influence of oxalic acid on the catalytic s

INFLUENCE OF OXALIC ACID ON THE CATALYTIC S(IV) OXIDATION BY
OXYGEN UNDER CONDITIONS CLOSE TO THOSE IN THE ATMOSPHERE
Anna Mainka, Irena Wilkosz
Air Protection Department, Silesian University <strong>of</strong> Technology, Akademicka 2, 44-100 Gliwice, Poland
e-mail: Anna.Mainka@polsl.pl, Irena.Wilkosz@polsl.pl
ABSTRACT
The paper presents results <strong>of</strong> kinetic studies on the S(IV) oxidation process catalysed by Mn(II) ions in
the presence <strong>of</strong> <strong>oxalic</strong> <strong>acid</strong>. Laboratory experiments were carried out at concentrations <strong>of</strong> reactants and
pH <strong>of</strong> solutions representative for <strong>acid</strong>ified atmospheric water. The results indicate that <strong>oxalic</strong> <strong>acid</strong> has a
moderate inhibiting effect on the Mn(II)-catalysed S(IV) oxidation. Depending on the experimental
conditions the Mn(II) catalysed S(IV) oxidation in the presence <strong>of</strong> <strong>oxalic</strong> <strong>acid</strong> was from 1.3 to 13 times
slower than in the absence <strong>of</strong> this <strong>acid</strong>.
Key words: atmospheric chemistry; S(IV) oxidation; manganese; <strong>oxalic</strong> <strong>acid</strong>; catalysis; inhibition.
Introduction
Sulphur dioxide emission results in <strong>acid</strong>ic
deposition which is one <strong>of</strong> the serious global
problems. This process involves complex
transformation <strong>of</strong> SO2 to H2SO4 in gaseous and
liquid phases <strong>of</strong> the atmosphere. The major
reaction path is the oxidation <strong>of</strong> sulphur dioxide
in the aqueous phase. It is believed that 50-80%
<strong>of</strong> atmospheric sulphur dioxide reacts within
atmospheric water droplets (rain, fog and
clouds) (Langner and Rodhe, 1991).
Depending on conditions, three
different SO2 oxidation pathways are crucial in
the atmospheric aqueous phase. These are the
oxidation <strong>of</strong> dissolved SO2 by H2O2, O3 and by
O2. Oxidation by O2 may be important in the
presence <strong>of</strong> catalysts only. Potential catalysts
are transition metal ions with at least two
oxidation states (e.g. vanadium, chromium,
manganese, iron, cobalt, nickel, cooper) (Grgić
et al., 1991; Berglund et al., 1993; Brandt and
van Eldik, 1995; Berglund and Elding, 1995).
The most active catalysts in the atmosphere are
manganese Mn(II) and iron Fe(III). They both
are common constituents <strong>of</strong> tropospheric
aerosols in heavy polluted urban and industrial
areas. They are present even in remote areas,
due to their generation from erosion <strong>of</strong> the
earth’s crust.
The only source <strong>of</strong> these metals in the
atmospheric aqueous phase is the dissolution <strong>of</strong>
aerosol particles incorporated in water droplets.
The common particles containing trace metals
are soil dust, fly ash from power plants, and
exhaust from combustion engines and industrial
operations. Concentrations <strong>of</strong> manganese in
street dust and ambient particulate matter in
industrial areas and heavy traffic density areas
vary from 554 to 2335 mg/kg (Piao et al., 2008;
Lu et al., 2009). Concentrations <strong>of</strong> Mn(II)
dissolved in atmospheric waters in urban and
industrial areas range from 10 -7 to 10 -6 mol/dm 3
in rain (Deutsch et al., 1997; Manoj et al., 2000;
Patel et. al., 2001), and from 10 -6 to 10 -5
mol/dm 3 in fog and cloudwater (Millet et al.,
1995; Brandt and van Eldik, 1995).
Besides the transition metal ions some
other substances present in the atmospheric
water may also affect the S(IV) oxidation by O2.
Dissolved in atmospheric water organic
compounds such as carboxylic <strong>acid</strong>s may react
with sulphoxy radicals and transition metal ions,
and thus alter the rate <strong>of</strong> catalytic S(IV)
oxidation (Martin et al.,1991; Pasiuk-
Bronikowska et al., 1997; Grgić et al., 1998,
1999; Ziajka and Pasiuk-Bronikowska, 2005).
The sources <strong>of</strong> carboxylic <strong>acid</strong>s are
numerous and they comprise anthropogenic and
biogenic emissions and photochemical
transformations <strong>of</strong> precursors in atmospheric
aqueous, gaseous, and particulate phases
(Chebbi and Carlier, 1996). Direct
anthropogenic emissions <strong>of</strong> carboxylic <strong>acid</strong>s
(e.g. from incomplete combustion <strong>of</strong> fossil
fuels, wood and other biomass material) and/or
photo-oxidation <strong>of</strong> anthropogenic organic
compounds in the atmosphere are the main
sources <strong>of</strong> these compounds in urban and
industrial environments (Chebbi and Carlier,
1996; Kawamura et al., 1996a). It has been
reported that automobile exhaust can be a
significant source as well (Kawamura and
Kaplan, 1987; Souza et al., 1999; Kerminen et

138
al., 2000; Yao et al., 2004). Kawamura and
Kaplan (1987) found that the distribution <strong>of</strong>
dicarboxylic <strong>acid</strong>s measured in gasoline and
diesel motor exhausts is similar to that <strong>of</strong> urban
air samples. The concentrations <strong>of</strong> these <strong>acid</strong>s in
motor exhausts are 28 (gasoline) and 144
(diesel) times higher than the average <strong>of</strong>
atmospheric concentrations.
Because <strong>of</strong> their low vapour pressures
and high solubility in water, dicarboxylic <strong>acid</strong>s
are predominantly present in atmospheric
aerosols, cloud droplets, fog droplets and
precipitation (Chebbi and Carlier, 1996;
Kawamura and Ikushima, 1993; Souza et al.,
1999). Oxalic <strong>acid</strong> is the single most abundant
organic compound identified in ambient
aerosols. It is found both in PM10 and PM2.5
fractions. Its concentrations in PM10 vary from
0.38 to 1.84 µg/m 3 and in PM2.5 from 0.01 to
1.53 µg/m 3 (Wang et al., 2002, 2007; Huang et
al., 2005; Yao et al. 2004; Yang and Yu, 2008).
Concentrations <strong>of</strong> <strong>oxalic</strong> <strong>acid</strong> in the range 0.1 –
40 µmol/dm 3 have been measured in the
tropospheric aqueous phase samples from
different areas in the world (Chebbi and Carlier,
1996; Kawamura et al., 1996b, Löflund et al.,
2002).
Very little is known so far about the
<strong>influence</strong> <strong>of</strong> carboxylic <strong>acid</strong>s on the catalytic
S(IV) oxidation by O2. Current laboratory
studies <strong>of</strong> chemical reactions involving S(IV)
species and organic ligands have focused on the
iron-catalysed autoxidation (Grgić et al., 1995;
Grgić and Poznič, 1998; Grgić et al., 1998;
Grgić et al., 1999; Wolf et al., 2000). It has been
found that organic ligands such as oxalate,
acetate and formate inhibit this process. The
<strong>influence</strong> <strong>of</strong> some low molecular weight
monocarboxylic <strong>acid</strong>s (formic, acetic, glycolic,
lactic) and dicarboxylic <strong>acid</strong>s (<strong>oxalic</strong>, malic,
malonic) on the Mn(II)-catalysed S(IV)
oxidation has also been investigated (Grgić et al.
2002, Podkrajšek et al. 2006, Wilkosz and
Mainka, 2008). It has been established that
monocarboxylic <strong>acid</strong>s inhibit the oxidation <strong>of</strong>
S(IV). From among dicarboxylic <strong>acid</strong>s, <strong>oxalic</strong>
<strong>acid</strong> slows down the S(IV) oxidation, while
malic and malonic <strong>acid</strong>s have practically no
<strong>influence</strong>.
The investigated process is a very
complex free radical chain reaction. Its
mechanism and kinetics are so sensitive to the
reaction conditions that even a slight change in
them can cause a change <strong>of</strong> the dominant path
<strong>of</strong> the reaction course, and thus lead to diverse
results. Thus, despite the studies cited above the
effect <strong>of</strong> carboxylic <strong>acid</strong>s on the metal catalysed
oxidation <strong>of</strong> S(IV) in atmospheric water is still
poorly understood and more work in this area is
needed. The purpose <strong>of</strong> the present work was to
study the <strong>influence</strong> <strong>of</strong> <strong>oxalic</strong> <strong>acid</strong> on the Mn(II)catalysed
S(IV) oxidation under the conditions
representative for <strong>acid</strong>ified atmospheric water in
heavily polluted areas.
Materials and Methods
All reagents used were <strong>of</strong> analytical grade
(Merck). Milli-Q water was used for preparation
<strong>of</strong> all solutions. Stock solutions <strong>of</strong> Mn(II) and
<strong>oxalic</strong> <strong>acid</strong> were prepared from MnSO4⋅H2O and
H2C2O4⋅2H2O, respectively. The S(IV) solutions
were prepared freshly before each run by
dissolving Na2SO3 in water which was
deoxygenated by bubbling high purity argon
through the Milli-Q water for at least 30 min.
The initial pH <strong>of</strong> the solutions was adjusted with
H2SO4. The source <strong>of</strong> oxygen for oxidation <strong>of</strong>
S(IV) was synthetic air.
Kinetic experiments were conducted in
a 500 cm 3 glass cylindrical reactor with four
inlet connectors for: pH electrode, introducing
reagents, thermometer and teflon tube for
sample sipping. The reactor was filled with 450
cm 3 <strong>of</strong> the S(IV) solution <strong>acid</strong>ified to the
required pH and containing appropriate amount
<strong>of</strong> <strong>oxalic</strong> <strong>acid</strong>. The reactor was protected from
light and immersed into a thermostat to
guarantee a constant temperature <strong>of</strong> 25 ± 1 o C.
The air was introduced at the bottom <strong>of</strong> reactor
through a ceramal at a rate <strong>of</strong> 100 ± 2 dm 3 /h.
Under such conditions the gas and liquid phases
were good mixed and the reaction took place in
the kinetic regime, i.e. the global rate <strong>of</strong> S(IV)
oxidation was limited by the rate <strong>of</strong> chemical
reaction, not by the diffusion.
To start the reaction, the air flow was
turned on and just after that the Mn(II) solution
was injected into the reactor. At selected time
intervals the concentration <strong>of</strong> S(IV) was
measured by UV-VIS (Shimadzu, Model UV-
2101 PC) spectrophotometer equipped with
Sipper 260 (Model L) - using flow cell method.
The sipping time was set to 5 s, and the slit
width was set to 2.0 nm. The S(IV)
measurements were carried out at wavelength
203 nm for the initial pH 3.5 and 205 nm for the
initial pH 4.0 and 5.0. The pH measurements
were performed by an Orion pH meter (Model
710A) combined with a glass electrode. The
concentration <strong>of</strong> Mn(II) was determined by
AVANTA PM atomic absorption spectrometer
<strong>of</strong> the GBC.
Conditions <strong>of</strong> experiments were as
follows: [S(IV)] ≈ 1·10 -3 mol/dm 3 , 1·10 -6 ≤
[Mn(II)] ≤ 1·10 -5 mol/dm 3 , 1·10 -6 ≤ [H2C2O4] ≤
1·10 -4 mol/dm 3 , 3.5 ≤ the initial pH ≤ 5.0, T =
298 K.

Results and Discussion
Some typical results <strong>of</strong> the kinetic
measurements are shown in Figure 1 as the time
[S(IV)] t/[S(IV)] 0
[S(IV)] t/[S(IV)] 0
[S(IV)] t/[S(IV)] 0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
a)
139
dependence <strong>of</strong> [S(IV)]t/[S(IV)]0 ratios, where
[S(IV)]t is the concentration <strong>of</strong> S(IV) at time t,
and [S(IV)]0 is the initial concentration <strong>of</strong> S(IV).
0 40 80 120 160 200
time, min.
b)
0 40 80 120 160 200
time, min.
0 40 80 120 160 200
time, min.
Figure 1. Effect <strong>of</strong> <strong>oxalic</strong> <strong>acid</strong> on the S(IV) oxidation in the presence <strong>of</strong> Mn(II) at initial pH=4.0 a)
[Mn(II)]≈1⋅10 -6 mol/dm 3 , b) [Mn(II)]≈5⋅10 -6 mol/dm 3 , c) [Mn(II)]≈1⋅10 -5 mol/dm 3 .
c)

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).

In the presence <strong>of</strong> <strong>oxalic</strong> <strong>acid</strong> the reaction rate
deviates from the zero-order relation at higher
initial pH values. This tendency is barely visible
at the initial pH 4.0, but at the pH 5.0 it
becomes quite distinct (Table 2). At the initial
pH 4.0 the maximum increase in the reaction
order is 0.1, if any, whereas at the pH 5.0 it is
0.35. Thus, at the initial pH 5.0, the reaction
order is between 0 and 0.35. The reaction order
increases with increase in <strong>oxalic</strong> <strong>acid</strong>
concentration and with decrease in Mn(II)
concentration.
141
Using the determined kinetic parameters, the
S(IV) oxidation rates (r2) were calculated for
[S(IV)] = 1·10 -3 mol/dm 3 and different Mn(II)
and <strong>oxalic</strong> <strong>acid</strong> concentrations as well as for
different initial pH values (Table 3). These rates
(r2) are evidently smaller than those (r1) in the
absence <strong>of</strong> <strong>oxalic</strong> <strong>acid</strong> (Table 1, r1 = k1). To
estimate the inhibiting effect <strong>of</strong> <strong>oxalic</strong> <strong>acid</strong> on
the S(IV) oxidation catalysed by Mn(II), ratios
r2/r1 (inhibition factors) were calculated (Table
4).
Table 3. Rates <strong>of</strong> the Mn(II) catalysed S(IV) oxidation in the presence <strong>of</strong> <strong>oxalic</strong> <strong>acid</strong>
[H2C2O4]
mol/dm 3
Initial pH = 3.5
r2, (mol/dm 3 )⋅s -1
[Mn] ≈1⋅10 -6 mol/dm 3 [Mn] ≈ 5⋅10 -6 mol/dm 3 [Mn] ≈ 1⋅10 -5 mol/dm 3
1⋅10 -6 1.365⋅10 -8 6.623⋅10 -8 1.253⋅10 -7
1⋅10 -5 1.187⋅10 -8 2.733⋅10 -8 4.639⋅10 -8
1⋅10 -4 8.131⋅10 -9 1.422⋅10 -8 2.645⋅10 -8
Initial pH = 4.0
1⋅10 -6 2.356⋅10 -8 9.867⋅10 -8 1.982⋅10 -7
1⋅10 -5 1.797⋅10 -8 3.950⋅10 -8 6.648⋅10 -8
1⋅10 -4 1.509⋅10 -8 1.844⋅10 -8 4.001⋅10 -8
Initial pH = 5.0
1⋅10 -6 3.959⋅10 -8 1.177⋅10 -7 2.335⋅10 -7
1⋅10 -5 3.234⋅10 -8 4.914⋅10 -8 6.832⋅10 -8
1⋅10 -4 1.717⋅10 -8 2.163⋅10 -8 6.706⋅10 -8
Table 4. Ratios <strong>of</strong> rates for the Mn(II) catalysed S(IV) oxidation in the absence and in the presence <strong>of</strong>
<strong>oxalic</strong> <strong>acid</strong>
[H2C2O4]
mol/dm 3
[Mn] ≈ 1⋅10 -6
mol/dm 3
Wi = r1/r2
[Mn] ≈ 5⋅10 -6
mol/dm 3
[Mn] ≈ 1⋅10 -5 mol/dm 3
Initial pH = 3.5
1⋅10 -6 2.86 1.53 2.56
1⋅10 -5 3.29 3.70 6.91
1⋅10 -4 4.80 7.11 12.12
Initial pH = 4.0
1⋅10 -6 2.05 1.32 1.83
1⋅10 -5 2.69 3.31 5.45
1⋅10 -4 3.20 7.08 9.06
Initial pH = 5.0
1⋅10 -6 1.27 1.29 3.76
1⋅10 -5 1.55 3.09 12.84
1⋅10 -4 2.92 7.02 13.08

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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