13 3 Hydrogen Peroxide Iodine Clock Oxidation of Potassium Iodide by Hydrogen Peroxide Source B Z Shakhashiri 1992 Chemical Demonstrations A Handbook for Teachers of Chemistry vol 4 pp 37 43 Description Two colorless solutions are mixed After 10 seconds the colorless mixture suddenly turns blue Concept Demonstrates a typical clock reaction shows the effect of the interaction between chemical reactions that have different rates Materials Solution A o 0 6 grams Starch o 30 mLs of Acetic Acid o 4 1 grams of Sodium Acetate o 50 grams of Potassium Iodide o 4 7 grams of Sodium Thiosulfate Allow mixture to cool and dilute to 1 liter with distilled water o 1 liter flask Solution B o 500 mLs of 3 Hydrogen Peroxide o 500 mLs of distilled water o 1 liter flask o Safety Hydrogen Peroxide can be irritating to skin and eyes Wear safety goggles and gloves Procedure Mix the two solutions together Stir by stir bar and stir plate swirl the mixture in the flask by hand or mix by transferring the mixture back and forth between the two flasks Stir until the colorless solution turns blue about 10 seconds Clean Up Remaining blue solution can be washed down drain with water Background For the entire explanation of this demo see Shakhashiri s Chemical Demonstrations Vol 4 pp 42 43 The following is a excerpt from this volume The sudden change from colorless to deep blue solutions in this demonstration can be explained with the following sequence of equations 2 3 I aq H202 2 H aq I3 aq 2 H20 l I3 aq 2 S2O3 2 aq 3 I aq S4O6 2 aq 13 3 1 2 2 I3 aq starch starch I5 complex I aq 3 The first equation indicates that in an acidic solution iodide ions are oxidized by hydrogen peroxide to triiodide ions These triiodide ions are reduced back to iodide ions by thiosulfate ions as indicated in equation 2 This reaction is much faster than the reaction of equation 1 it consumes triiodide ions as fast as they are formed This prevents any readily apparent reaction of equation 3 However after all the thiosulfate ions have been consumed by the reaction of equation 2 triiodide ions react with starch to form the blue starch pentaiodide complex 3 Notes This demo is done by Prof Peters in his Chemistry Magic Show A similar reaction is studied in lab C126 See also Rossini William Tell and the Iodine Clock Reaction 13 3
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