Uranyl Chromate, UO₂CrO₄

Uranyl Chromate, UO₂CrO₄.3H₂O, crystallises in yellow needles on evaporating a saturated solution of uranium trioxide in chromic acid. The crystals effloresce on exposure to air, and become anhydrous, with partial decomposition, at 200° C. They are readily soluble in water, 1 part of the salt dissolving in 13.3 parts of water; the chromate also dissolves in alcohol, forming a solution which decomposes on exposure to sunlight with separation of a brown precipitate. The addition of potassium chromate to an aqueous solution of uranyl chromate precipitates the basic salt, UO₃.2UO₂CrO₄.8H₂O, potassium dichromate also being formed, thus:

3UO₂CrO₄ + K₂CrO₄ = UO₃.2UO₂CrO₄ + K₂Cr₂O₇.

The basic salt, UO₃.UO₂CrO₄.6H₂O, has also been obtained.

Complex uranyl chromates may be prepared by treating an alkali uranate with warm concentrated chromic acid solution and evaporating the filtered solution over sulphuric acid. The potassium salt, K₂(UO₂)₂(CrO₄)₃.6H₂O, which may also be obtained by evaporating a mixed solution of potassium chromate and uranyl nitrate, forms yellow monosymmetric crystals,

a:b:c = 0.7566:1:1.9714; β = 72° 38',

partially decomposed by water. The ammonium salt, (NH₄)₂(UO₂)₂(CrO₄)₃.6H₂O, is isomorphous with the potassium salt,

a:b:c = 0.8016:1:1.0196; β = 72° 31'.

The sodium salt, Na₂(UO₂)₂(CrO₄)₃.10H₂O, separates in yellow warty crystals, which readily dissolve in water.

A pyridine compound, (UO₃)₂(CrO₃)₃.2C₅H₅N, has been obtained in the form of lustrous orange-coloured crystals.