Oxidation of lactic acid by water soluble (Colloidal) manganese dioxide

Article abstract of DOI:10.1002/kin.20010

Spectrophotometric method has been used to characterize water-soluble colloidal manganese dioxide obtained by the redox reaction between sodium thiosulphate and potassium permanganate in neutral aqueous medium which shows a single peak in the visible region with λ_max = 425 nm. The kinetics of the oxidation of lactic acid by colloidal manganese dioxide (oxidant) has been investigated spectrophotometrically under pseudo-first-order conditions of excess lactic acid. The rate of the noncatalytic reaction pathway was slow which increased with increasing lactic acid concentration. The reaction was first-order with respect to [oxidant] as well as [lactic acid]. In presence of manganase(II) and fluoride ions, the noncatalytic path disappeared completely while the oxidation rate of autocatalytic path increased and decreased, respectively with increasing [Mn(II)] and [F^-]. A mechanistic scheme in conformity with the observed kinetics has been proposed with the rate-law: v = -d[MnO₂]/dt = κ₁κ₂[MnO₂] [H⁺][lactic acid]_T/ ([H⁺] K_a)(κ_{_}1 + ≤₂)

Full text of DOI:10.1002/kin.20010

Oxidation of Lactic Acid
by Water Soluble (Colloidal)
Manganese Dioxide
ZAHEER KHAN,1 RAJU,1 MOHD. AKRAM,2 KABIR-UD-DIN2
1Department of Chemistry, Jamia Millia Islamia, New Delhi 110 025, India
2Department of Chemistry, Aligarh Muslim University, Aligarh 202 002, India
Received 29 October 2002; accepted 4 February 2004
DOI 10.1002/kin.20010
ABSTRACT: Spectrophotometric method has been used to characterize water-soluble colloidal
manganese dioxide obtained by the redox reaction between sodium thiosulphate and potas-
sium permanganate in neutral aqueous medium which shows a single peak in the visible region
with λmax = 425 nm. The kinetics of the oxidation of lactic acid by colloidal manganese dioxide
(oxidant) has been investigated spectrophotometrically under pseudo-first-order conditions of
excess lactic acid. The rate of the noncatalytic reaction pathway was slow which increased with
increasing lactic acid concentration. The reaction was first-order with respect to [oxidant] as
well as [lactic acid]. In presence of manganase(II) and fluoride ions, the noncatalytic path dis-
appeared completely while the oxidation rate of autocatalytic path increased and decreased,
respectively with increasing [Mn(II)] and [F−]. A mechanistic scheme in conformity with the
observed kinetics has been proposed with the rate-law:
−d[MnO2]
dt
k1k2[MnO2][H+][lactic acid]T
([H+] + Ka)(k−1 + k2)
ν =
=
ꢀ
C
2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 359–366, 2004
between permanganate and sodium thiosulphate un-
der neutral conditions for the first time. The water-
soluble form of colloidal manganese dioxide presents
the advantage over water-insoluble form of possessing
a larger specific surface.
The kinetic aspects of the colloidal manganese diox-
ide have been studied with formic and oxalic acids
[8,9]. However, the details of lactic acid oxidation
by colloidal manganese dioxide are yet unknown al-
though the permanganate/lactic acid reaction has been
studied in acidic solutions [10]. The process of the
preparation of colloidal manganese dioxide by per-
manganate/sodium thiosulphate system is not avail-
able with sufficient details to describe the reaction
under all conditions. Therefore, the results presented
here supplement the previous studies of this system
INTRODUCTION
It is well established that manganese dioxide acts as
an oxidizing agent for both organic [1] and inorganic
compounds [2,3], but, due to solubility problem under
normal conditions, its use is limited [4]. Active forms of
manganese dioxide have earlier been obtained by redox
reaction of permanganate with hydrogen peroxide [5]
and manganese(II) ion [6]. Perez-Benito et al. [7] re-
ported a method for the preparation of water-soluble
colloidal manganese dioxide by the redox reaction
Correspondence to: Zaheer Khan; e-mail: drkhanchem@yahoo.
co.in.
c
ꢀ
2004 Wiley Periodicals, Inc.

360
KHAN ET AL.
and contribute to the elucidation of the mechanism of
oxidation of lactic acid by the water-soluble colloidal
manganese dioxide.
EXPERIMENTAL
Materials
Lactic acid, sodium thiosulphate, KMnO4, and H2SO4
were commercial products of highest purity and
were used as supplied. Stock solutions of all the
reagents were freshly prepared in Pyrex glass double-
distilled water. The KMnO4 solution was frequently
restandardized and stored in amber coloured bottle in a
refrigerator.
Kinetic Measurements
Requisite amounts of each reagent were pipetted out
into two-necked reaction flasks and the contents were
thermally equilibrated at the desired temperature in a
constant temperature paraffin oil bath of 0.1◦C accu-
racy. The reaction was initiated by adding lactic acid
as the last component and the rate of the disappear-
ance of colloidal MnO2 was followed spectrophoto-
metrically by monitoring the decay of absorbance of
colloidal MnO2 (λmax = 425 nm) at regular time in-
tervals. Neither lactic acid nor products showed any
absorbance at this wavelength. Other experimental pro-
cedure details are described elsewhere [11–14]. All the
kinetic runs were carried out for a period of more than
80% completion with lactic acid in excess in order
to work under pseudo-first-order conditions. Observed
pseudo-first-order rate constants (kobs, s−1) were ob-
tained from the slopes of log(A)425 versus time plots
(average correlation coefficient = 0.997). The exper-
imental second-order rate constants (kII, mol−1 dm3
Figure 1 UV–visible spectra of the mixtures containing a
fixed amount of KMnO4 (=4.0 × 10−4 mol dm−3) and vary-
ing amounts of Na2S2O3 at 30◦C.
400 nm (Fig. 1). At lower [Na2S2O3] (≤0.6 mol
dm−3), the mixture showed two bands at 400 and 525
nm (Fig. 1, – –). At higher [Na S O ] (≥2.0 × 10−4
◦
2
2
3
mol dm−3), the mixture showed only one broad band
covering the whole visible region of the spectrum with
λmax = 425 nm (Fig. 1, –ꢁ–, – –). The concentration
ꢀ
of Na2S2O3 needed to stabilize the new peak was
2.4 × 10−4 mol dm−3. It was also observed that at
higher [Na2S2O3] (≥2.5 × 10−4 mol dm−3), the mix-
ture turned turbid due to the precipitation of manganese
dioxide. Thus, we conclude that the new stabilized
spectrum (Fig. 1, – –) is that of water-soluble
ꢀ
colloidal manganese dioxide. The new oxidizing
solution (i.e., the water soluble colloidal manganese
dioxide) was, therefore, prepared by addition of
Na2S2O3 (1.0 cm3, 0.01 mol dm−3) to a permanganate
solution (2.0 cm3, 0.01 mol dm−3) diluted to 50 cm3
with water. The resulting solution was dark brown
and perfectly transparent and stable for several weeks.
Figure 2 clearly indicates the effect of [Na2S2O3] on the
absorbance of colloidal manganese dioxide at 425 nm.
In order to confirm the fulfillment of Beer’s law, a cali-
bration graph was constructed between (A)425 and con-
centration of colloidal MnO2. The law is obeyed for the
concentration range used in the present investigations.
The reaction equation has earlier been found to be
s
−1) were calculated from the relationship: kII = kobs
/
[lactic acid].
RESULTS AND DISCUSSION
Preparation and Characterization
of Colloidal Manganese Dioxide
It is well known that the spectrum of acidified perman-
ganate solution possesses an absorption band located
at λmax = 525 nm (Fig. 1, – –) in the visible range.
ꢀ
It is observed that the spectrum of permanganate
changes by the addition of Na2S2O3 solution. The
band at 525 nm gradually disappears with increase
in [Na2S2O3] and a new band begins to appear at
8MnO4− + 3S2O32− + 2H+
= 8MnO2 + 6SO42− + H2O
(1)

OXIDATION OF LACTIC ACID BY WATER SOLUBLE (COLLOIDAL) MANGANESE DIOXIDE
361
(0.18 mol dm−3), the reaction mixture shows two
peaks with λmax = 400 (colloidal MnO2) and 525 nm
(MnO4−) but at higher [H2SO4] (1.2 mol dm−3), there
is no peak in the visible region. The unstability of col-
loidal MnO2 in the presence of H2SO4 is probably due
to the excess of sodium thiosulphate under acidic pH
as an oxidant (similarly to MnO4−) with the forma-
2−
tion of Mn(II). Since the MnO4−:S2O3 molar ra-
tio is 2:1 in our experiments and on the basis of the
2−
reaction Eq. (1) the S2O3 has been present in ex-
cess. This excess can, therefore, react with the MnO2
formed after acidification (the oxidation ability of both
−
MnO4 and MnO2 depends on pH and it is increas-
ing with decreasing pH, since the value of [H+] is in-
creased in the Nernst form−ula). The results indicate
that amounts of both MnO4 and colloidal MnO2 are
simultaneously decreasing with increasing acidity of
the medium. The reactivity of MnO2, however, seems
larger toward the thiosulphate ion than the reactivity of
the MnO4− because decrease of MnO2 was observed at
lower[H2SO4]thanthedecreaseoftheMnO4− content.
These results are in agreement with the observations of
other investigators [15]. Thus, the water-soluble col-
loidal MnO2 can exist in neutral aqueous medium.∗
Figure 2 Effect of [Na2S2O3] on the absorbance (425 nm)
of water soluble colloidal MnO2. Reaction conditions:
[KMnO4]= 4.0 × 10−4 mol dm−3, temp. = 30◦C.
Effect of [H2SO4] on Permanganate
Spectrum in Presence of Na2S2O3
Figure 3 shows the effect of [H2SO4] on the ab-
sorbance of KMnO4 (=4.0 × 10−4 mol dm−3) and its
reduction product (colloidal MnO2). At lower [H2SO4]
Kinetics of Lactic Acid Oxidation
by Colloidal MnO2
Under the experimental conditions of (colloidal)
[MnO2] = 2.0 × 10−4 mol dm−3 and [lactic acid]T in
the range 0.03–0.6 mol dm−3, Fig. 4 shows examples
of some of the kinetic curves from which the rate con-
stants for the oxidation were obtained. As the plots of
log(absorbance) versus time deviate from linearity, it is
clear that the oxidation kinetics proceed in two stages
[16], i.e., initial slow stage followed by a relatively
faster step. The time at which the deviation commenced
was found to decrease with an increase in [lactic acid]T
(the noncatalytic path even disappeared completely at
higher [lactic acid]T, Table I).
In a set of experiments the reaction was studied as
a function of colloidal MnO2 concentration between
2.0 × 10−4 and 10.0 × 10−4 mol dm−3 at constant
ionic strength (0.5 mol dm−3 NaClO4), [lactic acid]T
(0.06 mol dm−3), and temperature (30◦C). Indepen-
dence of the rate constants over the range of concen-
tration used (Table I) is in agreement with a first-order
dependence on [MnO2], i.e.,
ν = −d [MnO2]/dt = kobs[MnO2]
(2)
Figure 3 Effect of [H2SO4] on the UV–visible spectra of
the mixtures containing fixed amounts of KMnO4 (=4.0 ×
10−4 mol dm−3) and Na2S2O3 (=2.0 × 10−4 mol dm−3) at
∗The authors are thankful to one of the reviewers for these
suggestions.
30◦C, except in (- -) where [Na S O ] = 0.0 mol dm−3
.
ꢂ
2
2 3

362
KHAN ET AL.
increased the oxidation rate (Table I). These results are
alsoshowngraphicallyinFig. 5. Theplotofkobs1 versus
[lactic acid]T is a straight line whereas a log–log plot
was linear with slope = 0.94. The reaction is, there-
fore, first-order with respect to [lactic acid]T. The plot
of kobs1 versus [lactic acid] shows a positive intercept
(extrapolation until zero concentration of reductant) on
the y-axis (Fig. 5). It should, however, be zero at [lactic
acid] = 0.0 mol dm−3 (see Eq. (11)). Inspection of the
figure brings to light that the aqueous solution of col-
loidal MnO2 is unstable possibly due to its reduction
to some extent [9].
In order to confirm/gain insight into the autocat-
alytic pathway (Fig. 4), the effect of manganese(II)
(a reduction product of colloidal MnO2, vide infra)
was studied. The kobs2, obtained as a function of
[Mn(II)] at constant [lactic acid] (0.13 mol dm−3),
(colloidal)[MnO2] (8.0 × 10−5 mol dm−3), and tem-
perature (30◦C), was found to increase with increas-
ing [Mn(II)] (Table II). The results are also shown
in Fig. 6 as kobs2 − [Mn(II)] profile which indicates
sigmoid dependence of kobs2 on [Mn(II)]. It was
also noticed that the extent of the noncatalytic re-
action pathway disappeared completely even at as
low as [Mn(II)] = 1.0 × 10−4 mol dm−3. Our results
seem to indicate that Mn(II) is the active autocata-
lyst (the Mn(II) formed as reaction product contributes
to accelerate the oxidation of lactic acid by colloidal
MnO2).
Figure4 Plotsoflog(absorbance)425 versustimefortheox-
idation of lactic acid by colloidal MnO2. Reaction conditions:
[lactic acid] = 1.2 (a); 3.0 (b); 18.0 (c); and 30.0 × 10−2 mol
dm−3 (d), [MnO2] = 2.0 × 10−4 mol dm−3, [NaClO4] =
0.5 mol dm−3, temp. = 30◦C.
In the second set of experiments, the effect of [lac-
tic acid] on the reaction rate was studied at constant
ionic strength, (colloidal)[MnO2], and temperature. A
It is well known that fluoride ions form complexes
with Mn(III) and Mn(IV) [9]. To substantiate the
variation of the [lactic acid]T (0.03–0.60 mol dm−3
)
Table I Effect of Colloidal MnO2 and Lactic Acid Concentrations on the pseudo-First-Order Rate Constants (kobs1
,
a
k
obs2) of Lactic Acid Oxidation by Colloidal MnO2
105 [MnO2]b (mol dm−3
)
[Lactic Acid]T (mol dm−3
0.06
)
103 kobs1 (s−1
)
103kobs2 (s−1
)
2.0
3.0
6.0
7.0
10.0
2.0
1.7
1.8
1.7
1.6
1.8
1.3
1.7
2.6
3.6
2.3
2.3
2.1
2.2
2.4
1.6
2.3
3.3
4.4
5.4
6.1
7.8
9.5
11.1
0.03
0.06
0.12
0.18
0.24
0.30
0.40
0.50
0.60
4.2
5.1
Not observed
Not observed
Not observed
a [NaClO4] = 0.5 mol dm−3, temp. = 30◦C.
b As there exists the possibility of flocculation of colloidal MnO2, the effect was seen in presence of gum arabic, which is well known
stabilizing agent of the oxidant in solution [8].

OXIDATION OF LACTIC ACID BY WATER SOLUBLE (COLLOIDAL) MANGANESE DIOXIDE
363
Table II Effect of Different Variable Concentrations on the Pseudo-First-Order Rate Constants (kobs1, kobs2) of Lactic
Acid Oxidation by Colloidal MnO2 (8.0 × 10−5 mol dm−3) at 30◦C
Noncatalytic
103 kobs1 (s−1
Autocatalytic
103 kobs2 (s−1
)
104 [Mn(II)] (mol dm−3
)
104 [F−] (mol dm−3
)
[Lactic Acid] (mol dm−3
)
)
0.0
1.0
2.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
2.0
0.13
0.13
0.13
0.13
0.13
0.13
0.13
0.13
0.13
0.13
0.13
0.13
0.13
0.13
0.13
1.9
2.6
2.5
2.8
2.9
3.0
3.6
4.4
4.6
1.7
1.8
1.7
1.6
1.4
1.5
1.3
Not observed
Not observed
Not observed
Not observed
Not observed
Not observed
Not observed
Not observed
Not observed
Not observed
Not observed
Not observed
Not observed
Not observed
6.0
10.0
14.0
20.0
30.0
0.0
0.0
0.0
0.0
0.0
6.0
10.0
14.0
20.0
30.0
40.0
0.0
0.0
formation of Mn(III) during the oxidation of lactic
acid by colloidal MnO2, different amounts of sodium
fluoride were added to the reaction mixture. As NaF
decreased the rate (Table II), the noncatalytic reaction
pathway also disappeared. This indicates that Mn(III)
may also be the reactive species in the autocatalytic
path.
On the basis of above results and previous obser-
vations [8,9], the following two mechanisms can be
proposed:
Figure 5 Effect of [lactic acid] on the pseudo-first-order rate constants for the oxidation of lactic acid by colloidal MnO2.
Reaction conditions: [MnO2] = 2.0 × 10−4 mol dm−3, [NaClO4] = 0.5 mol dm−3, temp. = 30◦C.

364
KHAN ET AL.
(i) Non-catalytic reaction pathway:
Scheme 1
(ii) Auto-catalytic reaction pathway:
Scheme 2
In Scheme 1, (MnO2)n stands for the colloidal man-
ganese dioxide and Eq. (4) represents the adsorption of
lactic acid on the surface of the colloidal particles to
give complex C1. We assume that C1 decomposes in a
one-step, two electron oxidation–reduction mechanism
directly to Mn(II) and pyruvic acid (Eq. (5), the rate
determining step). According to the proposed mecha-
nism, the noncatalytic reaction pathway should be of
first-order in both oxidant and reductant. Therefore, the
total reaction rate law is given by Eqs. (10) and (11)
Equation (11), explains the first-order dependence on
[lactic acid]T at constant [H+] (Fig. 5).
In Scheme 2, reaction (6) represents formation of
another complex (C2) with colloidal MnO2-lactic acid
complex (C1) and manganese(II). The equilibrium be-
tween Mn(II) and C1 is fast. Equation (7) is the rate
determining step of autocatalytic pathway to yield
Mn(III) as an intermediate. The reduction of colloidal
MnO2 by Mn(II) has also been reported by Perez-
Benito [18]. The reaction product is Mn(III) which may
be in equilibrium with its reactants, Eq. (8). Therefore,
Mn(III) would also participate in the reaction as an au-
tocatalyst. In presence of large amount of lactic acid,
the equilibrium (Eq. (8)) shifts to forward direction. Ul-
timately, the intermediate (Mn(III)) immediately gets
converted into the stable products (Eq. (9)). This mech-
anism is in agreement with the results of Perez-Benito
−d[MnO2]
dt
k1k2[MnO2][H+][lactic acid]T
([H+] + Ka)(k−1 + k2)
ν =
=
(10)
k1k2[H+][lactic acid]T
([H+] + Ka)(k−1 + k2)
kobs1
=
(11)

OXIDATION OF LACTIC ACID BY WATER SOLUBLE (COLLOIDAL) MANGANESE DIOXIDE
365
acid toward colloidal MnO2 is higher in comparison
to Mn(II) [19]. Therefore, the participation of the col-
loidal MnO2-Mn(II) reaction (Eq. (8)) in the autocat-
alytic pathway can be ruled out or might be of minor
significance.
The probability of the direct reaction between
Mn(IV) and Mn(II) to yield Mn(III) without the par-
ticipation of lactic acid can not be ruled out either
(Eq. (8)); the participation of Mn(III) as an interme-
diate is well known [20–22]). Disappearance of the
noncatalytic reaction pathway in the presence of very
low added Mn(II) adequately accounts for the criti-
cal concentration of Mn(III). Though Mn(III) is not
stable (it immediately disproportionates, Eq. (8), re-
verse direction), the effect of F− on the rate constants
(Table II) suggests formation of Mn(III) as an interme-
diate in view of the strong complexation of Mn(III) by
F−compared to MnO2 [23,24]. However, despite our
attempt to monitor Mn(III) at 470 nm (characteristic
of Mn(III) [25]), we failed to detect any build-up of
Mn(III) during the course of the reaction.
Figure 6 Effect of [Mn(II)] on the kobs2 for the oxidation
of lactic acid by colloidal MnO2. Reaction conditions were
the same as in Fig. 5.
Comparison of the second-order rate constants (kII)
with those of other oxidants involving the same re-
ductant (lactic acid) (Table IV) shows that the reactiv-
ity/oxidation ability of colloidal MnO2 is much higher
in comparison to KMnO4, K2Cr2O7, S2O82−, or chlo-
ramine T but lower than Mn(III) (caution: one should
keep in mind the reaction conditions! pH, temperature,
etc.).
et al. [8,9,18] and conforms to that of Pare et al.
[16].
The kinetics of the Mn(IV) reduction by Mn(II) was
also studied spectrophotometrically to decide the role
of Mn(II) under the similar experimental conditions.
The results are given in Table III which suggest that
the reactivity of Mn(II) is ca. 10-fold slower than lac-
tic acid. The effect of addition of Mn(II) shows con-
clusively that colloidal MnO2 is the main reactive ox-
idizing species in the noncatalytic reaction pathway.
Firstly, the disappearance of this path in presence of
Mn(II) indicates that the effective concentration of
colloidal MnO2 decreases by the addition of Mn(II)
(Eq. (8)). Secondly, the rate of the auto-catalysis in-
crease in presence of added Mn(II) indicates that
Mn(II) is the principal reactive species of the auto-
catalytic path. Our results seem to suggest that there
is a competition between Mn(II) and lactic acid to
react with colloidal MnO2. The reactivity of lactic
Table IV Comparison Between the Second-Order Rate
Constants (kII) for the Oxidation of Lactic Acid by
Different Oxidants
Temperature
(◦C)
102 kII
Oxidant
(mol−1 dm3 s−1) Remark
KMnO4
25
35
55
26
45
30
398.0
1.6
1.3
32.4
0.1
2.1
a
b
c
d
e
f
K2Cr2O7
2−
S2O8
Manganse(III)
Chloramine T
Colloidal MnO2
Table III The Pseudo-First-Order Rate Constants
(kobs3) of the Reduction of Colloidal MnO2 (8.0 × 10−5
mol dm−3) by Manganese(II) in Absence of Lactic Acid
at 30◦C
a [KMnO4] = 2.1 × 10−4 mol dm−3, [lactic acid] = 3.1 × 10−3
mol dm−3, [H2SO4] = 2.0 mol dm−3; Ref. [10].
b [K2Cr2O7] = 4.9 × 10−4 mol dm−3, [lactic acid] = 4.0 × 10−3
mol dm−3, [HClO4] = 0.5 mol dm−3; Ref. [26].
c [S2O82−] = 3.0 × 10−2 mol dm−3, [lactic acid] = 2.0 × 10−2
mol dm−3, [CuSO4] = 1.0 × 10−4 mol dm−3; Ref. [27].
d [Mn(III)] = 4.0 × 10−4 mol dm−3, [lactic acid] = 2.0 × 10−3
mol dm−3, [H2SO4] = 2.0 mol dm−3; Ref. [10].
104 [Mn(II)]
104 kobs3
(s−1
)
(mol dm−3
)
2.0
3.0
6.0
10.0
20.0
0.7
1.5
1.0
1.5
1.5
e [Chloramine T] = 4.0 × 10−3 mol dm−3, [lactic acid] = 5.0 ×
10−2 mol dm−3, [CuSO4] = 4.0 × 10−5 mol dm−3, [OH−] = 0.5 mol
dm−3; Ref. [28].
f
[MnO2] = 2.0 × 10−4 mol dm−3, [lactic acid] = 12.0 × 10−2
mol dm−3, pH ≈ 3.0; present work.

366
KHAN ET AL.
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