<Tr> <Th> Classification </Th> <Td> Colorimetric method </Td> </Tr> <Tr> <Th> Analytes </Th> <Td> Starch </Td> </Tr> <P> 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 . </P> <P> 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 cannot be performed at very low pH due to the hydrolysis of the starch under these conditions . </P>

Why is iodine used to test for starch