Answer to Question #329365 in Organic Chemistry for Sm miahara

Question

What is the function of starch in an iodometric titration? Why should starch solution be added just before the end-point of the titration?

Answer 1

Starch is used as an indicator for the presence of iodine. They react to form a deep blue black coloured complex. This makes the equivalence point much easier to observe.

Adding starch to the iodine solution before it has been diluted to pale yellow would give a starch-iodine complex that's difficult to dissociate, consequently requiring a higher volume of titrant because the release of I₂ from the starch-iodine complex is slow.