J. Phys. Chem. 1996, 100, 9389-9394
9389
Photoresponse of the Ferrocyanide-Bromate-Sulfite Chemical Oscillator under Flow Conditions Akiko Kaminaga, Gyula Ra´ bai,† Yoshihito Mori,‡ and Ichiro Hanazaki* Institute for Molecular Science, Myodaiji, Okazaki 444, Japan ReceiVed: January 17, 1996; In Final Form: March 20, 1996X
The photoresponse of the ferrocyanide-bromate-sulfite system, known as the pH oscillator, has been studied in the visible region. We have established a state diagram for this system spanned by the flow rate and irradiation light power in a continuous-flow stirred tank reactor. Photoinduced oscillations are observed for the system in the low-pH steady state in the dark, while the photoinhibition of oscillations are observed for the system oscillating in the dark. Batch experiments for its subsystems (the SO32--BrO3--H+, Fe(CN)64-BrO3--H+ and SO32--Fe(CN)63--H+ systems) have revealed that the positive feedback channels for H+ (the SO32--BrO3--H+ and SO32--Fe(CN)63--H+ reaction) are not sensitive at all to the visible light, while the negative feedback channel (the Fe(CN)64--BrO3--H+ reaction) is responsible for the observed photosensitivity of the whole system. We have confirmed that the enhanced formation of Fe(CN)5(H2O)3from photoexcited HFe(CN)63- is responsible for the photoacceleration of the negative feedback process.
Introduction Many chemical oscillators are known to be sensitive to photoirradiation, showing the photoinduction and photoinhibition of chemical oscillations.1,2 The photosensitivity of inhomogeneous systems has also been reported which leads to the photoassisted pattern formation.3-5 It has been revealed that light is an excellent external control parameter for photosensitive systems since we can take an opportunity of easy control of the illumination power as well as the wavelength.1,6 The illumination effect has now been studied for many oscillatory reactions in the homogeneous systems such as the BelousovZhabotinsky (BZ) reaction,6-8 the Briggs-Rauscher reaction,9,10 and the Bray-Liebhafsky reaction.11 The photoinduction and photoinhibition of oscillations in the Fe(CN)64--H2O2-H+ system has recently been studied thoroughly.12-15 It is known that the system consisting of bromate, sulfite, and ferrocyanide ions exhibits sustained oscillations in its pH as well as the high-pH and the low-pH steady states and the bistability between them at temperatures between 20 and 40 °C in a continuous-flow stirred tank reactor (CSTR).16 This system, classified to one of the pH oscillators,17,18 has been derived from the iodate-sulfite-ferrocyanide oscillator (the mixed Landolt reaction)19 by replacing iodate with bromate. Recent reports on the spatial pattern formation in the iodatesulfite-ferrocyanide system20,21 suggest the possibility of extending the study on photocontrolled spatial inhomogeneity to systems other than the BZ oscillator. The bromate-sulfiteferrocyanide system should also be an excellent candidate for studying the photoassisted pattern formation. In this work, we have studied the photoresponse of the bromate-sulfite-ferrocyanide system to establish fundamental aspects of its photoresponse which must be useful in the future study in its photocontrolled inhomogeneity. The threshold light intensity for the photoinduction and photoinhibition of oscillations for this system have been determined quantitatively and summarized in a state diagram spanned by the flow rate and † Permanent address: Institute of Physical Chemistry, Kossuth Lajos University, H-4010 Debrecen, Hungary. ‡ Present address: Department of Applied Chemistry, Nagoya Institute of Technology, Showa, Nagoya 466, Japan. X Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(96)00178-5 CCC: $12.00
the light intensity. We have an opportunity in this system to examine the photoresponse by dividing the system into three subsystems; namely, the H+-producing SO32--BrO3--H+ and SO32--Fe(CN)63--H+ systems and the H+-consuming Fe(CN)64--BrO3--H+ system. We have examined the effect of photoirradiation for each of these subsystems in a batch reactor to identify the primary photochemical process responsible for the photosensitivity of the present system. Experimental Section Reagent grade K4[Fe(CN)6]‚3H2O, Na2SO3, H2SO4 (Katayama Chemical), and NaBrO3 (Wako Pure Chemical) were used without further purification. Solutions were prepared immediately before each run in distilled water. In the CSTR experiments, they were pumped into a cell through four inlet tubes by a peristaltic pump (Eyela, MP-3). Initial concentrations (the concentration which would be realized in the cell if no reaction took place) were fixed at [Fe(CN)64-]0 ) 15 mM, [BrO3-]0 ) 75 mM, [SO32-]0 ) 90 mM, and [H2SO4]0 ) 7.5 mM for the CSTR experiments, while in the batch experiments, [H2SO4]0 ) 6.0 mM for the Fe(CN)64--BrO3--H+ subsystem and [SO32-]0 ) 15 mM for the SO32--Fe(CN)63--H+ subsystem were employed. In the batch experiments for the subsystem of SO32--BrO3--H+, the reaction was started by adding an appropriate amount of the bromate solution to the SO32--H+ solution. Similarly, in the batch reaction for the SO32--Fe(CN)63--H+ system, the ferricyanide solution was added to the SO32--H+ solution. For the Fe(CN)64--BrO3-H+ system, the ferrocyanide or bromate solution was added to start the reaction. The stock solution of Fe(CN)64- was continuously bubbled with the N2 gas (99.9%) to purge dissolved O2. In the batch experiments, the oxygen-free solution was kept under the N2 gas flow throughout the experiments. A double-jacketed acryl resin cell with an inner volume of 11 mL was thermostated at 35 ( 0.5 °C by circulating water through a thermostat (Eyela, NTT-1300). The cell was equipped with a combined glass pH electrode (Horiba, #6861-10C) and with quartz windows for light irradiation. Stirring was carried out with a Teflon-coated magnetic stirrer bar. The reaction was followed by monitoring pH with a pH-meter (Horiba, F-13). The absorption spectra were measured using a © 1996 American Chemical Society
9390 J. Phys. Chem., Vol. 100, No. 22, 1996
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Figure 1. Bifurcation diagram for the Fe(CN)64--BrO3--SO32oscillator. Pairs of open circles for a single k0 indicate the maximum and minimum pH values in oscillations. Closed circles give the steadystate pH value. The arrows indicate discontinuous transitions. T ) 35 °C.
spectrophotometer (Hitachi, U-3200). As a light source, we used a 500 W Hg lamp (Ushio, USH-500D) with a L40 filter (Hoya) which provides major lines at λ ) 405, 436, 545, and 577 nm. In some cases, a L42 filter (Hoya) was employed to eliminate the 405 nm line. In the batch experiments, infrared light was eliminated with a HA30 filter (Hoya) to avoid the undesired heating effect. Therefore, the main components are the 436 nm line for L42 and the 436 and 405 nm lines for L40. The intensity is regulated with a variable neutral density (ND) filter (Sigma, Σ78(2)U) driven by a stepping motor (Oriental, UPD534M-A) or a set of fixed ND filters (Hoya). The light power was monitored by a photodiode (Hamamatsu, S172305). The maximum light power (Pmax) at the incident cell window was 1.3 W.
Figure 2. Time profile of pH under illumination with λ > 375 nm. Panel A: k0 ) 0.36 × 10-3 s-1, P/Pmax ) 0.01 (a), 0.20 (b), and 0.30 (c), where Pmax ) 1.3 W. Panel B: k0 ) 1.52 × 10-3 s-1, P/Pmax ) 0.01 (a), 0.30 (b), and 0.40 (c). T ) 35 °C.
Results and Discussion The ferrocyanide-bromate-sulfite system is known as one of the flow-controlled oscillators which exhibit no chemical oscillations under the batch condition.22 Figure 1 shows a bifurcation structure of the system in a CSTR under the dark condition as a function of the normalized flow rate, k0 ≡ F/V, where F is the total flow rate (cm3 s-1) and V is the cell volume (cm3). When k0 increases, the system bifurcates from the lowpH steady state (SSL) to the oscillatory state (OSC). Upon further increase of k0, the system bifurcates again from the OSC to the high-pH steady state (SSH). A hysteresis is observed for the bifurcation between the OSC and SSH as indicated in Figure 1 with arrows. Response of this system to the visible light irradiation is illustrated in Figure 2. Figure 2A shows that the illumination of the system in the SSL causes a bifurcation into the OSC for the incident light power P beyond a critical value. On the other hand, when the system in the OSC at higher k0 is illuminated, it bifurcates into the SSH (Figure 2B). Hysteresis was again observed for the bifurcation between the OSC and SSH similar to that in the dark system. It has been reported that illumination of aqueous solution of Fe(CN)64- with visible light causes only photoaquation (1) as the primary process,23,24 while illumination with the UV light causes photo-oxidation (2), which gives Fe(CN)63- and the hydrated electron (e-aq).23
Figure 3. State diagram for the Fe(CN)64--BrO3--SO32- system, T ) 35 °C. Symbols: (O) oscillatory (OSC), (b) low-pH steady (SSL), ([) high-pH steady (SSH), and (]) bistable.
In the present work, we have restricted ourselves to the visible light irradiation to avoid any complication resulting from (2). Results in Figure 2 indicate that the light irradiation has a similar effect on the system behavior to the increase of k0 in the dark. To establish the relationship between P and k0 as external parameters, we have determined the light-induced bifurcation for various k0 values. The results are summarized in Figure 3 in the form of a state diagram drawn in the k0-P plane. As expected, the threshold values of k0 both for the SSL/ OSC and OSC/SSH bifurcations decrease as P increases. The mechanism of oscillatory reactions in the Fe(CN)64-BrO3--SO32- system in the dark has been discussed in the literature.16,25 Stoichiometrically, the overall scheme may be expressed as the composite of the positive feedback process for generating H+:
Fe(CN)64- + H2O a Fe(CN)5(H2O)3- + CN-
(1)
BrO3- + 3HSO3- f Br- + 3H+ + 3SO42-
Fe(CN)64- f Fe(CN)63- + e-aq
(2)
and the negative feedback process for consuming H+:
(3)
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BrO3- + 6Fe(CN)64- + 6H+ f Br- + 6Fe(CN)63- + 3H2O (4) In addition, there is a rapid equilibrium:
H+ + SO32- a HSO3-
(5)
The acid dissociation constant for HSO3- is so small (K ≈ 10-7 M at 35 °C)26 that most of H+ is trapped in HSO3- as far as SO32- is existing. In an abundance of SO32-, H+ regenerated by (3) is trapped immediately by SO32-, keeping the system pH high (SSH). At a low flow rate, SO32- will be consumed completely by (3) after some induction period since its external supply is not sufficient. Then, [H+] increases suddenly to bring the system to SSL. As k0 increases, more SO32- is supplied which, in corporation with the negative feedback channel (4), can decrease [H+] to the initial value. Oscillations start at an appropriate k0 value (k0L). As k0 increases, an increase of the external supply of SO32- tends to prevent [H+] from becoming high. At k0 ) k0H, consumption of H+ by the externally supplied SO32exceeds its regeneration by (3) even at the high-pH peak of oscillations, where the negative feedback (4) is not effective since ferrocyanide is consumed in the previous cycle of oscillations. On the other hand, when k0 is decreased back from SSH, ferrocyanide stays in its stationary concentration, which is sufficient to initiate (4) if there is any H+. Therefore it is necessary to decrease k0 down to k′0H (< k0H) to start oscillations. This explains the appearance of hysteresis qualitatively. For the positive feedback channel, Edblom et al.16 have proposed a detailed reaction scheme by which they succeeded in reproducing oscillations in their simulation calculation, while for the negative feedback process, they employed reaction 6 in their simulation calculation, which corresponds to an incomplete
BrO3- + 2Fe(CN)64- + 3H+ f HBrO2 + 2Fe(CN)63- + H2O (6) advance of (4). They assumed the direct reaction between BrO3- and Fe(CN)64- to be fast enough25,27 to serve as the negative feedback in this system.17 However, a more detailed analysis of available experimental data has lead Ra´bai and Epstein to propose an alternative scheme:25
Fe(CN)64- + H+ a HFe(CN)63-
(7)
HFe(CN)63- + H2O a Fe(CN)5(H2O)3- + HCN
(8)
Fe(CN)5(H2O)3- + BrO3- + 2H+ f Fe(CN)5H2O2- + BrO2• + H2O (9) Fe(CN)5H2O2- + Fe(CN)64- a Fe(CN)5H2O3- + Fe(CN)63- (10) 5Fe(CN)64- + BrO2• + 4H+ f 5Fe(CN)63- + Br- + 2H2O (11) Fe(CN)5H2O3- + Fe(CN)63- a Fe2(CN)116- + H2O (12) In the pH range 3-7, where this system exhibits oscillations, the protonation of Fe(CN)64- (reaction 7) takes place.28 A large equilibrium constant, K ) 1470 M-1, for (7)25 suggests that the major species in acidic solution is HFe(CN)63-. Reaction
Figure 4. Time profile of pH for the batch BrO3--SO32- reaction under the light irradiation with the 405 and 436 nm mercury lines. Pmax ) 1.3 W. The initial concentrations are [BrO3-]0 ) 75 mM, [SO32-]0 ) 90 mM, and [H2SO4]0 ) 7.5 mM. T ) 35 °C.
Figure 5. Time profile of pH for the batch BrO3--Fe(CN)64- reaction under the light irradiation at 436 nm. Pmax ) 1.3 W. The initial concentrations are [BrO3-]0 ) 75 mM, [Fe(CN)64-]0 ) 15 mM, and [H2SO4]0 ) 6.0 mM. T ) 35 °C.
8 is a slow, rate-determining step under dark conditions. In acidic solution, reaction 8 becomes irreversible since CNreleased by the reaction would escape out of the system as HCN. For the purpose of elucidating which process is responsible for the photosensitivity of the whole system, we have studied the photoresponse of subsystems in a batch configuration. Figure 4 shows the effect of light irradiation on the subsystem composed of SO32-, BrO3-, and sulfuric acid. It can be concluded that there is essentially no effect of light irradiation in this subsystem. On the contrary, a remarkable effect of light irradiation has been found for the subsystem composed of Fe(CN)6,4- BrO3-, and sulfuric acid (Figure 5). As the irradiation light power is increased, the pH of the reaction mixture rises faster. Therefore, we conclude that the negative feedback process, in which Fe(CN)64- is oxidized by BrO3with a consumption of H+, is responsible for the photosensitivity. To confirm the mechanism further and to determine which step in the negative feedback channel (7)-(12) is photosensitive, we have conducted some additional experiments under the batch condition. Figure 6 shows the time profiles of the optical absorbance at 460 nm for the neutral and acidic aqueous solutions of ferrocyanide with and without bromate measured under the dark condition. The absorbance without bromate increases slowly in the acidic solution (curve a) while it remains constant in the neutral solution (curve b). The increased absorbance should be due to Fe(CN)5(H2O)3- formed by (8),
9392 J. Phys. Chem., Vol. 100, No. 22, 1996
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Figure 6. Time profile of the absorbance at 460 nm for the Fe(CN)64-H2SO4 system under the batch condition at T ) 35 °C. [Fe(CN)64-]0 ) 15 mM. Broken curves were measured without BrO3-: (a) [H2SO4]0 ) 6 mM and (b) [H2SO4]0 ) 0 mM. Solid curves were measured with BrO3- ([BrO3-]0 ) 75 mM): (c) [H2SO4]0 ) 0 mM and (d) [H2SO4]0 ) 6 mM.
Figure 7. Absorption spectra taken in a 1 cm cell for the aqueous solution containing 2 mM K4Fe(CN)6 and 0.5 mM H2SO4 (solid curve) before and after the 120 min light irradiation at 436 nm. The difference is shown by a broken curve.
since this experiment has been conducted under the oxygenfree condition where no possibility exists for the formation of ferricyanide. Curve c is the result for the neutral solution with bromate, which is almost the same as curve b. The acidic solution with bromate (curve d) shows a quick rise of the absorbance due to the ferricyanide formation through reactions 7-11. The latter observation can be understood if we assume that the reaction with bromate proceeds exclusively through (9) and that the formation of Fe(CN)5(H2O)3- is extremely slow in the neutral solution where HFe(CN)63- is not formed. Figure 7 shows the absorption spectrum obtained for the acidic aqueous solution of ferrocyanide left for 2 h with light irradiation at 436 nm under oxygen-free conditions. The difference spectrum, which should be attributed to Fe(CN)5(H2O)3-, is very similar to that of ferricyanide in accordance with previous reports.23,24,29 The difference between curve a in Figure 6 due to Fe(CN)5(H2O)3- and curve d due to Fe(CN)63- can be understood by noting that six molecules of Fe(CN)63- are formed at the expense of one molecule of Fe(CN)5(H2O)3- in the reaction scheme (7)-(11), if we assume nearly the same absorption coefficient at 460 nm for both species and that the reaction of Fe(CN)5(H2O)3- with bromate is fast25 compared with the rate-determining step 8. The “0 min.” curve in Figure 7 corresponds to the absorption spectrum of Fe(CN)6.4- If the pH of the aqueous solution of
Figure 8. Panel A: Time profile of pH for the batch BrO3--Fe(CN)64reaction. Solid curves: the dark reaction preirradiated at 436 nm with P/Pmax ) 0.25, where Pmax ) 1.3 W. Broken curves: without preirradiation, but irradiated throughout the reaction at 436 nm with P/Pmax ) 0.25. Panel B: the results in A are converted to the time profiles in [H+]. Panel C: the initial rate of [H+] consumption determined from B plotted against the preirradiation time. P/Pmax ) 0.25 (O) and P/Pmax ) 0.50 (b). The initial concentrations are [BrO3-]0 ) 75 mM, [Fe(CN)64-]0 ) 15 mM. and [H2SO4]0 ) 6.0 mM. T ) 35 °C.
Fe(CN)64- is decreased to pH ≈ 2 by adding sulfuric acid and the solution is left for several hours under dark and oxygenfree conditions, the spectrum does not change at all. Since about 90% of Fe(CN)64- must be converted to HFe(CN)63- under this condition, we can conclude that the absorption spectrum of HFe(CN)63- is almost the same as that of Fe(CN)6.4- Therefore, as far as the absorbance in the 400 nm region is concerned, Fe(CN)5H2O3- or Fe(CN)63- seems to be more probable than Fe(CN)64- or HFe(CN)63- to be photoexcited to enhance the negative feedback channel. On the basis of the reaction scheme (7)-(11), photoexcited Fe(CN)5H2O3- would enhance the reaction if step 9 was ratedetermining. However, this seems to be in contradiction with the dark mechanism mentioned above, where step 8 is ratedetermining followed by the relatively fast reactions 9 and 11. To examine the role of photoexcitation of Fe(CN)5H2O3-, we have conducted the preirradiation experiments for the subsystem, Fe(CN)64--BrO3--H+, where the acidic aqueous solution of ferrocyanide is irradiated for some time before the reaction is started by adding bromate. Solid curves in Figure 8A show the time profiles for the acidic ferrocyanide solution in a batch reactor preirradiated at 436 nm with P ) 0.25Pmax (Pmax ) 1.3
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W). The solution is left in the dark after the reaction is started. The figure shows clearly that the rate depends on the time of preirradiation. The time profiles of [H+] calculated from Figure 8A are shown in Figure 8B. Figure 8C shows that the initial rate of decrease in [H+] obtained from the slope in Figure 8B as a function of the preirradiation time, τ. The rate increases in proportion to τ, suggesting that reaction 8 is enhanced by photoirradiation to accumulate Fe(CN)5(H2O)3- which determines the initial rate of reaction with bromate. Figure 8C also shows that the slope becomes twice as large for the irradiation at P ) 0.50Pmax, indicating that the preirradiation effect is determined simply by the total light energy (P × τ) injected into the solution before the reaction starts. This, in turn, suggests that the photoproduced Fe(CN)5(H2O)3- lasts at least for several minutes and is accumulated until the reaction starts with the addition of bromate. These results suggest clearly that the photoexcitation of HFe(CN)63- enhances step 8 to produce Fe(CN)5(H2O)3-. However, there still remains a possibility that the enhancement of step 9 by the photoexcitation of Fe(CN)5(H2O)3- may participate at least partly in the photoresponse in view of its appreciable absorption around 400 nm.25,30 In Figure 8A,B, the broken curves show the result of continuous irradiation throughout the measurement. The rate is much lower than the preirradiation case at the same light power (P ) 0.50Pmax) until most of H+ is consumed, though a slight enhancement is observed at the final stage. This suggests that the photoenhancement of (9), if any, has only a minor contribution to the whole process. In relation to this, it should also be noted that Fe(CN)5(H2O)3- can be accumulated appreciably only in the system without bromate. In the full system, where bromate coexists, relatively fast reactions 9-11 should keep Fe(CN)5(H2O)3- at a low concentration so that its photoenhancement is not effective. This system produces ferricyanide as one of the final products, which has also appreciable absorbance in the 400 nm region ( ) 1040 M-1 cm-1, at 420 nm).25 In some pH oscillators, the photoaquation of Fe(CN)63-,
Fe(CN)63- + H2O a Fe(CN)5H2O2- + CN-
(13)
as well as Fe(CN)64- has been claimed to be effective as the source of photoresponse.15,30,31 If (10) is reasonably fast in the dark, Fe(CN)5H2O3- would be regenerated from Fe(CN)5(H2O)2to enhance the overall reaction. We have examined this possibility by adding Fe(CN)63- to the Fe(CN)64--BrO3--H+ system in a batch reactor. As shown in Figure 9, the addition of Fe(CN)63- brings about a negative effect on both the irradiated and dark reactions, suggesting that at least one of the reactions 10 or 13 is too slow to enhance the overall reaction even under the light irradiation. The slight inhibition effect observed is most probably due to the backward reaction of (10),32 which decreases the concentration of Fe(CN)5(H2O)3-. In the irradiated case, the internal light filtering effect due to Fe(CN)63- may partly contribute to the negative effect.33 The above discussion leads us to conclude that the photoresponse is due to the light absorption by HFe(CN)63-. If sulfuric acid is dissolved into water at our typical concentration of [H+]0 ) 15 mM, it gives pH ≈ 2. Addition of 15 mM of ferrocyanide to this solution raises the pH to 3; namely, roughly 90% of ferrocyanide is protonated instantaneously. The absorption spectrum of ferrocyanide in the 400 nm region remains almost the same after the protonation, both exhibiting very low absorbance ( ) 4.73 M-1 cm-1 for ferrocyanide at 422 nm).30 Although the photosensitivity of HFe(CN)63- to the “white” light
(c) (b)
(a)
Figure 9. Effect of addition of Fe(CN)63- to the batch BrO3--Fe(CN)64- reaction: (a) dark, (b) irradiated at 436 nm, P/Pmax ) 0.25, (c) same as b except for P/Pmax ) 0.50. Solid curve: [Fe(CN)63-]0 ) 1.5 mM. Dotted curve: [Fe(CN)63-]0 ) 0. The initial concentrations are [BrO3-]0 ) 75 mM, [H2SO4]0 ) 6.0 mM, and [Fe(CN)64-]0 ) 15 mM. T ) 35 °C.
has been pointed out,30 it is rather striking that HFe(CN)63- is the main source of the photosensitivity in this wavelength region. In the above discussion, we have divided the whole system into the subsystems, SO32--BrO3--H+ and Fe(CN)64-BrO3--H+, and examined their photoresponse separately. In the actual system, there is a possibility of the reaction of Fe(CN)63-, which is produced in the course of reaction, with SO32- as
2Fe(CN)63- + SO32- + H2O f 2Fe(CN)64- + SO42- + 2H+ (14) To see if this process is photosensitive, the photoirradiation effect on the subsystem, SO32--Fe(CN)63- has been examined in a batch reactor. The result shows that, although the pH decreases gradually in the dark, possibly because of the incorporation of reaction 14, photoirradiation causes little effect on the behavior of this subsystem. Therefore, it is concluded that (14) is not a primary source of photoresponse of this system. It is also interesting to note that, if the rate of (10) is comparable with or faster than that of (9), a small amount of Fe(CN)5(H2O)3- formed by (8), either through dark or photoenhanced reaction, could act as a catalyst to consume Fe(CN)64and H+, since Fe(CN)5(H2O)3- consumed in (9) is regenerated by (10). If this were the case, the concentration of the catalyst Fe(CN)5(H2O)3- should increase gradually according to (8), which should increase the rate of proton consumption, introducing a remarkable nonlinearity into the reaction. Inspection of Figure 8B suggests, however, that the time profile for [H+] is rather close to a simple exponential decay: no clear indication of increasing rate is noticed. On the other hand, if the catalytic cycle is not effective, the rate should decrease much after the exhaustion of Fe(CN)5H2O3- produced by preirradiation. This is not observed in Figure 8. Further studies are required to evaluate the relative importance of the catalytic cycle. Summary and Conclusion In this paper, we have established the photoresponse of the ferrocyanide-bromate-sulfite chemical oscillator by determining a state diagram spanned by the irradiation light intensity and the flow rate. The nature of the photoresponse has also been clarified in detail; the observed photoresponse in the visible region is due to the photoexcitation of HFe(CN)63- which enhances the production of Fe(CN)5H2O3- and accelerates the
9394 J. Phys. Chem., Vol. 100, No. 22, 1996 negative feedback channel to modify the bifurcation structure. This result confirms the previous qualitative suggestion of enhanced production of Fe(CN)5H2O3- by daylight25 in a more quantitative manner. The state diagram (Figure 3) shows that all bifurcation lines (k0L, k0H, and k′0H) decrease as P increases. In the dark, the composite rate of the supply of SO32- and the negative feedback channel (6) competes with the rate of positive feedback (3). Since the rate of (6) increases as P increases, a lower supply of SO32- is sufficient to bring the system into OSC. Therefore, k0L decreases as P increases. Similarly, at the bifurcation point between the OSC and SSH, the balance between the positive and negative feedback channels and the SO32- supply is broken by the photoenhancement of the negative feedback channel (6) so that the k0H and k′0H decrease as P increases. The light-sensitivity observed in the present system seems to be interesting in view of the recent report of pattern formation in an analogous system with iodate.20,21 Our preliminary results have indicated that the photosensitivity is much higher for the bromate system than the corresponding iodate system, suggesting a possibility of photocontrolled pattern formation in this system. A more detailed analysis of the photoresponse of this system is on the way in this laboratory, as well as the examination of its possible application to the photocontrolled pattern formation. References and Notes (1) Hanazaki, I. J. Phys. Chem. 1992, 96, 5652. (2) Hanazaki, I.; Mori, Y.; Sekiguchi, T.; Ra´bai, Gy. Physica D 1995, 84, 228. (3) Kuhnert, L. Nature 1986, 319, 393. (4) Kuhnert, L.; Agladze, K. I.; Krinsky, V. I. Nature 1989, 337, 244. (5) Jinguji, M.; Ishihara, M.; Nakazawa, T. J. Phys. Chem. 1990, 94, 1226. (6) Vavilin, V. A.; Zhabotinskii, A. M.; Zaikin, A. N. Russ. J. Phys. Chem. 1968, 42, 1649.
Kaminaga et al. (7) Srivastava, P. K.; Mori, Y.; Hanazaki, I. Chem. Phys. Lett. 1992, 190, 279. (8) Sekiguchi, T.; Mori, Y.; Hanazaki, I. Chem. Lett. 1993, 1309. (9) Vanag, V. K.; Alfimov, M. V. J. Phys. Chem. 1993, 97, 1878. (10) Okazaki, N.; Mori, Y.; Hanazaki, I. Chem. Lett. 1993, 1135. (11) Sharma, K. R.; Noyes, R. M. J. Am. Chem. Soc. 1975, 97, 202. (12) Ra´bai, Gy.; Kustin, K.; Epstein, I. R. J. Am. Chem. Soc. 1989, 111, 8271. (13) Mori, Y.; Hanazaki, I. J. Phys. Chem. 1992, 96, 9083. (14) Mori, Y.; Hanazaki, I. J. Phys. Chem. 1993, 97, 7375. (15) Mori, Y.; Ra´bai, Gy.; Hanazaki, I. J. Phys. Chem. 1994, 98, 12968. (16) Edblom, E. C.; Luo, Y.; Orba´n, M.; Kustin, K.; Epstein, I. R. J. Phys. Chem. 1989, 93, 2722. (17) Ra´bai, Gy.; Orba´n, M.; Epstein, I. R. Acc. Chem. Res. 1990, 23, 258. (18) Luo, Y.; Epstein, I. R. J. Am. Chem. Soc. 1991, 113, 1518. (19) Edblom, E. C.; Orba´n, M.; Epstein, I. R. J. Am. Chem. Soc. 1986, 108, 2826. (20) Lee, K. J.; McCormick, W. D.; Ouyang, Q.; Swinney, H. L. Science 1993, 261, 192. (21) Lee, K. J.; McCormick, W. D.; Pearson, J. E.; Swinney, H. L. Nature 1994, 369, 215. (22) Luo, Y.; Epstein, I. R. AdV. Chem. Phys. 1990, 79, 269. (23) Ohno, S.; Tsuchihashi, G. Bull. Chem. Soc. Jpn. 1965, 38, 1052. (24) Ohno, S. Bull. Chem. Soc. Jpn. 1967, 40, 1765. (25) Ra´bai, Gy.; Epstein, I. R.; Inorg. Chem. 1989, 28, 732. (26) Wagman, D. D.; Evans, W. H.; Parker, V. B.; Schumm, P. H.; Halow, I.; Bailey, S. M.; Chiney, K. L.; Nuttall, R. L. The NBS Tables of Chemical Thermodynamic Properties, Selected Values for Inorganic & C1 and C2 Organic Substances in SI Units. J. Phys. Chem. Ref. Data 1982, 11, Suppl. No. 2. (27) Birk, J. P.; Kozub, S. G. Inorg. Chem. 1973, 12, 2460. (28) Jordan, J.; Ewing, G. J. Inorg. Chem. 1962, 1, 587. (29) Shirom, M.; Stein, G. J. Chem. Phys. 1971, 55, 3379. (30) Adamson, A. W.; Waltz, W. L.; Zinato, E.; Watts, D. W.; Fleischauer, P. D.; Lindholm, R. D. Chem. ReV. 1968, 68, 541. (31) Moggi, L.; Bolletta, F.; Balzani, V.; Scandola, F. J. Inorg. Nucl. Chem. 1966, 28, 2589. (32) Stasiw, R.; Wilkins, R. G. Inorg. Chem. 1969, 8, 156. (33) Asˇpergeˇr, S. Trans. Faraday Soc. 1952, 48, 617.
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