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Briggs–Rauscher oscillating reaction

This is one of a small number of known oscillating chemical reactions.

Well suited for demonstration purposes because of its visually striking colour changes: the freshly prepared colourless solution slowly turns an amber colour, suddenly changing to a very dark blue.

This slowly fades to colourless and the process repeats, about ten times in the most popular formulation, before ending as a dark blue liquid smelling strongly of iodine.

History

The first known homogeneous oscillating chemical reaction, reported by W. C. Bray in 1921, was between hydrogen peroxide (H2O2) and iodate (IO3−) in acidic solution. Due to experimental difficulty, it attracted little attention and was unsuitable as a demonstration.

In 1958 B. P. Belousov in the Soviet Union discovered the Belousov–Zhabotinsky reaction, but it met with skepticism, because such oscillatory behaviour was unheard of up to that time. It was verified by A. M. Zhabotinsky, also in the USSR, who in 1964 published his research.

In May 1972 a pair of articles in the Journal of Chemical Education brought it to the attention of science instructors at Galileo High School in San Francisco. They discovered the Briggs–Rauscher oscillating reaction by replacing bromate (BrO3−) in the BZ reaction with iodate and adding hydrogen peroxide.

They produced the striking visual demonstration by adding starch indicator. Since then, many other investigators have added to the knowledge and uses of this very unusual reaction.

Initial conditions

The initial aqueous solution contains:
* hydrogen peroxide, an iodate,
* divalent manganese (Mn2+) as catalyst,
* strong chemically unreactive acid (sulphuric acid (H2SO4) or perchloric acid (HClO4) are good),
* organic compound with an active (“enolic”) hydrogen atom attached to carbon.
-> This will slowly reduce free iodine (I2) to iodide (I−).

Starch is optionally added as an indicator to show the abrupt increase in iodide ion concentration as a sudden change from amber (free iodine) to dark blue (the “iodine-starch complex”, which requires both iodine and iodide.)

The reaction is “poisoned” by chloride (Cl−) ion, which must therefore be avoided. The reaction will oscillate under a fairly wide range of initial concentrations.

Behaviour in time

The reaction shows recurring periodic changes, both gradual and sudden, which are visible: slow changes in the intensity of colour, interrupted by abrupt changes in hue.

This demonstrates that a complex combination of slow and fast reactions are taking place simultaneously. For example, following the iodide ion concentration with a silver/silver iodide electrode shows sudden dramatic swings of several orders of magnitude separated by slower variations.

Oscillations persist over a wide range of temperatures. Higher temperatures make everything happen faster, with some qualitative change observable (see Effect of temperature). Stirring the solution throughout the reaction is helpful for sharp colour changes, otherwise spatial variations may develop (see Videos). Bubbles of free oxygen are evolved throughout, and in most cases, the final state is rich in free iodine

 

Chemical mechanism

Wikipedia description: There are two key processes

A (“non-radical process”): The slow consumption of free iodine by the malonic acid substrate in the presence of iodate. This process involves the intermediate production of iodide ion.

B (“radical process”): A fast auto-catalytic process involving manganese and free radical intermediates, which converts hydrogen peroxide and iodate to free iodine and oxygen.
This process also can consume iodide up to a limiting rate.

Process B can operate only at low concentrations of iodide, creating a feedback loop as follows:

Initially, iodide is low and process B generates free iodine, which gradually accumulates.

Meanwhile process A slowly generates the intermediate iodide ion out of the free iodine at an increasing rate proportional to its (i.e. I2) concentration.

At a certain point, this overwhelms process B, stopping the production of more free iodine, which is still being consumed by process A.

Thus, eventually the concentration of free iodine (and thus iodide) falls low enough for process B to start up again.
The cycle repeats as long as the original reactants hold out.

The overall result of both processes is (approximately):

IO3 + 2H2O2 + CH2(COOH)2 + H+ → ICH(COOH)2 + 2O2 + 3H2O

The colour changes seen during the reaction correspond to the actions of the two processes:
the slowly increasing amber colour is due to the production of free iodine by process B.

When process B stops, the resulting increase in iodide ion enables the sudden blue starch colour.

But since process A is still acting, this slowly fades back to clear.

The eventual resumption of process B is invisible, but can be revealed by the use of a suitable electrode

A negative feedback loop which includes a delay (mediated here by process A) is a general mechanism for producing oscillations in many physical systems.

This kind of oscillation is very rare in non-biological homogeneous chemical systems.

Explanation from chemed.chem.purdue.edu/demos

The reaction is believed to be an indirect route to decomposing H2O2 into H2O and O2, related to the H2O2/IO3- oscillating reaction.

If so, the amber color results from the production of I2 in the absence of I -, while the blue color results from the simultaneous presence of both I2 and I – leading to the blue starch-I3- complex.

The bubbles are oxygen gas. The following steps have been proposed:

5 H2O2 (aq) + I2 (aq) –> 2 HIO3 (aq) + 4 H2O (l)

5 H2O2 (aq) + 2 HIO3 (aq) –> 5 O2 (g) + I2 (aq) + 6 H2O (l)

The functions of the malonic acid and the manganese ion may be to increase the size and frequency of the I2 and I – fluctuations.

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Most of this text has been adapted from http://en.wikipedia.org/wiki/Briggs%E2%80%93Rauscher_reaction
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Step-by-step

The iodate ion is changed into iodine by hydrogen peroxide.
The color changes to amber:

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