Chlorite-iodide reaction mechanism

An example of such a comparison is seen in the modeling of the oscillating chlorite-iodide reaction. The model initially proposed by Epstein and Kustin [39] showed only fair agreement with the experimentally observed 1 evolution, and worse agreement with the experimentally observed I2 evolution, as seen in fig. 11.5(a,b). A revised mechanism proposed by Citri and Epstein [40] predicts oscillations quite similar in shape to the experimentally observed 1 and I2 oscillations (fig. 11.5c). In many oscillatory systems the temporal variation of only a few species (essential or nonessential) can be measured. The comparison of an experimental time series with a prediction of a proposed mechanism can be made with regard to the period of the oscillations, but becomes subjective with regard to the shape of the variation. The comparisons do not easily lead to suggestions for improvements of the proposed reaction mechanism. 

Fig. 11.5 Comparison of experimental and theoretical oscillatory traces of I2 absorbance and 1 potential for the chlorite-iodide reaction, (a) Experimental traces, (b) Theoretical traces from the Epstein-Kustin mechanism show fair qnahtative agreement, bnt wave shapes are snb-stantially different from experimental observations, (c) Theoretical traces from the Citri-Epstein mechanism show marked improvement in wave shape agreement with experiments. (Erom [2].)...

Epstein, I. R. Kustin, K. Systematic design of chemical oscillators. 27. A mechanism for dynamical behavior in the oscillatory chlorite iodide reaction. J. Phys. Chem. 1985, 89, 2275-2282. 

To what extent are we justified in thinking of a chemically coupled oscillator system as consisting of the two independent subsystems plus a set of cross-reactions that provide the coupling A partial answer can be found in a mechanistic investigation of the bromate-chlorite-iodide reaction (Citri and Epstein, 1988). The mechanisms that had been derived for the bromate-iodide and chlorite-iodide systems in independent studies of these reactions are shown, respectively, in Tables 12.1 and 12.2. 

Figure 12.5 Phase diagram of the bromate-chlorite-iodide system calculated using the mechanism in Table 12.3. Fixed constraints and symbols as in Figure 12.3. C signifies compound oscillation, C, signifies one compound oscillation plus j chlorite-iodide oscillations per cycle, (Reprinted with permission from Citri, O. Epstein, I. R. 1988. Mechanistic Study of a Coupled Chemical Oscillator The Bromate-Chlorite-Iodide Reaction, J. Phys. Chem. 92, 1865-1871. 1988 American Chemical Society.)...

Citri, O. Epstein, L R. 1987. Dynamical Behavior in the Chlorite-Iodide Reaction A Simplified Mechanism, J. Phys. Chem. 91, 6034-6040. 

Lengyel, I, Li, J. Kustin, K. Epstein, I. R. 1996. Rate Constants for Reactions between Iodine- and Chlorine-Containing Species A Detailed Mechanism of the Chlorine Dioxide/Chlorite-Iodide Reaction, J. Am. Chem. Soc. 118, 3708-3719. 

In a closed system, the chlorite iodide reaction is a clock reaction if 1 < [I ]o/[C102 ]o < 4. Its kinetics were determined and the basic elements of a mechanism were proposed first for the batch reaction by Kern and Kim [22]. Figure 1 shows kinetic curves (iodine absorbance vs time) of the reaction with different initial ratios of the reactants. In the first part of each curve, iodine is produced at an accelerating rate. During this period, the stoichiometry is... 

For most of the oscillating reactions, the knowledge of the reaction mechanism and rate constants is generally very sketchy. Since the details of a particular kinetic model are not relevant close to bifurcation conditions, we will consider the simplest ordinary differential equation model which accounts for the characteristic features of the chlorite-iodide reaction and of its variants [69-71], namely bistability, excitability and relaxation oscillations. Our model of the reaction term is a two-variable Van der Pol-like system [82, 83] (C = u,v)) ... 

O. Citri and I. R. Epstein, Systematic design of chemical osdllators. 42. Dynamic behavior in the chlorite-iodide reaction a simplified mechanism, Journal of Physical Chemistry, vol. 91, no. 23, pp. 6034-6040, 1987. 

A complete mechanism for the chlorite-iodide system entails many steps. In addition to those consistent with the rate laws for processes (VII) (A) and (B), the elementary steps of the Dushman reaction need to be included. A set of relevant steps for these processes in collected in Table 7. 

The development of an adequate mechanism for the BZ reaction required nearly 15 years from the discovery of oscillations in that system, and refinement of that mechanism is still under way56. It is a measure of the progress in the field of oscillating reactions that only 15 months after the design of the first chlorite oscillator, a mechanism for that system seems well within reach. Without setting forth a full mechanistic treatment, which is not yet available, we sketch here what we believe to be the key elements in the oscillation of the chlorite-iodate-arsenite oscillator and, by extension, several of the related systems to be discussed below. A partial mechanism for the prototype chlorite-iodide system will be presented in the following section. 

Despite the importance of the chlorite-iodide systems in the development of nonlinear chemical dynamics in the 1980s, the Belousov-Zhabotinsky(BZ) reaction remains as the most intensively studied nonlinear chemical system, and one displaying a surprising variety of behavior. Oscillations here were discovered by Belousov (1951) but largely unnoticed until the works of Zhabotinsky (1964). Extensive description of the reaction and its behavior can be found in Tyson (1985), Murray (1993), Scott (1991), or Epstein and Pojman (1998). There are several versions of the reaction, but the most common involves the oxidation of malonic acid by bromate ions BrOj in acid medium and catalyzed by cerium, which during the reaction oscillates between the Ce3+ and the Ce4+ state. Another possibility is to use as catalyst iron (Fe2+ and Fe3+). The essentials of the mechanisms were elucidated by Field et al. (1972), and lead to the three-species model known as the Oregonator (Field and Noyes, 1974). In this... 

Epstein and Kustin [39] set up a first reaction mechanism for the chlorite-iodide oscillator and reported a (diagonal) cross-shaped diagram in the species [CIO2 ]o-[I ]o- Later, Citri and Epstein [40] revised the first suggestion for the mechanism by reducing the original model considerably. Flere we employ the revised mechanism proposed by Citri and Epstein. The elementary steps are as follows ... 

Another set of pattern formation phenomena involve stationary, or Turing patterns (77), which arise in systems where an inhibitor species diffuses much more rapidly than an activator species. These patterns, which are often invoked as a mechanism for biological pattern formation, were first found experimentally in the chlorite-iodide-malonic acid reaction (72). Examples of typical spot and stripe patterns appear in Figure 3. Recently, experiments in reverse microemulsions have given rise not only to the waves and patterns described above, but to a variety of novel behaviors, including standing waves and inwardly moving spirals, as well (75). 

Table I. Mechanism of the Bromate-Chlorite-Iodide Oscillating Reaction...

We have used the FKN mechanism of the BZ reaction as an example to illustrate a general approach to constructing mechanisms. This mechanism was developed at a time when the range of experimental and computational methods was less extensive than it is today. It may be useful to look at a more recent example of a mechanistic study, in which a wide range of techniques was brought to bear on the chlorite-iodide and related reactions (Lengyel et al., 1996). 

Table 5.2 Mechanism of the Chlorite-Iodide and Related Reactions (Lengyel et al 1996) ...

With the single exception of the chlorite-bromate-reductant systems [14], for which a mechanism has been developed by joining the NFT model [26] with THOMPSON S [42] mechanism for the BrOj -ClOo reaction, no mechanism has been published for any chlorite oscillator. In Table 2, we give a recently developed mechanism [43] for the minimal chlorite-iodide oscillator. Of special significance is the fact that it contains no radical species, but rather the binuclear intermediate CIO,. Calculations with this mechanism give excellent agreement with a wide variety of experimental results. One example is given in Fig. 5. 

Recent unpublished work at Brandeis suggests that the fundamental chlorite-iodide oscillator (Dateo et al.,[31] ) can be understood in terms of a mechanism involving the key binuclear intermediate IC102 In contrast to the mechanism for the BZ reaction, this model would require only singlet, non-radical species, thus constituting a fundamentally different pathway to oscillation. 

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