Note 6 bis — Chapter XXIV · COPPER, SILVER, AND GOLD

Copper, besides the cuprous oxide, Cu2O, and cupric oxide, CuO, gives two known higher forms of oxidation, but they have scarcely been investigated, and even their composition is not well known. Copper dioxide (CuO2, or CuO2,H2O, perhaps CuOH2O2) is obtained by the action of hydrogen peroxide on cupric hydroxide, when the green colour of the latter is changed to yellow. It is very unstable, and is decomposed even by boiling water, with the evolution of oxygen, whilst the action of acids gives cupric salts, oxygen being also disengaged. A still higher copper peroxide is formed by heating a mixture of caustic potash, nitre, and metallic copper to a red heat, and by dissolving cupric hydroxide in solutions of the hypochlorites of the alkali metals. A slight heating of the soluble salt formed is enough for it to be decomposed into oxygen and copper dioxide, which is precipitated. Judging from Frémy's researches, the composition of the copper-potassic compound should be K2CuO4. Perhaps this is a compound of the peroxides of potassium, K2O2, and of copper, CuO2.