Zinc Arsenates

Zinc Orthoarsenate, Zn₃(AsO₄)₂, has been obtained in the anhydrous form by heating zinc carbonate with aqueous arsenic acid to 200° C. or by fusing a mixture of zinc chloride and arsenic pentoxide. The latter process yields prismatic crystals of density 4.913 at 15° C. The trihydrate, Zn₃(AsO₄)₂.3H₂O, is precipitated from zinc sulphate solutions by normal sodium orthoarsenate. The same substance is precipitated in a white voluminous form when excess of alkali is added to solutions of acid zinc arsenates in hydrochloric acid. The octahydrate, Zn₃(AsO₄)₂.8H₂O, occurs naturally, associated with cobalt and nickel ores, as kottigite, a pinkish mineral found in Germany in the massive form or in crystalline crusts with a fibrous structure; the crystals are isomorphous with vivianite. The octahydrate may be obtained artificially by adding a soluble arsenate to a solution of a zinc salt, the quantity added being insufficient to remove completely the acidity of the solution; or by mixing solutions of zinc monohydrogen orthoarsenate and zinc sulphate. The precipitate formed is amorphous but, if kept in contact with the solution for some days, the crystalline octahydrate is formed.

Zinc Monohydrogen Orthoarsenate, ZnHAsO₄.H₂O, is obtained as a jelly by treating zinc with excess of aqueous arsenic acid, heating, and filtering at 100° C.; or by mixing solutions of potassium dihydrogen orthoarsenate and zinc sulphate in appropriate proportions. The jelly gradually crystallises on keeping, or when treated with hot water; the crystals are either rhombic or monoclinic. Small white tabular or needle-like crystals of this salt may be prepared by dissolving zinc oxide in aqueous arsenic acid. The salt has also been obtained by prolonged contact between zinc orthophosphate trihydrate and aqueous arsenic acid, and by prolonged digestion at 100° C. of the precipitate obtained by adding ammonium arsenate to a solution of a zinc salt.

An acid salt, of composition 5ZnO.2As₂O₅.4H₂O, has been obtained as a precipitate by mixing aqueous solutions of zinc sulphate and sodium orthoarsenate. It loses one quarter of its water at 100° C. By heating the monohydrogen orthoarsenate in a sealed tube at 200° C. rhombic crystals of composition 5ZnO.2As₂O₅.nH₂O, probably Zn₃(AsO₄)₂.2ZnHAsO₄.(n-1)H₂O, are obtained.

Zinc Metarsenate, Zn(NaO₃)₂, has been described as a heavy white powder formed by allowing zinc orthoarsenate trihydrate to remain in contact with aqueous arsenic acid for more than a year and then filtering the solution.

Zinc Pyroarsenate, Zn₂As₂O₇, is obtained by dissolving zinc oxide in molten potassium metarsenate. It yields tabular crystals isomorphous with manganese pyroarsenate. The salt is decomposed by water. A basic salt, of composition Zn(OH)₂.Zn₂As₂O₇.7H₂O, has been obtained by mixing solutions of zinc orthoarsenate and zinc acetate, and boiling the precipitate with water for several weeks; silky crystalline needles are finally obtained, from which all the water may be expelled at 300° C.

The basic salt 4ZnO.As₂O₅.H₂O is produced when zinc monohydrogen orthoarsenate crystals are heated with water. It occurs naturally as adamite, which may be green, yellow, violet or colourless and is usually formulated Zn(Zn.OH)AsO₄. Legrandite, a bright yellow transparent mineral associated with massive blende in Mexico, has a composition approximating to Zn₁₄(OH)(AsO₄)₉.12H₂O.

The following double arsenates have been described: NaZnAsO₄, Na₂ZnAs₂O₇, K₂Zn₂As₂O₈ and NH₄ZnAsO₄.