why is starch not used in redox titrations having high acid concentration
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The starch indicator is added to the solution near the end of the titration, at the point where dilute iodine imparts a pale yellow color to the solution. There are two reasons why the indicator is not added at the beginning of the titration when the iodine concentration is high. First, a diffuse endpoint would result from the slow dissociation of the starch-iodine complex if a large amount of iodine were absorbed in the starch. Second, iodometric titrations are carried out in strongly acid media, a situation that promotes the reaction between oxidizing agents and iodide. Unfortunately starch has a tendency to hydrolyze (decompose) in acidic media, destroying its indicator qualities
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