Expt:Iodine clock reaction

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Introduction

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.

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.

Technical notes

20 volume hydrogen peroxide is an 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 results are obtained if the solutions are made up on the day.
2. The starch solution needs to be fresh.

Procedure

Health & Safety

Wear eye protection throughout.

Procedure

Solution X is made up as follows:

A. Dissolve 6.0 g of potassium iodide in approximately 800 cm³ of distilled water.

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.

The demonstration

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.

Additional notes

Hydrogen peroxide is capable of oxidising thiosulfate ions to tetrathionate ions but the reaction is too slow to affect this demonstration.

The acid will react slowly with sodium thiosulfate and produce a cloudy suspension of sulfur and release sulfur dioxide which is TOXIC, therefore dispose of the mixture immediately after use.

Reference

This experiment was written by Andrew Thompson on behalf of the RSC