Testing for sulfate SO42- ion

The sulfate ion is the anion of sulfuric acid H2SO4. This a strong acid, but somewhat weaker than the hydrogen halide acids ( HCl , HBr , HI ) , and nitric acid ( HNO3 ). SO42- ion is colourless.
Na2SO4 and K2SO4 solutions are neutral.

Sulfate ion exists in many compounds. Some of them dissolve in water and some of them make precipitates. Sulfate compounds of d block metals take colours. Sulfuric acid ( H2SO4 ) completely dissociate in water into sulfate ion and two hydrogen plus ions.

Sulfates of alkali metals

All sulfates of alkali metals are solutions and colourless too.
Examples: Na2SO4, K2SO4

Soluble solutions (in water ) of sulfates

  • All alkali metal sulfates
  • MgSO4
  • CuSO4

CaSO4 is somewhat soluble in water.

BaSO4 , SrSO4 , PbSO4 are sparingly soluble in water.

Sulfate ion testing

Sulfate and barium chloride

BaSO4, white precipitate forms. BaSO4 is not soluble in strong acids.

Sulfate and strontium chloride

strontium chloride ( SrCl2 ) gives white precipitate ( SrSO4 ) with SO42- solutions. SrSO4 is insoluble in acids.

AgNO3 and SO42- ions

AgNO3 does not form a precipitate with dilute SO42- solutions. Ag2SO42- is fairly soluble in water. However concentrated Ag2SO42- solution may be precipitated.

Sulfate and calcium chloride

If Ca2+ and SO42- ions concentration are high, a white precipitate ( CaSO4 ) forms.

Sulfate and lead acetate ( Pb(CH3COO)2 )

PbSO4, white precipitate forms. PbSO4 is soluble in caustic alkalies and in ammonium acetate.


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