J. Phys. Chem. 1W2,96, 9083-9087 At the upper extremities of the temperatures studied (407-477 K), the cations remain more restricted in their motion in XAD-4 than in XAD-2 because of the smaller pore diameter of the former; at these temperatures, however, both cations and anions exhibit only relatively weak binding to the resins. At temperatures in the range 101-367 K, the cation probes exhibit only one type of spectrum, characteristicof a slow tumbling motion and suggestive of interactions with resin framework only. This behavior is in sharp contrast with the behavior of the anion probe of the earlier study from which spectral evidence was obtained for the more complex motions characteristic of a relatively complex environment. Only at the highest temperatures examined do the cations attain comparable freedom of motion. Thus, the data are consistent with a model in which the cations are "anchored" to the resin framework through their hydrocarbon chains and the anionic components oocupy an outer region within the resin pores with water occupying the polar ionic region around the ion pairs.
Acknowledgment. We gratefully acknowledge the support of King Fahd University of Petroleum and Minerals for this work and the help of Mr. M. M. Saleem in running the spectra.
9083
Rdstry NO. 509, 78859-35-5; 531D, 114199-16-5; XAD-2, 906005-3; XAD-4, 37380-42-0; (styrene)(divinylbenzene) (copolymer), 9003-70-7; polystyrene, 9003-53-6.
References and Notes (1) Hwang, J. S.; Lyle, S.J. J. Phys. Chem. 1990, 94, 8727. (2) Hwang, J. S.;Mason, R. P.; Hwang, L. P.; Freed, J. H. J. Phys. Chem. 1975, 79, 489. (3) (a) Goldman, S.A.; Bruno, G. V.; Freed, J. H. J . Phys. Chem. 1972, 76, 1858. (b) Freed, J. H. In Spin Labeling: Theory and Applications; Berliner, L. J., Ed.; Academic Press: New York, 1976; Vol. 1, pp 53-132. (4) Li, A. S.W.;Hwang, J. S.J. Phys. Chem. 1985,89, 2556. (5) Kovankii, A. L.; Vauerman, A. M.; Buchachenko, A. L. Vysokomol. Soedin., Ser. A 1971, A13, 1647. (6) Hwang, J. S.;Pollet, P.; Saleem, M. M. J. Chem. Phys. 1972,56,716. (7) Eastman, M. P.; Gonzalez, J. A. J. Phys. Chem. 1985,89,488. ( 8 ) Romanelli, M.; Ottaviani, M. F.; Martini, G. J . Colloid Interface Sci. 1983, 96, 373. (9) Martini. G. Colloids Sur/. 1984. 1I . 409. (10) Moore,'W. J. Physical chemistry, 4th ed.; Prentice-Hall: Englewood Cliffs, NJ, 1972; p 766. ( I I) Martini, G.: Ottaviani, M.F.; Romanelli, M. J. Colloid Interface Sci. 1987, 115, 87. (12) (a) Kivelson, D. J . Chem. Phys. 1957,27, 1087. (b) Kivelson, D. J . Chem. Phys. 1960, 33, 1094.
Primary Photochemical Processes of Light-Induced pH Oscillation in the Fe(CN)eC/H202System Yoshihito Mori and Ichiro Hanazaki* Institute for Molecular Science, Myodaiji. Okazaki 444, Japan (Received: May 18, 1992; In Final Form: July 29, 1992)
The primary photochemical processes of the light-induced pH oscillation in the Fe(CN),"/H2o2 system have been studied. Oscillation is induced when the flowing solution is irradiated with monochromatic light in the visible and ultraviolet regions. The wavelength dependence of the relative cross section for the photoinduction of oscillations has been determined to identify the primary photochemicalprocess. The cross section has also been determined for the irradiation of a pure solution of Fe(CN),+ or Fe(CN):- in a prereactor before mixing. On the basis of these results, it is concluded that both Fe(CN),+ and Fe(CN)63are the primary light absorbers. The photoexcitation of Fe(CN)6e and/or Fe(CN),'- facilitates the production of monoaquapentacyanoferrate(II), Fe(CN)5(H20)3-,which enhances the autocatalytic reaction process.
Introduction The photosensitivity of chemical oscillators has been a topic of considerable interest in recent years.l-Is Effects of light irradiation, including photoinhibition and photoinduction of oscillations, on various chemical oscillator systems have been investigated.'-'' The synchronization of chemical oscillation in the Briggs-Rauscher system with periodic light perturbation was 0bser~ed.l~ The effect of light irradiation on the target patterns in the Fklousov-Zhabotinskii reaction has also been reported.'3-'s We have recently studied the photoresponse of chemical oscillators to monochromatic light irradiation for the purpose of elucidating the primary photochemical processesS8-'I The light-induction or inhibition effect appears for the light power above a critical value. One of the authors has recently proposed to use the relative cross section, which provides a basis of identifying primary photochemical processes in photoassisted induction and inhibition of chemical oscillations." Rabai et al. have studied the light-irradiation effect on the Fe(CN),;-/H2o2 system, suggesting that HFe(CN)63- is the primary light absorber.' We have recently reexamined this system using monochromatic light irradiation and found that light absorption by Fe(CN)63-,as well as Fe(CN),+, induces chemical In the present paper, we intend to give a full account of our study on this system to solve the controversy. We employed here the monochromaticlight irradiation to determine the relative cross section for light induction of oscillation as a function of 0022-3654/92/2096-9083!§03.00/0
wavelength. The cross section was also determined for the preirradiation of the Fe(CN)," and Fe(CN),'- solutions. On the basis of these experimental results, a possible mechanism is proposed for the light induction of oscillation in this system, which accounts for several experimental results consistently.
Experimental Section All chemical reagents were of reagent grade and used without further purification. Potassium hexacyanoferrate(II), potassium hexacyanoferrate(III), and sulfuric acid were purchased from Katayama Chemical Industry, and hydrogen peroxide was purchased from Mitsubisi Chemicals. Water was purified by deionizing distilled water through a filtering system (Millipore). Aqueous solutions of these chemicals were stored in the dark. The hexacyanoferrate(I1) solution was bubbled by nitrogen gas to remove dissolved oxygen and stored under a nitrogen atmosphere. Experimental arrangements are shown schematically in Figure 1. Solutions of potassium hexacyanoferrate(I1) (and potassium hexacyanoferrate(III)),hydrogen peroxide, and sulfuric acid were separately fed into a CSTR (continuous flow stirred tank reactor) from the bottom by a peristaltic pump (EYELA, MP-32) through silicon or polyethylene tubes and overflowed from the top. The whole system was covered by a black sheet of paper to avoid undesired light irradiation. Temperature of the solution in the CSTR was maintained at 25 f 0.1 OC by contact with circulated water regulated by a thermoregulator (EYELA, "-1 100). The 0 1992 American Chemical Society
9084 The Journal of Physical Chemistry, Vol. 96, No.22, 1992
a
I
glass electrode
I
Mori and Hanazaki
A
Ti m e 'e r
it 1
inflow of reactants
b main CSTR
pre-CSTR
Figure 1. Experimental arrangements. (a) Irradiation of reaction mixture in the main reactor and (b) irradiation of one of the reactants in a prereactor.
present chemical oscillator is extremely sensitive to temperature variation; e.g., the system in a steady state at 25 OC starts oscillation when the temperature is raised by 2.0 OC. Therefore, in the present experiments, the temperature fluctuation of the solution was suppressed to 0.1 OC to ensure that the induction of oscillation is purely due to light irradiation. The increase of temperature of the solution by irradiation was also suppressed to less than 0.1 OC throughout the present experiment. The solution in the CSTR was stirred by a magnetic stirrer. The reactor was made of acryl resin and equipped with two quartz windows for the irradiation and the measurement of absorption spectrum. The monochromatic light with a bandwidth of 10 nm and a beam area of 8 X 4 mm2 was supplied by a pair of a monochromator and a 500-W Xe lamp of a fluorospectrophotometer(SHIMADZU, RF-502). The light intensity was controlled by varying the lamp current and was measured with the precision of f 1 pW using a photodiode (HAMAMATSU, S1723-05) at the incident window. Relative errors in the cross section values were estimated to be 3% at QR = 6.59 X s and 0.016% at uR = 2.14 X s. Absorption spectra were measured by a spectrophotometer (SHIMADZU, UV-2100) in the CSTR with an optical path length of 5 mm. The pH of the reaction mixture was measured with a precision of *0.001 pH using a glass electrode (HORIBA, 6366). We have observed the light-induced pH oscillation for two cases. In one case (Figure la), all reagent solutions were fed directly into the main reactor where the reaction mixture was irradiated. In the other case (Figure lb), one of the reagent solutions was
Time
Figure 2. Time profiles of the pH values of the reaction mixtures irradiated at (a) 420 and (b) 320 nm. The arrow indicates the start of irradiation. Initial concentrations are [Fe(CN),'Io = 3.30 mM, [H20210 = 50.0 mM, and [H2S0410= 0.900 mM. The residence time is 368 s.
fed into a prereactor, where the solution was irradiated and fed into the main reactor. The other reactants were fed directly into the main reactor. The volume and optical path length of the main and prereactors were 10.91 mL and 20 mm and 6.72 mL and 15 mm, respectively. Results and Discussion The pH oscillation is induced when the reaction mixture is irradiated by monochromaticvisible or ultraviolet light. Figure 2a shows a time profile of pH of the reaction mixture irradiated at 420 nm. The pH value stays at 5.00 in the dark. At the light power of 146 pW, it rises and stays at a value higher than 5.00. At 339 pW,it stays still at a higher value. At 397 pW, the pH value starts oscillation with a period of 167 s and an amplitude of ApH = 1.61. Figure 2b shows a time profile for the reaction mixture irradiated at 320 nm. A similar nonoscillatory shift of the steady value is observed at 60 and 87 pW. At 165 pW, it starts oscillation with the period of 216 s and an amplitude of ApH = 1.45. We have defined the critical light power, Poc,to be the minimum power of the incident monochromatic light which can induce oscillation." We measured the wavelength dependence of Poc in the range from 460 to 290 nm for the main reactor irradiation with initial reactor concentrations of [Fe(CN)6e]o = 3.30 mM, [H2O2I0= 50.0 mM, and [H$O4I0 = 0.900 mM, at the residence time of 368 s. The relative cross section, uR,was calculated using the following relation:I6 UR
= (hvD/Poc)[l
- exp(-2.303D)]-'
(1)
where D is the optical density of the reaction mixture at each wavelength and hv is the photon energy. The derivation of eq 1 has been given e1sewhere.l1 Briefly, it is a relative value of the cross section for photochemical production of an "actuator" species to induce oscillation. As discussed below, the actuator in the present case is thought to be Fe(CN)5(H20)3-. Figure 3 shows uR as a function of wavelength, together with the absorption spectrum of the reaction mixture at pH = 5.00. It was found that the absorption spectrum of the reaction mixture (curve a) is a sum of those for the 2.1 1 mM Fe(CN)63-solution (curve b) and the 1.19 mM Fe(CN)6e solution (curve c). A peak at 420 nm and a shoulder at 320 nm in the spectrum of uR coincide with those in the absorption spectrum of Fe(CN)63-. The coincidence suggests strongly that Fe(CN)63-is the primary absorber to induce oscillation. Ribai et aL5 have mentioned that the preirradiation of the Fe(cN)6e solution is effective. They took this result as evidence that the Fe(I1) species absorbs photons to induce oscillation. This seems to be in contradiction with our result. To clarify this point, we have determined uR' as a function of wavelength for the prereactor irradiation of the Fe(CN)64-solution. Figure 4 shows the result for the same initial concentrations and residence time as above. The wavelength dependence of the relative cross section,
pH Oscillation in the F c ( C N ) ~ ~ / H System @~
The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 9085
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Figure 3. Relative cross section for the monochromatic light irradiation of the reaction mixture in the main reactor. Initial concentration and residence time are the same as those in Figure 2. Open circles: relative cross section. Curve a: absorption spectrum of the reaction mixture at pH = 5.00. Curve b the 2.1 1 mM solution of Fe(CN),>. curve c: the 1.19 mM solution of FC(CN)~&.The path length of the cell for absorption measurements is 5 mm.
360
380 400 420 Wavelength / nm
440
460
Figure 5. Measured and calculated relative cross sections for the irradiation of Fe(CN)64-. Open circles: uRmeasured for the main reactor irradiation, same as those for Figure 3. Closed circles: uRtcalculated from uR' (in Figure 4) for the prereactor irradiation of Fe(CN),&.
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Figure 4. Relative cross section for the irradiation of FC(CN)6& in the prereactor. Open circles: relative cross section. Initial concentration and residence time are the same as thosc in Figure 2. Solid curve: absorption spectrum of the 9.90 mM solution of Fe(CN)6&.
uR', shown in Figure 4 coincides well with the absorption spectrum of Fc(CN)~&. This suggests that Fc(CN)~&can also be the primary absorber to induce chemical oscillation. These results suggest that both Fe(CN)d- and Fe(CN)6e can absorb light to induce oscillation. If we restrict ourselves to the longer wavelength region, the effect in the main reactor irradiation is most likely due to the light absorption by Fe(CN)63-,while both are effective in the ultraviolet region. For further confiation, the relative cross section, uRf,for the irradiation of Fc(CN)~+in the prereactor (Figure 4) and uR
350 400 Wavelength / nm
450
Figure 6. Relative cross section for the irradiation of Fe(CN):- in the prereactor before mixing. Open circles: relative cross section, uR', for the system, [Fe(CN)63]o = 0.25 mM, [Fe(CN)6C]o = 3.30 mM, [H202]o = 50.0 mM, [H2S0410= 0.900 mM, and residence time = 414 s. Solid curve: absorption spectrum of the 1.0 mM aqueous solution of Fe(cN)6'-.
measured for the irradiation of the main reactor (Figure 3) may be compared using the following relation:" aR'/uRt
= (L'/L)(F/Fx)
(2)
where uR' is the cross section measured for the prereactor irradiation and uRt is the hypothetical uRwhich would be obtained for the main reactor irradiation experiment if the same photochemical process were responsible for the main reactor irradiation as for the prereactor. L and L'are the optical path lengths of the main reactor and prereactor, respectively, Fx is the flow rate
300 Mori and Hanazaki
9086 The Journal of Physical Chemistry, Vol. 96, No. 22, 1992
250
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.
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an initial reactant in addition to the previous system and is found to show the oscillation under certain conditions. When we employ the initial reactor concentrations of [Fe(CN)2-Io = 0.25 mM, [Fe(CN)64-]o= 3.30 mM, [H,O2], = 50.0 mM and [H2SO4Io = 0.900 mM, and the residence time of 414 s, the system does not oscillate under the dark. Oscillation is induced when Fe(CN)63-is irradiated in the prereactor at light power above a critical value. The relative cross section has been determined in this way and is shown in Figure 6 as a function of wavelength. The figure shows that uR’ coincides well with the absorption spectrum of Fe(CN)63-. It is therefore confirmed that the photoexcitation of Fe(CN)b3-can indeed induce chemical oscillation. Figure 7 compares uR measured for the main reactor irradiation of this solution and UR+ calculated from UR’ measured for the prereactor irradiation of Fe(CN),’-. In a way similar to Figure 5, uR! is the relative cross section expected for the main reactor irradiation if the same process as for the prereactor irradiation of Fe(CN)63-occurred in the main reactor irradiation. Coincidence of the band at 420 nm again supports our conclusion that Fe(CN)t- is effective as the primary light absorber. Discrepancy in the shorter wavelength region would reflect the contribution of Fe(CN)64-. It is also to be noted that the absolute value is different between uR and uRt by an order of magnitude. This is presumably because some additional chemical reactions and/or decay processes take place before the solution irradiated in the prereactor reaches the main reactor, which have not been considered in the derivation of eq 2. Mbai et ai.’ proposed a set of elementary steps accounting for the chemical oscillation in this system. The scheme is illustrated in Figure 8, where the light-sensitivesteps for both of Fe(CN)6e and Fe(CN)63-have been included (see discussion below). They assumed that the production of monoaquapentacyanoferrate(II), Fe(CN),(H20)3-, via Fe(CN),4-
+ H+ a HFe(CN),3-
HFe(CN)63-+ H 2 0
-
Fe(CN)5(H20)3-+ HCN
(3) (4)
plays a key role in the photosensitivity of oscillation: step 4 is a slow process which cannot produce sufficient amount of Fe(CN)5(H20)3-under a certain condition. Light irradiation enhances (4) to induce oscillation. BASIC
/
A
Figure 8. The reaction scheme. Designation of reaction steps (M3, M4, ...) is after Ribai et aL5 See text for discussion of the autocatalytic process (AC). The photochemical primary processes are designated by hv.
The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 9087
pH Oscillation in the Fe(CN)64-/H202System On the other hand, our measurements of uR indicate that its wavelength dependence corresponds to Fe(CN)6& or Fe(CN)6*, not to HFe(CN)63-,which seems to be in contradiction to the mechanism given above. Jordan and Ewing" have reported the pH dependence of the ratio [HFe(CN)6*]/[Fe(CN)64-]. It takes a maximum value of 2.3 at pH = 3.8 and decreases to 0.15 at pH = 5 and 0.0015 at pH = 7. We measured absorption spectra of the 9.90 mM aqueous solution of Fe(CN)6e. In spite of an extremely large change expected for the concentration of HFe(CN)63-,no change of the absorption spectrum was observed between pH = 3.8 and pH > 7. This leads us to conclude that the absorption spectrum is almost the same for Fe(CN)6e and HFe(CN),%. It is therefore impmsible to identify spectroscopically which is the primary light absorber. Shirom and Stein1*have studied the photoaquation of Fe(CN),& for pH = 3.8-10.55. The photochemicalreaction product is identified spectroscopically to be Fe(CN)s(H20)*. The yield is independent of pH in this range though the ratio [HFeat pH (CN)63-]/[Fe(CN)6'] is 2.3 at pH = 3.8 and 4.2 X = 10.55 as estimated on the basis of Jordan and Ewing's data.I7 If we assume that either of step 3 or 4 is photosensitive, the quantum yield should depend on [H+]. The independence of quantum yield on pH indicates that neither (3) nor (4) is photosensitive. It is most likely that the following process is responsible: k(CN)6& + H20 -% Fe(CN),(H20)'CN-
+ CN-
+ H+ P HCN
(5)
(6)
as postulated by Shirom and Stein. The formation of Fe(CN)s(H20)* by ( 5 ) is independent of pH, accounting for Shirom and Stein's result. This scheme also accounts for our results. These considerations lead us to the conclusion that the primary absorber for the light-induced pH oscillation is Fe(CN)6". Our experimental results suggest that the light absorption by Fe(CN)63-can also induce oscillation. It has been reported that the ratio [HFe(CN),"]/[Fe(CN)6*] < 10[H+],Le.,
