influence of oxalic acid on the catalytic s

140
Based on the measurement results, the kinetic
law parameters for the processes studied were
determined. Since the rate <strong>of</strong> Mn(II)-catalysed
S(IV) oxidation is independent <strong>of</strong> oxygen
where kobs is the observed rate constant, and n is
the reaction order with respect to S(IV)
concentration. The observed rate constant kobs is
a function <strong>of</strong> Mn(II) and <strong>oxalic</strong> <strong>acid</strong>
[H2C2O4]
mol/dm 3
Initial pH = 3.5
d[
S(IV) ]
r ]
dt
concentration (Huss et al., 1982; Connick and
Zhang, 1996), the reaction rate has been
described by the equation:
n
= − = kobs
[ S(IV)
(1)
concentrations as well as the initial pH <strong>of</strong> the
solution. The reaction orders and the observed
rate constants determined by the standard
integral technique are listed in Tables 1 - 2.
Table 1. Observed rate constants k1 for the Mn(II) catalysed S(IV) oxidation
k1, (mol/dm 3 )⋅s -1
Initial pH
[Mn]≈1⋅10 -6
mol/dm 3
[Mn]≈5⋅10 -6
mol/dm 3
[Mn]≈1⋅10 -5
mol/dm 3
3.5 3.904⋅10 -8 1.011⋅10 -7 3.205⋅10 -7
4.0 4.829⋅10 -8 1.306⋅10 -7 3.624⋅10 -7
5.0 5.015⋅10 -8 1.518⋅10 -7 8.772⋅10 -7
Table 2. Observed rate constants k2 for the Mn(II) catalysed S(IV) oxidation in the
presence <strong>of</strong> <strong>oxalic</strong> <strong>acid</strong>
[Mn] ≈ 1⋅10 -6 mol/dm 3 [Mn] ≈ 5⋅10 -6 mol/dm 3 [Mn] ≈ 1⋅10 -5 mol/dm 3
n k2
(mol/dm 3 ) (1-n) ⋅s -1
n k2
(mol/dm 3 ) (1-n) ⋅s -1
n k2
(mol/dm 3 ) (1-n) ⋅s -1
1⋅10 -6 0 1.365⋅10 -8 0 6.623⋅10 -8 0 1.253⋅10 -7
1⋅10 -5
1⋅10 -4
Initial pH = 4.0
0 1.187⋅10 -8 0 2.733⋅10 -8 0 4.639⋅10 -8
0 8.131⋅10 -9 0 1.422⋅10 -8 0 2.645⋅10 -8
1⋅10 -6 0.05 3.328⋅10 -8 0 9.867⋅10 -8 0.05 2.800⋅10 -7
1⋅10 -5
1⋅10 -4
Initial pH = 5.0
0 1.797⋅10 -8 0 3.950⋅10 -8 0 6.648⋅10 -8
0.1 3.011⋅10 -8 0 1.844⋅10 -8 0.05 5.651⋅10 -8
1⋅10 -6 0.15 1.116⋅10 -7 0.15 3.317⋅10 -7 0 2.335⋅10 -7
1⋅10 -5
1⋅10 -4
0.25 1.819⋅10 -7 0.20 1.956⋅10 -7 0.05 9.650⋅10 -8
0.35 1.926⋅10 -7 0.20 8.613⋅10 -8 0.15 1.890⋅10 -7
The S(IV) oxidation in the presence <strong>of</strong> Mn(II)
ions only (in the absence <strong>of</strong> <strong>oxalic</strong> <strong>acid</strong>) is zero
order with respect to S(IV) concentration (n = 0)
over the entire studied ranges <strong>of</strong> Mn(II)
concentration and the initial pH <strong>of</strong> the reaction
solution. The value <strong>of</strong> the observed rate constant
k1 is dependent on these parameters. It increases
from 3.91·10 -8 to 8.89·10 -7 (mol/dm 3 )·s -1 with
increasing Mn(II) concentration from 1·10 -6 to
1·10 -5 mol/dm 3 and with increasing the initial
pH from 3.5 to 5.0 (Table 1). In more detail the
S(IV) oxidation in the presence <strong>of</strong> Mn(II) ions
alone has been presented in our earlier work
(Wilkosz and Mainka, 2008).