Briggs-Rauscher Reaction - Oscillating Clock

The Briggs-Rauscher reaction is an oscillating clock chemical reaction that cycles from clear, amber, to deep blue several times before finally ending as a blue-black mixture. It is one of the most popular chemistry demonstrations because it’s easy and reliable.

How to Do the Briggs-Rauscher Reaction

Doing the Briggs-Rauscher reaction is simple. You prepare three solutions and then mix them to initiate the reaction.

Note: For best results, prepare the solutions a short while before the reaction (or at least store them away from light). Ultraviolet radiation from sunlight or fluorescent lights decomposes some reactants or causes them to react with one another, reducing the effect.

Materials

You need the following chemicals and materials:

43 g Potassium iodate (KIO₃)
Sulfuric acid (H₂SO₄)
15.6 g malonic acid (HOOCCH₂COOH)
3.4 g manganese sulfate monohydrate (MnSO₄ . H₂O)
4 g vitex starch (modified starch)
30% hydrogen peroxide (H₂O₂)
Distilled water
1-L beaker
Flasks or beakers for mixing
Stirring plate
Magnetic stir bar

Prepare the Solutions

Make the following three solutions:

Solution A: Place 43 g potassium iodate in about 800 ml distilled water. Add 4.5 ml sulfuric acid and stir the solution until the potassium iodate dissolves. Dilute to 1 L.
Solution B: Stir 15.6 g malonic acid and 3.4 g manganese sulfate monohydrate into about 800 ml of distilled water. Add 4 g vitex starch. Stir until the solids dissolve. Dilute to 1 L.
Solution C: Dilute 400 ml of 30% hydrogen peroxide to 1 L.

The concentration of the solutions is not critical. However, changing concentration affects the intensity of the colors and the rate of the “clock”.

How to Perform the Briggs-Rauscher Reaction

Place a stirring bar in a 1-L beaker and set the beaker on a stirring plate.
Pour 300 ml each of solutions A and B into the beaker.
Turn on the stirring plate and adjust its speed so a large vortex forms.
Add 300 ml of solution C.

Note: Be sure solutions A and B are fully mixed before adding solution C or else the reaction will not work as well.

Safety and Disposal

The chemicals include irritants and oxidizing agents. Do not touch the chemicals or solutions.
Wear gloves and goggles. Ideally perform the reaction under a fume hood. Otherwise, do the reaction in a well-ventilated space.
For disposal, add about 10 g sodium thiosulfate to neutralize the iodine to iodide. Stir until the mixture becomes colorless. This reaction is exothermic, so expect it to get hot. Once cool, wash the neutralized mixture down the drain with water.

How the Briggs-Rauscher Reaction Works

While the specific details of the reaction are somewhat complicated, the overall reaction is as follows:

IO₃^– + 2 H₂O₂ + CH₂(CO₂H)₂ + H⁺ → ICH(CO₂H)₂ + 2 O₂ + 3 H₂O

Two processes occur in this reaction, with each process containing many individual reactions.

A non-radical process involves slow consumption of free iodine by malonic acid. This process occurs in the presence of iodate.
A radical process involves manganese and free radicals, auto-catalytically converting iodate and hydrogen peroxide into iodine and oxygen.

The cycling that makes it a clock reaction works like this:

The radical process only occurs when iodide concentration is low. Initially, iodide is low, so the radical process forms free iodine. At the same time, the non-radical process generates iodide. When the non-radical process makes enough iodide, it inhibits the radical process. The radical process continues consuming iodine and iodide until the concentration falls low enough for the radical process to start again. The cycling continues until the reactants are consumed.

The increasing amber color comes from production of free iodine by the radical process. When the radical process stops, the increase in iodide ion forms a blue complex with starch. Since the non-radical process continues consuming iodine, eventually the blue color fades and the solution turns clear.

Interesting Variations

Using diluted solutions results in more cycles and faster cycles. However, the colors are weaker.
Replacing malonic acid with other substrates, such as acetylacetone (CH₃COCH₃COCH₃) or acetone (CH₃COCH₃) changes the oscillation pattern of the clock reaction.
Normally, the Briggs-Rauscher reaction eventually stops. However, an ongoing cycle results from using a continuous flow stirred tank reactor (CSTR) that adds reagents while removing excess liquid.
Replacing starch with a fluorescent dye produces a reaction that glows under ultraviolet (black) light.

Briggs-Rauscher Reaction FAQs

How long does the Briggs-Rauscher reaction last?

The number of cycles depends on temperature, whether or not you stir the solution, concentration, and light exposure. Typically, the reaction cycles between 10 and 20 times. Diluting the reactants makes the reaction cycle more rapidly, potentially up to 40 times.

Can you do the Briggs-Rauscher reaction at home?

You can do the Briggs-Rauscher reaction at home, but you’ll need to order the chemicals online. It’s safe to wash the waste down the drain after it is neutralized, but keep in mind the reactants are irritants. Unless you are specifically exploring clock reactions, it’s easier and more economical to do the “water into wine” or “blue bottle” reaction.

More Color Change Chemical Reactions

The Briggs-Rauscher reaction is the most common clock reaction. However, there are several other interesting color change chemical reactions.

Christmas chemistry reaction: This is a red to green clock reaction, perfect for the winter holidays.
Water into wine or water into blood reaction: This is a clear to red color change reaction.
Gatorade and drain cleaner reaction: This color change reaction uses everyday materials, making it suitable for home school experiments.
Blue bottle reaction: This redox reaction changes from blue to clear. While not a clock reaction, it is repeatable.
Hot and cold reaction: A solution changes colors from pink to clear and back again.
Vanishing Valentine: A solution changes from clear to pink and returns to its colorless state.

References

Belousov, B. P. (1958). “A Periodic Reaction and Its Mechanism”. Sbornik Referatov po Radiatsionni Meditsine. Medgiz, Moscow.
Bray, W. C. (1921). “A Periodic Reaction in Homogeneous Solution and Its Relation to Catalysis”. J. Am. Chem. Soc. 43 (6): 1262–1267. doi:10.1021/ja01439a007
Field, R. J. (1972). “A Reaction Periodic in Time and Space”. J. Chem. Educ. 49: 308. doi:10.1021/ed049p308
Furrow, Stanley D.; Cervellati, Rinaldo; Amadori, Giovanna (2002). “New Substrates for the Oscillating Briggs–Rauscher Reaction”. J. Phys. Chem. A. 106 (24): 5841–5850. doi:10.1021/jp0138779
Shakhashiri, B. Z. (1992) Chemical Demonstrations: A Handbook for Teachers of Chemistry Vol. II. University of Wisconsin Press, Madison, WI.

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