Testing for sulfate ion (Sulphate | SO₄^2-) in Qualitative Analysis

There are several tests for sulfate ion to identify from other anions in qualitative analysis. Some of metal sulfate compounds are soluble in water and some metal sulfates form precipitates. Sulfate compounds of several metal cations form precipitates with specific colours. In this tutorial we will discuss followings regarding how to identify sulfate ion by considering solubility, colours and more physical and chemical characteristics with example questions.

Compounds of sulfates

Sulfate ion exists as many compounds. Some of them dissolve in water and some of them give 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.

Sulfates of group IA metal cations

Here, we will discuss about properties, colours, solubility in water and other characteristics of sulfates of alkali metal cations and alkali earth metal cations.

Sulfate of alkali metal cations

All sulfates of alkali metals are soluble in water and give colourless aqueous solutions. Solubility of sulfates of alkali metal cations are mentioned in following table.

Alkali metal sulfate	Chemical formula	Solubility (g/100 ml)
Lithium sulfate	Li2SO4	34.9 at 250C
Sodium sulfate	Na2SO4	28.1 at 250C
Potassium sulfate	K2SO4	120 at 250C

Sulfate of alkali earth metal cations

Solubility of alkali earth metalsulfates vary from top to the bottom of the group IIA.

• Beryllium sulfate and Magnesium sulfate are soluble in water and give colourless solutions.
• Calcium sulfate and Strontium sulfate are dissolved to some extent in water. If concentration of cations and sulfate ion is high, there is a possibility of giving the respective precipitate. Both Calcium sulfate and Strontium sulfate are white colour precipitates.
• Barium sulfate is not soluble in water and give a white precipitate.

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Soluble compounds of sulfate ion (in water)

• All alkali metal sulfate are soluble in water. ( Examples: Li₂SO₄, Na₂SO₄ )
• From alkali earth metal sulfates, Beryllium sulfate and Magnesium sulfate are readily soluble and give colourless solution.
• Most of the 3d metal sulfate compounds are soluble. CuSO₄ aqueous solution is a blue solution.

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Sulfate ion testing experiments

Following experiments are conducted to test occurrence of sulfate ion. A summarized tests

Testing for Sulfate ion with aqueous barium chloride solution

Add aqueous barium chloride (BaCl₂) solution to the sulfate ion solution and observe the differences. Barium sulfate (BaSO₄), is a white precipitate. BaSO₄ is not soluble in strong acids and dilute acids.

Case study: In the presence of sulfite ion

In the presence of sulfite ion, a white precipitate (Barium sulfite) can be given similar to Barium sulfate. Therefore, a fault decision can be made by thinking of the presence of sulfate ion. To remove sulfite ion first, add dilute HCl to remove sulfite ion before adding barium chloride.

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Testing of sulfate ion with strontium chloride solution

Add aqueous strontium chloride (SrCl₂) solution to the sulfate ion solution and observe the physical changes. Strontium chloride ( SrCl₂ ) gives Strontium sulfate ( SrSO₄) which is a white precipitate with sulfate ion solutions. Strontium sulfate is insoluble in acids.

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Testing of sulfate ion with Silver nitrate

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

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Testing of Sulfate ion and calcium chloride

Calcium sulfate is soluble to some extent in water. If Ca²⁺ and SO₄^2- ions concentration in the water are high, a white precipitate ( CaSO₄ ) forms. When adding water, precipitate dissolves to give a colourless solution again.

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Testing of Sulfate ion solution with lead acetate (Pb(CH₃COO)₂)

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

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.

Testing for sulfate ions in dilute sulfuric acid solution

Add barium chloride to dilute sulfuric acid solution. It will give a white precipitate, barium sulfate. Separate the precipitate from the solution and add a dilute acid such as HCl to the precipitate. Precipitate does not dissolve in dilute acid. So that says us, there were sulfate ions in the initial solution as an anion.

Precipitate list of sulfate ion with colours

• Strontium sulfate - SrSO₄ - white
• Barium sulfate - BaSO₄ - - white
• Lead sulfate - PbSO₄ - white
• Calcium sulfate is precipitated in higher concentrations of calcium and sulfate ions.

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Questions about sulfate ion solution

Identifying sulphate and halide compounds of sodium ?

To identify sulphate and halide compound, a cation should form a precipitate with those one anion. Add aqueous barium nitrate solution to sodium halide solution and sodium sulfate solution.

Barium forms barium sulphate, the white precipitate in sodium sulphate solution and no changes in sodium halide solution because all barium halides are soluble and colourless solutions.

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 because precipitates of metal sulfates are do not dissolve in dilute acids.

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

How to identify dilute hydrochloric acid and sulfuric acid solutions?

Both acids dissolve in water very wel . Hydrochloric acid gives chloride ions and sulfuric acid gives sulfate ions. So testing should be tested for chloride ion and sulfate ion to identify two anions.

Add barium hydroxide to samples of HCl and H₂SO₄

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In one flask, a white precipitate forms. Write two reactions occurs in the flasks to know what are the products. Bioth reactions are acid - base reactions

HCl + Ba(OH)₂ = BaCl₂ + H₂0

H₂SO₄ + Ba(OH)₂ = BaSO₄ + 2H₂O

BaCl₂ is a colourless aqueous solution. But BaSO₄ is a white precipitate and deposited in the bottom of the solution which is clearly observed. So dilute hydrochloric acid and sulfuric acid solutions can be identified.

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.

How we distinguish sulphate ion using hcl and barium chloride?

First add dilute HCl to remove sulfite ion if sulfite ion is in the solution. Otherwise, sulfite ion will give barium sulfite as a white precipitate.

After HCl adding, then add barium chloride to the sulfate solution. Then a white precipitate, BaSO₄ is given.

colour of sulphate ion?

Sulphate ion does not cause for colours. When sulphate ion is combined with cation such as 3d metal ion which have the ability to show colours, those metal sulphate compounds show colours.

What is the colour of SO₄^2- ion

Metal SO₄^2- solutions can be colourless or has colours. As an example aqueous CuSO₄ is a blue colour solution. But, aqueous MgSO₄ solution is a colourless one. So you should understand, colour of the solution may not depend always on SO₄^2- ion.

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