J . Phys. Chem. 1986, 90, 4700
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Reply to the Comment on Blstabliity in a CSTR Explodator Model
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Sir: Huskey and Epstein have shown that the Explodator model can indeed show interesting forms of bistability in CSTR mode if our restriction @kE3> R is relaxed. This is an important result, especially as their calculated bistability is similar in form to bistability observed in some oscillating reactions other than the Belousov-Zhabotinskii oscillator. We wish to point out, however, that the Huskey-Epstein condition, Le. @kE3< R , means that the rate of removal of the intermediate Y by the CSTR flow is competitive with its autocatalytic formation in reaction E3. Thus for the calculated bistability to be observed, the CSTR residence time must be shorter, perhaps much shorter for a relaxation oscillator, than the period of CSTR oscillations in the same system. Thus we expect the Huskey-Epstein bistability to be a rather high flow rate phenomenon. Bistability is observed in the Belousov-Zhabotinskii oscillator in CSTR mode at residence times much longer than its basic oscillatory period.
Richard J. Field
Department of Chemistry University of Montana Missoula, Montana 5981 2 Received: July 17, 1986
Comment on “Heterogeneitiesand Stirring Effects in the Belousov-ZhaboUnsky Reaction” by M. Menzinger and P. Jankowskit Sir: Menzinger and Jankowski’ report small-amplitude irregularities observed with a platinum microelectrode in a ferroincatalyzed malonic acid Belousov-Zhabotinsky (BZ) reaction. A platinum macroelectrode under otherwise identical conditions showed a smooth undisturbed oscillatory behavior. These irregularities occurred primarily when the oscillator was in its reduced state (high bromide ion concentration), and Menzinger and
0 Figure 1. (A) Oscillatory behavior and irregular fluctuations of a ferroin-catalyzed methylmalonic acid (MeMA) BZ system recorded in Eugene with a platinum macro electrode (compare with Figure 1 of ref 2 ) . Initial concentrations: [H2SO4I0= 0.5 M, [MeMA], = 0.3 M, [NaBrO,], = 0.1 M, [KBr], = 0.1 M, [ferroin],, = 5 X lo4 M, temperature = 25 OC. 1, addition of ferroin; 2 , spontaneous transition from reduced to oxidized state; 3, beginning of the oscillatory period; 4, train of small-amplitude oscillations in the oxidized state; 5 , oxidation spikes. The system was open to the atmosphere. Reaction volume was 50 mL. (B) Same system as in (A) but recorded in Stavanger with a different experimental setup including different electrodes, magnetic stirrer, x-t recorder, and reactor. Reaction volume was 25 mL. Note that the direction of postive potential was opposite to that in (A). ‘This paper is No. 73 in the series “Chemical Oscillations and Instabilities”; No. 72 is Kedma Bar-Eli and Richard M. Noyes, J . Chem. Phys., in press.
0022-3654/86/2090-4700$01.50/0
n n n n o
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Br=m
Figure 2. (A) Phase diagram and oscillatory behavior of redox potentials of a “fresh“ untreated silver wire and a bromide ion selective electrode (Br--BE) in the malonic acid (MA)-cerium catalyzed BZ reaction. Note the irregularities when the system was in its reduced state. Initial concentrations: [H2SO4I0= 1.0 M, [MA], = 0.3 M, [KBrO,], = 0.1 M, [(NH4),Ce(N03)6]o = 2 X IO-) M, temperature = 25 OC. The system was open to the atmosphere. Reaction volume was 25 mL. (B) shows the same system as in (A) but recorded somewhat earlier and using a polished silver wire. Arrows indicate more negative potentials.
Jankowski’ interpreted them as due to local fluctuations in composition of the solution. In our study of the same title reaction with a different organic substrate2 using a platinum macroelectrode, we observed the same sort of irregularities at precisely the same portion of the cycle (Figure 1A). However, subsequent studies3 performed with another experimental setup showed no irregularities (Figure 1B). We therefore tend to believe that the small-amplitude irregularities we first observed were artifacts and were not related to concentration fluctuations in the bulk solution. We tentatively explained our irregularities as the result of response by the electrode potential to sparking on the brushes of the stirring motor.4 We now believe that surface properties of the recording electrode itself are the source of such fluctuations. In support of this interpretation, small-amplitude irregularities have recently been observed with “fresh” untreated silver wires (Figure 2A), while a (“fresh”) polished silver wire showed a smooth undisturbed behavior (Figure 2B).5 At present it is not clear what types of reactions are responsible for fluctuations of electrode potential in such a system. We are convinced that our2 fluctuations are artifacts, and we tend to believe the same interpretation is appropriate for the observations by Menzinger and Jankoswki.’ Acknowledgment. This work was supported in part by Grant No. CHE-8405518 from the National Science Foundation to the University of Oregon. Menzinger, M.; Jankowski, P. J. Phys. Chem. 1986, 90, 1217-1219. (2) Ruoff, P.; Noyes, R. M. J . Phys. Chem. 1985,89, 1339-1341. ( 3 ) Ruoff, P.; Noyes, R. M., to be submitted for publication. (4) Ruoff, P.; Noyes, R. M. J. Chem. Phys. 1986,84, 1423, footnote 26. (5) Schwitters, B.;Ruoff, P., unpublished results.
(1)
Peter Ruoff*
Department of Chemistry Rogaland Regional College Ullandhaug, 4001 Stavanger, Norway Department of Chemistry University of Oregon Eugene, Oregon 97403
Richard M. Noyes*
Received: May 12, 1986 0 1986 American Chemical Society
