4
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 rate /k [PAA][I ] with a value of k /4.22 /10 2 (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 H 2 O 2 [1 /3]. Based on this fact, many methods have been proposed for the analysis of PAA in the presence of H 2 O 2 [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 H 2 O 2 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 H 2 O 2 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 H 2 O 2 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 H 2 O 2 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. The effects 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 H 2 O 2 and PAA solutions of appropriate concentrations were prepared from their stock solutions (30% for H 2 O 2 and 39.4% for PAA). The equilibrium PAA (containing H 2 O 2 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 H 2 O 2 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:S0020-1693(02)01337-3

Kinetic studies on the oxidation of iodide by peroxyacetic acid

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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 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

Page 2: Kinetic studies on the oxidation of iodide by peroxyacetic acid

(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

Page 3: Kinetic studies on the oxidation of iodide by peroxyacetic acid

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

Page 4: Kinetic studies on the oxidation of iodide by peroxyacetic acid

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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43.

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(1997) 42.

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