Cadmium Oxide, CdO

Cadmium Oxide, CdO, occurs occasionally in nature as black, shining, regular octahedra of density 6.15 and hardness 5.

When cadmium is heated in the air it forms. the characteristic cinnamon-brown amorphous oxide, which can also be obtained by calcining the hydroxide or carbonate, or by heating cadmium fluoride in air or steam. Regnault obtained cadmium oxide (apparently the amorphous form) by heating the metal in steam.

The crystalline oxide may form when cadmium is sublimed in a tube containing air. It has been obtained in black crystalline cubes by burning cadmium in oxygen, in deep red cubes by heating the oxide similarly, and in microscopic octahedra by igniting cadmium nitrate.

The density of the artificial crystals has been variously given from 6.25 to 8.18.

Cadmium oxide is reduced by hydrogen at 400° C., and, though the action is reversible, the reduction takes place more easily than with zinc oxide.

Cadmium oxide is also more easily reduced by carbon than zinc oxide, and magnesium reduces it at a red heat. Chlorine, in the heat, readily converts it into the chloride, and when heated with sulphur or phosphorus vapour, cadmium sulphide or phosphides result.

It volatilises slowly or dissociates at 900°-1000° C., and the rate of loss becomes less as the oxide becomes denser or more crystalline. It is very insoluble in water, though acids dissolve it readily.

[Cd]+(O) = [CdO] (amorphous)+57.0 Cal.
[Cd]+(O) = [CdO] (mainly crystalline)+63.0 Cal.

The heat of solution in aqueous hydrofluoric acid is 23.450 Cal.