UREA. 
o°3 
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). Bui 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 Bubstance ammonium carbamate (formed by the action of 
dry C0 2 upon Nil >. it heated to 135°, or treated with alternating electric 
currents, splits up into urea and water. The hepatic cells have the 
pi iwer of dehydr< tlising ammonium carbonate itself to form urea. Nitrous 
acid and the hypobromites oxidise urea according to the following 
equations :— 
(1) CO(XH,).,+2XOOH=CO,+2X,+3H,0 
(2) CO(NH 2 ) 2 +3BrONa=3NaBH-C0 2 +N 2 +2H 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 is 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, Virclww's Archir, loc. cit.\ and 
Warrington, Journ. C/tem. Soc. London, 1888, vol. i. p. 727. 
