UREA, or CARBAMIDE, CO is \ NH; physiologically important compound, being one of the chief nitrogenous decomposition products of animal metabolism. It appears in large quantities in the urine of carnivora; the amount excreted by an adult human being is about one ounce in 24 hours. Urea is also present in small quantities in the blood, milk, perspiration, tears and other animal fluids. It• does not occur to any great extent in the vegetable kingdom, al though it has been detected in certain fungi, and in much smaller quantities in some higher plants.
Urea was discovered by Rouelle in 1773. Of historical interest is its synthesis by WW1ler in 1828, this being practically' the first organic compound prepared by synthetic methods. Willer obtained it from an aqueous solution of ammonium cyanate. Urea may also be pre (1) by treating phosgene (COCh) with ammonia; (2) by heating ammonium carbonate in a sealed tube at 140 (3) by treating cyanamide (CNN,) with dilute sulphuric acid; (4) by passing a mixture of ammonia and carbon dioxide through a red-hot tube. On a commercial scale urea has been obtained (1) by conducting phosgene into a mixture of phenol And dilute sodium hydroxide solution; diphenyl carbonate (CO(OGH3).) is formed which 'upon treatment with dry ammonia yields urea and phenol ; (2) by the action of catalysts, such as oxides or hydroxides of iron, chromium, tin, lead, etc., upon solution of cyanamide under vigorous agitation (Brit. 17,018); (3) by the treatment of a strong solution of a highly soluble cyanide with ammonia and carbon dioxide under special conditions of temperature and pressure (Can. 182,746) ; (4) by heating ammonium carbamate in the presence of one or more catalysts, the carbamate being obtained from solid carbon dioxide and liquid ammonia (Brit. 24,042). Urea may also be extracted from urine by treating the concentrated liquid with oxalic acid, whereby the sparingly soluble oxalate is obtained; 'this is decomposed with calcium carbonate and filtered. The filtrate is
decolorized with animal charcoal, concentrated and allowed to form crystals.
Urea crystallizes in rhombic prisms or dies which melt at 132° C. It is very soluble in water and alcohol, but insoluble in chloroform, ether, or ethyl acetate. Above its melting point it undergoes a series of changes yielding am monia, biuret, cyanuric acid and other products. Nitrous acid, sodium hypochlorite, or sodium hypobromite decompose urea into nitrogen, carbon dioxide, and water. Heated with dilute acids or alkalis it breaks down yielding am monia and carbon dioxide. With acids or salts it forms a series of addition compounds; with certain acids it forms condensation prod ucts or ureids.
The amount of nitrogen excreted in the urine in the form of urea is very often a measure of the extent of the decomposition of albuminoid substances in the body; methods have therefore been worked out by Morner Sjiiquist, Pfliiger-Bleibtreu and others, for the quantitative estimation of nitrogen liberated from urea in a known volume of urine. The urine is first freed from nitrogenous impurities by precipitation with either barium compounds, or with a mixture of hydrochloric and phos pbot ngstic acids. The filtrate containing urea in solution is decomposed into ammonia and carbon dioxide by heating with phosphoric acid, and the total nitrogen set free as ammonia is estimated • by Kjeldahl's method.
Urea has been used to a certain extent in the manufacture of dyes, Benzo-Fast Scarlet being formed from simple derivatives of the compound. Its value as plant food in some instances has been conclusively demonstrate& Its compounds with naphthalene derivatives have been used in therapeutics, and stabilized compounds of urea with starch and hydrogen peroxide have been prepared which give hydro gen peroxide when mixed with water.