On the Problem of Bromide Control in a Tl3 +-Perturbed Belousov-Zhabotinsky Oscillator Szilvia Murányi
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On the Problem of Bromide Control in a Tl3 +-perturbed Belousov-Zhabotinsky Oscillator Szilvia Murányi Institute of Inorganic and Analytical Chemistry, L. Eötvös University, H-1518 Budapest, Hungary Zoltän Noszticzius Institute of Physics, Technical University of Budapest, Budapest, Hungary, and Center for Nonlinear Dynamics and the Department of Physics, University of Texas, Austin, Texas 78712 Z. Naturforsch. 47 a, 605-613 (1992); received January 9, 1992 It is shown that Tl3+ as a bromide removing perturbant of the BZ oscillators is superior to Ag + in several aspects. A most important advantage is that the upper limit of the actual Br" concentra- tion can be calculated from the potential of a bromide selective electrode in the presence of the corrosive Tl3+ ions. Experimental Potentiometrie traces of a bromide selective electrode in a Tl3 + perturbed BZ system show that the Br- concentration stays much below its critical concentration during the oscillations. These observations suggest that free Br" is not the critical intermediate formed during reduction of Ce4+ and responsible for the negative feedback associated with the oscillations. Introduction is below is critical value and it is "switched off" above that level. The main features of the mechanism of the classical While there is a general agreement in the literature Belousov-Zhabotinsky (BZ) reaction [1] - the oscilla- regarding the positive feedback loop, opinions are tory oxidation of malonic acid by acidic bromate cata- more divided concerning the negative one. The first lyzed by the Ce4+/Ce3+ redox couple - were clarified problems of the negative feedback loop came into by Field, Koros, and Noyes (FKN) nearly two decades light when Noszticzius [4] observed oscillations in the ago [2, 3]. According to their theory the oscillatory classical BZ system in the presence of a relatively high behavior is due to an interplay between a fast acting concentration (10-3 M) of silver ions. At first it was positive and a delayed negative feedback loop. The accepted [5, 6] that these oscillations are not con- positive feedback forces the system to leave its un- trolled by bromide especially because the bromide stable steady state and, after a certain delay, the nega- level is much below its critical value in the silver per- tive loop pushes the system back again. The above turbed system. Later on, however, a different inter- interplay results in sustained oscillations. In the FKN pretation was suggested by Ruoff and Schwitters theory the positive feedback is identified as the auto- [7, 8]. They pointed out correctly that bromide con- catalytic oxidation of Ce3 + to Ce4+ by acidic bromate trol is still possible even in the presence of silver ions where the autocatalytic species are bromous acid provided that the precipitation reaction (Rl) (HBr02) and bromine dioxide radicals (Br02). The + - negative feedback is accomplished by an inhibition of Ag + Br AgBr (Rl) the autocatalytic reaction. The inhibitory species are is a relatively slow process. The above assumption bromide ions (Br") produced with a delay in a reac- stirred some debate in the literature and initiated new 4+ tion of Ce with bromomalonic acid. As bromide experiments, as well [9-14]. Recent calculations by reacts with bromous acid controlling this way the Russo [15] based on the latest experimental data of autocatalytic cycle it is a so called control intermedi- Meisel and coworkers [16] show that there are still ate. According to the theory a "critical bromide con- problems in the modeling of the silver ion perturbed centration" can be defined. The autocatalytic reaction BZ oscillator. is "switched on" whenever the bromide concentration Another problem is the interpretation of the signal of the bromide selective electrode in the presence of Reprint requests to Prof. Zoltän Noszticzius, Institute of Physics, Technical University of Budapest, H-1521 Buda- silver ions. If (Rl) is fast and, consequently, the solu- pest/Ungarn. bility equilibrium holds, then there is no such a prob- 0932-0784 / 92 / 0400-0545 $ 01.30/0. - Please order a reprint rather than making your own copy. 606 S. Muränyi and Z. Noszticzius • The Bromide Control in a Tl3 +-perturbed Oscillator lern. In equilibrium the electrode potential can be cal- with theoretical curves based on the corrosion poten- culated either from the silver or the bromide ion tial theory [18, 19]. concentration as the product of those ionic concentra- tions is known. On the other hand, if (RI) is a slow process then the solution becomes supersaturated and Experimental Section - at least in theory - the ionic product might be much higher than its equilibrium value. In that case the Chemicals. All chemicals used in the experiments, electrode potential would not be an exact measure of H2S04 (96% Merck), KBr03 (Merck), Ce(S04)2 the bromide concentration of the solution. This would (Fluka), and malonic acid (Fluka), were of reagent be so because the silver ion perturbant is able to affect grade and were used without further purification. A the electrode potential directly not only via removing stock solution of Tl3+ was prepared following the bromide ions from the solution. Thus a higher than instructions of Koros et al. [17 b]. The solutions were equilibrium bromide concentration would not be de- prepared with distilled water. tected by a bromide selective electrode in the presence Apparatus. The overall redox potential was mea- of silver ions. (The membrane potential of the silver sured by a bright platinum electrode. The home made bromide membrane at the tip of the bromide selective molten AgBr type bromide selective electrode was electrode would be affected mostly by the silver ions produced following directions of Noszticzius et al. which are in a much higher concentration compared [20]. As a reference an HgS04/K2S04 electrode was to bromide ions in the silver perturbed system. It is used. To decrease potential drifts due to diffusion important to emphasize again: such a far from equi- potentials the reference electrode was placed into a librium situation is possible only if (RI) is slow for cell filled with 10 M NH N0 . That cell was con- some reasons.) In other words one cannot be abso- 4 3 nected to the reactor via a salt bridge (a tubing with lutely sure that the bromide level is really under its diaphragm on both ends) containing 1 M sulfuric critical value during the silver perturbed BZ oscilla- acid. tions. Measurements. Calibration of the bromide selective Regarding the problems discussed above it seems electrode for Tl3+ ions (Fig. 1) was carried out in highly desirable to find a perturbant which is able to magnetically stirred and in unstirred 1 molar sulfuric remove bromide ions without precipitate formation acid solution. and which does not affect the electrode potential di- In the oscillatory experiments (Fig. 2) at first solu- rectly. Tl3+ meets both conditions; it forms a strong tions of sulfuric acid, malonic acid, and bromate were but soluble complex with bromide ions and affects the mixed. After the electrode potentials reached a con- electrode potential only by changing the bromide con- stant value (approximately 5 min.), the reaction was centration of the solution and by reacting with bro- started by the addition of the catalyst. TI3 + was added mide ions coming from the AgBr electrode (corrosion only at the second oscillation after the induction pe- potential). Moreover the effect of Tl3+ on the BZ riod as it was done in experiments of Koros et al. [17]. system is similar to the effect of Ag + ; Koros and All experiments were performed at laboratory tem- coworkers [17] reported high frequency oscillations of perature (22 ± 1 °C) in magnetically stirred solutions. the platinum electrode potential in TI3 + perturbed BZ Potentiometrie traces were recorded on a two channel systems. Kipp and Zonen BD 41 recorder. In this work we study a TI3 + perturbed BZ system using a bromide selective and a platinum electrode simultaneously. We point out that while the bromide Results and Discussion concentration crosses its critical value in the normal Response of the Bromide Selective Electrode oscillations of the unperturbed system, this is not the for 773 + Ions case with the Tl3+ perturbed system. The bromide concentration stays much below its critical value dur- The experimental calibration curve for Tl3+ ions - ing the high frequency Tl3+ induced oscillations. To the electrode potential vs. the logarithm of Tl3+ con- support the above conclusions we also calibrate the centration - is depicted in Figure 1. (The reference potential of a bromide selective electrode with respect potential is the solubility limit potential. The latter to the Tl3+ concentration and compare our results was determined as the mean of two electrode poten- 607 S. Muränyi and Z. Noszticzius • The Bromide Control in a Tl3 +-perturbed Oscillator Electrode potential/mV Fig. 1. Experimental and theoretical calibration curves of an AgBr electrode; the electrode potential as a function of the corrosive Tl3+ concentration. Theoretical calculations are given in the Appendix. tials measured in 10_5MAg + and in 10-5MBr- but the silver ions which determine the electrode po- solutions respectively.) tential directly, there is a functional relationship be- The potential response of the AgBr electrode can be tween the silver ion concentration at the surface of the explained by the corrosion potential theory. Accord- electrode and the bulk concentration of the corrosive ing to that theory Ag+ or Br~ ions produced in a thallium ions. The functional relationship is given by corrosion process of the electrode determine the po- the corrosion potential theory [18,19]. The theory tential of the electrode. In the present experiments assumes that concentration profiles near the electrode Tl3+ ions react with the AgBr electrode and in this inside a thin-non-stirred boundary layer are deter- case there are silver ions among the reaction products.