Introduction

Three colorless solutions are combined into a large beaker and the mixture becomes amber, then blue-black, and then colorless again. This sequence of color changes will repeat with a period of approximately 15 seconds at 25 oC. The reaction last about 5 min.

Changing color of colorless solution

The Briggs-Rauscher Reaction

IO3- + 2 H2O2 + CH2(CO2H)2 + H+ --> ICH(CO2H)2 + 2 O2 + 3 H2O                             (1)

This reaction can be broken into two component reactions:

IO3- + 2 H2O2 + H+ --> HOI + 2 O2 + 2 H2O                                                               (2)

HOI + CH2(CO2H)2 --> ICH(CO2H)2 + H2O                                                                  (3)

This reaction can occur by a radical process which is turned on when I- concentration is low, or by a non-radical process when the I- concentration is high. Both processes reduce iodate to hypoiodous acid. The radical process forms hypoiodous acid at a much faster rate than the non-radical process.

The second reaction (3) also consists of two component reactions:

I- + HOI + H+ --> I2 + H2O                                                                                         (4)

I2CH2(CO2H)2 --> ICH2(CO2H)2 + H+ + I-                                                                                     (5)

The amber color results from the production of the I2. The I2 forms because of the rapid production of HOI during the radical process. When the radical process is occurring, HOI is created faster than it can be consumed. Some of the HOI is used while excess is reduced by hydrogen peroxide to I-. The increasing I- concentration reaches a point at which the non-radical process takes over. However, the non-radical process does not produce HOI nearly as fast as the radical process, so the amber color begins to clear as I2 is consumed more quickly than it can be created. Eventually the I- concentration drops low enough for the radical process to restart so the cycle can repeat itself.

The deep blue color is the result of the I- and I2 binding to the starch present in the solution.

 

To Conduct Demonstration

Pour 500 mL of solution A and 500 mL of solution B into the beaker on the magnetic stirrer. Adjust the stirring rate to produce a large vortex. Pour 500 mL of solution C into the mixture. The initially colorless mixture will become amber almost immediately, then it will suddenly turn blue-black, fade to colorless and the cycle will repeat with a period of about 15 seconds. After a few minutes the solution will remain blue-black.

 Demo Time: ~5 minutes

Notes:

  1. Chloride ion concentrations in excess of 0.07M suppress the oscillations; therefore, the glassware used must be cleaned well.
  2. If contaminated (even from dust) 30% hydrogen peroxide will decompose and generate large quantities of oxygen gas.

Safety 

Safety:  This demonstration includes chemicals that are irritants and strong oxidizing agents. It also evolves large amounts of iodine in solution, in suspension and also as a vapor above the reaction mixture. The vapor and solid is irritating to eyes, skin, and mucous membrane. Avoid contact between 30% hydrogen peroxide and combustible materials..

Perform in a well ventilated  room, preferable in a hood. Wear goggles.

Reference:

B.Z. Shakhashiri, Chemical Demonstrations:  A Handbook for Teachers of Chemistry, Vol. 2, pp. 248-256, 1983.