CHROMIUM OXIDE.
=Chrome Green.=—Although very different substances are commercially
known as chrome green, this name is usually applied to the very
valuable pigment which consists of pure chromium oxide. There is hardly
another pigment for the preparation of which such varied directions
have been given and whose shade varies so much according to the manner
of production.
The cheapest process is to ignite potassium bichromate with sulphur,
extract the mass with very dilute sulphuric acid and wash the residue.
The chromic acid is reduced by the sulphur; when the mass is treated
with sulphuric acid sulphur dioxide is evolved, potassium sulphide and
sulphate are dissolved, whilst pure chromium oxide remains. The larger
the quantity of sulphur used the paler is the chromium oxide. The
purity of the potassium bichromate has considerable influence on the
shade of the pigment; if iron is present in any quantity a discoloured
product always results. Convenient proportions are: 19 parts of
potassium bichromate and 4 parts of sulphur, which produce 9·33 parts
of chromium oxide. If potassium bichromate free from iron cannot be
obtained, the shade of the product is somewhat improved by treatment
with dilute hydrochloric acid, in which ferric oxide dissolves more
easily than chromium oxide. When the latter has been strongly ignited
it is soluble with very great difficulty.
There are many formulæ for the preparation of chromium oxide by means
of sulphur: in all the above statement holds good, that the more
sulphur the paler the product.
According to A. Casali, a chrome green which satisfies every
requirement is obtained by strongly igniting 1 part of potassium
bichromate with 3 parts of burnt gypsum. After ignition the mass is
boiled with very dilute hydrochloric acid. In this process the chromic
oxide is produced according to the following equation:—
2K₂C₂O₇ + CaSO₄ = 2Cr₂O₃ + 2K₂SO₄ + 2CaO + 3O₂.
On boiling with hydrochloric acid the lime is dissolved; when the
liquid remains distinctly acid after long boiling it is poured off and
the chromium oxide washed with hot water and dried.
In another method ammonium chromate is very gradually heated. At a
certain temperature the salt suddenly becomes incandescent and is
converted into a dark green, almost black, mass which has a very
similar appearance to dry tea-leaves. The green obtained after
washing and powdering is handsomer the lower the temperature of the
decomposition.
Chromium oxide may also be made in the wet way, but the product does
not compare in beauty with that obtained in the dry way. When soda
solution is added to a solution of chrome alum, a greyish green
precipitate of chromium hydroxide is formed; on ignition of the washed
precipitate pure chromium oxide remains.
In a similar manner chromium oxide is produced by precipitating with
soda solution the solution of chromium chloride obtained by adding
hydrochloric acid to a solution of potassium bichromate, and then
alcohol in small quantities, so long as a reaction occurs and the green
liquid becomes deeper in colour.
A handsome and bright chrome green is only obtained when the potassium
bichromate is quite free from iron, a very small quantity of which has
a very harmful effect upon the brightness of the pigment. The author
has found that there is no great difficulty in rendering commercial
potassium bichromate tolerably free from iron by recrystallisation. As
much of the salt is dissolved in boiling water as it will take up, the
boiling solution is quickly filtered and rapidly cooled with continual
stirring; the fine crystalline meal is left on a strainer until no more
liquid filters through, and then washed with a little cold water to
remove the mother liquor. The salt obtained by this simple operation is
of great purity and always produces chrome green of a good shade.
When chrome green contains ferric oxide it shows a blackish colour.
From such chrome green, which has little value, a product of more pure
colour can be obtained with a little care. By treatment with dilute
hydrochloric acid the ferric oxide dissolves, even when it has been
somewhat strongly ignited; chromium oxide loses almost all solubility
in dilute hydrochloric acid after heating at a relatively low
temperature. To facilitate the solution of the ferric oxide, the green
to be treated is finely powdered; it is then covered with a mixture
of equal parts of hydrochloric acid and water. After several days the
mass is strained and washed with pure water until the acid reaction
disappears. Comparative analyses have shown that almost the whole of
the ferric oxide can be removed from chrome green in this manner; the
brightness of the colour is considerably increased. It is, however,
advisable to employ materials free from iron in the manufacture of
chrome green.
Chromium oxide can be obtained as a colour of particular brightness,
according to the process of Leune, by precipitating it very slowly
from a solution in which it is contained in the green modification.
For this purpose a solution of chrome alum is boiled until the violet
colour has changed to green; the green modification of chromium oxide
is now contained in the solution. It is cooled to about 10° C., and
either freshly precipitated alumina or zinc carbonate added, very
gradually and in small quantities. A fine green shade of chromium oxide
separates; after washing and drying it is obtained in a condition in
which it can be used as a pigment.
Chromium oxide can indeed be obtained by this process, but it is not
distinguished by particular beauty; the author has never been able
to obtain, even by using the precipitant in very small quantities,
a chromium oxide surpassing in shade that obtained by the direct
precipitation of a chromium salt with an alkali.
The emerald green described in the following chapter is obtained in a
similar manner by precipitation with zinc oxide.