Starch indicatorとは 意味・読み方・使い方

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 complex is unstable in low concentrations of iodine and highly stable in high concentrations of iodine. Thus during iodine titrations, concentrated iodine solutions must be reacted with some titrant, often thiosulfate, in order to reduce coloration. Then the starch is added, and the titration process is resumed taking into account the amount of thiosulfate added before adding the starch.