Zinc Chromate, ZnCrO₄

Zinc Chromate, ZnCrO₄, may be obtained by heating together potassium dichromate solution and excess of zinc hydroxide in a sealed tube at 220° C., or by heating zinc carbonate with chromic acid in aqueous solution. It forms small lemon-yellow crystals. According to Groger, who has investigated the conditions of equilibrium in the system ZnO-CrO₃-H₂O, the monohydrate, ZnCrO₄.H₂O, may be crystallised from solutions containing more than 9-7 molecules of CrO₃ (and 4.78 molecules of ZnO). It yields microscopic lemon-yellow prisms, which may be heated to 125° C. without loss of water.

Zinc chromate is decomposed by water, forming basic chromates, the following having been separated: 4ZnO.CrO₃.3H₂O; 3ZnO.CrO₃.2H₂O; 4ZnO.2CrO₃.3H₂O; 3ZnO.2CrO₃.H₂O - this being the order of their existence. Basic salts, for example, 4ZnO.CrO₃.H₂O, are also formed by precipitation of zinc salts with sodium chromate, or by decomposition of the double salts with water. These compounds are used as pigments under the names of zinc yellow, zinc chrome, and citron yellow, and possess the advantage over lead chromes of not being discoloured by hydrogen sulphide or by contact with sulphide pigments. In "body" they are not equal to the lead chromes, however, and are therefore not largely used as separate pigments. Mixed with Prussian blue they yield the various zinc greens.

Double chromates of zinc and the alkali metals have been described. The potassium compound K₂Zn(CrO₄)₂.2H₂O is obtained in the form of light yellow prismatic crystals when an excess of a concentrated potassium chromate solution is added to a solution of a zinc salt. The compounds K₂O.4ZnO.4CrO₃.3H₂O and (NH₄)₂O.2ZnO.2CrO₃.H₂O have been formulated, but their individuality has not been definitely established.

The following compounds containing ammonia have been obtained by the action of excess of ammonium chromate on salts of zinc, or by precipitating ammoniacal solutions of zinc chromate by means of alcohol: ZnCrO₄.4NH₃.3H₂O; ZnCrO₄.4NH₃.5H₂O; ZnCrO₄.NH₃.H₂O; 4ZnCrO₄.2(NH₄)₂CrO₄.3NH₃.3H₂O; ZnCrO₄.(NH₄)₂CrO₄.2NH₃; 2ZnO.3CrO₃.10NH₃.10H₂O; 2(ZnCrO₄.4NH₃).(NH₄)₂CrO₄.9H₂O.