Sulfite

Sulfite Ion, SO₃^2-

Acid Equilibria:

Sulfite ion is a weak base, but does undergo some hydrolysis to produce basic solutions. In acidic solution, the equilibria are shifted to form sulfurous acid, resulting in the evolution of SO₂ gas. Sulfur dioxide is a colorless gas with a characteristic choking odor.

SO₃^2-(aq) + H₂O(l) <==> HSO₃^-(aq) + OH^-(aq) K_b = 1.8 x 10^-7

HSO₃^-(aq) + H₂O(l) <==> H₂SO₃(aq) + OH^-(aq) K_b = 1 x 10^-12

H₂SO₃(aq) <==> H₂O(l) + SO₂(g)

Solubility:

The sulfites of Na⁺, K⁺, and NH₄⁺ are soluble in water. Most other sulfites are insoluble in water. However, due to the basic nature of SO₃^2-, all sulfites dissolve in acidic solution.

Oxidation-Reduction:

Sulfite ion is readily oxidized to sulfate. On prolonged exposure to air, this oxidation occurs with atmospheric oxygen:

2SO₃^2-(aq) + O₂(g) --> 2SO₄^2-(aq)

Sulfite or sulfur dioxide will decolorize permanganate. This decolorization serves as a convenient test for sulfur dioxide:

2MnO₄^-(aq) + 5SO₂(g) + 2H₂O(l) --> 5SO₄^2-(aq) + 2Mn²⁺(aq) + 4H⁺(aq)

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