Briggs-Rauscher Reaction

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In this reaction three colorless solutions are mixed and stirred on a magnetic stirrer. The solution turns yellow, blue-black and clear again. The solutions will oscillate berween these colors for several minutes. Also, the electric potential oscillates with the color.

Solution A:

4.0 M H₂O₂ (410 ml of 30% hydrogen peroxide diluted to l.0 liter with distilled water)

Solution B:

0.20 M in KIO₃ and 0.077 M in H₂SO₄ (Place 43 g of potassium iodate in about 800 ml of distilled water. Add 4.3 ml of concentrated H₂SO₄. Warm and stir the mixture until the potassium iodate dissolves. Dilute the solution to 1.0 liter with distilled water.)

Solution C:

Starch solution that is 0.15 M in maloric acid and 0.02 M in MnSO₄ (Dissolve 16 g of malonic acid and 3.4 g of manganese (II) sulfate monohydrate in about 500 ml of distilled water. Mix 0.30 g of soluble starch with about 5 ml of distilled water and stir the mixture to form a slurry. Pour the slurry into 50 ml of boiling distilled water and continue heating and stirring the mixture until the starch has dissolved. Pour this starch solution into the solution of malonic acid and manganese (II) sulfate. Dilute the mixture to 1.0 liter with distilled water.)

Procedure:

Mix 150 ml of each solution in a beaker and stir on a magnetic stirrer. Observe the color changes. The initially colorless solution will become amber almost immediately. Then it will suddenly turn blue-black. The blue-black will fade to colorless. Also, you may want to record the potential of the solution as the oscillating reaction proceeds.

Explanation:

This oscillating reaction was developed by Thomas S. Briggs and Warren C. Rauscher of Galileo High School in San Francisco. In their reaction the evolution oxygen and carbon dioxide gases and the concentrations of iodine and iodide ions oscillate. Iodine is produced rapidly when the concentration of iodide ions is low. .As the concentration of iodine in the solution increases, the amber color of the solution intensifies. The production of I increases as the concentration of I₂ increases. and these ions react with iodine molecules and starch to form a blue-black complex containing the pentaiodide ion (I₅).

Upon initial mixing of the solutions IO₃ reacts with H₂O₂ to produce a little HIO₂. The HIO₂ reacts with IO₃ and HOI is formed. The HOI is reduced to I in a reaction with H₂O₂. The large amount of HOI reacts with I producing I₂. The I₂ reacts slowly with malonic acid but the concentrations of HOI, I₂, and I all increase. The amber color is present when the HOI concentration is greater than the I concentration. The dark blue color signifies that the concentration is greater than the HOI concentration. The reaction mechanism is very complex with about l2 steps. The oscillating sequence repeats until the malonic acid or IO₃ is depleted.

Safety Precautions:

1. Hydrogen peroxide is an oxidizer and a skin and eye irritant.

2. Sulfulric acid is a strong acid and is corrosive to eyes, skin and other tissue.

3. Malonic acid is a strong irritant to skin, eyes, and mucous membranes.

4. The reaction produces iodine in solution in suspension. and also as a vapor above the reaction mixture. The vapor or the solid is very irritating to the eyes. skin and mucous membranes.

5. Wear chemical-resistant gloves and goggles.

Tips:

1. Use only distilled or deionized water. Make sure all glassware is clean and rinsed in distilled or deionized water.

2. The solutions can be mixed in any order as long as they are combined quickly

Disposal:

The reaction produces elemental iodine, I₂, which should be reduced to iodide ions before disposal. Therefore, under a hood add sodium thiosulfate crystals until the mixture becomes colorless. This will be an exothermic reaction. When it cools, it may be flushed down the drain with water.