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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.

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