Our chlorine test papers are based on the potassium iodide starch reaction. As such, many oxidizing species will cause the paper to turn purple. The iodide that is impregnated in the paper is oxidized to iodine when exposed to a solution containing an oxidizer. The iodine then complexes with the starch (also impregnated in the paper) and turns the paper blue/purple. The iodide and starch impregnated into the paper are in excess, so that the intensity of the color developed depends on the amount and strength of the oxidizing species.