UREA. 583 



which can produce the same result. On the other hand, urine may 

 develop many organisms which have no such power. 1 So long as the 

 bacteria which induce the change are alive, the enzyme is closely 

 associated with the living cell, and a filtered urine is ferment free 

 (Sheridan Lea). But when the cells are dead, a ferment may be 

 extracted from them which hydrolyses pure urea solutions. 



While urea is thus easily converted into ammonium carbonate, the 

 intermediate substance ammonium carbamate (formed by the action of 

 dry C0 2 upon NH 3 ), if heated to 135, or treated with alternating electric 

 currents, splits up into urea and water. The hepatic cells have the 

 power of dehydrolising ammonium carbonate itself to form urea. Nitrous 

 acid and the hypobromites oxidise urea according to the following 

 equations : 



(1) CO(NH 9 ) 2 +2NOOH=C0 2 +2N 2 +3H 9 

 (2) 



Separation of urea. To prepare pure urea from urine, advantage may 

 be taken of the insolubility of the nitrate. The urine is concentrated to a 

 small bulk, and pure nitric acid is added in excess ; the mixture being kept 

 thoroughly cool during the addition of acid. The crystals are strained off by 

 pouring through muslin, and freed from excess of acid by pressing between 

 thick filtering paper. They are mixed with excess of barium carbonate, 

 sufficient alcohol is added to form a paste, and the mixture dried on the water 

 bath. On extracting the dried residue with absolute alcohol, a fairly pure 

 solution of urea is obtained, from which crystals separate on evaporation. 



I find that fine crystals may be prepared by the following simpler method. 

 Half a litre of urine is evaporated to a thick syrupy consistence, and the 

 residue is exhausted with hot absolute alcohol. The spirit is filtered and 

 taken to dryness ; and the residue extracted on the water bath with successive 

 quantities of pure acetone, which should be filtered while hot. The mixed 

 acetone extracts are evaporated nearly, but not quite, to dryness. On cooling, 

 fine white crystals of urea separate, any pigment present remaining in solution 

 in the small quantity of the solvent which is allowed to remain. The crystals 

 may be washed with cold acetone. 



Tests. For the detection of urea, the formation of the characteristic 

 crystals of the nitrate or oxalate (and especially of the former) is of 

 practical value. On the small scale the process of crystallisation may 

 be watched under the microscope ; a drop of the suspected fluid, after 

 concentration if necessary, and another of nitric acid, being allowed to 

 run together on a glass slide. 



The formation of biuret is an excellent test for urea, if the crystals 

 are first obtained in moderate quantity and fairly pure. After heating 

 them, as described above, the residue is dissolved in water, excess of 

 caustic alkali is added, and one or two drops of a dilute copper sulphate 

 solution. A pink colour is produced like that given by peptones under 

 like circumstances (" biuret reaction "). 



Estimation of urea. No method is known by which urea can be separated, 

 as such, from the urine in a quantitative manner. The ease with which it is 

 hydrolised isr a fundamental difficulty in the way of such quantitative isolation. 

 We can, however, find precipitants for the other nitrogenous constituents, and 

 a determination of the remaining nitrogen after the removal of these gives the 

 best available measure of the urea. 



1 For information on this subject, vide Leube and Graser, Virchoufs Archir, loc. cit.; and 

 Warrington, Journ. Chem. Soc.. London, 1888, vol. i. p. 727. 



