Calcium Chromate, CaCrO₄

Calcium Chromate, CaCrO₄, is produced industrially as a byproduct in the manufacture of alkali chromates. Finely powdered chrome iron ore is roasted with chalk yielding calcium chromate, which by treatment with an alkali sulphate is converted into insoluble calcium sulphate and the soluble alkali chromate. On a small scale it is obtained by double decomposition of calcium chloride and sodium chromate, or by neutralisation of chromic acid solution by means of calcium hydroxide. It has a limited application as a pigment.

In many of its properties calcium chromate shows considerable resemblance to calcium sulphate. It yields four hydrates, all of which are slightly more soluble in water than the anhydride. The dihydrate, CaCrO₄.2H₂O, exists in two forms - the α form, which is monoclinic and isomorphous with gypsum, and the β form, which is rhombic and is the most stable of the hydrates. The former separates as amber yellow acicular crystals when a saturated solution of the chromate, prepared at 0° C., is allowed to stand at room temperatures. If the crystals are allowed to remain in contact with the solution for a few days, a darkening in colour takes place and the a variety completely disappears, the β form being produced. The β crystals effloresce slowly in the air, and lose all water of crystallisation at 100° C. Unlike the other hydrates and the anhydride, which are all less soluble in warm water than in cold, the solubility of the β form increases slightly with the temperature. A saturated solution containing 10.3 per cent, of the salt at 18° C. has a density of 1.105. Above 14° C. the monohydrate is more stable than the β dihydrate, and if a saturated solution of the latter, or of the a salt, be kept at 30° C. for several days, the mono- hydrate separates in the form of rhombic crystals:

a:b:c = 0.6296:1:0.6404,

of density 2.793 at 15° C. Like calcium sulphate, the chromate yields a hemihydrate, 2CaCrO₄.H₂O, which may be obtained by heating a 15 per cent, solution of calcium chromate with an equal volume of glycerol at 100° C. It forms small octahedral crystals, which only completely lose their water at 400° C.

Anhydrous calcium chromate may be conveniently prepared by dehydration of any of the above hydrates at high temperatures. It is less soluble than the hydrates, and is the most stable form. It dissolves in molten sodium nitrate.

A few double salts obtained by double decomposition of calcium chloride with excess of concentrated solutions of alkali chromate, or by neutralisation of dichromate solutions with lime, have been described. A basic calcium chromate, CaCrO₄.CaO.3H₂O, results as large polyhedral crystals when a solution of chromic acid supersaturated with calcium hydroxide is allowed to stand for several days.