Iodine–starch test

The iodine–starch test is a chemical reaction that is used to test for the presence of starch or for iodine. The combination of starch and iodine is intensely blue-black. The interaction between starch and the triiodide anion is the basis for iodometry.

History and principles
The iodine–starch test was first described in 1814 by Jean-Jacques Colin and Henri-François Gaultier de Claubry, and independently by Friedrich Stromeyer the same year.

The triiodide anion instantly produces an intense blue-black colour upon contact with starch. The intensity of the colour decreases with increasing temperature and with the presence of water-miscible organic solvents such as ethanol. The test cannot be performed at very low pH due to the hydrolysis of the starch under these conditions. It is thought that the iodine–iodide mixture combines with the starch to form an infinite polyiodide homopolymer. This was rationalized through single crystal X-ray crystallography and comparative Raman spectroscopy.

Starch as an indicator
Starch is often used in chemistry as an indicator for redox titrations where triiodide is present. Starch forms a very dark blue-black complex with triiodide. However, the complex is not formed if only iodine or only iodide (I−) is present. The colour of the starch complex is so deep, that it can be detected visually when the concentration of the iodine is as low as 20 μM at 20 °C. During iodine titrations, concentrated iodine solutions must be reacted with some titrant, often thiosulfate, in order to remove most of the iodine before the starch is added. This is due to the insolubility of the starch–triiodide complex which may prevent some of the iodine reacting with the titrant. Close to the endpoint, the starch is added, and the titration process is resumed taking into account the amount of thiosulfate added before adding the starch.

The color change can be used to detect moisture or perspiration, as in the Minor test or starch–iodine test. Starch is also useful in detecting the enzyme amylase, which breaks down starch into sugars. Many bacteria like Bacillus subtilis can produce such an enzyme to help scientists identify unknown bacterial samples -- the starch-iodine test is one of many tests needed to identify the exact bacterium. The positive test for bacteria that has starch hydrolysis capabilities (able to produce amylase) is the presence of a yellow zone around a colony when iodine is added to detect starch.

Medical Use
Although the starch-iodine test is predominantly employed in the lab, recent assessments have shown potential for clinical use, such as confirming the diagnosis of Horner's syndrome. Hospitals with limited technical accessibility can exploit this diagnostic tool since it requires resources that may be easily attainable. In order to perform the experiment, a patient's skin is first dried with 70% alcohol; with the iodine solution added, subsequently. After the skin dries completely once more, it will be dusted with a starch material. Inducing sweating conditions will cause the skin to turn dark blue. Physicians can then make a diagnosis if the test shows sweating of different intensities on the left and right side of the body.