242 THE URINE. 



CO(NH 2 ) 2 + SNaOBr == SNaBr + CO 2 -f 2H 2 O + 2N. 



In practical work an alkaline solution of the hypobromite is 

 employed, so that the carbon dioxide which is liberated is at once 

 absorbed, while the nitrogen remains. It is to be noted, however, 

 that while 1 gramme of urea should theoretically give rise to the 

 formation of 372.7 c.c. of nitrogen, 354.3 c.c. are at best obtained 

 at C. and a pressure of 760 Hgmm. In clinical work this 

 difference is unimportant, and it is in a measure equalized by the 

 evolution of a small amount of nitrogen from some of the other 

 nitrogenous constituents which are at the same time present. On 

 hydrolysis urea is transformed into ammonium carbonate : 



CO(NH 2 ) 2 + 2H 2 = (NH,) 2 C0 3 . 



This occurs during the process of ammoniacal fermentation, which 

 results when urine is allowed to stand exposed to the air. The car- 

 bonate in turn is decomposed with the liberation of ammonia and 

 carbon dioxide. On boiling with acids or alkalies the same result 

 is obtained. 



On heating the substance in aqueous solution in a sealed tube to 

 a temperature of 100 C. ammonia and carbon dioxide likewise 

 result. 



Nitrous acid when added in excess decomposes urea, with the 

 formation of nitrogen, carbon dioxide, and water, but the acid is at 

 the same time decomposed, as is seen in the equation : 



CO(NH 2 ) 2 + 2HNO 2 = CO 2 + 4N -f 3H 2 O. 



This reaction is utilized when it is desired to remove nitrous acid 

 from a solution. 



On heating the dry substance in a test-tube to a temperature of 

 about 150-170 C., fumes of ammonia are freely evolved, owing to 

 the decomposition of the urea with the formation of biuret, as shown 

 in the equation : 



X NH 2 



C0< 

 2CO(NH 2 ) 2 = NH 3 + >NH 



Biuret. 



On further heating, more ammonia is given off; the melted mass 

 finally solidifies, and may be shown to contain both biuret and 

 cyanuric acid. The reaction which takes place may be represented 

 as follows : 



(1) 3CO(NH 2 ) 2 == 3CONH + 3NH 3 

 Cyanic acid. 



(2) 3CONH == C 3 N 3 (OH) 3 

 Cyanuric aoid. 



To demonstrate the presence of the biuret, the residue is dis- 



