On boiling a chromate in hydrochloric acid mixed with alcohol, chromic
acid is first set free, and then decomposed, forming a green solution of
chloride of chromium. Sulphuretted hydrogen and sulphurous acid effect
similar changes. With acetate of lead the chromates give a yellow
precipitate; with nitrate of silver, a reddish-brown; with nitrate of
mercury, a red one.
=CHROME ALUM.= See ALUMS.
=CHROME GREEN.= See GREEN PIGMENTS.
=CHROME IRON.= See IRON.
=CHROME RED.= See RED PIGMENTS.
=CHROME YELLOW.= See LEAD, CHROMATE OF.
=CHROMIC ACID.= See CHROMIC ANHYDRIDE.
=CHRO'MIUM.= Cr. A metal discovered in native chromate of lead by
Vauquelin in 1797. It is found in the state of oxide, combined with oxide
of iron, in some abundance, in the Shetland Islands, and elsewhere; as
chromate of lead it constitutes a very beautiful material.
Prepared in an impure condition as a white, very infusible, hard metal, by
igniting the oxide with charcoal, at a white heat, in a lime crucible.
=Chromous Chloride.= CrCl_{2}. _Syn._ PROTOCHLORIDE. _Prep._ Ignite the
chromic chloride in a current of dry hydrogen. A white, foliated mass,
soluble in water (evolving much heat), and yielding a blue solution, which
absorbs atmospheric oxygen with astonishing rapidity, acquiring a
deep-green colour, and passing into the state of oxychloride of chromium.
It is the most powerful reducing or deoxidising agent known.
=Chromic Chloride.= Cr_{2}Cl_{6}. _Syn._ SESQUICHLORIDE. _Prep._ Pass dry
chlorine over a mixture of sesquioxide of chromium and charcoal, heated to
redness, in a porcelain tube. The chloride collects as a sublimate, of a
peach or violet colour, in the cool part of the tube.
Dissolve chromic oxide in hydrochloric acid and evaporate to dryness; the
residue is chromic chloride. It forms a dark green mass, containing water,
which is evolved by igniting at a temperature of 400°, turning a purplish
red.
=Chromium Oxides=:--
=Chromous Oxide.= CrO. _Syn._ PROTOXIDE OF CHROMIUM. This oxide has not
yet been obtained in a satisfactory manner, but the hydrate is prepared by
the addition of potassium hydrate solution to a solution of chromous
chloride or sulphate. A brownish-red powder, speedily passing to a deep
foxy-red, with disengagement of hydrogen, and forming pale blue-coloured
salts with the acids, which absorb oxygen with avidity, whilst the metal
passes into a higher state of oxidation.
=Chromic Oxide.= CrO_{3}. _Syn._ SESQUIOXIDE. Prepared by igniting
potassium bichromate at a red heat and well washing the residue, and as
hydrate by cautiously adding equal parts of hydrochloric acid and alcohol
or sugar to a boiling solution of chromate of potassa in water, in small
portions at a time, until the red tint disappears, and the liquid assumes
a green colour; pure ammonia, in excess, is next added, and the
precipitate which subsides is collected and washed with water.
_Prop., &c._ The anhydrous oxide is a rich crystalline, green powder,
insoluble in both water and acids; fused with borax and glass, it imparts
a beautiful green colour.
The hydrate is soluble in the acids and in alkaline lyes; with the first
it forms salts which have a green or purple colour. These compounds may be
made by direct solution of the hydrate in the dilute acids. Chromic
sulphate combines with the sulphates of potassium and ammonium, giving
rise to salts (CHROME ALUMS) which crystallise in magnificent octahedrons
of a deep claret colour. The finest crystals are obtained by spontaneous
evaporation.
These salts of chromium are the most important, the chromous salts being
seldom met with, and are best recognised by the following
reactions:--Caustic alkalies precipitate the hydrate, easily soluble in
excess of the precipitant. Ammonia the same, but the precipitate is nearly
insoluble. The carbonates of potassium, sodium, and ammonium throw down a
green precipitate of carbonate and hydrate, slightly soluble in a large
excess. Sulphuretted hydrogen causes no change.--Sulphydrate of ammonium
precipitates the hydrate of a bluish-green colour.
=Chromic Anhydride.= CrO_{3}. _Syn._ CHROMIC ACID, ANHYDROUS CHROMIC ACID,
CHROMIC TRIOXIDE. _Prep._ By conducting gaseous fluoride of chromium into
a silver or platinum vessel, the sides of which are just moistened with
water, and the aperture covered with a piece of moist paper, the anhydride
will be deposited under the form of red, acicular crystals, which will
nearly fill the vessel. When the process is skilfully conducted, the
product is of exquisite beauty and chemically pure. The fluoride referred
to above is obtained from fluor spar, 3 parts; chromate of lead, 4 parts;
fuming (or the strongest) sulphuric acid, 5 parts; mixed cautiously in a
silver or leaden retort. A red-coloured gas is evolved, which acts rapidly
on glass, forming fluosilicic acid gas, and upon water forming
hydrofluoric acid and chromic anhydride. The moisture of the atmosphere is
sufficient to effect the decomposition last referred to; the former
substance escaping as gas, and the latter being deposited in small
crystals.
It is also prepared nearly pure by adding a cold saturated solution of
potassium bichromate to once and a half its bulk of pure strong sulphuric
acid. As the liquor cools, the anhydrous chromic acid is deposited under
the form of brilliant crimson-red prisms; the mother-liquor is then poured
off, and the crystals, placed between two tiles of glass or porcelain, are
submitted to strong pressure for some time, under a bell-glass or jar,
when the anhydride will be found sufficiently dry. It may be deprived of a
little adhering moisture by placing it over sulphuric acid for a short
time _in vacuo_.