Nickel Arsenates

Nickel Orthoarsenate, Ni₃(AsO₄)₂, is obtained when the octahydrate is heated or treated with nitric acid. It occurs in Nature as the mineral xanthiosite. The dihydrate, Ni₃(AsO₄)₂.2H₂O, is precipitated when an alkali arsenate is added to a solution of a nickel salt. It yields apple-green crystals which give up water on heating, turning red and at a high temperature yellow. It is soluble in mineral acids and in aqueous ammonia. The octahydrate, Ni₃(AsO₄)₂. 8H₂O, occurs as the mineral annabergite near Annaberg in Saxony. It may be prepared by addition of ammonium orthoarsenate to a nickel salt, the solution being kept neutral. If ammonia is present in excess, the molecules of water are partially replaced by ammonia, yielding hydrated ammino-orthoarsenates, which are insoluble in water but readily soluble in acids and in ammonia. The following compounds have been distinguished: Ni₃(AsO₄)₂.NH₃.7H₂O; Ni₃(AsO₄)₂.2NH₃.6H₂O and Ni₃(AsO₄)₂.3NH₃.5H₂O. When heated to dull redness all these salts yield the anhydrous arsenate.

Nickel orthoarsenate also occurs in Nature associated with magnesium (cabrerite), cobalt (forbesite), copper (Iindackerite) and other metals.

By treating nickel carbonate with arsenic acid at varying temperatures, precipitates of the following compositions have been obtained: 4NiO.As₂O₅.14H₂O, 3NiO.As₂O₅.11H₂O, 2.5NiO.As₂O₅.8H₂O.

Nickel Monohydrogen Orthoarsenate, NiHAsO₄.H₂O, has been obtained by heating in a sealed tube at 160° C. finely divided nickel and an excess of concentrated aqueous arsenic acid. The pale green rhombic prisms are soluble in water and, on strong ignition, lose their water of crystallisation. An acid salt of composition 5NiO.2As₂O₅.3H₂O has also been prepared.