Cuprous sulphate, Cu₂SO₄

Cuprous sulphate, Cu₂SO₄, cannot be isolated by methods analogous to those employed for the preparation of the cuprous halides. Metallic copper dissolves in solutions of cupric sulphate containing free sulphuric acid, an equilibrium corresponding with the scheme

2Cu^•⇔ Cu^•• + Cu

being attained. At ordinary temperatures the proportion of cuprous sulphate formed is small, but it is increased by rise of temperature.

Cuprous sulphate was first isolated by Recoura by the interaction of molecular proportions of cuprous oxide and methyl or ethyl sulphate at 160° C. in absence of moisture. The presence of excess of the alkyl sulphate induces decomposition of the product with formation of cupric sulphate:

Cu₂O + (CH₃)₂SO₄ = Cu₂SO₄ + (CH₃)₂O.

The salt is dried by washing with ether and placing in a vacuum-desiccator over sulphuric acid.

The sulphate is a light-grey powder, instantly decomposed by water in accordance with the equation

[Cu₂SO₄] + Aq. = CuSO₄ (dissolved) + [Cu] + 21 Cal.

Since the heat of solution of anhydrous cupric sulphate in water is 15.8 Cal., the transformation of a gram-molecule of cuprous sulphate into cupric sulphate and copper is attended by the evolution of 5.2 Cal., the reaction being exothermic. The formation of cuprous sulphate is an endothermic reaction, although that of each of the other cuprous salts is accompanied by evolution of heat. At ordinary temperatures, in presence of dry air, cuprous sulphate is stable, but contact with moist air induces a very slow decomposition. The presence of ether renders the action of moist air very rapid, the black mass formed being converted by water into a solution of cupric sulphate and a black oxide with properties similar to those of the product assumed by Rose to be a suboxide, Cu₄O. Heating at 200° C. in air oxidizes cuprous sulphate to a brownish-black mixture of cupric oxide and cupric sulphate.

Cuprous sulphate dissolves in concentrated hydrochloric acid and in concentrated ammonium hydroxide. The ammoniacal solution yields a colourless, crystalline compound, Cu₂SO₄,4NH₃,H₂O, decomposed by water into metallic copper and an ammoniacal cupric compound. Cuprous sulphate is also dissolved to a slight extent by glacial acetic acid. The deep-violet solution thus produced is rapidly oxidized by air to a blue liquid, from which crystals of an acid cupric acetate are deposited.