UntitledThe Oscillating Clock Reaction or 
The Briggs-Rauscher Oscillating Reaction 

Source: B. Z. Shakhashiri, 1985, Chemical Demonstrations: A Handbook for 
Teachers of Chemistry, vol. 2, pp 248-256. 

Description: Three colorless solutions are mixed together. The color of the 
resulting mixture will oscillate back and forth from amber to blue for about 5 
minutes. The reaction ends as a blue-black mixture with the odor of iodine. 
Concepts: The basic concept that this reaction demonstrates is that two 
reactions can switch back and forth. The product of one is the reactant for the 
other. 
Materials: 
  Solution A: (I usually only make 300 mL of each solution unless I know I will 
  use 1 L). 
Put 43 g Potassium Iodate (KIO3) in about 800 mL of distilled water 
Add 4.5 mLs of Sulfuric Acid (H2SO4) 
Stir until KIO3 is dissolved 
Dilute to 1 L 
  Solution B: 
Put 15.6 g of Malonic Acid (HOOCCH2COOH) and 
3.4 g of Manganese Sulfate Monohydrate (MnSO4 . H2O) in about 
800 mLs of distilled water. 
Add 4 g of Vitex Starch. 
Stir until all is dissolved. 
Dilute to 1 L 
  Solution C: 
(Use only fresh solution B. If you make solution B the day before the demo, 
store it in the refrigerator.) 
Dilute 400 mLs of 30% H2O2 up to 1 L.

  300 mLs of each solution 
  1 L beaker 
  Lighted stirring plate 
  magnetic stir bar 
Procedure: 
Put the stir bar into the beaker. Put 300 mL of solutions A and B into the 
beaker. Turn on stir plate and stir plate light. Adjust the speed to get a large 
vortex. Put 300 mL of Solution C into the beaker. Be sure to add the solutions 
in this order or it will not work: A + B and then C. 
Safety: Iodine is produced. The vapor and solid are irritating to the eyes, 
skin, and mucous membranes. Wear safety goggles and gloves. Do this demo in a 
well ventilated room. 
Be careful if you prepare the solutions. Wear goggles and disposable gloves. 
Sulfuric acid and malonic acid are strong irritants and 30% H2O2 is a strong 
oxidizer. 
Clean Up: 
Neutralize the iodine by reducing it to iodide. Add about 10 g of Sodium 
Thiosulfate to the mixture and stir until the mixture becomes colorless. 
Caution: the reaction between iodine and thiosulfate is exothermic and the 
mixture may be hot. The cooled, neutralized mixture should be washed down the 
drain with water.

Background: 
For a detailed discussion of the Briggs-Rauscher Reaction see volume two of 
Chemical Demonstrations* by Bassam Z. Shakhashiri, pp 252 - 256. 
A great simplification^ of this discussion follows.

The BR reaction:

IO3- + 2 H2O2 + CH2(CO2H)2 + H+ = ICH(CO2H)2 + 2 O2 + 3 H2O

This reaction is accomplished by two component reactions A and B.

IO3- + 2 H2O2 + H+ = HOI + 2 O2 + 2 H2O (A) 

HOI + CH2(CO2H)2 = ICH(CO2H)2 + H2O (B)

Reaction A can occur via two different processes, 1 and 2.  Each process 
involves several different reactions.   The oscillation of the BR reaction is 
between these two processes. Reaction B connects the two processes. The 
interactions of the chemicals involved in process 1 and reaction B will 
eventually bring about conditions that favor process 2 and then process 2 and 
reaction B will do the same for process 1... thus there is an oscillation 
between the two processes. 
When process 1 and reaction B interact, the amber color is produced and process 
2 and reaction B lead to the blue color. 
See following diagram.

 


Notes: 
* The four volume set of Chemical Demonstrations is kept in Alice 
Dobie-Galuska's office C021D. There is also a set available in the Chemistry 
Library.

^ This may be as far as you want to go with your students. The chemistry of the 
Briggs-Rauscher Reaction is a bit complicated. 

This demo is used frequently in the C100 and C101 courses. 
It has also been done in C105, C106, and S106. It is often used at the beginning 
of the year for fun.