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.
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.
Here, we will discuss about properties, colours, solubility in water and other characteristics of sulfates of alkali metal cations and alkali earth 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|
Solubility of alkali earth metalsulfates vary from top to the bottom of the group IIA.
Following experiments are conducted to test occurrence of sulfate ion. A summarized tests
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.
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.
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.
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.
Calcium sulfate is soluble to some extent in water. If Ca2+ and SO42- ions concentration in the water are high, a white precipitate ( CaSO4 ) forms. When adding water, precipitate dissolves to give a colourless solution again.
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.
In here, we are going to study solubility and colours of 3d metal sulfate compounds.
Manganese sulfate is a white orthorhombic crystals at solid state. MnSO4 dissolves in water.
Aqueous copper sulfate solution is blue colour. But when dehydrated copper sulfate is white a powder.
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.
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.
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.
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.
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.
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.
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.
When dilute HCl is added to the aqueous silver sulfate solution, a white precipitate (silver chloride - AgCl) is given.
Lithium sulfate is soluble in water and it completely decomposes to lithium and sulfate ion.
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.
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.
Metal SO42- 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 SO42- ion.
Qualitative Analysis for anions and cations for Grade 12 ClassEmission of gases Identify ammonium salts identify d block cations by ammonia solution Solubility if inorganic Compounds, s,p,d block elements Identify halides Identify carbonate ions identify chromium compounds Testing for thiosulfate (S2O32-) Testing for phosphate ion