Qualitative analysis:

An acid and a base react to form a salt.

The salt consists of two parts known as radicals.

The cation or the positively charged part, derived from the base, called the basicradical

The anion or the negatively charged part derived from the acid termed as the acidicradical.

NaCl is a salt with basic radical Na⁺ and acidic radical Cl ^–.

In qualitative inorganic analysis ,the given compound is analysed for both the radicalsof the given salt

Characteristic tests of some acidic radicals or anions:

Preliminary tests:

1. Action of dil HCl and heat on the given salt :

• Brisk effervescence indicates presence of carbonate (CO₃ ^2–) or sulphite(SO₃ ^2– )
• Smell of rotten eggs indicates the presence of sulphide (S^2–)
• Reddish brown gas (NO₂ )indicates the presence of nitrite ( NO₂^– )

The above tests involve the following reactions:

2. Action of conc H₂SO₄ and heat in a dry test tube :

• Colourless pungent gas (HCl) indicates chloride Cl ^–
• Brown gas (Br₂) in cold condition , indicates bromide (Br ^– )
• Violet fumes (I₂) indicates Iodide ( I ^– )
• Brown gas (NO₂) on heating indicates NO₃^–

1.Tests for carbonate:

The carbonates of alkali metals and ammonium alone are water soluble.

a. With dil HCl:The carbonate salt gives brisk effervescence due to the evolution of CO₂ gas

This gas can be tested by passing it through lime water (Ca(OH)₂,which turns milky due to the formation of CaCO₃ precipitate.

(CO₃)^2- + 2H⁺ → CO₂↑ + H₂O

CO₂ + Ca²⁺ + 2OH^- → CaCO₃↓ + H₂O

Note: Wih prolonged passage of CO₂ the turbidity slowly disappears due to the formation of soluble calcium bicarbonate.

CaCO₃↓ + CO₂ + H₂O → Ca²⁺ + 2HCO₃^-

b. With Barium chloride solution :The carbonate salt gives a white precipitate of Barium carbonate.This precipitate is soluble in mineral acids.

(CO₃)^2- + Ba²⁺ → BaCO₃↓

BaCO₃ + 2H⁺ → Ba²⁺ + CO₂↑ + H₂O

c. With AgNO₃ solution:The carbonate solt gives a white precipitate of silver carbonate This precipitate is soluble in NH₃ and in nitric acid. On boiling or on addition of excess reagent, it turns brown due to the formation of Ag₂O.

Ag₂CO₃ + 2H⁺ → 2Ag⁺ + CO₂↑ + H₂O

Ag₂CO₃ + 4NH₃ → 2[Ag(NH₃)₂]⁺ + (CO₃)^2-

Tests for sulphide:

a. With dil HCl :The sulphide salt produces a pungent gas with the smell of rotten eggs(H₂S).The gas turns lead acetate paper black due to the formation of black lead sulphide.

S^2- + 2H⁺ → H₂S ↑
H₂S + Pb²⁺ → PbS↓

b.With AgNO₃ solution:The salt produces a black precipitate of Ag₂S which is insoluble in cold but soluble in hot dil HNO₃.

S^2- + 2Ag⁺ → Ag₂S↓

c.With sodium nitro prusside solution (Na₂[Fe(CN)₅NO], the sulphide salt gives a violet colouration in the presence of NaOH solution.

Tests for sulphate :

a.With Lead acetate : In the presence of acetic acid ,the sulphate salt gives a white precipitate of lead sulphate.This precipitate is soluble in excess ammonium acetate on warming.

Na₂SO₄ + Pb(CH₃COO)₂ → PbSO₄↓ + 2CH₃COONa

b. With barium chloride solution:The sulphate salt solution gives a white precipitate of barium sulphate which is soluble only in boiling conc HCl

c.With AgNO₃ solution: The sulphate salt gives a white crystalline precipitate of silver sulphate

(SO₄)^2- + 2Ag⁺ → Ag₂SO₄↓

Tests for nitrite (NO₂ ^– )

All nitrites are water soluble.Silver nitrite is sparingly soluble.

A nitrite salt with dil H₂SO₄ gives a brown gas NO₂ which turns starch iodide paper blue.

The NO₂ gas oxidises the iodide to free iodine which turns starch blue.

NO₂^- + H ⁺ → HNO₂

3HNO₂ → HNO₃ + 2NO ↑ + H₂o

2NO ↑ + O₂ ↑ → 2NO₂ ↑

2KI + 2NO₂ → 2KNO₂ + I₂

I₂ + starch → blue colour.

a. Brown ring test: To the nitrite salt solution ,freshly prepared ferrous sulphate solution and dil H₂SO₄ are added ,Keeping the test tube slanted conc H₂SO₄ is slowly added along the sides of the test tube.This produces a brown ring at the junction of two layers in the test tube due to the formation of nitroso ferrous sulphate.

b. With cobalt chloride and KCl :

The nitrite salt solution is first acidified with acetic acid . This liberates nitrous acid(HNO₂) Cobalt chloride is added along with KCl and warmed .A yellow precipitate appears after some time.The reactions involved are

NaNO₂ + CH₃COOH → CH₃COONa + HNO₂

COCl₂ + 2NaNO₂ → CO(NO₂)₂ + 2NaCl

2HNO₂ → H₂O + 2NO + [O]

2Co(NO₂)₂ + 2NaNO₂ + H₂O + [O] → 2Co(NO₂)₃ + 2NaOH

CO(NO₂)₃ + 3 NaNO₂ → Na₃[CO(NO₂)₆] soluble complex

Na₃[Co(NO₂)₆] +3KCl → K₃[CO(NO₂)₆] + 3NaCl yellow precipitate

c. With AgNO₃ solution :the nitrite salt solution gives a white precipitate of silver nitrite

NaNO₂ + AgNO₃ → AgNO₂ ↓+ NaNO₃

Tests for nitrate(NO₃^– )

All nitrate salts decompose on treatment with conc sulphuric acid ,liberating reddish brown NO₂ gas.

NaNO₃ + H₂SO₄ → NaHSO₄ + HNO₃

4HNO₃ → 2H₂O + 4NO₂ ↑+ O₂ ↑

a.Brown ring test:This is a confirmatory test for the presence of nitrate.The salt solution is acidified with dil H₂SO₄,then freshly prepared FeSO₄ is added followed by addition of a few drops of conc H₂SO₄.The formation of brown ring confirms the presence of nitrate.

NaNO₃ + H₂SO₄ → NaHSO₄ + HNO₃

2HNO₃ + 6FeSO₄ + 3H₂SO₄ → Fe₂(SO₄)₃ + NO + H₂O

FeSO₄ + NO → [Fe(NO)]SO₄

Tests for chloride(Cl ^– )

Chlorides react with conc H₂SO₄ liberating HCl gas .The gas turns moist blue litmus paper red.The presence of the gas can be tested with a glass rod dipped in NH₄OH.Dense white fumes of NH₄Cl are produced

Cl^– + H₂SO₄ → HCl ↑ + HSO₄ ^–

a. Chromyl chloride test:

The chloride salt is treated with potassium dichromate and conc sulphuric acid and heated.

This produces orange red vapours of chromyl chloride.

These vapours when passed through NaOH solution produce a yellow solution of sodium chromate.

If this solution is neutralised with acetic acid and lead acetate is added,The formation of a yellow precipitate (lead chromate) confirms the presence of chloride.

Note:

Neutralisation is necessary to remove excess NaOH,since lead chromate is soluble in NaOH.

This test is not given by chlorides of Ag,Hg,Pb,Sn and Sb

The reactions involved are:

4NaCl + K₂Cr₂O₇ +6 H₂SO₄ →4 NaHSO₄ + 2KHSO₄ + 2CrO₂Cl₂ ↑+ 3H₂O chromyl chloride

CrO₂Cl₂ + 4NaOH → Na₂CrO₄ + 2NaCl + 2H₂O

Na₂CrO₄ + (CH₃COO)2Pb → PbCrO₄↓ +2 CH₃COONa yellow precipitate

b. With MnO₂ and conc H₂SO₄ :The chloride salt liberates the greenish yellow gas of chlorine when treated with a pinch of MnO₂and a few drops of conc H₂SO₄.

MnO₂ + 2H₂SO₄ + 2Cl ^– → Mn²⁺ + 2SO₄ ^2– + Cl₂ ↑ + 2H₂O

c. With AgNO₃ solution :chlorides give a curdy white porecipitate of AgCl which is insoluble in water and HNO₃ but dissolves ammonia due to the formation of a soluble complex salt

Cl ^– + Ag⁺ → AgCl ↓

AgCl + 2NH₃ → [Ag(NH₃)₂]Cl

d. With Lead acetate solution : chlorides give a white precipitate od lead chloride.

2Cl ^– + Pb²⁺ → PbCl₂ ↓

Tests for Bromide (Br ^– )

Bromide salts react with conc H₂SO₄ liberating reddish brown fumes of Bromine gas.

2KBr + H₂SO₄ → K₂SO₄ +2 HBr↑

2HBr + H₂SO₄ → 2H₂O + SO₂ ↑ + Br₂↑

Note : H₃PO₄ cannot be used instead of H₂SO₄,since the reaction stops after the formation of HBr.

a.With MnO₂ and conc H₂SO₄: The Bromide salt when warmed with MnO₂ and conc H₂SO₄,produces brown vapours of bromine..H₂SO₄ first converts the bromide to HBr,which then reacts with MnO₂ liberating bromine gas

4HBr + MnO₂ → MnBr₂ + Br₂ ↑ + 2H₂O

b.With AgNO₃ solution : bromide salt solution gives pale yellow precipitate which is partially soluble in ammonia .The salt solution is first acidified with nitric acid before the addition of AgNO₃

NaBr+ AgNO₃ → NaNO₃ + AgBr↓ pale yellow ppt

AgBr + NH₃ (aq) → [Ag(NH₃)₂] Br

c.With lead acetate solution :The bromide salt solution gives a white precipitate of lead bromide PbBr₂.

2 Br ^– + Pb ²⁺ → PbBr₂ ↓

d. With chlorine water : chlorine being more reactive than bromine can displace bromine from the bromide salt.Hence when chlorine water is added to a bromide salt solution bromine gas is evolved and the solution turns orange red.

Addition of excess of clorine water results in the formation of a pale yellow solution of BrCl ,an inter halogen compound.

Cl₂ +2 Br ^– → Br₂ + 2Cl ^–

Br₂ + Cl₂ → 2BrCl

Note: When an aqueous solution of bromine is shaken with organic solvents like CHCl₃, CS₂, CCl₄,bromine dissolves in them forming a reddish brown solution below the colourless aqueous layer.

e.With potassium dichromate and conc H₂SO₄: The bromide salt gives vapours of bromine which when passed into water give an orange red solution.

6 KBr + K₂Cr₂O₇ + 7 H₂SO₄ → 3Br₂ + Cr₂(SO₄)₃ + 4K₂SO₄ + 7H₂O

Tests for Iodide (I ^– )

When the solid iodide salt is warmed with conc H₂SO₄, violet vapours of Iodine are evolved which turn starch paper blue.The sulphuric acid is reduced to SO₂,S and H₂S.

a.With AgNO₃ solution: The iodide salt solution gives a yellow precipitate of AgI which is insoluble in dil HNO₃,slightly soluble in conc ammonia and highly soluble in hypo and KCN due to the formation of soluble complexes.

I ^– + Ag⁺ → AgI yellow ppt

AgI + 2CN ^– → [Ag(CN)₂] ^– + I ^–

AgI + 2S₂O ₃ ^2– → [Ag(S₂O₃)₂]^3– + I^–

b. With lead acetate: The iodide salt solution gives a yellow precipitate of lead iodide which dissolves in hot water and on sudden cooling gives shiny yellow particles called golden spangles.

2I ^– + Pb²⁺ → PbI₂ ↓

c.With sodium nitrite solution : The iodide salt solution,liberates iodine in acid medium(CH₃COOH or dil sulphuric acid).This test diffrentiates it from chloride and bromide.The iodine produced turns starch paste blue and CCl₄ violet

2 I ^– + 2NO₂^– + 4 H⁺ → I₂ +2 NO + 2H₂O

Characteristic tests of some basic radicals or cations:

Different basic radicals have different solubility products and are precipitated by different reagents.Thus based on the solubility products and the reagents used cations are classified into six groups.