Kinetics and mechanisms of aqueous chlorine reactions with chlorite ion in the presence of chloride ion and acetic acid/acetate buffer

Article abstract of DOI:10.1021/ic010762a

The kinetics and mechanism of the reaction between Cl₂ and CIO₂^- are studied in acetate buffer by stopped-flow spectrometric observation of CIO₂ formation. The reaction is first-order in [CI₂] and [CIO₂^-], with a rate constant of k₁ = (5.7 ± 0.2) × 10⁵ M^-1 s^-1 at 25.0 °C. Nucleophilic attack by CIO₂^- on CI₂, with CI⁺ transfer to form CIOCIO and CI^-, is proposed as the rate-determining step. A possible two-step electron-transfer mechanism for CI₂ and CIO₂^- is refuted by the lack of CIO₂ suppression. The yield Of CIO₂ is much less than 100%, due to the rapid reactions of the metastable CIOCIO intermediate via two competing pathways. In one path, CIOCIO reacts with CIO₂^- to form 2CIO₂ and CI^-, while in the other path it hydrolyzes to give CIO₃^- and CI^-. The observed rate constant also is affected by acetate-assisted hydrolysis of CI₂. The rate of CI₂ loss is suppressed as the concentration of CI^- increases, due to the formation of CI₃^-. In excess CIO₂^-, a much slower formation of CIO₂ is observed after the initial CI₂ reaction, due to the presence of HOCI, which reacts with H⁺ and CI^- to re-form steady-state levels of CI₂.

Full text of DOI:10.1021/ic010762a

Inorg. Chem. 2002, 41, 342−347
Kinetics and Mechanisms of Aqueous Chlorine Reactions with Chlorite
Ion in the Presence of Chloride Ion and Acetic Acid/Acetate Buffer
Jeffrey S. Nicoson and Dale W. Margerum*
Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907-1393
Received July 17, 2001
-
The kinetics and mechanism of the reaction between Cl
spectrometric observation of ClO
2
and ClO
2
are studied in acetate buffer by stopped-flow
-
2
formation. The reaction is first-order in [Cl
) (5.7 ± 0.2) × 10 M s at 25.0 °C. Nucleophilic attack by ClO
and Cl , is proposed as the rate-determining step. A possible two-step electron-transfer mechanism for Cl
is much less than 100%, due to the rapid reac-
tions of the metastable ClOClO intermediate via two competing pathways. In one path, ClOClO reacts with ClO
2
] and [ClO
2
], with a rate constant of
5
-1 -1
-
+
k
1
2
2
on Cl , with Cl transfer to form ClOClO
-
2
and
-
ClO
2
is refuted by the lack of ClO
2
suppression. The yield of ClO
2
-
2
-
-
-
to form 2ClO
also is affected by acetate-assisted hydrolysis of Cl
increases, due to the formation of Cl -
. In excess ClO
initial Cl
2
and Cl , while in the other path it hydrolyzes to give ClO
3
and Cl . The observed rate constant
2
. The rate of Cl
2
loss is suppressed as the concentration of Cl-
-
3
2
, a much slower formation of ClO
2
is observed after the
+
-
2
reaction, due to the presence of HOCl, which reacts with H and Cl to re-form steady-state levels of Cl
2
.
-
1,2
Introduction
Reactions of oxochlorine species that involve a Cl
intermediate were first discussed by Taube and Dodgen.
Cl
2
and ClO
2
is shown in eqs 1 and 2. Acidic conditions
-
and high concentrations of ClO
ClO
favor the formation of ClO
2
favor the formation of
2
O
2
1
-
2
, while basic conditions and low concentrations of ClO
2
-
2
3
.
Emmenegger and Gordon supported the existence of such
-
an intermediate in their study of the reaction between ClO
and HOCl/Cl
alternative oxidant in water treatment and pulp bleaching
2
-
-
-
2
(aq). The emergence of chlorine dioxide as an
Cl
2
+ 2ClO
2
f 2ClO
2
+ 2Cl
(1)
(2)
3,4
5
-
-
-
6
-11
and the development of chlorite-based oscillating reactions
has stimulated continued discussion about the formation and
Cl
2
+ ClO
2
+ 2OH f ClO
3
+ 2Cl + H
2
O
rapid reactions of XOClO (or XClO ) intermediates (X )
2
Jia et al.15 and Furman and Margerum14 have shown that
Cl, Br, I).1
2-16
The stoichiometry for the reaction between
-
the reactions of HOCl and HOBr with ClO
intermediate by halogen cation (X ) transfer (eqs 3-4).
2
form an XOClO
+
*
Author to whom correspondence should be addressed. E-mail:
margerum@purdue.edu.
These studies have also provided detailed schemes for the
(
(
(
1) Taube, H.; Dodgen, H. J. Am. Chem. Soc. 1949, 71, 3330-3336.
2) Emmenegger, F.; Gordon, G. Inorg. Chem. 1967, 6, 633-635.
3) Aieta, E. M.; Roberts, P. V. In Water Chlorination; Jolley, R. L., Bull,
R. J., Davis, W. P., Katz, S., Roberts, M. H., Jr., Jacobs, V. A., Eds.;
Lewis Publishers: Chelsea, MI, 1985; Vol. 5, pp 783-794.
rapid reactions of ClOClO and BrOClO to produce ClO
2
and
-
ClO
3
(eqs 5 and 6).
HOX + ClO2 h HOXOClO-
-
(3)
(4)
(
4) Vel Leitner, K. N.; De Laat, J.; Dore, M.; Suty, H. In Disinfection
By-Products in Water Treatment; Minear, R. A., Amy, G. L., Eds.;
CRC Press: Boca Raton, FL, 1996; pp 393-407.
-
-
HOXOClO + HA f XOClO + H O + A
(
5) Hart, P. W.; Hsieh, J. S. AIChE Symp. Ser. No. 319 1998, 94,
2
7
3-81.
(
6) Grant, J. L.; De Kepper, P.; Epstein, I. R.; Kustin, K.; Orban, M. Inorg.
Chem. 1982, 21, 2192-2196.
(12) Aieta, E. M.; Roberts, P. V. EnViron. Sci. Technol. 1986, 20, 50-55.
(
(
7) Epstein, I. R.; Kustin, K. J. Phys. Chem. 1985, 89, 2275-2282.
8) Valdes-Aguilera, O.; Boyd, D. W.; Epstein, I. R.; Kustin, K. J. Phys.
Chem. 1986, 90, 6696-6702.
(13) Gordon, G.; Tachiyashiki, S. EnViron. Sci. Technol. 1991, 25, 468-
474.
(14) Furman, C. S.; Margerum, D. W. Inorg. Chem. 1998, 37, 4321-
(
9) Rabai, G.; Beck, M. T. Inorg. Chem. 1987, 26, 1195-1199.
4327.
(
10) Peintler, G.; Nagypal, I.; Epstein, I. R. J. Phys. Chem. 1990, 94, 2954-
(15) Jia, Z.; Margerum, D. W.; Francisco, J. S. Inorg. Chem. 2000, 39,
2614-2620.
(16) Fabian, I.; Gordon, G. Inorg. Chem. 1992, 31, 2144-2150.
2958.
(11) Rabai, G.; Orban, M. J. Phys. Chem. 1993, 97, 5935-5939.
342 Inorganic Chemistry, Vol. 41, No. 2, 2002
10.1021/ic010762a CCC: $22.00 © 2002 American Chemical Society
Published on Web 12/27/2001

Aqueous Chlorine Reactions with Chlorite Ion
-
-
an Applied PhotoPhysics stopped flow spectrophotometer (APPSF)
model SX-18MV with an optical path length of 0.962 cm. The
APPSF was calibrated to determine the mixing efficiency of the
instrument using well-characterized chemical reactions such as
XOClO + ClO2 f 2ClO + X
(5)
(6)
2
-
-
+
XOClO + H O f ClO3 + X + 2H
2
2-
4-
-
21-23
IrCl
6
/Fe(CN)
6
2
, NH Cl/I , and the Dushman reaction.
The
-
The reaction between Cl
2
and ClO
2
is much more rapid
observed rate constants were corrected for the mixing efficiency
of the APPSF on the basis of eq 7
than the corresponding HOCl and HOBr reactions. Aieta and
Roberts12 estimated the rate constant for the Cl
/ClO
-
2
2
4
-1 -1
1
1
-1
reaction to be k ) 1.62 × 10 M
s
(20 °C, µ ) 0 M)
kr )
-
(7)
(k
kmix)
using mass-transfer theory for the difference in absorption
obsd
-
of Cl
2
(g) into a ClO
2
solution versus absorption into an
HCl solution. However, in their experiments, impure NaClO
78-96%) was used, there was no pH control, and the effect
and the dead time of the instrument (t
d
) was determined
2
from eq 8.
(
-
of Cl
3
was ignored.
∆
Aobsd
Apred
-
ln
Historically, the reactions of Br
been assumed to follow the same general mechanism of
HOBr and HOCl. However, Toth and Fabian have recently
2
and Cl
2
with ClO
2
have
(
)
∆
td ) -
(8)
kobsd
17
shown an alternative mechanism for the reaction between
At a drive pressure of 120 psi, the value of kmix is 4620 ( 120 s-1
and t is 2.0 ( 0.1 ms. However, visual inspection showed that t
varied between 2 and 3 ms over the course of this study, so each
stopped-flow trace was examined individually to determine the
starting point of the first-order fit.
-
Br
2
+
and ClO
2
that proceeds by electron transfer rather than
d
d
Br transfer. Detailed kinetic data are required to determine
whether the Cl reaction with ClO proceeds by Cl transfer
2 2
-
+
or electron transfer. The present work uses stopped-flow
-
spectroscopy to study the Cl
2
/ClO
2
reaction to obtain a more
Results and Discussion
reliable rate constant and determine the mechanism of the
reaction. Chlorine hydrolysis pathways that compete with
-
Competition between Cl
Reaction. Prior to mixing with the ClO
2
Hydrolysis and the Cl
2
/ClO
2
-
the reaction are discussed, along with a comparison of the
2
solution, the
-
-
reactivity and mechanisms of ClO
and Br
2
2
with Cl , HOCl, HOBr,
chlorine solution is typically kept at pH 2 and 1.0 M Cl .
2
.
2
Under these conditions, the solution consists of 78% Cl ,
-
-
1
4% Cl
3
, and 8% HOCl. The ClO
2
solution is buffered at
Experimental Section
Reagents. Doubly deionized, distilled water was used for all
-
pH 3.6-6.0 with HOAc/OAc in order to avoid reactions
of HClO
buffered ClO
2
. Upon mixing the chlorine solution with the
-
2
solution, the chlorine solution experiences a
solution preparation. Stock solutions of NaClO were recrystallized
4
solutions,14 NaOCl solutions,14 and “Br -
-
rapid increase in pH. The rapid increase in pH causes a shift
in the Cl /HOCl equilibrium toward formation of HOCl.
in water before use. ClO
2
free” NaCl solutions18 were prepared as previously reported. The
hypochlorite solutions were standardized spectrophotometrically at
2
However, this equilibrium is not established instantaneously
2
92 nm (ꢀOCl- ) 362 M cm ). Commercial sodium chlorite
-
1
-1 14
compared to the Cl /ClO reaction. The loss of Cl through
-
2
2
2
14
-
was purified as previously reported. The ClO
standardized spectrophotometrically at 260 nm (ꢀClO2
2
solutions were
-
hydrolysis competes with the Cl
2
/ClO
2
reaction, and the
-
1
-
) 154 M
, was used to
solutions.
kinetic analysis must account for this. The reverse reaction
-
1 14
cm ). Perchloric acid, prepared from 70% HClO
acidify NaOCl solutions in the preparation of Cl
4
-
+
of HOCl, Cl , and H to form Cl
2
is much slower than the
2
-
Cl
2
/ClO
2
reaction. Therefore, the concentration of HOCl
Methodology and Instrumentation. The measured pH was
in the chlorine solution will not affect the rate of the reaction.
+
converted to p[H ] on the basis of electrode calibration at µ ) 1.0
2
4
Lifshitz and Perlmutter-Hayman and Wang and Marg-
M. NaClO
4
or NaCl was used to control ionic strength. A T-mixer
erum2 showed that Cl
5
-
+
2
hydrolysis in acetic acid/acetate
was used to mix solution A (containing Cl and H ) and solution
B (containing Cl and OCl ) to confine the resulting Cl solution
2
-
-
buffer proceeds through two parallel pathways: an acetate-
2
4
assisted hydrolysis pathway (eq 9)
within a syringe. The total chlorine concentration is defined as [Cl
2
]
T
-
)
[Cl
2
] + [Cl
3
] + [HOCl]. On the 0-50 ms time scale, HOCl is
OAc-
ka
1
5
unreactive, and its contribution to total chlorine is neglected. The
-
-
Cl + OAc + H O yk-aHOAcz HOCl + HOAc + Cl (9)
2
2
reaction was initiated by mixing the A + B solution with solution
-
-
-
C (containing Cl , ClO
2
, and HOAc/OAc ) and followed at 359
-1
-1 14
and a conventional hydrolysis pathway25 (eq 10).
nm for ClO formation (ꢀ359 ) 1230 M cm ). The concentration
2
of HClO
2
(pK
+
a
) 1.61)19 is negligible under the conditions of this
ka
H2O
study (p[H ] ) 3.6-6.0). When only chlorine hydrolysis pathways
+
-
Cl + H O y z HOCl + H + Cl
(10)
2
2
H
+
were studied, the reaction was followed at 240 nm for the loss of
k
-a
-
-1
-1 20
Cl
3
(ꢀ240 ) 7000 M cm ). Kinetic data were obtained by using
(
21) Baron, C. D.; Margerum, D. W. Unpublished results.
(
(
(
17) Toth, Z.; Fabian, I. Inorg. Chem. 2000, 39, 4608-4614.
18) Jia, Z.; Margerum, D. W. Inorg. Chem. 1999, 38, 5374-5378.
19) Smith, R. M.; Martell, A. E. Critical Stability Constants; Plenum
Press: New York, 1976; Vol. 4, p 134.
(22) Dickson, P. N.; Margerum, D. W. Anal. Chem. 1986, 58, 3153-3158.
(23) Xie, Y.; McDonald, M. R.; Margerum, D. W. Inorg. Chem. 1999, 38,
3938-3940.
(24) Lifshitz, A.; Perlmutter-Hayman, B. J. Phys. Chem. 1961, 65, 753-
(20) Wang, T. X.; Kelley, M. D.; Cooper, J. N.; Beckwith, R. C.; Margerum,
757.
D. W. Inorg. Chem. 1994, 33, 5872-5878.
(25) Wang, T. X.; Margerum D. W. Inorg. Chem. 1994, 33, 1050-1055.
Inorganic Chemistry, Vol. 41, No. 2, 2002 343

Nicoson and Margerum
Figure 1. Dependence of the first-order rate constant of Cl2 hydrolysis
-
+
on acetate concentration. [Cl2]T ) 0.43 mM, [Cl ] ) 0.78-0.96 M, p[H ]
)
4.42(3), µ ) 1.0 M (NaCl), 25.0 °C, λ ) 240 nm.
Figure 2. (a) Absorbance versus time for the Cl2/ClO2- reaction, where
3
(
5% of the reaction has occurred in the dead time (2.5 ms). The solid line
only visible when t < 2.5 ms) is the exponential fit to the data. (b)
Absorbance versus time for the subsequent HOCl reaction. [Cl2]T ) 20
H O
The values of k
a
2
and k-aH+ obtained by Wang and
25
-1
-2 -1
Margerum, 22.3 s and 21 400 M s , respectively, were
-
-
-
+
µM, [ClO2 ] ) 0.300 mM, [OAc ]T ) 58 mM, [Cl ] ) 0.50 M, p[H ] )
.44, µ ) 1.0 M (NaClO4), 25.0 °C, λ ) 359 nm.
determined at 25.0 °C with an ionic strength of 0.50 M. The
4
H O
k
a
2
value is also valid at µ ) 1.0 M, since both species are
H+
uncharged. However, the value of k-a will change with
ionic strength and can be extrapolated using the Debye-
2
5
H+
H u¨ ckel-Brønsted-Davies relationship. The value of k-a
at m ) 1.0 M is 22 900 M-2 s .
-1
2
4
Since Lifshitz and Perlmutter-Hayman studied the reac-
tion in eq 9 at 9.5 °C with no ionic strength control, the rate
constants for the acetate-assisted pathway under the condi-
tions of this study (25.0 °C, m ) 1.0 M) were measured.
HOAc
The acetate concentration is determined using pK
a
)
)
2
6
-
OAc-
4
.61. A plot of kobsd vs [OAc ] has a slope of k
a
3
-1 -1
H O
2
(
(
2.06 ( 0.03) × 10 M
s
and an intercept of k
a
) 22
-
1
H O
4 s (Figure 1). The k
a
2
value agrees with that obtained
25
HOAc
by Wang and Margerum. The value of k-a
conditions can be calculated from k-a
K
the reaction of Cl
of Cl
under these
HOAc
OAc-
H+
) (k
a
k
-a
/
-
Figure 3. Dependence of the first-order rate constant for the Cl2/ClO2
HOAc H O
-2 -1
k
a
2
) ) 52 M s . All observed rate constants for
-
reaction on chlorite concentration. [Cl2]T ) 15-85 µM, [OAc ]T ) 0.05-
0.43 M, [Cl ] ) 0.50-1.0 M, p[H ] ) 4.20-6.04, µ ) 1.0 M (NaCl),
-
-
+
2
with ClO
2
are corrected for hydrolysis
25.0 °C, λ ) 359 nm.
2
.
+
Trichloride Suppression. At p[H ] ) 4.5, large concen-
trations of Cl are required to shift the equilibrium in eq 10
-
Kinetics and Mechanism. Under typical conditions, the
-
reaction between Cl
2
and ClO
2
goes to completion within
toward formation of Cl
2
. In this study, the concentration of
-
10-20 ms. The dead time on the APPSF instrument is 2-3
ms, so the first 25-50% of the reaction cannot be followed.
Cl is 0.5-1.0 M. Under these conditions, an appreciable
-
amount of Cl
3
2
is in rapid equilibrium with Cl , according
-
-
1
20
Upon mixing under pseudo-first-order conditions (ClO
at least 10-fold excess of Cl
order increase in absorbance due to the formation of ClO
2
in
to eq 11 (KCl3- ) 0.18 M ). Since NaCl is also used to
control ionic strength, the concentration of Cl varies from
one reaction to another.
-
2
), the reaction shows a first-
2
,
indicating a first-order dependence on [Cl
2
] (Figure 2a). After
-
K
-
correction of kobsd for mixing, Cl
, and hydrolysis pathways,
Cl3
3
-
-
Cl + Cl y z Cl3
(11)
+
-
-
2
T
kcorr values are independent of p[H ], [OAc ] , and [Cl ].
This indicates that acids, bases, and chloride ion do not
contribute to the reaction during the rate-determining step.
Therefore, kobsd is corrected for the fraction of total chlorine
-
(
[Cl
2
]
T
) [Cl
2
] + [Cl
3
]) present as Cl
2
. Combining the
-
As the concentration of ClO
a first-order dependence on [ClO
2
increases, kcorr increases with
-
correction for Cl
corrected pseudo-first-order rate constant for the Cl
reaction is defined in eq 12.
3
, hydrolysis, and mixing efficiency, a
-] (Figure 3). The least-
2
-
2 2
/ClO
squares slope of the data gives a rate constant of k
1
) (5.7
2
5
-1 -1
(
ClO
0.2) × 10 M
s
at 25.0 °C for the reaction of Cl with
-
2
. This rate constant is 35 times larger than the
-
-
OAc
-
H2O
kcorr ) k (1 + K [Cl ]) - ka
Cl3-
[OAc ] - ka
(12)
12
r
previously estimated value by Aieta and Roberts.
Using the known behavior of the system with respect to
-
-
-
+
(26) Portanova, R. et al. Inorg. Chim. Acta 1974, 8, 233-240.
2
[ClO ], [OAc ], [Cl ], and [H ], a detailed mechanism is
344 Inorganic Chemistry, Vol. 41, No. 2, 2002

Aqueous Chlorine Reactions with Chlorite Ion
Scheme 1. Proposed Mechanism for the Reaction of Chlorine and
Chlorite Ion
Chlorine is also a strong oxidizing agent, and the pos-
-
sibility exists for an electron to transfer from ClO
2
to Cl
2
-
to form ClO
2
and a reactive Cl
2
species (eq 13).
ke
-
-
Cl + ClO y z Cl + ClO2
(13)
2
2
2
k-e
-
A second electron transfer would yield 2Cl and another
ClO (eq 14).
2
kee
Cl2- + ClO2-
98 2Cl + ClO
-
(14)
2
-
proposed in Scheme 1. Cl
2
reacts with ClO
2
in the rate-
+
The steady-state rate expression for this mechanism would
have a complex order in [ClO ] and a [ClO ] term in the
determining step by transfer of Cl to form ClOClO and
Cl . Since there is no inhibition of the rate by Cl (other
-
-
-
2
2
-
denominator (eq 15).
than from formation of unreactive Cl
3
, for which the
observed rate constant has already been corrected) and a first-
-
2
-
-d[Cl2]
k k [ClO ] [Cl ]
e ee 2 2
order dependence in [ClO
intermediate must react rapidly with ClO
or rapidly hydrolyze to produce ClO
2
] is observed, the ClOClO
)
(15)
-
-
2
to produce ClO
2
dt
k [ClO ] + k [ClO ]
-
e
2
ee
2
-
3
. The yield of ClO
2
-
1
-1
(
determined from ∆A, ꢀ359 ) 1230 M cm ) is typically
0-60%, according to the stoichiometry in eq 1, but is
uncertain due to Cl volatility and the small absorbance
Since the experimental data show a first-order dependence
], the reaction between Cl and ClO (k ) must be
2 2 2 e
the rate-determining step if a pathway such as this contributes
2
-
-
in [ClO
2
changes (∆A ) 6-100 mAU). In a competing reaction, Cl
2
27
to kobsd. Using reduction potentials (eqs 16 and 17)
-
hydrolyzes to form HOCl. At 0.3 mM ClO
2
and 0.05 M
-
-
-
OAc , this hydrolysis accounts for the loss of 30% of the
Cl + e f Cl
E° ) 0.70 V
E° ) 0.934 V
(16)
(17)
2
2
initial Cl
2 2
. However, if the Cl lost to hydrolysis is taken
-
-
into consideration, the yield of ClO
2
is never greater than
ClO + e f ClO
2
2
-
80%. Therefore, a significant amount of ClO
3
must be
under these
-
and self-exchange rate constants28 (eqs 18 and 19),
formed from the oxidation of ClO
2
by Cl
2
-
conditions. Details about the formation of ClO
3
and ClO
2
-
-
from the ClOClO intermediate under similar conditions have
been discussed by Jia et al.15
Cl * + Cl f Cl * + Cl log k ) - 0.2
(18)
2
2
2
2
-
-
Rate constants for the reaction of ClOClO (or Cl
2
O
2
) with
ClO * + ClO f ClO * + ClO log k ) 2.3 (19)
2 2 2 2
-
9,11,16
ClO
2
have been postulated in several studies
of
complex chlorite reactions for the purpose of making accurate
Marcus theory predicts the rate constant for electron transfer
-
1
-1
4
-
-1
-1 -1
simulations of their behavior. Values (M s ) of 1 × 10
between Cl
2
and ClO
2
to be ∼10
M
s . This is more
9
8
16
(
1
Rabai and Beck), 5 × 10 (Fabian and Gordon), and 2 ×
than 6 orders of magnitude smaller than the rate constant
calculated from experimental data.
5
11
0 (Rabai and Orban) have been suggested. Our kinetic
-
study of the Cl
2
/ClO
2
system shows that the rate constant
The simplified Marcus equation used to determine the rate
constant assumes that the work required to bring the two
molecules together is negligible, which is a safe assumption
for a reaction between a neutral and a -1 species in a high
ionic strength medium. Another assumption requires that the
reaction is an outer-sphere electron-transfer process. While
this is appropriate for many reactions between metal
complexes, nonmetal redox reactions are more difficult to
predict due to the ease of interaction between the two
reactants. Any orbital overlap during electron-transfer results
in a larger rate constant than simplified Marcus theory
-
for the reaction between ClOClO and ClO
0 M s . This inequality is necessary to have a first-
order dependence in [ClO
2
must be .6 ×
5
-1 -1
1
-
2
]. Thus, in Scheme 1, the k
1
step
is the rate-determining step and the reactions of ClOClO (k
and k ) are rapid. Therefore, the rate constant used by Fabian
and Gordon is plausible.
2
3
Atom Transfer vs Electron-Transfer Mechanism. The
proposed mechanism (Scheme 1) indicates a nucleophilic
-
attack on Cl
2
by an oxygen atom on ClO
2
, followed by the
+
-
transfer of Cl and elimination of Cl to give ClOClO. The
rapid reactivity of Cl in this case is attributed to its ability
to behave as an electrophile.
The reaction between Br and ClO
2
9-31
predicts.
However, the large discrepancy between the
2
-, as reported by Toth
2
(27) Stanbury, D. M. AdVances in Inorganic Chemistry; Academic: New
York, 1989; Vol. 33, pp 69-138.
2
1
7
and Fabian, proceeds through two successive electron-
transfer steps. The first step involves the transfer of an
(28) Stanbury, D. M. AdVances in Chemistry Series: Electron-Transfer
Reactions; American Chemical Society: Washington, DC, 1997; pp
1
65-182.
-
-
electron from ClO
2
to Br
2
to form ClO
2
and Br
2
. In the
(
29) Stanbury, D. M.; de Maine, M. M.; Goodloe, G. J. Am. Chem. Soc.
1989, 111, 5496-5498.
30) Stanbury, D. M.; Martinez, R.; Tseng, E.; Miller, C. E. Inorg. Chem.
-
-
second step, Br
2
reacts with another ClO
2
to form ClO
2
(
-
and 2Br . Evidence of ClO
2
suppression in their reaction
1988, 27, 4277-4280.
rate supports the electron-transfer mechanism.
(31) Bennett, L. E.; Warlop, P. Inorg. Chem. 1990, 29, 1975-1981.
Inorganic Chemistry, Vol. 41, No. 2, 2002 345

Nicoson and Margerum
Figure 4. Lack of dependence of the first-order rate constant of the Cl2/
Figure 5. Observed versus predicted rate constants (eq 20) for the HOCl
-
-
-
-
ClO2 reaction on ClO2 concentration. [Cl2]T ) 26 µM, [ClO2 ] ) 0.572
reaction with ClO2 via Cl2 formation. [Cl2]T ) 20-80 µM, [OAc ]T )
-
-
+
-
+
mM, [OAc ]T ) 0.102 M, [Cl ] ) 0.95 M, p[H ] ) 4.27, µ ) 1.0 M
NaCl), 25.0 °C, λ ) 359 nm. Error bars represent one standard deviation.
0.05-0.10 M, [Cl ] ) 0.79-0.99 M, p[H ] ) 3.65-6.04, µ ) 1.0 M
(NaCl), 25.0 °C, λ ) 359 nm.
(
increase in absorbance occurs with kobsd ) 0.1-6 s- (Figure
2b). Small amounts of HOCl are present initially in the Cl2
solution (typically 8% of the total chlorine) and HOCl is
2
1
estimated Marcus value and the experimentally determined
rate constant suggests that electron-transfer is an unlikely
pathway.
Experimentally, the electron-transfer mechanism is tested
also produced from the hydrolysis of Cl . The direct reaction
of HOCl with ClO2 is much too slow to account for the
observed reaction rate. Due to the rapid initial reaction with
ClO , the concentration of Cl is depleted in 10-50 ms.
As this reaction reaches completion, the Cl /HOCl equilib-
rium will adjust to form more Cl (the reverse reaction in
eqs 9 and 10). The formation of Cl from HOCl is much
-
2
by varying the concentration of ClO . The following argu-
15
ment is based on the expected behavior if the reaction
proceeds through an electron-transfer mechanism, as in eqs
-
2
2
1
3 and 14. The experimental value for the forward path is
) 5.7 × 10 M s , and the equilibrium constant for
2
5
-1 -1
k
e
2
the electron-transfer calculated from the reduction potentials
is K
2
-
3.96
e
) 10
. The reverse path would then have k-e ) 5.2
slower than the initial reaction, and this maintains a steady-
9
-1 -1
-
×
10 M s , close to the diffusion limit. The steady-state
state amount of Cl that can react rapidly with ClO2 to form
2
-
expression requires k-e[ClO
2
-
] , kee[ClO
2
] to observe a first-
additional ClO . The mechanism for HOCl reacting through
general-acid-assisted pathways to form a steady-state Cl2
2
order dependence in [ClO
2
] and not observe suppression
due to [ClO ]. The rate constant kee can be no larger than
2
species has a rate expression as shown in eq 20.
9
-1 -1
∼
5 × 10 M s , the diffusion limit for a reaction between
+
-
-
HOAc
H
+
two -1 species. With k-e = kee, this simplifies the condition
k [ClO ][Cl ](k
-a
[HOAc] + k-a [H ])
1
2
-
to [ClO
added [ClO
2
] , [ClO
2
]. Data have been taken with an initial
-
-
OAc
-
H2O
k [ClO ] + k
[OAc ] + ka
-
1
2
a
2
] in excess of [ClO
2
], and no suppression is
kpred
)
(20)
1 + KCl3
[Cl-]
observed (Figure 4). According to the previous argument,
-
-
when [ClO
2
] ) [ClO
2
], at least a 50% decrease in the
1 a
Using the k , k
OAc-, and k-a
HOAc
observed rate constant is expected, if the reaction proceeds
by electron transfer. Hence, the kinetic data indicate that the
values determined from
H O
H+
this work and the KCl3-, k
2
, and k-a (extrapolated to µ
a
-
25
dominant pathway for the Cl
Cl transfer. The lack of ClO
2
/ClO
2
reaction must be through
) 1.0 M) from Wang and Margerum, the value of kobsd for
this slower reaction can be predicted and compared to the
experimental observed rate constant. The data for kpred vs
obsd with various concentrations of ClO
H fall on a straight line with a slope of 0.95 ( 0.04 (Figure
+
-
2
suppression in the Cl
reaction is contrary to the behavior of the Br
where Toth and Fabian17 found that the initial rate of the
2
/ClO
reaction,
2
-
2
/ClO
2
-
-
k
2
, HOAc/OAc , and
+
reaction decreased by a factor of 50 when [ClO
increased from 0 to 1 mM.
2
] was
5), which indicates that the proposed rate expression and
-, adduct
previously determined rate constants accurately predict the
25
For both Cl
2
and Br
2
reactions with ClO
2
-
-
formation to give BrBrOClO and ClClOClO are reason-
able pathways. Experimental evidence suggests that in one
observed reaction. This also clearly shows that HOCl is not
in rapid preequilibrium with Cl
2
and does not affect the
-
-
-
case Br
2
and ClO
2
are formed, while in the other Cl and
kinetics of the initial Cl
2
/ClO
2
reaction. The data show that,
-
+
ClOClO are formed. This is followed by more rapid reactions
in the presence of Cl and H , HOCl reacts through a Cl
2
-
of the respective intermediates, Br
electrophilicity of Cl
2
or ClOClO. The greater
intermediate much faster than through direct reaction with
-
2
compared to Br
for Cl versus Br elimination appear to favor the Cl -
2
and the preference
ClO
2
.
-
-
+
Comparison of the Reactivity and Mechanism of Cl
2
15
-
transfer mechanism in the Cl
2
/ClO
2
system as opposed to
vs HOCl and HOBr. The reaction mechanisms for HOCl
-
14
-
the electron-transfer mechanism in the Br
2
/ClO
2
system.
and HOBr with ClO
2
-
are similar. HOX (X ) Cl, Br) forms
-
+
HOCl Reaction with ClO
2
in the Presence of H and
reaction with ClO
an adduct with ClO
2
that undergoes a general-acid-assisted
-
-
Cl . After the rapid Cl
2
2
, a slower
decomposition to XOClO (eqs 3 and 4). The XOClO
346 Inorganic Chemistry, Vol. 41, No. 2, 2002

Aqueous Chlorine Reactions with Chlorite Ion
intermediate then reacts in rapid steps to produce ClO
2
and
HOBr, and Cl
2
, the rate constants for adduct formation with
-
-
-
ClO
3
. The reaction of Cl
2
with ClO
2
differs from HOCl
ClO
2
can be used. For HOCl and HOBr, the rate constants
-
1
-1
and HOBr because of the absence of acid assistance. The
2
are 1.6 and 97 M s , respectively. For Cl , the formation
-
5
-1
Cl
2
/ClO
2
interaction can be considered to proceed by adduct
of the adduct must have a rate constant g5.7 × 10 M
-1 -
2
formation (eq 21)
s . Therefore, when reacting with ClO
, the relative
], where x is HOCl,
x
HOCl
electrophilicity (defined as log[k
1
/k
1
Cl + ClO2 h ClClOClO-
-
(21)
HOBr, or Cl ) for HOCl:HOBr:Cl
2
2
is 0:1.8:g5.6.
2
Acknowledgment. This work was supported by National
Science Foundation Grant CHE-98-18214.
-
followed by a loss of Cl (eq 22).
-
-
Supporting Information Available: Tables of kinetic data and
graphs of kcorr vs p[H ], [OAc ], and [Cl ]. This material is
ClClOClO f ClOClO + Cl
(22)
+
-
-
available free of charge via the Internet at http://pubs.acs.org.
Thus, k
1
is a combination of the reactions in eqs 21 and 22.
To make a comparison between the reactivity of HOCl,
IC010762A
Inorganic Chemistry, Vol. 41, No. 2, 2002 347