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The Iodine test is used to test for the presence of starch. Iodine solution — iodine dissolved in an aqueous solution of potassium iodide — reacts with the starch, producing a purple-black colour. The colour can be detected visually with concentrations of iodine as low as 0.00002 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. Also, the test cannot be done at very low pH values due to the hydrolysis of the starch under these conditions.
The starch-iodide complex is formed as charge - recall electrons are charged particles - is transferred between the starch and iodide ions - tri-iodide or pentaiodide.
The transfer of charge between the starch and the iodide ion changes the spacing between the energy levels/ orbitals.
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.
The procedure for the iodine test for starch depends on whether the test sample is a solid or liquid.
SOLID SAMPLE Peel off the skin of any vegetables e.g. potato and fruits as these are often impermeable. Use a clean spatula to remove samples of powdered food. Avoid cross contamination with other foods. Add a few ( 2-3) drops of bench iodine solution potassium to a piece of solid food on a white tile. Make observation.
LIQUID SAMPLE Add 10 cm3 of the liquid food sample to a clean, dry test tube. Add about 5 drops of iodine solution to the test tube. Note any colour changes. To prepare a control, perform steps 1 -3 for de-ionized water. 
- Vogel's Textbook of Quantitative Chemical Analysis, 5th edition.
- Iodine test at Braukaiser