2275
J. Phys. Chem. 1985,89,2275-2282
A Mechanism for Dynamicai Behavlor in the Oscillatory Chlorite-Iodide Reaction' Irving R. Epstein* and Kenneth Kustin Department of Chemistry, Brandeis University, Waltham, Massachusetts 02254 (Received: January 22, 1985)
A mechanism is proposed for the reaction between chlorite and iodide in acidic solution. The proposed mechanism, which involves 13 elementary steps and 9 independent chemical species, accounts for the "clock" behavior observed in batch and for the bistability and periodic oscillation obtained in a flow reactor. The mechanism invokes no radical intermediates, but rather a binuclear singlet species, IC102. In one stage of the oscillations,an autocatalytic process produces HOI and ultimately converts I- to 12. When [I-] falls sufficiently with respect to the level of HOCl which is produced in the autocatalytic process, HIOZis oxidized by HOCl to iodate, and the iodide and iodine levels increase and decrease, respectively, as a result of the flow. The cycle then repeats.
Introduction In less than a decade and a half, oscillating chemical reactions have developed from a nearly unknown laboratory curiosity to a significant object of study in several fields of chemistry.2 Probably the single most important step in the growth of interest in these systems was the formulation of the Field-Karos-Noyes (FKN) mechanism3 and its subsequent success4in explaining and predicting the remarkable variety of behavior observed in the Belousov-Zhabotinsky and related systems. A second major breakthrough was the first deliberate synthesis of a new chemical o~cillator,~ which soon led to the discovery of a large family of oscillatory systems6 involving chlorite and an iodine-containing species. A next logical step in solidifying the understanding of chemical oscillation would be the development of a mechanism for the chlorite-iodide oscillator-the minimal' or prototype member of the chlorite-iodine family of oscillators. Such a mechanism is presented here. Later studies will focus on developing mechanisms for more complex chlorite oscillators, explaining the rich array of dynamic phenomena found in these reactions, and producing a simplified model for chlorite oscillators analogous to the Oregonators for the BZ reaction. As we shall see, although the chlorite-iodide oscillator is also an oxyhalogen system, its mechanism is fundamentally different from that of the BZ reaction. Experimental Background A satisfactory mechanism for the chlorite-iodide oscillator must be consistent not only with the observed oscillation in the CIOz--Ireaction but with a considerable number of other experimental results in a variety of related systems as well. These observations may be divided into batch (closed system) experiments on reactions which constitute subsystems of the full oscillator, and experiments in flow (open) systems on the CIOz--I- reaction itself as well as on more complex chlorite-containing systems. Batch Behavior In an acidic aqueous solution containing chlorite and iodide as well as the products and intermediates generated from these (1) Part 27 in the series "Systematic Design of Chemical Oscillators". Part 26: Orbin, M.; Epstein, I. R J . Am. Chem. Soc., in press. (2) Epstein, I. R.; Kustin, K.; De Kepper, P.; Orban, M. Sci. Am. 1983, 248(3), 112-123. (3) Field, R. J.; KBrBs, E.; Noyes, R. M. J. Am. Chem. SOC.1972, 94, 8649-8664. (4) For example: Edelson, D.; Noyes, R. M.; Field, R. J. Int. J . Chem. Kiner. 1979, 11, 155-164. ( 5 ) De Kepper, P.; Epstein, I. R.; Kustin, K. J. Am. Chem. Soc. 1981, 103,
2133-2134. (6)-0rMn, M.; Dateo, C.; De Kepper, P.; Epstein, I. R. J. Am. Chem. SOC. 1982, 104, 591 1-5918.
(7) Epstein, I. R.; Orbin, M. In "Oscillations and Traveling Waves in Chemical Systems": Field, R. J., Burger, M., Eds.; Wiley: New York, 1985; pp 257-286. (8) Field, R. J.; Noyes, R. M. J . Chem. Phys. 1974, 60, 1877-1884.
0022-3654/85/2089-2275$01.50/0
TABLE I: Rate Constants for the Chlorite-Iodide Reaction
Kern and KimIo k,,,M-2 s-l
4.6 X
lo2
de Meeus and Siaalla" 1.7 x 103 2.0 x 10-2
k i b r S-l
2.6 x 10-3
k,,,M-l s-l ionic strength temp, O C pH range
a
9.2 X
0.5 M 25
0.3; 0.07 M 25,b 30e 6.5-7.9
4-8
"k,, term not observed. bFor k,, determination. cFor k l b and k,, determinations. initial reactants, a number of reactions are possible. The most important of these is the chlorite-iodide reaction, but the reaction between chlorite and iodine must also be taken into account, while other processes such as the generation of I, from I- and IO3- or the decomposition of Cl(II1) will be of importance in other chlorite-iodine oscillators. We summarize here what is known about the stoichiometry and kinetics of the relevant reactions. Chlorite-Zodide. The stoichiometry of the reaction between Cl(II1) and I- was established by Bray9 as C10,-
+ 41- + 4H+ = 21z + C1- + 2 H 2 0
(1)
The kinetics of the reaction have been studied by Kern and Kimlo and by de Meeus and Sigalla." In both cases, the reaction was found to be autocatalytic in I2 and inhibited by I-. The rate law may be summarized as 1 d[CI2I --2
dt
+
where [ C I z J = [I2] [I3-], and the values found for the rate constants are given in Table I. In view of the rather different conditions employed, the two sets of determination of kla and klb appear quite consistent. The failure of Kern and Kimlo to observe the k,, term can be accounted for by the fact that they worked at [I-] levels (104-10-2 M) at which the klb term would dominate the k l , term, while de Meeus and Sigalla" used I- concentrations that were 1-2 orders of magnitude higher. Under typical oscillatory conditions M < [I-] < M), the k,, term is probably of little significance compared with the k l bterm. The batch reaction is a clock reaction, with the autocatalytic iodine formation producing a sudden burst of I, and disappearance of I-. Typical traces for the concentrations of these species are shown in Figure 1. The rapid disappearance of I, after the peak results from the subsequent very rapid reaction between the 1, (9) Bray, W. C. 2.Phys. Chem. 1906, 54, 731-749. (10) Kern, D. M.; Kim, C. H. J . Am. Chem. SOC.1965,87, 5309-5313. (1 1) De Meeus, J.; Sigalla, J. J . Chim. Phys. Phys.-Chim.Biol. 1966, 63, 453-459.
0 1985 American Chemical Society
Epstein and Kustin
2276 The Journal of Physical Chemistry, Vol. 89, No. 11, I985
a large number of kinetics investigations by a variety of techniques. While the stoichiometry is generally agreed to be that of eq 5 there
IO
