Overview
This lecture covers qualitative analysis techniques to identify common negative ions (anions) and positive ions (cations) in inorganic compounds.
Testing for Carbonate Ions
- Add any dilute acid to a sample; if carbonate is present, CO₂ gas is produced (observed as fizzing).
- Bubble the gas through limewater; CO₂ turns limewater cloudy.
- Example reaction: 2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂.
Testing for Sulfate Ions
- Add acidified barium chloride (BaCl₂) solution; a white precipitate indicates sulfate ions.
- Reaction: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
- Acid eliminates carbonate impurities, which could otherwise give a false positive.
- Do not use sulfuric acid to acidify, as its own sulfate ions interfere.
Testing for Halide Ions
- Add nitric acid to test solution, then add silver nitrate dropwise.
- Nitric acid removes carbonate interference, preventing Ag₂CO₃ precipitate.
- Chloride ions produce a white precipitate (AgCl).
- Bromide ions produce a cream precipitate (AgBr).
- Iodide ions produce a pale yellow precipitate (AgI).
- Fluoride ions produce no precipitate.
Differentiating Silver Halide Precipitates
- Silver chloride dissolves in dilute ammonia to form a colorless solution.
- Silver bromide dissolves in concentrated ammonia to form a colorless solution.
- Silver iodide does not dissolve in ammonia due to low solubility.
- Hydrochloric acid should not be used to acidify, as it would add chloride ions and give a false result.
Sequence of Tests
- Always test in the order: carbonate, sulfate, then halide to prevent false positives from insoluble barium carbonate or silver sulfate.
Testing for Ammonium Ion (NH₄⁺)
- Warm the sample with NaOH(aq); ammonia gas (NH₃) forms if ammonium is present.
- Ammonia gas is detected by its pungent smell or because it turns red litmus paper blue.
Key Terms & Definitions
- Carbonate (CO₃²⁻) — Ion that reacts with acids to release CO₂ gas.
- Sulfate (SO₄²⁻) — Ion forming a white precipitate with barium chloride.
- Halide ions (Cl⁻, Br⁻, I⁻, F⁻) — Produce different colored precipitates with silver nitrate.
- Ammonium ion (NH₄⁺) — Cation that forms ammonia gas when heated with NaOH.
Action Items / Next Steps
- Practice writing balanced equations for each test.
- Memorize test sequences and expected observations for carbonate, sulfate, halide, and ammonium ions.