Amylose triiodide ions

The reactions of potassium iodide in aqueous solutions are those of iodide ion, r. In iodometric titration I combines with iodine to form triiodide ion, I3. The latter adds to (i-amylose fraction of the starch to form a blue complex. 

Deep Blueing Mechanism of Triiodide Ions in Amylose Being Associated with Its Conformation... 

The intensely blue color is due to a charge-transfer complex formed by the triiodide ion and starch. It results from the inclusion of tri-iodide ions into the straight-chain fractions of a-amylose of starch, which form a helix. The formed clathrate exhibits a narrow band of charge transfer near 620 nm (Fig. 18.2). 

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