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Nickel Monosulphide, NiS

Nickel Monosulphide, NiS, occurs in nature as the mineral millerite. It may be prepared by heating nickel and sulphur together or by the action of hydrogen sulphide on nickel heated to redness. As obtained in either of these ways it is a bronze-yellow mass, insoluble in hydrochloric or sulphuric acid, but soluble in nitric acid and in aqua regia. Its density is 4.60, and specific heat 0.1248. Heated to redness in hydrogen, the sulphide remains unchanged, but in oxygen a basic sulphate is produced. Chlorine and water vapour attack it but slowly. The sulphide is also obtained as a grey amorphous mass by treating a solution of nickel sulphate at 80° C. with hydrogen sulphide under pressure.

It has been prepared in the crystalline form similar to the mineral by heating solutions of potassium sulphide and nickel chloride to 160-180° C. in a sealed tube. Ammonium thiocyanate and nickel chloride solutions under similar conditions produce the same sulphide.

Nickel sulphide may also be obtained by precipitation from solutions of nickel salts. It then occurs in three different polymeric forms, α, β, and γ, according to circumstances. Of these, α-nickel sulphide is soluble in dilute mineral acids, even with as low a concentration as 0.01 normal. β-nickel sulphide dissolves easily in 2-normal hydrogen chloride, whilst γ-nickel sulphide is insoluble unless oxidising agents are present.

When dilute reagents are used - for example, nickel sulphate and ammonium sulphide in dilute aqueous solution - probably the α compound is first formed, and then, when the solubility of this substance is exceeded, a precipitate separates out which subsequently more or less completely polymerises to the β and γ forms. This suffices to explain the well-known fact in qualitative analysis that whilst nickel sulphide cannot be precipitated in acid solution by hydrogen sulphide, yet when once precipitated in alkaline solution it is very difficult to dissolve again completely in dilute mineral acid.

The three varieties of nickel sulphide can be obtained in more or less pure forms separately as follow:

When dilute solutions of nickel sulphate and an alkali sulphide are slowly mixed at the ordinary temperature in the absence of air, 85 per cent, of the precipitate consists of α-NiS. This sulphide is stable in the absence of air if kept in contact with pure water. In contact with solutions that dissolve it to a slight extent it yields the β and γ varieties. β-nickel sulphide results, mixed with a little γ, when a solution of nickel acetate acidified with acetic acid is treated with hydrogen sulphide. Boiling with acetic acid converts it into the γ-form. It appears to be crystalline, γ-nickel sulphide may be obtained in a pure crystalline condition by boiling the mixed sulphides with 2-normal hydrochloric acid.
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