HYDRIDES. 63- Chromic Oxide In the Dry Way from a Chromate.
The preparation of chromic oxide by the reduction of an aqueous solution
of a chromate to one of a chromic salt, with subsequent precipitation and
ignition of chromic hydroxide, is not a convenient process because of the
difficulty and tediousness of filtering and washing the very voluminous pre-
cipitate. The method originated by Wohler, 1827, which is that given in the
following directions, is, therefore, preferable. The chromate is heated in a
crucible with ammonium chloride, whereby the ammonium radical is oxidized
to water and nitrogen while the chlorine combines with the alkali metal of the
chromate.
Mix 147 g. potassium dichromate (one-half mol.) intimately
with an equal weight of ammonium chloride, and heat the mixture
in a clay crucible, in a charcoal furnace, until no more vapors
are given off. After cooling, boil the brittle contents of the
crucible repeatedly with fresh portions of water until all the
soluble salt has been removed. Compute the yield of the dried
preparation in percentage of the theoretical. Dependent prep-
aration, Chromium No. 3.- Cuprous Oxide.
In a 1750 cc. Erlenmeyer flask mix a solution of 55 grams of
copper sulphate crystals in 500 cc. of water with one of 25 grams
of hydroxylamine hydrochloride in 125 cc. of water. While
cooling in running water and shaking vigorously, slowly add a
solution of 55 grams of stick potassium hydroxide in 750 cc of
water. A yellow precipitate of cuprous oxide will form. After
the precipitate has settled well, pour off the supernatant liquid
and wash the precipitate by decantation four times with very
dilute hydrochloric acid and then with pure water until the wash-
ings show no test for either chloride or sulphate. For the decan-
tations, it is well to support the flask in an inclined position so
that the precipitate is not disturbed when the liquid is poured off.
Finally drain the precipitate with suction, wash with a little
alcohol and finally with some ether. Dry in a vacuum desiccator,
which causes a slight darkening. Yield about 14 grams.
HYDRIDES.
A few metals combine directly with hydrogen at definite temperatures to
form hydrides (e.g., cerium hydride No. 32).
Copper hydride is formed in aqueous solution by treating cupric salts with
very strong reducing agents (No. 33).