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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 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 8C.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Peroxyacetic acid; Hydrogen peroxide; Molybdate; Kinetics; Stopped-flow spectrophotometry
1. Introduction
It has long been known that the rate of oxidation of
iodide by PAA is much faster than that by H2O2 [1�/3].
Based on this fact, many methods have been proposed
for the analysis of PAA in the presence of H2O2 [1�/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 I� by PAA, in contrast to the
oxidation of I� by H2O2 the kinetics of which has been
extensively studied under various experimental condi-
tions [4�/12]. The kinetic information on both these
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 I� by PAA based on a
stopped-flow spectrophotometric technique that enables
a high time-resolved monitoring of the absorbance. Theeffects of the various experimental conditions, such as
pH and molybdate concentration (as catalyst), on the
kinetic parameters were also investigated.
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 bySully and William [3]. The concentrations of PAA and
H2O2 in their mixture were determined to be 5.5 and 1.7
M, respectively. Ammonium heptamolybdate,
* Corresponding author. Tel.: �/81-45-924 5404; fax: �/81-45-924
5489
E-mail address: [email protected] (T. Ohsaka).
Inorganica Chimica Acta 344 (2003) 253�/256
www.elsevier.com/locate/ica
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
(NH4)6Mo7O24 �/4H2O ( abbreviated as Mo(VI)) was
purchased from Kanto Chemicals Co., Inc., Japan.
Different concentrations of Mo(VI) solutions were
prepared by dissolving the proper weight in Milli-Qwater.
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 spectracould 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 cm�2. 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 absorbancedue to the generation of I2 with time was monitored at
352 nm [5].
3. Results and discussion
3.1. Theoretical background
The overall oxidation of I� by PAA is expressed as
follows:
CH3COOOH�2I��2H� 0k
Mo(VI)CH3COOH�H2O
�I2 (1)
Consequently, the rate of this reaction can be expressedgenerally as
rate�d[I2]=dt��d[PAA]=dt
�k[PAA]a[I�]b[Mo(VI)]c[H�]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, I�, Mo(VI) and
H�, respectively. a�/b�/c�/d 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, [I�]o�/[PAA]oand the experiment is conducted at constant pH and in
the absence of molybdate, then the concentration of I�
is supposed to be constant during the experiment.
Accordingly, Eq. (2) can be reduced to:
�d[PAA]=dt�k?[PAA]a; k?�k[I�]b[H�]d (3)
The order of the reaction with respect to PAA can be
then determined using Eq. (3). If the reaction is assumed
to be first order with respect to PAA, then Eq. (3) can be
integrated to:
lnf[PAA]o=[PAA]tg�k?t (4)
where [PAA]t represents the concentration of PAA attime 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 [I�]b [H�]d .
The plot of ln k ? versus ln[I�] should be a straight line
with slope b (which is the order of the reaction with
respect to I�). 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,I� 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
absorbance�/time curves at 352 nm for the reaction
between 0.22 mM PAA and different concentrations of
KI (the lowest ratio of I� concentration to PAA
concentration is 25) in 0.05 M acetate buffer solution
(pH 5.4). In each case, the increase in the absorbance isattributed to the increase in the concentration of the
liberated I2. A leveling of the absorbance occurred after
Fig. 1. Typical absorbance�/time curves at 352 nm for the reaction of
PAA and I� in 0.5 M acetate buffer solution (pH 5.4). The
concentration of PAA was kept constant (0.22 mM), while the
concentration of I� 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.
M.I. Awad et al. / Inorganica Chimica Acta 344 (2003) 253�/256254
some time of mixing which decreases with increasing the
concentration of I�, e.g. in the case of 15.0 mM of I�
(curve 3), the leveling of the absorbance occurred after
approximately 300 ms of mixing, which indicates thecompletion of the reaction between PAA and I�. 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
each case), that is (A��/At) (where At and A� 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 eachcase (i.e. at different I� concentrations) by plotting
log(A��At) 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 s�1 at pH 5.4.
3.2.2. Order in iodide
The order with respect to I� 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 I�,
it is clear that the rate is increased with the increase in
the I� concentration. However, the value of theabsorbance obtained after the reaction was completed
was found to be changed with the I� concentration, that
is, it increases with increasing the concentration of I�
due to the change in the molar absorptivity of the
liberated I2 with the change in the concentration of the
excess I�. Similar behavior has been previously ob-
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 [I�] (see Eq. (4)). A plot of
log k ? versus log[I�] shows a good straight line with acorrelation 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 I�.
The rate constant for the uncatalyzed reaction can be
obtained from the intercept of the log k ? versus log[I�]
plot in Fig. 3 and it was estimated to be 4.22�/102
(mole/L)�1 s�1. So for the uncatalyzed reaction at pH
5.4 the rate could be expressed by the followingequation:
runcatalyzed�4:22�102 [PAA][I�] (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 molybdateconcentration was varied while the concentrations of
I�, H�, 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
examined range (B/0.088 mM). In contrast to thiscase, the oxidation of I� 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. 2. First-order kinetic plots for the data obtained from Fig. 1. I�
concentration: (1) 10, (2) 12.5, (3) 15.0, (4) 17.5, (5) 25.0 and (6) 35.0
Fig. 3. Dependence of the pseudo-first-order rate constant on I�
concentration at constant PAA concentration (0.22 mM) at pH 5.4
and 25 8C.
M.I. Awad et al. / Inorganica Chimica Acta 344 (2003) 253�/256 255
Mo(VI) on the oxidation of I� by different peroxides is
different depending on the type of peroxide. The
oxidation I� 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 I� 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
pH in the examined range of pH 3.5�/5.4 (see Fig. 4(b)),
in which PAA exists in the neutral undissociated form
(the pKa value is 8.2 [14]). This is also in contrast to thecase of H2O2, in which the oxidation I� by H2O2 is
more accelerated at lower pH [9]. Since the rate constant
is independent of pH and the Mo(VI) concentration, the
total reaction rate can be expressed by Eq. (5), suggest-
ing no contribution of both species in the rate determin-
ing step and that the reaction in Eq. (1) proceeds only
via the uncatalyzed path.
4. Conclusions
The kinetics of the oxidation of I� 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 rangeof pH 3.5�/5.4 as well as the molybdate concentration
(B/0.088 mM), in contrast to the oxidation of I� by
H2O2. The reaction is totally second order, i.e. first
order in PAA and first order in I�.
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 theGovernment of Japan for the Monbu-kagakusho fellow-
ship.
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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).
M.I. Awad et al. / Inorganica Chimica Acta 344 (2003) 253�/256256