Kinetics and mechanisms of bromine chloride reactions with bromite and chlorite ions

Article abstract of DOI:10.1021/ic048982m

Chloride ion catalyzes the reactions of HOBr with bromite and chlorite ions in phosphate buffer (p[H⁺] 5 to 7). Bromine chloride is generated in situ in small equilibrium concentrations by the addition of excess Cl ^- to HOBr. In the BrCl/ClO₂^- reaction, where ClO₂^- is in excess, a first-order rate of formation of ClO₂ is observed that depends on the HOBr concentration. The rate dependencies on ClO₂^-, Cl^-, H⁺, and buffer concentrations are determined. In the BrCl/BrO₂^- reaction where BrCl is in pre-equilibrium with the excess species, HOBr, the loss of absorbance due to BrO₂^- is followed. The dependencies on Cl^-, HOBr, H⁺, and HPO₄ ^2- concentrations are determined for the BrCl/BrO₂ ^- reaction. In the proposed mechanisms, the BrCl/ClO₂ ^- and BrCl/BrO₂^- reactions proceed by Br ⁺ transfer to form steady-state levels of BrOClO and BrOBrO, respectively. The rate constant for the BrCl/ClO₂^- reaction (k₂^Cl) is 5.2 x 10⁶ M^-1 s^-1 and for the BrCl/BrO₂^- reaction (k ₂^Br) is 1.9 × 10⁵ M^-1 s ^-1. In the BrCl/ClO₂^- case, BrOClO reacts with ClO₂^- to form two ClO₂ radicals and Br ^-. However, the hydrolysis of BrOBrO in the BrCl/ BrO ₂^- reaction leads to the formation of BrO₃ ^- and Br^-.

Full text of DOI:10.1021/ic048982m

Inorg. Chem. 2004, 43, 7412−7420
Kinetics and Mechanisms of Bromine Chloride Reactions with Bromite
and Chlorite Ions
Ihab N. Odeh, Jeffrey S. Nicoson, Kara E. Huff Hartz, and Dale W. Margerum*
Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907
Received July 27, 2004
Chloride ion catalyzes the reactions of HOBr with bromite and chlorite ions in phosphate buffer (p[H+] 5 to 7).
Bromine chloride is generated in situ in small equilibrium concentrations by the addition of excess Cl- to HOBr. In
-
-
the BrCl/ClO2 reaction, where ClO2 is in excess, a first-order rate of formation of ClO2 is observed that depends
on the HOBr concentration. The rate dependencies on ClO2-, Cl-, H+, and buffer concentrations are determined.
-
In the BrCl/BrO2 reaction where BrCl is in pre-equilibrium with the excess species, HOBr, the loss of absorbance
-
2-
due to BrO2 is followed. The dependencies on Cl-, HOBr, H+, and HPO4 concentrations are determined for the
BrCl/BrO2- reaction. In the proposed mechanisms, the BrCl/ClO2- and BrCl/BrO2- reactions proceed by Br+ transfer
to form steady-state levels of BrOClO and BrOBrO, respectively. The rate constant for the BrCl/ClO2 reaction
-
(kC2 l) is 5.2
×
106 M- s- and for the BrCl/BrO2 reaction (kB2r) is 1.9
×
105 M- s-1. In the BrCl/ClO2 case,
1
1
-
1
-
BrOClO reacts with ClO2 to form two ClO2 radicals and Br-. However, the hydrolysis of BrOBrO in the BrCl/
-
BrO2 reaction leads to the formation of BrO3 and Br-.
-
-
Introduction
reactions have been examined because of the limited com-
mercial availability of sodium bromite.14-16 A recent bromite
preparation and purification method17 has permitted the
Bromine chloride, BrCl, is a very reactive species that is
of relevance to several environmental and chemical areas.
Tropospheric ozone depletion in the Arctic region at sunrise
is dependent in part on BrCl.1 Also, the formation of BrO3 ,
a known carcinogen, in bromide-containing water is at-
tributed to BrCl reaction with hypochlorite ion.2 Recently,
studies of BrCl reactions have been conducted to gain a better
understanding of its chemistry. These include its hydrolysis
(eq 1)3-5 and its reactions with N2H5 , HOCl,2 HOBr,2
p-xylene,7 and ascorbic acid.8
-
investigation of several systems including BrO2 reactions
-
with O3,18 S(IV),19 ClO2,17 HOCl,20 and HOBr.21
In the present study the reactions of BrCl with ClO2- and
BrO2 are investigated in the p[H+] range of 5 to 7 by
-
stopped-flow spectroscopy. We propose that both reactions
proceed through BrOXO (X ) Cl or Br) intermediates
+ 6
(7) Voudrias, E. A.; Reinhard, M. EnViron. Sci. Technol. 1988, 22, 1049-
1056.
(8) Urbansky, E. T. J. EnViron. Monit. 1999, 1, 471-476.
(9) Masschelein, W. J.; Rice, R. G. Chlorine Dioxide: Chemistry and
EnVironmental Impact of Oxychlorine Compounds; Ann Arbor Sci-
ence: Ann Arbor, MI, 1979.
(10) Johnson, R. W.; Edwards, J. O. Inorg. Chem. 1966, 5, 2073-2075.
(11) Stanbury, D. M.; Lednicky, L. A. J. Am. Chem. Soc. 1984, 106, 2847-
2853.
HOBr + Cl- + H+ a BrCl + H2O
(1)
Many kinetic studies of chlorite reactions have been
performed over the years;9-13 however, only a few bromite
* Author to whom correspondence should be addressed. E-mail:
margerum@purdue.edu.
(1) Spicer, C. W.; Plastridge, R. A.; Fodter, K. L.; Finlayson-Pitts, B. J.;
Bottenheim, J. W.; Grannas, A. M.; Shepson, P. B. Atmos. EnViron.
2002, 36, 2721-2731.
(2) Margerum, D. W.; Huff Hartz, K. E. J. EnViron. Monit. 2002, 4, 20-
26.
(12) Nagypa´l, I.; Epstein, I. R. J. Phys. Chem. 1986, 90, 6285-6292.
(13) Fa´bia´n, I.; Gordon, G. Inorg. Chem. 1992, 31, 2144-2150.
(14) Faria, R. de B.; Epstein, I. R.; Kustin, K. J. Am. Chem. Soc. 1992,
114, 7164-7171.
(15) Lee, C. L.; Lister, M. W. Can. J. Chem. 1979, 57, 1524-1530.
(16) Jhanji, A. K.; Gould, E. S. Int. J. Chem. Kinet. 1991, 23, 229-236.
(17) Wang, L.; Nicoson, J. S.; Huff Hartz, K. E.; Francisco, J. S.; Margerum,
D. W. Inorg. Chem. 2002, 41, 108-113.
(18) Nicoson, J. S.; Wang, L.; Becker, R. H.; Huff Hartz, K. E.; Muller,
C. E.; Margerum, D. W. Inorg. Chem. 2002, 41, 2975-2980.
(19) Huff Hartz, K. E.; Nicoson, J. S.; Wang, L.; Margerum, D. W. Inorg.
Chem. 2003, 42, 78-87.
(20) Nicoson, J. S.; Perrone, T.; Huff Hartz, K. E.; Wang, L.; Margerum,
D. W. Inorg. Chem. 2003, 42, 5818-5824.
(21) Nicoson, J. S. Ph.D. Thesis, Purdue University, 2003.
(3) Wang, T. X.; Kelley, M. D.; Cooper, J. N.; Beckwith, R. C.; Margerum,
D. W. Inorg. Chem. 1994, 33, 5872-5878.
(4) Liu, Q.; Margerum, D. W. EnViron. Sci. Technol. 2001, 35, 1127-
1133.
(5) Becker, R. H.; Bartlett, W. P.; Urbansky, E. T.; Margerum, D. W. J.
Chem. Soc., Dalton Trans. 2002, 695-700.
(6) Jia, Z.; Salaita, M. G.; Margerum, D. W. Inorg. Chem. 2000, 39, 1974-
1978.
7412 Inorganic Chemistry, Vol. 43, No. 23, 2004
10.1021/ic048982m CCC: $27.50
© 2004 American Chemical Society
Published on Web 10/19/2004

-
-
BrCl Reactions with BrO2 and ClO2 Ions
Table 1. Equilibrium Constants for a System Containing HOBr, Br-,
-
analogous to those formed in the chloride-free HOBr/XO2
reactions.21,22
Cl-, and H+a
reaction
equilibrium
constant values
Experimental Section
KB1 rCl
Kh1
-
BrCl + Cl- a BrCl2
3.8 M-1
BrCl + H2O a HOBr + Cl- + H+
Br2 + H2O a HOBr + Br- + H+
Cl2 + H2O a HOCl + Cl- + H+
BrCl + Cl- a Cl2 + Br-
1.3 × 10-4 M2
6.1 × 10-9 M2
1.04 × 10-3 M2
9.1 × 10-7
16.1 M-1
Reagents. All solutions were prepared with doubly deionized
distilled water. HOBr solutions were prepared by adding liquid Br2
slowly to ice-chilled 0.2 M NaOH solution with rapid stirring.
Bromide was removed from the NaOBr solution by mixing the stock
with AgOH solution (freshly prepared by adding AgNO3 to NaOH)
and filtering out AgBr.23 NaOBr solutions were stored in a
refrigerator. Bromide-free NaCl was prepared by mixing distilled
bromide-free HCl with saturated NaOH followed by gravimetric
standardization.5 NaClO2 and NaBrO2 were prepared and purified
by methods described earlier.17,22 The stock NaBrO2 is 63.4%
pure by weight with 7.0% NaBrO3, 1.3% NaBr, 1.9% Na2SO4,
14.1% NaOH, and 12.2% H2O. Analytical-reagent grade NaH2PO4
(pKa ) 6.26)22 and Na2HPO4 were used without further purification.
Ionic strength was adjusted to µ ) 1.0 M with recrystallized
NaClO4.
Kh2
Kh3
KB8 rCl
KB4 r2
-
Br2 + Br- a Br3
KB3 r2
Br2 + Cl- a Br2Cl-
1.3 M-1
KH7 OBr
KC5 l2
HOBr + Cl- a HOCl + Br-
6.5 × 10-6
-
Cl2 + Cl- a Cl3
0.18 M-1
pKHa OBr
pKHa OCl
KB2 rCl
KC6 l2
HOBr a OBr- + H+
HOCl a OCl- + H+
BrCl + Br- a Br2Cl-
8.59
7.47
1.8 × 104 M-1
4.2 × 106 M-1
8.7 × 10-10 M2
7.6 × 10-3
-
Cl2 + Br- a BrCl2
BrCl + H2O a HOCl + Br- + H+
2BrCl a Br2 + Cl2
Kh4
KBDrCl
a Reference 4 and references within. All equilibrium constants are at 25
pH Measurement. An Orion model 720A pH meter equipped
with a Corning combination electrode was used for pH measure-
ment. The electrode was calibrated with previously standardized
HClO4 and NaOH to correct pH to p[H+], where p[H+] ) - log-
[H+] and pKw is 13.60 (25.0 °C, 1.0 M NaClO4).24
°C and µ ) 1.0 M.
was used for regression analysis, and MathCad 828 was used for
complex algebraic treatments.
d[HOBr]T
Spectrophotometry and Kinetics. A Perkin-Elmer Lambda 9
Br
-
-
) k [BrO2
]
(6)
obsd
-
dt
UV-vis-NIR spectrophotometer was used to standardize BrO2
(ꢀ295 ) 115 M-1 cm-1),25 ClO2 (ꢀ260 ) 154 M-1 cm-1),22 and
-
d[HOBr]T
OBr- (ꢀ329 ) 332 M-1 cm-1).26 An Applied PhotoPhysics SX18
MV stopped-flow spectrophotometer (APPSF, optical path length
) 0.962 cm) equipped with a PD.1 photodiode array for simulta-
neous multiwavelength detection and a photomultiplier tube for
single-wavelength detection was used in studying the kinetics of
-
-
) kBI r[HOBr]T[BrO2-] + kBr[BrCl][BrO2
]
(7)
(8)
II
dt
Br
obsd
k
) kBI r[HOBr]T + RkBr[HOBr]T
II
Product Analysis. A Dionex DX-500 chromatograph was used
to identify the products of the BrCl/BrO2 reaction by a method
-
the reactions. The kinetics of the BrCl reaction with excess ClO2
-
were followed at the λmax of the product, ClO2 (ꢀ359 ) 1230 M-1
cm-1),22 eqs 2-4. The term [HOBr]T equals the sum of [HOBr]
and [OBr-] in the absence of Cl-. Upon the addition of chloride,
similar to EPA 300.1.29 Samples were injected via an autosampler
(AS 40) through a 25 µL injection loop onto quaternary amine
anion-exchange guard (AG9 HC) and separation (AS9 HC)
columns. Analytes were eluted with 9 mM Na2CO3 at a flow rate
of 1.0 mL/min. Gas-assisted suppressed-conductivity detection (ED
40), with an ASRS-Ultra suppressor in the self-regenerating mode
and a current of 100 mA, was used to determine the analytes.
Residual NaOBr was removed by the addition of Na2SO3 to each
sample immediately prior to injection to prevent column damage.30
d[HOBr]T
-
) kCoblsd[HOBr]T
(2)
dt
d[HOBr]T
-
) kCI l[ClO2-][HOBr]T + kCl[ClO2-][BrCl] (3)
II
dt
Cl
obsd
-
k
) kCI l [ClO2-] + RkCl [ClO2
]
(4)
II
Results and Discussion
Chloride Catalysis of HOBr/ClO2- Reaction via a BrCl
Intermediate. The reaction of HOBr with ClO2 is known
to produce ClO2 with an established stoichiometry (eq 9).22
The reaction kinetics are on the stopped-flow time scale. Our
low levels of the other nine species in Table 1 form, including the
highly reactive BrCl. The R term is the BrCl concentration that
forms relative to [HOBr]T (eq 5) as will be discussed in a later
-
-
section. The BrCl/BrO2 kinetics were studied by following the
HOBr + 2ClO2- + H+ f 2ClO2 + Br- + H2O (9)
[BrCl]
R )
(5)
[HOBr]T
studies show a large increase in the reaction rate even with
millimolar levels of Cl-. We assign this acceleration to BrCl
because we know it is a highly reactive species2 and chloride
is not. We also know that BrCl forms readily from HOBr
and Cl- (eq 1).3-5 The large chloride acceleration levels off
when [Cl-] is ∼0.1 M. This is consistent with a mechanism
where BrCl and ClO2- react to produce BrOClO as a steady-
loss of BrO2- in the 240 to 245 nm region where [HOBr]T is present
-
in large excess with respect to BrO2 (eqs 6-8). SigmaPlot 8.027
(22) Furman, C. S.; Margerum, D. W. Inorg. Chem. 1998, 37, 4321-4327.
(23) Noszticzius, Z.; Noszticzius, E.; Schelly, Z. A. J. Phys. Chem. 1983,
87, 510-524.
(24) Mollino, M.; Melios, C.; Tongolli, J. O.; Luchiari, L. C.; Jafelicci,
M. J. Electroanal. Chem. Interfacial Electrochem. 1979, 105, 237-
246.
(25) Lee, C. L.; Lister, M. W. Can. J. Chem. 1971, 49, 2822-2826.
(26) Troy, R. C.; Margerum, D. W. Inorg. Chem. 1991, 30, 3538-3543.
(27) SigmaPlot 8.0 for Windows; SPSS Inc.: Chicago, IL, 2002.
(28) MathCad 8 for Windows; MathSoft: Cambridge, MA, 1998.
(29) EPA Method 300.1; U.S. EPA: Cincinnati, OH, 1997.
(30) Liu, Q. Ph.D. Thesis, Purdue University, 2000.
Inorganic Chemistry, Vol. 43, No. 23, 2004 7413

Odeh et al.
state species (eqs 10 and 11). BrOClO as an intermediate
has been proposed in an earlier study.22 Furman and
Cl
k2
-
BrCl + ClO2
{
} BrOClO + Cl-
(10)
Cl
k-2
k3
BrOClO + ClO2- 98 2ClO2 + Br-
(11)
Margerum have found that BrOClO can either hydrolyze or
react with a chlorite ion as shown in eqs 12-14 and 11,
respectively.
HOBr + ClO2- a HOBrOClO-
HOBrOClO- + HA f BrOClO + H2O + A- (13)
(12)
BrOClO + ClO2- f 2ClO2 + Br-
(11)
(14)
BrOClO + H2O f ClO3- + Br- + 2H+
In the current study, the rate of ClO2 formation is followed
at 359 nm upon the addition of Cl- where ClO2- is in large
excess over HOBr to give ClO2 as the main product. Data
collected with the photodiode array APPSF confirm that ClO2
is a product of the reaction (Figure 1a). The large absorbance
band at lower wavelengths is due to chlorite that is present
in large excess, ꢀ260 ) 154 M-1 cm-1.22 The ClO2 formation
follows first-order kinetics whether Cl- is present or absent.
However, the presence of chloride ion leads to a greater rate
of ClO2 formation (Figure 1b). Subtraction of the rate
constant for the uncatalyzed HOBr/ClO2- reaction (kHOBr/ClO
-
2
Cl
Cl
-
) kI [ClO2 ]) from the observed rate constant (kobsd) yields
a rate constant that we attribute to the BrCl/ClO2 reaction
-
Cl
BrCl
-
(k
) RkII [ClO2 ]) as shown in eqs 15 and 16.
-
-
RkCIIl [ClO2 ] ) kCl - kCI l [ClO2 ]
(15)
(16)
obsd
Cl
Cl
obsd
k
) k
- kHOBr/ClO
-
BrCl
2
Figure 2a shows the saturation kinetics exhibited at high
[Cl-]. An increase in ClO2- and H+ concentrations (Figures
Cl
BrCl
3 and 4a) also increases the k
values.
Figure 1. (a) APPSF photodiode array spectra of ClO2 formation in the
Small amounts of BrCl (<10-7 M) build up when HOBr
is mixed with Cl- under our experimental conditions. The
dependence of the rate on [BrCl] requires an accurate
evaluation of the [BrCl] relative to the total concentration
of hypobromous acid and hypobromite, [HOBr]T, (eq 5). The
involvement of BrCl in a complex set of equilibria,4 Table
1, must be taken into account. We developed an equation
(Supporting Information Appendix A) and used MathCad 8
to calculate the distribution of all of the species in the system
based on the concentrations of total HOBr, Br-, Cl-, and
H+ and the known equilibrium constants. This algorithm is
an expansion of one established by Liu.30 Our equation
accounts for the effect of the Br- formed on the distribution
of the species shown in Table 1. Table 2 shows the levels
of Cl2, BrCl, and Br2 that form as a function of chloride ion
concentration under the conditions of the BrCl/ClO2- study.
In the course of the reaction, Br- forms as a product (eqs
BrCl/ClO2 reaction: [Cl-] ) 19.1 mM, [PO4]T ) 5.0 mM, [HOBr]T
)
-
0.252 mM, [ClO2-] ) 28.3 mM, 25.0 °C, and µ ) 1.0 M. The inset shows
the typical absorption spectrum of the ClO2 product measured with a narrow
band-pass spectrophotometer. The large absorbance at lower wavelength
is due to the excess species ClO2-. (b) Kinetic traces for the first-order
formation of ClO2 in (i) 0.0 mM Cl- and (ii) 0.192 M Cl-: [HOBr]T
)
0.114 mM, [ClO2-] ) 25 mM, [PO4]T ) 5 mM, p[H+] ) 6.68, 25.0 °C, µ
Cl
Cl
obsd
) 1.0 M, and λ ) 359 nm. (i) k
) 0.30 s-1 and (ii) k
) 6.10 s-1
.
obsd
11 and 14). We find that R is nearly constant throughout
the reaction (Figure S1), which means that as [HOBr]T
decreases the BrCl concentration decreases. Figure S1
presents [HOBr]T in descending order to reflect the con-
sumption of HOBr during the course of the reaction.
Cl
The plot of k /R versus [Cl-] (Figure 2b) shows that
BrCl
high [Cl-] suppresses the rate where kCBlrCl/R is the rate
constant for the BrCl/ClO2- reaction divided by the relative
concentration of BrCl with respect to the total HOBr
7414 Inorganic Chemistry, Vol. 43, No. 23, 2004

-
-
BrCl Reactions with BrO2 and ClO2 Ions
Figure 3. Greater than first-order dependence in [ClO2-] for the rate
-
constants of the BrCl/ClO2 reaction. The solid line is a fit of the data to
eq 20. Conditions: [Cl-] ) 37.6 mM, [HOBr]T ) 0.305 mM, [PO4]T
5.0 mM, p[H+] ) 6.01, 25.0 °C, and µ ) 1.0 M.
)
4b). On the basis of these data, the mechanism in eqs 10,
11, and 17 is proposed. The term B- in eq 17 represents all
bases that catalyze BrOClO hydrolysis including H2O, OH-,
kC2 l
-
BrCl + ClO2
{
} BrOClO + Cl-
(10)
(11)
Cl
k
-2
k3
BrOClO + ClO2- 98 2ClO2 + Br-
-
Figure 2. (a) Saturation behavior of the BrCl/ClO2 reaction upon the
addition of Cl-. (b) Chloride ion suppression is observed after accounting
for the relative concentration of BrCl (i.e., accounting for the R term). The
k4B
BrOClO + B- 98 ClO3- + Br- + 2H+ + B- (17)
-
solid line is a fit of the data to eq 20. Conditions: 49.25 mM ClO2
,
[HOBr]T ) 0.295 mM, p[H+] ) 6.04, [PO4]T ) 5.0 mM, 25.0 °C, µ ) 1.0
M, and λ ) 359 nm. kC2 l ) 5.2(1) × 106 M-1 s-1, kC-l2/k3 ) 0.59(3).
and HPO42-. Treating BrOClO as a steady-state species leads
to eq 18. The solid lines in Figures 2b and 3 show that the
experimental data fit well to eq 18.
Table 2. Concentration of Molecular Halogens in the Presence of
-
Chloride Ion for BrCl/XO2 Reactions
Cl-, M
Cl2, M
BrCl, M
Br2, M
R ) [BrCl]/[HOBr]T
kB4 [B]
BrCl/ClO2- Reactiona
-
-
kC2 l[ClO2 ] [ClO2 ] +
Cl
BrCl
1.0 × 10-3 1.21 × 10-12 2.01 × 10-9 5.81 × 10-8
5.0 × 10-3 1.35 × 10-11 1.00 × 10-8 1.29 × 10-7
1.00 × 10-2 3.81 × 10-11 1.99 × 10-8 1.81 × 10-7
3.00 × 10-2 1.96 × 10-10 5.9 × 10-8 3.07 × 10-7
5.00 × 10-2 4.21 × 10-10 9.75 × 10-8 3.91 × 10-7
7.04 × 10-6
3.50 × 10-5
6.96 × 10-5
2.06 × 10-4
3.41 × 10-4
(
)
k
k3
)
(18)
kB4 [B]
R
k-2
[Cl-] + [ClO2 ] +
-
k3
k3
BrCl/BrO2- Reactionb
1.0 × 10-3 2.49 × 10-12 1.51 × 10-8 1.59 × 10-6
5.0 × 10-3 2.78 × 10-11 7.55 × 10-8 3.55 × 10-6
1.00 × 10-2 7.87 × 10-11 1.51 × 10-7 5.01 × 10-6
3.00 × 10-2 4.09 × 10-10 4.51 × 10-7 8.60 × 10-6
5.00 × 10-2 8.81 × 10-10 7.48 × 10-7 1.10 × 10-5
3.26 × 10-6
1.63 × 10-5
3.25 × 10-5
9.71 × 10-4
1.61 × 10-4
On the basis of the dependence in [Cl-], the rate constants
obtained are kC2 l ) 5.2(1) × 106 M-1 s-1 and kCl /k3 ) 0.59-
-2
-
(3). Although Cl- accelerates the HOBr/ClO2 reaction
beacause of BrCl production, high levels of Cl- suppress
this acceleration (Figure 2b), as can be seen in eq 18. The
a [HOBr]T ) 0.28 mM, p[H+] ) 6.04, µ ) 1.0 M, and 25.0 °C.
b [HOBr]T ) 4.64 mM, p[H+] ) 6.37, µ ) 1.0 M, and 25.0 °C.
-
results obtained for the [ClO2 ] dependence are in agreement
with those in the [Cl-] plot. The yield of ClO2 from the
Cl
-
-
concentration. Also, kBrCl/R versus [ClO2 ] yields a qua-
HOBr/ClO2 reaction is dependent on the concentration of
-
dratic dependence in [ClO2 ] (Figure 3). After correction
the buffer (Figure 5). On the basis of the percent of ClO2
formed and using eq 19, kH4 PO /k3 is 0.19 and kH O/k3 is 1 ×
-
4
2
for the relative [BrCl], the rate constant for the BrCl/ClO2
reaction shows no dependence in H+ concentration (Figure
4
-
10-4 M. In the study of the BrCl/ClO2 reaction, low
Inorganic Chemistry, Vol. 43, No. 23, 2004 7415

Odeh et al.
-
Figure 5. Dependence of ClO2 yield on [HPO42-] for the HOBr/ClO2
reaction: [HOBr]T ) 0.102 mM, [ClO2-] ) 6.00 mM, p[H+] ) 6.24, 25.0
°C, µ ) 1.0 M, and λ ) 359 nm. kH4 PO /k3 ) 0.19, k /k3 ) 1.0 × 10-4 M.
H2O
4
4
The solid line is a fit of the data to eq 19.
to produce BrO3- with an established stoichiometry (eq 21).21
The reaction kinetics are slower than the kinetics of the
analogous HOBr/ClO2- reaction but are still on the stopped-
Cl
BrCl
Figure 4. (a) Increase of k
as a function of acid concentration. (b)
-
Lack of acid effect on the BrCl/ClO2 rate constants after accounting for
the R term. Conditions: [ClO2-] ) 49.4 mM , [PO4]T ) 5.0 mM, [HOBr]T
) 0.253 mM, 19.1 mM Cl-, 25.0 °C, µ ) 1.0 M, and at 359 nm.
HOBr + BrO2- f BrO3- + Br- + H+
(21)
concentrations of phosphate and a high concentration of
ClO2 are used to make the reaction in eq 11 the dominant
-
flow time scale. The present study confirms the rate of the
pathway (relative to eq 17) for BrOClO loss. Thus, eq 18 is
reduced to eq 20, which is used to calculate the kC2 l and k-2/
k3 values. This explains the lack of effect of acid (Figure
-
HOBr/BrO2 reaction but also shows that chloride ion
increases the rate. We attribute this acceleration to the BrCl/
BrO2- reaction. As in the HOBr/ClO2- case, the acceleration
levels off at ∼0.1 M Cl-. A mechanism consistent with these
findings is proposed in eqs 22 and 23 where BrOBrO is a
-
4b) and buffer (Figure S4) on the BrCl/ClO2 reactions. In
fact, the acid dependence in Figure 4a is due to the formation
of BrCl (eq 1) and to shifts in the other equilibria in Table
1. Division of kCl /k3 by the kB/k3 values obtained in Figure
kB2 r
} BrOBrO + Cl-
(22)
-
-2
4
BrCl + BrO2
{
Br
k
5 gives relative rate constants for the reaction of BrOClO
-2
with Cl- compared to the base-catalyzed hydrolysis reactions
k5B
Cl
-2
Cl
-2
with values of k /kH4 O ) 5.9(3) × 103 M-1 and k /kH4 PO
BrOBrO + B- 98 BrO3- + Br- + 2H+ + B- (23)
2
4
) 3.1(2).
steady-state species. BrOBrO is a species similar to the
BrOClO proposed in the HOBr/ClO2- reaction. The reaction
-
100(2k3[ClO2 ])
2-
in eq 23 is base catalyzed (B- ) HPO4 or H2O), as will
% yield ClO2 )
(19)
(20)
2k3[ClO2 ] + kH4 PO [HPO42-] + kH4 O
-
4
2
be discussed later. Products analysis shows that the concen-
-
tration of BrO3 formed is not affected by the presence of
Cl
BrCl
- 2
kC2 l[ClO2 ]
k
chloride ion.
)
R
k-2
k3
[Cl-] + [ClO2 ]
-
In the present study, we confirm the rate of the HOBr/
BrO2- reaction in the absence of Cl- and determine its rate
-
in the presence of Cl- by following the loss of BrO2 . To
-
Chloride Catalysis of HOBr/BrO2- Reaction via a BrCl
Intermediate. The reaction of HOBr with BrO2 is known
minimize its self-decomposition, BrO2 is used as the
-
limiting species. Figure 6a shows data collected with the
7416 Inorganic Chemistry, Vol. 43, No. 23, 2004

-
-
BrCl Reactions with BrO2 and ClO2 Ions
Figure 6. (a) APPSF photodiode array spectra of BrO2- loss in the BrCl/
BrO2 reaction: [HOBr]T ) 2.52 mM, [Cl-] ) 15.3 mM , [PO4]T ) 80
-
-
Figure 7. (a) Saturation behavior of the BrCl/BrO2 reaction upon the
mM, p[H+] ) 6.28, [BrO2-] ) 70 µM, 25.0 °C, and µ ) 1.0 M. (b) Kinetic
addition of Cl-. (b) Chloride ion suppression is observed after accounting
for the relative concentration of BrCl (i.e., accounting for the R term). The
solid line is a fit of the data to eq 26. Conditions: [HOBr]T ) 4.87 mM,
p[H+] ) 6.37, [PO4]T ) 80 mM, [BrO2-] ) 0.1 mM, 25.0 °C, µ ) 1.0 M,
traces for the first-order decay of BrO2- in (i) 0.0 mM Cl- and (ii) 20 mM
Cl-: [HOBr]T ) 0.724 mM, [BrO2-] ) 70 µM, p[H+] ) 5.95, [PO4]T
)
Br
obsd
80 mM, 25.0 °C, µ ) 1.0 M, and λ ) 240 nm. (i) k
) 4.61(3) × 10-3
s-1 and (ii) kBobrsd ) 1.16(1) × 10-2 s-1
.
and λ ) 245 nm. k2 ) 1.9(2) × 105 M-1 s-1, k-Br2/k5HPO ) 2.6(6).
4
Br
photodiode array for the loss of bromite absorption. This
loss follows first-order kinetics (Figure 6b). A step similar
to that in eq 11 is not included in the mechanism because a
Table 2 shows the concentrations of Cl2, BrCl, and Br2
that are present under these conditions. A [Cl-] dependence
Br
BrCl
study shows that the increase in k
with [Cl-] also
-
simple first-order dependence in BrO2 is observed. In a
-
exhibits saturation behavior (Figure 7a). A [Cl-] suppression
trend is observed when the relative concentration of BrCl,
R, is taken into account (Figure 7b). The term R that was
defined in eq 5 is used. The purpose of this term is to define
[BrCl] in terms of total HOBr concentration. An increase of
behavior similar to the HOBr/ClO2 reaction, we find that
Cl- increases the rate of BrO2- loss. Subtraction of the rate
-
-
constant for the HOBr/BrO2 reaction (kHOBr/BrO
)
2
kBr[HOBr]T) from the observed rate constant (kBobrsd) yields a
I
-
rate constant that we attribute to the BrCl/BrO2 reaction
-
Br
BrCl
) RkBr[HOBr]T) as shown in eqs 24 and 25. The BrCl
[HOBr]T, which is present in large excess over BrO2 , also
(k
II
Br
BrCl
increases the k /R value (Figure 8). In addition, the
levels are low under the conditions of this study, [BrCl] <
results indicate that the BrCl/BrO2- reaction is base-catalyzed
(Figures 9a and 9b). Ion chromatographic data show that
10-6 M.
Br
II
Br
obsd
- kBI r [HOBr]T
(24)
(25)
-
Rk [HOBr]T ) k
BrO3 is a product of this reaction (Figure S10).
When BrOBrO is treated as a steady-state species, eqs 26-
Br
Br
obsd
k
) k
- kHOBr/BrO
-
Br
BrCl
BrCl
2
28 are obtained. Equation 26 shows the dependence of k
Inorganic Chemistry, Vol. 43, No. 23, 2004 7417

Odeh et al.
Br
BrCl
Figure 8. Linear dependence k
on [HOBr]T. Conditions: [BrO2-] )
70 µM, p[H+] ) 5.91, [Cl-] ) 20 mM, [PO4]T ) 80 mM, 25.0 °C, µ )
1.0 M, and λ ) 240 nm.
on Cl- and total hypobromous acid concentrations. The
rearrangements of eq 26 to the forms in 27 and 28 show the
dependence in H+ and HPO42- concentrations. The solid line
Br
BrCl
k
kB2 r[HOBr]T
)
(26)
(27)
(28)
kBr [Cl- ]
R
-2
1 +
kB5 [B]
kH5 PO [PO4]T
4
kB2 r[HOBr]T
Br
BrCl
Br
-
+
HPO4
(
)
k
k-2[Cl ](1 + [H ]/Ka
)
)
-
kH5 PO [PO4]T
k [Cl- ](1 + [H+]/KaHPO
4
Figure 9. (a) Acid suppression of the BrCl/BrO2 reaction. Conditions:
[HOBr]T ) 2.87 mM, [Cl-] ) 15.3 mM, [BrO2-] ) 0.1 mM, [PO4]T ) 80
mM, 25.0 °C, µ ) 1.0 M, and λ ) 245 nm. The solid line is a fit of the
data to eq 27. (b) The rate of the BrCl/BrO2- reaction is accelerated by the
basic form of phosphate buffer. [HOBr]T ) 2.78 mM, p[H+] ) 5.99(2),
[Cl-] ) 15.3 mM, [BrO2-] ) 0.1 mM, 25.0 °C, µ ) 1.0 M, and λ ) 245
nm. The solid line is a fit of the data to eq 28.
R
+ 1
Br
-2
4
)
kH5 PO4[HPO4
]
2-
kB2 r[HOBr]T
Br
BrCl
kBr [Cl-]
(
)
k
-2
)
2-
dependence in [HOBr]T, [H+], [HPO42-], and k2 ) 1.9(2)
Br
kH5 PO [HPO4
]
4
R
× 105 M-1 s-1, the value of k-Br2/kH5 PO4 is 2.6(6).
+ 1
kBr [Cl-]
-2
Comparison of BrCl/Bromite and BrCl/Chlorite Reac-
-
tions. We propose that reactions of both BrCl/ClO2 and
in Figure 7b shows the fit of the data to eq 26, which yields
-
BrCl/BrO2 proceed by Br+ transfer mechanisms to form
kB2 r ) 1.9(2) × 105 M-1 s-1 and kBr /kB5 ) 60(20) M-1.
-
BrOXO and Cl- (eqs 10 and 22). The BrCl/ClO2 reaction
-2
Similarly, experimental data in Figure 8 fit to eq 26 as
[HOBr]T is increased. A linear fit of the data in Figure 8
gives a slope of 8.1(2) × 104 M-1 s-1. We find the reaction
in eq 23 to be catalyzed by the basic form of phosphate,
HPO42-, and the data in Figures 9a and 9b fit the rate
expressions in eq 27 and 28 (solid lines). From the
is 27.4 times faster than the BrCl/BrO2- reaction. This kinetic
behavior is attributed to the relative stabilities of BrOClO
versus BrOBrO. Gas-phase calculations by Guha and Fran-
cisco31 show that BrOClO is thermodynamically favored over
(31) Guha, S.; Francisco, J. S. J. Phys. Chem. A 1997, 101, 5347-5359.
7418 Inorganic Chemistry, Vol. 43, No. 23, 2004

-
-
BrCl Reactions with BrO2 and ClO2 Ions
-
-
Y- loss. The Lewis acid strength has a less important
contribution on the rate. The rate constants for X2/ClO2- are
in agreement with the electrophilicity scale where BrCl(8.9)
> Cl2(8.2) > Br2(7.3). However, the fact that electron
transfer is the dominant pathway for Br2 implies that if a
Table 3. Summary of Constants for BrCl/BrO2 and BrCl/ClO2
Reactions
constants
values
kC2 l
5.2(1) × 106 M-1 s-1
kH4 PO /k3
4
0.19
-
Br+ transfer mechanism existed for Br2/ClO2 it must be
kH4 O/k3
1.0 × 10-4
M
2
negligible with a rate constant much smaller than kBr ) 1.3
Cl
-2
2
0.59(3)
k
/k3
× 103 M-1 s-1.
kC-l2/kH4 PO
4
3.1(2)
kC-l2/kH4 O
5.9(3) × 103 M-1
1.9(2) × 105 M-1 s-1
2.6(6)
2
-
The BrCl/BrO2 reaction is also compared to the Cl2/
kB2 r
-
-
BrO2 and Br2/BrO2 systems (eqs 33 and 34). Similar to
Br
-2
Br
k
k
/kH5 PO
/kH5 O
4
the ClO2- case, we find that the reactions in eq 33 (kCl/Br
)
2
negligible
-2
8.7 × 105 M-1 s-1)20 and in eq 34 (kBr/Br ) 58.4 M-1 s-1)32
BrOBrO. Table 3 summarizes all the constants obtained for
kCl/Br
-
Cl2 + BrO2
8 BrOClO + Cl-
(33)
(34)
-
-
-
the BrCl/BrO2 and BrCl/ClO2 reactions. The BrCl/XO2
reactions proceed by a different mechanism than the O3/XO2-
kBr/Br
-
reactions.18 However, both BrCl and O3 react with ClO2- at
Br2 + BrO2
8 BrOBrO + Br-
-
a greater rate than with BrO2 . Nicoson et al. attributed the
-
-
have negligible effects on the BrCl/BrO2 study. When
greater rate of electron transfer in O3/ClO2 , relative to that
-
-
comparing the reactions of BrO2 with Cl2, BrCl, and Br2,
in O3/BrO2 , to the stability of the initial product, ClO2 (eqs
a trend similar to that of X2/ClO2- is not observed. The order
of Cl2 and BrCl is reversed. We attribute this inconsistency
to the greater stability of the interhalogen intermediate,
29 and 30).18 In the current study, the stability of BrOClO
-
O3 + ClO2- f ClO2 + O3
(29)
(30)
-
BrOClO, in the Cl2/BrO2 reaction over BrOBrO for the
O3 + BrO2- f BrO2 + O3
-
-
-
BrCl/BrO2 and Br2/BrO2 reactions, respectively. This is
in agreement with what has been discussed in an earlier
section regarding the difference in the rate of the BrCl
reactions with bromite and chlorite ions.
over BrOBrO leads to a larger rate constant for the BrCl/
ClO2- case. The suppression by Cl- in the overall reactions
(Figures 2b and 7b) provides evidence that Br+ transfer rather
Characteristics of XOXO Molecules. Several authors
have proposed the presence of ClOClO31,34 and BrOClO31,33,35
as intermediates in the gas phase and in solution. In the
current study we propose BrOClO as an intermediate in the
BrCl/ClO2 reaction. We also introduce BrOBrO as the
analogous intermediate for the BrCl/BrO2 reaction. These
are proposed as steady-state intermediates as opposed to
previous studies in which the intermediates were kinetically
metastable species that reacted very rapidly to give prod-
ucts.22,32,36 The distinction of the BrOClO and BrOBrO
species as steady-state intermediates in this case is a result
of an appreciable reverse path (k-2) that is in competition
with the forward path (k3 and k5) to give products. In other
systems22,32,36 BrOClO and BrOBrO were reactive intermedi-
ates that were not restricted to the steady-state approximation
because the reverse path was not appreciable compared to
the forward path. Yet these intermediates were required to
give the observed reaction products.
-
than electron transfer is the process by which BrCl/XO2
reactions proceed.
Comparison of Molecular Halogen Reactions. There is
a need to establish whether Br2 and Cl2, which are present
at low levels along with BrCl, contribute to the observed
reaction rate under our experimental conditions. The rate
-
-
-
constants for ClO2 reactions with Cl2 and Br2 have been
reported.32,33 The concentrations of Br2 and Cl2 present in
-
the BrCl/ClO2 system are calculated using our algorithm
as detailed in Table 1 and Supporting Information Appendix
5
A. Nicoson and Margerum32 reported a kCl ) 5.7(2) × 10
2
M-1 s-1 for Cl2/ClO2 (eq 31), whereas To´th and Fa´bia´n33
-
reported kBr ) 1.3(2) × 103 M-1 s-1 for the Br2/ClO2
2
reaction (eq 32). By use of the molecular halogen concentra-
kCl
Cl2 + ClO2
Br2 + ClO2
28 ClOClO + Cl-
(31)
(32)
-
kBr
28 Br2- + ClO2
-
As the evidence in the HOCl/BrO2 study indicates,20
-
BrOClO has a chainlike structure. We also favor the chainlike
structure for BrOBrO rather than a Y-shaped form. On the
basis of the electrophile/nucleophile argument, it is more
tions in Table 2 and the rate constants for X2/ClO2- reactions,
we find that the effects of Br2 and Cl2 on the BrCl/ClO2
reaction are negligible.
-
-
likely for the more electronegative O atoms of BrO2 , the
We can compare the reactivity of molecular halogens with
nucleophile, to attack the electrophilic bromine of BrCl to
displace Cl-. Such interaction would form a chainlike
structure analogous to that of the BrOClO intermediate.
ClO2- on the basis of the electrophilicity scale described by
+
Jia et al.6 In that study Jia et al. found that the rate of N2H5 /
XY (X ) Br or Cl; Y ) I, Br, or Cl) was dependent on the
energetics of both the new N-X bond formation and the
(34) Taube, H.; Dodgen, H. J. Am. Chem. Soc. 1949, 71, 3330-3336.
(35) Schmitz, G.; Rooze, H. Can. J. Chem. 1987, 65, 497-501.
(36) Jia, Z.; Margerum, D. W.; Francisco, J. S. Inorg. Chem. 2000, 39,
2614-2620.
(32) Nicoson, J. S.; Margerum, D. W. Inorg. Chem. 2002, 41, 342-347.
(33) To´th, Z.; Fa´bia´n, I. Inorg. Chem. 2000, 39, 4608-4614.
Inorganic Chemistry, Vol. 43, No. 23, 2004 7419

Odeh et al.
Conclusions
the formation of the highly reactive BrCl if the system
includes bromine-containing species.
-
This work shows that chloride ion catalyzes HOBr/XO2
Acknowledgment. This work was supported by National
Science Foundation grant CHE-0139876. We thank Dr. Lu
Wang for her contribution to the preparation and purification
of NaBrO2.
reactions. The catalysis is proposed to proceed through a
BrCl intermediate. Although BrCl is present in small
concentrations in the systems, it has a significant effect on
the rates of the studied reactions. The results are in agreement
with previous reports that discuss the role of BrCl as a
reactive species in ozone depletion and in drinking water
disinfection.1,2 The findings in this study also serve as a
cautionary reminder that researchers should not use NaCl to
control ionic strength. High levels of chloride can lead to
Supporting Information Available: Supplemental kinetic and
stoichiometric data and an algorithm for [BrCl] calculation. This
material is available free of charge via the Internet at http://pubs.
acs.org.
IC048982M
7420 Inorganic Chemistry, Vol. 43, No. 23, 2004