Zinc Arsenates

The normal orthoarsenate occurs naturally, as an octahydrate, as the pinkish mineral kottigite, Zn₃(AsO₄)₂.8H₂O. It crystallises in the monoclinic system with a density of 31 and a hardness of 2.5-3.0.

The trihydrate, Zn₃(AsO₄)₂.3H₂O, is precipitated from zinc sulphate solutions by trisodium arsenate. The same substance is precipitated in a white voluminous form when excess of alkali is added to solutions of acid zinc phosphates in hydrochloric acid.

The anhydrous salt has been prepared by heating zinc carbonate and a solution of arsenic acid to 200° C., and, in prismatic crystals of density 4.913 at 15° C., by fusing zinc chloride and arsenic acid.

When the trihydrate remained in contact with a solution of arsenic acid for more than a year, the metarsenate, Zn(AsO₃)₂, was obtained from the filtered solution as a heavy white powder.

A precipitated jelly of the acid orthoarsenate, ZnHAsO₄.H₂O, is obtained by treating zinc with excess of arsenic acid solution, heating, and filtering at 100° C., or by mixing solutions of potassium dihydrogen arsenate and zinc sulphate in appropriate proportions. The jelly gradually crystallises, or forms crystals which are either rhombic or monoclinic if treated with hot water. Small white tabular or needlelike crystals of this salt were prepared by dissolving zinc oxide in a solution of arsenic acid. It has also been obtained by prolonged contact between zinc orthophosphate trihydrate and a solution of arsenic acid, and by prolonged digestion at 100° C. of the precipitate obtained by adding ammonium arsenate to a solution of a zinc salt.

The salt 5ZnO.2As₂O₅.4H₂O, which lost one quarter of its water at 100° C., was obtained by precipitating zinc sulphate solution with sodium arsenate.

The basic salt 4ZnO.As₂O₅.H₂O is produced when ZnHAsO₄.H₂O is heated with water. It occurs naturally as adamine. This mineral crystallises in the orthorhombic system with a density of 4.35 and a hardness of 3.5. It is green, yellow, violet, or colourless, and is usually formulated as Zn(Zn.OH)AsO₄.

The following double arsenates have been described: - NaZnAsO₄, Na₂ZnAs₂O₇, K₂Zn₂As₂O₈, and NH₄.ZnAsO₄.

Yellow flocculent zinc thioarsenate, Zn₃(AsS₄)₃, has been prepared by interaction between zinc sulphate and sodium thioarsenate. The double thioar senate, NaZnAsS₄.4H₂O, was also obtained in yellow crystals.