influence of oxalic acid on the catalytic s

142
The ratio r2/r1 indicates how many times the
Mn(II)-catalysed S(IV) oxidation in the
presence <strong>of</strong> <strong>oxalic</strong> <strong>acid</strong> is slower compared with
the Mn(II)-catalysed oxidation in the absence <strong>of</strong>
this <strong>acid</strong>. The results show that <strong>oxalic</strong> <strong>acid</strong> has a
moderate inhibiting effect on the Mn(II)catalysed
S(IV) oxidation. The ratio r2/r1
changes from 1.27 to 13.08 depending on <strong>oxalic</strong>
<strong>acid</strong> and Mn(II) concentrations as well as the
initial pH <strong>of</strong> the solution.
At a given Mn(II) concentration and
initial pH, the inhibiting effect increases with
<strong>oxalic</strong> <strong>acid</strong> concentration. The strongest
inhibiting effect was observed at the highest
<strong>oxalic</strong> <strong>acid</strong> and Mn(II) concentrations and at the
highest initial pH, whereas the weakest
inhibiting effect was found at the lowest <strong>oxalic</strong>
<strong>acid</strong> and Mn(II) concentrations and at the
highest initial pH.
At lower Mn(II) concentrations (1·10 -6
- 5·10 -6 mol/dm 3 ), the inhibition factor decreases
with increase in the initial pH. However, at the
highest concentration <strong>of</strong> Mn(II) (1·10 -5
mol/dm 3 ) such dependence was not observed -
the highest inhibition factors were found at the
initial pH 5.0, and the lowest ones were at the
initial pH 4.0.
At higher <strong>oxalic</strong> <strong>acid</strong> concentrations
(1·10 -5 - 1·10 -4 mol/dm 3 ) the inhibiting effect
increases with increase in Mn(II) concentration
(at all initial pH values). At <strong>oxalic</strong> <strong>acid</strong>
concentration <strong>of</strong> 1·10 -6 mol/dm 3 and the initial
pH 5.0 the similar dependence was observed,
whereas at the lower initial pH the inhibiting
effect was the weakest at Mn(II) concentration
<strong>of</strong> 5·10 -6 mol/dm 3 .
The inhibiting effect <strong>of</strong> <strong>oxalic</strong> <strong>acid</strong> on
the Mn(II)-catalysed S(IV) oxidation is also
reported by Podkrajšek et al. (2006). They
found that <strong>oxalic</strong> <strong>acid</strong> slows down the oxidation
about 3-times at pH 4.5, and at pH 3.5 the
inhibition is even lower. Our results for similar
Mn(II) and <strong>oxalic</strong> <strong>acid</strong> concentrations as well as
pH indicate that <strong>oxalic</strong> <strong>acid</strong> may have a little
stronger inhibiting effect, which can be
explained in terms <strong>of</strong> the difference in the other
experimental parameters. In the work <strong>of</strong>
Podkrajšek et al. (2006), S(IV) concentration
was about 1·10 -4 mol/dm 3 , while in our study it
was about 1·10 -3 mol/dm 3 . Mn(II) and <strong>oxalic</strong>
<strong>acid</strong> concentrations were similar in both the
studies. However, our experiments have been
performed in a wider concentration range,
especially for <strong>oxalic</strong> <strong>acid</strong>. The pH range <strong>of</strong> the
reaction solutions was similar in both studies,
but in experiments <strong>of</strong> the above authors, pH was
maintained constant during the reaction course,
while in ours it was left to change.
Conclusions
Oxalic <strong>acid</strong> affects the Mn(II)-catalysed
S(IV) oxidation changing the reaction rate
and in some cases the reaction order in
S(IV) concentration.
Oxalic <strong>acid</strong> has no <strong>influence</strong> on the reaction
order in S(IV) at the initial pH 3.5 (n = 0).
A tendency towards the change in the
reaction order begins to be barely visible at
the initial pH 4.0 and it becomes quite
distinct at the initial pH 5.0.
A change in the reaction order is the largest
at the highest concentration <strong>of</strong> <strong>oxalic</strong> <strong>acid</strong>
and at the lowest concentration <strong>of</strong> Mn(II).
The reaction order in S(IV) changes from 0
(at the initial pH 3.5) to 0.35 (at the initial
pH 5, <strong>oxalic</strong> <strong>acid</strong> concentration <strong>of</strong> 1·10 -4
mol/dm 3 , and Mn(II) concentration <strong>of</strong> 1·10 -6
mol/dm 3 ).
Oxalic <strong>acid</strong> inhibits the Mn(II)-catalysed
S(IV) oxidation. The inhibiting effect <strong>of</strong>
this <strong>acid</strong> is moderate and depends on both
<strong>oxalic</strong> <strong>acid</strong> and Mn(II) concentrations as
well as the initial pH <strong>of</strong> the solution.
Depending on these parameters the S(IV)
oxidation rate decreases from 1.27 to 13.08
times. The strongest inhibiting effect is
observed at the highest <strong>oxalic</strong> <strong>acid</strong> and
Mn(II) concentrations (1·10 -4 and 1·10 -5
mol/dm 3 , respectively) and at the highest
initial pH (5.0).
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