Cupric carbonates

The normal salt has not been prepared. Malachite, CuCO₃,Cu(OH)₂, occurs in monoclinic crystals, density 3.7 to 4. It has been produced artificially. Azurite, 2CuCO₃,Cu(OH)₂, forms monoclinic crystals, density 3.5 to 3.88. It has been obtained by a laboratory method.

The formation of basic carbonates of copper by the interaction of solutions of cupric sulphate and of the carbonates of sodium has been investigated by Pickering. Sodium carbonate precipitates a blue, basic carbonate, 5CuO,2CO₂,nH₂O, which is converted by drying over sulphuric acid at 100° C. into another hydrate of green colour, 5CuO,2CO₂,3H₂O. In moist air the green hydrate becomes reconverted into the blue form:

5CuSO₄ + 8Na₂CO₃ + 3H₂O = 5CuO,2CO₂ + 5Na₂SO₄ + 6NaHCO₃.

The blue carbonate is transformed by concentrated aqueous sodium carbonate into cupric hydroxide, and by aqueous sodium hydrogen carbonate into malachite, 2CuO,CO₂,H₂O. Pickering considered ordinary commercial copper carbonate to be similar in constitution to malachite, a view questioned by Dunnicliff and Lai.

Sodium hydrogen carbonate and cupric sulphate react to precipitate a blue, basic carbonate, 5CuO,3CO₂,nH₂O, converted by drying at 100° C. into another blue hydrate, 5CuO,3CO₂,7H₂O. Another basic carbonate is also produced in the same reaction. It has the formula 8CuO,3CO₂,6H₂O, is dark blue in colour, and becomes green at 100° C. No other basic carbonate was isolated by Pickering. All the products are insoluble in water and sodium-carbonate solution, but dissolve slightly in solutions of carbon dioxide and of sodium hydrogen carbonate, with production of the normal carbonate or a double carbonate.

Feist has prepared a basic carbonate, 7CuO,4CO₂,H₂O, by powdering together crystallized cupric sulphate and sodium carbonate, and then adding water. It is difficult to separate the substance from a basic cupric sulphate simultaneously formed. Auger has described an amorphous basic carbonate of the formula 8CuO,5CO₂,7H₂O. Another basic carbonate, 7CuO,2CO₂,5H₂O, has been prepared by the interaction of a mixture of sodium carbonate and sodium hydrogen carbonate with cupric sulphate in aqueous solution. Complex carbonates of copper with sodium and potassium have also been obtained. An example of this type of double salt of the formula Na₂Cu(CO₃)₂,3H₂O crystallizes on addition of a solution of cupric acetate to one of sodium carbonate and sodium hydrogen carbonate at 50° C. It forms needles or rosettelike agglomerations, and above 100° C. it is converted into cupric oxide and sodium carbonate with elimination of water and carbon dioxide. It is decomposed by water, but can be recrystallized from a concentrated solution of sodium carbonate containing sodium hydrogen carbonate.