Oscillating reactions, laboratory

Autocatalysis, although not rare, is not common either. A study of this phenomenon is crucial for the treatment of oscillating reactions, which are presented in Section 8.8. If the data give an indication of autocatalytic behavior, one quick laboratory test is to use the leftover solution from a completed reaction as the solvent for the next. If the replicate is faster than the first trial, autocatalysis is suggested. 

About two weeks after having sent the manuscript, I received it back with a nice short letter from Belousov saying that he was glad that somebody continued the work that he was too busy to carry on. He also attached a manuscript of his own in which I found a reference to the only published communication by Belousov on the oscillating reactions. I immediately went to a library and found his communication in a booklet entitled Short Communications on Radiation Medicine, an obscure and very unlikely place for such a paper. The booklet was published by a medical research institute where Belousov was the head of the analytical chemistry laboratory. I added a reference to Belousov s paper to my manuscript and sent it off to the Russian journal Biofizika. The paper appeared two years later. 

For most of the reactions carried out in the laboratory, the concentrations of the participating species vary steadily as time progresses those of reactants decrease, those of products increase and those of intermediates rise initially and then fall again. But this need not necessarily be so. For certain reactions under limited conditions, the concentrations of some of the species in the reaction mixture are found to oscillate, rising and falling repeatedly. Such reactions are called oscillating reactions. 

One of the great appeals of nonlinear chemical dynamics lies in the striking visual demonstrations that can be created. It is a rare person who is not impressed the first time he or she sees a clear solution repeatedly turn brown, then blue, then back to clear In this appendix, we explain how to perform demonstrations of oscillating reactions, fronts, and waves. In the next section, we provide information on how some of these systems can be systematically studied in the upper-level undergraduate laboratory. 

Higgins, J. 1967. The Theory of Oscillating Reactions, Ind. Eng. Chem. 59. 18-62. Hindmarsh, A. C. 1974. GEAR—Ordinary Differential Equation System Solver. UCID-30001 Rev. 3. Lawrence Livermore Laboratory Livermore, Calif. 

There has not always been such a wide spread interest in these amazing reactions. In the late 60 s the small number of oscillating reactions known were still regarded as mere laboratory curiosities, and more often as artifacts resulting from poorly controlled experimental conditions. A misinterpretation of the phenomenon, then communly spread, led to believe that such a behaviour would transgress the second principle of thermod3mamics. 

Subsequent to a fire in a teaching laboratory, it was discovered that a mixture of equal weights of the three dry solids, itself stable, reacted violently when wetted with up to two parts of water and was capable of igniting paper. All components (which exhibit an oscillating chemical reaction in solution) were necessary for this effect. 

Until the 1950s, the rare periodic phenomena known in chemistry, such as the reaction of Bray [1], represented laboratory curiosities. Some oscillatory reactions were also known in electrochemistry. The link was made between the cardiac rhythm and electrical oscillators [2]. New examples of oscillatory chemical reactions were later discovered [3, 4]. From a theoretical point of view, the first kinetic model for oscillatory reactions was analyzed by Lotka [5], while similar equations were proposed soon after by Volterra [6] to account for oscillations in predator-prey systems in ecology. The next important advance on biological oscillations came from the experimental and theoretical studies of Hodgkin and Huxley [7], which clarified the physicochemical bases of the action potential in electrically excitable cells. The theory that they developed was later applied [8] to account for sustained oscillations of the membrane potential in these cells. Remarkably, the classic study by Hodgkin and Huxley appeared in the same year as Turing s pioneering analysis of spatial patterns in chemical systems [9]. 

The released U(VI) from the U()2 matrix will continue to be dissolved until saturation with secondary U(VI) solid phases is reached. The observations from both laboratory and natural systems would indicate that the kinetically preferred phase is hydrated schoepite. This will be denoted as U02(0H)2(s) for the sake of description of the model, although the correct notation would be U03-xH20, with x oscillating between 0 and 2. Depending on the presence of carbonates in the contacting solution, the reactions can be described as ... 

These studies demonstrate the general mechanism of synchronization of biochemical systems, which I expect to be operative in even more complex systems, such as the mitochondrial respiration or the periodic activity of the slime mold Dictyostelium discoideum. As shown in a number of laboratories under suitable conditions mitochondrial respiration can break into self-sustained oscillations of ATP and ADP, NADH, cytochromes, and oxygen uptake as well as various ion transport and proton transport functions. It is important to note that mitochondrial respiration and oxidative phosphorylation under conditions of oscillations is open for the source, namely, oxygen, as well as with respect to a number of sink reactions producing water, carbon dioxide, and heat. 

The phrase laboratory curiosity was an apt characterization of a reaction that first saw the light of day in the late 1950 s1. This reaction - the acidic oxidation of citric acid by bromate in the presence of the dual catalysts bromide and cerium(IV)/(III) - displays oscillations in the concentrations of two component species in the course of proceeding towards completion. Curiosity and skepticism were engendered by oscillation in a homogeneous reaction mixture, even though such observations had been well documented in the past. 

In the following, we will discuss DMC simulations on the CO oxidation on the Pt(lOO) surface, that were done in our laboratories. The simulations show oscillations in the CO2 production rate as well as several types of spatio-temporal pattern formation. In essence, it is an extension of the ZGB model with desorption and diffusion of A, finite reaction rates and surface reconstruction. We will discuss it to illustrate the complexity of the models with which DMC simulations can be done nowadays. For clarity, we will stick to the A and B2 notation employed in the previous section. Species A corresponds to CO and B2 corresponds to 02- Furthermore, we will speak in terms of reaction rates instead of relative reaction probabilities. This terminology is entirely justified in the DMC approach that we used. 

HIE) 1973 Eckert, E., Hlavacek, V., Marek, M. Catalytic Oxidation of CO on Cu0-A1203, I. Reaction Rate Model Discrimination, Chem. Eng. Comm. vol. 1, 89-94. II. Measurement and Description of Hysteresis and Oscillations in a Laboratory Catalytic Recycle Reactors Chem. Eng. Com. vol. 1, 95-102... 

The iron (lll)-nitrate catalyzed reaction of ethanol with hydrogen peroxide forming acetaldehyde may exhibit temperature oscillations. In what follows the laboratory experiment described in [9] is modelled using the reaction kinetics and details on the experimental reactor given there. The reaction is described by ... 

Let me encourage those who have not yet experimented with the Belousov-Zhabotinskii reaction to give it. a try. For your convenience I have given recipes for producing homogeneous oscillations (p. 50) propagating waves (pp. 70f ). The chemicals and glassware are readily available in almost any wet-chemistry laboratory. Just ask ... 

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