Calcium Chromate, CaCrO₄

Calcium Chromate, CaCrO₄, may be obtained by fusing a mixture of calcium chloride with potassium and sodium chromate. It crystallises in fine needles. It may be precipitated from a concentrated solution of a calcium salt with sodium chromate, or formed by the action of chromic acid on calcium carbonate. Reddish-yellow crystals of the dihydrate are usually obtained. Calcium chromate is prepared industrially by the action of the oxygen of the air on a strongly heated mixture of powdered chrome iron and lime. It is used as a pigment.

Calcium chromate is much more soluble in both water and alcohol than the corresponding strontium and barium salts, and this affords a means of separation from the latter.

Solubility of calcium chromate

The relations between anhydrous calcium chromate and its different hydrates have been carefully studied by Mylius and von Wrochem. The α-dihydrate, CaCrO₄.2H₂O, forms yellow monoclinic crystals isomorphous with gypsum. These effloresce when exposed to air, and when in contact with a saturated solution pass readily into the rhombic β-dihydrate. At 12° C. the β-compound loses water and passes into a rhombic monohydrate, CaCrO₄.H₂O. In contact with a solution at 100° C., especially if mixed with calcium chloride or glycerine, a hemihydrate, 2CaCrO₄.H₂O, also rhombic, is formed. It does not begin to lose its water of crystallisation readily until 400° C. is reached. If a supersaturated solution of calcium chromate is heated above 36° C. the salt gradually separates in the anhydrous form, passing through the intermediate stages of the α-di- and the mono-hydrate. Even after prolonged contact with water the anhydrous form does not pass into the hydrated condition, showing that all the other modifications are unstable with respect to it within the range of temperature considered. This is borne out by the solubility relations, the anhydrous chromate being the least soluble. As may be seen from Fig., the solubility curves of the various forms show no breaks such as would indicate the passage of one modification into another. With the exception of that for the β-dihydrate, the curves all show a decrease of solubility with rise of temperature.

The solubilities at 18° C. are as follows: -

Modification	α-dihydrate.	β-di- hydrate	Mono- hydrate	Hemihydrate	Anhydrous
Solubility in per cent. CaCrO4	14.3	10.3	9.6	4.4	2.3

A double potassium calcium chromate in the form of yellow needles, K₂CrO₄.CaCrO₄.2H₂O, may be obtained by mixing concentrated solutions of the two salts and shaking for some time. The same salt has been obtained at temperatures below 45° C. in the form of large orthorhombic prisms. In contact with a solution of potassium chromate at 60° C., these are gradually transformed into small hexagonal prisms of anhydrous salt, K₂CrO₄.CaCrO₄.

Rakowski obtained two different modifications of the dihydrate, the α-form belonging to the rhombic system, and the β- to the hemihedral triclinic. The former is unstable with respect to the latter above about 0° C., which is the transition point. The heat of solution of the α-di- hydrate is 6.993 Cal., and that of the β-compound 5.459 Cal.

At 97° C. the salt breaks up into its constituents. A solution saturated at 20° C. becomes cloudy at 55°-60° C., and a precipitate is formed at 90° - 100° C. corresponding with the formula K₂CrO₄.4CaCrO₄.25H₂O.

Several other double salts have been mentioned, K₂CrO₄.5CaCrO₄, K₂CrO₄.4CaCrO₄.2H₂O, K₂CrO₄.CaCrO₄.H₂O, 2K₂CrO₄.CaCrO₄, and K₂CrO₄.Na₂CrO₄.CaCrO₄.H₂O.