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J. Phys. Chem. 1992, 96, 7331-7333 (10) Seetula, J. A.; Gutman, D. J. Phys. Chem. 1990, 94, 7529. (11) Seetula, J. A.; Russell, J. J.; Gutman. D. J . Phys. Chcm. 1990,112, 1347. (12) Richards, P. D.; Ryther, R. G.; Weitz, E. J. Am. Chem. Soc. 1990, 94, 3663. (13) Scctula, J. A.; Gutman, D. J . Phys. Chem. 1991, 95, 3626. (14) (a) Chen, Y.; Tschuikow-Roux, E.; Rauk, A. J. Phys. Chem. 1991, 95,9832. (b) Chen, Y.; Rauk, A.; Tschuikow-Roux,E. J. Phys. Chem. 1991, 95.9900. (1 5) Pople, J. A.; Head-Gordon, M.; Fox, D. J.; Raghavachari, K.; Curtis, L. A. J . Chem. Phys. 1989.90.5622. (16) Corbett, P.; Tarr, A. M.; Whittle, E. Truns. Furuduy Soc. 1963,59, 1609. (17) Tarr, A. M.; Coomber, J. W.; Whittle, E. Tram. Furuday Soc. 1965, 61, 1182.
(18) Coomber, J. W.; Whittle, E. Trans. Furuduy SOC.1966, 62, 1553. (19) Coomber, J. W.; Whittle, E. Trans. Faruduy Soc. 1966,62, 2183. (20) Ahonkhai. S. I.: Whittle. E. znt. J. Chem. Kincr. 1984., 16. 543. .(21) Pedley, J. B.; Naylor, R. D.; Kirby, S. P. Thermochrmk Dura of Orgunic Compounds, 2nd 4.;Chapman and Hall: London, 1986. (22) Hudgens, J. W.; Johnson, R. D., 111; Timonen, R. S.; Seetula, J. A.; Gutman, D. J. Phys. Chem. 1991, 95, 4400. (23) Kaufman, E. D.; Reed, J. F. J. Phys. Chem. 1963.67, 896. (24) Foon, R.; Tait, K. B. J . Chem. Soc., Furuduy Trum. 1 1972,68, 1121. (25) Leyland, L. M.; Majer, J. R.; Robb, J. C. Tram. Faruduy Soc. 1970, 66, 898. (26) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J . Phys. Chem. Ref. Datu 1988, 17, Suppl. No. 1. (27) Luke, B. T.; Loew, G. H.; McLean, A. D. J. Am. Chem. Soc. 1987, 109, 1307.
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Surface- Induced Stirring Effects In the Mn-Catalyzed Belousov-Zhabotinskii Reaction with a Mixed Hypophosphite/Acetone Substrate in a Batch Reactor John A. Pojman,* Herbert Dedeaux, and Dionne Fortenberry Department of Chemistry and Biochemistry, University of Southern Mississippi, Hattiesburg, Mississippi 39406-5043 (Received: January 6, 1992; In Final Form: June 2, 1992)
A manganese-catalyzed Belousov-Zhabotinskii (BZ) reaction with a mixed hypophosphite/acetone substrate was found to be remarkably sensitive to stirring in a batch reactor. The reaction d a t e d in a container with a gas/liquid interface (either air or nitrogen). However, when the reaction was stirred in a sealed flask with no interface, the system remained in the reduced state. When the slow stirring (about 100 rpm) was stopped, a pink color from Mn(II1) was observed to ascend from the Teflon-coated stir bar. When a glass stir bar was used, the system would oxidize much more slowly than with the Teflon one. A possible mechanism is considered in which bromine loss to the gas phase is crucial for oscillation. In the sealed system, oxidation occurs when the stirring is stopped because bromine can adsorb to the surface of the Teflon, allowing local oxidation which spreads throughout the system.
Oscillating reactions are known to be sensitive to the rate of stirring.'-' In continuous-stirred tank reactors (CSTRs) the stirring effects can occur because of imperfect mixing of the feedstream? However, in batch reactors two mechanisms have been proposed. The first mechanism involves the transfer of gases into and from the solution. Batch reactions of the Belousov-Zhabotinskii (BZ) reaction are sensitive to oxygen, and the rate of stirring affecp the amount of O2entrained into the ~ystem.'*~JO However, Sevcik and Adamcikovg found stirring effects even with an inert atmosphere.' Batch BZ reactions are sensitive to the transfer of bromine out of the system. Noszticzius and Bodiss demonstrated that a BZ system with an oxalic acid substrate (which cannot react with bromine) can oscillate if a stream of inert gas is bubbled through the solution and that the oscillating period and amplitude depend on the rate of gas The system would oscillate without a gas stream if acetone was present as a bromine removal agent.15 Lbpez-Tomb and SaguC observed sensitivity to the rate of stirring in the cerium/malonic acid BZ system when using a nitrogen atmosphere, which they attributed to the stirring affecting the rate of bromine loss.2 Noszticzius et al.3 reported stirring effects in both the batch and CSTR for the cerium/oxalic acid/acetone BZ system in a poly(methy1 methacrylate) reactor. The system would not start oscillating unless stirring was stopped. They proposed that bromine could be adsorbed by the reactor walls in the absence of stirring. The autocatalytic bromous acid productionlo(and oxidation of the metal ion catalyst) could start in the surface layer and then spread throughout the bulk when stirring was resumed. Noszticzius et ala4recently proposed that the rate of recombination reactions between very low concentration radicals could 0022-3654/92/2096-133 1$03.00/0
be affected by the rate of stirring (varying between 300 and 2000 rpm) in a batch reactor due to hydrodynamic turbulence. They proposed this mtchanism for the stirring effects in batch reactions of the BZ reaction (cerium catalyst with malonic acid substrate). We have observed interesting stirring effects with a modified version of the Belousov-Zhabotinskii reaction developed by Ouyang et al. in which the catalyst is manganese with a mixed substrate of hypophosphite and acetone.16 This system is noteworthy because of its lack of bubble formation. In this system, spontaneous oscillations would not occur unless a gas/liquid interface was present during stirring. If no such interface was present, then the system remained in a reduced state until stirring was stopped, at which time a wave of oxidation could be observed to spread from the Teflon-coated stir bar. Our o b servations support the mass transfer of bromine mechanism and not the radical recombination mechanism in this case. Experimental Section All chemicals, namely, acetone (Baker), MnS04-H20(Baker), NaH2P02(Sigma), concentrated H2S04(Fisher), and NaBrO, (Aldrich), were of reagent grade and used without further purification. The reaction solution was made with the following composition from reagent grade chemicals: acetone (0.33 M), H2SO4 (2.0 M), NaH2P02 (0.022 M), NaBrO, (0.0095 M), MnS04 (0.0095 M). Stirring was accomplished with a magnetic stirrer. All reactions were performed at ambient temperature (23
"C). The effect of a gas/liquid interface was determined by running the reaction in two different reactors. One reactor consisted of a 250-mL round bottom flask with a glass stopper and no gas/ liquid interface. The platinum electrode (Rainin) and Hg/HgS04 reference electrode (Rainin) were sealed into the sides of the flask. @ 1992 American Chemical Society
Pojman et al.
7332 The Journal of Physical Chemistry, Vol. 96, No. 18, 199‘2 Stmg
on
sllmng
on
Platinum No GaslLlquid lnterfam
Oxidized
s m g ofl
200 m V
sumng off
system is to r e m e bromine. The hypophosphite serve8 to reduce the Mn(II1). If a gas/liquid interface is present, then bromine can escape. However, in the sealed reactor, bromine cannot escape, and the system remains in the reduced state (Figure 1). This hypothesis is supported by the observation that injecting bromine into the solution forces the system into the reduced state. Following the Field-KBriis-Noyes mechanism for bromate oscillators,” oxidation of the manganese should occur via the following reaction: Br03-
+ HBr02 + 2Mn2++ 3H+
GaaRiquid hlcrface
00
30
60
90
Tune (nun)
Figure 1. The effect of a gas/liquid interface on the Occurrence of oscillations in the manganesc/hypophosphitt/acetone BZ reaction. The arrows indicate where the stirring was stopped and started in the sealed
system. The other reactor consisted of a 100-mL beaker open to the air. Both systems were stirred with Teflon-coated magnetic stir bars at approximately 100 rpm. The signals from the bromide selective electrode (Orion 9435) and the platinum electrode (Rainin) (both referenced to a Hg/ HgS04 electrode (Rainin) were digitized on a Strawberry Tree 12-bit A/D board and collected on a Macintosh IIcx computer every second. The platinum electrode signal from the sealed system was processed through a high-impedance instrumentation amplifier before being digitized. Mass spectral data were gathered on a Hewlett-Packard 5985 GC/MS system.
ReSdtS Figure 1 shows the potentiometric traces for the two systems. The stirred system with a gas/liquid interface oscillated with a 40-mV amplitude in the platinum electrode potential. However, in the sealed system without such an interface, no oscillations occurred with stirring. Instead, the system remained in a reduced state. If the stirring was stopped, the system oxidized with a change in platinum potential of 120 mV. The oxidation could be observed to start on the Teflon-coated stir bar and spread throughottt the solution. If a glass stir bar was used, the oxidation only occumd after several minutes without stirring. The oxidation was observed to begin on a small part of the bar. Oscillations occurred with a nitrogen atmosphere. It was not possible to determine if there was a difference between having an air or nitrogen atmosphere because of the sensitivity to the rate of stirring. The period shortened as the stirring rate increased. Analysis of gases in the headspace by GC/mass spectroscopy showed molecular fragments consistent with bromoacetone and bromine. Oscillations stopped (with the system in the reduced state) when pure bromine was injected into the headspace above a reactor sealed with a septum.
Discussion The bromine concentration is important to the Occurrence of oscillations. Menzinger et a1.6 proposed that bromine must leave the ferroin-catalyzed BZ system for oscillations, and therefore, a gas/liquid interface must be present. They also observed that bromine cannot readily adsorb onto glass. In our system, bromine was detected above the reactor which means it is escaping during the reaction. The presence of oxygen is not aential to oscillations because they occurred with either a nitrogen or air atmosphere. The hydrodynamic turbulence mechanism proposed by Noszticzius et aL4 is not operative in our experiments with the very low rate of stirring employed. Instead we invoke the foliowing mechanism to explain our observations. The bromine concentration must remain below a critical level for oscillations to occur. This phenomenon has been observed by Noszticzius et al. in the oxalic acid/acetone system.’ In fact, the role of acetone in the
-
+
2HBr02 + 2Mn3+ H20 (1)
This reaction can only occur when the bromide concentration is below a critical value. Eigen and Kustinl* determined that bromine is in a fast equilibrium with bromide: HOBr
+ Br- + H+
-
Br2 + H 2 0
(2) Thus, reducing the bromine concentration also reduces the bromide ion concentration. While Teflon is normally assumed to be chemically inert, bromine can adsorb to the surface. Nagy and Bazsa19 measured a 15%disappearance (over 2 h) of bromine (5.0 X lo4 M) from an acidic solution due to adsorption on a Teflon cuvette cap. They also observed that iodine production in the iodide-nitric acid reaction can be observed to occur first on a Teflon-coated stir bar. If bromine is adsorbed onto the Teflon surface, and the concentration of bromide is reduced below a critical level through reaction 2, then oxidation can occur. Bromine is continuously adsorbed onto the Teflon allowing HBr02 to be produced. On the basis of the work of Nagy and Bazsa,lg we can reasonably conclude that the small amount of bromine present would not saturate the Teflon over the course of an experiment. When stirring is stopped, the local concentration of HBrOz near the surface will increase because it will not be removed by the hydrodynamic flow caused by stirring. Because the oxidation reaction 1 is autocatalytic, it spreads throughout the entire solution via convection and diffusion with the stir bar surface acting as a nucleation site for the oxidation. It is not the Teflon itself which causes the oxidation but rather the locally reduced bromine concentration. This oxidation wave w a observed with the unaided eye (Mn(II1) is pmk while Mn(I1) is colorless). Glass stir bars are not effective m adsorbing bromine. However, scratches on the glass can adsorb bromine. With a glass bar, oxidation was observed to occur on a scratch on the surface and then to spread throughout the solution. However, the oxidation occurred after 4 min compared to the few seconds required with a Tefloh-coated stir bar. The adsorption of bromine on the scratch lowers the bromine concentration locally, allowing the autocatalytic oxidation to begin and then spread. However, this pnxxss required several minutes because of the much lower surface area for nucleating the oxidation. We further tested this proposed surface effect by adding shards of broken glass to a sealed system stirred with a glass stir bar. When stirring was stopped, oxidation was observed to begin on the sharp edges of the shards. Because Noszticzius et al. proposed that poIy(methy1 methacrylate) could adsorb bromine and affect the cerium/acetone/oxalic acid BZ ~ystem,~ we prepared a poly(methy1 methacrylate)-coated stir bar. In the sealed flask with no gas/liquid interface, the system remained in the reduced until stirring was stopped and then the oxidation began on the surface of the stir bar. CoaClUSiaa The effect of slow stirring on the appearance of oscillations in the manganese/acetone BZ reaction can be explained by including the loss of bromine from the system. Surfaces of Teflon and poly(methy1 methacrylate) can adsorb bromine and effect the oxidation of a closed system. Acknowledgment. We thank Istvin P. Nagy and Zoltgn Noszticzius for many helpful discussions and support from the National Science Foundation’s Mississippi EPSCoR program. We
7333
J. Phys. Chem. 1992,96,1333-7331 thank Kenneth McMurtrey for performing the mass spectral analysis and Emory Howell for a critical reading of the manuscript.
References and Notes (1) Farage, V. J.; Janjic, D. Chimia 1980, 34, 342. (2) Mpez-Tomb, L.; SaguC, F. J . Phys. Chem. 1991, 95, 701-705. (3) Noszticzius, Z.; Horsthemke, W.; McCormick, W. D.; Swinney, H. L. Stirring Effects in the BZ Reaction with Oxalic Acid-Acetone Mixed Substrate in a Batch Reactor and in a CSTR. In Spatial Inhomogeneities and Transient Behavior in Chemical Kinetics; Gray, P., Baras, G. N. F.,Borckmans, P.,Scott, S.K.,Eds.; Manchester University Press: Manchester, U.K., 1990; pp 647-652. (4) Noszticzius, Z.; Bodnir, Z.; Garamszegi, L.; Wittmann, M. J. Phys. Chem. 1991,9, 6575-6580. (5) Menzinger, M.; Jankowski, P.J. Phys. Chem. 1986,90, 1217-1219. (6) Menzinger, M.; Jankowski, P. J . Phys. Chem. 1986, 90, 6865.
(7) Menzinger, M.; Jankowski, P.J. Phys. Chem. 1990, 94,4123-4126. (8) Dutt, A. K.; Menzinger. M. J . Phys. Chem. 1990, 94, 4867-4870. (9) Farage, V. J.; Janjic, D. Chimia 1981, 35, 289. (10) Field, R. J.; Burger, M. Oscillations and Traueling Waves in Chemical Systems; Wiley: New York, 1985. (11) Sevcik, P.;Adamkcikovi, I. Chem. Phys. Lert. 1988,146,419-421. (12) Sevcik, P.; Adamkcikovi, I. J . Chem. Phys. 1989, 91, 1012-1014. (13) Noszticzius, Z.; Bodiss, J. J . Am. Chem. SOC.1979, 101, 3177. (14) Noszticzius, Z.; Stirling, P.;Wittmann, M. J . Phys. Chem. 1985, 89, 49 14. (15) Noszticzius, Z. M a w . Kem. Foly. 1979, 85, 330. (16) Ouyang, Q.; Tam, W. Y.; DeKepper, P.; McCormick, W. D.; Noszticzius. Z.; Swinney, H. L. J. Phys. Chem. 1987, 91, 2181-2184. (17) Field, R. J.; Koros, E.; Noyes, R. J . Am. Chem. Soc. 1972, 94, 8649-8664. (18) Eigen, M.; Kustin, K. J. Am. Chem. SOC.1962,84, 1355. (19) Nagy, I. P.;Baua, G. React. Kinet. Catal. Lert. 1991, 45, 15-25.
Analysis of the Transient Effect for a Bimolecular Fluorescence Quenching Reaction between Ions in Aqueous Solution A. D. Scully, S. Hirayama, Laboratory of Chemistry, Kyoto Institute of Technology, Sakyo-ku, Kyoto 606, Japan
D. Hachisu, and T. Tominaga* Department of Applied Chemistry, Okayama University of Science, 1 - 1 Ridai-cho, Okayama, 700. Japan (Received: February 12, 1992; In Final Form: May 13, 1992)
The influence of diffusion on the quenching of fluorescence from electronically excited 5,10,15,20-tetrakis(4-sulfonatopheny1)porphine by methylviologen in aqueous solution was investigated as a function of ionic strength using time-resolved fluorescence decay measurements. The resulting nonexponential fluorescencedecay curves were analyzed using the expression for the timedependent rate coefficient derived by Hong and Noolandi for bimolecular diffusion-influenced reactions between ions in solution. The results of this analysis indicate that this expression provides a satisfactory description of the kinetics for this reaction under the experimental conditions used in this work. Values for the reaction distance and the intrinsic reaction rate constant of 1.4 f 0.1 nm and (2.7 f 0.5) X 1O'O M-' s-I, respectively, were calculated on the basis of the results of this analysis.
Introduction The verification experimentally of equations derived from theory to describe the time dependence of the rate coefficient for diffusioncontrolled bimolecular reactions in solution, k(t), has been the subject of a number of recent publications.'-'* The measurement of fluorescence decay curves has been found to be an effective method for probing the influence of diffusion on chemical reactions. In particular, fluorescence decay data obtained using the technique of time-correlated single photon counting (TCSPC)'-5 for fluorophore solutions containing a known concentration of quencher, where both the fluorophore and quenching species are ionic, have been analyzed according to the equations for k ( t ) derived by either Flannery13 or Hong and Noolandi14 which are based on the DebyeSmoluchowski model with the C~llins-Kimball'~boundary condition (DSCK model), Analysis using the expression for k ( t ) derived by Flannery of data obtained by using TCSPC is not straightforward due to correlations between the fitted parameters3s4and it has been found necessary to fu at least one of the fitted parameters in order to recover physically realistic values for the other parameters. However, despite this severe limitation in the analysis of data, the results of recent e~periments~-~ indicate that this equation provides a satisfactory description of the kinetics of bimolecular reactions between ions in solution. The Hong-Noolandi expression, which is effectively a long-time approximation of the Author to whom correspondence should be addressed.
Flannery equation for k(t), has a form identical with that of the long-time approximation to the equation for k ( t ) derived for diffusion-controlled bimolecular reactions in the absence of Coulombic interactions between the reactants. Periasamy et al.' have demonstrated the general validity of the Hong-Noolandi expression for k(t) by analyzing fluorescence decay curves measured using TCSPC for a number of fluorescence quenching reactions between ions in aqueous solution. The breakdown of the DSCK model in the subpicosecond time regime has been proposed recently4s5based on the results of analysis using the Flannery equation for k(t) of fluorescence decay curves measured using the technique of fluorescence upconversion. Measurements using the technique of TCSPC of fluorescence decay curves for aqueous solutions of a tetraanionic porphyrin in the presence ofthe methylviologen dication over a range of ionic strengths are described in this report. The results of analysis of these decay curves are discussed in terms of the DSCK model for the kinetics of diffusion-controlled bimolecular reactions between ions in solution. Theory
The expression for the time-dependent rate coefficient for the quenching of an electronically excited ionic fluorophore, A*, by an ionic quencher, Q,that was derived by FlanneryI3 from the DSCK model is k(t) = a b exp(6t) erfc(ctl/*) (1) where
+
0022-3654/92/2096-7333$03.00/00 1992 American Chemical Society
