Barium Orthoarsenates

Tribarium Orthoarsenate, Ba₃(AsO₄)₂, is formed in large colourless plates when barium oxide is fused with sodium or potassium orthoarsenate and chloride. A trihydrate, Ba₃(AsO₄)₂.3H₂O, is prepared by precipitating excess of barium chloride solution with sodium arsenate, either cold or warm. A lower hydrate, 2Ba₃(AsO₄)₂.3H₂O, which still retains its water at 130° C., has been mentioned.

Barium chlorarsenoapatite, 3Ba(AsO₄)₂.BaCl₂, can be obtained by fusing a mixture of barium oxide, an alkali chloride, and potassium or sodium metarsenate containing more than 60 per cent, of the latter. Bromine and iodine arsenoapatites are also known.

The double salts, BaKAsO₄ and BaNH₄AsO₄.½H₂O, have been prepared.

H₂.Dibarium Orthoarsenate, BaHAsO₄.H₂O, or barium haidingerite, may be obtained by treating a fused mixture of barium oxide and potassium metarsenate with water, or by the action of disodium hydrogen arsenate on a hot solution of barium chloride. The precipitate may be dried at 100° C., but loses half its water at 120° C. It is slightly soluble in both precipitants. It forms transparent rhombic plates of density 3.926 at 15° C. From acetic acid it may be recrystallised in the form of octahedra.

A double salt, Ba(NH₄)₂H₂(AsO₄)₂, has been obtained.

Monobarium Orthoarsenate is obtained by dissolving the di- or tri-barium compound in arsenic acid and evaporating.