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While investigating the rate of reaction between potassium iodate (KIO3) and sodium sulphite (Na2SO3) with starch solution as the indicator at different concentrations and temperatures, which precaution should be followed?
- The sodium thiosulphate solution must always be kept at a lower concentration than the potassium iodate solution.
- Use starch solution that has been prepared freshly.
- Carry out the experiments using fresh solutions of H2O2 and KI.
- All of the above.
Correct answer: Use starch solution that has been prepared freshly.
Solution
In the iodate-sulphite (iodine clock) experiment the starch indicator must be freshly prepared, since old starch loses sensitivity and gives a weak or delayed blue end-point. The other options mention sodium thiosulphate and H2O2/KI, which belong to different experiments, so 'All of the above' is not correct.
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