Testing for sulfate ion | Qualitative Analysis for SO42-

The sulfate ion is the anion of sulfuric acid H2SO4 which is a strong acid, but somewhat weaker than the hydrogen halide acids ( HCl , HBr , HI ), and nitric acid ( HNO3 ).

Na2SO4 and K2SO4 solutions are neutral and colourless.



Sulfate ion exists as many compounds. Some of them dissolve in water and some of them make white or different colour precipitates. Sulfate compounds of 3d metals show colours. Sulfuric acid ( H2SO4 ) is a strong acid and completely dissociate in water to sulfate ion and two hydrogen plus ions. H2SO4 is a dibasic acid.



Sulfate of alkali metals

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


Soluble compounds of sulfate ion (in water)

  • All alkali metal sulfate are soluble in water. (Ex: Li2SO4, Na2SO4
  • MgSO4 is well soluble and colourless solution.
  • CuSO4 is a blue solution.

CaSO4 is somewhat soluble in water. When ion concentration is low, it is soluble. When ion concentration is high, it forms a white precipitate.

BaSO4 , SrSO4 , PbSO4 are sparingly soluble in water and form white precipitates.


Sulfate ion testing experiments

Following experiments are conducted to test occurrence of sulfate ion.



Sulfate ion and barium chloride solution

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


Sulfate and strontium chloride solution

Strontium chloride ( SrCl2 ) gives white precipitate ( SrSO4 ) with sulfate ion solutions. SrSO4 is insoluble in acids.


Silver nitrate and sulfate ions

AgNO3 does not form a precipitate with dilute sulfate solutions. Silver sulfate (Ag2SO4) is fairly soluble in water. However concentrated Ag2SO4 solution may be deposited as a precipitate.



Sulfate ion and calcium chloride

If Ca2+ and SO42- ions concentration in the water are high, a white precipitate ( CaSO4 ) forms. When adding water, precipitate dissolve to give a colourless solution again.


Sulfate and lead acetate ( Pb(CH3COO)2 )

When sodium sulfate is added to the lead acetate aqueous solution, lead sulfate (PbSO4), white precipitate is given. PbSO4 is soluble in caustic alkalis and in ammonium acetate solutions. But PbSO4 is not soluble in dilute acids.




testing-for-sulfate-ion

3d metal sulfate compounds

In here, we are going to study solubility and colours of 3d metal sulfate compounds.


Manganese sulfate | MnSO4

Manganese sulfate is a white orthorhombic crystals at solid state. MnSO4 dissolves in water.


Copper sulfate | CuSO4

Aqueous copper sulfate solution is blue colour. But when dehydrated copper sulfate is white a powder.




Test for sulfate ions in dilute sulfuric acid

Add barium chloride to dilute sulfuric acid. It will give a white precipitate strontium sulfate. Separate the precipitate from the solution. Then add dilute acid such as HCl to the precipitate. Pricipitate does not dissolve. SO that says us there were sulfate ions in the initail solution.



Precipitate list of sulfate ion

  • Strontium sulfate - SrSO4
  • Barium sulfate - BaSO4
  • Lead sulfate - PbSO4
  • Calcium sulfate is precipitated in higher concentrations

Questions


How to separate sulfate ion and sulfite ion?

Think, there are two colourless solutions. One solution contains sulfate ion and other one has sulfite ion. You are said to identify which solution have sulfate ion.

Add barium chloride to both solutions separately. You can see a forming white precipitates in both solutions.

Add dilute HCl to the both solutions. Then one precipitate dissolves and give colourless solution. That is for solution which contains sulfite ion.



How do you identify calcium carbonate and calcium sulfate?

There are two two solids and your teacher said that one of them is calcium carbonate and other one is calcium sulfate. Now you have to identify which solid is calcium carbonate by doing some tests.

This can be done by simple experiment. Add both solids to little amount two water samples separately. Two white precipitates form. Now add water to both flask. At one moment you can see, one white precipitate dissolve completely and give colourless solutions. That is calcium sulfate.



Can I identify sulfate ion from dilute HCl?

You cannot identify sulfate ion from dilute HCl. If sulfate ion exists as a precipitate, it does not dissolve in dilute HCl.

But if metal ion has a abililty to make a precipitate with chloride ion, we can see a precipitate is forming in the solution.


Silver sulfate and HCl reaction

When dilute HCl is added to the aqueous silver sulfate solution, a white precipitate (silver chloride - AgCl) is given.



Li2SO4 soluble or insoluble in water?

Lithium sulfate is soluble in water and it completely decomposes to lithium and sulfate ion.



How do you identify copper sulfate?

  • Add dilute NaOH to copper sulfate solution. It will give copper hydroxide which is blue precipitate.


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