🎯 Objective
To determine the total acidity (mineral acidity and CO₂ acidity) of a given water sample by NaOH titration.
📖 Principle / Theory
Acidity of water is its capacity to neutralize bases. It is mainly due to:
- Mineral acidity (Strong acid acidity): Due to HCl, H₂SO₄, HNO₃ — determined using methyl orange indicator (endpoint pH ~4.5).
- Carbon dioxide acidity (CO₂): Dissolved CO₂ forming H₂CO₃ — determined using phenolphthalein indicator (endpoint pH ~8.3).
Both are expressed as mg/L of CaCO₃ equivalents.
BIS limit: No specific limit, but pH of drinking water = 6.5–8.5.
🧰 Apparatus Required
Burette (50 mL), conical flask, pipette (100 mL), white tile.
🧪 Chemicals Required
N/50 NaOH standard solution, methyl orange indicator, phenolphthalein indicator, water sample.
⚗️ Procedure
- Pipette 100 mL of water sample into a 250 mL conical flask.
- Add 2 drops of methyl orange indicator. If solution is red/orange, mineral acidity is present.
- Titrate with N/50 NaOH until colour changes from red/orange to yellow. Volume = V₁ mL (mineral acidity).
- To the same flask, add 2 drops of phenolphthalein indicator.
- Continue titrating with N/50 NaOH until solution turns light pink. Volume used (additional) = V₂ mL (CO₂ acidity).
- Mineral acidity (mg/L as CaCO₃) = V₁ × 1000 × 50 / 100
- Total acidity (mg/L as CaCO₃) = (V₁ + V₂) × 1000 × 50 / 100
📊 Observations & Calculations
ℹ️
Mineral acidity = (V₁ × N × 50,000) / 100 mg/L as CaCO₃. CO₂ acidity = (V₂ × N × 50,000) / 100 mg/L as CaCO₃.
| 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
Mineral acidity = ______ mg/L as CaCO₃. CO₂ acidity = ______ mg/L as CaCO₃. Total acidity = ______ mg/L as CaCO₃.
⚠️ Precautions
- Use freshly collected and sealed water sample to avoid CO₂ loss.
- Methyl orange endpoint is gradual — approach it slowly.
- The phenolphthalein endpoint should be a faint permanent pink.
- NaOH must be standardised before use.
❓ Viva-Voce Questions
1. What is the difference between acidity and alkalinity of water?
Refer to your lab manual, textbook (Rattan or Vogel), and lecture notes. Discuss with your batch partners and prepare for the viva-voce examination.
2. What causes mineral acidity in natural water?
Refer to your lab manual, textbook (Rattan or Vogel), and lecture notes. Discuss with your batch partners and prepare for the viva-voce examination.
3. What are the health effects of acidic drinking water?
Refer to your lab manual, textbook (Rattan or Vogel), and lecture notes. Discuss with your batch partners and prepare for the viva-voce examination.
4. Why is CO₂ dissolved in water acidic?
Refer to your lab manual, textbook (Rattan or Vogel), and lecture notes. Discuss with your batch partners and prepare for the viva-voce examination.
5. What is acid rain and how does it affect water pH?
Refer to your lab manual, textbook (Rattan or Vogel), and lecture notes. Discuss with your batch partners and prepare for the viva-voce examination.
6. How is pH related to the acidity of water?
Refer to your lab manual, textbook (Rattan or Vogel), and lecture notes. Discuss with your batch partners and prepare for the viva-voce examination.
📚 References
- IS 3025 (Part 22): Methods of Sampling and Test for Industrial Water – Acidity
- Standard Methods for the Examination of Water and Wastewater, APHA