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NEET levelSingle correct optionDifficulty 1 of 3eq075

In the iodometric titration of liberated iodine with sodium thiosulphate, starch is added only near the end point because

  1. Astarch reacts with thiosulphate at the start of the titration
  2. Bthe blue colour of starch-iodine appears only when iodine is completely consumed
  3. Ca large amount of iodine forms a starch-iodine complex that is not easily decomposed by thiosulphate
  4. Dstarch is oxidised by iodine at high concentration to a coloured product

Answer: C

At high iodine concentration the starch-iodine complex is strongly adsorbed and reacts only slowly with thiosulphate, giving a sluggish and indistinct end point. Starch is therefore added when the colour has faded to pale yellow, and the blue colour then disappears sharply.
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