Iodine reacts with starch in water to produce an intensely blue color. Beta amylose, which is a form of soluble starch, traps iodine ions within its highly coiled structure to achieve this effect.
Iodine is not very water-soluble. An iodine reagent is formed by dissolving iodine in water containing potassium iodide. The mix of potassium iodide and iodine in water forms a linear triiodide complex. The linear nature of this chemical structure is important because it allows the complex to slip into the beta amylose coils of starch.
There is some transfer of electronic charge between the beta amylose coils and the iodine complex. As a result, electronic configurations and energy levels are reconfigured. The spacing of energy levels is sufficient to alter the absorption spectrum for visible light. This reaction causes an intense blue-black color that can be used as a chemical indicator in redox titrations.
For example, the reagent could be used when there is excessive concentrations of oxidising agent or reducing agent. Excess oxidising agent causes the complex to turn blue, whereas excess reducing agent causes the complex to turn colorless because the iodine and iodide ions break up and become separated. In other words, both iodine and iodide ions are required for the color change to occur.