Kinetics and mechanism of oxidation of l -ascorbic acid by peroxomonophosphate in aqueous acid medium

Article abstract of DOI:10.1002/kin.20741

The kinetics of oxidation of l-ascorbic acid (H₂A) by peroxomonophosphate in acid aqueous medium has been studied. The stoichiometry of the reaction corresponds to the reaction as represented by the equation where A is dehydroascorbic acid. The reaction is second order versus first order with respect to each reactant. The rate is retarded by hydrogen ion concentration. A plausible reaction mechanism has been suggested. The derived rate law (2) from such a mechanism accounts for all experimental observations: Such pH dependence is somewhat different from that observed in the case of metal ion oxidants.

Full text of DOI:10.1002/kin.20741

Kinetics and Mechanism of
Oxidation of L-Ascorbic Acid
by Peroxomonophosphate
in Aqueous Acid Medium
NARENDRA K. SONI, RIYA SAILANI, C. L. KHANDELWAL, P. D. SHARMA
Department of Chemistry, University of Rajasthan, Jaipur 302 055, India
Received 16 July 2011; revised 18 November 2011, 2 February 2012; accepted 25 March 2012
DOI 10.1002/kin.20741
Published online 20 November 2012 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: The kinetics of oxidation of L-ascorbic acid (H2A) by peroxomonophosphate in
acid aqueous medium has been studied. The stoichiometry of the reaction corresponds to the
reaction as represented by the equation
H3PO5 + H2A → H3PO4 + H2 O + A
(1)
(2)
where A is dehydroascorbic acid. The reaction is second order versus first order with respect
to each reactant. The rate is retarded by hydrogen ion concentration. A plausible reaction
mechanism has been suggested. The derived rate law (2) from such a mechanism accounts for
all experimental observations:
d[H2 A]
dt
(k1k2[H+] + K2 K1 K2)
=
[H3PO5][H2A]
(K1 + [H+])(K2 + [H+])
Such pH dependence is somewhat different from that observed in the case of metal ion
oxidants. ꢀ 2012 Wiley Periodicals, Inc. Int J Chem Kinet 45: 41–46, 2013
C
medium. Also, L-ascorbic acid is considered to be an
important biological compound of immense signifi-
cance as a water-soluble vitamin C [27]. Its application
in photoconversion is of an added importance.
Peroxomonophosphate (PMP) is a hydrolytic prod-
uct of peroxodiphosphate and is considered to be a
more potential oxidant than its parent analogue [28–
33]. The title study is taken from the following view-
points:
INTRODUCTION
The oxidation studies of ascorbic acid (H2A) are im-
portant both biochemically [1–17] and biomedically
[18–21]. The kinetic studies of such reactions have
further generated an interest to understand its oxida-
tion reactions by metal ions, nonmetal ions, and metal
ion complexes as oxidants [22–26] in acid aqueous
First, the L-ascorbic acid (H2A) is a dibasic acid. The
ascorbate (HA−) is considered to be a reactive species
in most of its reactions. However, free radical (A•) is
Correspondence to: Riya Sailani; e-mail: l.p riya@yahoo.co.in.
2012 Wiley Periodicals, Inc.
c
ꢀ

42
SONI ET AL.
unreactive and decays to ascorbic acid (H2A), ascor-
bate (HA−), and dehydroascorbic acid (A). Whether
or not the same trend of reactivity pattern, L-ascorbic
acid is observed in lower pH of the title reaction. It
will be interesting to further probe whether or not the
hydrogen ion dependence exhibits a similar trend as
reported in other reactions of ascorbic acid.
Second, whether or not there is any possibility of
complexation between the oxidant and substrate in
view of the complexation reported in the case of metal-
complex ion oxidants with the substrate.
Kinetic Procedure. The reactions were allowed to oc-
cur in glass-stoppered Erlenmeyer flasks, which were
painted black from the outside. These flasks were im-
mersed in a thermostated water bath at 0.1◦C unless
stated otherwise. The reaction mixtures containing all
other ingredients except H2A were allowed to attain the
temperature of water bath. The temperature preequili-
brated solution of PMP of required concentration was
added to the reaction mixture. The time of initiation
of the reaction was recorded when half of the contents
from the pipette was released into the reaction mixture.
An aliquot (5 cm3) of the reaction mixture was with-
drawn at different time intervals and then discharged
into ice-cold water to assay the remaining ascorbic acid
iodimetrically. PMP, however, did not interfere with the
pH of the titrating mixture.
Third, ascorbic acid species in acid medium ex-
hibits the following sequence of reactivity toward the
oxidants of varied nature:
A2− > HA− > H2A
Initial rates (ki, mol dm−3 −1) were computed
s
employing a plane mirror method. Since most of the
reactions were conducted taking comparable concen-
trations of the reactants, viz. [PMP] = 2.0 × 10−3
mol dm−3, [H2A] = 2.0 × 10−3 mol dm−3, pH
1.7 at 25◦C, second-order plots of [H2A]t /[PMP]t
or [PMP]t /[H2A]t versus time were made (Fig. 1).
However, an interesting observation was of an ini-
tial fast reaction to the extent of 10%–30% on mix-
ing of the reagents. This is an ample evidence of trace
metal ion catalysis of the reaction in doubly distilled
water.
Whether or not the species H2A is kinetically reactive
at lower pH, H2A species has been least reactive as
reported in a large number of reactions of ascorbic
acid.
EXPERIMENTAL
Materials and Method
Materials. L-Ascorbic acid (E. Merck) was employed
as supplied. PMP is the hydrolytic product of perox-
odiphosphate (a gift from FMC Corp., Philadelphia,
PA). It was prepared by hydrolyzing [34–36] per-
oxodiphosphate (0.05 mol dm−3) in perchloric acid
(0.5 mol dm−3) at 45◦C in a thermostated water bath
for 1.5 h. The solution was standardized iodometri-
cally [37]. H2O2 as a minor hydrolytic product was
negated by the tests with either cerium(IV) or per-
manganate. The concentrations of the H2O2 (∼0.1 ×
10−6) can be tested qualitatively by KMnO4 in acid
solutions. However, Maruthamuthu and Neta [38] de-
tected H2O2 only to the extent of ∼1 × 10−6 mol
dm−3 in hydrolysis of peroxodiphosphate conducto-
metrically. Perxomonophosphate and ascorbic acid so-
lutions were kept in bottles painted black from the out-
side to check decomposition by diffused light. Such so-
lutions are quite stable, if kept, at refrigerated temper-
ature (∼5◦C). Afresh solutions of the oxidant and sub-
strate, respectively, were, however, always employed
in kinetics studies.
All other chemicals were of the reagent grade, and
these were used as received.
Triply distilled water was employed throughout the
study; it was obtained by redistilling doubly distilled
water in the presence of EDTA, which eliminated traces
of metal ions such as Cu(II) or iron(II).
Figure 1 Second-order plots in the reaction of ascorbic acid
and PMP. [PMP] = 2.0 × 10−3 mol dm−3; pH 1.65, 25◦C.
[H2A] = (1) 8.0 × 10−4; (2) 1.2 × 10−3; (3) 1.4 × 10−3
;
(4) 1.6 × 10−3; (5) 1.8 × 10−3 mol dm−3
.
International Journal of Chemical Kinetics DOI 10.1002/kin.20741

KINETICS AND MECHANISM OF OXIDATION OF L-ASCORBIC ACID
43
Rates in triplicate were reproducible to within 6%
centration conforming to empirical rate law (3):
when peroxodiphosphate of the same batch was em-
ployed for preparation of PMP.
d [H2A]
B [H3PO5] [H2A]
=
(3)
dt
C+ [H+]
Stoichiometry. The reaction mixtures with an excess
concentration of ascorbic acid (H2A) over that of PMP
under kinetic conditions were allowed to react in a
thermostated water bath for ca. 5 h; the excess ascor-
bic acid was estimated iodimetrically. However, when
[PMP] was taken in excess over [H2A], the excess con-
centration of PMP was estimated ceremetrically. The
stoichiometric results under both these conditions rep-
resent that 1 mol of ascorbic acid reacts with 1 mol of
per acid corresponding to the stoichiometry as repre-
sented by Eq. (1):
where C is another empirical constant.
The concentration of copper(II) was varied by em-
ploying copper sulfate at fixed concentrations of other
reaction ingredients and pH 1.7 at 25◦C. The rate
increases with increasing concentration of copper(II)
(Fig. 2). It is also clear from such a plot that an intercept
accounts for catalysis by traces of metal ions present
in the reaction mixture in doubly distilled water.
(1)
RESULTS
The kinetics of the reaction was also studied
at 15, 20, and 25◦C, respectively, at fixed con-
centrations of reaction ingredients. The energy of
The concentration of PMP was varied at fixed concen-
trations of other reaction ingredients at 25◦C, Second-
order plots were made between log [PMP]t /[H2A]t or
log [H2A]t /[PMP]t versus time (Fig. 1) because of
comparable concentrations of the reactants to evalu-
ate second-order rate constants.
The concentration of L-ascorbic acid was similarly
varied keeping constant concentrations of other reac-
tion ingredients at 25◦C. Second-order rate constants
calculated in such a variation are in agreement with
the rate constants obtained in variation of [PMP]. Such
an agreement confirms that the reaction is second
order versus first order with respect to oxidant and
substrate, respectively. Thus an empirical rate law as
given by Eq. (2) conforms to the second-order nature
of the reaction:
−d [H2A]
= B [H2A] [H3PO5]
(2)
dt
where B is an empirical rate constant.
The pH variation was made by employing perchlo-
ric acid at fixed concentrations of other reaction ingre-
dients at ionic strength (I) = 0.5 mol dm−3 (I was
adjusted by employing lithium perchlorate) and 25◦C.
The rate decreases with increasing hydrogenion con-
Figure 2 Copper(II) concentration variation in the reaction
of ascorbic acid and PMP. [H2A] = 2.0 × 10−3 mol dm−3
;
[PMP] = 2.0 × 10−3 mol dm−3; pH 1.7 and 25◦C.
International Journal of Chemical Kinetics DOI 10.1002/kin.20741

44
SONI ET AL.
2 kJ mol−1and entropy of ac-
The dissociation constants K1, K2, and K3 of peracid
reported by Edwards et al. [36] are 8.0 × 10−2, 4.2 ×
10−6, and 1.6 × 10−13 mol dm−3, respectively.
H2PO−5 and not HPO52− or PO35− appears to be the
reactive form of peracid in view of the pH of the
medium. On similar arguments, HA− species of ascor-
bic acid is more reactive than H2A. Therefore, consid-
ering H2PO−5 to be the reactive form of peracid, H2A
and HA− to be the reactive species of ascorbic acid, the
following reaction mechanism consisting of steps (4)
and (7)–(9) can be envisaged to account for the kinetic
observations:
activation to be 49
tivation to be –65 0.4 J K−1 mol−1 were calculated
in a conventional manner. The energy of activation is
of the same order as usually obtained for second-order
reactions.
DISCUSSION
Ascorbic acid (H2A) is a dibasic acid and dissociates
into monoanion (HA−) and dianion (A2−) species rep-
resented as follows:
pK1 exhibits a hydroxyl group at C-3 of the enol group
of H2A and pK2 for the hydroxyl group at C-2 of the
enol group of HA−.
k2
ꢀ
ꢁ
−
+
H3PO5
H PO + H
(4)
2
5
There is a significant difference in the oxidizing
properties of L-ascorbic acid [H2A] and its conjugate
base species such as HA− and A2−. Since the acid
[39] is diabasic with pK1 and pK2 being 4.04 and
11.3, respectively, at I = 1.0 mol dm−3, the oxida-
tion reactions of ascorbic acid, therefore, should ex-
hibit a characteristic pH dependence at which the ox-
idizing trend of such species is exclusively delineated
provided that hydrolytic equilibria governing oxidant
species are not significant. Thus, if pH of the medium
is any guide, H2A species in strongly acid medium
(0 < pH < 1), HA− species in dilute acid medium
(2.5 < pH < 5.5), and A2−species toward higher pH >
6, respectively, are the reactive species of ascorbic
acid.
k1
ꢀ
ꢁ
−
+
H A
2
HA + H
(7)
(8)
(9)
k1
H2PO−5 + H2A −→ Products
k2
H2PO−5 + HA− −→ Products
The loss of ascorbic acid leads to the rate law (10) or
(11)
d[H2A]
(k1K2[H+] + k2K1K2)
=
[H3PO5][H2A]
dt
(K1 + [H+])(K2 + [H+])
(10)
where [H3PO5] and [H2A] are the gross analytical con-
centrations of peracid and L-ascorbic acid, respectively.
Similarly, peroxomonophosphoric acid species be-
ing hydrogen ion dependent are governed by equilibria
(4)–(6)
k1K2[H+] + k2K1K2
k =
(11)
(K1 + [H+]) (K2 + [H+])
−
+
ꢀ
ꢁ
H3PO5
H PO + H
(4)
(5)
(6)
2
5
where k is an observed second-order rate constant.
Equation (12) is obtained on rearranging Eq. (11)
H2PO−
HPO2− + H+
ꢀ
ꢁ
5
5
k(K1 + [H+])(K2 + [H+]) = k1K2[H+] + k2K1K2
HPO25−
PO3− + H+
ꢀ
ꢁ
(12)
5
International Journal of Chemical Kinetics DOI 10.1002/kin.20741

KINETICS AND MECHANISM OF OXIDATION OF L-ASCORBIC ACID
45
ꢀ
ꢁ
2+
CuII − H2A
→ CuI + Other products
(17)
(18)
(19)
ꢀ
ꢁ+
CuII − HA− → CuI + Other products
Fast
CuI + PMP −→ Products
Such a mechanism accounts for the rate to be inde-
pendent of peroxomonophosphoric acid. This allows
operation of CuII/CuI catalyst redox cycle. A com-
plete kinetic study of such a catalyzed reaction will
be undertaken separately to establish such a tentative
mechanism.
Figure 3 A plot of kꢁ([H+] + K1) (K2 + [H+]) versus
[H+]. [H2A] = 2.0 × 10−3 mol dm−3; [PMP] = 2.0 ×
10−3 mol dm−3; I = 0.5 mol dm−3; and 15◦C, ꢂ 20◦C,
ꢀ
CONCLUSIONS
25◦C.
◦
The kinetics of oxidation of L-ascorbic acid by PMP
in aqueous acid medium has been studied. The stoi-
chiometry of the reaction corresponds to a reaction of
1 mol of peracid with 1 mol of ascorbic acid. The re-
action is second order versus first order with respect to
the oxidant and substrate, respectively.
A plot of the left-hand side of Eq. (12) versus [H+]
yielded a straight line with nonzero intercept (Fig. 3).
k1K2 from the slope and k2K1K2 from the intercept
were 2.7 × 10−2 s−1 and 5.0 × 10−5 mol dm−3 s−1
,
respectively, at 25◦C.
The reaction kinetics exhibits complex hydrogen
ion dependence. The rate law and proposed mecha-
nism correspond reasonably well to the experimental
observations. The reaction is also catalyzed by cop-
per(II) ions, and a tentative mechanism for copper(II)-
catalyzed oxidation of L-ascorbic acid with PMP has
been suggested.
It is well established [40] that copper(II) forms
complexes with various species of ascorbic acid.
A qualitative portrait of the reaction events in
copper(II)-catalyzed oxidation of ascorbic acid by per-
oxomonophosphoric acid can be envisaged as follows:
−
+
ꢀ
ꢁ
H A
HA + H
(7)
(13)
(14)
(15)
(16)
2
ꢀ
ꢁ
2+
Cu2+ + H A
CuII − H2A
ꢀ
ꢁ
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International Journal of Chemical Kinetics DOI 10.1002/kin.20741