158 UKEA. 



and afterwards gives off ammonia, and if heated to 150 for some 

 time is converted largely into biuret : 2(NH 2 ) 3 CO=NH 2 .CO.NH.CO 

 (NH 2 )+NH 3 . On further heating to a higher temperature (200 ) 

 it is largely converted into cyanuric acid. When biuret is dis- 

 solved in water and treated with caustic soda and dilute sulphate 

 of copper it yields the well-known pink colour employed for the 

 detection of peptones, and hence called the ' biuret reaction.' In 

 the application of the test to urea some caution is requisite while 

 heating the suspected substance to avoid carrying the decomposi- 

 tion beyond the biuret stage. When boiled in aqueous solution 

 with strong sulphuric acid or alkalis it is gradually decomposed, 

 under assumption of two molecules of water, into carbonic acid and 

 ammonia ; the same decomposition ensues by simple heating of the 

 aqueous solution in sealed tubes, to 180. This forms the basis 

 for the older 'Bunsen method' of estimating urea. A similar 

 change (hydration) is produced under the influence of several 

 micro-organisms which are found in urine undergoing alkaline 

 fermentation. Of these the best known is the Micrococcus ureae 1 

 from which a soluble hydrolytic enzyme may be extracted. 2 (See 

 above, p. 70.) 



2. When treated with nitrous acid, e.g. impure yellow nitric 

 acid, it is decomposed finally into carbonic anhydride, nitrogen, and 

 water: (NH 2 ) 2 CO -f 2HNO. 2 = C0 2 -|-2N 2 + 3H 2 0. A similar de- 

 composition is obtained by the action of sodium hypochlorite or 

 hypobromite : (NH 2 ) ? CO -f 3NaBrO = SNaBr + C0 2 +N 2 -f-2H 2 0. 

 Since the volume of nitrogen evolved is constant for a given weight 

 of urea, this latter reaction forms the basis of a method for the 

 quantitative determination of urea. (Knop-Hiifner. ) 



3. When a crystal of urea is treated with a drop of concentrated 

 freshly prepared aqueous solution of furf urol - C 5 H 4 2 (aldehyde 

 of pyromucic acid) and then immediately with a drop of hydro- 

 chloric acid (sp. gr.= riO) a play of colours is observed which 

 passes rapidly from yellow through green, blue, and violet to a final 

 brilliant purple. The test may be also applied by the addition of 

 three drops of the acid to a mixture of one drop of 1 p.c. aqueous 

 urea solution and '5 cc. of aqueous furf urol solution. 3 



Detection in Solutions. In addition to the microscopic appear- 

 ance of the crystals obtained on evaporation, the nitrate and oxa- 

 late should be formed and examined. Another part should give a 

 precipitate with mercuric nitrate, in the absence of sodium chloride 

 but not in the presence of this last salt if in excess ; in presence 

 of sodium chloride the mercuric nitrate reacts first with the sodium 

 salt in preference to the urea. A third portion is treated with 



1 Pasteur, Compt. Rend. T. L. (1860), p. 869. Van Tieghem, Ibid. T. LVIII. (1864), 

 p. 210. Jaksch, Zt. f. ph^siol. Cttem. Bd. v. (1881), S. 395. 



2 Musculus, Pfluger's Arch. Bd. xn. (1876), S. 214. Lea, Jl. of Phusiol. Vol. vi. 

 (1885), S. 136. 



3 Schiff, Ber. d. d. ckem. Gesell 1877, S. 773. 



