Iodine test

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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.

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