Note 20 — Chapter XX · SULPHUR, SELENIUM, AND TELLURIUM

The sulphide P4S is obtained by cautiously fusing the requisite proportions of common phosphorus and sulphur under water; it is a liquid which solidifies at 0°, and may be distilled without undergoing change, but it fumes in air and easily takes fire. The higher sulphide, P2S, has similar properties. But little heat is evolved in the formation of these compounds, and it may be supposed that they are formed by the direct conjunction of whole molecules of phosphorus and sulphur; but if the proportion of sulphur be increased, the reaction is accompanied by so considerable a rise of temperature that an explosion takes place, and for the sake of safety red phosphorus must be used, mixed as intimately as possible with powdered sulphur and heated in an atmosphere of carbonic anhydride. The higher compounds are decomposed by water. By increasing the proportion of sulphur, the following compounds have been obtained: P4S3 as prisms (fuses at 165°, Rebs), soluble in carbon bisulphide, and unaltered by air and water; phosphorus trisulphide, P2S3, is the analogue of P2O3; it is a light yellow crystalline compound only slightly soluble in carbon bisulphide, fusible and volatile, decomposed into hydrogen sulphide and phosphorous acid by water, and, like the highest compound of sulphur and phosphorus, P2S5, it forms thio-salts with potassium sulphide, &c. This phosphorus pentasulphide corresponds with phosphoric anhydride; like the trisulphide it gives hydrogen sulphide and phosphoric acid with an excess of water. It reacts in many respects like phosphoric chloride. The sulphide PS2 is also known; the vapour density of this compound seems to indicate a molecule P3S6.

Phosphorus sulphochloride, PSCl₃, corresponds with phosphorus oxychloride. It is a colourless, pleasant-smelling liquid, boiling at 124°, and of sp. gr. 1·63; it fumes in air and is decomposed by water: PSCl₃ + 4H₂O = PH₃O₄ + H₂S + 3HCl. It is obtained when phosphoric chloride is treated with hydrogen sulphide, hydrochloric acid being also formed; it is also produced by the action of phosphoric chloride on certain sulphides—for example, on antimonious sulphide, also by the (cautious) action of phosphorus on sulphur chloride: 2P + 3S₂Cl₂ = 2PSCl₃ + 4S, by the action of PCl₅ upon certain sulphides, for example, Sb₂S₃, by the reaction: 3MCl + P₂S₅ = PSCl₃ + M₃PS₄ (Glatzel, 1893), and in the reaction 3PCl₃ + SOCl₂ = PCl₅ + POCl₃ + PSCl₃, showing the reducing action of phosphorus trichloride, which is especially clear in the reaction SO₃ + PCl₃ = SO₂ + POCl₃. Thorpe and Rodger (1889), by heating 3PbF₂ or BiF₃ with phosphorus pentasulphide (and also by heating AsF₃ and PSCl₃ to 150°), obtained thiophosphoryl fluoride as a colourless, spontaneously inflammable gas (see further on, Note 74 bis, and Chapter XIX., Note 25). The action of PSCl₃ upon NaHO gives a salt of monothiophosphoric acid (Würtz, Kubierschky), H₃PSO₃, which gives soluble salts of the alkalis.