🎯 Objective
To determine the dissolved oxygen (DO) content of a given water sample by Winkler's iodometric method.
📖 Principle / Theory
Dissolved oxygen reacts with manganous hydroxide [Mn(OH)₂] in alkaline conditions to form manganic hydroxide [Mn(OH)₃]. In acidic conditions, Mn(OH)₃ oxidizes I⁻ to liberate I₂, which is titrated against Na₂S₂O₃.
Reactions:
Mn²⁺ + 2OH⁻ → Mn(OH)₂ (white)
4Mn(OH)₂ + O₂ → 4Mn(OH)₃ (brown)
2Mn(OH)₃ + 2KI + 3H₂SO₄ → 2MnSO₄ + I₂ + K₂SO₄ + 6H₂O
I₂ + 2Na₂S₂O₃ → Na₂S₄O₆ + 2NaI
Normal DO: 8–10 mg/L; BIS limit for drinking water: ≥ 6 mg/L
🧰 Apparatus Required
BOD bottles (300 mL), burette, pipette, iodine flask.
🧪 Chemicals Required
MnSO₄ solution, alkaline KI (KOH + KI), conc. H₂SO₄, N/40 Na₂S₂O₃, starch indicator.
⚗️ Procedure
- Collect 300 mL of water sample in a BOD bottle carefully to avoid air bubbles.
- Add 1 mL of MnSO₄ solution and 1 mL of alkaline KI-KOH solution using pipettes below the surface. Cap carefully.
- Mix by inverting several times. A brown/orange precipitate forms (Mn(OH)₃).
- Add 1 mL of concentrated H₂SO₄ carefully along the side. Cap and mix. Precipitate dissolves and iodine is liberated (yellow solution).
- Transfer 100 mL of this solution to a conical flask.
- Titrate with N/40 Na₂S₂O₃ until pale yellow, add starch indicator, continue to colourless endpoint.
- DO (mg/L) = (V × N × 8 × 1000) / (volume of sample in mL)
📊 Observations & Calculations
ℹ️
DO (mg/L) = (V × 0.025 × 8 × 1000) / 100 = 2V mg/L
| Observation | Trial 1 | Trial 2 | Trial 3 |
|---|---|---|---|
| Initial burette reading (mL) | ______ | ______ | ______ |
| Final burette reading (mL) | ______ | ______ | ______ |
| Volume of titrant used (mL) | ______ | ______ | ______ |
| Concordant volume (mL) | ______ | ||
Calculation:
Volume of titrant (V) = ______ mL
Result = ______ (using appropriate formula)
Volume of titrant (V) = ______ mL
Result = ______ (using appropriate formula)
✅ Result
The dissolved oxygen content of the given water sample is ______ mg/L.
⚠️ Precautions
- Collect sample without trapping air bubbles.
- Add reagents gently below the surface.
- Ensure all precipitate dissolves before transferring to conical flask.
- Perform titration immediately after adding H₂SO₄.
❓ Viva-Voce Questions
1. What is dissolved oxygen and why is it important in water quality assessment?
Refer to your lab manual, textbook (Rattan or Vogel), and lecture notes for the answer to this question. Discuss with your batch partners and prepare for the viva-voce examination.
2. What is BOD and how is it related to DO?
Refer to your lab manual, textbook (Rattan or Vogel), and lecture notes for the answer to this question. Discuss with your batch partners and prepare for the viva-voce examination.
3. What factors affect the dissolved oxygen level in water?
Refer to your lab manual, textbook (Rattan or Vogel), and lecture notes for the answer to this question. Discuss with your batch partners and prepare for the viva-voce examination.
4. What is the minimum DO level acceptable in drinking water (BIS standard)?
Refer to your lab manual, textbook (Rattan or Vogel), and lecture notes for the answer to this question. Discuss with your batch partners and prepare for the viva-voce examination.
5. How does temperature affect the solubility of oxygen in water?
Refer to your lab manual, textbook (Rattan or Vogel), and lecture notes for the answer to this question. Discuss with your batch partners and prepare for the viva-voce examination.
6. What is the significance of DO in aquatic ecosystems?
Refer to your lab manual, textbook (Rattan or Vogel), and lecture notes for the answer to this question. Discuss with your batch partners and prepare for the viva-voce examination.
📚 References
- IS 3025 (Part 38): Dissolved Oxygen Determination
- Standard Methods for the Examination of Water and Wastewater, APHA