Cadmium Orthophosphate, Cd₃(PO₄)₂

When cadmium pyrophosphate is heated in sealed tubes to about 300° C. with water, a solution of acid phosphate and an insoluble tribasic phosphate result. Amorphous Cd₃(PO₄)₂ is also precipitated from solutions of cadmium salts by normal sodium phosphate. When its solution in fused cadmium chloride is slowly cooled, long hexagonal prisms of cadmium chlorapatite, 3Cd₃(PO₄)₂.CdCl₂, of density 5.46 at 15° C., separate. The analogous cadmium bromapatite occurs in flattened oblique lamellae of density 4.965 at 15° C.

The acid phosphate, Cd₅H₂(PO₄)₄.4H₂O, is precipitated by hydrogen disodium phosphate from hot solutions of cadmium chloride or sulphate. The amorphous precipitate quickly becomes crystalline and, after purification, forms small prismatic hexagons of density 3.98 at 15° C. The density of the crystals becomes 4.12 at 15° C. when they are precipitated from their solution in phosphoric acid.

When a saturated solution of the above salt in cold dilute phosphoric acid is slowly evaporated at normal temperature, orthorhombic prisms of monocadmium phosphate, H₄Cd(PO₄)₂.2H₂O, crystallise.