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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

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

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.

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