J . Phys. Chem. 1992,96,8441-8449 I,,isi,,/d[CF4] is the slope of a semilog plot of CO(a) emission signal vs added CF4. No evidence of CO(a) electronic quenching or vibrational relaxation by CF4was observed in our measurement. From an analysis of the semilog plots an upper limit of 5 X cm3 molecules-l s-I can be. placed on kF,CF,.
Conclusions The corrected CO(a,v’) population ratio for an unrelaxed N2(Ap’) reacting with CO(X,v’’=O) is determined to be 1.00:0.85 for v’ = 0 and 1, respectively. Our data show that a significant fraction of the signature emission observed from the N,(A) + CO(X) ET is the result of the N2(A,v’21) + CO(X) ET, as evidenced by the relative inefficiency of the N2(A,v’=O) + CO(X) ET process. The vibrational level population of CO(a) was found to be insensitive to collisions with CF4, k I 5 X cm3 molecules-l s-l, at least for v’ = 0 and l . Acknowledgment. We would like to thank Ms.Laura Pisani who assisted in this investigation. Ms. Pisani spent 10 weeks at Phillip’s Lab/GIM as part of the Wellesley College Dana Award for student research. Registry NO. Nz, 7727-37-9; CO, 630-08-0; CF4, 75-73-0. References and Notes (1) 272. (2) 1. (3) (4) (5)
Callear, A. B.; Wood, P. M. J. Chem. Soc., Faraday Trans. 1971,67, Meyer, J. A.; Setser, D. W.; Clark, W. G. J . Phys. Chem. 1972, 76, Slanger, T. G.; Wood, B. J.; Black, G. J . Phorochem. 1973, 2, 63. Dreyer, J. W.; Perner, D.; Roy, C. R.J. Chem. Phys. 1974,61,3164. Clark, W. G.; Setser, D. W. J . Phys. Chem. 1980, 84, 2225.
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Spectra of Diatomic Molecules, 2nd ed.; Van Nostrand Reinhold Co.: New York, 1950. (22) Lawrence, G. M. Chem. Phys. Lett. 1971, 9, 575. (23) James, T. C. J . Chem. Phys. 1971,53,4118. (24) Black, G.; Slanger, T. G. J. Chem. Phys. 1971, 55, 2164. (25) Fournier, J.; Mohammed, H. H.; Deson, J.; Vermeil, C.; Schamps, J. J. Chem. Phys. 1980, 73,6039. (26) Hinchfelder, J. 0.;Curtiss, C. F.; Bird, R.B. In Molecular Theory of Gases and Liquids, 1st ed.; John Wiley & Sons: New York, 1954. (27) Cvetanovic, R. J.; Singleton, D. L.; Paraskevopoulos, G. J . Phys. Chem. 1979.83. 50. (28) Young, R.A.; Van Volkenburgh, G. J . Chem. Phys. 1971,55,2990. (29) Langhaar, H. L. J . Appl. Mech. 1942, 6, ASS. (30) Marcoux, P. J.; Piper, L. G.; Setser, D. W. J . Chem. Phys. 1977,66, 121 ”“..
(31) Peterson, K. A.; Woods, R.C. J . Chem. Phys. 1990, 93, 5029.
Effect of Stirring and Temperature on a Belousov-Zhabotinskii-like Reaction in a Batch Reactor Arun K. Dutt**fand M. Menzinger Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S I AI (Received: June 10, 1992)
The effect of stirring and temperature on the BZ reaction is reported using gallic acid (C6H2(0H),COOH) as the organic substrate in a batch reactor. It is found that increased stirring increases both the induction and oscillation periods. The effect of stirring and temperature on oscillation amplitude does follow a regular order when the probe used is a microelectrode. An unusually large stirring effect on this oscillatory system in the low-temperatureregion is reported. The results give an indication of the qualitative role of stirring and temperature as bifurcation parameters in a Hopf bifurcation experiment.
Introduction For the past few years, there has been growing interest in the effect of stirring1-12in nonlinear chemical systems. Menzinger et aI.I3have interpreted the effects of stirring on the BZ reaction in terms of noiseinduced transitions mediated through exchange at the gas-liquid interface. It was suggested that the effect of stirring on the oscillations in a batch BZ reaction results from increased oxygen transfer from the gaseous phase to the liquid phase. D’Alba and Di Lorenzoi4proposed an oscillatory model for the BZ reaction based on phase-exchange kinetics and pulsating supersaturation of the solution with elementary bromine molecules. Sevcik et al.I5 recently studied a closed stirred batch BZ reaction and showed that the oscillation parameters depend on stirring even in an oxygen-free atmosphere. Lopez-Thomas et a1.I6 have shown that the effects of stirring strongly depend on the initial condition Author to whom correspondence should be addressed. Present address: Max-Planck-Institut fiir ErnahrunnsDhvsiolonie. - ‘ , Rheinlanddamm 201, D4600, Dortmund-1, FRG.
of the BZ mixture and on the detailed chemical dynamics inherent in each phase of the reaction. In this article we report a very strong effect of stirring on the BZ oscillating system in a batch reactor using gallic acid”-I9 (C6H2(0H)3COOH)as organic substrate and ferroin as catalyst specially at low temperature. It is found that the induction and oscillation periods and the amplitude of oscillation depend remarkably on the stirring rate and the temperature of the batch experiment. Some results of our experiment agree well with the work of Sevcik et al.I5 on BZ reaction using malonic acid as the organic substrate.
Experimental Section The experiments were conducted in a thermostated cylindrical batch reactor (diameter 2.8 cm)with appropriate concentrations of gallic acid, ferroin, and potassium bromate in 1 M sulfuric acid medium. The monitor was a datinum microelectrode (dimensions 0.25 X 0.25 mm) with referknce to a Ag-AgClsodium sulfate (saturated) The tips Of the were placed cm above the bottom of the reactor. The solution was stirred with
0022-3654/92/2096-~447%03.00/0 0 1992 American Chemical Society
Dutt and Menzinger
8448 The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 400
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Figure 2. Amplitude of oscillation as a function of stirring rate at three different temperatures; composition same as in Figure I .
4 7
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?io
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Figure 1. (a, top) Induction period (TI)as a function of stirring rate at three different temperatures; (b, bottom) time period of oscillation, To, as a function of stirring rate at three Werent temperatures. Composition of the reactor: [gallic acid] = 0.0347 M, [ferroin] = O.OOO4 M, [KBrO,] = 0.104 M, and [HlS04] = 1.0 M.
a Teflon-coated magnetic stirrer (dimensions 14 X 3.5 mm) at different constant stirring rates. The stirring rates were calibrated using an oscilloscope. The reactor was kept open, and gas exchange between air and the reacting chemicals was possible during the course of the reaction. The reacting solutions were prepared from analytical grade reagents. The experiments were done at three different temperatures. For each temperature the experiments were conducted at five different stirring rates (300,400, 500,650, and 833 rpm). For every set of experiments, the concentrations of the reacting chemicals were kept unchanged. The accuracy of the thermostat used was within iO.1 OC. The potentials were recorded on a x-t chart recorder. Result and Discussion The plots of induction period TI vs stirring rate at different temperatures are presented in Figure la. The induction period occurs because the system persists in an oxidized state until oscillations commence. Due to depletion of the reacting chemicals in a batch reactor, the oscillation period (ampliude) gradually increases (decreases) as the reaction progresses. Therefore, we present results by considering the period and amplitude of any particular oscillation in every experiment (the third oscillation in the present case) and have drawn the figures. The choice of either second or fourth oscillation also gives qualitatively similar results. The plots of oscillation period, To, vs stirring rate at different temperatures are shown in Figure 1b. The plots of oscillation amplitude vs stirring rate are shown in Figure 2. The
Figure 3. Potential traces of microelectrode at 15.5 OC for two stirring rates: (a) S = 300 rpm, TI = 16 min; (b) S = 833 rpm, T,= 22 min; the arrows indicate the initiation of oscillation; composition same as in Figure 1.
potential traces of Pt electrode during oscillation at initial concentrations are shown in Figure 3 at two different stirring rates (300 and 833 rpm) for 15.5 OC. The Figures 1-3 are reproducible within 5% in a separate run of the experiment. From Figure l b it is seen that at 31 OC,the oscillation period (To)increases by about 15% when the stirring rate is increased from 300 to 833 rpm; but at 15.5 OC this increase of Tois about 57% due to the same increase of stirring rate. This tremendous stirring effect at low temperature is a new feature in the kinetics of the BZ reaction using gallic acid as organic substrate. K6ros et a1.I8 reported that the oxidation of gallic acidIg by bromate in sulfuric acid medium exhibits a few oscillations even in the absence of any catalyst and subsequently reportedm a large number of aromatic polyphenols and amines that exhibit oscillations in the absence of any catalyst. These polyphenols and amines have at least one unblocked ortho or para position. Orban et aLZ'proposed a mechanism (OKN) for these uncatalyzed class of oscillators which is closely related to the FKNZZmechanism of the cerium ion catalyzed BZ reaction, and Herbme et al?3 made a computer simulation of the OKN mechanism. The present system under investigation has a subsystem gallic acidbromatesulfuric acid which belongs to the uncatalyzed class of oscillators and the mechanism of this subsystem is given by the steps of the OKN*' mechanism. If one considers adding ferroin catalyst to this uncatalyzed subsystem oscillator, the following points should be noted. The standard reduction potential of the ferriin/ferroin couple (1.06 V) is lower24than that of cerium(IV)/cerium(III) (1.44 V) in sulfuric acid medium. Therefore, it may be possible2e26 that ferroin, unlike cerium(II1). could directly be oxidized by some oxybromine species besides its reaction
Effect of Stirring and Temperature on BZ Reaction with BrOz radical and would generate a few more step in addition to the uncatalyzed OKNzl steps. Here we prefer not to make further mechanistic propositions because the information about this system from the chemical literature is not enough for this purpose. From the potential traces of Figure 3, it is seen that at high stirring rate the oxidizing phase continues for a longer time resulting in the increase of the oscillation period. We suggest that Br2 is related to the Br- production (via step K9 of ref 21) that eventually inhibits the oxidative phase of the reaction. The higher stirring rates allow Brz to transfer from the reaction mixture to the atmosphere faster by the gas-air interchange at the liquidair interface, delaying the production of Br- and the end of the oxidative phase of a cycle. This agrees with our experimental observation that at high stirring rates the maxima and the minima of the trace of electrochemical potential of Pt electrode is shifted to higher values. At low temperatures, chemical relaxation becomes much slower due to Arrhenius effect and the stirring effect is increased enormously. We also observe that, in the absence of ferroin, an uncatalyzed gallic acid system ends oscillations within a short time (we obtained maximum 7-8 oscillations in our experiment) and at the end of the cecillations, the system becomes stable at the oxidizing state. The presence of ferroin helps to continue oscillations for a long time. Ferroin enhances the overall rate of production of Br- ion which controls the oscillatory feedback loop compared to that in the uncatalyzed subsystem. Therefore, the switching of the oxidizing phase to the reducing phase presumably continues for a longer time, increasing the total number and time of oscillation. Figure 2 demonstrates that the local oscillation amplitude decreases with increase of temperature (this order was destroyedz8 when we did the experiment using a macroelectrode (dimension 9.5 X 0.6 mm) for amplitude measurements). In the low-temperature region the amplitude of local oscillation increases with the increase of stirring and attains an almost constant maximum value with a slight decreasing trend at high stirring rate. At high temperature (viz.3 1 "C), the amplitude of local oscillation initially increases with increase of stirring and then starts to decrease after it has attained a maximum value at an intermediatestirring rate. This result indicates that the amplitude of local oscillation is a function of both stirring and temperature. On the basis of the relative increase or decrease of the amplitude of local oscillation in every batch experiment, we point out that high stirring seems to shift this nonlinear chemical system further away from the Hopf point and high temperature closer to the same. Obviously we are quite away from the Hopf point and a controlled experiment in CSTR with continuous feeding of the reacting chemicals are appropriate for this kind of conclusion. The batch experiment
The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 8449 presented here only gives an indication regarding these ideas.
Acknowledgment. We are indebted to an anonymous referee for useful suggestions during revision. Registry No. Br03-, 15541-45-4; gallic acid, 149-91-7; ferroin, 14708-99-7.
References and Notes (1) Bar-Eli, K.; Haddad, S.J . Phys. Chem. 1979, 83, 2952. (2) Treindl, L.; Fabian, P. Collect. Czech. Chem. Commun. 1980,45, 1168. (3) Roux, J. C.; Rossi, A. Compt. Rend. Acad. Sci. (Paris) 1978,287C, 151. (4) Varadi, Z. B.; Beck, M. T. J. Chem. Soc., Chem. Commun. 1973, 30. (5) Vidal, C.; Roux, J. C.; Rossi, A. J. Am. Chem. Soc. 1980,102, 1241. (6) Trac, I.; Treindl, L.; Trac, A. Chem. Papers 1985,39, 147. Trac, I.; Treindl, L. Ibid. 1985. 39, 161. Trac, I.; Treindl, L. Ibid. 1985, 39, 175. (7) Rouff, P.;Noyes, R. M. J. Phys. Chem. 1989, 93, 7394. (8) Gyorgyi, L.; Field, R. J. J . Phys. Chem. 1992, 96, 1220. (9) Ruoff, P. Chem. Phys. Lett. 1982, 90, 76. (10) Kuhnert, L. Reacr. Kiner. Caral. Left. 1986, 31, 227. (1 1) (a) Dutt, A. K.; Menzinger, M. J. Phys. Chem. 1990,94,4867; 1991, 95, 3429. (b) Menzinger, M.; Dutt, A. K. J . Phys. Chem. 1990, 94, 4510; React. Kinet. Caral. Lett. 1990, 42, 419. (12) Nagypal, I.; Eptein, I. R. J. Phys. Chem. 1986, 90, 6285. (13) Menzinger, M.; Jankowski, P. J . Phys. Chem. 1986.90, 1217. (14) D'Alba, F.; Di Lorenzo, S . J. Chem. Soc., Faraday Trans. 1983,179, 39. (15) Sevcik, P.; Adamcikova, L. Chem. Phys. Left. 1988, 146, 419; J. Chem. Phys. 1990.91, 1012. (16) Lopez-Thomas, L.; Sagues, F. J . Phys. Chem. 1991, 95, 701. (17) Babu, J. S.; Srinivasulu, K. Proc. Indian Nail. Sci. Acad. (India) 1976,42A, 361; Bull. Chem. Soc. Jpn. 1976,49,2875; J. Chem. Soc., Faraday Trans. I 1911, 73, 1843. (18) KBros, E.; Orban, M. Nature 1978, 273, 371. (19) (a) Jwo, J. J.; Chang, E. F. J. Phys. Chem. 1989,93,2388. (b) Dutt, A. K.; Banerjee, R. S . Chem. Phys. Lett. 1983, 99, 186. (20) Orban, M.; Koros, E. J . Phys. Chem. 1978, 82, 1672. (21) Orban, M.; Koros, E.; Noya, R. M. J. Phys. Chem. 1979,83,3056. (22) Field, R. J.; Koros, E.; Noyes, R. M. J. Am. Chem. Soc. 1972, 94. 8649. (23) Herbine, P.; Field, R. J. J. Phys. Chem. 1980, 84, 1330. (24) Latimer, W. M. Oxidation Potenrials, 2nd ed.; Prentice-Hall: New York, 1952. (25) Noyes, R. M. J. Am. Chem. Soc. 1980, 102, 4644. (26) Keki, S.; Magyar, I.; Beck, M. T.; Gaspar, V. J . Phys. Chem. 1992, 96, 1725. (27) Epstein, I. R. Nafure 1990, 346, 16. (28) The kind of batch experiments presented here are not well-controlled like the experiments in a CSTR. So we decided to use also a microelectrode after we failed to get any insight from amplitude-measurement experiments using a huge macroelectrode (dimensions 9.5 X 0.6 mm). However, one should note that even in a batch reactor significant concentration gradients may exist (ref 27) between weakly and strongly stirred regions because of high nonlinearity in the systkm. We placed the tip of the microelectrode in a well-stirred region but the macroelectrode had regions of both strong and weak stirring which probably made it not good for amplitude measurements where the two successive stirring rates are not widely different.
