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The Briggs-Rauscher oscillating reaction is one of a small number of known oscillating chemical reactions. It is especially well suited for demonstration purposes because of its visually striking color changes: the freshly prepared colorless solution slowly turns an amber color, suddenly changing to a very dark blue. This slowly fades to clear and the process repeats, about ten times in the most popular formulation, before ending as a dark blue liquid smelling strongly of iodine.
Additional recommended knowledge
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 (BZ reaction), which is suitable as a demonstration, but it too met with skepticism (largely because such oscillatory behavior was unheard of up to that time) until A. M. Zhabotinsky, also in the USSR, learned of it and in 1964 published his research. In May of 1972 a pair of articles in the Journal of Chemical Educationbrought it to the attention of two science instructors at Galileo High School in San Francisco. They discovered the Briggs-Rauscher oscillating reaction by replacing bromate (BrO3-) in the BZ reaction by 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.
The initial aqueous solution contains hydrogen peroxide, an iodate, divalent manganese (Mn++) as catalyst, a strong chemically unreactive acid (sulfuric acid (H2SO4) or perchloric acid (HClO4) are good), and an organic compound with an active ("enolic") hydrogen atom attached to carbon which will slowly reduce free iodine (I2) to iodide (I-). (Malonic acid (CH2(COOH)2) is excellent for that purpose.) 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.)
NOTES: 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. For recipes suitable for demonstration purposes, see Shakhashiri or #Preparations.
Behavior in time
The reaction shows recurring periodic changes, both gradual and sudden, which are visible to the eye: slow changes in the intensity of color, 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. This is shown by the oscillogram above.
Stirring the solution throughout the reaction is helpful for sharp color 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.
Changing the initial concentrations
As noted above, the reaction will oscillate in a fairly wide range of initial concentrations of the reactants. For oscillometric demonstrations, more cycles are obtained in dilute solutions, which produce weaker color changes. See for example the graph, which shows more than 40 cycles in 8 minutes.
Changing the organic substrate
Malonic acid has been replaced by other suitable organic molecules, such as acetone (CH3COCH3) or 2,4 Pentanedione (CH3COCH2COCH3) ("acetylacetone"). More exotic substrates have been used. The resulting oscillographic records often show distinctive features, for example as reported by Szalai.(http://www.chem.elte.hu/departments/anal/szalai/chd.html)
Continuous flow reactors
The reaction may be made to oscillate indefinitely by using a continuous flow stirred tank reactor (CSTR), in which the starting reagents are continuously introduced and excess fluid is drawn.
Two dimensional phase space plots
By omitting the starch and monitoring the concentration of I2 photometrically, (i.e. measuring the absorption of a suitable light beam through the solution) while simultaneously monitoring the concentration of iodide ion with an iodide-selective electrode, a distorted spiral XY-plot will result. In a continuous-flow reactor, this becomes a closed loop (limit-cycle).
By replacing the starch with a fluorescent dye, Weinberg (2007) produced a demonstration visible in total darkness under UV illumination.
Use as a biological assay
The reaction has been proposed as an assay procedure for antioxidants in foodstuffs. The sample to be tested is added at the onset of oscillations, stopping the action for a period proportional to its antioxidant activity. Compared to existing assay methods, this procedure is quick and easy and operates at the pH of the human stomach.
The detailed mechanism of this reaction is quite complex.. Nevertheless, a good general explanation can be given.
The essential features of the system depend on two key processes (These processes each involve many reactions working together):
But 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 and the cycle repeats as long as the original reactants hold out.
The overall result of both processes is (again, approximately):
The color changes seen during the reaction correspond to the actions of the two processes: the slowly increasing amber color 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 color. 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, but is very rare in nonbiological homogeneous chemical systems (The BZ oscillating reaction has a somewhat similar feedback loop).
Categories: Chemical reactions | Non-equilibrium thermodynamics
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Briggs-Rauscher_reaction". A list of authors is available in Wikipedia.|