Chlorite-thiosulfate system

D) Iodine-free chlorite oscillators. In view of our almost total ignorance of how the chlorite-thiosulfate system functions, we place it for the moment in a class of its own, though further study may ultimately situate it in an expanded category A. The recently discovered chlorite-bromide-bromate oscillator may be analogous to the chlorite-iodide-iodate system of class A ) above, though one may view it alternatively as a bromate driven oscillator in which CIOJ plays the role of the metal catalyst. 

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. 

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.)...

Figure 9.7 Polar coordinate plot of front velocity vs. direction of propagation in the chlorite-thiosulfate system. [CIO2 ] = 0.009 M, [8203 "] = 0.0045, [OH ] = 0.002 M. Distance from the origin is proportional to the speed of propagation. Tube diameter (i.d., mm) (a) 0.794, (b) 3.17, (c) 4.76. Reprinted with permission from Nagypal, I. Bazsa, G. Epstein, I. R. 1986. Gravity Induced Anisotropies in Chemical Waves, J. Am. Chem. Soc. 108, 3635-3640.

A chlorite-thiosulfate system showed oscillations in a CSTR, and is the first chlorite-based oscillator involving no iodine-containing species. ... 

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). 

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... 

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. 

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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