Qualitative Analysis of Anions

Our Objective

Our objective is to determine the anion present in the given salt.

The Theory

What is Qualitative Inorganic Analysis?

The qualitative inorganic analysis is a method of analytical chemistry that seeks to find out the elemental composition of inorganic compounds through various reagents. It is mainly focused on detection of ions in an aqueous solution, so the substances in other states need to be converted into an aqueous solution before starting the experiment.

The procedure for detecting ions (cations and anions) in aqueous solutions are called Cation Analysis and Anion Analysis.

Let's discuss the Qualitative Analysis of Anions.

Qualitative Analysis of Anions

Preliminary Tests

Some preliminary tests are done before going to the anion analysis.

(A) Physical Examination: Colour and Smell

The physical examination of the unknown salt involves the study of colour, smell and density. Characteristic smell helps to identify some anions such as acetate, sulphide etc.

(B) Dry Heating Test

This test is performed by heating a small amount of salt in a dry test tube. Quite valuable information can be gathered by carefully performing and noting the observations. On heating, some salts undergo decomposition, thus evolving the gases.

(C) Identification of Anions (Acid Radicals)

The identification of the radicals is first done on the basis of the preliminary tests. The Dry heating test is one of the preliminary tests performed earlier which may give some important information about the acid radical present. The other preliminary tests are based on the fact that:

  • CO32-, S2-, NO2- and SO32- react with dil. H2SO4 to give out CO2, H2S, NO2 and SO2 gases respectively. These gases on identification indicate the nature of the anion present in the salt.
  • Cl-,Br-,I-,NO3- and C2O42- and CH3COO- react with conc. H2SO4 but not with dil. H2SO4 to produce characteristic gases.
  • SO42- and PO43- react neither with dil H2SO4 nor with conc. H2SO4. These are, therefore, identified by individual tests.

Thus, these anions may be identified by performing the following tests below:

1) Dil. H2SO4 Tests

Treat a pinch of the salt with dil. H2SO4 and identify the gas evolved.

Chemical Reactions Involved in Dil.H2SO4 Test:

Dilute H2SO4 decomposes carbonates, sulphides, sulphites and nitrites in cold to give gases. These gases on identification indicate the nature of the anion present in the salt.

(a) Carbonate: On treating the solid carbonate, CO2 is given off in the cold with brisk effervescence.

(b) Sulphide: Sulphides when treated with dil. H2SO4 give H2S gas.

(c) Sulphite: On heating solid sulphite with dil.H2SO4, SO2 gas is evolved

(d) Nitrite: On treating solid nitrite with dil. H2SO4, nitric oxide (NO) gas is evolved which readly gives brown fumes of  NO2 with the oxygen of the air.

2) Conc. H2SO4 Test

If no action takes place with dil. H2SO4, warm a pinch of the salt with conc. H2SO4 and identify the gas evolved.

Chemical Reactions Involved in Conc. H2SO4 Test

(a) Chlorides: Chloride salts react with conc. H2SO4 to evolve hydrogen chloride (HCl) gas.

(b) Bromides: Bromide salts react with conc. H2SO4 to evolve bromine gas.

(c) Iodides: Iodide salts react with conc. H2SO4 to evolve vapours of iodine.

(d) Nitrates: Upon reaction with conc.H2SO4 nitrates evolve NO2 gas. 

(e) Acetates: Acetates react with conc. H2SO4 to produce vapours of acetic acid.

(f) Oxalates: Oxalates salts react with conc. H2SO4 to evolve a mixture of carbon dioxide and carbon monoxide. 

3) Independent Group. (SO42- and PO43-) Test

If the salt does not react with dil H2SO4 as well as with conc. H2SO4 test for SO42-and PO43- by performing their individual tests.

4) Potassium permanganate Test

This test is performed by using dilute sulphuric acid and potassium permanganate as reagents. This test helps in the detection of Cl-, Br-, I- and C2O42-.

Chemical reactions involved in Potassium permanganate test

Wet Tests for Acid Radicals (Anions)

Let us discuss the chemical reactions involved in the confirmation of Anions:

Confirmation of Carbonate (CO32-)

(a) Reaction with di.l HCl

Carbonate on reaction with dil. HCl gives CO2 gas that reacts with lime water to produce a white precipitate of calcium carbonate that turns lime water milky. In case of soluble carbonate, this test is performed with water extract and in case of insoluble carbonates, this test is performed with the solid salt.

(b) Magnesium sulphate test

This test is performed in the case of soluble carbonates only. Soluble carbonates react with the magnesium sulphate solution to form a white precipitate of magnesium carbonate.

Confirmation of Sulphite (SO32-)

(a) Barium chloride test

Sulphites on reaction with barium chloride to form a white precipitate of barium sulphite. Barium sulphite dissolved in dil. HCl with the evolution  of sulphur dioxide gas.

(b) Potassium permanganate test

The colour of potassium permanganate is discharged when it reacts with sulphite. The reaction is given below.

(c) Potassium dichromate test

A green colour is obtained when sulphites react with potassium dichromate solution.

Confirmation of Sulphide (S2-)

(a) Sodium nitroprusside test

Sulphides give a violet or purple colouration with the sodium nitroprusside solution due to the formation of Na4[Fe(CN)5NOS].

(b) Lead acetate test

Sulphides react with lead acetate to form a black precipitate of lead sulphide.

(c) Cadmium carbonate test:

Sulphides react with a suspension of cadmium carbonate to form a yellow precipitate of cadmium sulphide.

Confirmation of Nitrite (NO2-)

(a) Ferrous sulphate test

Nitrites give a dark brown or black colouration in Ferrous sulphate test due to the formation of FeSO4.NO.

(b) Starch - Iodide test

Nitrites react with potassium iodide in the presence of dilute sulphuric acid to liberate iodine. Iodine forms a blue-black complex with starch.

(c) Diphenylamine test

In the presence of nitrites, diphenylamine is oxidised, giving a blue colouration.

Confirmation of Nitrate (NO3-)

(a) Diphenylamine test

In the presence of nitrates, diphenylamine is oxidised, giving a blue colouration.

(b) Copper chips test

In this reaction copper chips reduces nitrates to reddish brown NO2 gas.

(c) Brown-ring test

This test can be performed by adding a solution of iron (II) sulphate to a solution of nitrate, followed by the slow addition of concentrated sulphuric acid, such that the sulphuric acid forms a layer below the aqueous solution. The formation of a brown ring at the junction of two layers indicates the presence of nitrate.

The overall reaction is the reduction of nitrate ion by iron (II) which reduced to iron (I) and formation of a nitrosonium complex where nitric oxide is oxidised to NO+.

Confirmation of Chloride (Cl-)

(a) Silver nitrate test

Chlorides on reaction with silver nitrate solution to form a white precipitate of silver chloride which is soluble in ammonium hydroxide.

(b) Manganese dioxide test

When chloride salts react with manganese dioxide and concentrated sulphuric acid, chlorine gas is liberated.

(c) Chromyl chloride test

When chloride salts react with potassium dichromate and conc. sulphuric acid red fumes of chromyl chloride is formed which reacts with sodium hydroxide to form yellow solution of sodium chromate. Sodium chromate reacts with lead acetate in presence of dil. acetic acid to form yellow precipitate of lead chromate.

Confirmation of Bromide (Br-)

(a) Silver nitrate test

Bromides on reaction with silver nitrate solution forms a pale yellow precipitate of silver bromide which is sparingly soluble in ammonium hydroxide.

(b) Manganese dioxide test

When bromide salts react with manganese dioxide and concentrated sulphuric acid, bromine gas is liberated.

(c) Chlorine water test

Bromine liberated in this test being soluble in carbon disulphide imparts an orange colour to the carbon disulphide layer.

Note: Carbon tetrachloride, cyclohexane, chloroform etc can be used instead of carbon disulphide.

Confirmation of Iodide (I-)

(a) Silver nitrate test

Iodides on reaction with silver nitrate solution forms an yellow precipitate of silver iodide which is insoluble in ammonium hydroxide.

(b) Manganese dioxide test:

When iodide salts react with manganese dioxide and concentrated sulphuric acid, iodine gas is liberated.

(c) Chlorine water test

Iodine liberated in this test being soluble in carbon disulphide imparts a violet colour to the carbon disulphide layer.

Note: Carbon tetrachloride, cyclohexane, chloroform etc can be used instead of carbon disulphide.

Confirmation of Acetate (CH3COO-)

(a) Oxalic acid test

Oxalic acid reacts with acetate salt to form acetic acid which has a characteristic vinegar like smell.

(b) Ester test

Acetate salts react with conc. sulphuric acid and ethyl alcohol to form the ester, ethyl acetate which has a fruity smell.

(c) Ferric chloride test

The reaction takes place in the ferric chloride test is given by the following equations.

Confirmation of Oxalate (C2O42-)

(a) Calcium chloride test

Oxalate salts react with calcium chloride to form white precipitate of calcium oxalate.

(b) Potassium permanganate test

In this test, the pink colour of potassium permanganate is decolourised with the evolution of carbon dioxide gas.

Confirmation of Sulphate (SO42-)

(a) Barium chloride test

Sulphates react with barium chloride to form white precipitate of barium sulphate.

(b) Match stick test

Violet streaks are produced during this test.

(c) Lead acetate test

Sulphates react with lead acetate to form white precipitate of lead sulphate.

Confirmation of Phosphate (PO43-)

(a) Ammonium molybdate test

Phosphate salts react with ammonium molybdate solution to form a deep yellow precipitate of ammonium phosphate molybdate. The chemical reaction is as follows:

(b) Magnesia mixture test

Phosphate salts react with magnesia mixture to form white precipitate of magnesium ammonium phosphate.

Note: To prepare magnesia mixture, add solid NH4Cl to magnesium chloride solution. Boil, cool and add NH4OH till a strong smell of ammonia ia obtained.

Learning Outcomes:

  • Students understand different types of anions.
  • Students understand various tests to identify the anion present in a given salt.
  • Students understand the chemical reactions and their balanced equations that takes place during each test.
  • Students acquire the skill to perform the experiment in the real lab once they understand different steps in the procedure.

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