Cupric oxide, CuO

Cupric oxide, CuO, is obtained as a black, amorphous powder by igniting cupric hydroxide, carbonate, or nitrate. It is also formed on copper anodes in electrolytic oxidation. The amorphous oxide can be converted into lustrous, cubic tetrahedra by heating with potassium hydroxide, the crystalline variety being also produced by ignition to redness in a platinum crucible of a small amount of cuprous chloride.

A blue variety of cupric oxide is said to have been prepared by precipitating cupric sulphate with sodium hydroxide in presence of dissolved aluminium. On heating strongly, it blackens, the change being probably due to an agglomeration of the particles. On the other hand, Muller and Ernst state that agitation of cupric oxide or hydroxide with sodium hydroxide produces a blue precipitate of sodium cuprite. On warming the mixture, this substance dissolves, and on cooling separates in crystals. In contact with excess of water, these crystals decompose to form the black oxide.

Cupric oxide occurs as the hexagonal tenorite, and also as the rhombic or monoclinic melaconite. According to Slade and Farrow, the oxide melts above 1148° C., with partial decomposition into cuprous oxide; but Smyth and Roberts state that it does not melt with dissociation below 1233° C. Its density is 6.32 to 6.43. Its mean specific heat is 0.1420 between 12° and 98° C., and its heat of formation 37.16 Cal.

At 1020° C. it is said to lose half its oxygen, forming cuprous oxide. Heating in steam reduces it to cuprous oxide. Under the influence of heat, hydrogen reduces cupric oxide to metallic copper, the reaction being considerably accelerated by the presence of a dehydrating agent, such as barium monoxide. The amorphous form displays a remarkable power of sorption towards gases such as oxygen, nitrogen, and carbon dioxide. For the hydrogenation of unsaturated organic compounds it appears to be a better catalyst than reduced copper. A thin layer precipitated on asbestos and heated ignites an oxy-hydrogen jet. It is reduced by aluminium in accordance with the equation

3CuO + 2Al = Al₂O₃ + 3Cu,

the heat evolved being 650 cal. per kilogram of mixture. Cupric oxide is completely dissociated at the temperature of the electric furnace. When heated at 1000° C. with fused potassium chloride containing sodium chloride, it yields cuprous oxide, oxygen, and a basic cupric chloride of the formula 3CuO,CuCl₂,4H₂O. Cupric oxide is somewhat soluble in fused caustic alkalies, and in their concentrated solutions. Several coloured hydrates have been described, including the blue or green CuO,H₂O, the olive-green CuO,0.8H₂O, and the brown CuO,0.35H₂O. A magnetic, crystalline cupric ferrite, CuO,Fe₂O₃, is formed by precipitating a mixture of cupric and ferric salts with alkali.

The affinity-relations of cupric oxide and of cupric hydroxide have been investigated by Allmand by tensimetric and electrometric methods. His results prove crystalline cupric hydroxide to be stable in ordinary moist air, but to be converted into cupric oxide through elimination of water by agitation with alkaline or ammoniacal solutions at 25° C. To explain the apparent contradiction, Allmand assumes that lapse of time induces an ageing of cupric oxide, due to an irreversible augmentation of molecular complexity. Crystalline cupric hydroxide is stable towards freshly prepared cupric oxide, but unstable towards aged samples of the oxide. The unsaturation with respect to fresh cupric oxide of a saturated solution of cupric hydroxide, and its super- saturation with respect to aged cupric oxide, explain the apparently anomalous stability-relations of cupric hydroxide and of cupric oxide and water. Allmand has deduced the dissociation-pressure of cupric oxide at 1030° C. to be 200 mm., a value in good agreement with the pressure 170-180 mm. experimentally determined at the same temperature by other investigators.