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Hint Starch iodide paper consists of potassium iodide and starch. It is used for detection of nitrite and free chlorine atoms. Free chlorine and nitrite ion readily accept an electron to gain their stable configuration. As they get reduced, they end up oxidizing the iodide ion present in the form of potassium iodide.
The Potassium Iodide Starch test paper contains potassium iodide as an active ingredient. In the presence of an oxidant, such as peroxide or chlorine, iodide is converted to iodine which then binds to starch molecules in the paper forming the blue to purple color.
Chlorine turns moist starch iodide paper blue-black because the iodide ions on the paper are oxidised to iodine and the starch react to it to give blue black compound.
When moist starch iodide paper is introduced into chlorine gas, chlorine oxidizes iodide to iodine, which shows up as blue when complexed with starch.