Systematic Qualitative Analysis: A Concise Guide

This document provides a concise overview of systematic qualitative analysis, a chemical process for identifying the ionic components (cations and anions) in inorganic salts. It covers essential preliminary tests, specific confirmatory tests, and critical laboratory safety measures.

1. Introduction to Qualitative Analysis

Qualitative analysis identifies the nature and identity of substances and their constituents. For inorganic salts, this means pinpointing cations (from bases) and anions (from acids). It’s performed on various scales: macro (0.1-0.5g substance, 20mL solution), semimicro (0.05g substance, 1mL solution), and micro (very small amounts).

Key observable reactions include:

• Precipitate formation
• Colour change
• Gas evolution

The process involves:

1. Preliminary examination (dry tests): Initial clues from physical properties (colour, smell, solubility) and specific tests (heating, flame, borax bead, charcoal cavity).
2. Anion determination: Wet tests and confirmatory tests.
3. Cation determination: Wet tests and confirmatory tests.

Salt solubility and solution pH also offer important insights. Preliminary tests are always conducted before confirmatory tests.

2. Experiment: Detecting Cations and Anions

Aim: To identify one cation and one anion from a given salt, choosing from a specified list of common ions (excluding insoluble salts).

Theory: Based on:

1. Solubility Product (K_sp​): Precipitation occurs when the ionic product exceeds Ksp​.
2. Common Ion Effect: Used to control ionic product and influence precipitation.

Materials: Standard lab glassware and reagents.

3. Systematic Analysis of Anions

Step I: Preliminary Test with Dilute Sulphuric Acid

This test identifies anions (CO₃2⁻​, S²⁻, SO₃²⁻​, NO₂⁻​, CH_3​COO⁻) that evolve characteristic gases with dilute H₂​SO₄​.

Procedure: Add 1-2 mL dilute H₂​SO₄​ to ~0.1 g salt. Observe at room temperature, then warm. Test evolved gas.

Confirmatory Tests for Dilute Acid Group Anions

Tests use water extract (soluble salts) or sodium carbonate extract (insoluble salts).

Preparation of Sodium Carbonate Extract: Boil 1g salt + 3g Na2​CO3​ in 15mL distilled water for 10 min. Filter.

Step II: Preliminary Test with Concentrated Sulphuric Acid

Performed if dilute H2​SO4​ tests are negative. Identifies Cl−, Br−, I−, NO3−​, C2​O42−​.

Procedure: Add 3-4 drops conc. H2​SO4​ to ~0.1 g salt. Observe cold, then warm.

Step III: Test for Sulphate and Phosphate

Performed if previous tests are negative.

4. Systematic Analysis of Cations

Step I: Preliminary Examination of the Salt for Cation Identification

1. Colour Test: Salt colour indicates possible cations (e.g., blue for Cu²⁺, light green for Fe²⁺).
2. Dry Heating Test: Colour change of residue on heating/cooling (e.g., Zn²⁺ salts are yellow hot, white cold).
3. Flame Test: Platinum wire + conc. HCl + salt paste in non-luminous flame → characteristic flame colours (e.g., green for Cu²⁺, crimson red for Sr²⁺).
4. Borax Bead Test: For coloured salts. Borax bead + salt in oxidizing/reducing flame → characteristic bead colours (e.g., blue/green for Cu²⁺ in oxidizing flame).
5. Charcoal Cavity Test: Salt + Na_2​CO₃​ in charcoal cavity (reducing flame) → coloured residue/metallic bead (e.g., yellow hot/white cold for Zn²⁺).
6. Cobalt Nitrate Test: For white charcoal cavity residues. Residue + cobalt nitrate (heat) → characteristic colours (e.g., green for Zn²⁺, pink for Mg²⁺).

Step II: Wet Tests for Cation Identification (Group Analysis)

Preparation of Original Solution (O.S.): Dissolve salt sequentially in: distilled water → dil. HCl→ conc. HCl→ dil. HNO₃​→ aqua regia.

Flow Chart for Cation Separation (Summary): Cations are precipitated group-wise using specific reagents.

• Zero Group (NH₄⁺​): No group reagent. Tested first.
• Group I (Pb²⁺): Precipitated by dil. HCl.
• Group II (Pb²⁺, Cu²⁺, As³⁺): Precipitated by H₂​S in dil. HCl.
• Group III (Al³⁺, Fe³⁺): Precipitated by NH₄​OH in presence of NH₄​Cl.
• Group IV (Co²⁺, Ni²⁺, Mn²⁺, Zn²⁺): Precipitated by H_2​S in NH₄​OH.
• Group V (Ba²⁺, Sr²⁺, Ca²⁺): Precipitated by (NH₄​)₂​CO₃​ in NH₄​OH.
• Group VI (Mg²⁺): No group reagent. Tested last.

(I) Zero Group Cation (NH₄⁺​)

Test: Salt + NaOH (heat) → ammonia gas (smell, white fumes with HCl). Confirmed by brown ppt. with Nessler’s reagent.

(II) Group-I Cation (Pb²⁺)

Test: White ppt. with dil. HCl. Confirmed by:

• Yellow ppt. with KI (soluble in hot water, shining crystals on cooling).
• Yellow ppt. with K₂​CrO₄​ (soluble in NaOH).
• White ppt. with dil. H₂​SO₄​ (soluble in ammonium acetate).

(III) Group–II Cations (Pb²⁺, Cu²⁺, As³⁺)

Test: Precipitate with H₂​S in dil. HCl. Black (Pb²⁺, Cu²⁺), yellow (As³⁺).

• Group II-A (Copper Group – Insoluble in yellow ammonium sulphide):
  • Pb²⁺: White ppt. (PbSO₄​) after dissolving sulphide in HNO₃​ and adding H₂​SO₄​. Confirmed by yellow ppts. with K₂​CrO₄​ and KI.
  • Cu²⁺: Blue solution with excess NH₄​OH. Confirmed by chocolate brown ppt. with K₄​[Fe(CN)₆​].
• Group II-B (Arsenic Group – Soluble in yellow ammonium sulphide):
  • As³⁺: Yellow ppt. (As₂​S₅​) after acidification. Confirmed by canary yellow ppt. with ammonium molybdate.

(IV) Group–III Cations (Fe³⁺, Al³⁺)

Test: Precipitate with NH₄​Cl + excess NH₄​OH. Brown (Fe³⁺), gelatinous white (Al³⁺).

• Al³⁺: White gelatinous ppt. with NaOH (soluble in excess NaOH). Confirmed by blue lake test.
• Fe³⁺: Confirmed by blue ppt. with K₄​[Fe(CN)₆​] (Prussian blue) or blood red colour with KSCN.

(V) Group–IV Cations (Co²⁺, Ni²⁺, Mn²⁺, Zn²⁺)

Test: Precipitate with H₂​S. White (Zn²⁺), flesh (Mn²⁺), black (Ni²⁺, Co²⁺).

• Zn²⁺: White ppt. with NaOH (soluble in excess NaOH). Confirmed by bluish white ppt. with K₄​[Fe(CN)₆​].
• Mn²⁺: White ppt. with NaOH (turns brown on standing).
• Ni²⁺: Black sulphide. Confirmed by bright red ppt. with dimethylglyoxime.
• Co²⁺: Black sulphide. Confirmed by yellow ppt. with potassium nitrite (potassium hexanitritocobaltate(III)).

(VI) Group–V Cations (Ba²⁺, Sr²⁺, Ca²⁺)

Test: White ppt. with (NH₄​)₂​CO₃​ + NH₄​OH. Dissolve ppt. in acetic acid.

• Ba²⁺: Yellow ppt. with K₂​CrO₄​. Flame test: grassy green.
• Sr²⁺: White ppt. with (NH₄​)₂​SO₄​. Flame test: crimson-red.
• Ca²⁺: White ppt. with ammonium oxalate. Flame test: brick red (greenish-yellow through blue glass).

(VII) Group–VI Cation (Mg²⁺)

Test: If Group-V is absent. Original solution + NH₄​OH + Na₂​HPO₄​ (heat, scratch) → white crystalline ppt. (Mg(NH₄​)PO₄​).

5. Result

Example: The given salt contains Anion: SO₄²⁻​ and Cation: Mg²⁺.

6. Precautions

Always prioritize safety:

• Wear PPE (apron, eye protector, gloves).
• Read labels, never taste chemicals.
• Fan vapours, don’t inhale directly.
• Never add water to acid.
• Point test tubes away from people when heating.
• Handle explosives, flammables, poisons, electricity, and hot items with extreme care.
• Maintain a clean workspace, use dustbins.
• Wash hands after lab work.
• Use minimum reagent quantities.