Determination of Residual Chlorine in Water

To determine the residual chlorine content of a given water sample by iodometric / DPD colorimetric method.

Residual chlorine in water reacts with potassium iodide (KI) to liberate iodine (I₂), which is then titrated with sodium thiosulphate (Na₂S₂O₃) using starch as indicator.

Reactions:

Cl₂ + 2KI → 2KCl + I₂

I₂ + 2Na₂S₂O₃ → Na₂S₄O₆ + 2NaI

The blue colour of starch-iodine complex disappears at endpoint.

BIS Limit: Residual chlorine should be 0.2–0.5 mg/L in treated drinking water.

1. Take 200 mL of water sample in a 500 mL iodine flask.
2. Add 1 g of KI crystals and 5 mL of dilute H₂SO₄ (1:5). Swirl to dissolve.
3. Keep in dark for 5 minutes for complete reaction of Cl₂ with KI.
4. Titrate the liberated iodine with N/100 Na₂S₂O₃ from the burette.
5. Add starch indicator (1 mL) when the solution turns light yellow.
6. Continue titrating until the blue colour just disappears. Note volume V mL.
7. Residual Cl₂ (mg/L) = (V × N × 35450) / (volume of sample in mL)

The residual chlorine content of the given water sample is ______ mg/L.

• Conduct titration away from direct sunlight as iodine is light-sensitive.
• Add starch indicator only when solution is light yellow, not dark.
• Use freshly prepared starch solution.
• Keep the flask closed during the 5-minute reaction period.

• IS 3025 (Part 26): Method of Testing for Industrial Water – Residual Chlorine
• Standard Methods for the Examination of Water and Wastewater, APHA