We’ll take a brief break from chemical safety today in order to participate in the “favorite reaction” blog carnival.
I first recall learning about the Belousov–Zhabotinsky reaction several years ago when I was working on a story about photos of chemical reactions on display at the University of Wisconsin, Madison. It’s quite possible that I’d learned and forgotten about the BZ reaction before, but this time it stuck. We tend to think of reactions as proceeding to completion or reaching some sort of equilibrium, but they can also oscillate: In simplest form, the products of one reaction become the reactants of another that regenerates the original reactants.
The actual mechanisms of oscillating reactions can be quite complex, however. The classic BZ reaction involves potassium bromate, cerium(IV) sulfate, and propanedioic acid (aka malonic acid) in dilute sulfuric acid. Ten equations (shown) make up the overall reaction. The color changes are due to the oscillating oxidation state of cerium–Ce(IV) is yellow and Ce(III) is colorless. If ferroin is used in place of cerium, the color switches between blue and red. According to a Journal of Chemical Education article by Purdue University chemistry professor Arthur T. Winfree, Russian chemist Boris P. Belousov (1893-1970) first observed his namesake reaction when working in the Laboratory of Biophysics at the USSR Ministry of Health, but he couldn’t get it published. Moscow State University graduate student Anatol Zhabotinsky (1938-2008) followed up on and extended the work. Richard J. Field and Richard M. Noyes of the University of Oregon, along with Endre Körös of Hungary’s L. Eötvös University, published the mechanism in 1972.
Feedback loops along the lines of what happens in BZ or other oscillating reactions are common in biology. My college p-chem textbook tells me that oscillating reactions are the source of heartbeat rhythms, for example. Others point to the interplay of predator and prey populations. Seeing oscillations happen in real time in a dish or beaker is, for me, a lovely example of the wonder and power of chemistry.