Cadmium Arsenates

The hydrated normal cadmium orthoarsenate, 2Cd₃(AsO₄)₂.3H₂O, falls as a white voluminous precipitate on adding alkali to a solution of cadmium mono- or di-hydrogen arsenate in hydrochloric acid, or on addition of sodium orthoarsenate to a solution of cadmium sulphate. On drying at 100° C. the anhydrous salt remains.

Cadmium Monohydrogen Orthoarsenate, CdHAsO₄.H₂O, may be prepared by dissolving cadmium carbonate in aqueous arsenic acid. The white crystalline substance dissolves easily in dilute hydrochloric acid, and it is decomposed by water to form a salt of composition 5CdO.2As₂O₅.5H₂O. The existence of this salt appears to be well established and it may be regarded as cadmium dihydrogen tetraorthoarsenate, Cd₃(AsO₄)₂.2CdHAsO₄.4H₂O. It is formed as a precipitate when solutions of cadmium sulphate and sodium monohydrogen orthoarsenate are mixed. If its saturated solution in arsenic acid is heated, the monohydrogen salt, CdHAsO₄.H₂O, separates, the crystals of which have density 4-164 at 15° C. If, on the other hand, crystallisation of the solution occurs at ordinary temperatures, rhombic prisms of cadmium dihydrogen orthoarsenate, Cd(H₂AsO₄)₂.2H₂O, of density 3.241 at 15° C., separate. This salt is isomorphous with the corresponding phosphate, loses water with partial decomposition at 70° to 80° C., and with excess of water reverts to the acid salt, 5CdO.2As₂O₅.5H₂O.

Firm transparent jellies are produced when solutions of potassium dihydrogen 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: Na₄Cd(AsO₄)₂, Na₈Cd₂As₂O₁₁, K₂Cd₂(AsO₄)₂. Cadmium chlorarsenoapatite, 3Cd₃(AsO₄)₂. CdCl₂, is produced by fusing normal ammonium arsenate or the acid salt 5CdO.2As₂O₅.5H₂O 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 similarly prepared as long yellow prisms of density 6.017.

Cadmium Metarsenate, Cd(NaO₃)₂, has been obtained by dissolving cadimum chloride in arsenic acid at 200° C. and drying the precipitate at 100° C. When fused with cadmium oxide or carbonate, cadmium pyroarsenate, Cd₂As₂O₇, is obtained as colourless crystals.