Briggs-Rauscher reaction
Encyclopedia
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, 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
.
(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 behaviour 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 1972 a pair of articles in the Journal of Chemical Education brought 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 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.
(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.)
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 in the external links.
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.
(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
).
of the human stomach. For a detailed description suitable for high school chemistry, see Preparations.
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 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, but is very rare in nonbiological homogeneous chemical systems. (The BZ oscillating reaction has a somewhat similar feedback loop.)
Chemical clock
A chemical clock is a complex mixture of reacting chemical compounds in which the concentration of one or more components exhibits periodic changes....
. 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, 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
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
.
History
The first known homogeneous oscillating chemical reaction, reported by W. C. Bray in 1921, was between hydrogen peroxideHydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
(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 behaviour 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 1972 a pair of articles in the Journal of Chemical Education brought 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 with iodate and adding hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
. They produced the striking visual demonstration by adding starch indicator
Starch indicator
Starch is often used in chemistry as an indicator for redox titrations where triiodide is present. Starch forms a very dark blue-black complex with triiodide which can be made by mixing iodine with iodide . However, the complex is not formed if only iodine or only iodide is present...
. 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 manganeseManganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
(Mn2+) as catalyst
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
, a strong chemically unreactive acid (sulphuric acid (H2SO4) or perchloric acid
Perchloric acid
Perchloric acid is the inorganic compound with the formula HClO4. Usually encountered as an aqueous solution, this colourless compound is a strong acid comparable in strength to sulfuric and nitric acids. It is a powerful oxidizer, but its aqueous solutions up to appr. 70% are remarkably inert,...
(HClO4) are good), and an organic compound with an active ("enolic")
Keto-enol tautomerism
In organic chemistry, keto-enol tautomerism refers to a chemical equilibrium between a keto form and an enol . The enol and keto forms are said to be tautomers of each other...
hydrogen atom attached to carbon which will slowly reduce
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
free iodine (I2) to iodide
Iodide
An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...
(I−). (Malonic acid
Malonic acid
Malonic acid is a dicarboxylic acid with structure CH22. The ionised form of malonic acid, as well as its esters and salts, are known as malonates. For example, diethyl malonate is malonic acid's ethyl ester...
(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"
Starch indicator
Starch is often used in chemistry as an indicator for redox titrations where triiodide is present. Starch forms a very dark blue-black complex with triiodide which can be made by mixing iodine with iodide . However, the complex is not formed if only iodine or only iodide is present...
, 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. For recipes suitable for demonstration purposes, see Shakhashiri or Preparations in the external links.
Behaviour in time
The reaction shows recurring periodic changes, both gradual and sudden, which are visible to the eye: 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 (see Videos) shows sudden dramatic swings of several orders of magnitude separated by slower variations. This is shown by the oscillogram above.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.
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 colour 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 acetoneAcetone
Acetone is the organic compound with the formula 2CO, a colorless, mobile, flammable liquid, the simplest example of the ketones.Acetone is miscible with water and serves as an important solvent in its own right, typically as the solvent of choice for cleaning purposes in the laboratory...
(CH3COCH3) or 2,4 Pentanedione (CH3COCH2COCH3) ("acetylacetone
Acetylacetone
Acetylacetone is an organic compound that famously exists in two tautomeric forms that rapidly interconvert. The less stable tautomer is a diketone formally named pentane-2,4-dione. The more common tautomer is the enol form. The pair of tautomers rapidly interconvert and are treated as a single...
"). 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-cycleLimit-cycle
In mathematics, in the area of dynamical systems, a limit-cycle on a plane or a two-dimensional manifold is a closed trajectory in phase space having the property that at least one other trajectory spirals into it either as time approaches infinity or as time approaches negative infinity. Such...
).
Fluorescent Demonstration
By replacing the starch with a fluorescent dye, Weinberg and Muyskens (2007) produced a demonstration visible in 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 pHPH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
of the human stomach. For a detailed description suitable for high school chemistry, see Preparations.
Chemical mechanism
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):
- 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-catalyticAutocatalysisA single chemical reaction is said to have undergone autocatalysis, or be autocatalytic, if the reaction product itself is the catalyst for that reaction....
process involving manganese and free radicalRadical (chemistry)Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
intermediates, which converts hydrogen peroxide and iodate to free iodine and oxygen. This process also can consume iodide up to a limiting rate.
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):
- 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, but is very rare in nonbiological homogeneous chemical systems. (The BZ oscillating reaction has a somewhat similar feedback loop.)
External links
Videos
- Continuously stirred demo showing rapid and uniform colour changes
- Continuously stirred demo showing 16 colourful oscillations gradually increasing in intensity
- Unstirred demo showing minor spatial variations
- Unstirred demo showing extreme spatial variations
- This demo runs to completion in 19 cycles. Here the blue starch complex appears late, so the variations in free iodine are plainly visible
- This demo completes in 13 cycles. An iodide-selective electrode is used to produce a graph of I− in real time
Effect of Temperature
- This series of four videos vividly shows the effect of temperature on the oscillations: 10°C 22°C 40°C 60°C