Oscillatory reactions chlorite-iodide 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 ... 

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. 

Figure 12.3 Phase diagram of the bromate-chlorite-iodide reaction in the /co-[I ]o plane. Fixed constraints [BrO Jo = 2.5 x 10 M, [ClOJJo = 1.0 x 10 M, [H2S04]o = 0.75 M. Symbols open circles, low-frequency oscillatory state filled circles, high-frequency oscillatory state open triangles, low-potential stationary state filled triangles, high-potential stationary state open squares, intermediate-potential stationary state. Combinations of two symbols imply bistability between the corresponding states. (Reprinted with permission from Alamgir, M. Epstein, I. R. 1983. Birhythmidty and Compoimd Oscillation in Coupled Chemical Oscillators Chlorite—Bromate-Iodide System, J. Am. Chem. Soc. 105, 2500-2502. 1983 American Chemical Society.)...

Luo, Y. Epstein, I. R. 1986. Stirring and Premixing Effects in the Oscillatory Chlorite-Iodide Reaction, J. Chem. Phys. 85, 5733-5740. 

Another factor to keep in mind is the stirring rate. Although one generally assumes that mixing is essentially perfect in the CSTR, recent experiments by ROUX et. al. [50] on the chlorite-iodide reaction have shown marked dependence of the stability of steady states on the rate of stirring even at such "high" speeds as 700 rpm. Similar effects are to be expected for oscillatory states. 

M7) be slow in some accessible range of pH and reactant fluxes. One oxidant which fails to enhance the range of constraints over which a chlorite-iodide system will oscillate is peroxydisulfate. In spite of its highly favorable reduction potential (E° = 2.01 V), S2Og reacts too slowly with I" in reaction (M4) to be a useful oscillatory substrate. 

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 15.4 Phase portraits of the chlorite-iodide system with pH = 1.56, [r]o = 1.8 X 10 M, [CIOJlo = 5.0 x lO" M. Points with Roman numerals are steady states, closed curves are limit cycles. Arrows indicate how the system evolves in concentration space. Row A. shows evolution of the system without piemixing at low stirring rate (< 550 rpm) as flow rate increases Row B shows intermediate stirring rate without premixing Row C show high stirring rate or premixing. (Reprinted with permis.sion from Luo, Y. Epstein, I. R. Stirring and Premixing Eflects in the Oscillatory Chlorite Iodide Reaction," J. Chem. Phys. 85, 5733 5740. it) 1991 American Institute of Physics.)...

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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