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Nickel Monoxide, NiO

Nickelous Oxide or Nickel Monoxide, NiO, occurs in nature as the mineral Bunsenite in regular octahedra, of density 6.4. In the laboratory it may be obtained by heating the hydroxide, sesqui-oxide, nitrate, or carbonate, when it results as a green crystalline powder, which, on heating, assumes a deep yellow colour. When nickel is heated in steam to a dull red heat, or in nitric oxide to at least 200° C., nickelous oxide is formed. The oxide may also be prepared by igniting nickel borate and lime at a high temperature, and separated as green crystals by treating the cooled mass with aqueous hydrogen chloride. Its density ranges from 6.6 to 6.8.

Nickelous oxide may be reduced more or less completely to the metallic condition by heating in a current of hydrogen at 220° C., in carbon monoxide at 120° C., and in ammonia above 200° C. Solid carbon reduces it at 450° C. These temperatures of reduction, however, depend upon the mode of preparation and subsequent chemical history of the oxide. Below 300° C. reduction is not complete, but the product- is very active catalytically.

When ignited in air, nickelous oxide absorbs oxygen, but loses it again as the temperature is raised. When prepared by ignition of nickel nitrate, the oxide contains occluded oxygen and nitrogen, the latter gas predominating.

In the electric furnace it melts and on cooling solidifies to green crystals. By repeated heating to 1000° C. in excess of potassium chloride, amorphous nickel monoxide yields minute octahedral crystals, light yellowish green in colour, and of density 7.45. It becomes incandescent in fluorine, and in hydrogen sulphide yields an oxysulphide, 2NiS.NiO. Acids dissolve it, yielding nickel salts.

Nickel oxide unites with oxides of several other metals to yield crystalline substances of varied composition. With alumina a blue aluminate, NiO. Al2O3, is obtained by fusing the two oxides together at 900° C., using potassium chloride as a flux. It crystallises in small octahedra or octahedral cubes, and even a small excess of either oxide can be distinguished microscopically. With manganese oxide, series of homogeneous mixed crystals have been obtained in an analogous manner, with compositions ranging from NiO.5MnO to 5NiO.MnO. The crystals are slightly yellowish green in colour, and form octahedra and octahedral cubes.

The oxides of magnesium, zinc, and tin likewise yield interesting mixtures.
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