🎯 Objective
To determine the amount of oxalic acid (H₂C₂O₄) and sulphuric acid (H₂SO₄) present in one litre of their mixture by permanganate and NaOH titrations.
📖 Principle / Theory
The mixture contains two acids. Oxalic acid (a reducing agent) is determined first by titration with KMnO₄ in acidic medium:
2KMnO₄ + 5H₂C₂O₄ + 3H₂SO₄ → 2MnSO₄ + 10CO₂ + K₂SO₄ + 8H₂O
After removing oxalic acid (by precipitation as CaC₂O₄ or by evaporation), the total acid or H₂SO₄ is determined by NaOH titration:
H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
Alternatively, total acid is first determined with NaOH, then oxalic acid separately, and H₂SO₄ is obtained by difference.
🧰 Apparatus Required
Burette (50 mL), conical flask, pipette (20 mL), water bath, burner.
🧪 Chemicals Required
N/10 KMnO₄ solution, N/10 NaOH solution, dilute H₂SO₄, phenolphthalein indicator, sample solution.
⚗️ Procedure
- Pipette 20 mL of the given mixture into a conical flask. Heat to 60–70°C.
- Titrate with N/10 KMnO₄ (self-indicating, endpoint = permanent pink). Volume used = V₁ mL. This gives total (H₂C₂O₄ + H₂SO₄ combined with oxalic acid equivalent).
- Pipette another 20 mL sample into a second flask. Add 2 drops phenolphthalein.
- Titrate with N/10 NaOH until pink endpoint. Volume used = V₂ mL. This gives total acid equivalents.
- Oxalic acid (g/L) = V₁ × N_KMnO₄ × M_oxalic / (2 × 1000 × 20) × 1000
- Total acid equivalents = V₂ × N_NaOH. H₂SO₄ equivalents = Total − oxalic acid equivalents.
- H₂SO₄ (g/L) = calculated from the difference.
📊 Observations & Calculations
| Observation | Trial 1 | Trial 2 | Trial 3 |
|---|---|---|---|
| Initial burette reading (mL) | ______ | ______ | ______ |
| Final burette reading (mL) | ______ | ______ | ______ |
| Volume of titrant used (mL) | ______ | ______ | ______ |
| Concordant volume (mL) | ______ | ||
Volume of titrant (V) = ______ mL
Result = ______ (using appropriate formula)
✅ Result
Amount of oxalic acid = ______ g per litre. Amount of sulphuric acid = ______ g per litre.
⚠️ Precautions
- Heat the solution to 60–70°C before KMnO₄ titration (catalyst for the slow reaction).
- Add KMnO₄ slowly — each drop must decolourise before adding the next.
- The pink endpoint must persist for at least 30 seconds.
- NaOH must be standardised against a primary standard before use.
❓ Viva-Voce Questions
📚 References
- Vogel's Textbook of Quantitative Chemical Analysis
- Day, R.A. – Quantitative Analytical Chemistry