Cobalt Monoxide, CoO

Cobaltous Oxide, Cobalt Monoxide, CoO, may be obtained from cobaltie oxide by reduction in a current of ammonia, or of hydrogen at about 350° C. At higher temperatures metallic cobalt is obtained.

Cobaltous oxide also results when cobaltic oxide or cobalt carbonate is heated to redness in carbon dioxide or nitrogen; when cobalt itself is heated in nitric oxide to 150° C.; and when cobalt sulphate is heated to whiteness.

Obtained by any of the foregoing methods cobaltous oxide is a stable powder, greyish in colour, which on heating in air takes up oxygen forming cobalto-cobaltic oxide. Its density is about 5.68 when obtained by calcination from a higher oxide. Prepared from the sulphate it retains the crystalline form of the latter, and its density is 6.70. It is reduced to metallic cobalt when heated in carbon monoxide at temperatures above 450° C., or in hydrogen above 250° C. An allotropic modification of cobaltous oxide is formed by heating cobalto-cobaltic oxide with 2 to 3 per cent, of carbon at about 900° C. It is yellowish green in colour.6 Neither allotrope is magnetic.

When heated gently in air it yields tri-cobalt tetroxide, Co₃O₄. In hydrogen sulphide the oxysulphide CoO.CoS is produced.

It unites with acids to form cobaltous salts. It replaces ammonia in its salts and is dissolved by potassium hydroxide, yielding a deep blue solution.

Cobalt oxide unites with or forms solid solutions with many other metallic oxides at high temperatures. Thus, with alumina at 1100° C. a blue aluminate, CoO.Al₂O₃, is formed, whilst at higher temperatures a green compound, 4CoO.3Al₂O₃, is produced. With stannic oxide, cobalt oxide yields a dark green ortho-stannate, 2CoO.SnO₂, or Co₂SnO₄. The same compound is produced by precipitating cobalt metastannate from a solution of a cobalt salt by addition of potassium metastannate, and heating the precipitate. The colour is then greenish blue, the blue tinge resulting from the excess of stannic oxide. With chromic oxide a green chromite, CoO.Cr₂O₃, is obtained; the vanadate and tantalate are likewise green, whilst the uranate is yellow.

Cobalt Blue consists essentially of oxides of cobalt and aluminium with more or less zinc oxide. It is obtained by calcining a mixture of alum and cobalt sulphate, and is used by artists in painting porcelain. Save for this it has now no commercial value. Cobalt blue is also known as Cobalt Ultramarine, Thenard's Blue, and King's Blue.

Cobalt Green or Rinmann's Green was discovered in the eighteenth century by a Swede named Rinmann, who obtained it by heating a mixture of zinc oxide and cobalt oxide. It can be obtained in a variety of other ways. Large crystals may be prepared by heating zinc oxide with cobalt carbonate to 1100° C., potassium chloride being used as a flux. The product is cooled in carbon dioxide when thin tabular crystals are usually obtained, green in colour, and of density 5.69. Analysis indicates a composition corresponding to CoO.4.8ZnO. It is not a definite chemical entity, but a solid solution of the two oxides in one another. Turquoise Green is similar, but contains some oxide of chromium in addition. Both pigments are used in porcelain painting. Cobalt Red is obtained in a similar manner to Rinmann's green, save that the zinc oxide in the latter is replaced by magnesium oxide.