The potassium-iodide starch test utilizes test strips to detect levels of free iodine, chlorine and peroxides, or starches, in biological materials. Normally yellow-brown in color, the potassium-iodide solution turns deep blue when it encounters starch. Conversely, starch may also be used to detect free iodine.
Iodine takes about 60 seconds to react, while chlorine and any peroxide react immediately. A reaction initially shows as a bluish hue that grows darker from amylose in the starch. Free iodine held physically inside the amylose coil forms a concentration of iodine that is easily visible. With a high concentration of starch, the strip goes from dark blue to purple.