Lente et al.
chlorophenols,2,24 HSO5 was also shown to be a useful
-
Fast kinetic measurements were performed with an Applied
Photophysics DX-17 MV sequential stopped-flow apparatus using
1.00 cm optical path length. The dead time of the instrument was
measured to be 1.09 ( 0.02 ms by published methods.26,27 In some
experiments, an Applied Photophysics PD.1 Photodiode Array
accessory was used as a detector of the stopped-flow instrument.
Quantitative kinetic measurements were also performed in the HP-
8543 diode-array spectrophotometer using an RX-2000 Applied
Photophysics Rapid Kinetics Accessory. The softwares Scientist28
and Matlab29 were used in the fitting procedures. A Fluorolog
FL3-22 spectrofluorimeter equipped with a 450 W xenon lamp
was used to detect fluorescence spectra of Ce(III).
reagent to clarify mechanistic details of catalytic oxidation
reactions of hydrogen peroxide. HSO5- frequently acts as a
two-electron oxidant in redox reactions that involve hetero-
lytic cleavage of the peroxo bond:
HSO-5 + A f HSO-4 + OA
(1)
In eq 1, A is a reagent with an oxygen acceptor site. In a
much smaller number of reactions, HSO5 is proposed to
be a one-electron oxidant forming the sulfate ion radical:
-
18,20
Quantitative kinetic measurements on the photochemical reaction
between HSO5- and Ce(III) were performed in the HP-8543 diode-
array instrument using the method and general operating procedures
described in earlier publications.30,31 The solutions were kept
homogeneous during the photochemical experiments using the built-
in magnetic stirrer of the standard cell compartment of the HP-
8543 instrument. A 3 mm stirring rod was used inside standard
quartz cuvettes (optical path length: 1.000 cm). The geometry of
the setup was carefully tested, and it was made sure that the stirring
rod never disrupted the light beam. The light source was calibrated
by both ferrioxalate actinometry32 and reproducing observations
on the known photoreaction of 2,6-dichloro-1,4-benzoquinone.30
HSO5- + e- f SO4·- + OH-
(2)
This process is analogous to the first step of Fenton-type
reactions of hydrogen peroxide.
In the present work, our objective was to find a systematic
way to distinguish between one- and two-electron oxidation
-
in the reactions of HSO5 . We chose the simplest possible
reducing agents: halide ions and some reducing metal ions.
Despite the fact that the kinetic studies on some of these
processes have already been published,15,17,18 we reproduced
all experiments in these cases, usually in a much wider
concentration and temperature range. These experiments were
designed to resolve some of the discrepancies in earlier
literature data. Whenever we confirmed published rate
equations, the main text will only state this fact and detailed
results will be found in the Supporting Information.
Results
Halide Ions. Earlier studies of the oxidation of halide ions
-
by HSO5 showed that the process was first-order with
respect to both reductant and oxidant.15,17 However, these
experiments were carried out exclusively at halide excess
and in acidic medium. Our own results showed that the
kinetic results cannot be interpreted without considering the
aqueous hydrolysis of halogens and trihalide ion formation,
which are given in the following series of equilibria:
In addition to the fundamental mechanistic interest in the
-
reactions of HSO5 , the reductions with halide ions, and
especially with Cl-, have great practical importance as well.
Oxidation of ubiquitous Cl- to Cl2 in water treatment
technologies using Oxone may pose a serious concern
regarding such applications and oxidation of Br- to carci-
[X-][H+][HOX]
-
X2 + H2O ) HOX + H+ + X-
K3 )
nogenic BrO3 is also a potential danger. Therefore, very
reliable information is needed about this process.
[X2]
(3)
(4)
Experimental Section
[X-3 ]
[X2][X-]
X2 + X- ) X-3
K4 )
K5 )
Materials. Potassium peroxomonosulfate stock solutions were
freshly prepared every day from Oxone (2KHSO5 ·KHSO4 ·K2SO4,
Aldrich) and standardized by iodometric titration. These acidic stock
solutions were shown to be stable for at least 24 h. The known
[H+][OX-]
[HOX]
HOX ) H+ + OX-
(5)
-
decomposition of HSO5 in basic medium is orders of magnitude
slower than the processes we studied.14 Perchloric and sulfuric acid
was used to set the pH. In some of the experiments, the ionic
strength was set with NaClO4, which was prepared as described
earlier.25 In these cases, extra care was taken to make sure that the
oxidant concentration is sufficiently low to avoid the precipitation
of KClO4 (Oxone contains K+ ions). In other cases, H2SO4 or
Na2SO4 was used to provide constant ionic strength. Halide stock
solutions were prepared by weighing NaX salts and dissolving them
to a known volume. All other chemicals used in this study were of
analytical reagent grade and purchased from commercial sources.
Doubly deionized and ultrafiltered water from a Millipore Q system
was used to prepare the stock solutions and samples.
The equilibrium constants for each of these three processes
are known from previous literature and are summarized in
Table 1.33-38 It is also well established that these processes
equilibrate on a time scale that is very short compared to
(26) Tonomura, B.; Nakatani, H.; Ohnishi, M.; Yamaguchi-Ito, J.; Hiromi,
K. Anal. Biochem. 1978, 84, 370–383.
(27) Peintler, G.; Nagy, A.; Horva´th, A. K.; Ko¨rtve´lyesi, T.; Nagypa´l, I.
Phys. Chem. Chem. Phys. 2000, 2, 2575–2586.
(28) SCIENTIST, Version 2.0; Micromath Software: Salt Lake City, UT,
USA, 1995.
(29) MatLab for Windows, Version 4.2c1, The Mathworks, Inc.: Natick,
MA, USA, 1994.
Instrumentation and Computation. UV-vis spectra and kinetic
curves were recorded on PerkinElmer Lambda 25 or PerkinElmer
Lambda 2S scanning and HP-8543 diode-array spectrophotometers.
(30) Lente, G.; Espenson, J. H. J. Photochem. Photobiol., A 2004, 163,
249–258.
(31) Kerezsi, I.; Lente, G.; Fa´bia´n, I. J. Am. Chem. Soc. 2005, 127, 4785–
4793.
(32) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A 1956,
235, 518–536.
(25) Gordon, G.; Tewari, P. J. Phys. Chem. 1966, 60, 200–204.
1764 Inorganic Chemistry, Vol. 48, No. 4, 2009