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Oscillating chemical reaction

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 colour changes: the freshly prepared colourless solution slowly turns an amber colour, then suddenly changes 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.

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. Because of experimental difficulty, it attracted little attention and was unsuitable as a demonstration. In 1958 Boris Pavlovich Belousov discovered the Belousov–Zhabotinsky reaction (BZ reaction). The BZ reaction is suitable as a demonstration, but it too met with skepticism, largely because such oscillatory behaviour was unheard of up to that time, until Anatol Zhabotinsky learned of it and in 1964 published his research. In May 1972 a pair of articles in the Journal of Chemical Education brought it to the attention of Thomas Briggs and Warren Rauscher, 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 with iodate and adding hydrogen peroxide. They produced the strikingly colorful 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 (Mn2+) as catalyst, a strong chemically unreactive acid (sulphuric 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.) Recently it has been shown, however, that the starch is not only an indicator for iodine in the reaction. In the presence of starch the number of oscillations is higher and the period times are longer compared to the starch free mixtures. It was also found that the iodine consumption segment within one period of oscillation is also significantly longer in the starch containing mixtures. This suggests that the starch probably acts as a reservoir for the iodine and iodide because of the starch triiodide equilibrium, thereby modifying the kinetics of the steps in which iodine and iodide are involved. The reaction is "poisoned" by chloride (Cl−) ion, which must therefore be avoided, and will oscillate under a fairly wide range of initial concentrations. For recipes suitable for demonstration purposes, see Shakhashiri or Preparations in the external links.

The residual mixture contains iodinated malonic acid, inorganic acid, manganous catalysts, unreacted iodate and hydrogen peroxide. After the oscillations cease, the iodomalonic acid decomposes and iodine is produced. The rate of decomposition depends on the conditions. All of the components present in the residual mixture are of environmental concern: Iodate, iodine and hydrogen peroxide are strong oxidants, the acid is corrosive and manganese has been suggested to cause neurological disorders. A simple method has been developed employing thiosulfate and carbonate – two inexpensive salts – to remove all oxidants, neutralize the acidity and recover the manganous ion in the form of manganese dioxide. such as acetone (CH3COCH3) or acetylacetone (CH3COCH2COCH3, pentane 2,4 dione). More exotic substrates have been used. The resulting oscillographic records often show distinctive features, for example as reported by Szalai.

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.

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 and Muyskens (2007) produced a demonstration visible in darkness under UV illumination.

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. For a detailed description suitable for high school chemistry, see Preparations. In contrast to the findings referring predominantly to polyphenolic compounds reported in the above cited literature, it was found that the salicylic acid – a simple monophenolic compound – did not stop the oscillations immediately after it was added into the active Briggs Rauscher mixture. In the low concentration interval the salicyclic acid only damped the oscillations, while in higher concentrations the damping effect was much stronger and complete inhibition was also observed. Sulfosalicylic acid, a derivative of salicyclic acid, practically did not affect the oscillations.