CO + = ,CO, Urea Carbonate of Ammonia.
A similar change takes place at ordinary tem peratures in urine exposed for some time to the air. Here the transformation is effected through a process of fermentation caused by certain micro-organisms producing, when killed, an enzyme that hydrolyzes urea with great rapidity. This enzyme is probably secreted also by the mucous membrane of the bladder in certain in flammatory conditions, the fermentation then taking place while the urine is still in the bladder.
The following are some of the most important of the compounds of urea: Nitrate of area, and oxalate of area, 2C0 are readily crystallizable salts, formed by the direct addition of the respective acids to a moderately strong solution of urea. As nitrate of urea requires eight parts of cold water, and is still more insoluble if an excess of free nitric acid is present, and the oxalate is more insoluble than the nitrate, while urea dis solves in its own weight of water, these salts may be employed to test for and approximately deter mine the quantity of urea. Among the com pounds of urea with metallic oxides, those which it forms with the oxides of mercury are especially interesting, and have been completely examined by Liebig. A result of his researches is his celebrated method, which is now in universal Ilse. of determining the amount of circa volumetrical ly. The following are other accurate methods of determining urea quantitatively: (1) Knop-Htif ner's method consists in decomposing urea into nitrogen and carbonic acid, with sodium hypo bromite, and determining the volume of nitrogen produced from which the amount of urea can be readily calculated. (2) An excellent method de
vised by Bunsen consists in heating urea in a. sealed glass tube with barium chloride, which breaks up urea into carbonic acid and ammonia, the former combining with barium chloride to give the insoluble barium carbonate which is readily isolated, dried and weighed; the weight of barium carbonate gives the weight of carbonic acid produced in the decomposition, and hence, by a simple calculation, the weight of urea decom posed. The presence of urea may be readily de tected by means of the biuret reaction. It has been mentioned above that when urea is decom posed by heating, biuret is produced along with several other products of decomposition. NOW, when biuret is dissolved in water, then some caus tic soda and a little dilute copper sulphate so lution added, a characteristic pink coloration is produced. To detect urea, therefore, the dry substance submitted for examination, o• the residue left on evaporating a solution, is heated for some time at 150° C., then dissolved and treated with caustic soda and copper sulphate, a pink coloration produced proving the presence of urea.
Urea was discovered in urine in 1773, and it was the first organic compound produced arti ficially in the chemical laboratory—a synthesis of vast importance in the history of chemistry. (See CHEMISTRY.) It was then (1828) obtained by Withler (q.v.) by heating a solution of am monium eyanate, the transformation taking place according to the following chemical equation: