Carbamide, as it is called by systematic chemists, or _urea_, is next
to water the largest constituent of urine, and forms about one-third
of its total solids. Derived from ammonic carbonate by abstracting two
molecules of the elements of water, it is readily converted by
putrefaction into that salt, and the urine under these circumstances
becomes strongly alkaline in reaction. Earthy phosphates then fall
naturally out of solution, so that the putrid fluid is always well
furnished with sediment. Nitrogen that has served its purpose as
muscle or other proteid leaves the animal economy chiefly in the form
of urea, and its proportion in the urine, therefore, is a fair index
of the activity of wasting influences.
For its determination Knop's sodic hypobromite method, on account of
its convenience, is now generally preferred. The volumetric process of
Liebig, which depends on the formation of an insoluble compound of
urea with mercuric nitrate, possesses no advantages and is troublesome
to work. The principle of the hypobromite process is simple. In a
strongly alkaline solution urea is broken up by sodic hypobromite, its
nitrogen being evolved in the gaseous state, and its carbon and
hydrogen oxidized to carbonic anhydride and water respectively. The
volume of free nitrogen obtained bears a direct ratio to the amount of
urea decomposed.
[Illustration]
Among the number of instruments which have been introduced for the
purpose of conveniently measuring the evolved gas, that of Gerrard, an
illustration of which we give, is one of the simplest, cheapest, and
best. The ureometer tube, _b_, is connected at the base with a movable
reservoir, _c_, and by means of a rubber tube passing through a cork
at the top to the generating bottle, _a_. To use the apparatus, fill
_b_ to zero with water and have the reservoir placed so high that it
contains only an inch or so of the liquid. Replace the cork with
attached tube tightly in _b_. Now pour into the generating bottle 25
c.c. of a solution prepared by dissolving 1 part of caustic soda in 2½
parts of distilled water, and dexterously break in the liquid a tube
containing 2.2 c.c. of bromine. The tubes will be found very
convenient, obviating entirely the suffocating fumes diffused in the
act of measuring bromine. Allow to stand in the solution of sodic
hypobromite thus prepared a test tube containing exactly 5 c.c. of the
urine under examination. Cork the bottle as shown in the illustration,
see that the water is at zero, and that the liquid in the reservoir is
at the same level, and then allow the urine to gradually mix with the
hypobromite solution. Cool the evolved gas by placing the bottle in
cold water, adjust the levels of the water in the tube and reservoir
(to obviate a correction for pressure), and read off the percentage of
urea in terms of which the tube is graduated. Stale urine, the urea of
which has largely been converted into ammonic carbonate, still yields
a very fair result, that salt being also completely split up by the
powerful oxidant employed. Should the urine contain albumen, it is
advisable to remove it by boiling and filtering, as, although only
slowly decomposed by the hypobromite solution, it communicates to the
liquid such a tendency to froth that the disengagement of the nitrogen
is seriously impeded. Most of those alkaloids which might possibly be
present do not yield the gas when treated in this manner, and
therefore may be disregarded.