Hydrogen Pentasulphide, H₂S₅

Hydrogen Pentasulphide, H₂S₅, is a thin, pale yellow oil, obtained by treating pure dry ammonium pentasulphide crystals with carefully dried formic acid. No free sulphur is produced in this preparation as is the case when a mineral acid, or even 90 per cent, formic acid, is used. The liquid has a density of 1.67 at 16° C. Cryoscopic measurements in benzene give the molecular weight as 152.5; H₂S₅ requires 162. It cannot be distilled, but at 40° C. and 15 mm. it froths considerably. On cooling it suddenly becomes viscous at -25° C., almost solid at -35° C. and glassy at -50° C., melting to a clear liquid again on warming. It readily dissolves sulphur.