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iodine test
The iodine test is used to test for the presence of starch. When treated with IKI solution—iodine dissolved in an aqueous solution of potassium iodide—the triiodide anion (I3−) complexes with starch, producing an intense blue/purple colour. The colour can be detected visually with concentrations of iodine as low as 2×10-5 M at 20 °C. However, the intensity of the color 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.
==Principle==
The starch-iodide complex is formed as charge is transferred between the starch and triiodide or pentaiodide ions.
The transfer of charge between the starch and the iodide ion changes the spacing between the energy levels.
This change results in the starch-iodide complex absorbing light at a different wavelength—than any other species aforementioned—resulting in an intense purple colour; Biologists call this colour blue-black.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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