cooling taste. It is readily soluble in water or alcohol but not in ether. It melts at 132° C and may be vaporised in vacuo, but when more strongly heated it yields ammonia, biuret, ammelide and cyanuric acid.
Commercial Synthesis of Urea and its Derivatives.—Urea contains 46% of combined nitrogen in a form which is easily con verted into ammonia in the soil, and it is therefore a much more concentrated nitrogenous fertiliser than either ammonium sulphate or sodium nitrate. It is, moreover, easy to handle and stable
under ordinary factory and farm conditions, so that it possesses advantages over calcium cyanamide and calcium nitrate. Hence it is now manufactured on an extensive scale for use as an artificial manure. (See FERTILIZERS.) A century ago the synthetic production of an ounce of urea was a noteworthy scientific achievement ; at present a daily output of zoo tons is the routine practice of a modern fertiliser plant. The following processes are available : The regulated acidic hydrolysis of cyanamide (made from calcium cyanamide) in pres ence of an iron oxide as catalyst, =0 :C(NH2)2. (2) The heating under pressures of 33-53 atmospheres of ammo nium carbonate obtained from carbon dioxide and ammonia, NH2.0O2.NH4,=11120+NH2.CONH2: equilibrium is reached at 130--150° C with yields of 30-45% of urea. (3) Dry ammonia is passed into phenyl carbonate obtained from carbonyl chloride and phenol in presence of alkali, the phenol is recovered and utilised again. (4) The process of treating carbonyl chloride with ammonia is more practicable for urea derivatives than for urea itself. Thus, carbonyl chloride and ethyl aniline aive diphenyldiethylcarbamide, a crystalline substance known as "Centralite" and employed as a stabiliser in smokeless gunpowders.
By heating together urea and p-aminoacetanilide a diacetyl carbamide is produced which on hydrolysis yields pp-diamino diphenylcarbamide, a diamine giving rise to important azo-dyes such as benzo fast yellow. (See DYES, SYNTHETIC.) Applications of Urea and its Derivatives.—For use as a fertiliser, urea is frequently employed in combination with cal cium hydrogen phosphate (superphosphate), the product being known as "Phosphazote." Its complex salt with calcium nitrate is also recommended for agricultural purposes.