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. CrO3. 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.
Commercially, it is prepared by one of the two following processes:—
To a saturated solution of chromate of potassium, 100 parts, add oil of vitriol (sp. gr. 1·845), 49 parts; and let the whole cool. This is the common process. The product contains sulphate of potassium, but this does not much interfere with its value as a bleaching agent.
From chromate of barium, decomposed by concentrated nitric acid. The anhydrous chromic acid is separated from the nitrate of barium by decantation, or, which is still better, by filtration through glass or asbestos. It is then evaporated to dryness, when the nitric acid is volatilised, and pure chromic anhydride left behind. The volatilised nitric acid may be condensed, and again used for the same purpose. The only precautions necessary to ensure the purity of the anhydrous chromic acid prepared by this plan are—to use a sufficient quantity of nitric acid and to take care that the nitric acid is sufficiently concentrated and pure.
Prop., &c. Forms ruby-red anhydrous prisms, very soluble in water, with formation of true chromic acid, and extensively manufactured for the purpose of oxidising and bleaching substances.
CHROME IRON-STONE. Syn. Chrome Iron-ore. FeO.Cr2O3. This, which is the principal ore of chromium, corresponds in composition to brown oxide of chromium and to the magnetic oxide of iron; part of the iron, however, is generally displaced by the isomorphous metal magnesium, and part of the chromium by aluminium.
Chrome iron-stone is often met with in the form of octohedral crystals. Acids fail to dissolve it, and it cannot be fused in the furnace, but when heated it absorbs oxygen from the air. This oxidation may be effected very readily if the chrome ore reduced to very fine powder be mixed with a carbonate of one of the metals of the alkalies or alkaline earths, a chromate of the base being formed.
CHRYSENE. C18H12. A hydrocarbon found by Laurent in crude anthracene. It occurs in bright yellow, glistening scales. It