Kinetics and mechanism of oxidation of L-ascorbic acid by platinum(IV) in aqueous acid medium

Article abstract of DOI:10.1007/s11243-013-9770-0

The oxidation of l-ascorbic acid (H₂A) by platinum(IV) in aqueous acid medium exhibits overall second-order kinetics, being first order with respect to each reactant. Increasing both hydrogen and chloride ion concentrations inhibits the rate. The stoichiometry involves reaction of one platinum(IV) ion with H₂A to give dehydroascorbic acid. A reaction mechanism consistent with all the experimental observations is proposed.

Full text of DOI:10.1007/s11243-013-9770-0

Transition Met Chem (2014) 39:41–45
DOI 10.1007/s11243-013-9770-0
Kinetics and mechanism of oxidation of L-ascorbic acid
by platinum(IV) in aqueous acid medium
N. K. Soni
P. D. Sharma
•
Riya Sailani
•
C. L. Khandelwal
•
Received: 11 August 2013 / Accepted: 7 October 2013 / Published online: 31 October 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract The oxidation of l-ascorbic acid (H A) by
2
Trace metal ion catalysis in the oxidation of ascorbic
platinum(IV) in aqueous acid medium exhibits overall
second-order kinetics, being first order with respect to each
reactant. Increasing both hydrogen and chloride ion con-
centrations inhibits the rate. The stoichiometry involves
acid, for example by peroxodiphosphate [13] in acetate
buffers and peroxo-bound chromium(V) [14] in acid med-
ium, is another important category of the reactions of
ascorbic acid. The title reaction has been studied previously
[15], but the proposed reaction mechanism is not complete
as it lacks the effect of chloride ion concentration on the
rate. This has prompted us to undertake a study of the
kinetics of oxidation of ascorbic acid by platinum(IV) in
aqueous acid medium, so that a comprehensive mechanism
accounting for the effect of chloride ion can be delineated.
L-ascorbic acid can exhibit one-equivalent or two-
equivalent nature in oxidations. Further, characteristic
hydrogen ion dependence is reported in a large number of
oxidations of ascorbic acid by metal ion oxidants. We were
interested to see if such hydrogen ion dependence would
also be observed in the present system.
reaction of one platinum(IV) ion with H A to give dehy-
2
droascorbic acid. A reaction mechanism consistent with all
the experimental observations is proposed.
Introduction
Platinum(IV) complexes find applications in cancer che-
motherapy and treatment of various other diseases and as
gel components for several types of medical implants
(
breast implants, joint replacement prosthetics, artificial
lumbar disks, vascular access ports, etc.). Ascorbic acid
vitamin C), being an important biochemical compound,
(
is also suspected to play some role in the treatment of
various tumors. Hence, the interactions of platinum(IV)
with ascorbic acid appear to be worthy of investigation.
L-ascorbic acid in recent years has been extensively
studied as a reductant both for free metal ions and for
metal complexes [1–12]. These reactions are categorized
into three main groups, namely outer-sphere, inner-
sphere, and mixed outer-and-inner-sphere electron-trans-
fer reactions.
Experimental
Materials and methods
L-ascorbic acid (Merck) (AnalaR grade) was used as
received, and its aqueous solution was standardized iodi-
metrically [16]. A stock solution of platinum(IV) was
prepared by dissolving the requisite quantity of hexachlo-
3
roplatinic acid (Acros) in 0.3 mol dm HCl and standard-
ized iodometrically. Both solutions were kept in bottles
painted black from the outside to suppress photodecom-
position. Other reagents were either of AnalaR or guaran-
teed reagent grade and were employed as supplied. Triply
distilled water was used to prepare the solutions; the sec-
ond and third distillations were from alkaline permanga-
nate and edta, respectively, in an all-glass still.
N. K. Soni ꢀ R. Sailani (&) ꢀ C. L. Khandelwal ꢀ P. D. Sharma
Department of Chemistry, University of Rajasthan,
Jaipur 302055, India
e-mail: l.p_riya@yahoo.co.in
123

4
2
Transition Met Chem (2014) 39:41–45
-
3 -1
s
O
Table 1 Initial rates (k
3
i
-1 -1
, mol dm
) and second-order rate con-
O
stants (k, dm mol s ) in the reaction of Pt(IV) and ascorbic acid
?
in aqueous acid medium ([H ] = 0.5 mol dm ; 30 °C)
-3
OH
OH
O
O
4
0 [Pt(IV)]
-3
3
10 [H
mol dm
7
10 (k
mol dm
1
mol dm
2
A]
i
)
(k)
mol dm s
O
-3
-3 -1
s
-1
3 -1
O
2
4
5
7
8
.0
.0
.0
.0
.0
0.5
0.5
0.5
0.5
0.5
0.5
1.0
2.0
3.0
4.0
5.0
0.5
1.0
1.5
2.0
0.46
0.90
1.10
1.50
1.80
2.00
1.90
4.17
6.46
8.75
10.42
1.10
2.50
5.20
6.60
0.46 (0.45)
0.45 (0.48)
0.44 (–)
HOHC
HOH C
H
HOHC
HOH C
H
0.44 (0.48)
0.45 (0.49)
0.40 (0.46)
0.40 (0.46)
0.42 (0.40)
0.43 (–)
2
2
H A
10.0
2
A
5
5
5
5
5
5
1
1
2
.0
Scheme 1 Structural representation of Ascorbic acid and De-hydro
ascorbic acid
.0
.0
.0
0.44 (–)
Kinetic procedure
.0
0.42 (–)
.0
– (0.43)*
– (0.47)*
– (0.42)*
– (0.42)*
The reactions were conducted in glass-stoppered Erlen-
meyer flasks immersed in a water-bath thermostated at
0.0
5.0
0.0
±
0.1 °C unless stated otherwise. The reactions were initi-
ated by adding Pt(IV) solution to temperature pre-equili-
brated reaction mixtures containing all the other reagents.
The time of initiation was recorded when the solution of
platinum(IV) from the pipette was half-released into the
Second-order rate constants in parenthesis are from second-order
plots
*
plots
Marked asterisk second-order rate constants are from stoichiometric
3
reaction mixture. Aliquots (5 cm ) of the reaction mixture
were withdrawn at different time intervals and then dis-
charged into ice-chilled cerium(IV) sulfate solution of
known concentration. The excess cerium(IV) was esti-
mated by titrating against iron(II) sulfate solution
employing ferroin as an indicator. Further experiments
showed no difference whether the reaction was initiated by
The stoichiometry of the reaction was further confirmed
by studying the kinetics of a few reactions using stoichi-
ometric concentrations of the reactants under identical
experimental conditions. Second-order plots of 1/[H A]t-
2
-
3
adding Pt(IV) or ascorbic acid. Initial rates (k , mol dm
i
versus time were made, and the second-order rate constants
calculated from the gradients of the resulting straight lines
were in agreement with the rate constants calculated from
initial rates and second-order plots made for comparable
concentrations of the reactants (Table 1).
-
1
s
) were computed [17] by employing the plane mirror
method. Second-order plots were made for comparable
concentrations of the reactants. Triplicate rate measure-
ments were reproducible to within ± 5 %.
Stoichiometry
Results
The stoichiometry of the reaction was determined by
conducting a set of reactions with an excess concentration
of Pt(IV) over that of ascorbic acid, or vice versa, in a
thermostated water-bath maintained at (30.0 ± 0.1) °C for
ca. 6 h. The excess ascorbic acid was estimated iodimet-
rically in ice-cold solution. The results corresponded to the
stoichiometry of the reaction shown in Eq. (1);
þ
The order with respect to the oxidant was determined by
varying the concentration of Pt(IV) at fixed concentrations
of other reactants at (30.0 ± 0.1) °C. Initial rates (k
,
i
-
3
-1
mol dm
s ) were computed by employing the plane
mirror method [17]. A plot of initial rate versus concentra-
tion of Pt(IV) yielded a straight line passing through the
origin, conforming that the reaction is first order with respect
to the oxidant. Similarly, initial rates for varying concen-
trations of ascorbic acid at constant concentrations of other
reactants were calculated. The plot of initial rate against the
concentration of ascorbic acid also yielded a straight line
passing through the origin, conforming to first-order
dependence with respect to the substrate. Second-order rate
PtðIVÞ þ H2A ! PtðIIÞ þ 2H þ A
ð1Þ
where H A and A represent ascorbic acid and dehydroa-
2
scorbic acid, respectively (Scheme 1); the latter was also
detected qualitatively [18]. Dehydroascorbic acid being of
triketo structure has also been reported [19, 20] in other
oxidation reactions of ascorbic acid.
1
23

Transition Met Chem (2014) 39:41–45
43
(Ionic strength (I) was kept constant by employing lithium
perchlorate). The rate decreased with increasing hydrogen
ion concentration.
The effect of ionic strength was studied by employing
lithium perchlorate; the rate increased with increasing ionic
strength, consistent with an interaction between like-
charged species of the reactants (Fig. 2). The effect of
temperature on the rate of the reaction was studied at
constant concentrations of all reactants. The energy and
entropy of activation were calculated as (48.24 ± 0.49)
-
1
-1
-1
kJ mol and (-92.31 ± 1.65) JK mol , respectively,
by employing Eyring’s equation [21].
Discussion
If one takes into account the percentage distribution of
-
ascorbic acid species, namely H A, HA , and A con-
2-
2
-
sidering the following equilibria (2) and (3), HA appears
2
Fig. 1 Second-order plots in the reaction of Pt(IV) and ascorbic acid (H A).
-
4
-4
-4
Pt(IV) = (1) 4.0 9 10 (2) 7.0 9 10 (3) 8.0 9 10 (4) 1.0 9 10
-3
to be the predominant species of ascorbic acid under the
experimental conditions used in this study.
[
-
4
-3
5) 2.0 9 10 mol dm ]; [H
-4
-3
(
2
A] = 5.0 9 10 mol dm ; 30 °C
K
1
ꢁ
þ
H A ꢀ HA þ H
ð2Þ
ð3Þ
2
K
2
HA ꢀ A þ Hþ
ꢁ
2ꢁ
where the values of pK and pK for steps (2) and (3) are
1
2
4.03 and 11.3, respectively.
In view of the decelerating effect of chloride ion, the
speciation of platinum(IV) appears to be governed by
equilibrium (4). This effect of chloride ion also has been
observed previously [15].
The effect of ionic strength on the rate has been studied by
employing lithium perchlorate; contrary to the effect of
chloride ion, the rate increases with increasing ionic strength
under these conditions. This pattern of reactivity can be
explained by considering a mechanism consisting of step (2)
above and steps (4–6). It is worth mentioning that the effect
of chloride ion was assumed to be equivalent to the effect of
ionic strength in an earlier study [15], whereas the present
study shows that this is an oversimplification, due to the
observed rate increase when using lithium perchlorate to
increase the ionic strength. Such an effect requires an
interaction of like-charged species of reactants. Therefore,
step (6) in the proposed mechanism is predominant. This
revised mechanism is more reasonable; in that, it accounts
for the effects of both chloride ion and ionic strength on the
rate.
?
-3
Fig. 2 A plot of slope versus [H ]. I = 1.0 mol dm ; 30 °C
constants were evaluated from second-order plots of
log[Pt(IV)] versus [H A] or log[H A] versus [Pt(IV)]
t
t
2
t
2
t
(
Fig. 1) for comparable concentrations of the reactants, and
found to be in close agreement with those calculated from
initial rates and stoichiometric plots (Table 1).
The concentration of sodium chloride was varied in the
-
3
range of (0.05–0.5) mol dm , keeping fixed concentrations
of other reactants. Initial rates were found to decrease with
increasing concentration of chloride ion. Hydrogen ion
K2
2
6
ꢁ
ꢁ
5
ꢁ
PtCl ꢀ PtCl þ Cl
ð4Þ
ð5Þ
-
3
k
concentration was varied from 0.1 to 0.5 mol dm at fixed
-
concentrations of other reactants at I = 1.0 mol dm
ꢁ
PtCl þ H2A ꢁ! Products
5
3
123

4
4
Transition Met Chem (2014) 39:41–45
1
G ¼
ð11Þ
0
k K K ðAÞ
1
2
and
1
I ¼
ð12Þ
k0K1ðAÞ
the value of K was calculated from the ratio of intercept
2
-
3
and gradient as (0.37 ± 0.10) mol dm
.
Equations (11) and (12) in terms of hydrogen ion con-
centration can be rewritten and rearranged as Eqs. (13) and
(
14), respectively, as follows:
þ
1
0
½H ꢂ
G ¼
þ
ð13Þ
k K2 kK1K2
and
þ
1
0
½H ꢂ
I ¼ k
þ
ð14Þ
0
k K
1
?
-3
Fig. 3 A plot of intercept versus [H ]. I = 1.0 mol dm ; 30 °C
Further plots according to Eq. (13) between gradient
?
(G) and [H ] (Fig. 2) and (I) versus [H ] from Eq. (14)
?
0
k
ꢁ
PtCl þ H A ꢁ! Products
ð6Þ
5
2
(
Fig. 3) also yielded straight lines with nonzero intercepts.
0
-
-
0
1
The values of k K and k K K were calculated from the
2
Thus, the interaction between PtCl5 and HA in step
6) can be considered to be the rate limiting step, leading to
2
intercept and slope of Figs. 2 and 3, and the ratio of these
0
(
-
two yielded K as (0.51 ± 0.10) mol dm . Similarly, k
3
the rate law (7)
1
was calculated from the intercept as (0.38 ± 0.10)
0
ꢁ
d½H2Aꢂ
dt
k K1K2½H2Aꢂ½PtðIVÞꢂ
-
mol dm .
3
¼
ð7Þ
ꢁ
þ
ðK þ ½Cl ꢂÞðK þ ½H ꢂÞ
2
1
where [H A] and [Pt(IV)] are the gross analytical
2
concentrations of ascorbic acid and platinum(IV),
respectively. Rearrangement of Eq. (7) gives Eq. (8);
Conclusion
The previous kinetic analysis of this system was deficient;
in that, the effect of chloride ion concentration on the rate
of the reaction was not treated properly. In fact, the ionic
strength has a pronounced effect on the rate, whereas
chloride ion inhibits the rate. The revised mechanism
proposed in this paper accounts separately for the effects of
both ionic strength and chloride ion. The reaction appears
to occur via an outer-sphere mechanism.
0
ꢁ
k K1K2
k ¼
ð8Þ
þ
ðK2 þ ½Cl ꢂÞðK1 þ ½H ꢂÞ
where k is the observed second-order rate constant
(
Table 1).
Since the variation of chloride ion concentration was
made at constant hydrogen ion concentration, Eq. (8) can
be simplified to Eq. (9),
0
k K1K2ðAÞ
k ¼
ð9Þ
ꢁ
ðK2 þ ½Cl ꢂÞ
1
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