Sulfate

Sulfate Ion, SO₄^2-

Acid Equilibria:

SO₄^2-(aq)+ H₂O(l) <==> HSO₄^-(aq) + OH^-(aq) K_b = 1 x 10^-12

HSO₄^-(aq) + H₂O(l) <==> H₂SO₄(aq) + OH^-(aq) K_b = 1 x 10^-15

Sulfate ion is a very weak base, while HSO₄^- is a fairly strong acid, with K_a = 0.01. On the other hand, H₂SO₄ is a very strong acid. Because it is such a weak base, sulfate ion undergoes negligible hydrolysis in aqueous solution.

Solubility:

Most sulfates, including those of Na⁺, K⁺, and NH₄⁺, are soluble in water. Exceptions that are insoluble are white lead(II) sulfate and white barium sulfate:

BaSO₄(s) <==> Ba²⁺(aq) + SO₄^2-(aq) K_sp = 1.4 x 10^-8

PbSO₄(s) <==> Pb²⁺(aq) + SO₄^2-(aq) K_sp = 1.1 x 10^-10

Formation of white BaSO₄ upon addition of Ba²⁺ to a solution of SO₄^2-, even if it is acidic, is a reliable test for sulfate. Other insoluble sulfates are those of calcium, strontium, and mercury(I).

Oxidation-Reduction:

Sulfate is a very weak oxidizing agent. Since sulfur is in its maximum oxidation number in sulfate ion, this ion cannot act as a reducing agent.

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