The Determination of Urea. By Professor Liebig.—The new me-
thod proposed by Liebig has the advantage of simultaneously determin-
ing the amount of chlorine and of urea. It is as follows : 100 grammes
of mercury carefully purified from bismuth and lead, are dissolved in
pure nitric acid ; the salt is evaporated to a syrup like consistence, and
then enough water is added to reach accurately to 1400 cubic centime-
tres. Every 100 cubic centimetres contains 7.140 grammes of mercury.
If this solution be added to a solution of pure urea, a snow-white preci-
pitate falls, which is a compound of urea and oxide of mercury
(U+4HgO.) When the urea is all precipitated, and when consequently,
nitrate of mercury is the solution, a yellow precipitate, hydrated oxide
is thrown down by the addition of a little carbonate of soda to a drop
of the urea. In order to determine the exact moment when the yellow
precipitate occurs, the solution of the nitrate of mercury is of course
added drop by drop from a burette, and from time to time a drop of the
urine is taken and tested with the soda. One cubic centimetre of the
solution corresponds to 10 milligrammes of urea, and from the quantity
used, the amount of urea is calculated.
Although nitrate of mercury will thus precipitate urea, the bichloride
will not do so, and on this fact is founded the determination of the
chlorine. If the solution of urea be not pure, but mixed with chloride
of sodium, no precipitate at first occurs with the nitrate of mercury,
because sublimate is formed; if the nitrate continue to be added, then
at last, the chloride being exhausted, the mercury combines with urea,
as in the pure solution. Therefore the quantity of solution used before
the white precipitate appears, shows the amount of chlorine which must
have combined with the mercury first of all. A cubic centimetre of
the mercurial solution corresponds to 10 milligrammes of chloride of
sodium.
In testing urine the phosphoric and sulphuric acids are first precipi-
tated with barytic water; and after filtration, the urine is weakly acidi-
fied with nitric acid.
Although both the chlorine and the urea may be determined by the
same quantity of urine, it is advisable to use two portions, one for the
chlorine and one for the urea. The quantity of nitrate used for the
chlorine alone in the first specimen, can be deducted from that used for
the urea and chlorine together in the second, and the remainder gives,
of course, the quantity which has been used for the urea.—Brit, and
For. Med.-Chir. Review, from Annalen d. Chem. und Pharm., Band
lxxxv. 1853.