Sulphur Hexafluoride, SF₆

Sulphur Hexafluoride, SF₆, the first hexahalide to be discovered, is prepared by submitting sulphur to the action of fluorine in a copper tube. The issuing gas on condensation in a spiral tube of the same metal at - 80° C. becomes partly solidified; by allowing the solid to vaporise gradually and passing the vapours through potassium hydroxide solution and solid potassium hydroxide successively, the substance is rendered purer, complete purification being effected subsequently by re-solidification followed by fractional evaporation.

Sulphur hexafluoride is a colourless, odourless and incombustible gas of density 5.03 (air = 1). When solidified it forms a colourless crystalline mass of melting-point -56° C. As the vapour pressure of the solid attains one atmosphere at -62° C. the solid has no melting-point under ordinary pressures but volatilises without melting. The critical temperature is +54° C. The gas is only sparingly soluble in alcohol or water. It is remarkably inert, approaching even nitrogen in this respect. It is unaffected by the silent electric discharge, and even the spark discharge causes only slight decomposition. Mixed with hydrogen it withstands a high temperature, but under the influence of powerful electric sparks formation of hydrogen sulphide and hydrogen fluoride can be effected. At a red heat copper and silver are without action on the gas, although magnesium and sodium effect its decomposition; at lower temperatures, however, it resists even these two metals and sodium can be melted unchanged in an atmosphere of the gas, as also can the hydroxides of the alkali metals. Hydrogen chloride and ammonia are unaffected by sulphur hexafluoride, but hydrogen sulphide reacts rapidly according to the equation:

SF₆ + 3H₂S = 6HF + 4S.