J . Phys. Chem. 1990, 94, 2915-2921
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Evidence of Malonyl Radical Controlled Oscillations in the Belousov-Zhabotinsky Reaction (Malonic Acid-Bromate-Cerium System) Horst-Dieter Forsterling,* Szilvia Mursinyi,+ Fachbereich Physikalische Chemie, Philipps- Universitat Marburg, D- 3550 MarburglLahn, Federal Republic of Germany
and Zoltsin Noszticziusf Department of Physics, University of Texas at Austin, Austin, Texas 78712 (Received: July 5, 1989; In Final Form: October 27, 1989)
I n the original theory of the Belousov-Zhabotinsky (BZ) reaction it is assumed that the oscillations are exclusively controlled by bromide ions. We have observed oscillations in the malonic acid-bromate-cerium-sulfuric acid system that cannot be explained by bromide control. We conclude that the oscillations are controlled by malonyl radicals instead of bromide in these cases. A new model (“Radicalator”) is presented which is based on radical control of the BZ reaction.
Introduction In the classical Belousov-Zhabotinsky (BZ) system] Ce4+ is reduced by malonic acid (process 1); the resulting Ce3+ is oxidized by bromate during an autocatalytic reaction starting with the oxidation of Ce3+by BrO,’ radicals (process 22,3). Process 2 can be switched off by species reacting with Br02’ directly (like malonyl radicals formed in process 14) or with HBrO, (like Brformed in a reaction of Ce4+with bromomalonic acid5S6or directly from bromate’). Process 2 is switched on again as soon as the concentration of the control intermediate is lower than a critical level, and oscillationscan occur. Most authors believe that bromide ion is the only important control intermediate. Indeed, there are only a few examples in which the nature of the control intermediate is not completely clear. The most extensively studied system of this kind is the silver ion perturbed oscillator, which was first studied by Noszticzius;she observed that after the addition of silver ions into an oscillating BZ system oscillationsof the redox potential continue, contrary to the signal of a bromide selective electrode. He concluded that the observed oscillations are not bromide controlled. The chemistry in such a system is rather complex, however, and an explanation of all experimental facts has not yet been a ~ h i e v e d . ~The main problem in that case is that silver bromide precipitate is formed during the experiment and presently it is not clear how the kinetics of the silver bromide reaction and the subsequent formation of precipitate affects the dynamics of the oscillating reaction. To avoid such problems the discovery of non-bromide-controlled oscillations without a silver bromide precipitate would be highly desirable. Recently, Forsterling and Noszticius4proved that malonyl radicals (which are formed during the reaction of cerium(1V) with malonic acid) react with Br02’ (which is formed during the autocatalytic reaction step) at a nearly diffusion controlled rate. From this point of view it seems to be reasonable to include the malonyl radical as a possible control intermediate in the BZ reaction. It is well-knownlO-llthat one single oscillation occurs in some BZ systems immediately after the mixing of the compounds provided that oxygen is excluded from the solution. Oxygen can be considered as a scavenger of malonyl radicals, and really the single oscillation does not appear in solutions saturated with air or oxygen. With increasing malonic acid concentration more malonyl radicals are formed, and there is a greater chance to detect more pronounced radical-controlled oscillationsin such systems. In fact, in systems with a high malonic acid/bromate ratio in 3 M sulfuric acid oscillations have been
* Address correspondence and reprint requests to Prof. Dr. H. D. Forsterling, Physikalische Chemie, Fb 14, Universitat Marburg, HansMeerwein-Str., D-3550 Marburg, Federal Republic of Germany. ‘Permanent address: Institute of Inorganic and Analytical Chemistry, L. EotvGs University, Budapest, Hungary. 8 Permanent address: Institute of Physics, Technical University, Budapest, Hungary. 0022-3654/90/2094-2915$02.50/0
observed by Ricz12 which start without any induction period. Those systems will be analyzed in this paper.
Experiments Chemicals. H2SO4 (96%), NaBr, Ce(S04)2,and Ce2(S04)3 (all Fluka purissimum) were used without further purification. Malonic acid (Fluka purissimum, Serva p.A.) was purified by using a procedure proposed by Noszticzius et al.” (recrystallization from acetone/chloroform and from nitric acid/acetone). We were not able to perform the experiments at high malonic acid concentrations with the commercially available products; it turned out that traces of chloride impurities are able to inhibit the reactions performed at high malonic acid concentrations. On the other hand, in experiments at low malonic acid concentration (e.g., 0.1 M as usual in the BZ system) these impurities did not disturb the measurements at all. The N a B r 0 3 (Fluka purissimum) was recrystallized twice from hot water. All solutions were prepared from doubly distilled water. Apparatus. The kinetics of Ce4+ and of BrO,’ were followed spectroscopically (Ce4+by single beam technique at 401,433, or 450 nm, Br02’ by the dual wavelength methodI4 at 549 nm, reference 649 nm). The optical path length was 10.7 cm in a reaction cell with a volume of 140 mL; the solution was stirred by a magnetic stirrer. A homemade solid AgBr electrodels was inserted into the cell, and the potential was measured with a WTW DIG1 610 pH meter (reference 1 M KClsilver chloride electrode, connected to the cell by a salt bridge with sintered glass diaphragms on both ends of the tubing filled with 3 M H2S04). All three signals were measured simultaneously. The temperature ( I ) Oscillations and Traveling Waves in Chemical Sysrems; Field, R. J., Burger, M., Eds.; Wiley-Interscience: New York, 1985. (2) Field, R. J.; Forsterling, H. D. J . Phys. Chem. 1986, 90, 5400. (3) Forsterling, H. D.; Lamberz, H. J.; Schreiber, H. Z . Narurforsch. 1985, 400, 368. (4) Forsterling, H . D.; Noszticzius, Z. J . Phys. Chem. 1989, 93, 2740. ( 5 ) Field, R. J.; Koros, E.; Noyes, R. M. J . Am. Chem. Soc. 1972, 94, 8649. (6) Noszticzius, Z.; GBspir, V.;Forsterling, H. D. J . Am. Chem. Soc. 1985, 107, 2314. (7) Ruoff, P.; Varga, M.; Koros, E. Arc. Chem. Res. 1988, 21, 326. (8) Noszticzius, Z. J . Am. Chem. SOC.1979, 101, 3660. (9) Noyes, R. M.; Field, R. J.; Forsterling, H. D.; Koros, E.; Ruoff, P. J . Phys. Chem. 1989, 93, 270. (IO) Forsterling, H. D.; Lamberz, H.; Schreiber, H. Presented at the Discussion Meeting held by the Deutsche Bunsengesellschaft fur Physikalische Chemie, Aachen.1979. ( 1 1) Forsterling. H. D.; Pachl. R.; Idstein, H.; Schreiber, H. Z . Narurforsch. 1984, 39a,-993. (12) RBcz, K. Ph.D. Thesis, L. EotvGs University, Budapest, 1984. ( I 3) Noszticzius, Z.; McCormick, W. D.; Swinney, H. L.J . Phys. Chem. 1987, 91, 5129. (14) Forsterling, H. D.; Schreiber, H.; Zittlau, W. Z . Naturforsch. 1978, 330, 1552. (1 5 ) Noszticzius, Z.; Noszticzius, E.; Schelly, Z. A. J . Am. Chem. Ssoc. 1982, 104, 6194.
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within the cell was electronically controlled at 25 OC. It must be mentioned that commercially available AgBr electrodes (Orion, Radelkis) are sensitive to Ce4+, most probably due to a reaction of Ce4+ with the Ag2S which is part of the AgBr pellets in order to reduce their electric impedance and sensitivity to light. In our experiments the response of those electrodes to concentration changes of Ce4+ was much larger than their response to changes of the HOBr or bromide concentration in the same system. For this reason we used electrodes with pellets made from pure AgBr. In some experiments a reaction cell with optical path length of 2 cm and a volume of 15 mL was used; in this case no electrode measurements were possible, but a continuous gas stream could be applied through the solution during the measurement and the change from N, to O2and vice versa was much easier than in the 140-mL cell. Evaluation of Experimental Data and Numerical Methods. The analog signals of the measuring system were digitized by a 12-bit analog-digital converter, and the data were stored and evaluated in the memory of an Apple I1 computer connected to a Taxan plotter. The numerical integration of the kinetic differential equations was performed with the program DIFFGL, which is based on Gear's method,I6 by using an IBM PS/2 personal computer.
Forsterling et a].
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Kinetic Measurements A 140-mL volume of a mixture of malonic acid in 3 M sulfuric acid was bubbled with nitrogen gas (99.99% purity) outside the reaction cell for 15 min in order to remove dissolved oxygen. After that, the solution was transferred into the optical cell (path length 10.7 cm) supplied with a bromide-selective electrode. The potential of the electrode was recorded, and a solution of NaBr03 (dissolved in water) was added. The change of the electrode potential (most probably due to the formation of H B r 0 2 from acidic bromate) was recorded until a constant value was established after 5 min. Ce2(S04)3(dissolved in 1 M sulfuric acid) was added to start the reaction. Both the bromate and the cerium solutions were bubbled with nitrogen before the injection. During the measurement a stream of N, was applied above the solution in order to prevent contact with air. First we investigated a system described by Ricz.'* In that experiment we started with 140 mL of a 0.6 M solution of malonic acid in 3 M sulfuric acid; 2.1 mL of a 1 M solution of N a B r 0 3 in water was added, leading to an initial concentration [BrO
