Ferric sulphide, Fe₂S₃

Hydrated ferric sulphide, Fe₂S₃.Aq., is readily produced by treating moist ferric hydroxide (or the hydroxide suspended in water) with hydrogen sulphide: -

2Fe(HO)₃ + 3H₂S = Fe₂S₃ + 3H₂O.

It is thus produced during the commercial purification of coal gas from sulphuretted hydrogen. In the laboratory preparation of the pure substance the reaction should be continued for several hours in the absence of air, the precipitated sulphide being black when the reaction is complete.

A precipitate of ferric sulphide is also obtained when ammonium sulphide is added to a solution of a ferric salt, the alkali remaining in excess. If, however, the ferric salt is present in excess, the precipitate appears to consist of a mixture of ferrous sulphide and free sulphur. As obtained by either of the foregoing methods, the sulphide is hydrated and unstable in air. Dilute hydrochloric acid decomposes it completely into ferrous chloride, with evolution of hydrogen sulphide and a simultaneous deposition of sulphur. When boiled with water it yields ferric hydroxide and hydrogen sulphide.

In the moist condition in the absence of air, or in the presence of excess hydrogen sulphide, ferric sulphide is transformed into a mixture of disulphide and ferrous sulphide: -

Fe₂S₃ = FeS₂ + FeS.

This transformation, which takes a week at the ordinary temperature, may be effected in a few hours at 60° C. Upon exposure to air in the presence of alkaline substances, ferric sulphide becomes light yellow in colour, and sulphur is deposited.

Anhydrous ferric sulphide,Fe₂S₃, is obtained by exposing the hydrated compound in a vacuum over phosphorus pentoxide. It is then pyrophoric if suddenly brought into the air; otherwise it is quite stable.

Anhydrous ferric sulphide is produced in massive form by gently heating iron and sulphur together. As obtained in this way it is a yellow, non-magnetic mass, soluble in dilute mineral acids, density 4.4.

Ferric sulphide yields stable compounds with the sulphides of certain other metals. Some of these occur free in nature. Thus copper pyrites is generally regarded as Cu₂S.Fe₂S₃; Barnhardtite, 2Cu₂S.Fe₂S₃; Cubanite, CuS.Fe₂S₃.

These are amongst the best known. Other more or less stable double sulphides have been prepared in the laboratory.