Rate Constants for Reactions between Iodine- and Chlorine-Containing Species: A Detailed Mechanism of the Chlorine Dioxide/Chlorite-Iodide Reaction ² Istva ´ n Lengyel, ‡ Jing Li, Kenneth Kustin,* and Irving R. Epstein Contribution from the Department of Chemistry and Center for Complex Systems, Brandeis UniVersity, Waltham, Massachusetts 02254-9110 ReceiVed NoVember 27, 1995 X Abstract: The chlorite-iodide reaction is unusual because it is substrate-inhibited and autocatalytic. Because analytically pure ClO 2 - ion is not easily prepared, it was generated in situ from the rapid reaction between ClO 2 and I - . The resulting overall reaction is multiphasic, consisting of four separable parts. Sequentially, beginning with mixing, these parts are the (a) chlorine dioxide-iodide, (b) chlorine(III)-iodide, (c) chlorine(III)-iodine, and (d) hypoiodous and iodous acid disproportionation reactions. The overall reaction has been studied experimentally and by computer simulation by breaking it down into a set of kinetically active subsystems and three rapidly established equilibria: protonations of chlorite and HOI and formation of I 3 - . The subsystems whose kinetics and stoichiometries were experimentally measured, remeasured, or which were previously experimentally measured include oxidation of iodine(-1,0,+1,+3) by chlorine(0,+1,+3), oxidation of I - by HIO 2 , and disproportionation of HOI and HIO 2 . The final mechanism and rate constants of the overall reaction and of its subsystems were determined by sensitivity analysis and parameter fitting of differential equation systems. Rate constants determined for simpler reactions were fixed in the more complex systems. A 13-step model with the three above-mentioned rapid equilibria fits the overall reaction and all of its subsystems over the range [I - ] 0 < 10 -3 M, [ClO 2 - ] 0 < 10 -3 M, [I - ] 0 /[ClO 2 - ] 0 ) 3-5, pH ) 1-3.5, and 25 °C. The derived model with all experimentally determined rate and equilibrium constants fits both the overall reaction and all of its subsystems within 1% relative accuracy. Introduction Rate constants of reactions between iodine- and chlorine- containing species are very important in the mechanisms of chlorite/chlorine dioxide and iodide/iodine/iodate-containing chemical oscillators. Among the "chlorite-driven" chemical oscillators 1 one of the most versatile is the chlorite-iodide reaction, which has become, next to the Belousov-Zhabotinsky (BZ) reaction, perhaps the most widely studied reaction in nonlinear chemical dynamics. The past 15 years has witnessed publication of more than 200 articles on this reaction. These studies have been concerned mainly with the system's nonlinear features, such as oscillations, bistability, stirring and premixing effects, and spatial phenomena. 2 However, only a few of these articles report detailed mechanistic investigations. Previous attempts to model these complex dynamical systems utilized neither rate constant determinations by parameter fitting to experiments nor direct kinetics measurements of elementary reactions; essentially these studies were qualitative descriptions of oscillations in the chlorite-iodide reaction. Not surprisingly, rate constants vary considerably among the different models. For example, the rate constant for the reaction between HOCl and HOI was reported to be 0, 3 2 × 10 3 , 4 5 × 10 5 , 5 and 2 × 10 8 M -1 s -1 , 6 in different modeling efforts. Similarly large variation is seen in the rate constant found for the HOCl + HIO 2 reaction: 0, 5 1 × 10 3 , 3,4 and 2 × 10 8 M -1 s -1 . 6 The treatment of iodine hydrolysis in the models differs substan- tially: the forward reaction is described as a pH-dependent 3,7 or a pH-independent process. 4,6 There are also smaller but significant variations for the rate constants of the HIO 2 + I - (+ H + ), HIO 2 + HOI, HIO 2 disproportionation, HClO 2 + HOI, and HClO 2 + HIO 2 reactions. Since most of the rate constants of previous models had not been measured directly, we decided to reinvestigate the chlorite-iodide reaction, which is at the heart of chlorite- and iodine-containing oscillators. Among small molecule inorganic oxidation-reduction reac- tions, the chlorite-iodide reaction is quite unusual, because it is both substrate inhibited and autocatalytic. Iodide, which is a reactant, inhibits the reaction, and iodine, one of the products, accelerates the reaction. This feature was first recognized by Bray 8 and later studied in more quantitative detail. 9,10 Kern and Kim 9 established a very accurate empirical rate law valid up to 90% of iodide conversion to iodine. They proposed that the reaction is controlled by HOI-generating iodine hydrolysis: HOI reacts rapidly with chlorous acid followed by even faster reactions of the intermediates with iodide. This picture is preserved in the subsequent detailed mechanisms of Epstein and Kustin 6 and Citri and Epstein. 3 The Citri-Epstein model (C&E), which is the most widely used model of the reaction, reproduces many qualitative features of the experiments, but significant differences remain, as shown by Ross and co-workers. 11,12 ² Systematic Design of Chemical Oscillators. 94. Part 93: Epstein, I. R.; Kustin, K.; Lengyel, I. In Taube Insights: From Electron Transfer Reactions to Modern Inorganic Chemistry; Isied, S., Ed.; American Chemical Society: Washington, DC, 1996; In press. ‡ Current address: Chemical Engineering Department, Massachusetts Institute of Technology, 66-250, Cambridge, MA 02139-4307. X Abstract published in AdVance ACS Abstracts, April 1, 1996. (1) Epstein, I. R.; Orba ´n, M. In Oscillations and TraVeling WaVes in Chemical Systems; Field, R. J., Burger, M., Eds.; Wiley: New York, 1985; p 257. (2) De Kepper, P.; Boissonade, J.; Epstein, I. R. J. Phys. Chem. 1990, 94, 6525. (3) Citri, O.; Epstein, I. R. J. Phys. Chem. 1987, 91, 6034. (4) Ra ´bai, G.; Beck, M. T. Inorg. Chem. 1987, 26, 1195. (5) Citri, O.; Epstein, I. R. J. Phys. Chem. 1988, 92, 1865. (6) Epstein, I. R.; Kustin, K. J. Phys. Chem. 1985, 89, 2275. (7) Noyes, R. M.; Furrow, S. D. J. Am. Chem. Soc. 1982, 104, 45. (8) Bray, W. C. Z. Physik. Chem. 1906, 54, 741. (9) Kern, D. M.; Kim, C.-H. J. Am. Chem. Soc. 1965, 87, 5309. (10) De Meeus, J.; Sigalla, J. J. Chim. Phys. 1966, 63, 453. 3708 J. Am. Chem. Soc. 1996, 118, 3708-3719 0002-7863/96/1518-3708$12.00/0 © 1996 American Chemical Society