Briggs Rauscher Reaction-JAIPRAKSH

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.



briggs reaction

 

Variants

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Malonic acid has been replaced by other suitable organic molecules, such as  (CH₃COCH₃) or 2,4 Pentanedione (CH₃COCH₂COCH₃) ("acetylacetone").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.

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).

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 pH 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-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.

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.)


by Jaiprakash Srivastav