Hypohalite ion catalysis of the disproportionation of chlorine dioxide

Article abstract of DOI:10.1021/ic020440m

The disproportionation of chlorine dioxide in basic solution to give ClO₂^- and ClO₃^- is catalyzed by OBr^- and OCl^-. The reactions have a first-order dependence in both [ClO₂] and [OX^-] (X = Br, Cl) when the ClO₂^- concentrations are low. However, the reactions become second-order in [ClO₂] with the addition of excess ClO₂^-, and the observed rates become inversely proportional to [ClO₂^-]. In the proposed mechanisms, electron transfer from OX^- to ClO₂ (k₁^OBr- = 2.05 ± 0.03 M^-1 s^-1 for OBr^-/ClO₂ and k₁^OCl- = 0.91 ± 0.04 M^-1 s^-1 for OCl^-/ClO₂) occurs in the first step to give OX and ClO₂^-. This reversible step (k₁^OBr-/k_-1^OBr- = 1.3 × 10^-7 for OBr^-/ClO₂, k₁^OCl-/k_-1^OCl = 5.1 × 10^-10 for OCl^-/ClO₂) accounts for the observed suppression by ClO₂^-. The second step is the reaction between two free radicals (XO and ClO₂) to form XOClO₂. These rate constants are k₂^OBr = 1.0 × 10⁸ M^-1 s^-1 for OBr/ClO₂ and k₂^OCl = 7 × 10⁹ M^-1 s^-1 for OCl/ClO₂. The XOClO₂ adduct hydrolyzes rapidly in the basic solution to give ClO₃^- and to regenerate OX^-. The activation parameters for the first step are ΔH₁^? = 55 ± 1 kJ mol^-1, ΔS₁^? = - 49 ± 2 J mol^-1 K^-1 for the OBr^-/ClO₂ reaction and ΔH₁^? = 61 ± 3 kJ mol^-1, ΔS₁^? = - 43 ± 2 J mol^-1 K^-1 for the OCl^-/ClO₂ reaction.

Full text of DOI:10.1021/ic020440m

Inorg. Chem. 2002, 41, 6099−6105
Hypohalite Ion Catalysis of the Disproportionation of Chlorine Dioxide
Lu Wang and Dale W. Margerum*
Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907-1393
Received July 8, 2002
-
-
-
The disproportionation of chlorine dioxide in basic solution to give ClO
2
and ClO
3
is catalyzed by OBr and
-
-
-
OCl . The reactions have a first-order dependence in both [ClO
2
] and [OX ] (X ) Br, Cl) when the ClO
] with the addition of excess ClO
]. In the proposed mechanisms, electron transfer
2
concentrations are low. However, the reactions become second-order in [ClO
2
2
-,
-
and the observed rates become inversely proportional to [ClO
2
-
-
-
OBr
-1 -1
-
OCl
-1 -1
-
from OX to ClO
2
(k1 ) 2.05 ± 0.03 M s for OBr /ClO
2
and k1 ) 0.91 ± 0.04 M s for OCl /ClO )
2
OCl
-
-
-. This reversible step (k1 /k ) 1.3 × 10 for OBr /ClO
OBr OBr
-1
-7
-
, k1
/
occurs in the first step to give OX and ClO
2
2
OCl
) 5.1 × 10-10 for OCl /ClO
-
-
k
2
) accounts for the observed suppression by ClO
reaction between two free radicals (XO and ClO ) to form XOClO . These rate constants are k2 ) 1.0 × 10 M
2
. The second step is the
-
1
OBr
8
-1
2
2
s- for OBr/ClO
1
and k2 ) 7 × 10 M
3
OCl
9
-1 -1
s
for OCl/ClO
. The XOClO
adduct hydrolyzes rapidly in the basic
2
2
2
-
-
‡
solution to give ClO
mol , ∆S
and to regenerate OX . The activation parameters for the first step are ∆H
1
) 55 ± 1 kJ
) − 43 ±
-
1
‡
-1 -1
-
‡
-1
‡
1
) − 49 ± 2 J mol
K
for the OBr /ClO
2
reaction and ∆H
1
) 61 ± 3 kJ mol , ∆S
1
J mol K-1 for the OCl /ClO
-
1
-
reaction.
2
2
previously10,11 and was protected from light and stored at 5 °C.
The ClO stock solution was standardized spectrophotometrically
on the basis of the molar absorptivity of ClO
Introduction
It has long been known1 that chlorine dioxide dispro-
2
-6
-1
-1
2
, ꢀ ) 1230 M cm
at 359 nm. Working solutions of NaOCl were prepared from a
stock solution obtained by slowly bubbling Cl (g) into a well-stirred
-
-
portionates in basic solutions to give ClO
2
and ClO
3
.
11
Recent work has shown that oxygen is also a product of the
2
7
2
decomposition at low ClO concentrations. The dispropor-
solution of NaOH (∼0.3 M) maintained at 0-4 °C. Concentrations
8
tionation reaction is catalyzed by hypochlorite ion and by
of the NaOCl solutions were determined spectrophotometrically at
9
-1
-1 11
bromite ion, and the present work shows that hypobromite
292 nm (ꢀ ) 362 M cm ). Sodium chlorite was purified and
9
ion also catalyzes ClO
2
disproportionation. We also show
recrystallized as described previously, and stock solutions were
-
1
-1
-
-
standardized spectrophotometrically (ꢀ ) 154.0 M cm at 260
that excess ClO
the OCl reactions. Hence, a new mechanism is needed for
2
suppresses the rates of both the OBr and
-
nm).11 The ionic strength (µ) was adjusted with NaClO
4
that was
-
recrystallized from water, redissolved, and standardized gravimetri-
cally. Solutions of phosphate and carbonate buffer were prepared
from the reagent-grade salts. For kinetic measurements, the pH was
hypohalite catalysis that takes the ClO
account.
2
suppression into
buffered by mixing stock solutions of 1.0 M Na
PO (pK (pK
) 6.26)11 or 1.0 M NaHCO ) 9.48)11 and Na
to yield the desired pH upon 10-fold dilution.
Preparation of NaOBr Solutions. We discovered in the process
2
HPO
4
and NaH
2
-
Experimental Section
4
a
3
a
2
CO
3
Reagents. All solutions were made with distilled, deionized
water. Chlorine dioxide stock solution was prepared as described
-
-
of this work that trace levels of BrO
2
in HOBr/OBr solutions
*
To whom correspondence should be addressed. E-mail: margerum@
can affect the observed rate of catalytic disproportionation of ClO
2
.
chem.purdue.edu.
-
(
(
(
(
1) Bray, W. C. Z. Anorg. Allg. Chem. 1906, 48, 217-250.
2) Halperin, J.; Taube, H. J. Am. Chem. Soc. 1952, 74, 375-380.
3) Granstrom, M. L.; Lee, G. F. Public Works 1957, 88, 90-92.
4) Gordon, G.; Keiffer, R. G.; Rosenblatt, D. H. In Progress in Inorganic
Chemistry; Lippard, S. J., Ed.; Wiley-Interscience: New York, 1972;
pp 201-287.
This is due to electron-transfer reactions in which BrO
2
and ClO
2
- 9
give BrO
formation of BrO
of HOBr/OBr disproportionate to BrO
2
and ClO
2
. Precautions were taken to minimize the
-
-
2
in our preparation of OBr solutions. Solutions
-
-
-
2
and Br with a rate that
2
-
depends on [HOBr]
T
, where [HOBr]
T
) [OBr ] + [HOBr];
(
(
(
(
5) Emerich, D. E. Ph.D. Dissertation, Miami University, 1981.
6) Gordon, G. Pure Appl. Chem. 1989, 61, 873-878.
7) Odeh, I. N.; Francisco, J. S.; Margerum, D. W. Inorg. Chem., in press.
8) Csord a´ s, V.; Bubins, B.; F a´ bi a´ n, I.; Gordon, G. Inorg. Chem. 2001,
(9) Wang, L.; Nicoson, J. S.; Huff Hartz, K. E.; Francisco, J. S.; Margerum,
D. W. Inorg. Chem. 2002, 41, 108-113.
(10) Masschelein, W. J. J. Am. Water Works Assoc. 1984, 76, 70-76.
(11) Furman, C. S.; Margerum, D. W. Inorg. Chem. 1998, 37, 4321-4327.
40, 1833-1836.
1
0.1021/ic020440m CCC: $22.00 © 2002 American Chemical Society
Inorganic Chemistry, Vol. 41, No. 23, 2002 6099
Published on Web 10/24/2002

Wang and Margerum
-
12
however, OBr is much slower to react than HOBr (pK
)
a
of HOBr
8.59 at 25.0 °C, µ ) 1.0 M).12 Therefore, hypobromite was
2
prepared by adding small amounts of Br (l) very slowly to a
vigorously stirred 0.3 M NaOH solution. Bromide ion was removed
with freshly precipitated AgOH in a method similar to that of
Noszticzius et al.13 in which 50 mL of 0.1 M AgNO
and 10 mL
3
of 1 M NaOH were mixed in a 200 mL flask. After the AgOH
precipitate was freed from its colloid fraction by 4-5 decantations
-
with 0.1 M NaOH, 25 mL of the OBr stock was added and shaken
with the precipitate. After filtration, the clear light-yellow NaOBr
solution was analyzed spectrophotometrically at 329 nm on the basis
-
1
-1 14
-
of its molar absorptivity of 332 M cm
.
The OBr stock
solution (∼0.1 M) was prepared daily and kept in 0.1 M NaOH.
-
The OBr reactant solutions were diluted prior to the reaction. In
-
+
-
the presence of 0.1 M OH , the [Ag ] in the OBr stock solution
-
7
-2
would be 2 × 10 M. In our kinetic studies, 1.45 × 10 M or
-
-
less of OBr was used, which is a 7-fold dilution of the OBr stock.
-
8
+
Accordingly, 2.8 × 10 M or less Ag would be present in the
reactant solution. Additional tests of (OBr + Br ) solutions
without a AgOH treatment) gave the same kinetic results as
solutions that were treated with AgOH. Ion chromatographic (IC)
analysis was used to measure the BrO
solution. In this analysis, excess of (NH
-
-
-
Figure 1. Spectral evidence of no OBr loss in the catalyzed decay of
(
-
ClO2. Conditions: [OBr ]T ) 1.01 mM, [ClO2]i ) 0.994 mM, [CO3]T )
0.10 M, p[H+] ) 11.6, 25.0 °C, 1 cm cell, reaction time ) 1.55 h, 100 s
-
-
content in the OBr
SO solid was added
time interval per scan. Appreciable amounts of ClO2 are already lost when
the first scan was made because of a 3 min time interval between mixing
and the first scan.
2
4
)
2
4
-
+
into the OBr sample solution at p[H ] ) 9.5 in order to remove
-
-
OBr /HOBr, which reacts with the column packing. BrO
2
does
Table 1. Ion Chromatographic Determination of Products for the
+
15
- a
not react with NH
that the [BrO
3
/NH
4
.
The ion chromatographic results showed
Reaction of ClO2 with OBr
-
-
2
] present in OBr stock was only 0.07 mol % of
the initial [OBr ]. Interference from BrO
+
b
-
-
p[H ] of reaction mixture
yield ratio [ClO2 ]/[ClO3 ]
-
-
-
2
in the OBr stock is
1
1
0.9
1.4
1.00 ( 0.04
1.00 ( 0.04
negligible for kinetic studies in basic solutions. However, interfer-
-
+
ence from the BrO
.7.
Products and Stoichiometry. Ion chromatography was used to
identify and quantitatively determine the reaction products. Excess
2
reaction becomes appreciable below p[H ]
a
-
-
Conditions: [OBr ]i ) [OBr ]f ) 0.960 mM, [CO3]T ) 0.10 M, [ClO2]i
7
-
)
1.7 mM, 25.0 °C. b Ethylenediamine was used to remove OBr from
the solution.
-
-16
with [HOBr] or [HOCl] ()[HOCl] + [OCl ]) in large excess
ethylenediamine was added to remove the OBr before the product
solutions were analyzed on the Dionex DX-500 IC. Detailed
T
T
over [ClO
2
]. Reactions were maintained at an ionic strength of 1.0
M. The rates of the reactions were monitored at 359 or 380 nm for
the disappearance of ClO . SigmaPlot 4.0 was used for the
regression analysis.
experimental procedures for the determination of the products by
IC were described in earlier work.9
17
2
A Perkin-Elmer Lambda-9 UV-vis-NIR spectrophotometer
used in conjunction with PECSS (Perkin-Elmer Computerized
Spectroscopy Software) was used to acquire spectra for the reaction.
Spectral scans were taken from 250 to 450 nm at 100 s intervals
and after the reaction was complete.
Results and Discussion
-
I. Reaction between OBr and ClO
Stoichiometry. When OBr and ClO solutions are mixed
to give 1.0 mM concentrations of each at p[H ] 11.6, a
2
.
-
2
+
pH Measurement. An Orion model 720 A digital pH meter
equipped with a Corning combination electrode was used. The elec-
significant absorbance drop at 359 nm is observed, and a
final absorbance from 250 to 450 nm remains, in which the
dominant absorbing species is OBr (Figure 1). Ion chro-
4
trode was calibrated by titrations of standard HClO with standard
+
+
NaOH to correct the measured pH values to -log[H ] ()p[H ])
at 25.0 ( 0.1 °C and an ionic strength (µ) adjusted to 1.0 M with
-
-
-
matographic results show that ClO
products of the reaction with a 1:1 stoichiometry (Table 1).
After all the ClO reacts, the remaining absorbance from 250
to 450 nm is in good agreement with the sum of the
2
and ClO
3
are the
4
NaClO .
Kinetic Measurements. Kinetic traces for slow reactions were
acquired with the Perkin-Elmer Lambda-9 UV-vis-NIR spectro-
photometer. The kinetics of the fast reactions were followed on an
Applied PhotoPhysics SX.18 MV stopped-flow spectrophotometer
2
-
-
absorbance spectra of the added OBr and the ClO
generated from the reaction where [ClO
no OBr is lost. The rate of the depletion of ClO increases
with the increase of [OBr ] added. It is apparent that the
2
-] ) 1/
[ClO ] and
2 2 i
2
(
APPSF, optical path length ) 0.962 cm). The molar absorptivities
-
-
-
of ClO
2
, OCl , and ClO
2
1
determined on the Lambda-9 UV-vis-
2
-
1
NIR spectrophotometer agreed with values measured on the
APPSF. All reactions were run under pseudo-first-order conditions
-
disproportionation of ClO
2
is catalyzed by OBr (eq 1).
OBr-
-
8ClO2- + ClO3- + H O
2
ClO + 2OH
(1)
(
12) Beckwith, R. C.; Margerum, D. W. Inorg. Chem. 1997, 36, 3754-
760.
13) Noszticzius, Z.; Noszticzius, E.; Schelly, Z. A. J. Phys. Chem. 1983,
2
2
3
Kinetics. Without initial addition of excess ClO
-, the
2
(
8
7, 510-524.
-
rates of loss of ClO
2
with added OBr show a first-order
(
(
(
14) Troy, R. C.; Margerum, D. W. Inorg. Chem. 1991, 30, 3538-3543.
15) Hashmi, M. H.; Ayaz, A. A. Anal. Chem. 1963, 35, 908-909.
16) Furman, C. S. Ph.D. Thesis, Purdue University, 1998.
(17) SigmaPlot 4.0 for Windows; SPPS Inc., 1997.
6100 Inorganic Chemistry, Vol. 41, No. 23, 2002

Disproportionation of Chlorine Dioxide
-
Figure 2. Dependence of the observed first-order rate constant on [OX ]
-
-
for the reaction of OX with ClO2. (a) Conditions for the OBr /ClO2
reaction: [CO3]T ) 0.10 M, µ ) 1.0 M, [ClO2]i ) 0.41 mM, no added
-
+
-1 -1
[
ClO2 ]i, p[H ] ) 9.9-10.0, 25.0 °C. Slope ) (4.09 ( 0.09) M
s
;
-
4
-1
-
intercept ) (6 ( 7) × 10
s
. (b) Conditions for the OCl /ClO2
reaction: [CO3]T ) 0.10 M, µ ) 1.0 M, [ClO2]i ) 0.1 mM, no added
-
+
-1 -1
[
)
ClO2 ]i, p[H ] ) 10.6, 25.0 °C. Slope ) (1.81 ( 0.07) M
s ; intercept
-
4
-1
(8 ( 6) × 10
s .
Table 2. No Effect of Carbonate Buffer or Bromide Ion on the
Hypohalite Catalysis of ClO2 Disproportionationa
-
-
OBr /ClO2 reaction
OCl /ClO2 reaction
-
-
kobsd/[OBr ],
kobsd/[OCl ],
-
1
-1 b
-1 -1 g
[CO3]T, M
M
s
[CO3]T, M
M
s
0.00
0.05
0.30
4.20c
0.05
0.30
1.83
1.85
d
4.15
e
4.28
-
2nd
obsd
, M-1 s-1 f
obsd T
Figure 3. Dependence of k /[HOX] on p[H ] for the catalytic reactions
+
[
CO3]T, M
added [Br ], M
k
-
-
of OBr and OCl in ClO2 disproportionation. The open circles (O) in
Figure 3a are from measured rate constants under conditions where BrO2
0
0
0
.05
.10
.10
0.0
0.0
33.2
32.2
31.3
-
HOBr
12
HOCl
27
interference is expected. pKa
) 8.59 and pKa
) 7.47 were used
0.05
+
for the calculations. Conditions: [HPO4]T ) 0.10 M at p[H ] < 8, [CO3]T
+
a
b
-
-
) 0.10 M at p[H ] > 8.5, [ClO2]i ) 0.1-0.01 mM, µ ) 1.0 M, 25.0 °C.
Conditions: 25.0 °C, µ ) 1.0 M. [ClO2 ]i ) 0 M. The kobsd/[OBr ]
c
values refer to eq 9. Conditions: [HOBr]T ) 14.3 mM, [ClO2]i ) 0.22
+
d
mM, p[H ] ) 8.73. Conditions: [HOBr]T ) 5.32 mM, [ClO2]i ) 0.26
the pH value should be uniform throughout the solution. The
obsd/[OBr ] data (the points at p[H ] 11.14, 8.73, 8.95 in
Figure 3a) obtained by using this OBr stock solution are
consistent with our other data. Additional evidence supports
contamination does not interfere
with data at higher pH. Because the BrO
] while the OBr /ClO
] dependence, appreciable amounts
present in the OBr stock solution will cause a
significant difference in the overall rates when CO
present compared to when it is absent. Our experiments show
that, when excess HOBr/OBr was used to buffer the reaction
at p[H ] 8.73 (no carbonate buffer was added), the kobsd/
[OBr ] value (shown in Table 2 and Figure 3a) is in good
+
e
mM, p[H ] ) 9.87. Conditions: [HOBr]T ) 5.32 mM, [ClO2]i ) 0.26
-
+
k
+
f
-
mM, p[H ] ) 10.14. [ClO2 ]i ) 12.3 mM, [HOBr]T ) 13.32 mM, [ClO2]i
-
+
2nd
obsd
)
0.12 mM, p[H ] ) 11.0-10.9. The k
values refer to eq 11.
Conditions: [HOCl]T ) 2.62 mM, [ClO2]i ) 0.10 mM, p[H ] )
.87-10.13.
g
+
9
-
the conclusion that BrO
2
-
9
2
2
/ClO
2
reaction
dependence upon the concentration of chlorine dioxide (eq
). The observed first-order rate constants (kobsd) have a first-
2
-
-
is greatly accelerated by [CO
3
2
2
-
reaction has no [CO
3
-
order dependence in [OBr ] (Figure 2a).
-
-
of BrO
2
2-
-
d[ClO ]/dt ) kobsd[ClO2]
(2)
3
is
2
-
Our studies show that the reaction has no dependence on
the concentration of carbonate buffer (Table 2) or on
hydroxide ion concentration from p[H ] 10.0 to 11.2 (pK
+
+
-
W
18
-
-
)
13.6). As additional tests, we also prepared OBr stock
agreement with the kobsd/[OBr ] values for the reactions
by bubbling Br
Contamination of BrO
because locally high concentrations of Br
2
vapor into a well-stirred NaOH solution.
buffered with carbonate.
Interference from Bromite Ion below p[H ] 8.5. Prior
-
+
2
should be minimized by this method
are avoided and
-
2
to our efforts to minimize BrO concentrations in the
2
-
preparation of OBr solutions, we experienced difficulty in
(
18) Molina, M.; Melios, C.; Tognolli, J. O.; Luchiari, L. C.; Jafelicci, M.,
Jr. J. Electroanal. Chem. Interfacial Electrochem. 1979, 105, 237-
46.
-
obtaining reproducible rate constants for the OBr reaction
-
2
2
with ClO . After the preparation of OBr solution with only
Inorganic Chemistry, Vol. 41, No. 23, 2002 6101

Wang and Margerum
-
Table 3. Calculations for the Interference from [BrO2 ] When It Is
.1% [HOBr]T
0
HOBr] a
[ClO
]
b
Σk [Nu]c
Nu
r
-d
r
-d
r
-
/
-
[
T
2
i
OBr
BrO2
BrO2
p[H+]
(mM)
(mM)
(M
-1
s
-1
)
(M s-1)
(M s-1)
r
OBr
1.25e
14.5
14.5
14.5
14.3
2.37
2.72
0.248 1.56 × 10 7.66 × 10
7
-6
-6
-6
-6
-9
-9
6.34 × 10
5.70 × 10
4.38 × 10
1.13 × 10
7.20 × 10
9.80 × 10
-8
0.008
0.007
0.006
0.003
0.2
1
1
0.17e
0.248 1.29 × 10 7.66 × 10
7
-8
-8
-8
-10
-9
e
6
9
8
7
6
.55
.95
0.248 8.51 × 10 6.91 × 10
e
6
0.22
0.10
0.10
1.11 × 10 2.82 × 10
4.27 × 10
.64f
.41f
3.80 × 10
2.6
a
b
[
HOBr]T is the concentration used in the kinetic study. [ClO2]i is the
c
Nu
H O
OH
-
initial concentration of ClO2. ∑k [Nu] ) k
2
× 55.5 + k [OH ] +
CO
2-
+
-
Nu
k
3
[CO3 ]. At p[H ] < 7.64, [ClO2 ] ) 0, the ∑k [Nu] term does not
d
contribute to the rate. rOBr- and rBrO2- are the OBr and BrO2 reaction
rates at the first half-life of the ClO2 decay. [CO3]T ) 0.10 M. [HPO4]T
0.10 M.
-
-
e
f
)
-
0
.07% BrO
2
content, reproducibility was no longer a
+
problem above p[H ] 8.5. However, there are several reasons
interference still occurs at lower pH. First, the
disproportionation of HOBr to give BrO
orders of magnitude faster at pH 5-8 than it is at pH 11.
Therefore, at low pH, the concentration of BrO
-
why BrO
2
-
-
2
and Br is 3
-
-
Figure 4. Kinetic traces of the OX reactions with ClO2 in excess [ClO2 ].
1
2
-
-
(
a) Conditions for the OBr /ClO2 reaction: [OBr ] ) 17.02 mM, [CO3]T
-
-
+
increases
) 0.10 M, [ClO2]i ) 0.20 mM, [ClO2 ] ) 15.71 mM, p[H ] ) 10.50, µ
2
)
1.0 M, 25.0 °C, λ ) 390 nm, cell path ) 0.962 cm. (b) Conditions for
during the time of the reaction. Second, HOBr is a poor
catalyst (or a noncatalyst) of ClO disproportionation, and
-
-
the OCl /ClO2 reaction: [OCl ] ) 18.77 mM, [CO3]T ) 0.10 M, [ClO2]i
-
+
2
)
0.045 mM, [ClO2 ] ) 7.98 mM, p[H ] ) 10.30, µ ) 1.0 M, 25.0 °C,
λ ) 380 nm, cell path ) 1.0 cm.
longer reaction times are needed to observe the reaction at
+
p[H ] values below 8.5 where more HOBr is present. This,
-
in turn, allows more BrO
2
to form. Third, HOBr reacts
-
-
- 11
rapidly with ClO
The removal of ClO
2
at pH 5-8 to generate ClO
accelerates the BrO
3
and Br .
-
-
2
9
2
catalysis of ClO
2
-
disproportionation. Higher concentrations of ClO
2
are
-
needed to suppress the OBr catalysis. Because the pK
a
of
1
9
2
HBrO is 3.59, its formation is not significant in the pH
range of this study. However, low pH increases the rate of
-
BrO
2
formation because of general-acid assistance from
-
9,11,12
2 4
H PO . The combination of all these factors is complex.
Table 3 shows that, from our measured rate constants, the
-
effect of 0.1% BrO
2
T
in [HOBr] would be negligible for
-
the OBr /ClO
2
studies at pH 8.95 or higher but would be
significant at p[H ] 7.6 and lower (the details for the
+
calculation are shown in the Supporting Information). Figure
-
3
a shows that the pH dependence of the OBr /ClO
2
reaction
+
from p[H ] 8.73 to 11.23 (b) fits the assumption that HOBr
-
Figure 5. Suppression by [ClO2 ] of the second-order rate constant for
the reaction of OX with ClO . (a) Conditions for the OBr /ClO reaction:
2 2
-
-
does not react (eq 3). The open circles (O) in Figure 3a are
-
+
-
3 T 2 i
[OBr ] ) 17.02 mM, [CO ] ) 0.10 M, [ClO ] ) 0.20 mM, p[H ] )
from measured rate constants under conditions where BrO
2
-
1
0.50, µ ) 1.0 M, 25.0 °C, λ ) 390 nm. (b) Conditions for the OCl /ClO2
interference is expected. These are not included in the fit.
-
reaction: [OCl ] ) 18.77 mM, [CO3]T ) 0.10 M, [ClO2]i ) 0.045 mM,
+
2nd
obs
p[H ] ) 10.30, µ ) 1.0 M, 25.0 °C, λ ) 380 nm. Regression fit: k
)
HOBr
OX- OX
-
-
kobsd
Ka
2K1
k [OX ]/[ClO2 ].
OBr-
2
)
k
(3)
HOBr
+
[
HOBr]T
Ka
+ [H ]
[
HOBr] is approximately 1% of the [HOBr]
T
, so the reaction
-
between HOBr and ClO
slower than the OBr catalysis of ClO
We also find no dependence on Br and carbonate concen-
trations in the presence of excess ClO
2
under these conditions is much
-
Effect of Chlorite. When excess ClO
2
is added to the
-
2
disproportionation.
-
OBr /ClO
2
mixture, the reaction order shifts from first-order
-
to second-order dependence in [ClO
2
] (Figure 4a). The
-
2
(Table 2). This is
-
+
inhibitory effect of ClO
2
on the rate at p[H ] 10.5 is shown
in marked contrast to the large effect of many nucleophile
in Figure 5a. The observed second-order rate constant is
2-
-
species (including CO
3
and Br ) on the kinetics of the
and ClO because of nucleophile
assistance of the electron transfer between the intermediate
-
inversely proportional to the excess [ClO
2
]. It is known that
-
reaction between BrO
2
2
-
-
HOBr reacts with ClO
2
to generate ClO
2
and ClO
3
and
11
+
that the reaction is general-acid catalyzed. At p[H ] ) 10.5,
9
species BrO
Mechanism. The rate inhibition by ClO
cally significant and strongly suggests that an electron
2 2
with ClO .
-
2
is mechanisti-
(
19) Huff Hartz, K. E.; Nicoson, J. S.; Wang, L.; Margerum, D. W. Inorg.
Chem., accepted for publication.
6102 Inorganic Chemistry, Vol. 41, No. 23, 2002

Disproportionation of Chlorine Dioxide
-
transfer occurs from OBr to ClO
2
. The proposed mechanism
for the reaction is given in eqs 4-7, where BrO is a highly
reactive radical.
OBr-
1
k
-
-
-
OBr
ClO + OBr {
} ClO2 + BrO K1
(4)
2
OBr
k
-1
OBr
k2
BrO + ClO2
8 BrOClO2
(5)
(6)
(7)
rapid
rapid
-
-
BrOClO + OH
8OBr + HClO3
2
-
-
HClO + OH
8ClO3 + H O
3
2
-
It could react with ClO
with excess ClO
of various BrClO
by ab initio methods.20 Calculations show that formation of
BrClO by the reaction between BrO and ClO is very
favorable. The BrOCl(O)O conformation was found to be
2
to go back to the reactants or react
. The stabilities
species in the gas phase have been studied
2
to form an adduct, BrOClO
2
3
-
Figure 6. Eyring plots for the reaction of OX /ClO2. Conditions for the
-
-
OBr /ClO2 reaction: [HOBr]T ) 0.58 mM, [ClO2 ] ) 0.0 M, [ClO2]i )
0.010 mM, [CO ] ) 0.10 M, µ ) 1.0 M, p[H+] ) 10.70. Slope ) -(6.6
3
2
3
T
3
-
(
0.3) × 10 K, intercept ) 17.9 ( 0.9. Conditions for the OCl /ClO2
-
-
1
reaction: [HOCl]T ) 2.51 mM, [ClO2 ]i ) 0.0 M, [ClO2]i ) 0.10 mM,
3
the most stable of the BrClO isomers; it is 34.3 kJ mol
+
[
CO3]T ) 0.10 M, µ ) 1.0 M, p[H ] ) 10.60. Slope ) -(7.2 ( 0.4) ×
20
more stable than (BrO + OClO). This calculation by
Francisco and Clark20 supports the second step in our
proposed mechanism. Water solvation will undoubtedly have
a large effect on the stability and reactivity of the intermedi-
ates. However, gas phase calculations provide a useful model
to evaluate the plausibility of the proposed intermediate. The
103 K, intercept ) 19 ( 1.
OBr
-
Because the k
value is large, the addition of excess
1
could lead to the inequality, k-1 [ClO ] .
], in the denominator in eq 8. Elimination of the
] term gives the rate expression in eq 10. Equa-
-
OBr
-
ClO
2
2
OBr
k
[ClO
[ClO
2
2
OBr
k
2
2
-
-
2
BrOClO adduct can then be attacked by OH to give OBr
tions 10 and 11 are consistent with our experimental
observations that the addition of excess ClO
reaction to exhibit a second-order dependence in [ClO
and HOClO
2
(eq 6), and chloric acid is rapidly converted to
-
2
causes the
] and
-
ClO
3
as a final product in basic solution (eq 7).
2
A steady-state approximation in regard to BrO gives the
rate expression in eq 8, where the factor of 2 accounts for
the stoichiometric ratio in eqs 4 and 5. Typically, the
that the reciprocal of the observed second-order rate constant
-
is directly proportional to the excess [ClO
2
].
-
k
concentration of ClO
2
used in this study was low (<0.2 mM).
OBr OBr
-
2
2
K1
[OBr ][ClO ]
2
2
-
d[ClO ]/dt )
(10)
-
2
-
OBr
1
OBr
2
-
2
2
k
k
[OBr ][ClO ]
[ClO ]
2
2
-
d[ClO ]/dt )
(8)
2
OBr
-
OBr
2
k
[ClO ] + k [ClO2]
OBr OBr
-
k
[OBr-]
-
1
2
2
K1
2
obsd
nd
2
k
)
(11)
-
-] generated
-
When no initial ClO
2
was added, the [ClO
2
[ClO ]
2
from the disproportionation of ClO
2
was low, and the reverse
OBr
step (k-1 ) in eq 4 could be neglected. Under this condition
-
2nd
obsd
-
The slope of the plot of [OBr ]/k
versus [ClO ]
2
-
OBr OBr
-
(
[ClO
2
]/[ClO
2
] . k-1 /k2 ), the rate expression in eq 8
OBr
1
OBr
2
(
(
K
M
Supporting Information Figure 2) gives 1/(2K
k
) )
simplifies to eq 9. To meet the required condition, the data
for the first 75% of the reaction were used to determine the
observed first-order rate constants.
-
2
-1 -1
3.86 ( 0.09) × 10
M
s . This slope value and the
to be evaluated as 1.0 × 10
-
OBr
1
OBr
2
8
value allow k
-
1
-1
s .
-
OBr
-
Activation Parameters Measurement. The temperature
-
d[ClO ]/dt ) 2k1 [OBr ][ClO ] ) k [ClO ] (9)
2 2 obsd 2
dependence of the kinetics was determined over the tem-
-
+
From the slope of the kobsd versus [OBr ] plot in Figure
is determined as 2.05 ( 0.03 M s . Reduction
potential values for OBr/OBr and ClO
perature range from 5 to 25 °C at p[H ] ) 10.70 with
-
-
OBr
-1 -1
[
HOBr]
T
) 0.58 mM, [ClO
2
]
i
) 0.010 mM, no added ClO
2
,
2
a, k1
‡
-
-
and µ ) 1.0 M. The activation enthalpy, ∆H ) 55 ( 1 kJ
/ClO
2
are reported
1
2
-
1
‡
-1
21
mol , and the activation entropy, ∆S ) -49 ( 2 J mol
to be 1.34 and 0.934 V, respectively. These potentials lead
1
-
-1
-
7
OBr
1
K , of the reaction are obtained from an Eyring Plot (Figure
6a). The activation parameters determined under this condi-
tion correspond to the electron-transfer step in eq 4. The
to a value of 1.3 × 10 for the equilibrium constant K
-
-
OBr
OBr
-1
OBr
1
(
k
)k
/k ) for the first step (eq 4). From the K
and
-1 -1
1
-
OBr
OBr
7
values, k-1 is estimated to be 1.6 × 10 M s .
1
‡
‡
positive ∆H
need to bring OBr and ClO
rangement prior to electron transfer to give OBr and ClO
1
and negative ∆S
1
values correspond to the
-
2
together as well as their rear-
(
(
20) Francisco, J. S.; Clark, J. J. Phys. Chem. A 1998, 102, 2209-2214.
21) Stanbury, D. M. AdV. Inorg. Chem. 1989, 33, 69-138.
-
2
.
Inorganic Chemistry, Vol. 41, No. 23, 2002 6103

Wang and Margerum
-
II. Reaction between OCl and ClO
2
. An early study of
that the rate constant for the solvated electron reaction with
-
9
-1 -1 26
9
-1 -1 27
the decomposition of ClO in aqueous solution in the
presence of HOCl was reported in 1959 by Flis and
co-workers.22 In a more recent investigation of the reaction
2
ClO
2
was 2.5 × 10 M
s
and 4.4 × 10 M s .
As will be shown, we find a value of 3.6 × 10 M s-1 for
9 -1
-
-
the ClO reaction with ClO
2 2
to give ClO and OCl (eq 12,
8
OCl
-1
by Csord a´ s and co-workers, a mechanism was proposed in
k
). This strongly suggests that ClO may be an interme-
-
-
-
which ClO
2
and OCl form an adduct in a rate-determining
step, followed by a rapid electron transfer from the adduct
to the second ClO . Our results from a spectrophotometric
study of the reaction stoichiometry agree with their conclu-
sion that OCl catalyzes the decomposition of ClO
2
diate in the reaction of e(aq) with ClO , and the presence
of excess ClO2 causes the formation of ClO and OCl .
-
-
2
2
Hence, our results are in substantial agreement with those
2
6
27
of Eriksen et al. and Zuo et al.
-
2
. We also
] and [OCl ]
According to the mechanism in eqs 12-15, the rate
-
-
observed a first-order dependence in both [ClO
2
expression for the reaction of OCl and ClO is given in eq
2
-
(
Figure 2b) when ClO
2
and OCl reactant solutions were
16.
mixed. However, we found important evidence that Csord a´ s
-
k
-
OCl OCl
-
2
et al. missed, in which addition of excess ClO
2
changes
2k1
[OCl ][ClO ]
2
2
-
d[ClO ]/dt )
(16)
the reaction order from first-order to second-order depen-
dence in [ClO ] (Figure 4b) and suppresses the rate (Figure
b). These observations indicate that an electron transfer must
2
OCl
-
OCl
k
[ClO ] + k [ClO2]
2 2
-
1
2
5
Equations 17 and 18 give the simplified rate expressions.
-
2
occur from OCl to ClO in the first step, which therefore
leads to a different mechanism from the one proposed by
these authors. We find no buffer dependence for the reaction
-
OCl
1
-
-
-
d[ClO ]/dt ) 2k
[OCl ][ClO ]
2
2
OCl
OCl
-1
-
(
Table 2). The mechanism we propose (eqs 12-15) is
(when k2 [ClO ] . k [ClO ]) (17)
2
2
-
analogous to that for the reaction of OBr with ClO
2
(eqs
-
OCl OCl
[OCl-]
4-7).
2k1
k
2
2
d[ClO ]/dt )
[ClO2]
2
-
OCl
-
OCl
k1
k
-
1
[ClO ]
2
-
-
-
OCl
ClO + OCl { } ClO2 + ClO K1
(12)
OCl
-
OCl
2
2
OCl
(
when k-1 [ClO ] . k [ClO ]) (18)
k1
2
2
OCl
k2
We followed the reaction for only 50% loss of ClO
the [OCl ] dependence study to meet the requirement for
in
2
ClO + ClO2
8 ClOClO2
(13)
(14)
(15)
-
-
OCl OCl
-1
eq 17 that [ClO
2
]/[ClO
2
] . k /k2 . From the value of
rapid
-
-
-
ClOClO + OH
8 OCl + HClO3
OCl
1
2
the slope in Figure 2b, k
is determined as 0.91 ( 0.04
s . By using the reported potential values, E°(OCl/
-
1
-1
21
M
rapid
-
8 ClO3- + H O
HClO + OH
-
-
3
2
OCl ) ) 1.50 V, E°(ClO
2
/ClO
) ) 0.924 V, the equilibrium
for the electron-transfer step in eq 12 is
2
-
OCl
23
constant K1
Calculation of the stability of Cl
formation of ClOClO from the reaction in eq 13 would be
favorable by 45.6 kJ mol in the gas phase.
Csord a´ s et al. excluded the possibility of an electron
2
O
3
shows that the
-
10
calculated to be 2.55 × 10 . To have second-order de-
2
-
-
1
pendence in [ClO ] (eq 18), concentrations of ClO ranging
2
2
-
8
from 7.98 to 14.63 mM were used in the [ClO ] dependence
study to meet the required condition: k
2
-
OCl
1
[ClO
2
-] .
transfer between the reactants on the basis of the work by
OCl
-
F a´ bi a´ n et al.24 and Buxton et al. However, the mechanism
25
k
[ClO
2
]. However, at these high [ClO
2
], the kinetic
2
-
proposed for the ClO reaction with ClO
F a´ bi a´ n et al. actually supports our proposed k-1 and k2
steps in eqs 12 and 13. When Buxton and Subhani
performed their pioneering pulse radiolysis study in 1972,
it was erroneously thought that an OH-type radical was the
primary product of the reaction between a hydrated electron
and a halogen oxoanion. Later pulse radiolytic studies by
Eriksen et al.26 and Zuo et al.27 detected and demonstrated
2
and ClO
2
by
traces deviate slightly from a second-order fit because of
2
4
OCl
OCl
-
-
28
the interference of the OCl /ClO
2
reaction. To avoid this
2
5
problem, the rates were analyzed by an initial rate method
2
9,30
with a five-point smoothing derivative program.
From
-
-
OCl
1
, k1 , and the slope of the plot [OCl-]/
OCl
the values of K
2
nd
-
k
k
2
against [ClO ], the following values were obtained:
obsd
OCl
9
-1 -1
OCl OCl
OCl
) 3.6 × 10 M s , k /k2 ) 0.260, k2 ) 1.4 ×
-
1
10
-1
-1 -1
OCl
1
0 M s . The resolved k value exceeds the diffusion
2
-
that ClO
2
and OCl are the products of the reaction between
9
-1 -1
controlled rate constant value (7 × 10 M s ), which is
-
OCl
the solvated electron and ClO
2
. Their interpretations were
not reasonable. However, because the k value is calcu-
2
-
-
lated indirectly from E°(OCl/OCl ) and E°(ClO
2
/ClO
2
), and
(
22) Flis, I. E.; Mishchenko, K. P.: Salnis, K. Yu. J. Appl. Chem. 1959,
-
the E°(OCl/OCl ) value was estimated on the basis of some
assumptions, a small error in this value could cause a
deviation of the k2 value. If we set the k2 to 7 × 10
3
2, 295-301.
2
1
(
23) Clark, J.; Francisco, J. S. J. Phys. Chem. A 1997, 101, 7145-7153.
24) F a´ bi a´ n, I.; Szucs, D.; Gordon, G. J. Phys. Chem. A 2000, 104, 8045-
(
OCl
OCl
9
8049.
(
(
(
25) Buxton, G. V.; Subhani, M. S. J. Chem. Soc., Faraday Trans. 1 1972,
8, 947-955.
26) Eriksen, T. E.; Lind, J.; Mer e´ nyi, G. J. Chem. Soc., Faraday Trans.
6
(28) Gordon, G.; Tachiyashiki, S. EnViron. Sci. Technol. 1991, 25, 468-
474.
(29) The program, provided by H. L. Pardue, Department of Chemistry,
Purdue University, is based on ref 30.
1
, 1981, 77, 2115-2123.
27) Zuo, Z.; Katsumura, Y.; Ueda, K.; Ishigure, K. J. Chem. Soc., Faraday
Trans. 1997, 93, 1885-1891.
(30) Savitzky, A.; Golay, M. Anal. Chem. 1964, 36, 1627-1639.
6104 Inorganic Chemistry, Vol. 41, No. 23, 2002

Disproportionation of Chlorine Dioxide
Table 4. Summary of Rate Constants and Activation Parameters for Hypohalite Ion Catalysis of the Disproportionation of Chlorine Dioxidea
b
∆H‡
‡
rate
constant
value
∆S
-
1
(kJ mol-1)
(J mol K-1
)
-1
reaction
(M s-1
)
OBr-
OBr-
1
k
{
2.05(3)
55(1)
61(3)
-49(2)
1
k
-
-
ClO + OBr
} ClO2 + BrO
2
OBr
k-1
OBr
7
8
k
1.6 × 10
.0 × 10
-
1
OBr
k
2
OBr
2
1
k
BrO + ClO2
8 BrOClO2
OCl-
1
OCl-
1
OCl
k
0.91(4)
-43(2)
-
-
k
k
k
ClO + OCl
{
} ClO2 + OCl
2
OCl
k-1
9
3.6 × 10
-
1
OCl
2
k
9
OCl
2
7
× 10
ClO + ClO2
8 ClOClO2
a
Conditions: µ ) 1.0 M, [CO3]T ) 0.10 M. b 25.0 °C
-
M- s-1, then K O1 Cl and E°(OCl + ClO
1
rate constants and the activation parameters obtained in the
present study are summarized in Table 4.
Conclusions. The first step of the hypohalite ion catalysis
2
2
f ClOClO ) are
-
10
calculated to be 5.11 × 10 and -0.549 V, respectively.
-
Correspondingly, E°(OCl/OCl ) equals 1.48 V assuming that
-
the value of E°(ClO
between this value and the reported value is
.02 V, which is within the error of the literature value for
E°(OCl/OCl ). On the basis of this argument, we consider
the formation of ClOClO
As shown in Figure 3b, the excellent fit of the pH
dependence data to eq 19 indicates the lack of an HOCl
) 7.47 at 25.0 °C , µ ) 1.0 M) path. The level of
generated from the disproportionation of ClO
low to inhibit the reaction. In addition, the HOCl/ClO
reaction will convert ClO
2
/ClO
2
) is accurate. The difference
of the disproportionation of ClO
2
is the reaction between
-
-
OX and ClO
2
via electron transfer to give OX and ClO
2
.
-
-
OBr
OCl
-
0
At 25.0 °C, k1 /k1 ) 2.3, and ClO
2
suppresses the rate
-
of both reactions. Subsequent steps between the XO and ClO
2
OBr
2
2
in eq 13 to be diffusion controlled.
radicals to give XOClO are extremely fast (k
2
) 1.0 ×
8
-1 -1
OCl
9
-1 -1
10 M
s
and k2 ) 7 × 10 M s ), and the base
- -
2 3
hydrolysis of XOClO to give OX and ClO is also fast.
HOCl
31
-
(
pKa
2 2
Catalysis of ClO disproportionation by BrO is also very
effective, and the rate constant for the first electron-transfer
-
9
ClO
2
2
is too
-
-
-1 -1
2
step between BrO and ClO is 36 M
2
-
2
s
(17 times larger
-
-
- 32
2
to ClO
3
and Cl .
than that of OBr ).
Acknowledgment. This work was supported by National
Science Foundation Grants CHE-9818214, CHE-0139876
and the Purdue Research Foundation. The authors wish to
thank one of the reviewers for thoughtful suggestions about
the agreement between our work and the pulse radiolysis
studies.
HOCl
kobsd
Ka
OCl-
)
2k1
(19)
HOCl
+
[
HOCl]T
Ka
+ [H ]
The activation parameters for the reaction were determined
‡
-1
(
∆
Figure 6b) as the following: ∆H
1
) 61 ( 3 kJ mol ,
) -43 ( 2 J mol K . These values are in good
agreement with the results reported by Csord a´ s et al. The
‡
-1
-1
S
1
Supporting Information Available: Tables and figures with
supplemental data and the details for the calculation for the data in
Table 3. This material is available free of charge via the Internet
at http://pubs.acs.org.
8
(
31) Gerritsen, C. M.; Margerum, D. W. Inorg. Chem., 1990, 29, 2757-
762.
32) Jia, Z.; Margerum, D. W.; Francisco, J. S. Inorg. Chem. 2000, 39,
614-2620.
2
(
2
IC020440M
Inorganic Chemistry, Vol. 41, No. 23, 2002 6105