Cuprous oxide, Cu₂O

This Cuprous oxide, Cu₂O, occurs as the mineral cuprite or ruby copper. It is formed by reduction of alkaline solutions of complex cupric salts with a reducing sugar, such as dextrose, an example being the reduction of Fehling's solution, the oxide being deposited as a red, crystalline powder.

Cuprous oxide is produced in the form of an orange-yellow, amorphous gel containing water by the reduction of an alkaline cupric solution with sodium hyposulphite:

2CuO + Na₂S₂O₄ + 2NaOH = Cu₂O + 2Na₂SO₃ + H₂O.

A better method is the action of hydroxylamine on a cupric salt in presence of alkali. The initial light-yellow product is probably a hydroxide. In absence of air, the colour quickly changes to orange or brick-red, the phenomenon being probably due to elimination of water. The dry product contains 2 to 3 per cent, of water, but above low red heat this water is expelled, the metastable, yellow, amorphous oxide becoming transformed into the stable, red, crystalline variety.

Other methods of formation are the addition of sodium carbonate to a solution of cupric sulphate and sodium chloride reduced with sulphurous acid, and that of an alkaline solution of sodium potassium tartrate to a solution of cuprous chloride and sodium chloride. At temperatures below 350° C. copper reacts with nitrous oxide to form cuprous oxide; above this temperature the product is cupric oxide. Cuprous oxide is also formed at the anode in the electrolysis of a solution of cupric sulphate,12 and by heating cupric oxide in steam.

The oxide crystallizes in cubic octahedra, melting above 1230° C. according to Truthe, and at 1235° C. under a pressure of 0.6 mm. according to Roberts and Hastings Smyth, and of density 5.75 to 6.09. Its solubility in water is very slight, but it dissolves readily in aqueous solutions of ammonia, less readily in potassium hydroxide, and easily in hydrogen halides, with formation of complex derivatives. The solutions absorb oxygen, yielding cupric compounds. When heated in air, the oxide is stable only between 1025° and 1070° C. At 155° C. reduction of the oxide to metallic copper by hydrogen becomes perceptible, the velocity of the reaction increasing rapidly with rise of temperature. It is much accelerated by the presence of a dehydrating agent, such as barium monoxide. At 250° C. the oxide is converted by a solution of chlorine in carbon tetrachloride into a dark-brown, amorphous compound of the formula Cu₂OCl₂. At 1250° C. in an atmosphere of nitrogen it combines with ferric oxide to the crystalline, slightly magnetic cuprous ferrite, Cu₂O₅Fe₂O₃.