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.
All sulfates of alkali metals
are solutions and colourless too.
Examples: Na2SO4, K2SO4
BaSO4, white precipitate forms. BaSO4 is not soluble in strong acids.
strontium chloride ( SrCl2 ) gives white precipitate ( SrSO4 ) with SO42- solutions. SrSO4 is insoluble in acids.
AgNO3 does not form a precipitate with dilute SO42- solutions. Ag2SO42- is fairly soluble in water. However concentrated Ag2SO42- solution may be precipitated.
If Ca2+ and SO42- ions concentration are high, a white precipitate ( CaSO4 ) forms.
PbSO4, white precipitate forms. PbSO4 is soluble in caustic alkalies and in ammonium acetate.