Kinetics and mechanism of the oxidation of sulfite by chlorine dioxide in a slightly acidic medium

Article abstract of DOI:10.1021/jp060246b

The sulfite-chlorine dioxide reaction was studied by stopped-flow method at I = 0.5 M and at 25.0 ± 0.1 °C in a slightly acidic medium. The stoichiometry was found to be 2 SO₃^2- + 2-ClO₂ + H₂O → 2SO₄^2- + Cl^- + ClO ₃^- + 2H⁺ in ClO₂ excess and 6SO ₃^2- + 2·ClO₂ → S₂O ₆^2- + 4SO₄^2- + 2Cl^- in total sulfite excess ([S(IV)] = [H₂SO₃] + [HSO ₃^-] + [SO₃^2-]). A nine-step model with four fitted kinetic parameters is suggested in which the proposed adduct SO₃C1O₂^2- plays a significant role. The pH-dependence of the kinetic traces indicates that SO₃^2- reacts much faster with ClO₂ than HSO₃^- does.

Full text of DOI:10.1021/jp060246b

J. Phys. Chem. A 2006, 110, 4753-4758
4753
Kinetics and Mechanism of the Oxidation of Sulfite by Chlorine Dioxide in a Slightly Acidic
Medium
Attila K. Horva´th* and Istva´n Nagypa´l
UniVersity of Szeged, Department of Physical Chemistry, Szeged, P.O. Box 105, H-6701, Hungary
ReceiVed: January 13, 2006; In Final Form: February 21, 2006
The sulfite-chlorine dioxide reaction was studied by stopped-flow method at I ) 0.5 M and at 25.0 ( 0.1
2 -
°C in a slightly acidic medium. The stoichiometry was found to be 2 SO32- + 2‚ClO2 + H2O f 2SO4
+
-
2-
2-
2-
Cl- + ClO3 + 2H+ in ‚ClO2 excess and 6SO3 + 2‚ClO2 f S2O6 + 4SO4 + 2Cl- in total sulfite
excess ([S(IV)] ) [H2SO3] + [HSO3 ] + [SO32-]). A nine-step model with four fitted kinetic parameters is
-
2-
suggested in which the proposed adduct ‚SO3ClO2 plays a significant role. The pH-dependence of the
2-
-
kinetic traces indicates that SO3 reacts much faster with ‚ClO2 than HSO3 does.
Introduction
pH dependence of ka strongly suggests that the reactivity of
-
HSO3 is greater than that of H2SO3 toward chlorine dioxide,
The chlorite-thiosulfate reaction displays a remarkable range
of “exotic” kinetic phenomena including complex periodic and
aperiodic oscillation1,2 and chaos3 in CSTR, and shows extreme
sensitivity to stirring rate effects and fluctuations4 in batch
condition. So far no attempt has been made to unravel the
kinetics and mechanism of this reaction because of its complex-
ity. Three long-lived intermediates and/or end productss
tetrathionate, hypochlorous acid, chlorine dioxideshave been
established in this reaction that can react further with not only
the reactants but also with each other, making the mechanism
particularly complicated. Systematic investigations in the last
15 years have provided the kinetics and mechanism of several
subsystems of the parent reaction such as that of the hypochlo-
rous acid-chlorite,5 thiosulfate-chlorine dioxide,6 tetrathionate-
hypochlorous acid,7 and tetrathionate-chlorine dioxide8 and that
of the decomposition of chlorous acid.9 Very recently a five-
step model has been published10 for interpreting the unusual
kinetic behavior of the chlorite-tetrathionate reaction in which
HSO3- plays a crucial role to regulate the concentration of the
autocatalyst HOCl. Since the oxidation of sulfite by chlorine
dioxide is a relatively rapid process, it may also have a great
impact on the kinetics of the parent reaction.
2-
and still nothing is known about that of SO3
.
The stoichiometry and the kinetics of the sulfite-chlorine
dioxide was later thoroughly reinvestigated by Suzuki and
Gordon12 in alkaline solutions. Slight stoichiometric excess of
sulfite has been applied, the ratio of ([S(IV)]0/[‚ClO2]) was
varied between 1.5 and 2.0. Under these circumstances the
formation of dithionate was ruled out. Three competitive
stoichiometries have been found
2‚ClO2 + SO32- + 2OH- f SO42- + 2ClO2- + H2O
(II)
2-
2‚ClO2 + 2SO32- + 2OH- f Cl- + ClO3- + 2SO4
+
H2O (III)
2‚ClO2 + 3SO32- + 2OH- f ClO2- + Cl- + 3SO4
+
2-
H2O (IV)
It has also been found that the disappearance of chlorine
dioxide obeyed a simple second-order kinetics
The first article in the literature dealing comprehensively with
the kinetics and mechanism of the reaction between sulfite and
chlorine dioxide was published by Halperin and Taube.11 The
thermodynamically most favorable stoichiometry would be
d[‚ClO2]
-
) kb[‚ClO2][S(IV)]
(2)
dt
5S(IV) + 2 ‚ClO2 + 2H+ f 5SO42- + 2Cl- + H2O (I)
They have also noticed that the value of kb increases with
increasing pH; at 10 °C kb was found to be 7.5 × 105 M-1 s-1
and 1.2 × 106 M-1 s-1 as the pH was adjusted to 8.7 and 10.0,
respectively. Moreover, the authors have also reported a slight
buffer dependence on kb. The following mechanism was
proposed based on their measurements
chlorate formation was, however, always detected in appreciable
amounts in their experiments. Note that S(IV) stands for all the
differently protonated sulfite species, i.e, [S(IV)] ) [H2SO3] +
[HSO3-] + [SO32-] along this paper. They have also found that
the kinetics can be best described by
SO32- + ‚ClO2 f ‚SO3- + ClO2
(3)
(4)
-
d[‚ClO2]
-
) ka[‚ClO2]2[H2SO3]
(1)
SO32- + ‚ClO2 f SO42- + ‚ClO
dt
in strong acidic solution (0.1 M HCl), where ka decreases as
the acidity increases. This fact raises a question about the
reactivity of differently protonated S(IV) species. In fact the
‚SO3- + ‚ClO2 + 2OH- f SO42- + ClO2- + H2O (5)
10.1021/jp060246b CCC: $33.50 © 2006 American Chemical Society
Published on Web 03/23/2006

4754 J. Phys. Chem. A, Vol. 110, No. 14, 2006
Horva´th and Nagypa´l
TABLE 1: Initial Concentrations of the Reactants
‚SO3- + ‚ClO + 2OH- f SO42- + OCl- + H2O (6)
[‚ClO2]0
(mM)
[S(IV)]0
(mM) pH
SO32- + OCl- f SO42- + Cl-
(7)
run no.
1-7
0.265, 0.395, 0.55, 0.77, 1.035, 1.79, 2.50
0.267, 0.385, 0.55, 0.76, 1.035, 1.84, 2.46
0.29, 0.395, 0.55, 0.765, 1.038, 1.88, 2.46
0.28, 0.395, 0.55, 0.785, 1.035, 1.90, 2.49
0.27, 0.392, 0.55, 0.775, 1.02, 1.88, 2.50
0.28, 0.392, 0.55, 0.77, 1.02, 1.98, 2.54
0.27, 0.396, 0.56, 0.77, 1.02, 1.88, 2.52
0.285, 0.405, 0.565, 0.79, 1.05, 1.88, 2.51
0.275, 0.39, 0.55, 0.77, 1.08, 1.88, 2.57
0.277, 0.385, 0.545, 0.76, 1.07, 1.88, 2.62
0.278, 0.395, 0.55, 0.765, 1.06, 1.88, 2.62
0.27, 0.395, 0.55, 0.785, 1.07, 1.89, 2.63
0.26, 0.385, 0.545, 0.775, 1.06, 1.88, 2.63
0.265, 0.39, 0.54, 0.77, 1.07, 1.88, 2.57
0.2
0.3
0.5
0.7
1.0
2.0
3.0
4.0
0.2
0.3
0.5
0.7
1.0
2.0
3.0
4.0
0.2
0.3
0.5
0.7
1.0
2.0
3.0
4.0
0.2
0.3
0.5
0.7
1.0
2.0
3.0
4.0
4.55
4.55
4.55
4.55
4.55
4.55
4.55
4.55
4.25
4.25
4.25
4.25
4.25
4.25
4.25
4.25
3.95
3.95
3.95
3.95
3.95
3.95
3.95
3.95
3.65
3.65
3.65
3.65
3.65
3.65
3.65
3.65
8-14
Note that further reaction of chlorite with S(IV) was not
involved since in alkaline solution this reaction is very slow.13
This mechanism, however, cannot be incorporated with the three
competitive stoichiometry, since no chlorate formation is
indicated in the mechanism. Stoichiometry II comes forward
with addition of eqs 3 and 5, while stoichiometry IV with
addition of eqs 4, 6, and 7. Stoichiometry III would easily be
interpreted after inclusion of the following step into the
mechanism
15-21
22-28
29-35
36-42
43-49
50-56
57-63
64-70
71-77
78-84
85-91
92-98
2‚ClO + H2O f Cl- + ClO3- + 2H+
(8)
99-105 0.26, 0.39, 0.54, 0.77, 1.06, 1.88, 2.58
106-112 0.27, 0.395, 0.55, 0.77, 1.07, 1.88, 2.55
113-119 0.29, 0.405, 0.548, 0.79, 1.115, 1.89, 2.52
120-126 0.29, 0.405, 0.548, 0.78, 1.11, 1.91, 2.64
127-133 0.29, 0.405, 0.555, 0.785, 1.10, 1.9, 2.63
134-140 0.288, 0.4, 0.55, 0.78, 1.10, 1.85, 2.635
141-147 0.285, 0.4, 0.545, 0.78, 1.10, 1.86, 2.62
148-154 0.27, 0.395, 0.545, 0.73, 1.07, 1.89, 2.68
155-161 0.27, 0.39, 0.545, 0.75, 1.10, 1.88, 2.61
162-168 0.27, 0.39, 0.545, 0.75, 1.05, 1.75, 2.60
169-175 0.283, 0.385, 0.54, 0.765, 1.07, 1.82, 2.52
176-182 0.283, 0.377, 0.533, 0.775, 1.055, 1.84, 2.64
183-189 0.282, 0.375, 0.536, 0.765, 1.055, 1.85, 2.63
190-196 0.275, 0.38, 0.534, 0.765, 1.05, 1.85, 2.635
197-203 0.27, 0.365, 0.53, 0.765, 1.05, 1.82, 2.62
204-210 0.25, 0.365, 0.53, 0.73, 1.04, 1.82, 2.68
211-217 0.255, 0.365, 0.53, 0.74, 1.0, 1.80, 2.61
218-224 0.260, 0.36, 0.53, 0.73, 1.0, 1.75, 2.60
since the appropriate linear combination of eqs 4 and 8 would
directly refer to this stoichiometry. The most important conclu-
sion of this work was that formal oxygen transfer from chlorine
dioxide to sulfite must occur parallel to a single electron transfer
from S(IV) to chlorine dioxide to give the branching stoichi-
ometry.
More recently the electron-transfer process from sulfite to
chlorine dioxide has been investigated14 in 1 M ClO2- solution.
The forward and the reverse rate coefficients of the following
equation
-
SO32- + ‚ClO2 h ‚SO3- + ClO2
(9)
was found to be 2.6 × 106 M-1 s-1 and 1300 M-1 s-1
,
respectively, in huge excess of chlorite (1 M). A very similar
value of 2.82 × 106 M-1 s-1 was used for the forward reaction
in the interpretation of pH oscillations found in the chlorite-
sulfite system.15 Furthermore, formation of dithionate was also
considered in Frerichs’s and co-workers’ simulation throughout
a fast dimerization of sulfite radical.
acidic medium, so the unreacted sulfite can easily be determined.
In this case the stoichiometric ratio (SR) is calculated by
([S(IV)]0 - [S(IV)]∞)/[‚ClO2]0. In excess of chlorine dioxide
spectrophotometric measurements were used to calculate SR
) [S(IV)]0/([‚ClO2]0 - [‚ClO2]∞).
To give an adequate answer whether the S(IV)-chlorine
dioxide reaction may play significant role describing the kinetics
of the parent reaction or not we decided to reinvestigate it in
slightly acidic medium.
For the qualitative determination of the end products, Raman
spectroscopy has been used by a Bio-rad Digilab Division
dedicated FT-Raman spectrometer with laser type Nd:YVO4
lasing at 1064 nm. After the reaction was completed, the solution
was vacuum-evaporated at 35 °C. The Raman spectrum of the
solid material was registered for identification of the dithionate.
The kinetic measurements were carried out by a Hi-Tech SF-
61 stopped-flow apparatus attached to a single wavelength
spectrophotometer that provides monochromatic light. The
reaction was followed at 400 nm where the only absorbing
Experimental Section
Materials. The chlorine dioxide solution was prepared as
described previously.8 The stock solution was kept refrigerated
and was protected from light. The purity of the commercially
available sodium sulfite (Merck) was checked by standard
iodometric titration and was found to be better than 95%, and
the main contamination was found to be sulfate. Separate
experiment has shown that addition of sulfate disturbs neither
the stoichiometry nor the kinetic curves. All the other chemicals
were of the highest purity commercially available (sodium
acetate, acetic acid). All the solutions were prepared by four
times distilled water. Before preparing the sulfite solution the
water was even boiled for 20 min and deoxygenated with
bubbling N2. All the solutions used for the kinetic measurements
were prethermostated at 25.0 ( 0.1 °C and protected from light.
The ionic strength was adjusted to 0.5 M by using 0.5 M sodium
acetate. The desired pH was achieved by addition of the
appropriate amount of acetic acid.
species is chlorine dioxide (ꢀ
) 552 M-1 cm-1). Table 1
‚ClO2
summarizes the pH and concentration range of the reactants
used in the kinetic experiments.
Data Treatment. Only measurements up to 1.4 absorbance
units were used in our analysis, because the relative error of
absorbance measurements above this value is unacceptably high.
Because each kinetic curve contained 512 data points, the
number of points in each run was reduced to 45-50 to avoid
unnecessary and time-consuming calculations. The point reduc-
tion algorithm was based on the principle of equivalent arclength
in order to avoid losing any significant chemical information.
Each kinetic curve was repeated at least five times; the average
of them was used for the fitting procedure.
Methods. The stoichiometry was determined by standard
iodometry in excess of sulfite. After mixing the reactants in
the desired buffer solution only chloride and chlorate remained
since both chlorite and hypochlorous acid react rapidly13,16 with
sulfite and sulfite cannot be oxidized by chlorate in slightly
The experimental curves were analyzed with the program
package ZiTa,17 developed recently for fitting kinetic data
simultaneously. Altogether, 10700 experimental points from the
224 absorbance-time series were used for simultaneous fitting.
This method required a new evaluation procedure of the stopped-

Oxidation of Sulfite by Chlorine Dioxide
J. Phys. Chem. A, Vol. 110, No. 14, 2006 4755
TABLE 2: Determination of the Ratio of Consumed S(IV)
TABLE 3: Determination of the Ratio of Consumed S(IV)
and ‚ClO2 in Excess of Sulfitea
and ‚ClO2 in Excess of Chlorine Dioxidea
c0(S(IV)) c0(‚ClO2) c∞(S(IV))
pH
[S(IV)]conc/[‚ClO2]conc
c0(S(IV))
c0(‚ClO2)
A∞
pH
[S(IV)]conc/[‚ClO2]conc
3.64
4.20
4.76
5.31
5.85
6.37
6.89
7.40
7.90
8.40
8.33
8.26
8.19
8.12
8.05
7.99
7.93
8.47
8.54
15.01
14.23
13.42
12.58
11.72
10.83
9.91
1.91
1.89
1.87
1.86
1.84
1.83
1.81
1.79
1.78
1.76
2.33
2.89
3.44
3.98
4.51
5.03
5.55
1.19
0.60
1.05
1.07
1.08
1.10
1.12
1.14
1.16
0.11
0.53
1.06
1.16
1.29
2.11
2.38
3.19
3.93
3.56
2.95
2.39
1.48
0.64
0.29
0.09
0.04
4.92
6.73
12.08
11.00
10.59
9.66
8.64
7.40
6.73
4.55
4.55
4.55
4.55
4.02
4.55
4.02
4.02
4.55
4.55
4.55
4.55
4.55
4.55
4.55
4.55
4.55
4.55
4.55
4.55
4.05
4.55
4.25
4.02
3.85
4.05
1.85
1.94
1.98
2.23
2.48
2.33
2.49
2.35
2.23
2.75
2.31
2.03
1.95
1.88
1.72
1.57
1.42
2.98
3.02
2.79
3.02
2.62
2.65
2.75
3.01
2.74
0.2
0.3
0.5
0.7
1.0
2.0
3.0
4.0
0.2
0.3
0.5
0.7
1.0
0.2
0.3
0.5
0.7
1.0
0.2
0.3
0.5
0.7
1.0
2.50
2.46
2.46
2.49
2.50
2.54
2.52
2.51
0.77
0.76
0.77
0.77
0.77
0.405
0.405
0.405
0.40
0.40
0.54
0.533
0.536
0.534
0.53
1.286
1.234
1.158
1.097
0.989
0.635
0.270
0.020
0.348
0.364
0.334
0.292
0.245
0.170
0.153
0.118
0.075
0.018
0.243
0.221
0.184
0.140
0.091
4.55
4.55
4.55
4.55
4.55
4.55
4.55
4.55
4.25
4.25
4.25
4.25
4.25
3.95
3.95
3.95
3.95
3.95
3.65
3.65
3.65
3.65
3.65
1.18
1.34
1.38
1.39
1.41
1.44
1.48
1.62
1.81
1.94
2.08
2.15
2.21
2.06
2.35
2.62
2.65
2.72
2.00
2.26
2.47
2.50
2.74
a A∞ is the final absorbance at 400 nm measured after the reaction
is completed. All the concentrations are indicated in mM units. The
molar absorbance of ·ClO2 Is 552 M-1 cm-1 at this wavelength.
a c∞(S(IV)) is the final concentration of S(IV) calculated from the
titration after the reaction is completed. All the concentrations are
indicated in mM units.
contamination after purging the remaining chlorine dioxide from
the sample. Only a trace amount (≈0.1%) of the reacted chlorine
dioxide is transformed into chlorite; thus, its role is negligible
in determining the stoichiometry.
These studies convincingly demonstrate that in chlorine
dioxide excess the limiting stoichiometry is
flow measurements. Details of this method were published
previously.6,18 Absolute and relative average deviations have
also been calculated in order to judge between the different
models. The criterion of the goodness of fit was to reach the
uncertainty of the spectrophotometer ((0.005) in case of the
absolute fit and 3% in case of relative fit.
2SO32- + 2‚ClO2 + H2O f 2SO42- + ClO3- + Cl- +
2H+ (S1)
Results
but as the [S(IV)]0/[‚ClO2]0 ratio increases the following
Stoichiometry. Table 2 summarizes the results of the
stoichiometric measurements in sulfite excess. The consumed
S(IV)/‚ClO2 ratio (SR) varies between 1.43 and 3.02; the larger
the excess of S(IV) is, the higher this ratio is. Moreover, in a
number of cases this ratio exceeds 2.5, which cannot be
explained if sulfite is uniquely oxidized to sulfate. As eqs I-IV
indicate, this ratio must be between 0.5 and 2.5 if sulfate is the
only sulfur-containing end product. Our stoichiometric measure-
ments, however, clearly suggest that, beside sulfate, dithionate
should also be present to be responsible for the values of SR
found to be higher than 2.5. To strengthen the formation of
dithionate among the end products, we have analyzed the Raman
spectrum of the vacuum-evaporated solution of the reacted
mixtures. The appearance of the characteristic bands of dithio-
nate around 291, 708, and 1100 cm-1 strongly supports the result
of the classical titration method. Separate experiments have
shown that dithionate is not oxidized by chlorine dioxide under
our experimental circumstances.
Table 3 summarizes the results of the stoichiometric measure-
ments in chlorine dioxide excess. As it is clearly seen when
([S(IV)]0/[‚ClO2]) ratio is small, the consumed S(IV)/‚ClO2 ratio
(SR) approaches unity, but increase of the initial concentration
ratio of the reactants results in the increase of SR. In some cases,
when almost all the chlorine dioxide is consumed, SR slightly
exceeds 2.5, supporting again the formation of dithionate. In
excess of chlorine dioxide, formation of chlorite is also
conceivable; therefore, the end product was analyzed for chlorite
stoichiometry gradually takes over the governing role.
6SO32- + 2‚ClO2 f 4SO42- + S2O62- + 2Cl- (S2)
The latter reaction may seem unnecessarily complicated to
give sulfate and dithionate, but it is easily understood from our
proposed mechanism (see below). The stoichiometry of sulfite-
chlorine dioxide reaction at any initial concentration ratios of
the reactants can therefore be described by an appropriate linear
combination of S1 and S2 in slightly acidic medium.
Initial Rate Studies. Since the only absorbing species in the
given wavelength is ‚ClO2, (dA/dt) is directly proportional to
(d[‚ClO2]/dt) former values were used to analyze the initial rates.
These studies have revealed that the kinetic order of the reactants
are greater than 1. Figure 1 indicates typical kinetic orders of
the reactants. The kinetic order of chlorine dioxide varies
between 1.03 and 1.22, while that of the sulfite changes between
0.99 and 1.33. The rate of the reaction depends reversely on
the H+ concentration in agreement with earlier studies11,12 found
at very different experimental circumstances.
Proposed Mechanism. The approach used to generate the
proposed model resembles more some of the techniques used
to construct large models in atmospheric19 or combustion20
chemistry than the earlier studies of this system, takes advantage
of the significant advancing in computing power. The first,
probably the most critical part of this procedure is to choose
all the species that may be involved in the chemical model. Of

4756 J. Phys. Chem. A, Vol. 110, No. 14, 2006
Horva´th and Nagypa´l
course the reactants, the end products, and the solvent must be
included. This group consists of SO32-, HSO3-, H2SO3 (SO2),
‚ClO2, Cl-, ClO3-, SO42-, S2O62-, H+, and H2O. Additional
intermediates ‚SO3ClO22-, ‚SO3-, ‚ClO, Cl2O2, HOCl, and
-
ClO2 were chosen for the following reasons:
• Several studies of the reactions of chlorine dioxide have
suggested an initializing step in which a weak adduct is
2-
formed.6,8,21,22 By analogy, we propose ‚SO3ClO2
.
• Single electron transfer from sulfite to chlorine dioxide is
easily conceivable such as in the oxidation of phenol, indoles,
and metal complexes by chlorine dioxide.14,23 Therefore, inclu-
-
-
sion of ‚SO3 and ClO2 seems plausible.
• Suzuki and Gordon12 have proven experimentally that in
alkaline solution formal oxygen transfer from chlorine dioxide
to sulfite plays a significant role in describing their results beside
the electron-transfer process. Thus, we included ‚ClO and its
dimeric form Cl2O2.
• Since chlorite might also be an intermediate and its reaction
with sulfite goes through the intermediate HOCl,13 this com-
pound was also included among the possible intermediates.
This set of reactants, intermediates and end products leads
to 40 plausible mono- or bimolecular steps. Since the initial
rate also depends on [H+], the H+ and OH--catalyzed pathways
of the 40 reactions were also considered initially. From the 120
initially included steps, the following simple model emerged
from the systematic reduction procedure.
Figure 1. Dependence of the initial rate on the concentration of the
reactants. [S(IV)]0 (mM) ) 0.7 and pH ) 4.55 (0), [‚ClO2]0 (mM) )
2.50 and pH ) 3.65 (b). Note furthermore that the latter curve is shifted
by -0.1 along Y-axis to avoid overlapping of the curves.
below which the importance of step R4 is gradually lost and
therefore the average deviation between the measured and
calculated absorbance would increase significantly. Therefore
k-2 was fixed to a feasible 1300 s-1 value chosen from the
following reason. Mere´nyi and co-workers14 determined the
forward and reverse rate of the electron-transfer process from
sulfite to chlorine dioxide in alkaline medium at constant
[ClO2-]0 ) 1 M concentration. They have found 2.6 × 106 M-1
s-1 and 1300 M-1 s-1 values for the forward and reverse rate
coefficients, respectively. Since the concentration of chlorite
was not varied in their experiments, it cannot be distinguished
unambiguously whether the reverse reaction is a second-order
reaction between sulfite radical and chlorite or a simple first-
order decomposition of the adduct ‚SO3ClO22-. The adduct
formation is preferred from the following reasons:
SO32- + H+ h HSO3
(R1)
(R2)
(R3)
(R4)
(R5)
(R6)
-
SO32- + ‚ClO2 h ‚SO3ClO2
2-
‚SO3ClO22- f SO42- + ‚ClO
2‚SO3ClO22- f 2SO42- + Cl2O2
Cl2O2 + H2O f Cl- + ClO3- + 2H+
HSO3- + ‚ClO f ‚SO3- + HOCl
• A weak adduct formation between a substrate and chlorine
dioxide is generally accepted.6,8,21,22
• If single electron transfer occurs, it would yield appreciable
amount of dithionate from the recombination of sulfite radical
in sulfite excess at alkaline pH. The formation of dithionate,
however, was clearly ruled out by Suzuki and Gordon.12
• Chlorite formation can easily be taken into account by the
following equation that is likely to be hydroxide-catalyzed.
‚ClO2 + ‚ClO + H2O f HOCl + ClO3- + H+ (R7)
SO32- + HOCl f SO42- + Cl- + H+
(R8)
(R9)
‚SO3ClO22- + ‚ClO2 + H2O f SO42- + 2ClO2- + 2H+
(10)
2-
2‚SO3- f S2O6
• Experiments carried out by Suzuki and Gordon12 strongly
support this reaction since not only the increase of kb with
increasing [OH-] can be explained with it, but also the lack of
chlorate at higher pH.
The corresponding rate laws are given by either the law of
mass action (for equilibria) or by simple first or second-order
kinetics (the concentration of water is not included in the rate
equations).
• The kinetic orders of the reactants slightly higher than one
can easily be taken into consideration by the second-order
decomposition of the adduct.
Discussion
We have calculated (2.08 ( 0.13) × 106 M-1 s-1 value for
the forward rate coefficient, which is in reasonable agreement
with the values found in the literature.14,15
Step R1 is the simple protonation equilibrium of sulfite. The
forward and reverse rate coefficients were fixed to sufficiently
high values in such a way that the ratio of them gives the
Any efforts intended to include the direct electron-transfer
process
- 24
association constant pK ) 6.30 of HSO3
.
Step R2 is the equilibrium formation of the adduct ‚SO3ClO22-
.
The rate constants of the forward and reverse reactions can be
determined independently, but k-2 is in total correlation with
k3 and k4 (see later). Since step R4 is a second-order process
and k4 correlates with k-2, the diffusion control limit of the
bimolecular step (R4) provides an upper limit for the value of
k-2 found to be 6500 s-1. Our calculations have also revealed
-
SO32- + ‚ClO2 h ‚SO3- + ClO2
(11)
that must be augmented with the well-known13 further oxidation
of S(IV) by chlorite, i.e.
HSO3- + ClO2- f SO42- + HOCl
that k-2 must have a lower limit that was found to be 350 s-1
,

Oxidation of Sulfite by Chlorine Dioxide
J. Phys. Chem. A, Vol. 110, No. 14, 2006 4757
-
-
TABLE 4: Fitted and Fixed Rate Coefficients Used in the
Fitting Procedurea
V ) 5.5[HSO3 ][ClO2 ] +
-
-
6.26 × 106[HSO3 ][ClO2 ][H+] (12)
parameter
value
k1 (M-1 s-1
)
1.995 × 1010
have failed; we therefore concluded that direct electron transfer
is unlikely to occur.
k-1 (s-1
)
104
k2 (M-1 s-1
k-2 (s-1
k3 (s-1
k4 (M-1 s-1
)
(2.08 ( 0.13) × 106
1.3 × 103
)
Step R3 is a formal oxygen transfer from chlorine dioxide to
sulfite yielding sulfate and chlorine monoxide radical. This
formal oxygen transfer process was already proposed by Suzuki
and Gordon12 since significant amount of chlorate was detected
in their experiments at alkaline pH. If, however, either from a
formal electron-transfer process or from a reversible adduct
formation followed by eq 10 only chlorite was formed, there
would be no other route for chlorate to form only the sulfite-
chlorite reaction. In an alkaline solution, however, no reaction
takes place between them, and in acidic solution, no chlorate
could be detected among the end products of the reaction.13
The rate coefficient of this step cannot be determined indepen-
dently, because it is in complete correlation with the reverse
rate constant of the adduct formation in Step R2. This ratio of
k3/k-2 was found to be 0.79 ( 0.13.
)
(1.02 ( 0.18) × 103
(1.43 ( 0.58) × 108
102
)
k5 (s-1
)
k6 (M-1 s-1
k7 (M-1 s-1
k8 (M-1 s-1
k9 (M-1 s-1
)
)
)
)
109
(5.53 ( 0.41) × 107
7.6 × 108
3.1 × 108
a No error indicates that the value in question was fixed during the
fitting procedure. Note that the actual calculations yielded k3/k-2, k4/
(k-2)2 and k7/k6.
Step R4 is the second-order decay of the ‚SO3ClO22- adduct.
This step is responsible for the kinetic order of the reactants
slightly higher than 1. It is also a possible route for chlorate to
form throughout the hydrolysis of Cl2O2 and gives back
stoichiometry S1. Since k4 correlates with k-2, only the ratio of
k4/(k-2)2 could be calculated, and it was found to be 84.6 (
3.5 M-1 s. We have also tried to neglect the second-order
decomposition of the adduct from the model without any
success, the average deviation in case of the absolute fit
increased dramatically from 0.0054 to 0.0389; thus, we con-
cluded that the second-order decomposition is needed for the
adequate fit.
Step R5 is the well-known hydrolysis of Cl2O2 into chloride
and chlorate that is quite often involved in many of the reactions
of chlorite and chlorine dioxide, and it is even central part of
general model used to interpret the kinetic behavior of the
chlorite-based oscillators.25 Several other reactions of Cl2O2 were
also considered to be inserted into the model. The most obvious
Figure 2. Measured (symbols) and calculated (solid lines) kinetic
curves at pH ) 3.65 and [‚ClO2]0 ≈ 0.00075 M. [S(IV)]0 (mM) ) 0.2
(b), 0.3(0), 0.5(2), 0.7(]), 1.0(+), 2.0(9), 3.0(O), 4.0([).
The average deviation was increased to 0.0065 absorbance unit
in case of absolute fit and 3.8% in case of relative fit. Inclusion
of the branching ratio means that when the production of
dithionate is decreased, so too is the consumed S(IV)/‚ClO2 ratio.
If, however, we include the following reaction
HSO3- + Cl2O2 + H2O f SO42- + 2HOCl + H+
SO32- + ‚SO3ClO22- + 2H+ f S2O62- + ‚ClO + H2O
(13)
(14)
was found to be unnecessary to give adequate fit even if it was
considered as a proton- or hydroxide-catalyzed path. All the
efforts to implant any other reactions of Cl2O2 were unsuccess-
ful.
Step R6 is one of the possible routes for ‚ClO to react. The
absolute value of this rate coefficient cannot be determined
independently since it is in complete correlation with that of
Step R7. Therefore k7 was fixed to a reasonable fast 109 M-1
s-1 value and k8/k7 was found to be (5.53 ( 0.41) × 10-2. It is
also clearly seen that addition of Steps R2, R3, R6, and R9
gives stoichiometry S2, while that of Steps R2, R3, R7, and R8
gives stoichiometry S1.
Step R8 is the well-known rapid oxidation of sulfite by
hypochlorous acid. This rate coefficient was fixed to 7.6 × 108
M-1 s-1, which was determined by Fogelman et al.16
Step R9 is the fast recombination of sulfite radical. The
absolute value of this rate coefficient cannot be determined from
our experiments; therefore, we fixed it to 3.1 × 108 M-1 s-1
that was determined by Fischer and Warneck.26 A very similar
value was given by Waygood and McElroy27 in quite different
experimental circumstances. Therefore, we have also tried to
take the branching ratio26 of the recombination into account.
fitting its rate coefficient (k14) at a fixed branching ratio value,
almost the same average can be reached by adding a fixed and
a fitted parameter to the final model (0.006 in case of absolute
and 3.4% in case of relative fitting). Since the value of k14 is in
total correlation with the value of the branching ratio, they
cannot be determined independently. The final consequence of
this fitting procedure is that if the branching ratio is included
then an additional source of dithionate might also have to be
taken into consideration (preferably eq 14) for an adequate fit.
Table 4 summarizes the fixed and fitted rate coefficients used
in the fitting procedure. Figures 2-4 demonstrate that the model
works pretty well. The average deviation between the measured
and calculated data was found to be 0.0054 absorbance units in
case of absolute fit and 3.2% in case of relative fit, which we
believe is close to the experimentally achievable limit of error.
Conclusions
Despite the fact that formation of chlorate, chloride, chlorite
(only in alkaline solution), sulfate, and dithionate might make
the sulfite-chlorine dioxide reaction quite complex, the mech-
anism turned out to be very simple indeed, which can easily be

4758 J. Phys. Chem. A, Vol. 110, No. 14, 2006
Horva´th and Nagypa´l
far from the pKa of HSO3-. Instead of direct electron transfer
therefore, eq 10 is suggested that involves the possibility of the
explanation of the observed characteristics of the kinetic curves.
Unfortunately, the rate coefficient of this step cannot be
estimated from the present experiments; it would require further
extension of pH-dependence studies as well as changing the
buffer components. Changing the buffer components, however,
may establish the appearance of general hydroxide catalysis in
this system, the phenomenon which was already mentioned by
Suzuki and Gordon.12
Acknowledgment. This work was supported by the Hungar-
ian Research Fund (Grant No. T047031). A.K.H is grateful for
the financial support of Be´kesy Gyo¨rgy (B12/2003) postdoctoral
fellowship. The authors are thankful to Dr. Otto´ Berkesi for
his valuable assistance in the Raman measurements. The authors
greatly appreciate the thought-provoking suggestions of an
anonymous reviewer.
Figure 3. Measured (symbols) and calculated (solid lines) kinetic
curves at pH ) 4.25 and [S(IV)]0 ) 0.0007 M. [‚ClO2]0 (mM) ) 0.27
(b), 0.395 (0), 0.55 (2), 0.77 (]), 1.07 (+), 1.89 (9), 2.63 (O).
Supporting Information Available: Files containing the
measured and calculated absorbances for all the 224 kinetic
curves are available free of charge via Internet at http://
pubs.acs.org.
References and Notes
(1) Orba´n, M.; De Kepper, P.; Epstein, I. R. J. Phys. Chem. 1982, 86,
431.
(2) Orba´n, M.; Epstein, I. R. J. Phys. Chem. 1982, 86, 3907.
(3) Maselko, J.; Epstein, I. R. J. Chem. Phys. 1984, 80, 3175.
(4) Nagypa´l, I.; Epstein, I. R. J. Phys. Chem. 1986, 90, 6285.
(5) Peintler, G.; Nagypa´l, I.; Epstein, I. R. J. Phys. Chem. 1990, 94,
2954.
(6) Horva´th, A. K.; Nagypa´l, I. J. Phys. Chem. A 1998, 102, 7267.
(7) Horva´th, A. K.; Nagypa´l, I.: Int. J. Chem. Kin. 2000, 32, 395.
(8) Horva´th, A. K.; Nagypa´l, I.; Epstein, I. R. J. Phys. Chem. A 2003,
107, 10063.
(9) Horva´th, A. K.; Nagypa´l, I.; Peintler, G.; Epstein, I. R.; Kustin, K.
J. Phys. Chem. A 2003, 107, 6966.
Figure 4. Measured (symbols) and calculated (solid lines) kinetic
curves at [S(IV)]0 ) 0.004 M and [‚ClO2]0 ≈ 0.00105 M. pH ) 4.55
(b), 4.25 (0), 3.95 2), 3.65 (]).
(10) Horva´th, A. K.; Nagypa´l, I.; Epstein, I. R. J. Am. Chem. Soc 2004,
126, 6246.
(11) Halperin, J.; Taube, H. J. Am. Chem. Soc. 1952, 74, 375.
(12) Suzuki, K.; Gordon, G. Inorg. Chem. 1978, 11, 3115.
(13) Huff Hartz, K. E.; Nicoson, J. S.; Wang, L.; Margerum, D. W.
Inorg. Chem. 2003, 42, 78.
SCHEME 1
(14) Mere´nyi, G.; Lind, J.; Shen, X. J. Phys. Chem. 1988, 92, 135.
(15) Frerichs, G. A.; Mlnarik, T. M.; Grun, R. J.; Thompson, R. C. J.
Phys. Chem. A 2001, 105, 929.
(16) Fogelman, K. D.; Walker, D. M.; Margerum, D. W. Inorg. Chem.
1988, 27, 1672.
(17) Peintler, G.: ZiTa, Version 5.0, a Comprehensive Program Package
for Fitting Parameters of Chemical Reaction Mechanism. Attila Jo´zsef
University, Szeged, Hungary, 1989-1998.
(18) Peintler, G.; Nagy, A.; Horva´th, A. K.; Ko¨rtve´lyesi,T.; Nagypa´l, I.
Phys. Chem. Chem. Phys. 2000, 2, 2575.
2-
visualized by Scheme 1. Formation of an adduct ‚SO3ClO2
has been proposed to initiate the reaction. Further reactions of
this adduct yield the final products throughout a mixed first-
and second-order decomposition. The second-order decomposi-
tion of the intermediate uniquely leads to stoichiometry S1. The
first-order decomposition leads to the reactive ‚ClO species
(formal oxygen transfer) of which further reactions with the
reactants yield the branching stoichiometry. High excess of
chlorine dioxide also leads to stoichiometry S1, while high
excess of sulfite shifts the stoichiometry toward S2. In other
words it means that S1 can be reached if either the excess of
chlorine dioxide or the product of the concentration of the
reactants is high; otherwise stoichiometry S3 rules. Direct
electron transfer from sulfite to chlorine dioxide is not likely
to happen because there would be no plausible explanation for
the kinetic order slightly higher than 1 with respect to both
reactants and for the OH- dependence of kb in alkaline solution
(19) Jenkin, M. E.; Saunders, S. M.; Philling M. J. Atmos. EnViron.
1997, 31, 81.
(20) Broadbelt, L. J.; Stark, S. M.; Klein, M. T. Chem. Eng. Sci. 1994,
49, 4991.
(21) Ni, Y.; Wang, X. Can. J. Chem. Eng. 1996, 75, 31.
(22) Fa´bia´n, I.; Gordon, G. Inorg. Chem. 1997, 36, 2494.
(23) Lednicky, L. A.; Stanbury, D. M. J. Am. Chem. Soc. 1983, 105,
3098.
(24) IUPAC Stability Constant Database; Royal Society of Chemistry
1992-1997.
(25) Ra´bai, Gy.; Orba´n, M.; J. Phys. Chem. 1993, 97, 5935.
(26) Fischer, M.; Warneck, P. J. Phys. Chem. 1996, 100, 15111.
(27) Waygood, S. J.; McElroy, W. J. J. Chem. Soc., Faraday Trans.
1992, 88, 1525.