Qualitative Analysis (Inorganic)

Qualitative Inorganic Analysis – Overview

Qualitative inorganic analysis deals with the detection of cations (basic radicals) and anions (acid radicals) present in an inorganic salt or mixture.

Parts of Qualitative Analysis

• Dry tests
• Confirmatory tests for basic radicals
• Confirmatory tests for acid radicals

Classification of Basic Radicals (Cations)

The classification is based on solubility product, solubility and common ion effect.

Group I – Dilute HCl

• Ag⁺ → AgCl (white ppt)
• Pb²⁺ → PbCl₂ (white ppt)
• Hg₂²⁺ → Hg₂Cl₂ (white ppt)

Group II – H₂S in Dilute HCl

• II A: Hg²⁺, Pb²⁺, Bi³⁺, Cu²⁺, Cd²⁺
  • Sulphides: black (HgS, PbS, Bi₂S₃, CuS), yellow (CdS)
• II B: As³⁺, As⁵⁺, Sb³⁺, Sb⁵⁺, Sn²⁺, Sn⁴⁺
  • Sulphides: yellow (As₂S₃, SnS₂), orange (Sb₂S₃), brown (SnS)

Group III – NH₄OH in Presence of Excess NH₄Cl

• Fe³⁺ → Fe(OH)₃ (reddish brown)
• Cr³⁺ → Cr(OH)₃ (dirty green)
• Al³⁺ → Al(OH)₃ (white gelatinous)

Group IV – H₂S in Presence of NH₄OH

• Co²⁺ → CoS (black)
• Ni²⁺ → NiS (black)
• Zn²⁺ → ZnS (bluish white)
• Mn²⁺ → MnS (buff coloured)

Group V – (NH₄)₂CO₃ in NH₄OH

• Ba²⁺ → BaCO₃ (white)
• Sr²⁺ → SrCO₃ (white)
• Ca²⁺ → CaCO₃ (white)

Group VI – No Group Reagent

• Mg²⁺ → Mg(NH₄)PO₄ (white crystalline ppt)

Zero Group – NH₄⁺

• On heating with NaOH → NH₃ gas evolved
• NH₃ gives white fumes with HCl
• Brown ppt with Nessler’s reagent

Dry Tests

Flame Test

• Li⁺: Crimson red
• Na⁺: Golden yellow
• K⁺: Violet
• Ca²⁺: Brick red
• Sr²⁺: Crimson
• Ba²⁺: Apple green

Note: Flame test is not performed in presence of As, Sb, Bi, Sn and Pb.

Borax Bead Test

On heating borax, a colourless glassy bead is formed which gives characteristic colours with metal oxides.

Microcosmic Salt Bead Test

Similar to borax bead test; sodium metaphosphate bead forms coloured orthophosphates with metals.

Tests for Acid Radicals (Anions)

Carbonate (CO₃²⁻)

• Brisk effervescence of CO₂
• Turns lime water milky

Sulphite (SO₃²⁻)

• Colourless gas with smell of burning sulphur
• Turns acidic K₂Cr₂O₇ from orange to green

Sulphide (S²⁻)

• H₂S gas with rotten egg smell
• Black ppt with lead acetate paper
• Violet colour with sodium nitroprusside

Acetate (CH₃COO⁻)

• Vinegar smell on heating with conc. H₂SO₄
• Blood red colour with neutral FeCl₃

Chloride (Cl⁻)

• White fumes of HCl
• Curdy white ppt with AgNO₃
• Chromyl chloride test gives red vapours

Bromide (Br⁻)

• Reddish-brown fumes of Br₂
• Pale yellow ppt with AgNO₃

Iodide (I⁻)

• Violet vapours of iodine
• Blue colour with starch

Nitrate (NO₃⁻)

• Brown fumes of NO₂
• Brown ring test with FeSO₄ and conc. H₂SO₄