Copper Arsenates

Cupric orthoarsenate, Cu₃(AsO₄)₂, is formed when metallic copper is heated at 180° to 200° C. with aqueous arsenic acid in a sealed tube; after some hours the product consists of green triclinic crystals of the arsenate mixed with arsenious oxide and unchanged copper. The tetrahydrate, Cu₃(AsO₄)₂.4H₂O, is produced by heating cupric nitrate solution with calcium arsenate, by the interaction of cupric chloride solution and silver arsenate, or by the action of aqueous arsenic acid on copper oxide. A pentahydrate is found in Nature as trichalcite.

Commercial methods of producing copper arsenate consist in heating copper arsenite at 600° to 700° C. in an oxygen-enriched atmosphere, or in heating basic copper chloride with an arsenate or arsenic acid.

When copper orthoarsenate is subjected to the action of hydrogen at high temperature and pressure it is reduced and a residue of approximate composition Cu₃As (domeykite) remains.

Concentration at 70° C. of a solution of cupric carbonate in an excess of arsenic acid yields pale-blue leaflets of cupric monohydrogen orthoarsenate, CuHAsO₄.H₂O. When boiled with water this yields a basic salt of composition Cu₃(AsO₄)₂.CuO.H₂O.

Several other acid salts have been described, including 2Cu₄H(AsO₄)₃.11H₂O and Cu₅H₂(AsO₄)₄.xH₂O (x = 2, 7, 9½ and 11½). Many basic salts are also known and are found frequently in Nature. Thus olivenite, clinoclasite, cornwallite and erinite occur in Cornwall; chalcophyIlite, euchroite, leucochalcite and tyrolite are also basic copper arsenates. Ammino-arsenates of composition Cu₃(AsO₄)₂(NH₃)₃.4H₂O and CuHAsO₄(NH₃)₂ (anhydrous and with 1H₂O) have been prepared by the action of alcoholic or aqueous ammonia on the respective arsenates. The compounds are stable at ordinary temperatures but lose water and ammonia when heated.

Complex orthoarsenates with the alkali metals, for example - NaCuAsO₄, 2Na₃AsO₄.Cu₃(AsO₄)₂, KCuAsO₄.Cu₃(AsO₄)₂, Na₂HAsO₄. 4Cu₃(AsO₄)₂.11H₂O and NaH₂AsO₄.2Cu₃(AsO₄)₂.5H₂O - and complex pyroarsenates of composition MCuAs₂O₇ (M = Ca, Sr or Ba), have been prepared.