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Nickel oxide, whose chemical formula is NiO, is the main oxide of nickel and an alkali metal oxide. Several million kilograms of the compound of different qualities are produced annually, mainly as intermediates for nickel alloy production. It occurs naturally as the mineral arsenite in combination with sulfur and other minerals; the typical compounds are nickel sulfide, NiS; nickel arsenide, NiAs; nickel antimony oxide, NiSbO; and nickel thioarsenide, NiAsS. It can also be formed by the pyrolysis of nickel hydroxides and nitrates at high temperatures. Nickel oxide is in the +2 oxidation state, and it readily reacts with molten salts and other reducing agents to form metallic nickel.

It is soluble in acids and potassium cyanide but insoluble in cold water and caustic solutions. It has a gray to black color in the pure state, but it turns green upon exposure to air. This color change is attributed to non-stoichiometry, i.e., the ratio of nickel atoms to oxygen atoms deviates from 1:1. This property is exhibited by many other binary metal oxides, which are colored green in their stoichiometric state but black when they are not.

Upon inhalation, nickel oxide may cause lung irritation and pulmonary edema. It has a low acute toxicity, but is a potent irritant and causes skin irritation. It has also been reported to have an inflammatory effect on the kidney. A 2-year National Toxicology Program study of green NiO inhalation gave some evidence of carcinogenicity in F344/N rats and equivocal evidence in B6C3F1 mice.

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