Iodine test - wikipedia Iodine test This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please help to improve this article by introducing more precise citations. ( October 2012 ) Iodine test Schematic view of I ions embedded in amylose helix Classification Colorimetric method Analytes Starch The iodine test is used to test for the presence of starch. Starch turns an intense `` blue - black '' colour upon addition of aqueous solutions of the triiodide anion, due to the formation of an intermolecular charge - transfer complex. In the absence of starch, the brown colour of the aqueous solution remains. This interaction between starch and triiodide is also the basis for iodometry. Contents 1 Principles 2 Starch as an indicator 3 See also 4 References 5 Further reading 6 External links Principles To produce the triiodide anion ( I ), elemental iodine is dissolved in an aqueous solution of potassium iodide. The resulting complex with starch produces an intense `` blue - black '' colour. The intensity of the colour decreases with increasing temperature and with the presence of water - miscible organic solvents such as ethanol. The test can not be performed at very low pH due to the hydrolysis of the starch under these conditions. Starch as an indicator See also : iodometry 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 which can be made by mixing iodine with iodide ( often from potassium iodide ). 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 0.00002 M 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 end - point, the starch is added, and the titration process is resumed taking into account the amount of thiosulfate added before adding the starch. See also Lugol 's Iodine Counterfeit banknote detection pen References Jump up ^ `` Iodine Test for Starch ''. Brilliant Biology Student Master Biology Labs. Retrieved 2015 - 12 - 01. Further reading http://antoine.frostburg.edu/chem/senese/101/redox/faq/starch-as-redox-indicator.shtml Vogel 's Textbook of Quantitative Chemical Analysis, 5th edition. External links How does starch indicate iodine ? General Chemistry Online Iodine test at Braukaiser Titrations.info : Potentiometric titration -- Solutions used in iodometric titrations Retrieved from `` https://en.wikipedia.org/w/index.php?title=Iodine_test&oldid=801446274 '' Categories : Laboratory techniques Chemical tests Hidden categories : Articles lacking in - text citations from October 2012 All articles lacking in - text citations All articles with unsourced statements Articles with unsourced statements from June 2015 Talk Contents About Wikipedia Deutsch Español Polski Simple English Svenska Edit links This page was last edited on 19 September 2017, at 18 : 41. About Wikipedia

Question: why is iodine used to test for starch
Answer: Starch turns an intense "blue-black" colour upon addition of aqueous solutions of the triiodide anion