Copper Tungstates

Basic copper tungstates are found native as cuprotungstite, while the mineral cuproscheelite contains a copper calcium tungstate. By fusing together sodium tungstate, sodium chloride, and cupric chloride, light green octahedral crystals are obtained which probably consist of basic copper tungstate. The normal cupric tungstate, CuWO₄.2H₂O, is precipitated by the addition of a cupric salt to a normal tungstate solution. It is a light green powder which becomes brownish on heating, with loss of water, and finally melts, yielding a crystalline mass on cooling. It is insoluble in water, but soluble in ammonia, phosphoric acid, or acetic acid.

Cupric ditungstate, CuO.2WO₃.4H₂O, is stated by Anthon to be formed as a light green precipitate when a cupric salt is added to a ditungstate solution, but the actual composition of such precipitates is doubtful.

A cuprous cupric tungstate, of composition Cu₂WO₄.2CuWO₄, results as a reddish-brown crystalline powder after lixiviation of the melt obtained by fusing together copper sulphate and sodium tungstate in molecular proportions, or a mixture containing sodium tungstate (2 parts), sodium chloride (4 parts), and cuprous chloride (3 parts).

Copper paratungstate, 3CuO.7WO₃.19H₂O or 5CuO.12WO₃.33H₂O, is formed as a light blue or bluish-green microcrystalline precipitate on the addition of excess of copper sulphate to a solution of sodium paratungstate at 70° C. The salt after heating becomes yellow. It is insoluble in water. When equimolecular proportions of the two salts are used, greenish crystals of the double salt, 3Na₂O.3CuO.14WO₃.32H₂O, are obtained. If excess of the copper salt is added to a boiling solution of sodium paratungstate, slender bright blue needles of composition 4Na₂O.CuO.12WO₃.32H₂O separate. Rosenheim obtained from a cold mixture of the solutions, a light blue crystalline powder to which he gives the formula Na₃CuH₅[H₂(WO₄)₆].11.5H₂O.