Iodine Crystals

Isolating Elemental Iodine from Potassium Iodide

1. Weigh out 2.0 g of potassium iodide and transfer it to a test tube. 2. Add about 1.5 mL of distilled water to the test tube and swirl to dissolve the potassium iodide. 3. Add 1.5 mL of concentrated hydrochloric acid (or about 1.8 mL of hardware store muriatic acid) to the test tube and swirl to mix the solutions. 4. Add about 10 mL of drugstore 3% hydrogen peroxide. The solution immediately turns dark brown as the iodide ions are oxidized to elemental iodine, which precipitates out. 5. Swirl the test tube to suspend the iodine and pour the liquid through a funnel with a folded piece of filter paper to capture the iodine crystals. 6. Rinse the iodine crystals on the filter paper several times with a few mL of distilled water. The rinse solution appears brown from dissolved iodine, but iodine is not very soluble in water, so you're not losing much of your yield. 7. Spread out the filter paper on a watch glass or saucer and allow the crystals to dry thoroughly. Iodine gradually sublimates (passes directly from solid to gaseous form) at room temperature, so don't leave the crystals exposed to air any longer than necessary to dry them. 8. Once the crystals are dry, transfer them to a sealed storage bottle or vial. These quantities produce a gram or so of iodine, which is sufficient for the experiments that require it. If you need more iodine, simply increase all quantities proportionately.