PHYSIOLOGICAL CHEMISTRY 417 



present in the hypobromite. This reaction is employed in the quanti- 

 tative estimation of urea (see below, p. 419). 



There are several reactions which are peculiarly interesting, since 

 they demonstrate the chemical relationships of urea to its probable 

 precursors in the tissues (see below). Thus, if urea be hydrolysed (i.e. 

 be caused to take up water) it forms ammonium carbonate : 



HOH 



\Urea) (Water) (Ammonium 

 carbonate) 



This process occurs in urine which has stood for some time, the 

 hydrolysis being effected by several kinds of microbes. It may also be 

 produced by boiling urea with strong acids or alkalies ; in both cases 

 the ammonium carbonate is further decomposed, liberating, in the case 

 of alkalies, ammonia gas (the carbon dioxide being absorbed by the 

 alkali present), and in the case of acids, carbon dioxide gas (the 

 ammonia being absorbed by the acid present). 



EXPERIMENT IV. Prepare a solution of pure urea, and divide it 

 into two portions, A and B. To A add about 10 drops of sulphuric 

 acid and boil, meanwhile collecting the vapour which comes off in a 

 second test tube containing lime or baryta water. By this becoming 

 milky, the presence of carbon dioxide gas is demonstrated. To B add 

 about 5 drops saturated caustic potash and boil. Ammonia gas is 

 evolved, so that a moistened strip of red litmus paper is turned blue if 

 held in the fumes, which smell strongly of ammonia. 



A substance intermediate between urea and ammonium carbonate, 



and having therefore the formula CO<OTTT 4 > can be formed by 



allowing dry carbon dioxide gas to act on dry ammonia. This is 

 called ammonium carbamate. If heated to 135 C. it splits up into 

 urea and water. A certain amount of ammonium carbamate is always 

 present in watery solutions of ammonium carbonate. 



Dry heat splits urea into ammonia gas and a body called Biuret. By 

 further heating, Biuret changes into cyanuric acid (HCN0 3 ), which 

 is isomeric with cyanic acid, HCNO. 



EXPERIMENT V. Heat some urea crystals in a dry test tube. 

 Note that they melt and give off ammonia. Continue heating for a 

 few minutes, then cool the test tube and dissolve the residue in water, 

 and to this solution apply the Biuret test. A rose pink colour results 

 (see Peptone, p. 302). 



Conversely, we can change cyanic acid into urea by evaporating an 

 aqueous solution of ammonium cyanate (NH 4 CNO) to dryness. This 



2D 



