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Iodine Clock
An eye-catching colour change demonstration in which a colourless solution suddenly changes to a dark-blue after an amount of time. It is known as the "iodine clock" as the colour change doesn't happen straight away. The demonstration is particularly well-suited as an introduction to reaction rates and kinetics.
About these notes These notes have been designed to accompany the film "Iodine Clock demonstration - Get set...demonstrate for Demo Day 2014", as part of the Get Set Demonstrate series of video guides. They are not a standalone guide to performing this demonstration. The aim is to encourage as many teachers as possible to try a new demonstration for National Science and Engineering Week. See www.getsetdemo.com for more information.
Equipment 2 large conical flasks marked "A" and "B" Several 250 ml beakers Stirring rod Stopclock/timer Safety screen used as white background Few sheets of white paper Balance Eye protection Volumetric flasks Measuring cylinders (2 x 100 cm^3 )
Chemicals 2dm^3 deionised/distilled water
For solution A 0.2g soluble starch 4.1g anhydrous sodium ethanoate (sodium acetate) - see CLEAPSS Hazcard 38A 50g potassium iodide - see CLEAPSS Hazcard 47B 9.4g sodium thiosulfate-5-water - see CLEAPSS Hazcard 95C
For solution B 30 cm^3 glacial (concentrated) ethanoic acid (CORROSIVE) - see CLEAPSS Hazcard 38A 500 cm^3 hydrogen peroxide solution, 20 'vol' (IRRITANT) - see CLEAPSS Hazcard 50
Making the solutions Solutions A and B 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.
1. Make a paste of 0.2 g of soluble starch with a few drops of water in a beaker. Pour onto
this approximately 100 cm^3 of boiling water and stir.
2. Pour the resulting solution into a 1 dm^3 beaker and dilute to around 800 cm^3.
3. Add 4.1 g of sodium ethanoate, 50 g of potassium iodide and 9.4 g of sodium thiosulfate.
Stir until all the solids have dissolved and allow to cool to room temperature.
4. Pour the mixture into a 1 dm^3 volumetric flask and make up to 1 dm^3 with water. Label
this flask solution A.
5. In a 1 dm^3 volumetric flask mix 500 cm^3 of 20 volume hydrogen peroxide with 30 cm^3 of
glacial ethanoic acid and dilute to 1 dm^3 with water. This flask should be labelled solution B.
Both solutions are colourless although solution A will be slightly cloudy.
Demonstration procedure
1. Measure 100 cm^3 of solution A and pour it into an empty 250 ml beaker.
2. Measure 100 cm^3 of solution B in a separate (clean) 100 cm^3 measuring cylinder.
3. Position the beaker containing solution A against a white or light coloured surface. Add
solution B to the 250 ml beaker and give it a good mix with a stirring rod.
4. Leave the mixture to stand. After about 20 seconds at room temperature the mixture will
suddenly turn dark blue.
The appearance of the blue colour may be timed; a pupil could start and stop the timer.
Safety notes
The acid will react slowly with sodium thiosulfate and produce a cloudy suspension of
sulfur and release sulfur dioxide which is TOXIC (see CLEAPSS Hazcard 97). To avoid this the acid and sodium thiosulfate are separated in solutions A and B.
Ethanoic acid is corrosive and you should refer to CLEAPSS Hazcard 38A.
Hydrogen peroxide solution, 20 'vol' is an irritant and you should refer to CLEAPSS Hazcard
Teaching notes In the main reaction the hydrogen peroxide oxidises iodide ions to iodine. This is quite slow and is the rate determining step in this demonstration (useful stimulus for advanced discussions):
H 2 O2(aq) + 2I–(aq) + 2H+(aq) → I2(aq) + 2H 2 O(l)
As soon as any iodine is formed, it reacts with the thiosulfate and turns back into iodide ions by this fast reaction:
2S 2 O 32 – (aq) + I2(aq) → S 4 O 62 – (aq) + 2I–(aq)
Therefore, there is no colour change as long as there is thiosulfate present in the solution. However, once all the thiosulfate is used up, the free iodine remains in solution and reacts with the starch to form the familiar blue-black complex.
The ethanoic acid/sodium ethanoate is added to buffer the pH.
The time for the blue colour to appear can be adjusted by diluting the hydrogen peroxide solution, as in the video. Alternatively, varying the amount of thiosulfate in solution A will also change the time for the blue colour to appear.
