This is the hydrogen peroxide/ potassium iodide ‘clock’ reaction. A solution of hydrogen peroxide is mixed with one containing potassium iodide, starch and sodium thiosulfate. After a few seconds the colourless mixture suddenly turns dark blue. This is one of a number of reactions loosely called the iodine clock. It can be used as an introduction to experiments on rates / kinetics.
This demonstration can be used at secondary level as an introduction to some of the ideas about kinetics. It can be used to stimulate discussion about what factors affect the rate of reaction. It also makes a useful starting-point for a student investigation. As described this is intended as a demonstration, best done on a large scale for the most visual impact. The demonstration itself takes less than 1 minute. For a student investigation, the quantities required would be smaller but volumes then need to be measured quite accurately with, for example, disposable plastic syringes. It also lends itself to a class competition aiming for a change at a teacher determined time.
Apparatus and chemicals
Eye protection Balance (1 or 2 d.p.) Volumetric flasks (1 dm3), Beakers (100 cm3), 5 Beaker (250 cm3) Beaker (2 dm3) Boiling tubes, 5 Boiling tube rack Measuring cylinder (50 cm3) Measuring cylinders (100 cm3), 2 Stirring rod or magnetic stirrer and follower (optional) Stopclock/timer, 5 0.2 g soluble starch 1M sulfuric acid (Irritant), 50 cm3 Potassium iodide (KI), 6.0 g. (Low hazard) Sodium thiosulfate-5-water (Na2S2O3.5H2O), 7.5 g (Low hazard) 20 volume hydrogen peroxide solution (H2O2(aq)), 100 cm3 (Irritant) Deionised/distilled water, 1 dm3.
20 volume hydrogen peroxide is Irritant. Refer to CLEAPSS® Hazcard 50. 1M sulfuric acid is Corrosive. Refer to CLEAPSS® Hazcard 98A 1 Solution X and the starch solution should be made up before the demonstration. The solutions will keep overnight, but best...