Table 2 Initial reactant concentrations and pH. Product yields in terms of concentrations (in units of 10Ϫ3 mol dmϪ3). Multiple entries in columns
5–7 give an impression of the degree of scatter in the results of these experiments
[Fe()]
[EDTA]
[DMSO]
pH
7.0
[CH3SO2H]
[HCO2H]
[CH2O]
1.5
2
10
0.23/0.26/0.21/0.29/0.28/
0.24/0.19
0.15/0.16/0.15/0.16/0.15/0.16
0.08/0.08/0.09/0.09
2.3
7.0
7.0
0.20/0.20/0.19/0.18/0.24/0.23
0.26/0.27/0.16/0.18
3.1/2.6/3.0/3.2/1.8/1.9/3.1/2.8
0.26/0.25/0.21/0.19/0.25
0.15/0.16/0.14/0.15
0.38/0.26/0.55/0.55/0.25/0.40/
0.31/0.35
0.06/0.07/0.06
0.08/0.08/0.09
0.50/0.47/0.52/0.42/
0.41/0.41
1.5
15
2
20
100
100
20 W. H. Koppenol and J. Butler, Adv. Free Radical Biol. Med., 1985, 1,
91.
the complete oxidation of ferrous to ferric. (As ferrous ion
becomes depleted in the course of the autoxidation, the attain-
ment of the endpoint is retarded on account of the reverse of
reaction (8),51 but this has no influence on the stoichiometry.)
In the (hypothetical) absence of peroxyl radical formation, the
ratio of methanesulfinic acid to ferric ion would be near 1:3.
However, the results in Table 2 indicate that these products are
formed in the ratio of between 1:5 and 1:6. This is in agree-
ment with the foregoing mechanism which suggests that this
ratio will incline toward the side of the smaller value when the
oxidation of the methyl-radical fragment stops at CH2O instead
of HCO2H. Indeed, the highest values for methanesulfinic acid:
Fe() are found where the ratio of CH2O:HCO2H is largest,
i.e. where the EDTA concentration is relatively large and the
buffering effect comes more strongly into play (Table 2).
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Acknowledgements
This work has been supported by the German Federal Ministry
for Education and Research (Project: 02-WT-9583). I. Yurkova
thanks the German Academic-Exchange Service DAAD for a
stipend.
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