Antimony Pentasulphide, Sb₂S₅

Antimony Pentasulphide, Sb₂S₅, appears to have been first described by Basil Valentine. Early writers knew it as sulphur auratum, while Glauber (1654) described it in his "Pharmacopoeia spagyrica" as Panacea Antimonialis.

It is doubtful if pure antimony pentasulphide has been prepared, most samples, and certainly all commercial products, containing free sulphur; possibly no higher sulphide exists than antimony tetrasulphide. A fairly pure product may be obtained by passing hydrogen sulphide through a solution containing antimony pentachloride (free from tervalent antimony) and 12 to 15 per cent, free hydrochloric acid. After treatment with carbon disulphide the resulting substance has a composition corresponding to Sb₂S₅, and does not react with ammoniacal silver nitrate.

Commercial antimony pentasulphide, or "golden antimony sulphide," is as already stated always impure, and is generally supposed to be a mixture of antimony tetrasulphide and free sulphur. The usual method of preparation is by the decomposition of an alkali thioantimonate, such as Schlippe's salt, Na₃SbS₄, by means of dilute acid:

2Na₃SbS₄ + 3H₂SO₄ = (Sb₂S₄ + S) + 3Na₂SO₄ + 3H₂S
2Na₃SbS₄ + 6HCl = (Sb₂S₄ + S) + 6NaCl + 3H₂S

Sulphurous acid has also been suggested, and is recommended in preference to the stronger acids since the evolution of hydrogen sulphide by the latter is objectionable. When sulphurous acid is used, the formation of excess of free sulphur (which would contaminate the product) may be prevented by the careful addition of sodium sulphite. In order to avoid the production of brownish products the solution must be kept acid.

Antimony pentasulphide is a reddish-yellow or brown powder with a faint smell and a sweetish taste. When heated it loses sulphur, darkens in colour, and at 170° C. is gradually converted into antimony trisulphide. When heated in air it is oxidised to antimony trioxide, which volatilises. It is reduced to metal by heating in a current of hydrogen, and is decomposed by exposure to moist air, antimony trioxide being formed. It is only partially oxidised by treatment with ammoniacal hydrogen peroxide.

The pentasulphide is partially soluble in ammonium hydroxide, forming a yellowish solution and leaving as residue a mixture of antimony trisulphide and sulphur. It is decomposed by nitric acid, and by hydrochloric acid (density 1.12) forming antimony trichloride, sulphur and hydrogen sulphide; treatment with sulphuryl chloride converts it into antimony pentachloride.

From most commercial samples of antimony pentasulphide sulphur can be removed by treatment with carbon disulphide, or other solvents of sulphur, the proportion removed depending upon the method of preparation of the pentasulphide.

Alkali hydroxides and carbonates react with antimony pentasulphide with formation of a mixture of alkali antimonates and thioantimonates, the former being precipitated. From the solutions obtained on filtering, antimony pentasulphide is reprecipitated by the addition of acids; while from solutions in alkali carbonate the precipitate consists of antimony trisulphide. Barium and strontium hydroxides behave in a somewhat similar manner. Fusion with potassium cyanide causes partial reduction, some potassium thioantimonite being formed.

Commercial antimony pentasulphide is a mixture of variable composition depending upon the method of manufacture. It is employed extensively in the rubber industry, and many methods of assay have been proposed.