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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3eq091

Which of the following statements about the pH conditions of iodine-thiosulphate or iodine-arsenite titrations are correct?

  1. ARaising the pH above 11 makes the iodine-thiosulphate stoichiometry more accurate.
  2. BIn strongly alkaline solution iodine disproportionates to iodide and hypoiodite/iodate, which spoils the stoichiometry.
  3. CIn strongly acidic solution thiosulphate decomposes to sulphur and sulphur dioxide.
  4. DArsenite is titrated with iodine in buffer, because in strongly acidic solution the reaction runs backwards.

Answer: B, C, D

The arsenite reaction produces H+, so strongly acidic conditions reverse it and bicarbonate buffering keeps it quantitative. In strong alkali I2 disproportionates (and hypoiodite oxidises thiosulphate partly to sulphate), while in strong acid thiosulphate gives S and SO2. Hence only the first three are correct.
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