ANALYTICAL cHEMISTRY - III

Experiment 4

Principle

• The available chlorine reacts with I⁻ in presence of acid and oxidizes it to iodine, I₂ which is titrated with 0.01 N Na₂S₂O₃.5H₂O solution. The chlorine tablet or bleaching powder liberates Cl₂ gas on the treatment with glacial acetic acid as per the following reaction,

Ca(OCl)₂ + 2 CH₃COOH → Ca(CH₃COO)₂ + H₂O + Cl₂

• The amount of chlorine so liberated is termed as available chlorine. The liberated chlorine can be used to oxidize potassium iodide (KI, taken in excess) in presence of an acid. This liberate out an equivalent amount of iodine, I₂ as per the following equation,

Cl₂ + 2 KI → 2 KCl + I₂

• This iodine can then be determined by titrating against a standardized solution of Na₂S₂O₃.5H₂O, using freshly prepared starch solution as an indicator. The chemical reaction involved can be given as,

2 Na₂S₂O₃ + I₂ → Na₂S₄O₆ + 2 NaI

Apparatus required

Beaker, Conical flask, Burette, Pipette, etc.

Chemicals required

Cl₂ tablets, glacial acetic acid, 0.01 N Na₂S₂O₃.5H₂O solution, 0.01 N CuSO₄.5H₂O solution, Starch indicator, KI, etc.

Procedure

• Weigh Cl₂ tablet and add it to the 100 mL water, immediately take 25 mL of this solution for titration in flask, add 10 mL glacial acetic acid and 2 g of KI.

• Titrate liberated I₂ with Na₂S₂O₃.5H₂O using starch solution as an indicator. This gives available chlorine content in the given sample.

• Now, take 25 mL of sample solution after 30 minutes using the same procedure as above. This will give residual chlorine content in the given sample.

Result

1. Amount of available chlorine = _________ ppm

2. Amount of residual chlorine = _________ ppm

Reference Material

1. G H Jeffery, J Bassett, J Mendham and R C Denney, Vogel's Textbook of Quantitative Chemical Analysis, 5th Edition

Questions

1. What is the difference between the available chlorine and residual chlorine content?

2. Is this titration Iodometric or Iodimetric? Discuss the difference between the two.

3. Identify the species being oxidized and reduced during this titration.

4. Why we need to standardize sodium thiosulphate solution? Which materials can be used to standardize sodium thiosulphate solution?