Cadmium Arsenates

The hydrated normal cadmium orthoar- senate, 2Cd₃(AsO₄)₂.3H₂O, falls as a white voluminous precipitate on adding alkali to a solution of an acid cadmium arsenate.

Cadmium Chlorarsenioapatite, 3Cd₃(AsO₄)₂.CdCl₂, is produced by fusing normal ammonium arsenate or the salt H₂Cd₅(AsO₄)₄.4H₂O with excess of cadmium chloride. Its density at 15° C. is 5.865, and it resembles the corresponding phosphatic compound. The analogous bromo compound is analogously prepared in long yellow prisms of density 6.017.

The precipitate from mixed solutions of cadmium sulphate and hydrogen disodium arsenate is 5CdO.2As₂O₅.5H₂O (or H₂Cd₅(AsO₄)₄.4H₂O). If the saturated solution of this salt in arsenic acid is heated, crystals of the acid salt, HCdAsO₄.H₂O, of density 4.164 at 15° C., separate. It has also been prepared by dissolving cadmium carbonate in a solution of arsenic acid. The white crystalline substance dissolves easily in dilute hydrochloric acid, and is said to be decomposed by water into 5CdO.2As₂O₅.5H₂O.

If the arsenic acid solution of H₂Cd₅(AsO₄)₄.4H₂O evaporates at ordinary temperatures, rhombic prisms of H₄Cd(AsO₄)₂.2H₂O, of density 3.241 at 15° C., separate. They are isomorphous with the corresponding phosphate, lose their water with partial decomposition at 70°-80° C., and excess of water converts them into flocculent H₂Cd₅ (AsO₄)₄.4H₂O.

Firm transparent jellies are produced when solutions of dihydrogen potassium arsenate and cadmium sulphate, of suitable concentrations, are mixed. They are unstable, and crystallise into 2Cd₃(AsO₄)₂.4CdHAsO₄.9H₂O.

The following double arsenates have been described: Na₄CdAs₂O₈, Na₈Cd₂As₂O₁₁, K₂Cd₂As₂O₈.