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).
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 (<7), HS− converts to H2S, hydrogen sulfide.
Sulfides are moderately strong reducing agents. They react with oxygen in the air in elevated temperatures to form higher-valence sulfur salts, such as sulfates and sulfur dioxide.
Read more about this topic: Sulfide
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