Zinc Chromate, ZnCrO₄

When solutions of zinc salts are precipitated with potassium or sodium chromate, basic zinc chromates or complex double salts are produced.

The anhydrous normal chromate, ZnCrO₄, has been obtained as a fine light crystalline powder by heating zinc hydroxide with a dichromate solution containing a slight excess of chromic acid in a sealed tube at 220° C. It was also obtained in microscopic citron-yellow prisms by heating equal weights of chromium trioxide and zinc carbonate with a little water in a sealed tube at 220° C., warming the precipitate formed with some concentrated chromic acid solution, and drying the washed residue over sulphuric acid.

The monohydrate, ZnCrO₄.H₂O, was obtained in microscopic citron- yellow prisms by shaking 11 grm. zinc oxide with 50 c.c. of a 5-molar solution of chromic acid, filtering, and crystallising in vacuo over sulphuric acid.

The anhydrous salt is insoluble in water, easily soluble in acids, and forms a greyish-yellow crystalline basic salt on boiling with water. The monohydrate does not lose its water at 125° C., and cannot be converted by heat into the anhydrous salt (when the water is liberated it seems to hydrolyse the monochromate).

The hydrated tetrammoniate, ZnCrO₄.4NH₄.5H₂O, has been prepared. The following double chromates and their ammoniates have been prepared: K₂O.5ZnO.4CrO₃.6H₂O; K₂O.4ZnO.8CrO₃.3H₂O; K₂CrO₄.ZnCrO₄.2H₂O; 2(NH₄)₂CrO₄.4ZnCrO₄.3NH₄.3H₂O; and (NH₄)₂CrO₄.ZnCrO₄.2NH₄.