Cobalt Arsenates

Cobalt Orthoarsenate, Co₃(AsO₄)₂.8H₂O, may be precipitated as pink microscopic needles by adding ammonium orthoarsenate to an aqueous solution of a cobalt salt, or by stirring cobalt oxide or carbonate with aqueous arsenic acid. It occurs widely distributed in Nature as the mineral erythrite or cobalt bloom in the form of monoclinic crystals isomorphous with vivianite, Fe₃(PO₄)₂. 8H₂O, and perhaps with nickel ochre, Ni₃(AsO₄)₂.8H₂O. Its crystal-lographic elements are a:b:c = 0.7937:1:0.7356; β = 74°51'. The indices of refraction along the three axes for sodium light are a = 1.629, β = 1.663 and γ = 1.701. The mineral is soluble in hydrochloric acid forming a blue solution which reddens on dilution. A cobalt hydrogen arsenate of composition Co₂H₂(AsO₄)₂.3.5H₂O is precipitated by adding ether to a mixture of syrupy arsenic acid (2 parts) and alcoholic cobalt nitrate (3 parts). The salt readily loses water. Cobalt dihydrogen tetrarsenate, Co₃(AsO₄)₂.2CoHAsO₄.2H₂O, has been obtained as red needle-shaped crystals by heating excess of arsenic acid with cobalt carbonate in a sealed tube at 235° C. It is insoluble in water and loses its water of crystallisation when heated.

A basic arsenate of composition Co₃(AsO₄)₂.CoO is obtained when sodium carbonate is added to a solution of cobalt orthoarsenate in nitric acid and the precipitate heated to fusion. On cooling, deep blue prismatic crystals are formed which, when powdered, turn a rose-red colour. The product, which is soluble in acids, has been used as a pigment. An impure basic arsenate is known under the name of zaffre and is used in painting porcelain. It may be obtained by roasting iron-free cobaltite, or by addition of sodium carbonate to a solution of cobaltite in nitric acid, when the zaffre is obtained as a precipitate. A salt of composition 5CoO.2As₂O₅.15H₂O (and also with 28H₂O) has been prepared.

Cobalt Pyroarsenate, Co₂As₂O₇, is formed when cobalt oxide is fused with potassium metarsenate; violet crystals are obtained on cooling. A dihydrate, Co₂As₂O₇.2H₂O, has also been described.

Ammino-orthoarsenates of composition Co₃(AsO₄)₂(NH₃).7H₂O, Co₃(AsO₄)₂(NH₃)₂.6H₂O and Co₃(AsO₄)₂(NH₃)₃.5H₂O have been obtained by the interaction in solution of a cobalt salt, arsenic acid or an arsenate and free ammonia, ammonium salts being present. The nature of the product depends on the amount of free ammonia present.

Colloidal cobalt arsenate has been obtained in the form of an opalescent jelly either by mixing in the cold a solution of a cobalt salt of a strong acid with potassium dihydrogen orthoarsenate, or by successively treating an aqueous solution of a cobaltous salt with ammonium sulphate, acetic acid and an excess of sodium orthoarsenate.

The following mixed arsenates have been prepared: NaCoAsO₄, KCoAsO₄, Na₄Co₄As₆O₂₁. Complex salts corresponding to the ferri-arsenates do not appear to be formed