A chemical clock is a complex mixture of reacting chemical compounds in which the concentration of one or more components exhibits periodic changes, or where sudden property changes occur after a predictable induction time. They are a class of reactions that serve as an example of non-equilibrium thermodynamics, resulting in the establishment of a nonlinear oscillator. The reactions are theoretically important in that they show that chemical reactions do not have to be dominated by equilibrium thermodynamic behavior.
In cases where one of the reagents has a visible color, crossing a concentration threshold can lead to an abrupt color change in a reproducible time lapse. Examples of clock reactions are the Belousov-Zhabotinsky reaction, the Briggs-Rauscher reaction, the Bray-Liebhafsky reaction and the iodine clock reaction. The concentration of products and reactants of oscillatory chemical systems can be approximated in terms of damped oscillations.
The earliest scientific evidence that such reactions can oscillate was met with extreme scepticism. In 1828, G.T. Fechner published a report of oscillations in a chemical system. He described an electrochemical cell that produced an oscillating current. In 1899, W. Ostwald observed that the rate of chromium dissolution in acid periodically increased and decreased. Both of these systems were heterogeneous and it was believed then, and through much of the last century, that homogeneous oscillating systems were nonexistent. While theoretical discussions date back to around 1910, the systematic study of oscillating chemical reactions and of the broader field of non-linear chemical dynamics did not become well established until the mid-1970s.
Theoretical models of oscillating reactions have been studied by chemists, physicists, and mathematicians. In an oscillating system the energy-releasing reaction can follow at least two different pathways, and the reaction periodically switches from one pathway to another. One of these pathways produces a specific intermediate, while another pathway consumes it. The concentration of this intermediate triggers the switching of pathways. When the concentration of the intermediate is low, the reaction follows the producing pathway, leading then to a relatively high concentration of intermediate. When the concentration of the intermediate is high, the reaction switches to the consuming pathway.
Different theoretical models for this type of reaction have been created, including the Lotka-Volterra model, the Brusselator and the Oregonator. The latter was designed to simulate the Belousov-Zabotinskii reaction.
BZ reaction 
In a Belousov–Zhabotinsky reaction, the only common element in these oscillating systems is the inclusion of bromine and an acid. 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.
Boris Belousov first noted, sometime in the 1950s, that in a mix of potassium bromate, cerium(IV) sulfate, propanedioic acid and citric acid in dilute sulfuric acid, the ratio of concentration of the cerium(IV) and cerium(III) ions oscillated, causing the colour of the solution to oscillate between a yellow solution and a colorless solution. This is due to the cerium(IV) ions being reduced by propanedioic acid to cerium(III) ions, which are then oxidized back to cerium(IV) ions by bromate(V) ions.
Briggs–Rauscher 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 color changes: the freshly prepared colorless solution slowly turns an amber color, suddenly changing to a very dark blue. This slowly fades to colorless and the process repeats, about ten times in the most popular formulation.
Bray–Liebhafsky reaction 
- 5 H2O2 + I2 → 2 IO3- + 2 H+ + 4 H2O
and the reduction of iodate back to iodine:
- 5 H2O2 + 2 IO3- + 2 H+ → I2 + 5 O2 + 6 H2O
See also 
There are other classes of chemical oscillators:
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- Bray, William C. (1921). Journal of the American Chemical Society 43 (6): 1262. doi:10.1021/ja01439a007.