Photo-response of the bromate-sulfite chemical oscillator with tris-(bipyridine)ruthenium(II) as a catalyst

Article abstract of DOI:10.1016/S0009-2614(00)00507-8

Photo-response of the chemical oscillator composed of BrO₃^-, SO₃^2- and Ru(bpy)₃²⁺ in the acidic aqueous solution was examined in a flow system. After establishing the dark state diagram, state diagrams have been determined taking P (the illumination light power) as one of the external parameters. In the P versus [SO₃^2-] plane, the bifurcation between the reduced steady state and the oscillatory or oxidized steady state was found to be independent of P, occurring at a fixed critical concentration of SO₃^2-. Qualitative discussions are given for this and other characteristic features in the system containing SO₃^2- as a reductant.

Full text of DOI:10.1016/S0009-2614(00)00507-8

16 June 2000
Ž
.
Chemical Physics Letters 323 2000 372–376
www.elsevier.nlrlocatercplett
Photo-response of the bromate-sulfite chemical oscillator with
ž
/
ž /
tris- bipyridine ruthenium II as a catalyst
N. Matsuyama, N. Okazaki, Y. Tanimoto, I. Hanazaki)
Department of Chemistry, Faculty of Science, Hiroshima UniÕersity, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan
Received 3 April 2000; in final form 18 April 2000
Abstract
Photo-response of the chemical oscillator composed of BrOy3 , SO32y and Ru bpy
2
3
q
Ž
.
in the acidic aqueous solution was
Ž
examined in a flow system. After establishing the dark state diagram, state diagrams have been determined taking P the
2
.
w
yx
illumination light power as one of the external parameters. In the P versus SO3 plane, the bifurcation between the
reduced steady state and the oscillatory or oxidized steady state was found to be independent of P, occurring at a fixed
critical concentration of SO32y. Qualitative discussions are given for this and other characteristic features in the system
y
containing SO32 as a reductant. q 2000 Elsevier Science B.V. All rights reserved.
1. Introduction
than the original BZ reaction and is appropriate for
elucidating detailed reaction mechanism.
Ž
.
The Belousov–Zhabotinsky BZ reaction is a
well-known redox chemical oscillator in which the
bromate ion and malonic acid are employed typically
as the oxidant and reductant, respectively, together
with an appropriate metal catalyst 1 . The minimal
bromate oscillator is an variation of the BZ oscillator
in which the reductant, malonic acid, is replaced
with the inorganic bromide ion 2 . This type of
chemical oscillator is often called as the inorganic
bromate oscillator. Another example of the inorganic
Photo-response has been studied in several types
w
x
of chemical oscillator systems 4,5 , including the pH
w
x
w x
oscillators 6–10 , the Briggs–Rauscher system 11 ,
w
x
and the BZ and related systems 12–22 . Effect of
light on the pattern formation has also been studied
w x
w
x
Ž
23–26 . Among them, the BZ system with tris- bi-
2q
.
Ž . Ž
Ž
.
.
as a catalyst is
3
pyridine ruthenium II Ru bpy
w x
especially light sensitive because of its intense ab-
w
x
sorption band in the visible region 12 . The
Ru bpy 2q-catalyzed minimal bromate oscillator has
Ž
.
2y
3
Ž
.
bromate oscillators utilizes the sulfite ion SO3 as
a reductant. Alamgir et al. have studied chemical
oscillations in the BrOy3 –SO32y system with the
w
x
also been reported to be sensitive to light 15–19 . It
will be interesting to examine how the
2q
BrOy3 rSO32yrRu bpy
system would respond to
Ž
.
Mn2q ion as a catalyst 3 . The chemical oscillators
3
w x
the illumination in the visible region.
without organic substance is more or less simpler
In the present work, we first examine the nonlin-
earity in the Ru bpy 2q-catalyzed BrOy3 rSO32yrHq
Ž
.
3
)
system under the dark condition, for which quantita-
Ž .
Corresponding author. Fax q81- 0 824-24-0731; e-mail:
tive state diagram has not so far been given. Then we
hanazaki@sci.hiroshima-u.ac.jp
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
Ž
.
PII: S0009-2614 00 00507-8

(
)
N. Matsuyama et al.rChemical Physics Letters 323 2000 372–376
373
examine its photo-response to see how the system
response is characterized by SO32y employed as the
reductant.
2. Experimental
Ž
.
Ž
Commercially available Ru bpy Cl2 P6H2O Al-
3
.
Ž
.
Ž
.
drich , NaBrO3 Katayama , Na2 SO3 Katayama ,
Ž
.
and H2 SO4 Katayama are reagent-grade and used
without further purification. The measurement is per-
formed in a continuous-flow stirred tank reactor
Fig. 1. Typical oscillation pattern in the dark observed at 25.08C
Ž
.
Ž
CSTR , with V the volume of reacting solution in
with k0 s5.59=10y3 sy1. Ru bpy
w
Ž
.
w
2q x
w
y x
0 s0.4 mM, BrO3 0 s
the cell s 12.0 mL and k0 s5.59=10y3 sy1
,
3
.
w
2y x0
90 mM, SO3
x
s 1.45 mM and H2 SO4 0 s0.60 M.
where k0 is defined as the total flow rate divided by
V. Hence the average residence time is ts1rk0 s
179 s. Three stock solutions containing BrOy3 , SO32y
,
and Ru bpy 2q, respectively, in acidic water provide
and SO3 0, with a constant value of Ru bpy
Ž
.
w
2yx
w
Ž
.
2qx0
3
3
the materials to the cell through a peristaltic pump
s0.4 mM, a state diagram is obtained as shown in
Ž
.
Eyela, MP-3 . Solution in the cell is stirred by a
Fig. 2, which takes a form of ‘cross-shaped’ diagram
w x
magnetic stirrer and is pumped out by an aspirator
1 . Two limiting curve representing the limits of
Ž . Ž
RSS the reduced steady state and OSS the oxi-
Ž
.
Yamato, mp-15 to keep the volume of the reacting
solution to V. In the following, the initial concentra-
.
dized steady state , respectively, are crossed at
w
yx
w
yx 2yx
w
tions, BrO3 0, etc., are defined to be the concentra-
tions in the cell calculated under the assumption that
no reaction took place. The temperature of the solu-
tion is kept at 25.0"0.18C.
BrO3 0 f85 mM and SO3 0 f0.9 mM. The left
Ž
side of the crossing point corresponds to BIS bista-
.
Ž
bility between RSS and OSS, while the right side to
OCS the oscillatory state . A similar diagram has
.
Illumination experiments are performed with a
been obtained by Alamgir et al. for the
500 W high-pressure mercury lamp USHIO UI-
Mn2qrBrOy3 rAsO33y system 3 . A characteristic
Ž
w x
.
501C . The light from the lamp is collected by a lens
feature in the present system is that the oscillation
w
2yx
into the cell through an ultraviolet-cut filter and a
ceases for SO3 0 )1.5 mM, independently of
w
Ž
.
yx0
band-pass filter 425–473 nm to restrict the light
BrO3
The effect of illumination is demonstrated in
2q
Ž
.
.
3
wavelength around the absorption peak of Ru bpy
w
2yx
w
yx
Light intensity is controlled with a combination of
Fig. 3 for SO3 0 f1.45 mM, BrO3 0 f90 mM,
Ž
.
w
Ž
.
2qx
neutral density filters Hoya . The maximum avail-
and Ru bpy
0 s0.4 mM. It is clearly shown that
3
able power is P0 s1.35 W.
increasing light intensity tends to depress oscillations
to bring the system to OSS. The illumination effect
is summarized in Figs. 4 and 5 as state diagrams
w
yx
3. Results and discussion
spanned by light intensity versus BrO3 0, and light
w
2yx
intensity versus SO3 0, respectively. It is clear also
from these figures that the illumination tends to
bring the system towards OSS.
Ž
.
In the following, temperature 25.0"0.18C and
y3
the flow rate k0 5.59=10 sy1 are kept constant
Ž
.
w
x0
w
yx0
was
throughout. H2 SO4 is also kept constant at 0.60
In constructing Figs. 2 and 4, BrO3
scanned for a fixed value of the other parameter
Žw 2yx0 2yx0
M.
.
w
First, we examine the system behavior under the
dark condition. Fig. 1 illustrates a typical oscillation
SO3 or PrP0 . In the case of Fig. 5, SO3
was scanned for a fixed value of the other parameter
w
Ž
.
2qx
w
yx
Ž
.
pattern for Ru bpy
0 s0.4 mM, BrO3 0 s90
PrP0 . In each case, existence of bistability near
3
w
2yx
w
yx0
mM and SO3 0 s1.45 mM. By varying BrO3
the boundary was checked carefully by watching the

(
)
374
N. Matsuyama et al.rChemical Physics Letters 323 2000 372–376
w
y x0
Fig. 4. State diagram spanned by the light power and BrO3
w
y x0
Fig. 2. The dark state diagram spanned by BrO3
w
2y x0
and SO3
.
.
w
2y x
SO3 0 s1.5 mM. The other constraints are the same as for Fig.
The other constraints are the same as for Fig. 1. The filled circles
and crosses represent the oscillatory and bistable regions, respec-
tively, while the open and filled triangles represent the oxidized
and reduced steady states, respectively.
2.
Ž .
Ž .
1 q 2 =2 gives the autocatalytic process with
respect to HBrO2:
2q
HBrO qBrOyq2Ru bpy q3Hq
Ž
.
3
2
3
bifurcation both in increasing and decreasing direc-
tions of the variable parameter. No bistability was
2HBrO q2Ru bpy 3qqH2O .
4
Ž
.
Ž .
The threshold for the initiation of autocatalysis is
Ž . Ž .
3
2
Ž
observed except for the BIS region in Fig. 2 marked
.
by crosses .
determined by the competition between 1 and 3
w
x
Main skeleton model for the nonlinear oxidation
27 ; autocatalysis should start if V1 s
y xw q x
w
x
w
xw
in the BZ type reaction is well known as 27
HBrO2 qBrOy3 qHq 2BrO2ØqH2O ,
k1 HBrO2 BrO3
w
H
is larger than V3 s
xw yxw qx
k3 HBrO2 Br
tion is regulated by a parameter defined by
H . Therefore the system condi-
1
Ž .
2q
Ru bpy qBrO2ØqHq Ru bpy 3qqHBrO2 ,
Ž
.
Ž
.
yxC
' k rk BrOy3
3
3
w
Br
The system stays in RSS for Br ) Br C , while
yx yxC
.
5
Ž
.
Ž .
1
3
2
Ž .
w
yx
w
w
yx
w
with the switching process,
it starts oscillations for Br - Br
.
In the original BZ system with malonic acid as a
reductant, the reduction process is composed of rather
HBrO2 qBryqHqs2HOBr .
3
Ž .
w
2y x0
Fig. 3. Effect of illumination on the oscillation waveform.
y x 2y x
Fig. 5. State diagram spanned by the light power and SO3
.
y x0
BrO3
w
w
w
BrO3 0 s90.0 mM and SO3 0 s1.45 mM. The other con-
straints are the same as for Fig. 2.
s 90 mM. The other constraints are the same as for
Fig. 2.

(
)
N. Matsuyama et al.rChemical Physics Letters 323 2000 372–376
375
3q
Ž
.
3
complicated organic reactions to reduce Ru bpy
An additional discussion may be required for the
RSSrOSS bifurcation under illumination. In Fig. 5,
the RSSrOSS bifurcation occurs at the same point
2q
Ž
.
to Ru bpy
as well as to produce Bry, which is a
key species to control oscillations through 3 . Alam-
3
Ž .
w x
Žw 2yx
.
gir et al. 3 listed some reaction steps inherent to the
SO3 0 f1.5 mM as for the RSSrOSC bifurca-
bromatersulfite oscillator with Mn2q. Among them,
tion independently of the light intensity. This can be
Ž .
the reaction step,
understood if the threshold like 7 operates for the
w
2yx
effect of changing SO3 0. On the other hand, the
RSSrOSS bifurcation in Fig. 4 depends on the light
intensity. This may be understood as follows; even in
RSS, HBrO2 is being produced more or less through
non-autocatalytic processes such as
HOBrqSO32y BryqSO42yqHq
6
Ž .
Ž
Ž
..
which corresponds to their R2 should be effective
to bring the system to the reduced side by reducing
yx
w
x
w
HOBr and increasing Br , bringing the equilib-
2q
2Ru bpy qBrOy3 q3Hq
Ž .
rium in 3 more to the right. Note that sulfite may
actually exist in protonated forms, such as HSOy3 or
H2 SO3, in the strongly acidic solution as employed
here. Even SO42y should partly take a form of
HSO4y. However these would not bring about any
essential problem into the qualitative discussion given
Ž
.
3
2Ru bpy 3qqHBrO2 qH2O .
8
Ž .
Ž
.
3
Ž .
HBrO2 produced in 8 is, however, consumed im-
Ž .
mediately by switching process 3 to prevent the
autocatalysis to initiate. Illumination enhances pro-
Ž .
here. The rate constant for 6 has been estimated as
Ž .
cess 8 to increase the production of HBrO2 , which,
k6 )108 My1 sy1, which could compete with pro-
in turn, enhances the consumption of Bry in 3 .
Ž .
9
cess 3 ; k3 s2=10 My2 sy1 3 .
Ž .
w x
Therefore, the autocatalysis would start at a lower
y
w
yx0
Ž .
In 6 , the rate of production of Br is propor-
value of BrO3 for increasing light intensity.
In the present letter, we have examined the effect
w
w
x
tional to HOBr , which is, in turn, roughly propor-
yx yx yxw 2yx
w
w
of stationary illumination on the BrOy3 rSO32yr
tional to BrO3 ; namely, Br A BrO3 SO3
.
2q
Ž .
Comparing this with 5 , we may get the critical
concentration of SO32y for the initiation of autocatal-
ysis as
Ž
.
Ru bpy
chemical oscillator system in a flow reac-
3
tor in a form of the state diagrams spanned by the
w
yx0
light power and BrO3 and by the light power and
w
2yx
SO3 0. We have also determined a state diagram in
SO32y A k rk
,
.
3
7
Ž .
Ž
C
w
yx0 2yx
w
1
the dark spanned by BrO3 and SO3 0. We
believe that we have established fundamental aspects
of the light sensitivity of the system. We have also
tried to give some discussions on the reaction mech-
anisms accounting for our observation. The role of
sulfite employed here as the reductant is interesting
in that it could produce HBrO2 by the reaction with
BrOy3 to enhance the oxidation of the system, as
well as it could reduce HBrO2 to produce Bry to
prevent the autocatalytic oxidation of the system.
Although the present discussion is only qualitative in
nature, it must be a starting point for a more eluci-
dated quantitative theoretical treatment, which should
clarify the twofold action of sulfite in more detail.
w
yx
which is independent of BrO3 . Although this dis-
cussion is rather intuitive without rigorous mathe-
matical foundation, we believe that a relation such as
Ž .
7 should correspond to the observed independence
of the RSSrOSC bifurcation line on BrO3
certain range of the latter.
w
yx0
in a
Under illumination, the RSSrOSC bifurcation in
w
2yx
Fig. 5 occurs at SO3 0 f1.5 mM, completely in-
dependent of the light intensity. Similarly in Fig. 4,
w
yx
the OSCrRSS bifurcation occurs at BrO3 0 f80
mM, nearly independently of the light intensity. This
Ž .
is understandable if step 1 is ideally rate-determin-
ing; the initiation of autocatalysis is controlled in the
same way as in the dark. The illumination excites a
2q
Ž
.
part of Ru bpy
to its excited state, increasing the
Ž .3
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.
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