iodinestarch

Iodinestarch, commonly called iodine-starch, is the blue-black complex that forms when iodine interacts with starch, particularly the amylose fraction. This complex is widely used in qualitative analysis as the starch-iodine test, which detects the presence of starch in a sample.

Mechanism: In aqueous iodine solutions, iodine molecules insert into the left-handed helical cavity of amylose to form an inclusion complex with polyiodide chains. The resulting charge-transfer interaction yields a characteristic blue color that is visible even at low starch concentrations. The color intensity depends on factors such as starch content, iodine concentration, temperature, and pH. Heating can disrupt the complex, causing decolorization as iodine is released from the helix.

Applications: The starch-iodine test is a standard qualitative assay in chemistry and biology labs, and is also used in food science to check for starch in ingredients and finished products. It can be used to monitor gelatinization of starch during heating, as the color changes when the helix structure is disrupted.

Limitations: The test is not perfectly specific to starch; some other polysaccharides or thickened solutions may give a weak color under certain conditions. Interfering substances that bind iodine or scavenge iodine can mask the result. The color can be unstable and may fade over time, requiring prompt observation.

Safety: Iodine solutions are potentially hazardous. Use appropriate personal protective equipment and dispose of iodine-containing waste according to regulations.