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<C8S URINE.
on cooling is purified by recrystallization from warm alcohol. A fur-
ther quantity of urea may be obtained from the mother-liquor by con-
centration. The urea is purified from contaminating mineral bodies
by redissolving in alcohol-ether. If it is only necessary to detect the
presence of urea in urine, it is sufficient to concentrate a little of the
urine on a watch-glass and, after cooling, treat it with an excess of nitric
acid. In this way we obtain crystals of urea nitrate.
Quantitative Estimation of the Total Nitrogen and Urea in Urine.
Among the various methods proposed for the estimation of the total
nitrogen, that suggested by Kjeldahl is to be recommended. Liebig’s
method for the estimation of urea is really a method for determining
the total nitrogen, but as it is very seldom used now, we can refer to
larger works in regard to details.
Kjeldahl’s method consists in transforming all the nitrogen of the
organic substances into ammonia by heating with a sufficiently con-
centrated sulphuric acid. The ammonia is distilled off, after super-
saturating with alkali, and collected in standard sulphuric acid. The
following reagents are necessary:
1. Sulphuric Acid. Either a mixture of equal volumes of pure con-
centrated and fuming sulphuric acid, or else a solution of 200 grams
phosphoric anhydride in 1 liter of pure concentrated sulphuric acid.
2. Caustic soda free from nitrates, 30-40 per cent solution. The quantity
of this caustic-soda sohition necessary to neutralize 10 cc. of the acid
mixture must be determined. 3. Metallic mercury or pure yellow mercuric
oxide. (The addition of this facilitates the destruction of the organic
substances.) 4. A potassium-sulphide solution of 4 per cent, whose
object is to decompose any mercuric amide combination which might
not evolve its ammonia completely during the distillation with caustic
:soda. 5. 1/5 normal sulphuric acid and 1/5 normal caustic soda solution.
In performing the determination 5 cc. of the carefully measured and
filtered urine are placed in a long-necked Kjeldahl flask, a drop of mercury
or about 0.3 gram of mercuric oxide added, and then treated with 10-15
cc. of the strong sulphuric acid. The contents are heated very care-
fully, placing the flask at an angle, until they just begin to boil gently;
this is continued for about half an hour after the mixture becomes color-
less. On cooling, the contents are transferred to a voluminous distilling-
flask, carefully washing the Kjeldahl flask with water and the greater
part of the acid is neutralized by caustic soda. A few zinc shavings are
added to prevent too rapid ebullition on distillation, and then an excess
of caustic-soda solution which has previously been treated with 30-40
cc. of the potassium-sulphide solution. The flask is quickly connected
with the condenser-tube and all the ammonia distilled off. In order
to prevent loss of ammonia it is best to lower the end of the exit-tube
below the surface of the acid, the regurgitation of the acid being prevented
by having a bulb blown on the exit-tube. Not less than 25-30 cc. of
the standard acid is used for every 5 cc. of urine, and on completion
[of the distillation the acid is retitrated with 1/5 normal caustic soda
u.-ing rosolic acid, tincture of cochineal, or lacmoid as indicator. Each
cubic centimeter of the acid corresponds to 2.8 milligrams nitrogen. As a
control and in order to test the purity of the reagents, or to eliminate
any error caused by an accidental quantity of ammonia in the air, we
always make a blank determination with the reagents.
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