Sulfide Ion (S²⁻)
- Page ID
- 97285
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 \(\ce{H2S}\) as a result of this hydrolysis.
\[\ce{S2^{-}(aq) + H2O(l) <=> HS^{-}(aq) + OH^{-}(aq) } \nonumber \]
with \(K_b = 8.3\) and
\[\ce{H^{-}(aq) + H2O(l) <=> H2S(aq) + OH^{-}(aq)} \nonumber \]
with \(K_b = 1 \times 10^{-7}\).
Solubility
Many sulfide salts are insoluble in acidic or basic solution:
- Acidic: \(\ce{PbS}\), \(\ce{Bi2S3}\), \(\ce{CuS}\), \(\ce{CdS}\), \(\ce{HgS}\), \(\ce{As2S3}\), \(\ce{Sb2S3}\), \(\ce{SnS2}\)
- Basic: \(\ce{CoS}\), \(\ce{FeS}\), \(\ce{MnS}\), \(\ce{NiS}\), \(\ce{ZnS}\)
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 \(\ce{H2S}\), then exposure to moistened lead acetate paper to form black \(\ce{PbS}\) on the paper:
\[\ce{Pb(OAc)2 + H2S -> PbS + 2HOAc} \nonumber \]
Oxidation-Reduction:
\(\ce{S2^{-}}\) or \(\ce{H2S}\) can be oxidized to yellow elemental sulfur in a colloidal form with fairly mild oxidizing agents, including nitric acid.