What Is a Chemical Clock?

Phil Riddel

A chemical clock is a scenario where reacting chemical compounds bring about a sudden, observable event after a time delay that can be set relatively precisely by adjusting the concentrations of the reactants. Often the event is indicated by a change in color, but it may take some other form, such as the production of gas causing effervescence. In some cases, the change is cyclical and involves a solution that periodically switches between two or more states, usually indicated by different colors.

A chemical clock scenario occurs when reacting chemical compounds bring about a sudden, observable event after a time delay that can be set by adjusting concentrations of the reactants.
A chemical clock scenario occurs when reacting chemical compounds bring about a sudden, observable event after a time delay that can be set by adjusting concentrations of the reactants.

One of the simplest chemical clocks is known as the “iodine clock” reaction. Two colorless solutions are mixed and after a pause, the resulting solution abruptly turns dark blue. In the most common version of the experiment, one solution contains a dilute mixture of sulfuric acid and hydrogen peroxide, and the other a mixture of potassium iodide, starch and sodium thiosulfate. On mixing the solutions, elemental iodine is released from the potassium iodide, but a faster reaction between the iodine and the sodium thiosulfate converts it back to colorless iodide ions. When all the thiosulfate has been used up the iodine is able to react with the starch to produce a dark blue compound.

Cyclic, or oscillating, chemical clock reactions are particularly fascinating. Normally, a chemical reaction proceeds in one direction until an equilibrium point is reached. After this, no further change will take place without the intervention of some other factor, such as a change in temperature. Oscillating reactions were initially puzzling as they seemed to defy this rule by spontaneously moving away from equilibrium and returning there repeatedly. In reality, the overall reaction does proceed toward equilibrium and stays there, but in the process, the concentration of one or more reactants or intermediate products varies in a cyclical way.

In an idealized oscillating chemical clock, there is a reaction that creates a product and another reaction that uses this product, with the concentration of the product determining which reaction takes place. When the concentration is low, the first reaction occurs, making more of the product. An increase in the product’s concentration, however, triggers the second reaction, reducing the concentration and prompting the first reaction to take place. This results in a cycle whereby the two competing reactions determine the concentration of a product, which in turn determines which reaction will take place. After a number of cycles, the mixture will reach equilibrium and the reactions will stop.

One of the first cyclic chemical clocks was observed by William C. Bray in 1921. It involved the reaction of hydrogen peroxide and an iodate salt. Investigation by Bray and his student Hermann Liebhafsky showed that the reduction of iodate to iodine, with production of oxygen, and the oxidation of iodine back to iodate took place in a periodic way with cyclic peaks in oxygen production and iodine concentration. This came to be known as the Bray-Liebhafsky reaction.

In the 1950s and 1960s, the biophysicists Boris P. Belousov and, later, Anatol M. Zhabotinsky investigated another cyclic reaction involving the periodic oxidation and reduction of a cerium salt, resulting in oscillating color changes. If the Belousov-Zhabotinsky, or BZ, reaction is performed using a thin layer of the chemical mixture, a remarkable effect is seen, with small local fluctuations in the concentrations of the reactants leading to the emergence of complex patterns of spirals and concentric circles. The chemical processes taking place are very complex, involving as many as 18 distinct reactions.

The science instructors Thomas S. Briggs and Warren C. Rauscsher, using the above reactions as a basis, created an interesting three-color oscillating chemical clock in 1972. The Briggs-Rauscher reaction features a solution that periodically changes from colorless to light brown to dark blue. If set up carefully, there may be 10-15 cycles before it settles into equilibrium in a dark blue color.

An unusual chemical clock that involves changes of shape rather than color is the mercury beating heart reaction. A drop of mercury is added to a solution of potassium dichromate in sulfuric acid, and an iron nail is then placed close to the mercury. A film of mercury I sulfate forms on the drop, reducing the surface tension and causing it to spread out and touch the iron nail. When this happens, electrons from the nail reduce the mercury I sulfate back to mercury, restoring the surface tension and causing the blob to contract again, losing contact with the nail. The process repeats many times, resulting in a cyclic change of shape.

Chemical clock reactions are an area of ongoing research. Cyclic or oscillating reactions in particular are of great interest in the study of chemical kinetics and self-organizing systems. It has been speculated that reactions of this type may have been involved in the origin of life.

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