Hydrogen Trisulphide, H₂S₃

Hydrogen Trisulphide, H₂S₃, distils from the crude "persulphide " at approximately 69° C. when the pressure is reduced to 2 mm. It is a pale yellow mobile oil at the ordinary temperature, but on cooling becomes colourless, whilst when warmed it becomes more viscous and deepens in colour; at 90° C. decomposition sets in, accompanied by a vigorous evolution of hydrogen sulphide. Under atmospheric pressure at ordinary temperatures it undergoes slow decomposition, the escape of hydrogen sulphide causing it to foam. The liquid has a disagreeable pungent odour, suggesting camphor and sulphur chloride; its vapour has an irritating effect on the eyes and nose. The density is 1.496. On strong cooling (-75° C.) the liquid yields a crystalline mass after passing through a very viscous stage, no sharp solidifying-point being apparent; on warming again, however, there is a short delay at -53° to -52° C. in the rise of temperature. In bromoform solution the molecular weight by cryoscopic measurement has been found to agree with the formula H₂S₃. The oil is soluble in liquid hydrogen sulphide.

Hydrogen trisulphide is much more easily combustible than the crude parent hydrogen polysulphide. Exposure to light tends to accelerate its decomposition. It slowly reduces concentrated sulphuric acid to sulphur dioxide, whilst on contact with dry silver oxide, cupric oxide, lead dioxide or mercuric oxide, it bursts into explosive combustion, a residue of the metallic sulphide being obtained. Many other metallic oxides and most salts bring about a less vigorous decomposition; metals in the massive condition only react with it slowly. With potassium permanganate or dichromate the reaction is violent.