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PROPERTIES AND REACTIONS OF UREA. C87
plates, with an angle of 82°. When crystallizing slowly, larger and
thicker rhombic pillars or plates are obtained. This compound is rather
easily soluble in pure water, but is considerably less soluble in water
containing nitric acid; it may be obtained by treating a concentrated
solution of urea with an excess of strong nitric acid free from nitroufl
acid. On heating this compound it volatilizes without leaving a residue.
This compound may bo employed with advantage in detecting small amounts
of urea. A drop of the concentrated solution is placed on a microscope slide and
the cover-glass placed upon it; a drop of nitric acid is then placed on the side
of the cover-glass and allowed to flow under. The formation of crystals begins
where the solution and the nitric acid meet. Alkali nitrates may crystallize
very similarly to urea nitrate when they are contaminated with other bodies;
therefore, in testing for urea, the crystals must be identified as urea nitrate by
heating and by other means.
Urea Oxalate, 2.CO(NH2)2-H2C204. This compound is nu re
sparingly soluble in water than the nitric-acid compound. It is obtained
in rhombic or six-sided prisms or plates on adding a saturated oxalic-
acid solution to a concentrated solution of urea.
Urea also forms combinations with mercuric nitrate in variable
proportions. If a very faintly acid mercuric-nitrate solution is added
to a 2 per cent solution of urea and the mixture carefully neutralized,
a compound is obtained of a constant composition which contains for
every 10 parts of urea 72 parts of mercuric oxide. This compound serves
as the basis of Liebig’s titration method. Urea also combines with
salts, forming mostly crystallizable combinations, as, for instance, with
sodium chloride, with the chlorides of the heavy metals, etc. An alka-
line but not a neutral solution of urea is precipitated by mercuric chloride.
If urea is dissolved in dilute hydrochloric acid and then an excess of formal-
dehyde is added, a thick, white, granular precipitate is obtained which is dif-
ficultly soluble and whose composition is somewhat disputed. 1
With phenyl-
hydrazine, urea in strong acetic acid gives a colorless crystalline compound of
phenylsemicarbazid, C\Jd6 NH.NH:CONH2 , which is soluble with difficulty in cold
water and melts at 172° C. (Jaffe 2
).
The method of preparing urea from urine is in the main as follows:
Concentrate the urine, which has been faintly acidified with sulphuric
acid, at a low temperature, add an excess of nitric acid, at the same time
keeping the mixture cool, press the precipitate well, decompose it in
water with freshly precipitated barium carbonate, dry on the water-
bath, extract the residue with strong alcohol, decolorize when necessary
with animal charcoal, and filter while warm. The urea which crystallizes
1
See Tollens and his pupils, Ber. d. deutsch. chem. Gesellsch., 29, 2751; Gold-
schmidt, ibid., 29; and Chem. Centralbl., 1897, 1, 33; Thorns, ibid., 2, 144 and 737.
2
Zeitschr. f. physiol. Chem., 22.
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