Ammonium ferro-heptanitroso sulphide, NH₄Fe₄(NO)₇S₃

Ammonium ferro-heptanitroso sulphide, NH₄Fe₄(NO)₇S₃.H₂O, is readily obtained by double decomposition of the sodium salt with ammonium carbonate. The precipitated salt is washed and recrystallised from warm water rendered faintly alkaline with ammonia. It may be prepared by passing nitric oxide for some ten hours into freshly precipitated ferrous sulphide suspended in water, without previous addition of ammonia. The salt is extracted from the excess of ferrous sulphide with warm alcohol, and recrystallised from water.

It also results when nitric oxide is passed through a mixture of ferrous hydroxide and carbon disulphide saturated with ammonia. The ferrous hydroxide is obtained by precipitation from ferrous sulphate, and the passage of nitric oxide is maintained for some eight hours. The ammonium salt is extracted from the mixture with hot alcohol. The same salt is formed even without the addition of ammonia, by the action of nitric oxide on ferrous hydroxide suspended in carbon disulphide.

The crystals are monoclinic prisms, less soluble than those of the sodium salt, but, like the latter, they begin to decompose at 80° C.

By warming the solutions of this salt with the hydroxides of the alkaline earth metals until ammonia ceases to escape, the corresponding salts of calcium, barium, and magnesium have been obtained. These, however, are less stable than the ammonium salt.