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R. Sailani et al. / C. R. Chimie 14 (2011) 1088–1094
1089
ꢀ
Secondly, transition metal-ion catalyzed oxidations of
ascorbic acid by H2O2 have been reported without any
agreement for specific rate; the title study becomes more
interesting to probe further.
made for comparable concentrations of the reactants.
Rates in triplicate were reproducible to within ꢂ 5%.
2.3. Stoichiometry
The reaction mixtures with [PMS] > [H2A] and
[H2A] > [PMS] were allowed to react in a thermostated
water-bath for 6 h, excess [PMS] was determined iodome-
trically (heretofore PMS was used for peroxomonosul-
phate). The reactions where excess [H2A] was taken over
[PMS], the excess [H2A] was determined iodimetrically.
These results subscribe to the reaction of one mole of PMS
with one mole of ascorbic acid as represented by Eq. (1).
2. Experimental
2.1. Material/Methods
L-ascorbic acid (BDH AnalaR) was employed as received.
The solutions of the acid were always prepared afresh as
and when required and were kept in refrigerated brown
colored glass vessels to minimize deterioration/oxidation.
O
C
O
C
C
O
O
O
C
O
C
C
OH
OH
+ HSO4– + H2O
+ PMS
C
C
H
CHOH
H
CHOH
(1)
CH2OH
CH2OH
(H2A)
(A)
The concentration of the solution was checked iodimetri-
cally [49,50] before use. Peroxomonosulphate (Aldrich) was
employed as supplied. Other reagents were of analytical
grade. Copper sulphate was employed as a catalyst.
Triply distilled waterwas employed in the preparation of
thereagentsandreactionmixtures,thethirddistillationwas
from edta of double distilled water in alkaline KMnO4 in an
all glass apparatus. Such a distillation with edta removed
traces of metal-ions known to catalyze the oxidation of
ascorbic acid by molecular oxygen or peroxides.
A similar stoiochiometry has been reported earlier in
reactions of ascorbic acid with peroxodisulphate, perox-
odiphosphate and peroxomonophosphate, respectively, in
acid media.
3. Results
3.1. Uncatalyzed reaction
3.1.1. Peroxomonosulphate dependence
The concentration of peroxomonosulphate was varied in
the range (2.0 to 10.0) ꢄ 10ꢁ3 mol dmꢁ3 at fixed concentra-
tions of other reaction ingredients viz. [H2A] = 2.0 ꢄ
10ꢁ3 mol dmꢁ3 and [H+] = 0.02 mol dmꢁ3 employing per-
chloric acid. Initial rates (ki, mol ꢁ3 sꢁ1) were calculated and
a plot of initial rate (ki) against the concentration of PMS
yielded a straight line passing through the origin indicating
first order with respect to PMS.
2.2. Procedure
The reaction mixtures containing all other ingredients
except peroxomonosulphate were taken in glass stoppered
Erlenmeyer flasks painted black from the outside to check
photodecomposition. These flasks were suspended in a
water-bath thermostated at ꢂ 0.1 8C unless specified other-
wise. The reactions were initiated by adding requisite solution
of the temperature pre-equilibrated peracid in to the reaction
mixture. The time of initiation was recorded when half of the
contents from the pipette were released into the reaction
mixture. A known aliquot (5 or 10 cm3) of the reaction mixture
was withdrawn periodically and then quenched in an ice-cold
H2SO4 (ꢃ1 mol dmꢁ3), the remaining ascorbic acid at different
time intervals was estimated ceremetrically employing n-
phenyl anthranilic acid indicator.
3.1.2. Ascorbic acid dependence
The concentration of ascorbic acid (H2A) was varied
from (2.0 to 10.0) ꢄ 10ꢁ3 mol dmꢁ3 at constant concentra-
tions
of
other
reaction
components
viz.
[PMS] = 5.0 ꢄ 10ꢁ3 mol dmꢁ3 and [H+] = 0.02 mol dmꢁ3
.
The plot of initial rate (ki) against the concentration of
ascorbic acid also yields a straight line passing through the
origin indicating first order dependence with respect to
organic acid. Second order plots of log of [PMS]t/[H2A]t or
[H2A]t/[PMS]t were also made for comparable concentra-
tions of the reactants. The second order rate constants
Initial rates were computed employing plane-mirror
method. Pseudo first order plots were also made wherever
reaction conditions permitted and second order plots were