Brief Description:

Two colorless solutions are mixed, and after several seconds, the mixture suddenly turns deep blue.


Procedure presents an example of a typical clock reaction.

Explanation of Experiment:

Sudden change from colorless to deep blue can be explained by the following sequence of reactions:

  1. 3 I- (aq) + H2O2 (aq) + 2 H+ (aq) → I3- (aq) + 2 H2O (l) (slow)
  2. I3- (aq) + 2 S2O32- (aq) → 3 I- (aq) + S4O62- (aq) (fast)
  3. 2 I3- (aq) + starch (aq) → starch-I5- complex (aq) + I- (aq)

Equation (1) 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, equation (2).

Reaction (2) is much faster than reaction (1) – it consumes triiodide ions as fast as they are formed.

This prevents any readily apparent reaction of equation (3). However, after all of the thiosulfate ions have been consumed by the reaction of equation (2), triiodide ions react with starch to form the blue starch-pentaiodide complex.

Materials Preparation:

  1. Stock solution 2.0 M sulfuric acid, H2SO4: Take 120 mL distilled water and add 22 mL concentrated sulfuric acid – dilute to 200 mL with water – allow to cool.
  2. Stock solution 1.0 M potassium iodide, KI: Dissolve 33.2 g KI in 160 mL distilled water – dilute to 200 mL. (The iodide ions in this solution will be oxidized slowly by oxygen in the air. The product is molecular iodine, I2, which darkens the solution. This air oxidation can be retarded by preparing the solution with water that has been deoxygenated by boiling or by passing nitrogen gas through it, and then storing the solution in an air tight container.)
  3. Stock solution 1% starch: Bring 100 mL distilled water to a boil. Make a slurry of 2.0 g soluble starch in 4 mL water. Pour slurry into boiling water – boil 5 minutes – dilute to 200 mL – allow to cool.
  4. Stock solution 3% hydrogen peroxide, H2O2 – available at local pharmacy.
  5. 0.20 g sodium thiosulfate, Na2S2O3
  6. Glassware: 2x 600 mL beakers, 250 mL graduated cylinder, 250 mL beaker, stirring rod
  7. In a 250 mL graduated cylinder, combine 25 mL of 2.0 M H2SO4 and 25 mL of 3% H2O2. Dilute the solution to 250 mL with distilled water. Pour the solution back and forth between the graduated cylinder and one of the 600 mL beakers to mix the solution. Leave the solution in the beaker. (This solution is 0.20 M in H2SO4 and 0.09 M in H2O2.)
  8. In a 250 mL beaker, dissolve 0.20 g Na2S2O3 in 50 mL of distilled water. Rinse the 250 mL graduated cylinder with distilled water. Add the Na2S2O3 solution, 13 mL 1.0 M KI, and 10 mL of 1% starch solution to the cylinder. Dilute the mixture to 250 mL with distilled water. Pour the solution back and forth between the graduated cylinder and the remaining 600 mL beaker to mix the solution. Leave the solution in the beaker. (This solution is 0.052 M in KI and 0.0032 M in Na2S2O3.)
  9. Place the two 600 mL beakers side by side on the display table and put a stirring rod in one of the beakers.


Quickly pour the contents of one beaker into the other, stir the mixture, and begin timing. The mixture will remain colorless for about 20-25 seconds and then suddenly turn deep blue.


Because sulfuric acid is both a strong acid and a powerful dehydrating agent, it must be handled with great care. The dilution of concentrated H2SO4 is a highly exothermic process and releases sufficient heat to cause burns. Therefore, when preparing dilute solutions from the concentrated acid, always add the acid to water slowly, and with stirring.

The 3% hydrogen peroxide solution can irritate the skin and eyes.


Waste solutions should be flushed down the drain with water.


General Concept:

  • Kinetics

Type of Reaction:

  • Oxidation/Reduction

Primary Reference:

  • Shakhashiri, Bassam Z., Chemical Demonstrations – A Handbook for Teachers of Chemistry – Volume 4.