A sulfide is an anion of sulfur in its lowest oxidation state with the number of −2. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.
Properties
The dianion S2− exists only in strongly alkaline aqueous solutions. Such solutions can form by dissolution of H2S or alkali metal salts such as Li2S, Na2S, and K2S in the presence of extra hydroxide. The ion S2− is exceptionally basic with a pKa > 14. It does not exist in appreciable concentrations even in highly alkaline water, being undetectable at pH < ~15 (8 M NaOH).
Hydrogen sulfide ion
Hydrogen sulfide
Instead, sulfide combines with protons to form HS−, which is variously called hydrogen sulfide ion, hydrosulfide ion, sulfhydryl ion, or bisulfide ion. At still lower pH’s (<7), HS− converts to H2S, hydrogen sulfide.
Safety
Many metal sulfides are so insoluble in water that they are probably not very toxic. Some metal sulfides, when exposed to a strong mineral acid, including gastric acids, will release toxic hydrogen sulfide.
Organic sulfides are highly flammable. When a sulfide burns to give sulfur dioxide (SO2) gas.
Hydrogen sulfide, some of its salts, and almost all organic sulfides have a strong and putrid stench; rotting biomass releases these.
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