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A Belousov-Zhabotinsky reaction, or BZ reaction, is one of a class of reactions that serve as a classical example of non-equilibrium thermodynamics, resulting in the establishment of a nonlinear chemical oscillator. The only common element in these oscillating systems are the inclusion of bromine and an acid. The reactions are theoretically important in that they show that chemical reactions do not have to be dominated by equilibrium thermodynamic behavior. These reactions are far from equilibrium and remain so for a significant length of time. In this sense, they provide an interesting chemical model of nonequilibrium biological phenomena, and the mathematical model of the BZ reactions themselves are of theoretical interest.

An essential aspect of the BZ reaction is its so called "excitability"- under the influence of stimuli, patterns develop in what would otherwise be a perfectly quiescent medium. Some clock reactions such as Briggs-Rauscher and BZ using the chemical ruthenium bipyridyl as catalyst can be excited into self-organising activity through the influence of light.

The discovery of the phenomenon is credited to Boris Belousov. He noted, sometime in the 1950s (the dates change depending on source, but it ranges from 1951 to 1958), that in a mix of potassium bromate, cerium(IV) sulfate, and citric acid in dilute sulfuric acid, the ratio of concentration of the cerium(IV) and cerium(III) ions oscillated. Belousov made two attempts to publish his finding, but was rejected on the grounds that he could not explain his results to the satisfaction of the editors of the journals to which he submitted his results.

Later, in 1961, a graduate student named A. M. Zhabotinsky rediscovered this reaction sequence; however, the results of these men's work were still not widely disseminated, and was not known in the West until a conference in Prague in 1968.

There are a number of BZ cocktails available in the chemical literature and on the web. A complex of phenanthroline and iron is a common indicator. These reactions, if carried out in petri dishes, result in the formation first of colored spots. These spots grow into a series of expanding concentric rings or perhaps expanding spirals. The colors disappear if the dishes are shaken, and then reappear. The waves continue until the reagents are consumed. The reaction can also be performed in a beaker using a magnetic stirrer.

Andrew Adamatzky, a computer scientist in the University of the West of England seems to have worked out how to make liquid logic gates using the BZ reaction.^[1]

• Excitable medium

1. ^ Motoike I., Adamatzky A. Three-valued logic gates in reaction-diffusion excitable media. Chaos, Solitons & Fractals 24 (2005) 107-114. [1]

• Pabian and Zarins, "Banded Agates; Origins and inclusions", University of Nebraska-Lincoln, Educational circular #12.

• The Belousov-Zhabotinski Reaction
• Gallery of BZ demonstrations
• The Belousov-Zhabotinsky Reaction
• The Phenomenology of the Belousov-Zhabotinsky Reacton, with pictures
• Belousow Zhabotinsky Reaction, with video
• The Belousov-Zhabotinski Reaction   PDF file