Sulfate Ion, SO42-


Acid Equilibria:

SO42-(aq)+ H2O(l) <==> HSO4-(aq) + OH-(aq) Kb = 1 x 10-12

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

Sulfate ion is a very weak base, while HSO4- is a fairly strong acid, with Ka = 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.


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) + SO42-(aq) Ksp = 1.4 x 10-8

PbSO4(s) <==> Pb2+(aq) + SO42-(aq) Ksp = 1.1 x 10-10

Formation of white BaSO4 upon addition of Ba2+ to a solution of SO42-, even if it is acidic, is a reliable test for sulfate. Other insoluble sulfates are those of calcium, strontium, and mercury(I).


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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