Magnesium Chromates

The heptahydrate, MgCrO₄.7H₂O, is formed when a solution of chromic acid is neutralised with magnesium oxide, and the resulting solution concentrated and allowed to crystallise at ordinary temperatures. It yields yellow transparent prisms, rhombic, and isomorphous with the sulphate, MgSO₄.7H₂O, with which it forms mixed crystals. Density = 1.761 at 16° C. The crystals are very soluble in water: 100 grams of water at 18° C. dissolve 72.3 grams of MgCrO₄, the density at saturation being 1.422.

The pentahydrate, MgCrO₄.5H₂O, results when crystallisation takes place above 30° C., or when the heptahydrate is exposed to the air for some time. It forms large yellow transparent crystals, of density 1.954, and isomorphous with copper sulphate, CuSO₄.5H₂O. On heating to 120° C. it loses water and yields the dihydrate, MgCrO₄.2H₂O, from which the remaining water is only expelled on decomposition. The anhydrous salt has not been obtained.

Double salts of magnesium chromate with alkali chromates may be obtained by mixing solutions containing the two components in suitable proportions, or by neutralising a solution of the alkali dichromate with magnesium oxide or carbonate. Yellow crystals separate on evaporation. In the case of ammonium, ccesium, and rubidium, isomorphous crystals of the general formula, R^•₂Mg(CrO₄)₂.6H₂O, are obtained, isomorphous also with the corresponding double sulphates. The corresponding potassium salt, K₂Mg(CrO₄)₂.6H₂O, is formed only at low temperatures, the dihydrate, K₂Mg(CrO₄)₂.2H₂O, being the stable form at ordinary temperatures. The sodium salt, Na₂Mg(CrO₄)₂.3H₂O, becomes anhydrous at 200° C.