CIMA reaction

The reaction involving chlorite and iodide ions in the presence of malonic acid, the CIMA reaction, is another that supports oscillatory behaviour in a batch system (the chlorite-iodide reaction being a classic clock system the CIMA system also shows reaction-diffusion wave behaviour similar to the BZ reaction, see section A3.14.4). The initial reactants, chlorite and iodide are rapidly consumed, producing CIO2 and I2 which subsequently play the role of reactants . If the system is assembled from these species initially, we have the CDIMA reaction. The chemistry of this oscillator is driven by the following overall processes, with the empirical rate laws as given ... 

Figure C3.6.12 a) Turing spot pattern in the CIMA reaction. (A) Tio-temporal turbulence near the Turing bifurcation. Reproduced by pennission from Ouyang and Swinney [59].

The Turing mechanism requires that the diffusion coefficients of the activator and inlribitor be sufficiently different but the diffusion coefficients of small molecules in solution differ very little. The chemical Turing patterns seen in the CIMA reaction used starch as an indicator for iodine. The starch indicator complexes with iodide which is the activator species in the reaction. As a result, the complexing reaction with the immobilized starch molecules must be accounted for in the mechanism and leads to the possibility of Turing pattern fonnation even if the diffusion coefficients of the activator and inlribitor species are the same 62. 

Figure 21. Examples of the stationary patterns observed in the CIMA reaction (a) Hexagonal structure (b) lamellar stripe structure (c) irregular (turbulent structure).

The Lengyel-Epstein model is a more realistic chemical reaction scheme. The Lengyel-Epstein model is a two-variable model for the chlorite-iodide-malonic acid (CIMA) reaction scheme and its variant, the chlorine dioxide-iodine-malonic acid (CDIMA) reaction scheme. In the model, the oscillatory behavior is related with ... 

Figure 4.11 Turing patterns observed in a CIMA reaction. Light and dark colors are the two states of an indicator for the Ig concentration. From Ouyang and Swinney (1991). Reprinted by permission from Macmillan Publishers Ltd Nature 352, 610-612, copyright 1991.

The first unambiguous observation of a Turing instability in any experimental system did not occur until 1990. That year, the Bordeaux group found convincing evidence for Turing patterns in an in vitro system, the CIMA reaction (see Sect. 1.4.9). The gap of almost 40 yr between Turing s theoretical prediction of diffusion-induced instabilities and the experimental realization of stationary chemical pattern was caused by two main factors. 

Fig. 12.4 Stationary patterns in the CIMA reaction in a CFUR. (a) and (b) hexagons, (c) stripes, (d) mixed state. The bar beside each picture represents 1 mm the reactor is 25 mm in diameter. Reprinted with permission from Macmillan Publishers Ltd Nature [337]. Copyright 1991...

In this reaction, ClOj is self-inhibitor and I is autocatalyst. Kepper et al. [40] investigated the pattern formation in CIMA reaction by using the above type of reactor. For the CIMA system, the composition of CSTR was kept constant whereas that of other CSTRs was varied stepwise. Different sequences of spatial and spatio-temporal patterns were observed. At large CIO2 flow rates, stationary patterns were observed. In the CIMA reaction in CR, the concentrations of Ij, MA and H were same in both the CSTRs but spatial gradient in CIOJ concentration may be imposed by applying different feed concentrations in the CSTR. Thus [ClOj] was allowed to vary with distance along the reactor but not with time. 

Detailed mechanistic interpretation of patterns in the case of CIMA reaction in the Couvette reactor has been proposed by using modified forms of R-D equations [40]. 

Quasi-two-dimensional structure in CIMA reaction (chlorite - - iodide -t- malonic acid -I- starch). Initially transient yellow circles emerge and start to grow in a blue surrounding. These structures dissolve and break into dot patterns demonstrating a wide distribution of sizes. The dots evolve to a stationary structure consisting of yellow hexagons of different orientations [51, 52]. Similar structures have been observed in petri dish experiments with PA-MBO reaction [53]. 

Hexagon stripes have been observed in PA-MBO system in which case gel is removed from the petri dish [53]. Similarobservationshavebeenreportedin CIMA reaction with disc reactor [52] and also PA-MBO system with petri dish [53]. 

Transient Turing-like patterns in PA-MBO reaction (Polyacrylamide-methylene blue-sulphide-oxygen) system are obtained. Non-Turing stationary patterns in FIS reaction (Hexacyanoferrate (Il)-iodate-sulphite system) are obtained using an experimental technique similar to that used in CIMA reaction. These patterns develop through propagation of chemical fronts from the initial perturbation [54],... 

Turing strucmres have been widely studied in CIMA reaction [67] and its derivatives. When the CIMA reaction is performed in gel media by using starch as an indicator, some striped and hexagonal (spotted) structures [68] are observed. These strucmres are shown in Fig. 1.7. First, a starch-iodide complex is formed. The activator (iodine species) and the starch-triiodide complex generate Turing strucmres which diffuse much more slowly in the gel medium than inhibitor species (chlorite or chlorine dioxide). 

An alternative approach to seeking an elementary step description of the CIMA reaction is to characterize the system in terms of a few overall stoichiometric processes and their empirical rate laws (Rabai et ah, 1979). Such an approach works well if there is no significant interaction among the intermediates of the component net processes, that is, no cross talk, and if none of the intermediates builds up to significant levels. The proof of the pudding lies, of course, in comparing the predictions of this empirical rate law approach with the experimental results for the system of interest. 

Historically, it was the CIMA reaction in which Turing patterns were first found. Under the conditions of these experiments, however, our analysis suggests that, after a relatively brief initial period, it is really the CDIMA reaction that governs the formation of the patterns. Even when the input feeds consist of chlorite and iodide, chlorine dioxide and iodine soon build up within the gel and play the role of reactants whose concentrations vary relatively slowly compared with those of C102 and I . We have therefore found it more practical to work with the CDIMA system, using chlorine dioxide and iodine along with malonic acid as the input species, since in this way the relevant parameters can more easily be measured and controlled. Working with the CDIMA system also leads us naturally toward a simpler version of the model described by eqs. (14.22)-( 14.24). 

The development of practical methods [56] for the systematic design of new oscillating reactions in continuous stirred tank reactors (CSTR) lead to the discovery of several dozens of different isothermal oscillating systems, including the CIMA reaction [57]. This reaction is one of the very few to also exhibit transient oscillatory behavior in batch conditions. This and the fact that it does not exhibit marked excitability character like the well-known Belousov-Zhabotinsky reaction [5], lead us to select the CIMA reaction for systematic research on stationary spatial structures in open spatial reactors [14]. 

Concerning open spatial reactors, a first approach was provided by the so-called Couette flow reactor . In this quasi-one-dimensionalreactor, the transport is ensured by turbulent diffusion. Fresh reagents permanently renewed at each end allow the system to be maintained at a controlled distance from equilibrium. When operated in this reactor, the CIMA reaction lead to various spatio-temporal structures (oscillating fronts) as well as to a nontrivial stationary spatial structure (three stationary fronts) [58, 59]. However, this is... 

The initial reagents of the CIMA reaction are chlorite (CIOJ), iodide (I ), and malonic acid (CH2(COOH)2). The overall reaction consists of the oxidation of iodide by chlorite complicated by the iodination of malonic acid. The oscillatory mechanism of the reaction was elucidated by Lengyel et al. [60]. They found that the oscillatory dynamics actually occurred when the initial chlorite and iodide ions were nearly completely consumed. Thereafter, besides the malonic acid, the major species are chlorine dioxide (CIO2) and iodine (I2) while iodide and chlorite become the true variables and play respectively the roles of the activator and of the inhibitor . 

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