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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.


0.6 grams Starch

30 mLs of Acetic Acid

4.1 grams of Sodium Acetate

50 grams of Potassium Iodide

4.7 grams of Sodium Thiosulfate

Allow mixture to cool and dilute to 1 liter with distilled water

1 liter flask

500 mLs of 3% Hydrogen Peroxide

500 mLs of distilled water

1 liter flask

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) (1)

I3- (aq) + 2 S2O3 2- (aq) = 3 I- (aq) + S4O6 2- (aq) (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]."


This demo is done by Prof. Peters in his Chemistry Magic Show

The kinetics of an iodine clock reaction is studied in C126 Experiment 3.

See also "Rossini, William Tell and the Iodine Clock Reaction"