Starch indicator

Starch is often used in chemistry as an indicator for redox titrations where triiodide is present. Starch forms a very dark blue-black complex with triiodide which can be made by mixing iodine with iodide (often from potassium iodide). However, the complex is not formed if only iodine or only iodide (I⁻) is present. The colour of the starch complex is so deep, that it can be detected visually when the concentration of the iodine is as low as 0.00002 M at 20 °C. The colour sensitivity decreases with increasing temperature (ten times less sensitive at 50 °C), and upon the addition of organic solvents such as ethanol or propanone. During iodine titrations, concentrated iodine solutions must be reacted with some titrant, often thiosulfate, in order to remove most of the iodine before the starch is added. This is due to the insolubility of the starch-iodine complex which may prevent some of the iodine reacting with the titrant. Close to the end-point, the starch is added, and the titration process is resumed taking into account the amount of thiosulfate added before adding the starch.