1014
J. Phys. Chem. 1992,96, 1014-1015
New Region of Oscillations in the Chlorite-Bromate-Iodlde System of Internally Coupled Chemical Oscillators Farzad Mabootian, David J. H a d , and Joseph E. Eprley* Department of Chemistry, Georgetown University, Washington, D.C. 20057 (Received: October 21, 1991; In Final Form: December 1 1 , 1991)
When bromate, chlorite, and iodide ions react (in 1.08 M H904 solution) in a continuously stirred tank reactor, complicated d a t i o n s occur at [TI0> 1 mM, as has been previously reported. If 6 mM < [I-], < 20 mM, oscillations with lars-complicated waveforms occur. The high [I-], and low [I-], oscillations are separated by an extensive region (in [1-lO-k,, space) in which the CSTR reaches a steady state. The mechanistic model that adequately fits the previously reported oscillations does not predict the high-[I-], oscillations now reported. An extension of that mechanism to involve autocatalyticformation of solid diiodine is suggested.
Introdaction When continuously stirred tank reactors (CSTRs) in which oscillating chemical reactions are in progress are connected, striking results are obtained.’ As the magnitude of the interaction between the oscillators is increased from zero, the separate oscillators at first continue to operate in a modified way, but when the coupling increases beyond a critical value? a new system (of which the two original oscillators may be regarded as components) arises. Sometimes a sort of “division of labor” occufs-one of the CSTRs remains in one steady state while the other stays in a quite different steady state. It is also possible to study two different chemical oscillators together in a single CSTR. When two oscillatory reaction networks that are operating simultaneously in one vessel share one or more common components, the combination may be regarded as a system of internally coupled chemical oscillators. These experimentally accessible systems share important characteristics with complicated natural entities such as marine ecosystems. A previously studied example of the class of internally coupled chemical oscillators is the combination (in a single CSTR) of the bromattiodide and chloriteiodide oscillators (each of which has been studied separately). Alamgir, Citri, and Epstein3described three steady states and a number of diverse types of oscillations (in various regions of the parameter space studied) for the bromate-chlorittiodide system of internally coupled chemical oscillators. They devised a multistep mechanism, based on mechanisms previously proposed for the component reactions, that adequately reproduces the interesting features that they observe. In this case, two chemical oscillators are coupled through several chemical species that m r in the mechanisms of both component reactions. We now report results of a reinvestigation of the bromate chlorite-iodide system of internally coupled chemical oscillators. We confirmed main features previously reported, but observed an additional region (in [I-lO-k, space) of oscillation that has not previously been observed. E x p e ~ t a Section l Sodium chlorite solutions were prepareddaily in 1.O mM NaOH to retard decomposition. Reagents were stored in a water bath at 25.0 OC. Input concentrations, [XI,,are concentrations after mixing but before reaction. Chlorite and bromate input concentrations were held constant at 0.10 and 3.25 mM, respectively. Iodide input concentrationsranged between 0.2 and 30 mM. The (1) Bar-Eli, K. E. J. Phys. Chem. 1990, 94, 2368. Bar-Eli, K.; Geisler, W. J. J . Phys. Chem. 1981,85, 3461. Stuchl, I.; Marek, J. J. Chem. Phys. 1982, 77, 2956. (2) Waller, I.; Kapral, R. Phys. Left. 1984. IOSA, 163. (3) Alamgir, M.;Epstein, I. R. J . Am. Chem. Sor. 1983,105,2500. Citri, 0.;Eptcin, I. R. J . Phys. Chem. 1988, 92, 1865.
reaction medium was 1.08 M H2S04. The CSTR was a 15-mL glass chamber containing a stirring bar, four input tubes, and platinum and saturated calomel electrodes, connected to a Beckman potentiometer and a Varian stripchart recorder. The functioning volume of the CSTR was 9.0 f 0.1 mL, and stirring (magnetic) was at the maximum pwible rate. Larger reaction chambers or slower stirring gave oscillations that were not as well defined. Buchler 2-1600 and COlePamer WZlRO31 peristaltic pumps were used. The [I]&, plane was generally scanned by varying pumping rate at constant [I-lo.
Results and Discussion Preliminary studies of the bromate40dide4and chlorit&xIid$ systems separately gave oscillations reasonably similar to those previously reported and in the same general regions of parameter space. The studies of Br03--c102--I- system by Alamgir, Citri, and Epstein3identified regions of oscillation at relatively low [I-], of 0.4-3.0 mM ([H+] = 0.75 M, [BrOq], = 2.5 mM, [C102-],, = 0.1 mM). At low [I-], and relatively lugh k,, (for conditions similar to those used in the previous studies) we observed periodic (or approximately periodic) waveforms with complicated shapes (Figure 2). Neither of the component chemical systems yield oscillations under these conditions. Removing either bromate or chlorite from the feedstream destroyed the oscillatory behavior. In this concentration range we sometimes observed spontaneous transitions between different complicated waveforms, each of which persisted for extended periods of time (Figure 2CJ)). We identify this region of oscillation (labeled OSC I in Figure 1) as the one that was previously observed. The low iodide region of oscillations (OSC I) separated two regions of parameter space in which CSTRs reached steady states. In the lowest iodide region, labeled SS I in Figure 1, steady-state reaction mixtures were colorless and electrochemical potential readings were low: in the higher iodide region (SS I1 in Figure 1) steady-state reaction mixtures were straw-colored or yellow and potential readings were higher. A computer simulation, using the previously published proposed mechanism2 and the Gear method6 satisfactorily generated results comsponding to SS I and ss 11. Relatively simple waveforms and several different sorts of combinations of simple waveforms were observed previously2at low [I-],,, and lower values of ko than those we employed. We did not observe these relatively simple, low iodide oscillations, but (4) Alamgir, M.;DeKepper, P.; Orban, M.;Eptcin, I. R. J. Am. Chem. Soc. 1983,105,2641.
(5) Dateo, C. E.; Orban, M.; DeKepper, P.; Eptein, I. R. J . Am. Chem.
SOC.1982, 104, 504.
(6) ODEPAC, double precision, graciously supplied by Dr. Alan C., Hindmarsh, of Lawrence Livermore National Laboratory, Nov 1990.
0022-365419212096-1014$03.00/0 @ 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1015
Letters
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io3bo(C) F i e 1. Behavior of the bromattchlorittiodide system in a continuously stirred tank reactor ([ClO,-]o = 0.10 mM, [BrO;]o = 3.25 mM, [H2S0,], = 1.08 M, T = 25 "C) as a function of pump rate and input iodide concentration (logarithmic scale). The dotted line corresponds to the region of oscillation reported in previous studies of this system: open lozenge, spiked complicated waveform (Figure 2A); closed lozenge, complicated waveform (Figure 2B-D); open circle, simple waveform (Figure 3A); half-filled circle, combination of simple and standard waveforms; fded circle, standard waveform (Figure 3B,F); fded triangle, diiodine precipitation, no oscillation;inverted open triangle, high-potential steady state; open triangle, low-potential steady state.
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Epgun 2. Oscillations in the low iodide region. Constraints as in Figure 1 and [I-I0 = 0.42 mM. (A) k0 = 21 X s-I; (B) ko = 35 X IF3s-l; (C) k0 = 44 X l F 3 s-I; (D) ko = 44 X s-l. Spontaneous change between two modes of oscillation is shown at D.
we did observe oscillations with quite simple waveforms at [I-lO of 15-20 mM (a concentration range higher than that covered in previous studies) and over a wide range of flow rates. During these high iodide oscillations the solution was yellow. At high [I-lO(OSC I1 in Figure 1) a single waveform (called the "standard" waveform) was observed over a range of flow rates
Figure 3. Oscillations in the high iodide region. Constraints are as in Figure 1. (A) [I-], = 15.0 mM, k0 = 30 X l F 3 s-l; (B) [I-I0 = 10.0 mM, k0 = 30 X l F 3 s-l; (C) [I-lO = 15.0 mM, ko = 15 X s-I; (D) [I-], PI; (E) [I-], = 12.5 mM, ko = 35 X = 15.0 mM, ko = 25 X s-I; (F) [I-lO = 7.5 mM, ko = 31 X 10-3s-1.
and iodide concentrations (Figure 3B,F). The standard waveform appears to be composed of two components. At lower flow rates and lower iodide concentration the wave tended to spread, while at higher iodide concentrations and flow rata the wave contracted and its shape became less complicated. (Figure 3A,C-E) Alamgir and Epstein reported that some waves they observed at lower were composed of two parts, one ascribable to the bromateiodide oscillator and one to the chlorite-iodide oscillator. In both oscillatory regions, ordinary waveforms (ca. 1 cycle/ min) were sometimes associated with a high-frequency signal (about 10-15 cycles/s)' (Figures 2A and 3F). No oscillations were observed when [I-],, was greater than 20 mM, but solid diiodine was clearly visible in such solutions. This region of [I-I0-k0 space is labeled IJs) in Figure 1. The previously proposed mechanism does not predict a second set of oscillations of concentrations in the high iodide region of [I-l0-kOspace. Computer simulations using this mechanism in this region of parameter space yielded only increase of diiodine to concentrations well in excess of the experimental solubility of 12. Autocatalytic precipitation of MnOz (involving a metastable form of Mn(II1)) has been proposed as important in the mechanism of permanganate oscillators.* It seems likely that the high iodide oscillatory region (OSC 11) in the bromattchlorittiodide system results from a nonlinearity, presumably autocatalysis, connected with precipitation of diiodine, combined with reactions like those included in the mechanism proposed for the low iodide region of oscillations (OSC I). (7) It seems likely that, as a referee has suggested, these more rapid oscillations are connected with the peristaltic pump employed. (8) Orban, M.; Eptein, I. R. J . Am. Chem. Soc. 1989,111,8543. 1990, 112, 1812. Mata-perez, F.; Perez-Benit, J. F. Can. J. Chem. 1985, 63, 988.
