Demo Photo

DemonstrationsChemical Kinetics › 14.1

Lecture Demonstrations

Chemical Kinetics

14.1 H2O2 I2 Clock: Oxidation of Potassium Iodide by H2O2

Subjects: Kinetics, titrations (iodometry)

Description: A clock reaction is demonstrated. Two clear, colorless solutions are mixed together. Upon mixing the solution remains clear for approximately 30 seconds and then suddenly turns dark blue.


Solution A:

25 mL 2.0 M sulfuric acid‡
25 mL 3% H2O2
450 ml distilled H2O

Solution B:

0.10 g sodium thiosulfate pentahydrate (Na2S2O3.5H2O)‡
50 mL DI H2O
13 mL 1.0 M potassium iodide, KI‡
10 mL 1% starch solution

‡Sulfuric acid is located in the cabinet under the hood.
Hydrogen peroxide is located in the refrigerator.
Sodium thiosulfate pentahydrate is located on the general chemical storage shelf.
1.0 M potassium iodide is located on the general storage solutions shelf.

  • 250 mL graduated cylinder
  • 50 mL graduated cylinder
  • 10 mL graduated cylinder
  • 250 mL beaker
  • 2 600 mL beakers
  • 2 stirring rods
  • timing device (optional)

Pre-class Preparation (allow 15-20 minutes for preparation)
Solution A:

1. In a 250 mL graduated cylinder, combine the 2.0 M sulfuric acid and 3% H2O2.
2. Dilute the solution to 250 mL with DI water. Pour the solution into a 600 mL beaker and stir. Leave the solution in beaker until you are ready to perform the demo.
3. Rinse the 250 mL cylinder with distilled water.

Solution B:
1. In a 250 mL beaker, dissolve 0.10 g of sodium thiosulfate pentahydrate in 50 mL of distilled water.  Pour this solution into the 250 mL graduated cylinder.
2. Add the KI and starch solutions to the cylinder. Dilute with DI water to 250 mL. Pour this solution into the second 600 mL beaker and stir.  Leave the solution in beaker until ready to perform the demo.


The reactions taking place to produce the blue color after the solutions are mixed are given below:

3I-(aq) + H2O2 (aq) + 2H+(aq) –> I3-(aq) + 2H20(l)             (1)

I3-(aq) + 2S2O32-(aq) –> 3I-(aq) + S4O62-(aq)                    (2)
2I3- + starch <–> blue starch-I5- complex + I-                      (3)

Equation 1 shows that the iodide ions are being oxidized by hydrogen peroxide to triiodide ions. These ions are reduced back to iodide ions by thiosulfate ions (equation 2). Reaction 2 is fast, consuming the triiodide as fast as it forms and keeping the concentration of I- relatively constant, thus preventing the reaction in equation 3 for a period of time (the clock period). However, the thiosulfate concentration is limited and will run out. Once this happens, the I3- concentration will increase and accumulate, allowing it to react with the starch to form the I5- complex shown in equation 3, which results in the blue color.

In addition to the above reactions, hydrogen peroxide is oxidizing thiosulfate ions to tetrathionate ions.

2S2O3 2-(aq) + H2O2(aq) + 2H+(aq) –> S4O62-(aq) + 2H2O(aq)     (4)

Varying the concentrations of the different species in these reactions helps to determine the rate equations for these reactions. Doubling the initial concentration of iodide halves the clock period. Doubling the initial concentration of H2O2 also halves the clock period.  Varying the concentration of thiosulfate shows that the clock period is inversely proportional to the initial concentrations. Doubling the concentration doubled the clock period.


Sulfuric acid is a strong dehydrating agent and strongly corrosive. It can cause burns. Use appropriate protective equipment including gloves and safety glasses. 3% hydrogen peroxide solution can be irritating to the skin and eyes.


The reaction solution should be disposed of in the appropriate aqueous waste container.


1. B.Z. Shakhashiri; Chemical Demonstrations: A Handbook for Teachers of Chemistry; Volume 4; Wisconsin; 1992; p. 37-43

Download a Printable Version