BZ Reaction in Electric Field Ben Huh Corinne Teeter Biophysics BGGN 266 The behavior of wavefronts in the BZ reaction were investigated under the influence of electric field It has been previously shown that depending on the polarity of the electric field wave fronts within the field are accelerated or slowed The results of this experiment did not exhibit this effect This could be due to experimental difficulties such as differences between experiments or difficulty in analysis due to a lack of steady state wave propagation being reached before too many bubbles took over Introduction The following introduction and basic methodology has been taken from Hill and Morgan s 2003 lab writeup I include it here with permission of the author instead of simply referencing it so that future students who may reference this paper will have all the necessary information in one place INTRODUCTION The Belousov Zhabotinskii BZ reaction is the earliest wellunderstood example of a chemical system that results in oscillations and pattern formation It was discovered in 1951 by the Soviet biophysicist Boris Belousov At the time chemists were skeptical of the possibility of chemical oscillators because of a misconception about the second law of thermodynamics As a result Belousov was unable to get his work published A decade later another Soviet scientist named Anatol Zhabotinskii reproduced Belousov s experiment and successfully persuaded more chemists to accept the idea of chemical oscillators In 1972 three researches at the University of Oregon Field Koros and Noyes published a complete mechanism describing the BZ reaction known as the FKN mechanism However their equations were too complex for numerical analysis by the computers of the time Consequently Field and Noyes distilled their model to a set of ordinary differential equations with only three variables dubbed the Oregonator Belousov s original experiment studied only temporal oscillations in a wellstirred solution however much more interesting is the formation of spatial patterns in an unstirred solution In these cases if the reaction begins at a given point the concentrations of intermediates will propagate outward via diffusion initiating the reaction in the adjacent regions This is known as a trigger wave Periodically the reaction will reinitiate at the nucleation point resulting in successive bands in a onedimensional test tube reaction or concentric rings in a two dimensional Petri dish reaction Systems of chemical oscillators are of great interest in biological systems For example the sinoatrial node the heart s pacemaker causes trigger waves which though electrical in nature propagate in much the same way as BZ trigger waves The generation of cyclic adenosine monophosphate in aggregating Dictyostelium discoideum slime mold colonies creates spiral patterns nearly identical to those of the BZ reaction RNA has been found to to self replicate with fronts of increased RNA concentration moving outward via diffusion All examples Epstein and Pojman THEORY As stated before early skeptics of the BZ reaction held that chemical oscillators would violate the 2nd law of thermodynamics They believed the second law meant that all chemical concentrations in a reaction must move directly towards equilibrium Many compared the BZ reaction to a damped pendulum which passes through its equilibrium position during each oscillation and eventually comes to rest A chemical system that did this would indeed violate the second law of thermodynamics Passing through the equilibrium point and then moving away from it would require an increase in Gibbs free energy which must always decrease However the BZ reaction like all other chemical oscillators does not reach its equilibrium point until after the oscillations are finished oscillatory behavior is a process involving only intermediates The BZ reaction is actually a system of several chemical reactions with dozens of elementary steps but the overall process is the oxidation of malonic acid by bromate producing carbon dioxide As the system slowly progresses towards equilibrium the concentrations of several intermediate species oscillate while the concentrations of products move monotonically towards equilibrium The following is a quick summary of the development of oscillatory behavior Normally bromide reduces the bromate This reaction is fast quickly using up the available bromide Once bromide drops below a critical level bromous acid takes over the reduction of bromate in a reaction that autocatalytically produces more bromous acid This leads to exponential growth in HBrO2 This is eventually checked by a reaction that converts HBrO2 to HOBr and bromate Meanwhile the decomposition of malonic acid results in the reduction of bromine to bromide nearly restoring the initial concentration and allowing the whole process to begin again Much of our understanding of the BZ reaction stems from the FKN mechanism Below is an overview of the FKN mechanism For a more thorough treatment see Tyson Important Points When Br is high 1 dominates the reduction of bromate When Br is low 5 dominates the reduction of bromate 5 includes the autocatalytic production of HBrO2 when 5 is dominant HBrO2 increases exponentially until limited by 6 7 8 and 9 are the final processes oxidizing malonic and bromomalonic acid The rates of 7 8 and 9 are much slower than those of the other reactions Double and triple brominated malonic acid also occur but in much smaller concentrations than BrMA so the FKN mechanism neglects these They would be oxidized in a manner similar to BrMA Oscillations occur in the following manner Process 1 lowers the bromide concentration and increases bromine allowing for the bromination of malonic acid by 3 When Br has been significantly lowered 5 causes an exponential increase in bromous acid and the oxidized form of the metal ion catalyst and indicator cerium Bromous acid is subsequently converted to bromate and HOBr Meanwhile the ratelimiting steps 7 8 and 9 reduce the cerium to Ce 3 and simultaneously increase bromide concentration Once the bromide concentration is high enough it reacts with bromate and HOBr in 1 and 2 to form bromine and the process begins again The different absorption spectra of the two ionization states of cerium causes the solution to change from yellow to clear and back as their relative concentrations change allowing the oscillations to be observed visually Two dimensional spatial patterns can