Hydrosulfide

Hydrosulfide, or the hydrosulfide ion, is the anion HS− formed by deprotonation of hydrogen sulfide (H2S). In aqueous solution it represents the conjugate base of H2S and participates in acid–base equilibria that determine the relative proportions of H2S, HS−, and S2−, with pKa values around 7.0 and 12.9 at 25 °C.

HS− is a soft base and a relatively good nucleophile, readily forming metal sulfide precipitates such as iron sulfide (FeS) and zinc sulfide (ZnS). It also forms various complexes with transition metals and can act as a reducing agent in some organic transformations. Hydrosulfide salts, especially sodium hydrosulfide (NaHS) and potassium hydrosulfide (KHS), are common laboratory and industrial sources of HS−.

Industrially, hydrosulfide is produced by neutralizing hydrogen sulfide with base: H2S + OH− → HS− + H2O, or by treating sulfide-containing solutions to generate HS− in situ. In mineral processing, HS− is used for flotation and for precipitation of metal sulfides; in wastewater treatment, it helps remove certain metal ions by forming insoluble sulfides.

Safety considerations are important: hydrogen sulfide gas is highly toxic and flammable. Hydrosulfide salts release H2S upon acidification and can pose inhalation and contact hazards. Handling requires adequate ventilation, appropriate containment, and monitoring to minimize exposure.