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

Wet Tests for Acid Radicals (Anions)

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

Confirmation of Carbonate (CO₃^2-)

(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 (SO₃^2-)

(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 (S^2-)

(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 (C₂O₄^2-)

(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 (SO₄^2-)

(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 (PO₄^3-)

(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.