Tri-cobalt Tetroxide, Co₃O₄

Cobalto-cobaltic Oxide, Tri-cobalt Tetroxide, Co₃O₄, is the essential constituent of commercial oxide of cobalt. It is produced when either of the other oxides is strongly heated in air, and when cobalt nitrate is ignited.

It is a black, hygroscopic powder, of density 5.8 to 6.3. It dissolves in hydrochloric acid, evolving chlorine.

A crystalline form of this oxide results when cobalt chloride or a mixture of this and ammonium chloride is heated in air or oxygen, microscopic octahedra possessed of metallic lustre being obtained.

The oxide differs from its iron analogue in not being magnetic. When strongly heated in the oxyhydrogen flame it yields metallic cobalt, which, as it cools, becomes tarnished with a film of oxide. The various actions of hydrogen, carbon monoxide, and aluminium upon this oxide have already been discussed.

Cobalto-cobaltic oxide has been prepared in three states of hydration. The trihydrate, Co₃O₄.3H₂O, results on warming cobaltous hydroxide with potassium persulphate and heating the resulting product to 100° C. with dilute nitric acid. It is also produced as a brown precipitate on boiling a solution of roseo-cobaltic sulphate.

The hexahydrate, Co₃O₄.6H₂O, occurs as a green precipitate on adding baryta water to a cold solution of cobalt chloride in vacuo.

The heptahydrate, Co₃O₄.7H₂O, results after exposing for several months in a vacuum desiccator the brown precipitate obtained by allowing cobaltous hydroxide to remain in contact with air.