Posted on December 18, 2009  Comments (0)

video showing the Briggs-Rauscher Oscillating Reaction. From Wikipedia:

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), 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 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 by iodate and adding hydrogen peroxide. They produced the striking visual demonstration by adding starch indicator.

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 autocatalytic 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 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.)

Related: Non-Newtonian Fluid Demo – Why Does Hair Turn Grey as We Age? – posts with scientific explanations of what we see – Cu2C03(OH)2 –