Iodate oscillations

While no confirmed reports of new iodate oscillators have yet appeared, it seems inevitable that systems based on the bistable arsenite-iodate [30,31] or related Landolt-type [32] reactions will soon be made to oscillate. The iodide-peroxydisulfate-oxalate reaction discussed by Chopin-Dumas elsewhere in this volume may be the first of the new iodate oscillators. 

Kinetic study of the self-oscillating reaction observed in a potassium iodate-hydrogen peroxide-cysteine-sulfuric acid (acid medium) system was carried out [57], It is found that according to an adequate model the feedback mechanism is associated with autocatalytic reaction... 

Table 4. Component processes of the arsenite-iodate-chlorite oscillator and their rate laws1...

The very narrow region of the constraint space where oscillations could occur in this system was not found until after the chlorite-iodate-arsenite oscillator had been discovered. Its occurrence is shown in the cross-shaped diagram for chlorite-iodide (Fig. 8.)... 

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. 

The chlorite-iodate-arsenite oscillator was the first oscillating reaction discovered which is based upon chlorite chemistry. The BZ reaction and its relatives are bromate oscillators, while the BL and Briggs-Rauscher oscillators are iodate systems. The initial chlorite oscillator was rapidly followed by a large family of related systems58"60, which are summarized in Table 8. We note that while most of these systems contain an iodine species (I-, I2, IOf) as well as the chlorite, at least two iodine-free chlorite oscillators exist. 

Some of these chlorite oscillators exhibit particularly interesting or exotic phenomena. Batch oscillations in the absence of flow may be obtained in the systems numbered 3, 10 a and 13, while the chlorite-iodide-malonic acid reaction gives rise to spatial wave patterns as well. These latter, which are strikingly similar to those observed in the BZ reaction61 are shown in Fig. 12. Addition of iodide to the original chlorite-iodate-arsenite oscillator produces a system with an extremely complex phase diagram58, shown in Fig. 13, which even contains a region of tristability, three possible stable steady-states for the same values of the constraints. 

A ) Chlorite-iodide-iodate. While this system fits into either categories B) or C) below, it may also be considered a sort of fundamental oscillator which is generated (via M4, M8 or M9) by the systems of type B) or C). 

C ) Chlorite-iodate-reductant. Oscillators 2, 7, 8 a, 9 a, 10 a, 11 and 12 of Table 8 are of this type. The requirements for a successful substrate appear to be that (M8) be thermodynamically favorable and relatively rapid while (M10) is slow. 

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. 

In 1921, Bray published the first description of an oscillating reaction in the liquid phase, the catalytic decomposition of hydrogen peroxide under the influence of iodate ion. Amazingly, the initial response of the chemical community, instead of undertaking a normal study of the reaction, was to try to prove that the cause of the oscillations was some unknown heterogeneous impurity. 

Another example of an oscillating reaction is provided by the Bray reaction, the first identified homogeneous isothermal chemical oscillator, which is a complex reaction of iodate, iodine, and hydrogen peroxide. As hydrogen peroxide decomposes to oxygen and water, the resulting rate of the evolution of oxygen and I2 vary periodically. 

Briggs and Rauscher discovered an oscillating reaction which is identified as an iodine clock. It resembles the iodate-hydrogen peroxide reaction of Bray (1921), and has some of the elements of the reaction of Belousov, see Section III.C. The chemicals involved are ... 

III A) 1967 Degn, H. Evidence of a Branched Chain Reaction in the Oscillating Reaction of Hydrogen Peroxide, Iodine and Iodate, Acta Chem. Scand. vol. 21, 1057-1066... 

III A), 1976 Sharma, K. R., Noyes, R. M. Oscillations in Chemical Systems, 13. A Detailed Molecular Mechanism for the Bray-Liebhafsky Reaction of Iodate and Hydrogen Peroxide, J. Amer. Chem. Soc. vol. 98,4345-4361... 

The behavior of the redox potential oscillations under the effects of hydrogen peroxide, iodate ions, Mn(II) ions, acetone, and sulfuric acid was examined. The interrelationship between redox potential oscillations and iodine oscillations was studied. Both the chloride ion and Cu(II) inhibit the iodate-hydrogen peroxide reaction where Mn(II) catalyzed production of iodine is the key step. 

By adding hydrogen peroxide to a moderately acidic solution of potassium iodate and perchloric acid changes in behavior observed at 50 °C with an iodide ion specific electrode, were reported by Odutola et al. (1982). As the concentrations of H+ and H202 were increased, following the addition of hydrogen peroxide, oscillations in the potential with varying periods were, recorded. 

Noyes and Furrow (1982) modelled the essential mechanistic features of the full iodate-peroxide-Mn(II)-malonie acid (IO, —H202—Mn(II)—CH2(C02H)2) system with a mechanism involving 30 pseudo-elementary processes. Eleven of these processes are believed to generate the oscillatory behavior, observed experimentally. The rate constants of seven of these processes were determined experimentally. The other four values are assigned to create a system that mimics the essential features of oscillations. This mechanism differs from, those of other known oscillators in that both radical and nonradical paths generate the same net chemical change. 

During the recent years a number of studies has been made to design homogeneous oscillators systematically. While the others are the iodate (Bray-Liebhavsky and... 

De Kepper, et al. (1981-1) designed a homogeneous oscillating reaction by coupling the autocatalytic oxidation of arsenite by I03 to the autocatalytic C102 -I03- reaction in a CSTR. Both I2 and I concentrations oscillate with the concentration of the latter changing by a factor of > 105 during each cycle. This arsenite-iodate-chlorite system was obtained in two separate reactions. The oxidation of arsenite by iodate, a reaction autocatalytic in iodide is ... 

In the iodate-hydrogen-platinum-acid system the oscillatory range was found to be narrow and sensitive to physical parameters. Under similar conditions, with anions such as Cr207 and Mn04 no oscillations were recorded. 

Iodate-Chlorite System, J. Am. Chem. Soc. 103, 2133-2134 (IIIL) De Kepper, P., Epstein, I. R., Kustin, K. Systematic Design of Chemical Oscillators 1981-2 Part 3. Bistability in the Oxidation of Arsenite by Iodate in Stirred Flow Reactor, J. Am. Chem. Soc. 103, 6121-6127... 

Oscillators, Part 4. New Family of Homogeneous Chemical Oscillators-Chlorite-Iodate-Substrate. Nature (London) 292,816-818... 

IIIA) Zueva, T. S., Protopopov, E. V. Study of Oscillating Conditions in the Potassium 1982-2 Iodate-Hydrogen Peroxide-Cysteine System in a Sulfuric Acid Medium. Izv. Vyssh. 

Self-oscillations of sweUing and deswelling for the poly(N-isopropylacry-lamide-co-acryhc acid) hydrogels were realized by coupling pH and temperatme in the external solution media. The pH oscillations were also observed for the system consisting of iodate-sulfite-thiosulfate and poly(2-acrylamido-2-methyl-1-propanesulfonic acid) when polymeric acid was substituted for sulfuric acid... 

Two reportshave appeared recently on the Landolt oscillating reaction. Where the oxidation of sulfite and ferrocyanide by iodate takes place in a continuously stirred reaction vessel, large-amplitude oscillations in pH at constant [I ] are observed. The nature of the intermediates and elementary steps have been discussed together with the detailed mechanistic profile. The overall processes may be described as shown in equations (31)-(37). Individual rate constants for the rate-determining steps in the above reactions have been identified. 

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