Expt:Iodine clock reaction
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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.
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 an irritant. Refer to CLEAPSS® Hazcard 50.
1M sulfuric acid is Corrosive. Refer to CLEAPSS® Hazcard 98A
- Solution X and the starch solution should be made up before the demonstration. The solutions will keep overnight, but best results are obtained if the solutions are made up on the day.
- The starch solution needs to be fresh.
Health & Safety
Wear eye protection throughout.
Solution X is made up as follows:
B. To the potassium iodide solution add 7.5 g of sodium thiosulfate and dissolve.
C. Transfer the solution to a 1 dm3 volumetric flask and make up the solution to 1 dm3 with distilled water. Ensure the solution is well mixed.
Starch Solution is made up as follows:
A. Make a paste of 0.2 g of soluble starch with a few drops of water in a 250 cm3 beaker. Pour onto this approximately 100 cm3 of boiling water and stir. Both solutions are colourless although solution X will be slightly cloudy on storage.
A. Stand five 100 cm3 beakers side by side on white paper or tiles.
B. To each beaker add 20 cm3 of solution X, 10 cm3 of 1M sulfuric acid and approximately 2 cm3 of starch solution.
C. Prepare five boiling tubes containing 30, 25, 20, 15, 10 cm3 respectively of 20 vols hydrogen peroxide. Maintain the overall volume by adding 0, 5, 10, 15, 20 cm3 of distilled water.
D. Add the contents of the five boiling tubes to the corresponding beakers at the same time whilst starting timing. Assistants will be required at this stage.
E. Record the time taken for the change in colour (blue/black) of each beaker.
This experiment was written by Andrew Thompson on behalf of the RSC