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 21/2 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.

SUGAR.