J . Phys. Chem. 1994,98, 8392-8395
8392
Photoinduced pH Oscillations in the Hydrogen Peroxide-Sulfite-Ferrocyanide System in the Presence of Bromocresol Purple in a Continuous-Flow Stirred Tank Reactor Vladimir K. Vanag,+Yoshihito Mori, and Ichiro Hanazaki' Institute for Molecular Science, Myodaiji, Okazaki 444, Japan Received: March 2, 1994; In Final Form: May 16, 1994"
The H,O, - SO:-- Fe(CN)Fsystem, one of the p H oscillators, is shown to be sensitive to visible light. Addition of a p H indicator, bromocresol purple (BCP), to this system is found to acquire new nonlinear interactions due to the screening effect of the indicator. The oscillatory region in a state diagram spanned by the flow rate and the irradiation light power shifts and becomes broader by the addition of indicator. This behavior is also confirmed by a simulation calculation. Because of extremely high extinction of light a t 590 nm due to the indicator, it is expected that the system can be used for experiments on wave propagation and spatial patterns in thin films.
Introduction At present, many oscillatory reactions are known to be sensitive to visible light: the Ru(bpy)?-catalyzed Belousov-Zhabotinskii (BZ) reaction,l-3 the Briggs-Rauscher (BR) system,a and the Bray-Liebhafsky' reaction with molecular iodine as a photosensitive species. Recently, the photosensitive oscillatory H20?- Fe(CN)c reaction has been thoroughly studied.8-10The species sensitive to visible light (antenna) in this reaction is the ferricyanide Being convenient systems for the study of periodic perturbationsII or chemical image processing,12most of these photosensitive reactions, except for the BZ system, possess a serious drawback; namely, optical density variation in the course of oscillations does not exceed a few tenths of an optical density unit in a 1 cm cell (the only exception is the reaction of I; with starchl3). When these reactions proceed in films of 0.1-1 mm thickness, the change in color is hardly discernible. In this connection, a new group of oscillatory reactions, pH 0sci1lators~~J~ with corresponding pH indicators,16is of particular interest, because of appreciablecolor change due to the indicators. The requirements which a pH indicator should satisfy for this purpose are as follows: its pK should lie in the pH variation range of oscillators, and it should not change the reaction mechanism, for instance, not capture any radical or intermediate essential to pH oscillations. Studies on the photosensitive pH indicator-pH oscillator system bring forth an additional interesting feature to pH oscillators; changing its color, a pH indicator begins screening light, thus affecting the intensity of incident light. This may suppress or enhance oscillations, depending on the relative position of absorption bands of the pH indicator and photoantenna. The competition for light absorption by the various intermediates arising as a result of Lambert-Beer's law is the key element of the mechanism leading to multiple steady states even in such simple reactions as17J8 hv
A*B A+A*+2A In more complicated systems, such as oscillatory reactions, the screening effect may lead to more complex effects. Permanent address: Department of Photochemistry, N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 117421, Novatorov Street 7A, Moscow, Russia. * Abstract published in Aduance ACS Abstracts. August 1, 1994. +
0022-3654/94/2098-8392$04.50/0
The goal of the present paper is to establish both theoretically and experimentally the behavior of photosensitive pH oscillators with a pH indicator. As a pH oscillator, the hydrogen peroxidesulfite-ferrocyanide system19 was chosen, which by our assumptions should be as sensitive to light as the hydrogen peroxideferrocyanide reaction8s9 but, unlike the latter, should be less sensitive to the addition of pH indicator, because the radical pathways are not as important for it.
Experimental Section Analytical grade K4[Fe(CN)6].3H~0,NazS03.7Hz0, HzS04, and HzOz (Katayama Chemical) were used without further purification. Water was distilled twice. Hydrogen peroxide was stabilizer free. Solutions were prepared immediately before each experiment. A continuous-flow stirred tank reactor (CSTR) was fed by a peristaltic pump MP-3 (EYELA) from two stock solutions, the first containing a mixture of a 4 mM F e ( C N ) r with 8 mM SO:-and another containing a mixture of 0.8 HzS04 with 12.9 mM Hz02. A solution with SO:- was continuously bubbled through with Nz. The CSTR was a thermostatic photometric cell 2 cm X 2 cm in cross section equipped with a hermetically sealed combined glass pH electrode. A spare space of the reactor was 7 mL. Stirring was carried out with a Teflon-coated magnetic stirring bar which was 3 mm in diameter and 1.5 cm long. The rotation rate was 500-600 rpm. The reaction was followed by monitoring pH and absorbance with a homemade spectrophotometric setup. The accuracy of absorption measurement was 0.0024.0 1 units of optical density. The optical density could be measured up to D 3.5. All experiments were run at 25 OC. As a source of analyzing light, we used a 25 W iodine-cycle incandescent lamp stabilized in current to 0.01%, the light from which was incident on the cell after being passed through a heat filter and corresponding interference filters. As a source of irradiation, we used a 500 W Hg lamp, the light from which passed through a heat filter, a series of neutral density filters (ND1, ND20, ND50, and so on), and a B480 filter (see Figure l ) , which cut off UV light and light with wavelengths X > 600 nm (the transmitted light corresponded to a very weak absorption band of ferricyanide in this region). The intensity of analyzing light was several orders of magnitude smaller than that of exciting light. As pH indicators, we examined p-nitrophenol (pH = 5-7.6), phenol red (pH = 6.8-8.4), methyl red (pH = 4.2-6.2), and bromocresol purple (pH = 5.2-6.3), all from Katayama Chemical. Some indicators inhibited oscillations (e.g.,p-nitrophenol), prob-
-
0 1994 American Chemical Society
Photoinduced pH Oscillations
The Journal of Physical Chemistry, Vol. 98, No. 34, 1994 8393
10 min
Figure 1. Optical spectra of (1) B480 filter (transparency,right axis), (2) protonated form of bromocresol purple, (3) deprotonated form of BCP, and (4) ferricyanide;[BCP] = 1.85 X 10-4M,[Fe(CN)i;l = 2 mM in 0.3 M H2S04. Curves 2-4 are calculated for a cell with optical pathlength of 2 cm. 1007
0
0
0
0
0
Figure 3. Oscillations in [Fe(CN)k] (upper curve) and in pH (lower curve) in the H202- SO:-- Fe(CN)F !ystem under the flow conditions ko = 1.66 X le3 s-] and Io = 1.2 W. Initial concentrationsare the same as in Figure 2.
A
lO0l
* *
[u
A
A
ao -
F
-
X 60Y
.-
v)
- D * * *
c
A
A
al
Y
-
*
*
* A
A
**An
A
0 , A, A , ^ 0
I
2
40-
. I
,
I
4
6
,
*
A
A
I
8
-’
k0x1O3, s Figure 2. An experimentally obtained phase diagram for the H202- SO:- - Fe(CN)c oscillator in the k& plane. T = 100% corresponds to the B480 filter, and the integral intensity of blue light 10 = 6 W. 10 was decreased by adding neutral density filters. The concentrations used are as follows: [SO:-], = 4 X lo-’ M, [Fe (CN):], = 2 X le3M,(H20210 = 6.45 X le3M,[HzS04] = 4 X 1V M. (0)high pH (pH > 7) steady state, (*)oscillations,(A)low pH (pH
< 5) steady state, ( 0 ) bistability.
ably by capturing OH’ or HO.2 radicals, so that the pH of the system remained low (-4.0). Bromocresol purple (BCP) proved to be the most promising pH indicator. Its acidic form spectrum overlaps the ferricyanide absorption spectrum, while the spectrum of its basic form is stained in deep blue and shows practically no overlapping with the ferricyanide spectrum (see Figure 1). BCP in a quantity of 50 mg/250 mL was stirred in water for 2-3 h up to complete dissolution; the recommended preliminary dissolving in 2-3 drops of alcohol was omitted. After dissolving, the solution of BCP was mixed with the solution of Fe(CN)c and
so:-.
Results To study the dependence of the behavior of the H,O, SO:-- Fe(CN)E oscillator on the intensity of exciting light, we constructed a phase diagram (Figure 2) in the ko-lo plane, where ko is the reciprocal of residence time and Zo is the incident light intensity. As is seen from Figure 2, the system is rather sensitive to light. The oscillations may be either initiated or suppressed by light. Characteristic shapes of temporal oscillations are presented in Figure 3. In all of theexperiments, the concentrations of the solutions were close to those used by RBbai et al.:I9 [H202]0 = 6.45 X 10-3 M, [SO:-], = 4 X 10-3 M, [Fe(CN)c] = 2 X 10-3 M, and [H+] = 0.8 X 10-3 M. The mechanism of photosensitivity for a given oscillator is likely to correspond to the mechanism for the H202 -
Fe(CN)k Fe(CN),(H,O)’-
+ H,O
hv
Fe(CN),(H,O)’-
+ Fe(CN)F
-+
Fe(CN)k
Fe(CN),(H20)3-
+ + + -
+ CN-
(1)
Fe(CN),(H,0)3- (2)
+ H202
Fe(CN),(H,O)*-
Fe(CN)F
OH’
+ OH’ + OH-
Fe(CN)k
+ OH-
(3) (4)
OH-
H+
H,O
(5)
CN-
H+
HCN
(6)
The result of the light effect is the consumption of protons, which is opposite of the autocatalytic multiplication of protons (see eq 7 below). It is noteworthy that the action of light on the BZ reaction results in the formation of Br-, which also opposes the autocatalytic multiplication of HBr02. A phase diagram for the system with BCP is presented in Figure 4. Characteristic temporal oscillations are shown in Figure 5 . A comparison of the phase diagrams of the system with and without the pH indicator shows that the oscillatory region has become much broader and moved up along the ordinate. Such a result can be expected, since the intensity of light should be increased to compensate the light screened by the pH indicator.
8394
The Journal of Physical Chemistry, Vol.98, No. 34, 1994
Vanag et al. We added to this model the following photosensitive reaction corresponding to reaction 1 above:
hv
C
products
(15)
The reaction rate 015of reaction 15 is assumed to be proportional to the ferricyanide concentration and to the photokinetic factor a = (1 - exp(-2.3D))/D involving the absorption of light by all the components of the system,20-21Le.,
Figure 5. Oscillations in the deprotonated form of BCP, [In] (upper curve), and in pH (lower curve) in the H,O, - SO:- - Fe(CN)t BCP (pH indicator) system under the flow conditions (a) ko = 0.95 X l e 3 s-l, 10= 3 W; (b) ko = 2.38 X l e 3 s-I, 10 = 3 W;(c) ko = 2.38 X 10-3s-I,I~=6W;(d)ko= 3.57 X 1 0 - 3 s - 1 , 1 ~ =W. 6 Initialconcentrations are the same as in Figure 4.
It should be noted that the phase diagram without a pH indicator cannot be obtained from one with the pH indicator by means of a simple linear contraction of the latter. Besides, according to Figures 3 and 5, the periods of oscillations becomes 2-3 times shorter on addition of pH indicator. All these facts testify that additional nonlinear effects occur due to the competitive absorption of light by the pH indicator and ferricyanide.
Model Now let us try to simulate the effect of screening of light by the indicator on the basis of a known reaction scheme modified to account for our system. RAbai et al.19 have shown that three main reactions are necessary to construct the oscillatory behavior in the hydrogen peroxide-sulfite-ferrocyanide reaction. These are the oxidation of HSO; by H202 (reaction 7), the oxidation of Fe(CN)F by Hz02 (reaction 8), and protonation and deprotonation reactions (reactions 9 and 10):
- + + - +
+ H202+ HSOj 2Fe(CN)F + H202+ 2H’ H+
SO:-
SO:-
HSO;
SO:-
+XH +H
H+
-
2H
(1 1)
X+H+XH
(13)
XH-X+H
(14)
C = where A H202, X H = HSO;, H H+, B = Fe(CN):, Fe(CN)k, and X = SO:-.Steps 11-14 correspond to reactions 7-10, respectively. Among them, reaction 12 is obtained by dividing reaction 8 into the following two steps:
HFe(CN)i-
InH
(17)
Factor a varies betwen 1.5 and 0.2 during the course of the reaction. Since the pH indicator is existing in much lower concentration than all other species, our crude model ignores its effect on [HI. A set of equations in the final form looks as follows:
(10)
(12)
+ H+
-
(9)
B+H-+C
Fe(CN);f
+H
(7)
To model the effect of light in a pH oscillator-pH indicator system, we considered the simplest theoretical model for the H,O, - SO:- - Fe(CN)F oscillator suggested by Luo and Epstein:Is A
In
2 F e ( C N ) k + 2 H 2 0 (8)
H+
HSO;
H 2 0 + 2H+
where kl5 = k’lslo. In our case, the total absorbance D a t the irradiation wavelengths is defined as D = (ec[C] + q n ~ [ I n H ] ) I , where e, and ~I,,H are the molar extinction coefficients of ferricyanide and the protonated form of the pH indicator (InH), respectively, and I is the optical pathlength. For calculations we used the following valuesof €,and t l a averaged over theabsorption band of ferricyanide: tc = 1000 M-l cm-l and eInH = 40 000 M-I cm-I. The deprotonated form of BCP (In) absorbs practically no light in the fericyanide absorption band region. We also suppose that In and InH are in equilibrium with the equilibrium constant KI,(for BCP, PKIn 5.75-5.8):
-
+ (1 / 2 ) H 2 0 2
HFe(CN)iFe(CN)i-
(8’)
+ H20
and assuming reaction 8’ to be rate-determining.
(8’9
whereull = ~ I I [ H I [ X H I [ A I , ~=Ik1~[Hl[Bl,v13 Z = kl~[Hl[Xl, = k14[XH], and [InH] = [In]o[H]/(K~n+ [HI). The term - ~ 1 5 in the expression of d[H]/dt results from the assumption that reaction 15 produces CN- as in reaction 1 and CN- is rapidly consumed by reaction 6 in the acidic media with consumption of H+. A phase diagram calculated on the basis of reactions 11-15 is presented in Figure 6 in the krkl5 plane. The observed oscillatory behavior and the screening effect due to the indicator seem to be reproduced qualitatively in this diagram. The shift of the boundary between the oscillatory and high-pH steady states due to the addition of indicator can reproduce the observation reasonably. However, the shift of the boundary between the oscillatory and low-pH steady states remains almost unchanged, in contrast to the observation. This is due to the crudeness of the model, which may be improved in the future. However, for our present purpose, it is sufficient to obtain qualitative theoretical support for the photoinduced oscillatory and screening behaviors observed in this system. If the deprotonated form of the pH indicator is assumed to absorb light in the ferricyanide absorption band region, the calculated oscillatory region shrinks from that for the system without pH indicator (Figure 6). In this case, we used the 014
The Journal of Physical Chemistry, Vol. 98, No. 34, 1994 8395
Photoinduced pH Oscillations
With BCP, an amplitudeof optical density oscillations measured in a 2 cm cell at 590 nm could exceed 10 optical density units; Le., it is possible to obtain oscillations with an amplitude of not less than 0.5 optical density units in a 1 mm thin film by using BCP with a concentration of 1 V M. This would provide a new possibility for visual observation of spatial pattern formation in films of chemical oscillators other than the BZ system.
9
-
20 X
n
-K
x
15
Acknowledgment. We thank Dr. G. RBbai for helpful comments, suggestions, and a critical reading of the manuscript. References and Notes
5
2 2
2
.0 _ J
o
0
2
G
4
k,xlO',
0
s-'
Figure 6. Calculated phase diagrams for models 11-15 in the k 0 - k ~ plane. The concentrations and constants are as follows: [HI0 = 2 X M, [AI0 = 6.45 X 10-3 M,[B]o 4 X M, [XI0 4 X le3M; kii = 3.077 X 10' M-2 s-I, k12 = 2 X 102 M-' s-I, kl3 = 5 X 10" M-' s-', k14 = 3 X lo3 s-l. The hatched region with inclined lines indicates the oscillatory region with p H indicator (BCP), the hatched region with horizontal lines indicates the oscillatory region without p H indicator, and the hatched region withvertical lines indicates the oscillatory region with p H indicator, whose deprotonated form absorbs light. For the case of pH indicators, [Inlo = 10-4 M.
following relations: D = (tc[C] + t ~ ~ [ I n ] )[In] l , = [InloK~n/(K~n + [HI), eln = 40 000 M-I cm-I.
Conclusion
In this paper, it is shown that the H,O, - SO:- - Fe (CN); oscillator is photosensitive. In addition, by selection of an appropriate pH indicator, such as BCP, it is shown that additional nonlinear interactions appear in the photosensitive system due to the screening of light by the pH indicator, leading to the shift and broadening of the oscillatory region.
(1) Gdspdr, V.; Bazsa, G.; Beck, M. T. Z . Phys. Chem. (Leiprig) 1993, 264,43. (2) Kuhnert, L. Nature 1986, 319,393. (3) Sekiguchi, T.;Mori, Y.; Hanazaki, I. Chem. Lett. 1993, 1309. Srivastava, P. K.; Mori, Y.; Hanazaki, I. Chem. Phys. Lett. 1992,190,279. Jinguji, M.; Ishihara, M.;Nakazawa,T.J. Phys. Chem. 1992,96,4279;1990. 94, 1226. (4) Briggs, T.S.;Rauscher, W. C. J . Chem. Educ. 1973,50, 96. (5) De Keppex, P.;Boissonade, J. In Oscillations and Traveling Waues in ChemicalSystems; Field, R. J., Burger, M., Eds.; Wiley-Interscience: New York, 1985;p 287. (6) Vanag, V. K.; Alfimov, M. V. J . Phys. Chem. 1993,97, 1878. (7) Sharma. K.R.: Noves. P. M. J . Am. Chem. SOC.1975,97,202. (8j Rdbai, Gy.; Kustin,j
