Qualitative Inorganic Analysis

Qualitative inorganic analysis is a fundamental branch of analytical chemistry that identifies the cations (positive ions) and anions (negative ions) present in an unknown inorganic compound. It follows a systematic approach, utilizing preliminary tests, wet tests, and group analysis to confirm the presence of different radicals.

1. Preliminary Tests

These tests provide initial clues about the composition of the compound before detailed analysis.

A. Physical Examination

• Color: Provides insight into certain cations.
  • Blue/Green → Cu²⁺ or Ni²⁺
  • Brown → Fe³⁺
  • Pink → Co²⁺
  • White → Likely absence of transition metals
• Smell: Some radicals release distinctive odors.
  • Ammonia-like → NH₄⁺ (Ammonium)
  • Vinegar-like → CH₃COO⁻ (Acetate)
  • Rotten eggs → S²⁻ (Sulphide)

B. Dry Heating Test

• Heating a sample in a dry test tube reveals decomposition products.
  • Gas Evolution:
    • CO₂ (Limewater turns milky) → CO₃²⁻ (Carbonate)
    • H₂S (Rotten egg smell, blackens lead acetate paper) → S²⁻ (Sulphide)
    • SO₂ (Pungent, turns potassium dichromate green) → SO₃²⁻ (Sulphite)

C. Flame Test

• Certain cations impart characteristic colors to a flame.
  • Na⁺ → Golden Yellow
  • K⁺ → Violet
  • Ca²⁺ → Brick Red
  • Ba²⁺ → Apple Green
  • Cu²⁺ → Bluish Green

D. Borax Bead Test

• Used for transition metals, forming colored beads in a borax flux.
  • Green → Cr³⁺
  • Blue → Co²⁺
  • Violet → Mn²⁺

2. Identification of Anions (Acid Radicals)

Anions are detected using Dilute H₂SO₄, Concentrated H₂SO₄, and specific wet tests.

A. Dilute Sulfuric Acid Test

• CO₂ bubbles (turns limewater milky) → CO₃²⁻ (Carbonate)
• SO₂ (suffocating smell, turns K₂Cr₂O₇ green) → SO₃²⁻ (Sulphite)
• H₂S (rotten egg smell, blackens Pb²⁺ paper) → S²⁻ (Sulphide)

B. Concentrated Sulfuric Acid Test

• HCl gas (White fumes with NH₃) → Cl⁻ (Chloride)
• Br₂ vapors (Reddish-brown, turns starch yellow) → Br⁻ (Bromide)
• I₂ vapors (Violet, turns starch blue) → I⁻ (Iodide)
• NO₂ gas (Reddish-brown, turns Fe²⁺ solution black) → NO₃⁻ (Nitrate)

C. Confirmatory Anion Tests

• BaCl₂ Test (White ppt.) → SO₄²⁻ (Sulfate)
• Ammonium Molybdate Test (Yellow ppt.) → PO₄³⁻ (Phosphate)
• Silver Nitrate Test
  • White ppt. → Cl⁻
  • Yellow ppt. → Br⁻
  • Dark yellow ppt. → I⁻

3. Identification of Cations (Basic Radicals)

Cations are classified into six groups, each identified by selective precipitation.

Group Zero: NH₄⁺ (Ammonium Ion)

• Heated with NaOH → NH₃ gas (Turns red litmus blue, white fumes with HCl).

Group I: Pb²⁺ (Lead Group)

• HCl Precipitation → PbCl₂ (White).
• Confirmed by:
  • KI (Yellow ppt.)
  • K₂CrO₄ (Yellow ppt.)

Group II: Cu²⁺, Pb²⁺, As³⁺ (Sulphide Precipitation in Acidic Medium)

• Cu²⁺ (Copper): Black CuS, confirmed by K₄[Fe(CN)₆] (Chocolate brown ppt.).
• As³⁺ (Arsenic): Yellow As₂S₃, confirmed by Ammonium Molybdate (Yellow ppt.).

Group III: Fe³⁺, Al³⁺ (Hydroxide Precipitation in NH₄OH + NH₄Cl Medium)

• Fe³⁺ (Iron): Reddish-brown Fe(OH)₃, confirmed by Prussian Blue Test.
• Al³⁺ (Aluminum): White Al(OH)₃, confirmed by Cobalt Nitrate Test (Blue mass).

Group IV: Zn²⁺, Ni²⁺, Co²⁺, Mn²⁺ (Sulphide Precipitation in Alkaline Medium)

• Zn²⁺: White ZnS, confirmed by Cobalt Nitrate Test (Green color).
• Co²⁺ (Cobalt): Black CoS, confirmed by Borax Bead Test (Blue).

Group V: Ca²⁺, Sr²⁺, Ba²⁺ (Carbonate Precipitation in (NH₄)₂CO₃ Medium)

• Ca²⁺: Brick-red flame, white ppt. with Ammonium Oxalate.
• Ba²⁺: Green flame, white ppt. with K₂CrO₄.

Group VI: Mg²⁺ (Soluble in Ammonium Carbonate Medium)

• White ppt. with Magnesia Mixture.

4. Systematic Analysis Procedure

1. Perform Preliminary Tests
2. Test for Acid Radicals (Anions)
3. Test for Basic Radicals (Cations)
4. Confirm with Specific Tests

5. Conclusion

Qualitative inorganic analysis follows a logical approach for identifying unknown compounds. The use of color changes, precipitations, and gas evolution provides reliable identification of chemical species. This process is widely used in academic laboratories, forensic investigations, and environmental testing.

1. Basic Concept Questions

1. What is qualitative inorganic analysis?
2. How is qualitative analysis different from quantitative analysis?
3. What are the two main types of radicals identified in qualitative inorganic analysis?
4. Why is it important to perform preliminary tests before group analysis?
5. What are the three main types of tests used in qualitative analysis?

2. Preliminary Test Questions

6. What information can be obtained from the color of a compound?
7. Which ion gives an ammoniacal smell when rubbed with water?
8. What does the dry heating test help to determine?
9. What gas turns lime water milky? Which radical does this indicate?
10. What flame color is observed for sodium (Na⁺) and potassium (K⁺) ions?
11. What is the borax bead test used for?

3. Anion (Acid Radical) Identification Questions

12. What gas is evolved when carbonates (CO₃²⁻) react with dilute H₂SO₄?
13. How can you distinguish between sulphite (SO₃²⁻) and sulphate (SO₄²⁻)?
14. What is the confirmatory test for phosphate (PO₄³⁻)?
15. What color does starch paper turn when exposed to iodine vapors?
16. How do you confirm the presence of chloride (Cl⁻) ions?
17. What happens when nitrate (NO₃⁻) salts are heated with conc. H₂SO₄ and copper chips?

4. Cation (Basic Radical) Identification Questions

18. Why is NH₄OH + NH₄Cl used for group III analysis?
19. What is the group reagent for detecting group II cations (Cu²⁺, Pb²⁺, As³⁺)?
20. What is the confirmatory test for Fe³⁺ ions?
21. How do you distinguish between Ba²⁺ and Ca²⁺ ions using a flame test?
22. What color does cobalt(II) (Co²⁺) give in the borax bead test?
23. What is the role of potassium ferrocyanide (K₄[Fe(CN)₆]) in Cu²⁺ identification?
24. What happens when ammonium chloride (NH₄Cl) is heated with NaOH?
25. Which radical gives a blue mass in the cobalt nitrate test?

5. Application-Based Questions

26. Why is lead chloride (PbCl₂) soluble in hot water but not in cold water?
27. What precautions should be taken while performing the dry heating test?
28. A white precipitate is obtained with BaCl₂ test, which anion could be present?
29. What happens when silver nitrate (AgNO₃) is added to a chloride solution?
30. How can you differentiate between iron (Fe²⁺) and aluminum (Al³⁺) in group III analysis?