Many of the above transformations are slow and afford in stances of "dynamic" allotropy. (See ALLOTROPY.) Hydrogen sulphide or sulphuretted hydrogen results when hydrogen is passed over molten sulphur at C; at higher temperatures it begins to dissociate into its elements again ; it is also formed when acids act on metallic sulphides, ferrous sulphide and dilute hydrochloric or sulphuric acid being most frequently used, but in this case the gas is contaminated with hydrogen. The purest hydrogen sulphide is obtained either by warming precipitated antimony sulphide with hydrochloric acid or by dropping water on to aluminium sulphide produced by a thermite process from sulphur and aluminium powder. It results from the putrefaction of organic substances containing sulphur or from the destructive distillation of coal; it is evolved when organic compounds (such as paraffin wax) are heated with sulphur.
Hydrogen sulphide, a colourless, poisonous gas having the odour of rotten eggs, is moderately soluble in water to give a very feebly acidic solution, which gradually deposits sulphur on ex posure to the air; it is much more soluble in alcohol. It burns with a pale blue flame to give sulphur dioxide and water. The gas may be liquefied at 18° C by a pressure of 17 atmospheres, and the liquid boils at 61.8° under ordinary pressure, and solidifies at —83°. It is decomposed by the halogens or by sulphuric acid with the liberation of sulphur. It reacts with many metals to form sulphides, which are also produced by its action on aqueous solutions of metallic salts; the latter reaction is extensively util ised in chemical analysis. Hydrogen sulphide is also used as a reducing agent : in acid solution it reduces ferric to ferrous salts, chromates to chromium salts, and manganates to manganese salts; in alkaline solution it reduces many organic nitro-compounds to the corresponding amino-derivatives. It is rapidly attacked by oxidising agents, yielding primarily sulphur and water.
When metallic polysulphides are acted upon by hydrochloric acid they give a mixture of hydrogen polysulphides, and by frac tional distillation under reduced pressure two of these have been separated as oils, viz., the disulphide, H2S2, b.p. and the
trisulphide, b.p. 69°/2 mm. They are readily decomposed by the action of light or of water.
Sulphur chloride, occurs as a by-product in the manu facture of carbon tetrachloride from carbon disulphide and chlorine, and may readily be prepared by passing chlorine over molten sulphur and condensing the issuing gas. It is an amber coloured, fuming liquid with a very irritating smell; it boils at 139° and freezes at —8o°. It is slowly decomposed by water to sulphur and a mixture of sulphur acids. Its chief use is in the vulcanisation of rubber, since it dissolves sulphur very readily.
Sulphur dichloride, had its entity definitely established by T. M. Lowry, L. P. McHatton, and G. G. Jones in 1927, before which it was frequently said to be a mixture of the mono- and tetra-chlorides. Chlorine reacts very slowly with the monochloride, but in the presence of a trace of iodine reaction becomes rapid, and by taking advantage of this discovery (among others), these authors were able to show that both and S3C14 exist ; the former, however, becomes unstable just about its melting point (-83°) and gives SC14 and lower chlorides; S3C14 has a narrow range of existence but melts at —103°.
Sulphur tetrachloride, SCI, is formed by saturating with chlorine at —22° ; it is a yellowish-brown liquid, solidifying to a crystalline mass at 30° C, and dissociating rapidly above It is violently decomposed by water: SC14+3H20 = H2S03+4HC1.
Sulphur bromide, S2Br2, is a dark red liquid boiling at under 0.2 mm. pressure and decomposing at about Sulphur forms no definite compound with iodine.