Photoinduced reaction between chlorine dioxide and iodine in acidic aqueous solution

Article abstract of DOI:10.1021/jp010370p

Photoinduced reaction between ClO₂ and I₂ has been discovered under illumination with 460 nm lightband. The photochemical reaction has a variable stoichiometry in acidic aqueous solution because the induced disproportionation of ClO₂ to ClO₃^- and Cl^- competes with the oxidation of I₂ to IO₃^- by ClO₂ in the illuminated reaction mixture. The reaction rate depends on the light power of illumination and on the concentration of I₂, but it is independent of the concentration of ClO₂. It is also independent of the pH in the range of 0-2.0 and of the ionic strength in the range of 0.01-1.0 M. Reversible dissociation of I₂ has been identified as the primary photochemical process and rate-determining step in the mechanism. Reactive I atoms are considered to initiate fast reaction steps, leading to the formation of products through reactive intermediates such as IClO₂, ClO, IO, and HOCl. This mechanism is proposed for explaining the photoresponses of the CDIMA oscillatory reaction system to the illumination with visible light.

Full text of DOI:10.1021/jp010370p

J. Phys. Chem. A 2001, 105, 6167-6170
6167
Photoinduced Reaction between Chlorine Dioxide and Iodine in Acidic Aqueous Solution
Gyula Ra´bai* and Kla´ra M. Kova´cs
Institute of Physical Chemistry, The UniVersity of Debrecen, H-4010 Debrecen, Hungary
ReceiVed: January 29, 2001; In Final Form: April 16, 2001
Photoinduced reaction between ClO2 and I2 has been discovered under illumination with 460 nm lightband.
The photochemical reaction has a variable stoichiometry in acidic aqueous solution because the induced
-
-
disproportionation of ClO2 to ClO3 and Cl- competes with the oxidation of I2 to IO3 by ClO2 in the
illuminated reaction mixture. The reaction rate depends on the light power of illumination and on the
concentration of I2, but it is independent of the concentration of ClO2. It is also independent of the pH in the
range of 0-2.0 and of the ionic strength in the range of 0.01-1.0 M. Reversible dissociation of I2 has been
identified as the primary photochemical process and rate-determining step in the mechanism. Reactive I atoms
are considered to initiate fast reaction steps, leading to the formation of products through reactive intermediates
such as IClO2, ClO, IO, and HOCl. This mechanism is proposed for explaining the photoresponses of the
CDIMA oscillatory reaction system to the illumination with visible light.
Introduction
semibatch reactor.4 However, according to Horva´th et al.,6
iodomalonic acid plays only a minor role, if any, in the
Light illumination is a convenient tool for controlling
photosensitive chemical oscillations and related nonlinear
dynamical behavior in homogeneous solutions. In some cases,
it is also applied for external forcing spatial structures in
unstirred reaction-diffusion systems. The observed photo-
responses are hoped to provide general information about the
responses of complex systems to external stimuli and to help
us understand the mechanism of composite chemical reactions
responsible for the complex dynamics in such systems.1
The oscillatory chlorite ion-iodide ion-malonic acid reaction
system (CIMA)2 and some of its derivatives have long been
known to be sensitive to the illumination by visible and UV
light. Both the spatial structures3 and exotic solution kinetics4
exhibited by these reactions can be influenced by illumination.
Recently, a related chlorine dioxide-iodine-malonic acid
system (CDIMA) was also found to show sharp photoresponses
both in a batch5 and in a continuous-flow stirred tank reactor
(CSTR) under illumination with light in the wavelength range
of 380-620 nm.6 Today, it is the CDIMA system that is often
used to study Turing structures. Its light sensitivity may provide
a convenient tool for controlling stationary Turing structures
in spatially extended systems. An important task is deciphering
which of the composite reactions or species can be held
responsible for the observed photosensitivity. In other words,
what is the primary photochemical process? For this question
to be answered, photoresponses of composite reactions of the
complex system should be studied separately, if possible.
Chlorine dioxide, iodomalonic acid, and iodine are among
the major species of the CIMA and CDIMA systems, and they
all appear to absorb light in the wavelength range of 380-620
nm applied in the recent photoperturbation experiments.6 The
photolysis of chlorine dioxide both in gas phase and in water
has been studied extensively.7 Intermediates of its photolysis
(e.g., ClO) are expected to be involved in composite reactions
in the oscillatory system. Photoinduced disproportionation of
iodomalonic acid has also been investigated in details8 and was
assumed to be responsible for the peculiar photoresponses of
the iodomalonic acid-chlorite ion reaction observed in a
photoresponses, but the iodine-iodide ion-chlorine dioxide
subsystem is largely responsible for the response to the
illumination in the CDIMA reaction. They suggest that the
photodissociation of iodine induces some interaction between
I2 and ClO2. However, we are not aware of any previous kinetic
study of such a photoinduced reaction. On the contrary, it is
known that this reaction is very slow, at least in the dark,9 and
it has not been considered as a component process in a model
for the oscillatory CDIMA system.10 Our present preliminary
experiments, however, support Horva´th’s suggestion, as we
found that the rate of ClO2 consumption increased significantly
under illumination. The most effective light band is around 460
nm, which corresponds to the maximum of the iodine absorption
spectrum. Discovery of this photochemical reaction confirms
the hypothesis that the composite reaction between I2 and ClO2
may be the main source and the primary reason for the
photosensitivity of the oscillatory CDIMA system.
In this paper, we report on the investigation of the stoichi-
ometry and the kinetics of the photoinduced reaction between
I2 and ClO2. We suggest a simple mechanism, in which the
primary photochemical process is the dissociation of molecular
iodine that initiates the reduction and disproportionation of ClO2
to Cl- and ClO3-. Meanwhile, a part of iodine is oxidized to
IO3-. On the basis of our results, we also suggest a modified
mechanistic explanation for the photoresponses of the CDIMA
system.
Experimental Section
Materials. Iodine and sulfuric acid were used as received.
Chlorine dioxide gas was prepared from potassium chlorate and
oxalic acid in sulfuric acid11 and was purified by bubbling
through saturated sodium carbonate solution to remove CO2 as
well as traces of HCl and Cl2. The purified ClO2 gas was
dissolved in cold water. Stock solutions of ClO2 were acidified
to pH 2.0 with sulfuric acid and were stored in darkness in a
refrigerator. The ClO2 concentration was determined by spec-
trophotometry at 358 nm (ꢀ ) 1050 cm-1 M-1).9 Acidic aqueous
10.1021/jp010370p CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/30/2001

6168 J. Phys. Chem. A, Vol. 105, No. 25, 2001
Ra´bai and Kova´cs
TABLE 1: Consumed [ClO2]/Consumed [I2] at Different
Initial [I2]0’sa
[I2]0 × 104 M 1.45 1.97 2.86 3.07 3.94 5.08 6.07 7.11
4[ClO2]/4[I2] 2.91 2.79 2.62 2.58 2.48 2.35 2.29 2.23
a [ClO2]0 ) 4.0 × 10-4 M, pH ) 2.0, and integrated light power )
14.6 mW dm-3
.
solution of saturated I2 (at 25 °C) was prepared by dissolving
I2 crystals in water. Its pH was adjusted to 2.0 with sulfuric
acid, and its concentration was measured at 460 nm (ꢀ ) 740
cm-1 M-1).9
Methods. For the stoichiometry, the consumed chlorine
dioxide (∆[ClO2], usually equal to [ClO2]0) and the consumed
iodine (∆[I2]) concentrations were determined after 24 h of
illumination time. For the kinetics, the reaction was started by
mixing the acidic solution of I2 with the acidic solution of ClO2.
The mixture was poured into a spectrophotometric cell. The
cell was sealed with a stopper to avoid any loss of volatile ClO2
and I2 to the air. The light source was a 100 W tungsten lamp,
the light of which was led through a 10 cm water layer to filter
out the heat of the lamp. An interference optical filter was
employed to obtain a light band in a wavelength interval of
454-466 nm. In this range, neither iodomalonic acid nor ClO2
absorbs light. The only light-absorbing species is I2. The optical
cell (1.0 × 1.0 × 3.0 cm3) filled with the reaction mixture was
illuminated with the filtered light band. The illuminated surface
of the cell was 3 cm2, and the optical path length in the solution
was 1.00 cm. The light intensity entering the cell was measured
with a power meter. Integrated light power entering the reaction
mixture was calculated and was divided by the cell volume
(P, in milliwatts per cubic decimeter). The reaction was followed
spectrophotometrically by measuring the absorbance at 358 nm
as a function of the time of the illumination. The ClO2
concentration was calculated from the measured absorbance.
Figure 1. ClO2 concentration as a function of time under illumination
with 460 nm light. P ) 1.46 mW dm-3, T ) 25 °C, and [I2]0 ) 0.0 (a),
1.45 × 10-4 (b), and 5.08 × 10-4 M (c).
maximum at 353 nm (ꢀ ) 26 400 M-1 cm-1),12 and it would
disturb the spectrophotometric determination of I2 and ClO2.
Since the optimum pH for the oscillatory behavior in the
CDIMA system is below pH 2, the results above pH 2 would
not be of importance for the oscillatory system.
Our results suggest that not only reduction of ClO2 takes place
(reaction 1) in the presence of I2, but some induced or
spontaneous disproportionation of ClO2 should also be consid-
ered which may be described by eq 213
6ClO2 + 3H2O f 5ClO3- + Cl- + 6H+
(2)
-
Disproportionation of ClO2 is known to be very slow in acidic
solution in the absence of I2. But intermediates of the oxidation
of I2 are expected to induce the disproportionation process. The
measured stoichiometry is assumed to result from the competi-
tion between reactions 1 and 2. Contribution of reaction 1 is
always rather high, especially when the ratio [I2]0/[ClO2]0 is
high, and somewhat smaller when the ratio is the opposite. The
pure limiting stoichiometry indicated by eq 1 could not be
reached in our experiments because of experimental limitations.
Kinetics. The observed kinetics turned out to be surprisingly
simple. Figure 1 shows the measured typical concentration traces
of ClO2 as a function of time under illumination. A slow
decrease in [ClO2] could be observed even in the absence of
iodine (curve a) indicating that spontaneous disproportionation
or some slow unknown escape of volatile ClO2 takes place. In
the presence of iodine, however, the decay of chlorine dioxide
concentration was much faster. An important finding is that the
rate does not depend on the ClO2 concentration. This is
concluded from curve c in Figure 1, where the concentration
of I2 is high enough to be considered constant during reaction.
Curve c is a straight line that corresponds to a constant
consumption rate with decreasing [ClO2]. In other experiments,
the initial rate was found to be independent of the initial
concentration of ClO2 as another indication of the zero order
dependence. When [I2]0 was small, its concentration decreased
significantly as the reaction proceeded. As a result, the reaction
rate decreased in time (curve b in Figure 1). The initial rate
(d[ClO2]/dt) was determined from the slope of the initial part
of the time-[ClO2] curves.
No I3 was formed in the reaction mixture under the experi-
mental conditions. All experiments were conducted at 25 ( 1
°C.
Results and Discussions
Stoichiometry. Reaction 1 seems to be the most reasonable
to assume for the stoichiometry
2ClO2 + I2 + 2H2O f 2Cl- + 2IO3- + 4H+
(1)
However, we found that eq 1 alone did not reflect exactly the
measured mole consumption ratio (∆[ClO2]/∆[I2]); that depends
on the relative concentrations of the reactants.
In our experiments, chlorine dioxide and iodine were allowed
to react in acidic aqueous solution under illumination overnight.
We applied initial concentrations where all the ClO2 has been
consumed but some I2 remained unreacted. The remaining I2
concentration was determined. Then the mole consumption ratio
was calculated at different initial concentrations of iodine. The
results are presented in Table 1. As seen, the measured values
vary with the ratio of the initial concentrations of the reagents
at pH 2.0. The limiting value (2) given by eq 1 is approached,
but never reached with increasing excess of [I2]0. Very similar
values and tendencies for the mole consumption ratio were
obtained when these experiments were repeated in 0.05 and 0.50
M sulfuric acid solutions, indicating that the stoichiometry does
not depend on the acidity of the reaction mixture. We have not
carried out experiments above pH 2 because the hydrolysis of
I2 to I- and HOI and the consequent formation of I3- would be
Shown in Figure 2 is the plot of the reaction rate as a function
of the integrated light power (P) of illumination at [I2]0 ) 3.74
-
significant at higher pH values. I3 absorbs strongly with a

Reaction between Chlorine Dioxide and Iodine
J. Phys. Chem. A, Vol. 105, No. 25, 2001 6169
From the slope of Figure 2, kobs ) 2.0 × 10-4 s-1. Here, we
neglect both the slow dark reaction between ClO2 and I2 and
the spontaneous disproportionation of ClO2. Of course, rate law
3 is not valid at very low [ClO2].
Mechanism. A mechanism of the I2-ClO2 reaction should
account for both the variable stoichiometry and the observed
photoresponses. The results of illumination experiments can be
explained by a single photosensitive process. In the first and
rate-determining step, I2 undergoes the well-known photoin-
duced dissociation (M1). The rate of the formation of reactive
I atoms depends on the I2 concentration and the light power.
The rate of the recombination of I does not depend on the light
power. As a result, the steady-state concentration of iodine atoms
in the photostationary state is much higher than that in the dark
equilibrium. I atoms react rapidly with ClO2 to form supposedly
-
the IClO2 intermediate (M2). IClO2 can either hydrolyze to IO3
and Cl- in aqueous solution in several steps, the net of which
is represented by reaction M3, or it can decompose according
to M4. Reactions M1-M3 constitute the main channel and give
the net reaction 1. ClO formed in M4 is a strong oxidizing agent,
and it can oxidize both ClO2 and I2 according to M5 and M6,
respectively
Figure 2. Reaction rate as a function of the light power. [I2]0 ) 3.74
× 10-4 M, and [ClO2]0 ) 2.78 × 10-4 M.
I2 + hV h 2I
(M1)
(M2)
I + ClO2 h IClO2
IClO2 + H2O f IO3- + Cl- + 2H+
(M3)
(M4)
IClO2 f IO + ClO
ClO2 + ClO + H2O f ClO3- + HOCl
I2 + ClO f ICl + IO
(M5)
(M6)
M5 is responsible for the disproportionation of ClO2, while M6
is a first step toward the oxidation of I2. In M5 and M6, ClO2
and I2 compete with each other for ClO. This competition results
in a variable net stoichiometry. Note that I is a strong oxidizing
agent. As the data derived from Latimer15 show, the reduction
potential of I/I- couple is higher than that of ClO3-/ClO2 in
acidic aqueous solution (1.23 and 1.175 V, respectively, at 25
°C). If the photostationary concentration of I is rather high, direct
formation of some ClO3- is also possible according to reaction
M7
Figure 3. Reaction rate (d[ClO2]/dt) as a function of I2 concentration.
P ) 8.5 mW dm-3, T ) 25 °C, and [ClO2]0 ) 4.1 × 10-4 M.
× 10-4 M and [ClO2]0 ) 2.78 × 10-4 M. The linear dependence
of the rate on the light power indicates that the rate-determining
step in the mechanism is a photoinduced process. The intercept
corresponds to the rate of the dark reaction.
ClO2 + I + H2O f ClO3- + I- + 2H+
(M7)
The I- ion is oxidized back to I2 immediately by excess ClO2.
Reaction M7 might be of stoichiometric significance under very
strong illumination. The fate of intermediates IO, ICl is to be
oxidized further to IO3-, while HOCl and ClO are reduced to
Cl- in further fast steps
Reaction rates at different iodine concentrations are shown
in Figure 3. The straight line suggests that the reaction order is
1 with respect to I2. The positive intercept in Figure 3 is an
indication of the spontaneous disproportionation of ClO2 which
takes place even in the absence of iodine. (Again, we emphasize
that some loss of volatile ClO2 to the air cannot be excluded).
The reaction rate was found to be independent of the
concentration of H2SO4 in the range of 0.005-1.0 M, indicating
that the concentration of the proton does not affect the rate.
Variation of the ionic strength in the range of 0.01-1.0 M was
also found to be without any noticeable effect on the rate.
According to the results of the kinetic experiments, a rate
law can be given for the photoinduced part of the reaction as
I2 + HOCl f ICl + IOH
ICl + H2O h IOH + Cl- + H+
I2 + H2O h IOH + I- + H+
(M8)
(M9)
(M10)
(M11)
IO + ClO2 f IO2 + ClO
IO2 + ClO2 + H2O f IO3- + HClO2 + H+ (M12)
IOH + HClO2 f HIO2 + HOCl
HIO2 + ClO2 f IO3- + ClO + H+
(M13)
(M14)
V ) -d[ClO2]/dt ) kobs[I2]
(3)
where kobs ) φkP and φ is the quantum yield (0.46),14 k is a
constant, and P is the integrated light power entering the cell.
Modified Mechanism of the Photoresponses of the CDIMA
Reaction System. Horva´th et al. suggested that illumination of

6170 J. Phys. Chem. A, Vol. 105, No. 25, 2001
Ra´bai and Kova´cs
the CDIMA reaction resulted in an increased rate of iodide ion
consumption, which led ultimately to a decrease in the steady-
state concentration of triiodide ion and that of starch-I3
oxidize ClO to ClO2-. A more likely reaction between ClO2
and ClO is M6.
-
Our suggestion for the additional photoinduced consumption
complex when the reaction is run in a CSTR.6 In addition to
the direct electron transfer between ClO2 and I-, they assumed
an indirect route for the oxidation of I- which is induced by
illumination. (Since the direct electron transfer between I- and
ClO2 is rather fast even in the dark,16 it is not likely that such
a fast dark reaction can be directly accelerated by photoillumi-
nation.) They proposed a mechanism for the additional
I--consuming channel in which the primary photochemical step
was the photodissociation of molecular I2. We agree with this
conclusion and emphasize the key role of the photosensitive
ClO2-I2 reaction in the photoresponses of the CDIMA system.
Our experiments indicated that the most effective light band is
460 nm, which is the absorption maximum of I2. We believe
that the increased rate or an extended degree of the iodide ion
consumption is due to the fast reactions between I- and some
reactive intermediates (ClO, HOCl, ICl, etc.) and products (IO3-)
formed in the photoinduced I2-ClO2 reaction.
of I- is the reaction sequence M15-M18
ClO + I- + H+ f I + HOCl
HOCl + I- f IOH + Cl-
(M15)
(M16)
(M17)
(M18)
IO + I- + H+ f I + IOH
IO3- + I- + 2H+ f IOH + HIO2
Since M15-M18 are all fast reactions, the photoresponses of
the CDIMA reaction is determined by the slow photodissociation
of I2.
Conclusion
It is known that the oxidation of I2 by ClO2 is a very slow
process in an aqueous acidic solution in the dark. In this work,
we have shown that the reaction accelerates significantly under
illumination with white light. The most effective wavelength
band is found to be around 460 nm, the absorption maximum
of I2. A logical conclusion is that photodissociation of I2
molecules is the primary photochemical step, which is followed
by fast steps between the reactive I atoms and ClO2, leading
through intermediates to the iodate, chlorate, and chloride ions
as products.
Earlier, this reaction was not considered as an important
composite reaction of the CIMA or CDIMA oscillatory system
although both I2 and ClO2 are major components of those
systems. We propose here that the I2-ClO2 reaction is the key
to the photoresponse of the CDIMA system. The reactive
intermediates of this reaction can react with the free iodide ions
in fast reactions, providing an additional route for the iodide
ion consumption. This mechanism explains the low steady-state
concentration of I- in a CSTR under illumination compared
with the higher steady-state I- ion concentration measured under
the same conditions in the dark.
Horva´th et al. derived a rate law for the additional photo-
induced route from their proposed mechanism. Their rate law
shows a complex dependence of the rate of the light-induced
route on the concentration of chlorine dioxide and iodide and
on the rate of absorption of actinic photons (eq 11 in ref 6).
This form is not consistent with our experimental rate law (eq
4) obtained for the photoinduced ClO2-I2 reaction because our
experiments showed that the rate is independent of [ClO2]. In
the following, we show several inconsistencies which may be
the source of the contradiction and propose alterations. Horva´th’s
mechanism consists of steps H1-H9
I2 + hV h 2I
(H1)
(H2)
I + ClO2 f IClO2
IClO2 + I- f I2ClO2
(H3)
(H4)
(H5)
(H6)
(H7)
-
I2ClO2- + IClO2 f I3- + 2ClO2
I2 + H2O h HOI + I- + H+
I3- h I2 + I-
Acknowledgment. This paper is dedicated to professor
Gyorgy Bazsa on the occasion of his 60th birthday. We
gratefully acknowledge the support of the Hungarian Science
Foundation (Grant OTKA 25076).
References and Notes
HIO2 + I- + H+ h 2HOI
(1) Hanazaki, I.; Mori, I.; Sekiguchi, T.; Ra´bai, Gy. Phys. D 1995, 84,
228.
(2) De Kepper, P.; Epstein, I. R.; Kustin, K.; Orba´n, M. J. Phys. Chem.
1982, 86, 170.
(3) Ouyang, Q.; Swinney, H. L. CHAOS 1991, 1, 411.
(4) Ra´bai, Gy.; Hanazaki, I. J. Phys. Chem. 1994, 98, 10550.
(5) Munuzuri, A. P.; Dolnik, M.; Zhabotinsky, A. M.; Epstein, I. R. J.
Am. Chem. Soc. 1999, 121, 8065.
IClO2 + ClO2 + H2O f ClO2- + HIO2 + ClO + H+
(H8)
ClO2 + ClO + H2O f 2ClO2- + 2H+
(H9)
(6) Horva´th, A. K.; Dolnik, M.; Zhabotinsky, A. M.; Epstein, I. R. J.
Phys. Chem. A 2000, 104, 5766.
(7) Vaida, V.; Simon, J. Science 1995, 268, 1443.
(8) Ra´bai, Gy.; Hanazaki, I. Int. J. Chem. Kinetics 1995, 27, 431.
(9) Ra´bai, Gy; Beck, M. T. Inorg. Chem. 1987, 26, 1195.
(10) Lengyel, I.; Ra´bai, Gy.; Epstein, I. R. J. Am. Chem. Soc. 1990,
112, 9104.
(11) Brauer, G., Ed. Handbook of PreparatiVe Inorganic Chemistry;
Academic Press: New York, 1963; Vol. 1, p 301.
(12) Chen, T. H. Anal. Chem. 1967, 39, 804.
(13) Gordon, G.; Kiefer, R. G.; Rosenblatt, D. H. Prog. Inorg. Chem.
1979, 15.
(14) Vanag, V.; Alfimov, M. V. J. Phys. Chem. 1993, 97, 1878.
(15) Latimer, W. M. Oxidation Potentials, 2nd ed.; Prentice-Hall: New
York, 1952.
First, reaction H4 seems unlikely in their mechanism since it
requires the bimolecular reaction of reactive intermediates.
-
Second, the net of reactions H1-H4 is the formation of I3
from I2 and I-. The light and ClO2 both play only the role of a
“catalyst” in this reaction sequence. Assumption of the catalytic
-
effect of the light and ClO2 on the formation rate of I3 is not
reasonable chemically because the direct charge-transfer I2-I-
reaction itself (H6) is a diffusion-controlled one,17 which is not
expected to be accelerated by a catalyst of any kind under the
conditions applied in the experiments. We propose that H3 and
H4 should be eliminated from the scheme. Third, reaction H9
is very unfavorable thermodynamically because ClO is a more
powerful oxidant than ClO2. It is not expected that ClO2 can
(16) Fa´bia´n, I.; Gordon, G. Inorg. Chem. 1997, 36, 2494.
(17) Meyers, E. J. Chem. Phys. 1958, 28, 1027.