Question

Why is glacial acetic acid used to find available chlorine?

Answer 1

Starch acts an indicator. The titration u have mentioned is iodometric one, where indirectly evolved iodine is titrated. Chlorine present in bleaching powder indirectly utilised in liberation of iodine from a standard iodine solution. This liberated iodine forms a violet colored solution with starch. So at the end point, when whole of the iodine is consumed by sodium thiosulphate (usually taken as titrant), colour of the solution changes from violet to colourless.

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Answer 2

Glacial acetic acid (also known as concentrated acetic acid) is used to measure the amount of chlorine that is present in a sample as it combines with hypochlorous acid (HOCl) to produce chloroacetic acid (CH3COOCl) and water. The reaction proceeds as follows:

• CH₃COOCl + H₂O = HOCl + CH₃COOH
• The amount of chloroacetic acid produced, which may be quantified with a spectrophotometer, can be used to calculate the amount of accessible chlorine in a sample.
• It is strong enough to react with hypochlorous acid but not so strong that it will also react with other forms of chlorine, including chloramines, glacial acetic acid is utilised.
• Glacial acetic acid is a perfect reagent for detecting the amount of accessible chlorine in a sample since it does not interfere with the measurement of chloroacetic acid.

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