Iodine test

Schematic view of in amylosehelice embedded I₃⁻-ions

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 color. The colour can be detected visually with concentrations of iodine as low as 0.00002M 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 pHs due to the hydrolysis of the starch under these conditions.

This reaction is the result of the formation of polyiodide chains from the reaction of starch and iodine. The amylose, or straight chain portion of starch, forms helices where iodine molecules assemble, forming a dark purple/black color. The amylopectin, or branched portion of starch, stains more strongly than amylose. As starch is broken down or hydrolyzed into smaller carbohydrate units, the purple-black color is not produced. Therefore, this test can determine completion of hydrolysis when a color change does not occur.

Iodine solution will also react with glycogen, although the color produced is browner and much less intense. It is rust colored normally.

See also

• Starch indicator
• Lugol's Iodine
• Counterfeit banknote detection pen

References

• http://antoine.frostburg.edu/chem/senese/101/redox/faq/starch-as-redox-indicator.shtml
• Vogel's Textbook of Quantitative Chemical Analysis, 5th edition.