Pulse radiolysis study of the oxidation of the I- ions with the radical anions Br2 >=- In an aqueous solution: Formation and properties of the radical anion BrI>=-

Article abstract of DOI:10.1007/s11172-008-0245-y

The radiation chemical redox transformations in solutions of bromides in the presence of minor additives of iodides were studied by pulse radiolysis. The change in the concentrations of the Br^- and I^- ions changes the ratio of the formed short-lived radical anions Br₂ ^?-, BrI^?-, and I₂ ^?-. The spectrum of the mixed radical anion BrI^?- contains a broad optical band at 370 nm with ₃₇₀ = 9650 L mol^-1 cm ^-1. The reduction potential of the BrI^?-/Br ^-, I^- pair is 1.25 V. The rate constants for the forward and backward reactions Br₂ ^?- + I^- BrI^?- + Br^- are k _f = 4.3?10 ⁹ and k _r = 1.0?10⁵ L mol^-1 s^-1, respectively; for the reactions BrI^?- Br ^- + I^?, k _f = 5.7?10⁸ s^-1 and k _r = 1.0?10¹⁰ L mol^-1 s^-1.

Full text of DOI:10.1007/s11172-008-0245-y

Russian Chemical Bulletin, International Edition, Vol. 57, No. 9, pp. 1821—1826, September, 2008
1821
Pulse radiolysis study of the oxidation of the I– ions
with the radical anions Br2•– in an aqueous solution:
formation and properties of the radical anion BrI•–
B. G. Ershov,a E. Janata,b and A. V. Gordeeva
aInstitute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
31 Leninsky prosp., 119991 Moscow, Russian Federation.
Eꢀmail: ershov@ipc.rssi.ru
bHahnꢀMeitner Institute,
100 Gluniker Str., 14109 Berlin, Germany
The radiation chemical redox transformations in solutions of bromides in the presence of
minor additives of iodides were studied by pulse radiolysis. The change in the concentrations of
the Br– and I– ions changes the ratio of the formed shortꢀlived radical anions Br2 , BrI•–, and
–
•
–
–
•
I2 . The spectrum of the mixed radical anion BrI• contains a broad optical band at 370 nm
with ε370 = 9650 L mol–1 cm–1. The reduction potential of the BrI•–/Br–, I– pair is 1.25 V. The
rate constants for the forward and backward reactions Br2 + I–
BrI• + Br– are kf =
–
–
•
4.3•109 and kr = 1.0•105 L mol–1 s–1, respectively; for the reactions BrI•
Br– + I•, kf =
–
5.7•108 s–1 and kr = 1.0•1010 L mol–1 s–1
.
Key words: pulse radiolysis, oxidation, reaction, chlorine, bromine, iodine, ions, radicals,
kinetics.
–
–
0
–
The reactions of oneꢀelectron oxidation of the Cl–, Br–,
and I– ions that occur upon the radiolysis or photolysis of
aqueous solutions of halides afford intermediate shortꢀlived
E0(Cl2 /2Cl ) = 2.09 V and E (Br•/Br ) = 1.93 V, as
well as E0(I•/I–) = 1.33 V. In the first case, the potential
difference is only 0.16 V, which is favorable for the organiꢀ
zation of a mixed orbital in ClBr•–. In the second case, the
difference reaches 0.76 V, and the transfer of an electron
•
radical anions Cl2 –, Br2 –, and I2 (see review1). They
exhibit properties of strong oxidants. The rate constants
for many oneꢀelectron oxidation reactions of organic
and inorganic compounds involving these radical anions
were measured by pulse radiolysis and photolysis.2
–
•
•
•
from I– to Cl2 – turns out to be energetically more favorable.
•
A comparison of E0(Br2 –/2Br–) = 1.66 V with
•
E0(I2 /2I ) = 1.03 V and E (I•/I ) = 1.33 V (see Ref. 6)
–
–
0
–
•
–
•
–
•
It was found3 that Cl2 interacts with the Br– ion to
form the mixed radical anion ClBr•–. This species occuꢀ
pies an intermediate position between the radical anions
suggests that the radical anions Br2 can oxidize the
I– ions. However, this reaction was not studied until presꢀ
ently. The potential difference between E0(Br2 –/2Br–)
•
Cl2 – and Br2 – in the properties. For instance, the maxiꢀ
mum of the optical band (λmax) in the spectrum of this
species lies at a wavelength of 350 nm, and the molar
absorption coefficient (ε) is 9.3•103 L mol–1 cm–1.3 The
and E0(I•/I–) is 0.33 V, and the formation of the mixed
•
•
radical anions BrI•– can be expected, as it is observed for
the reaction of Cl2 – with the Br– ion.
•
The study of the redox reactions involving chlorine,
bromine, and iodine is important due to their wide abunꢀ
dance in practical activities. For example, when chlorine
is used for the disinfection of drinking water, admixtures of
bromine and iodine are oxidized with chlorine to form
diverse oxide compounds. Bromine and iodine are obtained
by the chlorination of hydrochloric brines with the evoluꢀ
tion of molecular forms of these halogens. Therefore, the
problem of studying the mechanisms and kinetics of the
redox reactions involving chlorine, bromine, and iodine is
topical. The present work is devoted to the pulse radiolysis
–
•
corresponding parameters for Cl2
are 340 nm and
–
•
8.8•103 L mol–1 cm–1 4
, and for Br2 they are 360 nm
and 9.9•103 L mol–1 cm–1 5
.
The standard redox potenꢀ
tials of the Cl2 –/2Cl– and Br2 –/2Br– pairs are 2.09 and
•
•
1.66 V.6 The calculated E0 potential for the ClBr•–/Cl–,Br–
pair is 1.85 V.3 The radical anions Cl2 – interact with the
•
I– ion in a different manner.7 In this case, no mixed radiꢀ
cal anion is formed, but the I• atoms, which are further
transformed into the radical anions I2 –, are generated. In
•
authors´ opinion,7 this difference in the mechanism of
–
oxidation of the Br– and I– ions with the radical anions
study of the reaction of the Br2 radical anions with the
I– ions in an aqueous solution.
•
Cl2 – is a consequence of the difference between the values
•
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1787—1792, September, 2008.
1066ꢀ5285/08/5709ꢀ1821 © 2008 Springer Science+Business Media, Inc.

1822
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 9, September, 2008
Ershov et al.
A
Experimental
0.16
A pulse radiolysis technique8 and the van de Graaf generator
software9 were used. The duration of an electron pulse with an
energy of 3.8 MeV was varied in the interval from 3 to 20 ns.10
Optical signals were obtained by averaging the data from ten
pulses. The absorbed dose in a pulse was measured from the
optical absorption of the hydrated electron in water11 at 700 nm
and its molar absorption coefficient at this wavelength equal to
1.9•104 L mol–1 cm–1. The radiation chemical yield of electrons
in water at pH 7 is 2.6 particles per 100 eV of the absorbed
energy.12
0.12
0.08
0.04
1
2
3
4
300
400
λ/nm
The salts NaBr and NaI (Merck) were additionally recrystalꢀ
lized. Solutions were prepared using triply distilled water and
saturated with nitrous oxide (N2O).
Fig. 1. Absorption spectra of a 0.1 М solution of NaBr with
pH 3.3 containing 5•10–6 М NaI and saturated with N2O 0.7 (1),
10 (2), 32 (3), and 250 μs (4) after an electron pulse. The pulse
duration is 40 ns, and the absorbed dose is 15 Gy and corresponds
to the formation of the •OH radicals at a concentration of
Results and Discussion
8.6•10–6 mol L–1
.
Experiments were carried out with weakly acidic
(5•10–4 М HClO4) aqueous solutions of NaBr containing
NaI additives and saturated with N2O. Upon the radiolysis
of these solutions, the hydrated electrons eaq–, formed in
the primary processes along with the •OH hydroxy radiꢀ
cals, are transformed into the •OH radicals ([N2O] =
at 265 nm, as it was observed in the solution without I– ions.
In addition, the band intensity is approximately twice as
large. The increase in the absorption intensity at 258 nm is
accompanied by the clearing up of the solution at the waveꢀ
length 225 nm corresponding to the absorption band of the
I– ions (λmax = 225 nm, ε = 1.1•104 L mol–1 cm–1).15
This is induced, most probably, by the formation, along
= 2.6•10–2 mol L–1 and k1 = 9.1•109 L mol–1 s–1 13
due
)
to the reaction
with the Br3 ion, of the mixed trihalide ion Br2I– in the
–
eaq– + N2O + H2O
•OH + N2 + OH–.
(1)
reaction
Thus, the presence of N2O makes it possible to generate
predominantly the •OH radicals by the ionizing irradiaꢀ
tion of water. The H+ ions catalyze the oxidation of the
Br– ions with the •OH radicals in aqueous solutions.5
After an aqueous 0.1 М solution of NaBr (рН 3.3)
saturated with N2O was irradiated with a pulse of acceleratꢀ
Br2 + I–
Br2I–.
(4)
Simple and mixed trihalide ions appear in reactions
of molecular halogens with halogen ions. According to
earlier published data,16 the spectrum of Br2I– has an abꢀ
sorption band with a maximum at 253 nm and its ε is
–
•
ed electrons, an absorption band of the radical anion Br2
with a maximum at 360 nm (εBr •– = 9900 L mol–1 cm–1 5,14
appeared in the optical spect2rum. The disappearance of
5.5•104 L mol–1 cm–1, and Br3 absorbs at 266 nm with
–
)
ε = 4.1•104 L mol–1 cm–1 17
Under the conditions of our
.
experiments, we observed, most likely, a mixture of two
types of the indicated trihalide ions, whose concentration
ratio corresponds to the ratio of the bromide and iodide
ions present in the solution.
–
•
Br2 is accompanied by the appearance of a new optical
absorption: a band with a maximum at 265 nm induced by
the formation of the Br3– ion due to the reactions
Br2•– + Br2
Br2 + 2 Br–,
(2)
•–
The absorption spectra of a 0.1 М solution of NaBr
(pH 3.3) saturated with N2O and containing a higher conꢀ
centration of the I– ions (5•10–6 М NaI) are shown in
Fig. 2. 0.3 μs after an accelerated electron pulse with
a duration of 5 ns, the optical spectrum contains an abꢀ
sorption band with a maximum at 360 nm due to the radiꢀ
Br2 + Br–
Br3
.
(3)
–
We considered the mechanism of this process in detail.14
The rate constant for forward reaction (2) was determined
as kf = 3.0•109 L mol–1 s–1, and the rate constants for the
forward and backward reactions (3) were also calculated.
In the present study, we found that the addition of
5•10–6 М NaI to an 0.1 М solution of NaBr (pH 3.3)
saturated with N2O resulted in noticeable changes in the
optical spectrum. As can be seen from the data in Fig. 1, in
this case, the disappearance of the absorption of the radꢀ
cal anion Br2 –. The high concentration of the Br– ions
•
provided the nearly complete capture of the •ОН radicals,
formed by water radiolysis, with these ions rather than
with I–. Using the absorbance value and the known5 value
ε = 9.9•103 L mol–1 cm–1, one can determine the conꢀ
centration of the formed radical anions Br2 –, which is
•
8.6•10–7 mol L–1 for a solution containing 0.1 М NaBr.
For this concentration of the radical anions, their interacꢀ
tion with the I– ions prevails, most likely, over their reꢀ
ical anion Br2 – at 360 nm is accompanied by the appearance
•
of the band with a maximum at 258 nm instead of the band

Рulse radiolysis oxidation of I– with Br2
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 9, September, 2008
1823
–
A•102
combination according to the secondꢀorder law. In fact,
the data in Fig. 2 show that after a pulse the absorption
band maximum shifts with time toward long wavelengths,
reaching its position at ~370 nm. It is important that the
changes observed in an optical region of 300—400 nm are
accompanied by the noticeable clearing up of the solution
at 225 nm (Fig. 3). This clearing up is caused by the disapꢀ
pearance of the free I– ions. It seems natural to attribute
0.5
0.0
–0.5
–
•
these facts to the interaction of the radical anions Br2
–1.0
–1.5
0
with the I– ions. This reaction is rather fast and, as can be
seen in Fig. 2, in the presence of 5•10–5 М NaI the reacꢀ
tion ceases ~3 μs after an electron pulse. At long times
(~500 μs, when pulses with a duration of 5 ns are used), the
optical absorption shown in Fig. 2 decreases without apꢀ
preciable changes in the band shape. The decay kinetics is
described by the secondꢀorder kinetic equation, and τ1/2
decreases proportionally with an increase in the absorbed
dose in a pulse. Reactions (2)—(4) result in the appearance
of an absorption band in the spectrum with a maximum at
253 nm belonging to the Br2I– ion.
5
10
15
20
t/μs
Fig. 3. Kinetic curve of the change in the absorption at 225 nm.
The pulse duration is 10 ns, and the absorbed dose is 3.5 Gy;
the data are presented for the same solution as in Fig. 2.
length 725 nm caused by the radical anions Br2 – continꢀ
ues to increase within several microseconds after the end
of an electron pulse. Note that the I2 – radical anions give
a more intense (by ~3 times) absorption at the indicated
wavelength (ε725 = 3•103 L mol–1 cm–1).7,8,20 The abꢀ
sorption at 725 nm increases substantially with an increase
in the content of the I– ions in a solution (5•10–4 М NaI
and more), and this correlates with the transformation of
the radical anions Br2 – into I2 – observed at wavelengths
of 360—385 nm.
•
In a 0.1 М solution of NaBr containing 5•10–4 М NaI
and more, already 1 μs after an accelerated electron pulse
the primarily appeared absorption band of the radical anꢀ
•
ion Br2 – at 360 nm is transformed into an absorption band
•
– 18—20
•
at 385 nm. The latter is due to the radical anion I2
.
It can be concluded that the interaction of the radical
anions through the step of formation of the mixed product
ceases by the formation of the I2 – species.
•
•
•
It is known3,5 that the radical anion Br2 has a weak
absorption at 650—750 nm (ε700 = 400—600 L mol–1 cm–1).
The kinetic curves for the appearance of an optical absorpꢀ
tion at a wavelength of 725 nm in a 0.1 М solution of NaBr
containing 5•10–5 М NaI are shown in Fig. 4. In the
presence of the I– ions, the weak absorption at the waveꢀ
–
•
The experimental facts indicate that the radical anion
–
•
Br2 oxidizes the I– ions. Along with the radical anion
I2 –, which is formed at rather high content of the I–
•
ions in the solution (> 5•10–4 М NaI), at a lower concenꢀ
tration of the I– ions this reaction also affords another
A•103
A
1
2
3
1.5
0.08
0.04
0
4
1.0
0.5
0
–0.04
1
2
3
4
5
6
t/μs
300
400
λ/nm
Fig. 2. Absorption spectra of a 0.1 М solution of NaBr with
pH 3.3 containing 5•10–5 М NaI and saturated with N2O 0.3 (1),
1.5 (2), 3.0 (3), and 6.0 μs (4) after an electron pulse. The pulse
duration is 5 ns, and the absorbed dose is 1.6 Gy and corresponds
to the formation of the •OH radicals in a concentration of
Fig. 4. Kinetic curve of the change in the absorption at 725 nm.
The pulse duration is 10 ns, and the absorbed dose is 1.8 Gy; the
data are presented for the same solution as in Fig. 2. The smooth
line is the calculation by the scheme (see Table 1). The following
–
values were used for Br2 , BrI•–, and I2 –, respectively: ε = 560,
•
•
9.3•10–7 mol L–1
.
1700, and 2600 L mol–1 cm–1
.

1824
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 9, September, 2008
Ershov et al.
–
–
•
•
species. Its optical characteristics in the UV region resemꢀ
Br2 and I2 radical anions. The following values were
accepted for the initial yields of the products of water radiꢀ
ble those of Br2 and I2 –. So, the maximum of its abꢀ
sorption band lies at ~370 nm and the ε value does not
substantially differ from ε for the mentioned radical anꢀ
ions. Thus, an increase in the concentration of the I– ions
in the solution results in a decrease in the relative content
of the unknown intermediate species and, to the contrary,
the content of I2– increases.
–
•
•
olysis12: G(eaq ) = 2.7, G(H•) = 0.6, and G(OH•) = 2.8.
–
The presence of 0.1 М NaBr or other halides exerts no
substantial effect25 on the initial yields of the water radiolꢀ
ysis products. It was also taken into account that the
hydrated electrons formed in reaction (1) are transformed
into the hydroxy radicals. It is found that the use of the
indicated scheme allows the satisfactory simulation to be
performed for the kinetics of the change in the optical
absorption, generated by an electron pulse, at all waveꢀ
lengths. In this case, the known molar absorption coeffiꢀ
We believe that the interaction of the radical anions
–
Br2 with I– produces, in addition to I2 –, the mixed
•
•
radical anion BrI•–. Similar species, namely, the radical
anions ClSCN• and BrSCN•–, were found by pulse
–
radiolysis upon the oxidation of chloride21 and broꢀ
mide22 in the presence of minor additives of the SCN–
ions. The formation of the mixed radical anion ClBr•– has
recently been observed by pulse radiolysis during the oxiꢀ
cients of the radical anions Br2 – and I2 – and the I– ions
involved in the reactions and the εIBr – value chosen for
•
•
•
consistency were used for each wavelength. Figures 4, 5, a,
and 5, b illustrate the agreement between the calculations
and the change in the absorption of a 0.1 М solution of
NaBr containing 5•10–5 М NaI at 725, 340, and 400 nm,
respectively, after an electron pulse 5 ns long. The contriꢀ
butions of the absorption from particular radical anions
–
•
dation of the Br– ions with the radical anions Cl2 in
a 1 М solution of NaCl containing an admixture of the Br–
ions (less than 10–3 mol L–1). It is most likely that the
–
radical anions Br2 interact with the I– ions via an
analogous mechanism due to the following consecutive
reactions
•
–
–
•
Br2 , BrI•–, and I2 involved in the chemical transforꢀ
mations are also shown in the figures. Similar results were
obtained at other wavelengths using solutions with other
concentrations of the components and powers of the abꢀ
sorbed dose. It can be concluded that the proposed scheme
provides a satisfactory agreement with experiment.
•
Br2•– + I–
BrI•– + I–
BrI•– + Br–,
I2•– + Br–.
(5)
(6)
The ε values for BrI•– at different wavelengths, which
were obtained by the agreement between the experiꢀ
mental data and calculations using the aboveꢀindicated
scheme, are shown in Fig. 6. The absorption spectrum of
BrI•–, as can be seen, is a broad absorption band with
a maximum at 370 3 nm, and ε at this wavelength is
9650 200 L mol–1 cm–1. A broad optical absorption at
–
–
•
The ratio of the radical anions Br2 , BrI•–, and I2
•
formed upon the radiation chemical oxidation of the Br–
and I– ions is determined by the concentrations of the
halogen ions present in the solution.
The computer simulation of the process was performed
to describe the oxidation of the I– ions with the radical
anions Br2 in the solutions under study. The interacꢀ
tion of the Cl• atom with the Cl– ion affording the radical
anion Cl2 – and the analogous reaction for bromine proꢀ
ceed with very high rate constants: 1.0•1010 L mol–1 s–1
(see Ref. 5) and 1.1•1010 L mol–1 s–1, respectively.18,19
We accepted that the rate constant for the similar reaction
between the I• atom and the Br– ion affording the radical
anion BrI• is 1.0•1010 L mol–1 s–1. The rate conꢀ
stant for the reaction between the H atom and the radical
–
•
Table 1. Rate constants (k) for the reactions used in modeling
the oxidation kinetics of the I– ions with the Br2– radical anions
•
Reaction
k/L mol–1 s–1
Reference
Br– + OH• → Br•
1.1•1010
4.6•104 *
1.0•1010
3.4•109
7.0•109
7.8•109
4.3•109
<1.0•105
5.7•108 *
1.0•1010
5.8•109
4.3•106
1.1•105*
1.2•1010
3.0•109
3.0•109
5
5
5
3
23
24
–
–
–
•
Br2 → Br• + Br
–
–
•
Br• + Br → Br2
Br2 – + Br2 → Br3– + Br–
–
•
•
–
•
anion Br2 is unknown. We accepted that it is equal to
7•109 L mol–1 s–1, measured23 for an analogous reaction
of the H atom with the radical anion Cl2 –. We obtained
H• + Br2 → Br– + HBr
–
•
H• + H• → H2
•
Br2 – + I– → BrI•– + Br–
This work
This work
This work
This work
This work
This work
18, 19
18, 19
This work
18, 19
•
BrI•– + Br– → Br2 – + I–
the rate constant values for other unknown reactions in the
reaction scheme (Table 1) by the simulation of the kinetics
of the change in the optical absorption in the studied soluꢀ
tions at different wavelengths. The changes in the absorpꢀ
tion at 300—400 nm and 725 nm reflect the formation,
•
BrI• → Br– + I•
–
Br– + I• → BrI•
–
BrI•–+ I– → I2 – + Br–
•
I2 – + Br– → BrI•– + I–
•
–
•
–
–
•
–
I2 → I + I•
transformation, and decay of the radical anions Br2 , BrI•
,
–
I– + I• → I2
and I2 –, and the change at 225 nm shows the disapꢀ
pearance of the I– ions in these reactions. The recombinaꢀ
•
BrI• + BrI• → Br2I•– + I–
–
–
I2 – + I2 → I3– + I–
–
•
•
–
tion rate constant of the radical anion BrI• was selected
* In s–1
.
on the basis of the known values for similar reactions of the

Рulse radiolysis oxidation of I– with Br2
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 9, September, 2008
1825
–
Α (%)
100
ε/L mol–1 s–1
A•103
a
9300
7.0
6.0
5.0
4.0
–
•
4650
Br2
50
3.0
2.0
1.0
0
–
BrI•
–
•
I2
1
2
3
4
5
t/μs
300
400
500
600
700
800
λ/nm
Fig. 6. Optical absorption spectrum of the BrI•– radical anion.
A•103
b
From the calculated value K5 > 4.3•104 and known5 K9 =
= 4.6•10–6 mol L–1 we determine K8 < 1.1•10–10 mol L–1
.
–
4.0
2.0
A comparison of the K7 and K8 values shows that BrI•
–
•
decomposes exclusively via reaction (8). This is quite natꢀ
ural, because upon the decomposition of BrI•– an electron
preferentially adds to the bromine atom, whose oxidation
potential (1.93 V) is higher than that for the iodine atom
(1.33 V),6 i.e., reaction (8) is thermodynamically unfavorꢀ
able. An analogous situation is observed for the same reaꢀ
son in the case of another mixed radical anion ClBr•–, for
which the equilibrium constant for the decomposition with
the formation of the chlorine atom and bromine ion is
3.8•10–13 mol L–1, whereas that with the formation of the
Br2
–
BrI•
–
•
I2
1
2
3
4
5
t/μs
Fig. 5. Comparison of the experiment with the computer calcuꢀ
lation (smooth line) by the scheme (see Table 1) (the data are
presented for the same solution as in Fig. 2); a, kinetics of the
change in the absorption at 340 nm. The absorbed dose is 1.6 Gy.
chlorine ion and bromine atom is 8.5•10–3 mol L–1 3
The redox potentials of equilibria (5)—(7) are written
.
The following values were used for Br2 , BrI•–, and I2 –, resꢀ
pectively: ε = 8100, 5900, and 3600 L mol–1 cm–1. b, Kinetics
of the change in the absorption at 400 nm. The absorbed dose is
–
•
•
as follows: ΔE5 = E0(Br2 /2Br ) – E (BrI•–/Br–,I–),
0
–
–
0
•
ΔE6 = E (BrI•–/Br–,I–) – E0(I2 –/2I–), and ΔE7
=
0
0
0
•
1.2 Gy. The following values were used for Br2 , BrI•–, and
–
•
–
I2 –, respectively: ε = 5800, 7400, and 8700 L mol–1 cm–1
.
= E0(BrI• /Br–,I–) – E0(I•/I–). The ΔE0 value can be
expressed through the equilibrium constant K of the corꢀ
responding reaction in the form ΔE0 = 0.059•logK.
Using the known values K5, K6, and K7, as well as
•
650—750 nm is also characteristic of BrI•–, and its
ε725 value was calculated as approximately equal to
580 L mol–1 cm–1. Thus, its optical characteristics are inꢀ
termediate compared to the optical characteristics
E0(Br2 –/2Br–) = 1.66 V, E0(I2 –/2I–) = 1.03 V, and
E0(I•/I–) = 1.33 V,6 we find that the redox potential
E0(BrI•–/Br–,I–) is <1.39, 1.22, and 1.26 V for equilibria
(5), (6), and (7), respectively. The agreement between these
values can be recognized as very good, which indicates in
favor of the right choice of the reaction scheme that satisꢀ
factorily describes the radiation chemical oxidation of the
I– ions in aqueous solutions of NaBr. The E0(BrI•–/Br–,I–)
potential can be accepted as equal to 1.25 0.05 V. In fact,
•
•
–
•
of the radical anions Br2
(λmax = 360 nm and
ε = 9900 L mol–1 cm–1) and I2 (λmax = 385 nm and
–
•
ε = 9400 L mol–1 cm–1).
The equilibrium constant K is equal to the ratio of the rate
constants for the forward and backward reactions, i.e., kf/kb.
The calculated data (see Table 1) show that K5 > 4.3•104
and K6 = 1.3•103. The constant of the equilibrium
–
as could be expected, the reduction potential of the ClBr•
radical anion is intermediate between the potentials for the
BrI•–
Br– + I•
(7)
Br2 – (1.66 V) and I2– (1.03 V) radical anions.
•
The ratio between the equilibrium concentrations of
K7 is 5.7•10–2 mol L–1
The equation K8 = K9/K5 is valid for the equilibria
.
–
–
•
Br2 , BrI•–, and I2 in the absence of their decay at
different ratios of the concentrations of the I– and Br– ions
in the solution is shown in Fig. 7.
•
BrI•–
Br• + I–,
(8)
(9)
The results of the present study show that the radical
•–
•
Br2
Br• + Br–
anions Br2 – oxidize the I– ions to form the radical anions

1826
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 9, September, 2008
Ershov et al.
[X]/([I2–] + [Br2–] + [IBr–])
1.0
= 1.93 V, are 0.33 and 0.16 V, respectively. A slight differꢀ
ence between the potentials is favorable for the organizaꢀ
–
tion of a mixed orbital in BrI• and in ClBr•–. The differꢀ
ence in the potential values E0(Cl2 –/2Cl–) = 2.09 V and
•
–
•
–
•
Br2
I2
E0(I•/I–) = 1.33 V is 0.76 V, and the electron transfer from
0.8
0.6
0.4
I– to Cl2 – turns out to be energetically more favorable.
•
References
1. B. G. Ershov, Usp. Khim., 2004, 73, 107 [Russ. Chem. Rev.,
2004, 73 (Engl. Transl.)].
–
BrI•
2. P. Neta, R. E. Huie, A. B. Ross, J. Phys. Chem. Ref. Data,
1988, 17, 1027.
0.2
3. B. G. Ershov, M. Kelm, A. V. Gordeev, E. Janata, Phys.
Chem. Chem. Phys., 2002, 4, 1872.
4. G. G. Jayson, B. J. Parsons, A. J. Swallow, J. Chem. Soc.,
Faraday Trans. 1, 1973, 69, 1579.
10–6
10–5
10–4
10–3
10–3 [I–]/[Br–]
Fig. 7. Plots of the ratio of the concentrations of the radical
anions Br2 , BrI•–, and I2 – in solution vs I–/Br– value.
5. D. Zehahi, J. Rabany, J. Phys. Chem., 1972, 76, 312.
6. P. Wardman, J. Phys. Chem. Ref. Data, 1989, 18, 1711.
7. B. G. Ershov, E. Janata, A. V. Gordeev, Izv. Akad. Nauk,
Ser. Khim., 2005, 1336 [Russ. Chem. Bull., Int. Ed., 2005,
54, 1378].
8. E. Janata, Radiat. Phys. Chem., 1992, 40, 437.
9. E. Janata, Radiat. Phys. Chem., 1994, 44, 449.
10. E. Janata, W. Gutsch, Radiat. Phys. Chem., 1998, 51, 65.
11. G. L. Hug, Optical Spectra of Nonmetallic Inorganic Transient
Species in Aqueous Solution, NSRDSꢀNBS 69, Washington,
1981, 159.
12. G. V. Buxton, C. L. Greenstock, W. P. Helman, A. B. Ross,
J. Phys. Chem. Ref. Data, 1988, 17, 513.
13. E. Janata, R. H. Schuler, J. Phys. Chem., 1982, 86, 2078.
14. B. G. Ershov, A. V. Gordeev, E. Janata, M. Kelm, Menꢀ
deleev Commun., 2001, 4, 149.
15. D. W. Margerum, P. N. Dickson, J. C. Nagy, K. Kumar,
C. P. Bowers, K. D. Fogelman, Inorg. Chem., 1986, 25, 4900.
16. W. C. Troy, M. D. Kelley, J. C. Nagy, D. W. Margerum,
Inorg. Chem., 1991, 30, 4838.
17. T. X. Wang, M. D. Kelley, J. Cooper, R. C. Beckwith, D. W.
Margerum, Inorg. Chem., 1994, 33, 5872.
18. H. A. Schwarz, B. H. J. Bielski, J. Phys. Chem., 1986,
90, 1445.
19. A. J. Elliot, Can. J. Chem., 1992, 70, 1658.
20. B. G. Ershov, E. Janata, Chem. Phys. Lett., 2003, 372, 159.
21. M. Schoneshofer, A. Henglein, Ber. Bunsenges. Phys. Chem.,
1969, 73, 289.
–
•
•
BrI•– and I2 –. The equilibrium shifts to the predominant
•
formation of I2 – with an increase in the I– concentration.
•
These radical anions are highly reactive and shortꢀlived
species. The rate constants of their recombination are
~3.0•109 L mol–1 s–1 (see Table 1). Their decay results in
the formation of a large array of molecular (Br2, BrI, I2)
and ionic (Br3–, Br2I–, BrI2–, I3–) products, which exist at
equilibrium with each other. The mixed trihalide ions give
intense absorption bands in the UV region: for Br2I–
λmax = 253 nm, ε = 5.5•104 L mol–1 cm–1 and for BrI2
–
λ
max = 273 nm, ε = 6.1•104 L mol–1 cm–1 16
. These anions
are formed after the disappearance of the radical anions,
and their absorption bands are overlapped with those of
Br3 (λmax = 266 nm, ε = 4.1•104 L mol–1 cm–1) and I–
–
(λmax = 225 nm, ε = 1.1•104 L mol–1 cm–1).16,17 This fact
substantially impedes the rigid kinetic analysis of the proꢀ
cess of their formation. However, this problem was not
stated in our investigation, because the properties of these
spacies were studied in detail elsewhere15—17. They were
produced in the reactions of molecular halogens with haꢀ
lide ions. Nevertheless, at the concentration of the I– ion
about 5•10–6 mol L–1 in a 0.1 М solution of NaBr, when
the formation of the Br3 and Br2I– species prevailed
–
22. M. Schoneshofer, Int. J. Radiat. Chem., 1969, 1, 505.
23. S. Navaratnam, B. Parsons, A. J. Swallow Radiat. Phys.
Chem., 1980, 15, 159.
24. P. Pagsberg, G. Fenger, S. O. Nielsen, J. Phys. Chem., 1969,
73, 1029.
(see Fig. 1), we calculated the rate constant for reaction (1)
equal to 2•1010 L mol–1 s–1
.
Thus, the radical anions Br2 oxidize the I– ions to
–
•
form the intermediate mixed radical anion BrI•–. The reꢀ
25. E. Peled, D. Meisel, G. Czapski, J. Phys. Chem., 1972,
76, 3677.
action between Cl2 – and the Br– ion proceeds similarly to
•
form ClBr•–. At the same time, Cl2 – interacts with the I–
•
ion to form the I• atoms, which are further transformed
into the radical anions I2 –.7 The potential differences of
•
–
•
–
0
–
the pairs E0(Br2 /2Br ) = 1.66 V and E (I•/I ) = 1.33 V,
Received September 10, 2005;
in revised form January 16, 2008
as well as E0(Cl2 /2Cl ) = 2.09 V and E (Br•/Br ) =
–
–
0
–
•