Schematic view of I3− ions embedded in amylose helix
The Iodine test is used to test for the presence of starch. Iodine solution — iodine dissolved in an aqueous solution of potassium iodide — reacts with the starch, producing a purple-black colour. The colour can be detected visually with concentrations of iodine as low as 0.00002 M at 20 °C. However, the intensity of the colour decreases with increasing temperature and with the presence of water-miscible organic solvents, such as ethanol. Also, the test cannot be done at very low pH values due to the hydrolysis of the starch under these conditions.