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Kinetics and Mechanism for Manganese-Catalyzed Oxidation of Sulfur(IV) in Aqueous Solution

Berglund, Johan; Fronaeus, Sture and Elding, Lars Ivar LU (1993) In Inorganic Chemistry 1993(32). p.4527-4538
Abstract
The kinetics for manganese-catalyzed autoxidation of sulfur(IV) (SO2.nH2O, HSO3-, SO32-) in aqueous solution has been studied spectrophotometrically at 25-degrees-C, 1 < pH < 4, [S(IV)] almost-equal-to 2.3 X 10(-5) M, 1 X 10(-6) M less-than-or-equal-to [Mn(II)] less-than-or-equal-to 1.5 X 10(-3) M, [Mn(III)] less-than-or-equal-to 4 X 10(-8) M, [O2] almost 2.5 X 10(-4) M, and low ionic strength. In the absence of added manganese(III), the kinetic traces display an induction period followed by a reaction first-order in sulfur(IV). Addition of a small amount of manganese(III) increases the rate significantly and suppresses the induction period. At pH 2.4, the first-order rate constant is k(obsd) = k[Mn(II)](1 + B[Mn(III)]0)/(A +... (More)
The kinetics for manganese-catalyzed autoxidation of sulfur(IV) (SO2.nH2O, HSO3-, SO32-) in aqueous solution has been studied spectrophotometrically at 25-degrees-C, 1 < pH < 4, [S(IV)] almost-equal-to 2.3 X 10(-5) M, 1 X 10(-6) M less-than-or-equal-to [Mn(II)] less-than-or-equal-to 1.5 X 10(-3) M, [Mn(III)] less-than-or-equal-to 4 X 10(-8) M, [O2] almost 2.5 X 10(-4) M, and low ionic strength. In the absence of added manganese(III), the kinetic traces display an induction period followed by a reaction first-order in sulfur(IV). Addition of a small amount of manganese(III) increases the rate significantly and suppresses the induction period. At pH 2.4, the first-order rate constant is k(obsd) = k[Mn(II)](1 + B[Mn(III)]0)/(A + [Mn(II)]), where A and B are constants and [Mn(III)]0 is the concentration of initially added manganese(III). The experiments are interpreted in terms of a free-radical chain mechanism. The first step, with rate constant k8 = (1.3 +/- 0.6) X 10(6) m-1 s-1, is a reaction between manganese(III) and a manganese(II) hydrogen sulfite complex with stability constant beta1 = (3 +/- 1) X 10(4) M-1: Mn(III) + MnHSO3+ -->(k8) 2Mn(II) + SO3- + H+. The catalytic activity of Mn(III/II) may be explained by formation of an oxo- (or hydroxo-) bridged mixed-valence precursor complex Mn(III)-O-Mn(II)-HSO3, in which bridged electron transfer produces the SO3 radical. When [Fe(III)] < 10(-6) M is added to the reacting system, the oxidation rate becomes much faster than the sum of the individual contributions from the manganese and iron catalyses; i.e., a synergistic effect is displayed. Initiation of the manganese-catalyzed oxidation in the absence of initially added manganese(III) is shown to be due to a trace impurity of ca. (1-2) x 10(-8) M iron(III). The SO3- radical is generated by the oxidation of sulfur(IV) by iron(III). In subsequent steps, manganese(II) is oxidized to manganese(III) by SO5- formed by oxidation of SO3 with dissolved oxygen. Computer simulation of the overall kinetics shows that an iron(III) concentration of 2 x 10(-8) M is indeed sufficient to initiate the manganese-catalyzed oxidation and explains the autocatalysis. At pH 4.0, the first-order rate constant is k(obsd) = (k[Mn(II)] + k[Mn(II)]2)/(A + [Mn(II)]). The quadratic manganese(II) term indicates formation of a sulfito-bridged manganese(II) complex, MnSO3Mn2+, which can also be attacked by manganese(III), forming SO3- radicals. Several discrepancies in previous literature may be explained by the reaction mechanism derived. (Less)
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organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Sulfur dioxide, Oxidation, Manganese catalysis, Atmospheric chemistry, Kinetics , Reaction mechanism
in
Inorganic Chemistry
volume
1993
issue
32
pages
12 pages
publisher
The American Chemical Society
ISSN
1520-510X
DOI
10.1021/ic00073a011
language
English
LU publication?
yes
id
32719a3c-faab-4e2c-88bb-4697f306bf5f
date added to LUP
2017-01-07 21:57:54
date last changed
2017-05-29 11:37:38
@article{32719a3c-faab-4e2c-88bb-4697f306bf5f,
  abstract     = {The kinetics for manganese-catalyzed autoxidation of sulfur(IV) (SO2.nH2O, HSO3-, SO32-) in aqueous solution has been studied spectrophotometrically at 25-degrees-C, 1 &lt; pH &lt; 4, [S(IV)] almost-equal-to 2.3 X 10(-5) M, 1 X 10(-6) M less-than-or-equal-to [Mn(II)] less-than-or-equal-to 1.5 X 10(-3) M, [Mn(III)] less-than-or-equal-to 4 X 10(-8) M, [O2] almost 2.5 X 10(-4) M, and low ionic strength. In the absence of added manganese(III), the kinetic traces display an induction period followed by a reaction first-order in sulfur(IV). Addition of a small amount of manganese(III) increases the rate significantly and suppresses the induction period. At pH 2.4, the first-order rate constant is k(obsd) = k[Mn(II)](1 + B[Mn(III)]0)/(A + [Mn(II)]), where A and B are constants and [Mn(III)]0 is the concentration of initially added manganese(III). The experiments are interpreted in terms of a free-radical chain mechanism. The first step, with rate constant k8 = (1.3 +/- 0.6) X 10(6) m-1 s-1, is a reaction between manganese(III) and a manganese(II) hydrogen sulfite complex with stability constant beta1 = (3 +/- 1) X 10(4) M-1: Mn(III) + MnHSO3+ --&gt;(k8) 2Mn(II) + SO3- + H+. The catalytic activity of Mn(III/II) may be explained by formation of an oxo- (or hydroxo-) bridged mixed-valence precursor complex Mn(III)-O-Mn(II)-HSO3, in which bridged electron transfer produces the SO3 radical. When [Fe(III)] &lt; 10(-6) M is added to the reacting system, the oxidation rate becomes much faster than the sum of the individual contributions from the manganese and iron catalyses; i.e., a synergistic effect is displayed. Initiation of the manganese-catalyzed oxidation in the absence of initially added manganese(III) is shown to be due to a trace impurity of ca. (1-2) x 10(-8) M iron(III). The SO3- radical is generated by the oxidation of sulfur(IV) by iron(III). In subsequent steps, manganese(II) is oxidized to manganese(III) by SO5- formed by oxidation of SO3 with dissolved oxygen. Computer simulation of the overall kinetics shows that an iron(III) concentration of 2 x 10(-8) M is indeed sufficient to initiate the manganese-catalyzed oxidation and explains the autocatalysis. At pH 4.0, the first-order rate constant is k(obsd) = (k[Mn(II)] + k[Mn(II)]2)/(A + [Mn(II)]). The quadratic manganese(II) term indicates formation of a sulfito-bridged manganese(II) complex, MnSO3Mn2+, which can also be attacked by manganese(III), forming SO3- radicals. Several discrepancies in previous literature may be explained by the reaction mechanism derived.},
  articleno    = {0020-1669/93/1332-4527},
  author       = {Berglund, Johan and Fronaeus, Sture  and Elding, Lars Ivar},
  issn         = {1520-510X},
  keyword      = {Sulfur dioxide,Oxidation,Manganese catalysis,Atmospheric chemistry,Kinetics ,Reaction mechanism},
  language     = {eng},
  month        = {10},
  number       = {32},
  pages        = {4527--4538},
  publisher    = {The American Chemical Society},
  series       = {Inorganic Chemistry},
  title        = {Kinetics and Mechanism for Manganese-Catalyzed Oxidation of Sulfur(IV) in Aqueous Solution},
  url          = {http://dx.doi.org/10.1021/ic00073a011},
  volume       = {1993},
  year         = {1993},
}