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Potassium Chromate, K2CrO4

Potassium Chromate, K2CrO4, is usually prepared from the dichromate (vide infra) by neutralisation of the excess of CrO3 above that necessary to form the normal salt, e.g. by potassium carbonate in aqueous solution; by fusion of the dichromate with potassium nitrate or potassium carbonate, and extraction with water; by interaction of potassium dichromate with ammonium chloride or sodium hydrogen phosphate.

Potassium chromate is yellow in colour, and crystallises in the rhombic system (bipyramidal):

a:b:c = 0.5694:1:0.7298,

and is isomorphous with potassium sulphate. It has a density of 2.741, a specific heat of 0.189, and melts at 971° C. The solid exists in two crystalline modifications, the a form being stable above, and the β form stable below, a transition temperature of 666° C. The solubility of potassium chromate in water is as follows:

Temperature, ° C.Grams K2CrO4 in 100 Grams Water.
-11.3554.54
057.11
3065.13
6074.60
105.888.8


The aqueous solution of potassium chromate has a sharp metallic taste, is poisonous, and exhibits an alkaline reaction. The freezing- point of a saturated solution is -11.35° C., and the boiling-point 105.8° C. Potassium chromate is soluble in solutions of potassium sulphate; it is insoluble in alcohol. Its solution is readily reduced by the usual agents on treatment with weak acids, with ammonium chloride, or with certain other salts.

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