Sulfide Ion, S2-


Acid Equilibria:

Sulfide is a strong base, so solutions of sulfide in water are basic, due to hydrolysis. Sulfide solutions develop the characteristic rotten-egg odor of H2S as a result of this hydrolysis.

S2-(aq)+ H2O(l) <==> HS-(aq) + OH-(aq) Kb = 8.3

HS-(aq) + H2O(l) <==> H2S(aq) + OH-(aq) Kb = 1 x 10-7



Many sulfide salts are insoluble in acidic or basic solution:

Acidic: PbS, Bi2S3, CuS, CdS, HgS, As2S3, Sb2S3, SnS2

Basic: CoS, FeS, MnS, NiS, ZnS

Those salts that are insoluble in acidic solution are also insoluble in basic solution.

A common test for aqueous sulfide ion involves acidification to form H2S, then exposure to moistened lead acetate paper to form black PbS on the paper:

Pb(OAc)2 + H2S ---> PbS + 2HOAc


S2- or H2S can be oxidized to yellow elemental sulfur in a colloidal form with fairly mild oxidizing agents, including nitric acid.

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