Which of the following statements about the iodometric titration of an oxidant (for example $Cu X^{2 +}$ or $Cr X_{2} O X_{7} X^{2 -}$ ) with thiosulphate are correct?

AExcess KI is used both as reductant and to keep $I X_{2}$ in solution as $I X_{3} X^{-}$
BStarch added too early binds iodine strongly and causes a delayed, unclear end point
CThe titration can be carried out in strongly alkaline solution without error
DThe amount of thiosulphate used is a measure of the iodine liberated, not of the starch added

Answer: A, B, D

Excess iodide reduces the oxidant and forms soluble

I X_{3} X^{-}

, which also lowers loss of iodine by volatilisation. Starch is added only near the end point because early addition gives a strongly adsorbed complex that dissociates slowly. In strong alkali iodine disproportionates to hypoiodite/iodate and thiosulphate is partly oxidised to sulphate, so stoichiometry is lost. A, B and D are correct.

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Which of the following statements about the iodometric titration of an oxidant (for example CuX2+ or CrX2​OX7​X2−) with thiosulphate are correct?

Answer: A, B, D

More questions on Redox titration calculations

Which of the following statements about the iodometric titration of an oxidant (for example $Cu X^{2 +}$ or $Cr X_{2} O X_{7} X^{2 -}$ ) with thiosulphate are correct?