COMPOSITION AND CHARACTERS OF URINE 



1115 



ORGANIC CONSTITUENTS OF THE URINE 



Almost all these constituents contain nitrogen, which in man is dis- 

 tributed among the various urinary constituents as follows : 



Urea 

 Ammonia 

 Creatinine 

 Uric acid 



85-90 per cent. 

 2-4 

 3 

 1-3 



About 6 per cent, of the urinary nitrogen is in the form of other substances, 

 such as hippuric acid, pigments, &c. 



UREA or CARBAMIDE, CO 2 can be regarded as derived from 



/OH vNH 2 



carbonic acid, C0\n-rr ^7 the replacement of each OH group by an NH 2 



group. It is isomeric with ammonuim cyanate, NH 4 CNO. If a solution 



of potassium cyanate and ammonium chloride be warmed together and 



evaporated, crystals of urea may be 



obtained in long colourless prisms 



(Fig. 516) without any water of 



crystallisation. It is soluble in water 



and alcohol, and insoluble in ether. 



Its solutions are neutral in reaction, 



but it forms crystalline salts with 



strong acids. Thus urea nitrate, 



which is produced by treating strong 



solutions of urea with concentrated 



nitric acid, forms microscopic rhombic 



plates which are extremely insoluble, 



so that their formation may be used 



as a test for urea (Fig. 517). With 



oxalic acid urea solutions yield an 



insoluble oxalate, also in typical 



crystals. Urea when heated melts 



at about 130 C. On further heating, 



it undergoes decomposition, giving 



ofi ammonia and forming biuret, as 



follows : 



FIG. 517. Urea nitrate. Urea oxalate. 



At the same time a certain amount of cyanic acid is formed, and this 

 polymerises to cyanuric acid, H 3 C3N 3 03. On heating with alkalies or with 

 strong acids urea undergoes hydrolysis, with the production of carbon 

 dioxide and ammonia : 



CON 2 H 4 + H 2 = C0 2 + 2NH 3 . 



The same change is effected in urea by certain micro-organisms, e.g. the 



n 



FIG. 516. Urea. 



