How does starch help detect small amounts of iodine?

Starch is used to detect small amounts of iodine because it forms a distinct, highly sensitive blue or blue-black complex when combined with iodine in an aqueous solution.123

This reaction is so effective that it serves as a standard test for the presence of iodine. In analytical chemistry, this property is also utilized during volumetric titration to determine the concentration of iodine or other oxidizing substances; the starch acts as an indicator, where the disappearance of the characteristic blue color signals the end point of the reaction.13