They are precipitated from their alkaline solutions as cobalt sulphide by sulphuretted hydrogen, but this precipitation is prevented by the presence of citric and tartaric acids; similarly the presence of ammonium salts hinders their precipitation by caustic alkalis.
It may be prepared in the amorphous form by heating cobalt with sulphur dioxide, in a sealed tube, at 200° C. In the hydrated condition it is formed by the action of alkaline sulphides on cobaltous salts, or by precipitating cobalt acetate with sulphuretted hydrogen (in the absence of free acetic acid).
Certain yeasts exercise a reducing action, forming sulphuretted hydrogen, when sulphur is present.
It combines with sulphuretted hydrogen, in the .presence of water, to form the compound C 2 N 2 H 2 S, and in the presence of alcohol, to form the compound C 2 N 2.2H 2 S.
By the addition of sulphuretted hydrogen to the nitriles, or by the action of phosphorus pentasulphide on the acid-amides.
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