GIPHOTOSTOCK / SCIENCE PHOTO LIBRARY GIPHOTOSTOCK / SCIENCE PHOTO LIBRARY
Formation of iodine in a redox reaction, 3 of 3. Purple-black iodine (I2) forms when concentrated (18M) sulfuric acid (H2SO4) is added to white solid potassium iodide (KI) in a test tube. The initial reaction forms hydrogen iodide gas (HI), which is then oxidized by the sulfuric acid to form iodine (I2). The two reactions are: KI + H2SO4 -> KHSO4 + HI and HI + H2SO4 -> H2S + H2O + I2. The combined reaction is: KI + H2SO4 -> I2 + H2S + H2O + KHSO4. The fumes are hydrogen iodide contaminated with iodine.
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