Reactions chlorite-thiosulfate

IIIL) Orban, M., De Kepper, P., Epstein, I. R. Systematic Design of Chemical Oscillators, 1982-1 Part 77. An Iodine-free Chlorite-based Oscillator. The Chlorite-Thiosulfate Reaction in a Continuous Flow Stirred Tank Reactor. J. Phys. Chem., 86, 431-433... 

I. Nagypal and I.R. Epstein. Fluctuations and stirring rate effects in the chlorite-thiosulfate reaction. J. Phys. Chem., 90 6285-6292, 1986. 

Figure 4. Chaotic oscillation in the potential of a Pt redox electrode in the chlorite-thiosulfate reaction in a flow reactor. Note the aperiodic alternation between large and small amplitude peaks. Input concentrations, [ClOfJo = 5 x 10 M, [S203 ]o = 3 X 10 M, pH = 4, residence time in reactor = a) 6.8 min, b) 10.5 min, c) 23.6 min. Reproduced from Orbdn et al. (14). Copyright 1982 American Chemical Society.

Figure 8.9 Phase diagram of the chlorite-thiosulfate system in the [S203 ]q — ko plane with [C102]o = 5 X 10 MandpH4. SIMPLE and simple denote pure large-and pure small-amplitude oscillations, respectively. l iV denotes 1 multipeak oscillations. Vertical segments I show flow rate at which new 1 iV state appears. Filled-in boxes denote regions in which basic states are mixed to form aperiodic states. V, Low-potential steady state (SSI) A, high-potential steady state (SSII) o, bistability (SSI/SSII). (Reprinted with permission from Orban, M. Epstein, I. R. 1982. Complex Periodic and Aperiodic Oscillation in the Chlorite-Thiosulfate Reaction, J. Phys. Chem. 86, 3907-3910. 1982 American Chemical Society.)...

In some clock reactions, however, there is a narrow range of concentrations in which the quality of the mixing becomes critically important. Under these conditions, the time of the sharp transition from initial to final state becomes essentially unpredictable. The prototype system of this type is the chlorite-thiosulfate reaction (Nagypal and Epstein, 1986). Measurements of pH vs. time for five replicate experiments starting from the same initial concentrations are shown in Figure 15.8. For the first several minutes, all the curves are identical. The pH increases smoothly. In three of the curves, we observe a sharp drop in pH at approximately 3, 5, and 9 min in the other two, this decrease occurs at times greater than 20 min. When an acid-base indicator like phenolphthalein is added to the solution, the pH... 

Figure 15.8 Representative pH traces in the chlorite-thiosulfate reaction at 20.0 °C measured under identical conditions. Successive curves have been shifted by 0.07 pH units for easier viewing, since in the absence of a shift, the initial portions of the curves coincide. (Adapted from Nagypal and Epstein, 19861.)...

Mechanistic analysis of the chlorite-thiosulfate reaction, as well as consideration of the reaction time distributions, leads to the following qualitative explanation for the observed behavior. The pH rise and subsequent drop result from competition between two reaction pathways starting from chlorite and thiosulfate ... 

Although the behavior described above is rare, it is by no means unique to the chlorite-thiosulfate reaction. Similar distributions of reaction times are found in the chlorite-iodide reaction (Nagypal and Epstein, 1988) in a concentration range where it, too, shows high-order autocatalysis. More interesting is the fact that strikingly similar behavior occurs in two very different systems the polymerization of the mutant hemoglobin 8 molecules (Hb 8) that cause sickle cell anemia in their unfortunate hosts, and the combustion of simple hydrocarbons in a cool flame. 

Figure 2.4 Chaotic oscillation in the potential of a Pt redox electrode in the chlorite-thiosulfate reaction in a flow reactor. Adapted from Orbdn and Epstein [15].

Periodic chemical oscillations may be complex, and complex oscillations need not always be periodic. These observations are illustrated for the chlorite-thiosulfate reaction in Figures 10 and 11, where we see first complex periodic and then aperiodic or chaotic oscillation. 

We close this Section by mentioning that, despite initial controversies, the most complex type of dynamic behavior, chaos, has been shown to be also present in chemical systems, among which the most studied is again the BZ reaction (Scott, 1991). Chaos has also been observed, for example, in the chlorite-thiosulfate or the bromate-chlorite-iodide reactions, or in the gas-phase reaction between carbon monoxide and oxygen (Epstein and Pojman, 1998). 

Simple periodic chemical oscillation may now be said to be reasonably well understood. More complex behavior can arise when a single oscillator is pushed into new realms or when it is coupled either to other oscillators or to external influences. Some chemical oscillators that are simply periodic under one set of conditions can exhibit complex, multi-peaked, periodic or even aperiodic, chaotic (14) behavior at other concentrations and flow rates in an open reactor. Some examples of chaotic oscillations in the chlorite-thiosulfate system are shown in Figure 4. Coupling two or more reactions together can result in the... 

Other mixed-mode oscillations of this type have been seen in the Bray reaction (Chopin-Dumas, 1978) and several chlorite-based oscillators (Orban and Epstein, 1982 Alamgir and Epstein, 1985a, 1985b). In Figure 8.9, we present a phase diagram that shows the progression of 1" oscillations in the chlorite-thiosulfate system. 

Sodium Chlorite. An accurately weighed sample of about 2.0 g. of sodium chlorite is dissolved in 1 1. of water a 25-ml. aliquot serves as the sample for analysis. Two milliliters of 50 % potassium iodide and 10 ml. of acetic acid are added to the aliquot, and the ensuing reaction is allowed to proceed in the dark for 5 minutes. The liberated iodine is then titrated with 0.1 A standard sodium thiosulfate solution, using starch as an indicator. The equations for the reactions are written below.1... 

Orban et al. (1982-1) showed that the reaction between chlorite and thiosulphate in a CSTR exhibits oscillations. The oscillations in the potential of platinum redox electrode were recorded. These periodic oscillations showed different patterns as the input concentrations of chlorite and thiosulfate as well as pH (between 2 and 5) and flow rate varied. 

The first system on which this approach was tried (De Kepper, Epstein and Kustin, [ 18]) employed two coupled autocatalytic reactions, chlorite plus iodide, and arsenite plus iodate, which have key intermediates in common. As Figure h shows, the chlorite-iodate-arsenite system did indeed prove to oscillate, constituting the first systematically designed chemical oscillator. More recently, by starting from the fundamental or minimal chlorite-iodide bistable system and adding different feedback species, it has been possible to generate a family of nearly 20 different chlorite-iodine species oscillators (Orb n et al., [19]). In addition, two iodine free chlorite oscillators involving thiosulfate (Orban, De Kepper and Epstein, [ 20] ) and bromate (Orban and Epstein, [21]) have been found. 

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