Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

132 POTASSIUM CYANATB AND UREA
must be completely disintegrated and a sample may not produce a
film of moisture on the walls of a tube in which it is heated. In the
same way 150 g. of potassium dichromate are freed from adherent
moisture. After being powdered separately, the two salts are
thoroughly mixed in a mortar and the mixture, in portions of 4 to
5 g., is dropped into an iron basin or on to a large iron plate which is
strongly heated (but not, however, to red heat) by a powerful burner
(Teclu or triple burner). The temperature should be high enough
to cause vigorous glowing of each portion, but the spongy black
mass thus formed must on no account be allowed to melt. After
the very rapid completion of its oxidation, each portion is scraped
aside by means of a broad metallic spatula or is entirely removed
from the plate. The whole amount can be treated in this way in
one to one and a half hours. The various portions of the material,
collected in a round-bottomed flask, are covered with hot 80 per
cent alcohol (800 c.c), which is then kept boiling for three minutes
by heating the flask on a vigorously boiling water bath. The clear
solution which is formed is decanted from the black deposit into
a conical flask, which is immediately placed on ice and shaken well
so that its contents are cooled as quickly as possible. After the
flask has been allowed to stand for a short time, the mother liquor
from the deposited cyanate crystals is poured off into the roundbottomed
flask containing the melt and the extraction of the latter
is repeated until all the salt has been extracted. (Five to six
times. A sample in a test tube should no longer deposit crystals on
cooling.) The salt is now filtered as dry as possible at the pump,
washed twice with rectified spirit, and then three times with ether,
and finally thoroughly dried in a desiccator. Average yield 80 g.
Potassium cyanate is also conveniently prepared by oxidation of
cyanide with permanganate in aqueous solution. 1
(6) UREA
Potassium cyanate (40 g.) and ammonium sulphate (40 g.) are
dissolved in 500 c.c. of water in a porcelain basin, and the solution
is evaporated to dryness on the water bath. The residue is exhaustively
extracted in a round-bottomed flask with boiling absolute
alcohol, and the alcoholic solution is concentrated until, on
1 J. Volhard, Annalen, 1890, 259, 378 ; F. Ullmann and Uzbachian, Ber., 1903,
36, 1806 ; Marckwald, Ber., 1923, 56, 1325. The best method is that of Gall and
Lehmann, Ber., 1928, 61, 675.