Kinetic studies on the oxidation of iodide by peroxyacetic acid

Article abstract of DOI:10.1016/S0020-1693(02)01337-3

The kinetics of the oxidation of iodide by peroxyacetic acid (PAA) in aqueous media in the presence and absence of the heptamolybdate has been studied by a high time resolution spectrophotometric stopped-flow method. The time-dependent concentration of the liberated iodine was monitored by the change in absorbance at 352 nm. The effect of ammonium heptamolybdate as well as pH on the rate of the reaction was also studied and it was found that the rate of the reaction is independent of pH and molybdate concentration under the examined conditions. The results obtained show that the rate law of the reaction can be expressed as rate=k[PAA][I^-] with a value of k=4.22×10² (mole/l)^-1 s^-1 at pH 3.5-5.4 and 25°C.

Full text of DOI:10.1016/S0020-1693(02)01337-3

Inorganica Chimica Acta 344 (2003) 253ꢃ256
/
www.elsevier.com/locate/ica
Note
Kinetic studies on the oxidation of iodide by peroxyacetic acid
Mohamed Ismail Awad, Tadato Oritani, Takeo Ohsaka *
Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta,
Midori-ku, Yokohama 226-8502, Japan
Received 21 May 2002; accepted 2 September 2002
Abstract
The kinetics of the oxidation of iodide by peroxyacetic acid (PAA) in aqueous media in the presence and absence of the
heptamolybdate has been studied by a high time resolution spectrophotometric stopped-flow method. The time-dependent
concentration of the liberated iodine was monitored by the change in absorbance at 352 nm. The effect of ammonium
heptamolybdate as well as pH on the rate of the reaction was also studied and it was found that the rate of the reaction is
independent of pH and molybdate concentration under the examined conditions. The results obtained show that the rate law of the
reaction can be expressed as rateꢀ
/
k[PAA][Iꢁ] with a value of kꢀ
/
4.22ꢂ
/
102 (mole/l)ꢁ1 sꢁ1 at pH 3.5ꢃ
5.4 and 25 8C.
/
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Peroxyacetic acid; Hydrogen peroxide; Molybdate; Kinetics; Stopped-flow spectrophotometry
1. Introduction
oxidations is considered to be very useful in, for
example, analyzing both PAA and H2O2 in their
coexistence. The present paper thus aims to clarify the
kinetics of the oxidation of Iꢁ by PAA based on a
stopped-flow spectrophotometric technique that enables
a high time-resolved monitoring of the absorbance. The
effects of the various experimental conditions, such as
pH and molybdate concentration (as catalyst), on the
kinetic parameters were also investigated.
It has long been known that the rate of oxidation of
iodide by PAA is much faster than that by H2O2 [1ꢃ
Based on this fact, many methods have been proposed
for the analysis of PAA in the presence of H2O2 [1ꢃ3].
/
3].
/
Of these methods, the most typical one, is the volumetric
method originally proposed by Sully and Williams [3].
Saltzman and Gilbert [2] reported the spectrophoto-
metric analysis of binary mixtures of PAA with H2O2 or
methyl ethyl ketone peroxide by measuring the absor-
bance of the liberated iodine. Recently, Davies and
Deary [1] have succeeded in analyzing PAA in the
presence of up to 1000-fold excess of H2O2 using a rapid
spectrophotometric method that involves a simple linear
extrapolation to obtain the response for PAA. Irrespec-
tive of these successful analyses of PAA, to the best of
our knowledge, there is no report on the kinetic studies
of the oxidation of Iꢁ by PAA, in contrast to the
oxidation of Iꢁ by H2O2 the kinetics of which has been
extensively studied under various experimental condi-
2. Experimental
2.1. Reagents
All solutions were prepared in deionized water (Milli-
Q, Millipore, Japan) and all chemicals were of analytical
grade. The H2O2 and PAA solutions of appropriate
concentrations were prepared from their stock solutions
(30% for H2O2 and 39.4% for PAA). The equilibrium
PAA (containing H2O2 and acetic acid), which was
obtained from Mitsubishi Gas Chemicals Co., was
analyzed using the conventional method proposed by
Sully and William [3]. The concentrations of PAA and
H2O2 in their mixture were determined to be 5.5 and 1.7
tions [4ꢃ12]. The kinetic information on both these
/
* Corresponding author. Tel.: ꢄ
5489
E-mail address: ohsaka@echem.titech.ac.jp (T. Ohsaka).
/
81-45-924 5404; fax: ꢄ81-45-924
/
M,
respectively.
Ammonium
heptamolybdate,
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 3 3 7 - 3

254
M.I. Awad et al. / Inorganica Chimica Acta 344 (2003) 253ꢃ256
/
(NH4)6Mo7O24×
/
4H2O ( abbreviated as Mo(VI)) was
to be first order with respect to PAA, then Eq. (3) can be
integrated to:
purchased from Kanto Chemicals Co., Inc., Japan.
Different concentrations of Mo(VI) solutions were
prepared by dissolving the proper weight in Milli-Q
water.
lnf[PAA]o=[PAA]tgꢀk?t
(4)
where [PAA]t represents the concentration of PAA at
time t. The above equation shows that a plot of
ln{[PAA]o/[PAA]t} versus t would be linear if the
reaction is first order with respect to PAA. The slope
of the straight line is the pseudo-first-order rate constant
of the reaction, k?, which equals k[Iꢁ]b[Hꢄ]d.
2.2. Instrument and procedure
The measurements of absorbance were carried out
using an RA-401 stopped-flow spectrophotometer (Ot-
suka Electronics Co., Japan). The absorption spectra
could be observed with the minimum time interval of 4
ms after mixing of the reactants. N2 gas used for mixing
is of pressure of 5.0 kg cmꢁ2. The optical cell length is 2
mm and the slit width is 3.5 mm. One of the driving
syringes of the stop-flow unit was filled with KI solution
and the other one was filled with PAA solution. For
each run, equal volumes of both solutions were mixed in
the mixing chamber and the change in the absorbance
due to the generation of I2 with time was monitored at
352 nm [5].
The plot of ln k? versus ln[Iꢁ] should be a straight line
with slope b (which is the order of the reaction with
respect to Iꢁ). While the rate constant of the reaction,
k, can be determined from the intercept at a certain pH.
Similarly the effect of Mo(VI) and pH on the rate of the
reaction can be studied by changing Mo(VI) concentra-
tion or pH and using all the other species in excess.
3.2. Experimental results
3.2.1. Order in PAA
The isolation method was used for the determination
of the order of the reaction with respect to PAA. That is,
Iꢁ was used in a large excess with respect to PAA.
Under these conditions the reaction was expected to be
pseudo-first-order in PAA. Fig. 1 shows the typical
3. Results and discussion
3.1. Theoretical background
absorbanceꢃtime curves at 352 nm for the reaction
/
between 0.22 mM PAA and different concentrations of
KI (the lowest ratio of Iꢁ concentration to PAA
concentration is 25) in 0.05 M acetate buffer solution
(pH 5.4). In each case, the increase in the absorbance is
attributed to the increase in the concentration of the
liberated I2. A leveling of the absorbance occurred after
The overall oxidation of Iꢁ by PAA is expressed as
follows:
k
CH3COOOHꢄ2Iꢁ ꢄ2Hꢄ 0 CH3COOHꢄH2O
Mo(VI)
ꢄI2
(1)
Consequently, the rate of this reaction can be expressed
generally as
rateꢀd[I2]=dtꢀꢁd[PAA]=dt
ꢀk[PAA]a[Iꢁ]b[Mo(VI)]c[Hꢄ]d
(2)
in which Mo(VI) is the possible catalyst of the reaction,
[PAA] is the concentration of PAA, a, b, c and d are the
reaction orders with respect to PAA, Iꢁ, Mo(VI) and
Hꢄ, respectively. aꢄ
/
bꢄ
/
cꢄd then refers to the total
/
order of the reaction. The rate constant for the reaction
can be determined using the isolation method, that is, by
isolating one of the reactants by using the other
reactants in excess. For example, if the initial concen-
trations of the reactants are such that, [Iꢁ]oꢀ
[PAA]o
/
and the experiment is conducted at constant pH and in
the absence of molybdate, then the concentration of Iꢁ
is supposed to be constant during the experiment.
Accordingly, Eq. (2) can be reduced to:
Fig. 1. Typical absorbanceꢃ
/
time curves at 352 nm for the reaction of
acetate buffer solution (pH 5.4). The
ꢁd[PAA]=dtꢀk?[PAA]a; k?ꢀk[Iꢁ]b[Hꢄ]d
(3)
PAA and Iꢁ in 0.5
M
concentration of PAA was kept constant (0.22 mM), while the
concentration of Iꢁ was changed: (1) 10, (2) 12.5, (3) 15.0, (4) 17.5,
(5) 25.0 and (6) 35.0 mM at pH 5.4.
The order of the reaction with respect to PAA can be
then determined using Eq. (3). If the reaction is assumed

M.I. Awad et al. / Inorganica Chimica Acta 344 (2003) 253ꢃ
/256
255
some time of mixing which decreases with increasing the
concentration of Iꢁ, e.g. in the case of 15.0 mM of Iꢁ
(curve 3), the leveling of the absorbance occurred after
approximately 300 ms of mixing, which indicates the
completion of the reaction between PAA and Iꢁ. The
well-defined absorbance plateau indicates no interfer-
ence from the coexisting H2O2 in the studied time
domain. From Fig. 1, the undeveloped absorbance (in
served by Davies and Deary [1] and Saltzman and
Gilbert [2]. The slope (in Fig. 2) in each case represents
the dependence of k? on [Iꢁ] (see Eq. (4)). A plot of
log k? versus log[Iꢁ] shows a good straight line with a
correlation coefficient higher than 0.99 and is shown in
Fig. 3. The value of the slope was determined to be 0.95
and thus the order of the reaction is 1 with respect to Iꢁ.
The rate constant for the uncatalyzed reaction can be
obtained from the intercept of the log k? versus log[Iꢁ]
each case), that is (AꢁꢁAt) (where At and Aꢁ are the
/
absorbance at time t and at the end of the reaction,
respectively, and so referred to the unreacted PAA), was
obtained. Then, the data in Fig. 1 were analyzed in each
case (i.e. at different Iꢁ concentrations) by plotting
log(AꢁꢁAt) against t (see Fig. 2). An excellent straight
line with correlation coefficients higher than 0.999 was
obtained in all cases, indicating that the overall reaction
is a pseudo-first-order reaction with respect to PAA
under these conditions. In the presence of 15.0 mM KI,
for example, the pseudo-first-order rate constant was
thus determined to be 14.3 sꢁ1 at pH 5.4.
plot in Fig. 3 and it was estimated to be 4.22ꢂ
102
/
(mole/L)ꢁ1 sꢁ1. So for the uncatalyzed reaction at pH
5.4 the rate could be expressed by the following
equation:
runcatalyzed ꢀ4:22ꢂ102 [PAA][Iꢁ]
(5)
3.2.3. Order in molybdate and hydrogen ion
For studying the effect of molybdate, a series of
experiments were performed in which the molybdate
concentration was varied while the concentrations of
Iꢁ, Hꢄ, and PAA were held constant. From this series
of experiments, the pseudo-first-order rate constants
were obtained and are plotted against the molybdate
(Mo(VI)) concentration, as shown in Fig. 4, curve a. It is
clear that the pseudo-first-order rate constant is inde-
pendent of the heptamolybdate concentration in its
3.2.2. Order in iodide
The order with respect to Iꢁ can be also determined
from Fig. 1. From the slope of the rising part in Fig. 1
and the decrease in the time needed for leveling off of
the absorbance with increasing the concentration of Iꢁ,
it is clear that the rate is increased with the increase in
the Iꢁ concentration. However, the value of the
absorbance obtained after the reaction was completed
was found to be changed with the Iꢁ concentration, that
is, it increases with increasing the concentration of Iꢁ
due to the change in the molar absorptivity of the
liberated I2 with the change in the concentration of the
excess Iꢁ. Similar behavior has been previously ob-
examined range (B0.088 mM). In contrast to this
/
case, the oxidation of Iꢁ by H2O2 is highly catalyzed
by molybdate (i.e. the rate constant of the Mo(VI)-
catalyzed reaction is by about three orders of magnitude
larger than that of the uncatalyzed one [9]) and it is first
[9] or nonintegral [10] order in molybdate depending on
the conditions of the reaction. The catalytic effect of
Fig. 3. Dependence of the pseudo-first-order rate constant on Iꢁ
concentration at constant PAA concentration (0.22 mM) at pH 5.4
and 25 8C.
Fig. 2. First-order kinetic plots for the data obtained from Fig. 1. Iꢁ
concentration: (1) 10, (2) 12.5, (3) 15.0, (4) 17.5, (5) 25.0 and (6) 35.0

256
M.I. Awad et al. / Inorganica Chimica Acta 344 (2003) 253ꢃ256
/
ing step and that the reaction in Eq. (1) proceeds only
via the uncatalyzed path.
4. Conclusions
The kinetics of the oxidation of Iꢁ by PAA in
aqueous solutions has been studied using a stopped-
flow spectrophotometric technique. It was found that
the rate of the reaction is independent of pH in the range
of pH 3.5ꢃ
/
5.4 as well as the molybdate concentration
0.088 mM), in contrast to the oxidation of Iꢁ by
(B
/
H2O2. The reaction is totally second order, i.e. first
order in PAA and first order in Iꢁ.
Acknowledgements
The present work was financially supported by Grant-
in-Aids for Scientific Research (Nos. 12875164 and
14050038) and Scientific Research (A) (No. 10305064)
from the Ministry of Education, Culture, Sports,
Science and Technology, Japan. M.I.A. thanks the
Government of Japan for the Monbu-kagakusho fellow-
ship.
Fig. 4. (a) Effect of [Mo(VI)] on the pseudo-first-order rate constant
for the reaction of PAA (0.22 mM) with 10 mM KI at pH 5.4. (b)
Effect of pH on the pseudo-first-order rate constant for the reaction of
PAA (0.165 mM) with 5.0 mM KI in the absence of Mo(VI).
Mo(VI) on the oxidation of Iꢁ by different peroxides is
different depending on the type of peroxide. The
oxidation Iꢁ by H2O2, as mentioned above, is highly
affected by Mo(VI), while those by t-butyl hydroper-
oxide and peroxydisulfate ion are not affected by
Mo(VI) [9]. It has been also reported that the rate of
the oxidation of Iꢁ by organic peroxide decreases with
increasing the complexicity of the organic molecules
[2,13].
The effect of pH on the rate of the reaction was also
examined by changing pH and keeping the concentra-
tions of other species constant. The apparent first order
rate constants obtained were found to be independent of
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/
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