Barium Chromate, BaCrO₄

Barium Chromate, BaCrO₄, is obtained as a pale yellow amorphous precipitate when a barium salt is added to a solution of an alkali chromate. If a dichromate is used, the precipitate tends to dissolve in the acid formed. It has a density of 4.3. On heating, it darkens in colour, and if fused, it yields the crystalline form on cooling. The latter form is also obtained by heating a mixture of potassium and sodium chromates and barium chloride to a bright red heat, and allowing to cool. Greenish crystals are formed in the interior of the fused mass and can be isolated by lixiviation with boiling water. The crystals have density 4.60, and are isomorphous with barium sulphate. They are only very slightly soluble in water, and the solubility has been determined by conductivity measurements as follows:

Temperature ° C	0	10	20	30
Grams per litre of solution	0.002	0.0028	0.0037	0.0046

For analytical purposes the solubility has also been determined in dilute acetic acid and in solutions of ammonium salts, and Waddell has shown that for washing barium chromate, pure water is preferable to a one per cent, ammonium acetate solution, which latter dissolves about four times as much of the salt as an equal quantity of water.

Barium chromate readily dissolves in hydrochloric and nitric acids, forming the dichromate; concentrated sulphuric acid decomposes it, forming barium sulphate and chromic acid; it also dissolves in molten sodium nitrate. The simultaneous precipitation of barium chromate and barium carbonate, and of barium chromate and barium sulphate, has been studied.

Barium chromate finds its way into the market as a pigment under various names, such as barium chrome, lemon chrome, and yellow ultramarine. It is very permanent, but is deficient in brightness and body, and is therefore not widely used.

Double salts of barium chromate, for example, BaK₂(CrO₄)₂, Ba(NH₄)₂(CrO₄)₂, are obtained as light yellow precipitates by adding a large excess of the alkali chromate to a solution of a barium salt. They are decomposed by water.