Kinetics and mechanism of the homogeneous oxidation of quinaldine red by hydrogen peroxide

Article abstract of DOI:10.1524/zpch.2001.215.5.623

The kinetics of the oxidation of quinaldine red, a cationic dye, using hydrogen peroxide has been studied in aqueous solution. The rate of reaction was found to be first order in [quinaldine red] whereas in hydrogen peroxide it varies from one to zero with increasing [H₂O₂]. The rate of reaction increases with increasing pH as well as with the addition of sodium carbonate. The reaction rate attains a limiting value at higher carbonate concentrations which is attributed to the maximum formation of peroxide anions. The rate of hydrogen peroxide decomposition in the presence of sodium carbonate is much greater than that of bleaching of the dye under the same conditions. Moreover, the rate of reaction decreases with increasing ionic strength suggesting an interaction between two singly oppositely charged ions in the activated complex. A plausible reaction mechanism is proposed with a rate equation that is in agreement with the experimental data. by Oldenbourg Wissenschaftsverlag, Muenchen.

Full text of DOI:10.1524/zpch.2001.215.5.623

Zeitschrift für Physikalische Chemie, 215, 5, 623–636 (2001)
by Oldenbourg Wissenschaftsverlag, München

Kinetics and Mechanism of the
Homogeneous Oxidation of Quinaldine
Red by Hydrogen Peroxide
∗
By Ibrahim A. Salem and Mohamed El-Maazawi
Chemistry Department, Faculty of Science, U.A.E. University, Al-Ain, P.O. Box 17551,
U.A.E.
(Received July 3, 2000; accepted in revised form September 21, 2000)
Kinetics / Hydrogen Peroxide / Quinaldine Red / Oxidation /
Ionic Strength
The kinetics of the oxidation of quinaldine red, a cationic dye, using hydrogen peroxide
has been studied in aqueous solution. The rate of reaction was found to be first order in
[
quinaldine red] whereas in hydrogen peroxide it varies from one to zero with increasing
[
H
2
O
2
]. The rate of reaction increases with increasing pH as well as with the addition
of sodium carbonate. The reaction rate attains a limiting value at higher carbonate
concentrations which is attributed to the maximum formation of peroxide anions. The
rate of hydrogen peroxide decomposition in the presence of sodium carbonate is much
greater than that of bleaching of the dye under the same conditions. Moreover, the rate of
reaction decreases with increasing ionic strength suggesting an interaction between two
singly oppositely charged ions in the activated complex. A plausible reaction mechanism
is proposed with a rate equation that is in agreement with the experimental data.
1. Introduction
Oxidation processes are widely applied both in industrial preparations and in
degradation of hazardous pollutants [1–7]. Hydrogen peroxide has a prominent
position, among the oxidants, as a commercially available and cheap reagent of
low molecular mass. It contains a higher amount of active oxygen than other
oxidants and it is friendly oxidizer, since water and oxygen are the product
of its reaction. It can be used directly (uncatalyzed) [8–13], for simple oxi-
dation reactions, or in conjunction with an activator (catalyzed) [14–24], UV
light [25–36], or ultrasound/CuO [37] for advanced and complex oxidation
processes. Hydrogen peroxide, when used directly, oxidizes a range of organic
*
Corresponding author, on leave from Chemistry Department, Faculty of Science,
Tanta University, Tanta, Egypt. E-mail: ibrahims@uaeu.ac.ae, ibrahimsalem@hotmail.com

6
24
Ibrahim A. Salem and Mohamed El-Maazawi
nitrogen compounds to nitroso/nitro, amines, and amide compounds. Alde-
hydes are oxidized to their correspondig acids and organic sulfur compounds
are oxidized to disulfides, sulfonic acids, sulfoxides, and sulfones. However,
the drawbacks with using the uncatalyzed hydrogen peroxide lies in its failure
to oxidize some organic pollutants. Destruction of complex organic pollutants
from waste water in advanced oxidation processes involves the use of hydro-
gen peroxide in conjunction with an activator or UV light. This generates the
highly reactive and nonspecific hydroxyl radical that can oxidize and degrade
many complex organic compounds [14–36]. For the overwhelming majority of
organic substrates, the rate constant for their interaction with OH radicals lies
8
10
−1 −1
in the range of 10 –10 M
s
[38, 39].
Bleaching or removing color from textile dye baths and textile waste wa-
ters is a major problem facing the textile industry. The annual report published
by the National Textile Center at U.S.A. in 1995 demonstrated that, 100 billion
gallons of dye waste water are discharged per year to waste water treatment
systems in the southern United States. Untreated effluents may be highly col-
ored and, thus, particularly objectionable by federal regulations if discharged
into open waters. Different techniques were applied for removing colors, and
hydrogen peroxide was found to be an active oxidant especially when acti-
vated with a catalyst, UV light or both. Recently, some investigations dealing
with the oxidative color removal from organic dyes as model systems have
been reported [11–13, 22–24, 33–36, 40, 41]. For uncatalyzed reactions, the
hydroperoxide anion was reported to be the active species in the oxidation pro-
cesses [11, 12]. This nucleophile can react with electron-deficient alkenes such
as unsaturated ketones, aldehydes and sulfones to give epoxides [1]. This will
destroy the conjugated systems in the dye molecule and therefore removing the
color [11, 12]. Further attack with hydroperoxide anion was found to degrade
the phenolphthalein molecule to phthalic acid and 1,4-hydroquinone [12]. The
effective H O level required for degrading remazol black B was found to be
2
2
adjusted by controlling the initial mass ratio of H O to dye, or the light frac-
2
2
tion absorbed by H O [40]. On the other hand, removing color from congo red
2
2
dye with H O catalyzed by supported zirconium oxide showed a limiting rate
2
2
at higher concentrations of the oxidant as well as at higher pHs [41].
From the literature review cited above, it is clear that little work on the un-
catalyzed oxidation of organic dyes with hydrogen peroxide have been done.
The present work investigates the kinetics of the oxidative bleaching of one
of the cationic dyes, quinaldine red (2-[2-[4-(dimethylamino)phenyl]ethenyl]-
1-ethylquinolinium iodide). Cationic dyes were used initially for dyeing of silk,
leather, paper, and cotton mordanted with tannin, as well as for the production
of ink and copying paper in the office supplies industry. Their importance for
dyeing of textiles increased with the development of synthetic fibers. Cationic
dyes were also employed for printing of cellulose acetate but gained much
greater importance for dyeing of polyacrylonitrile, acid-modified polyester,
and polyamide fibers.

Kinetics and Mechanism of the Homogeneous Oxidation ...
625
2. Experimental
Materials and reagents
All chemicals used were of analytical reagent grade quality from Merck and
Aldrich. H O solutions were prepared by direct dilution and were standard-
2
2
ized iodometrically using sodium thiosulfate. Phosphate buffer (KH PO +
2
4
Na HPO ) was used throughout and few drops of sodium hydroxide solution
2
4
were added for attaining high pHs.
Determination of CO2
In CO free redistilled water (boiled and purged with nitrogen gas before use),
2
known concentrations of both quinaldine red and H O were mixed together
2
2
with the catalyst. CO , that may be evolved during the oxidation reaction, has
2
been trapped in barium hydroxide dissolved in CO free redistilled water [24].
2
Product analysis
−
3
2
.5·10 mole of quinaldine red was dissolved in 50 ml ethanol and com-
pleted with redistilled water to 200 ml. Hydrogen peroxide, 0.05 mole, was
then added and the pH of the mixture was adjusted with buffer to pH = 10.
◦
The mixture was thermostated at 35 C and after complete color removal, the
solution was evaporated. The solid residue was then dissolved in benzene and
the organic materials were extracted with ethyl acetate. Reaction products were
then separated using column chromatography. The isolated oxidation products
were characterized using NMR technique.
Physical measurements
The pH measurements were made on a Mettler Delta 320 pH-meter. The
UV/Vis. spectrophotometric measurements were recorded on a Shimadzu
2101 PC UV/Vis. spectrometer, operating via a Shimadzu data acquisition sys-
tem. Joel model JNM-LA 300 FT-NMR and JNM-LSH 30 solid NMR system
is used for characterizing the organic products.
Kinetic measurements
In a typical kinetic run, one ml of a known concentration of quinaldine red to-
gether with one ml buffer solution were placed in a thermostated quartz UV
cell of the spectrometer. The reaction is started by injecting one ml of H O
2
2
of a known concentration and the course of the reaction was followed by mon-
itoring the decay in absorbance at λmax = 498 nm. The change in the initial
absorbance, (dA/dt) , was determined from the intercept (at the initial ab-
0
sorbance) of the straight lines obtained by plotting (dA/dt) versus absorbance.

6
26
Ibrahim A. Salem and Mohamed El-Maazawi
The initial rate values were obtained by dividing (dA/dt) over the absorbitiv-
0
4
−1
ity of the substrate which is equal to 3.01·10 mol L at λmax = 498 nm.
The ionic strength was varied by the addition of sodium chloride prior to the
addition of hydrogen peroxide. On the other hand, the kinetics of the decom-
position reaction of hydrogen peroxide with sodium carbonate was followed
iodometrically by titrating aliquots at different time intervals. The initial reac-
tion rates were determined from d[H O ]/dt versus [H O ] plots.
2
2
2
2
3. Results and discussion
The stoichiometry of the reaction between hydrogen peroxide and quinaldine
red was determined, via two methods, to be 50 : 1. In the first method, a fixed
−3
concentration of quinaldine red (10 M) was allowed to react with a range of
−
4
concentrations of H O (10 M ≤ [H O ] ≤ 0.1 M) at pH = 8.45. A complete
2
2
2
2
color removal is achieved at [H O ] ≥ 0.05 M. In the second method, known
2
2
concentrations of quinaldine red and H O were mixed at pH = 8.45. After
2
2
complete color removal of quinaldine red, the unreacted H O was then titrated
2
2
and the stoichiometry of the reaction was determined.
13
1
Both C (Fig. 1) and the H-NMR spectra (not included) of the isolated
oxidation products are recorded. Two main products, from the oxidation of
quinaldine red by H O , are identified to be 4-(dimethylamino)benzaldehyde
2
2
and 2-(ethylamino)benzoic acid. This suggests that the oxidation reaction pro-
ceeds presumably according to Scheme 1.
The UV/Vis spectrum of quinaldine red in phosphate buffer is found to
have four absorption bands at λmax = 498, 317, 266, and 233 nm. When H O
2
2
was mixed with quinaldine red, a regular decrease in absorbance at 498 is ob-
served as the reaction progressed. However, the absorbance at λmax = 317 nm
increased and red shifted giving two new bands at 324 and 338 nm as shown in
Fig. 2. The formation of a good isosbestic point at 375 nm suggests an equilib-
rium between the oxidized and the reduced forms of the substrate. At the end
of the reaction, the solution became colorless.
Effect of reactants concentrations
Experiments carried out at constant concentration of quinaldine red equal to
−5
4
.16·10 M and at pH = 8.48, showed that the rate of reaction increases with

Kinetics and Mechanism of the Homogeneous Oxidation ...
627
1
3
Fig. 1. C NMR spectra for the products isolated after a complete reaction between
quinaldine red and hydrogen peroxide.
increasing H O concentration and attains almost a limiting rate at high con-
2
2
centrations as shown in Fig. 3. The order of reaction varied from first-order at
concentrations of H O up to 0.05 M to zero-order at higher concentrations. On
2
2
the other hand, the rate of reaction was found to have a first-order dependence
on the concentration of quinaldine red as demonstrated in Fig. 4. Therefore, the
experimental rate equation can be written in the form of a Michaelis-Menten
equation [22, 42]:
k[Dye] [H O ]
0
2
2 0
Rate =
.
K +[H O ]
m
2
2 0

6
28
Ibrahim A. Salem and Mohamed El-Maazawi
Scheme 1.
The constant K , known as the Michaelis constant, has the dimensions of
m
a concentration.
Effect of pH
An investigation of the pH dependence of the reaction shows that the rate of re-
action increases with increasing pH, especially in the pH range 10–12 as shown
in Fig. 5. No discernible reaction occurred at pH ≤ 6, suggesting that the basic
−
form of hydrogen peroxide, HOO , and the unprotonated form of quinaldine
red are the most reactive species. The rate enhancement at higher pHs was as-
cribed earlier to the enhanced formation of hydroperoxide anion in alkaline
solutions [43, 44]. Above pH = 12, we could not monitor the progress of the re-

Kinetics and Mechanism of the Homogeneous Oxidation ...
629
−5
Fig. 2. UV-VIS absorption spectra recorded with time for the oxidation of 4.16·10
M
◦
quinaldine red by 0.193 M hydrogen peroxide at 30 C and pH = 8.48.
Fig. 3. Variation of the initial rate with the initial concentration of hydrogen peroxide for
the oxidation of 4.16·10− M quinaldine red at pH = 8.48 and at different temperatures.
5
action because of the extremely high rate which requires the use of one of the
fast reaction techniques.
Effect of ionic strength
The effect of ionic strength on reaction rate was investigated in order to obtain
information about the charge numbers of the reacting species that constitute
the activated complex. The ionic strength was varied by the addition of sodium

6
30
Ibrahim A. Salem and Mohamed El-Maazawi
Fig. 4. Dependence of the initial rate on the initial concentration of quinaldine red for its
◦
reaction with hydrogen peroxide at 25 C and pH = 8.48.
Fig. 5. Dependence of the initial rate on the pH for the oxidation of quinaldine red with
hydrogen peroxide.
◦
chloride as an inert salt at 30 C and pH = 9.2. The plot of log k as a function
of ionic strength, according to Bronsted-Debye-Huckel equation [42] showed
√
√
that the rate of reaction decreased linearly with µ/1+ µ, where µ is the
ionic strength, as shown in Fig. 6. The slope of the straight line is equal to
◦
0
.992Z Z at 30 C, where Z and Z are the charge numbers of the two react-
A
B
A
B
ing species. This led to a value of Z Z = −1.02 which means that two singly
A
B
oppositely charged ions are interacting. The rate of oxidation of quinaldine red
by hydrogen peroxide under these conditions conformed to second-order ki-
netics. Therefore, one of the reacting species should be the positively charged
−
substrate and the other one is the hydroperoxide anion, HOO ; not H O . This
2
2
also explains the rate enhancement found with increasing pH due to the in-
crease in concentration of the hydroperoxide anions.

Kinetics and Mechanism of the Homogeneous Oxidation ...
631
Fig. 6. Variation of the rate constant with the ionic strength for the oxidation of quinaldine
red with hydrogen peroxide.
Detection of hydroxyl free radicals
Experiment which carried out for trapping CO with Ba(OH) , that might
2
2
evolve as a result of the oxidation reaction, showed that no CO is evolved. On
2
the other hand, the use of a free-radical trapping agent is a useful method for
discovering whether or not free radicals are implicated in the reaction mech-
anism [12, 45–47]. Radical species react preferentially with the trap and so
a decrease in the rate of the radical reaction should be observed. Tertiary bu-
tanol is known to trap a wide range of radical species and have been used in
peroxide reactions [12, 46, 47]. In the present work, experiments were carried
out in presence of 20% t-butanol as a radical scavenger. The results showed
no rate inhibition, therefore, suggesting that no radicals are involved during the
oxidation of the substrate with H O .
2
2
Reaction mechanism and the rate equation
Based on the results shown in this work, a mechanism which implies the disso-
ciation of H O in the first step with the production of hydroperoxide anions,
2
2
−
HO , is proposed. The latter then attacks the substrate, at the unsaturated dou-
2
ble bond, to give a peroxo-dye intermediate, which in turn decomposes in the
rate-determining step to give the final oxidation products according to the fol-
lowing mechanism:
K1
H O ꢀ HO +H+
−
(1)
(2)
2
2
2
k2
QR +HO2− ꢀ QR(HO )
+
2
k−2
Pe roxo-dye intermediate
k3
QR(HO ) −→ oxidation products.
(3)
2

6
32
Ibrahim A. Salem and Mohamed El-Maazawi
From Eq. (1):
K [H O ]
−
1
2
2
[
HO ] =
.
(4)
2
+
[
H ]
Applying steady state approximation for QR(HO ) assuming that k ≫ k ,
2
2
−2
therefore;
d[QR(HO )]
2
+
−
2
=
k [QR ][HO ]−k [QR(HO )] = 0
2
3
2
dt
+
−
2
k K [QR ][HO ]
2
1
[
QR(HO )] =
.
(5)
(6)
2
+
k [H ]
3
From Eqs. (3) and (5):
+
k K [QR ][H O ]
2
1
2
2
rate =
.
+
[
H ]
Expressing the rate of reaction in terms of the initial concentration of the dye
+
and the oxidant, [QR ] and [H O ] respectively, since:
0
2
2 0
[
H O ] = [H O ] −[QR(HO )]
(7)
(8)
2
2
2
2
0
2
+
+
[
QR ] = [QR ] −[QR(HO )]
0
2
+
+
0
k [H ][H O ] [QR ]
+
3
2
2
0
[QR ][H O ] =
.
(9)
2
2
+
+
k [H ]+k K [QR ] +k K [H O ]
3
2
1
0
2
1
2
2 0
Substituting in Eq. (6):
k k K [H O ] [QR ]
+
0
2
3
1
2
2
0
rate =
.
(10)
+
+
k [H ]+k K [QR ] +k K [H O ]
3
2
1
0
2
1
2
2
0
+
−5
According to our experimental conditions, [QR ] is in the range of 10 and
0
−
12
◦
+
0
since K = 1.5·10 at 25 C [48–50], therefore, the k K [QR ] term can be
1
2
1
omitted from the denominator. Eq. (10) then becomes:
+
k k K [H O ] [QR ]
2
3
1
2
2
0
0
0
rate =
.
(11)
+
k [H ]+k K [H O ]
3
2
1
2
2
As seen from this equation, the rate of reaction attains a first-order dependence
on the substrate, a first-order dependence on hydrogen peroxide only at lower
+
concentrations, and is inversely proportional to [H ]. However, the order of re-
action in H O decreases with increasing concentration and becomes zero at
2
2
very high concentrations. Reciprocal of Eq. (11) gives:
+
1
[H ]
1
=
+
.
(12)
rate k K [QR] [H O ]
k3[QR]0
2
1
0
2
2 0
Values for k and k were estimated (Table 1) from the slope and intercept of
2
3
1
rate
1
versus
plot (Fig. 7) using K values reported in [48–50].
1
[H2O2]0

Kinetics and Mechanism of the Homogeneous Oxidation ...
633
Table 1. Estimated values for the rate constants k
2
and k
3
according to Eq. (12) and Fig. 7.
◦
4
t C
k
2
10 k
3
25
30
35
40
4.31
6.62
9.36
4.13
6.27
7.78
14.01
12.2
Fig. 7. Reciprocal initial rate versus reciprocal initial hydrogen peroxide concentration for
the oxidation of 4.16·10− M quinaldine red at pH = 8.48 according to Eq. (12).
5
Effect of added carbonate
The curve in Fig. 8 shows that the rate of oxidation of quinaldine red, using
H O , increases with increasing concentration of the added sodium carbonate.
2
2
−
−2
However, the rate attained a limiting value at [CO ] ≥ 5.5·10 M. The en-
3
hanced rate in the presence of carbonate is ascribed to the enhanced formation
of the hydroperoxide anions according to the following equations:
KH
2−
+
CO3 +2H ꢀ H CO .
(13)
2
3
log K is equal to −11.82 which is very small as compared to the first (log K )
1
H1
and the second (log K ) protonation constants of the carbonate anion. Their
H2
◦
reported values are equal to 10.33 and 6.35, respectively, at 25 C [51]. This
means that all protons released in step 1 should be captured by carbonate ions
in the next step. The limiting rate observed at high carbonate concentrations

6
34
Ibrahim A. Salem and Mohamed El-Maazawi
−5
Fig. 8. Effect of added sodium carbonate of the initial rate for the oxidation of 4.16·10
quinaldine red by 0.193 M hydrogen peroxide.
M
Fig. 9. Decomposition – time curves for the reaction of 0.193 M hydrogen peroxide with
.15 M sodium carbonate at pH = 9.4.
0
is corresponding to the maximum formation of hydroperoxide ions, which are
the active species in the oxidation process [9]. The curves in Fig. 9 show the
decomposition of 0.193 M H O using 0.15 M sodium carbonate at pH = 9.4
2
2
and at different temperatures. The rate of decomposition of H O by carbon-
2
2
ate was found to be 100 times greater than the rate of oxidation of quinaldine
−
red under the same conditions. This means that, the rate of production of HO2
according to Eq. (1) is greater than the rate of their consumption by the sub-
strate. Therefore, step 3 should be the rate-determining step as indicated in the

Kinetics and Mechanism of the Homogeneous Oxidation ...
635
above mechanism. Otherwise, if step 2 is the rate-determining step, one can ex-
pect that the two rates, decomposition of H O and oxidation of the substrate,
2
2
to be equal.
4. Conclusion
Hydrogen peroxide caused the oxidation and color removal of quinaldine red
dye without the use of any activator. Hydroperoxide anion and the unpro-
tonated form of the dye are the active species in the reaction. 4-(Dimethyl-
amino)benzaldehyde and 2-(ethylamino)benzoic acid are the two main prod-
13
1
ucts isolated and characterized by C and H-NMR measurements. The reac-
tion rate increases with increasing pH as well as with the addition of sodium
carbonate. This was ascribed to the enhanced formation of the hydroperoxide
anion. No free radicals were detected using t-butanol as scavenger. The rate
of reaction decreased with increasing ionic strength suggesting that both the
positively charged substrate and the hydroperoxide anion is involved in the
formation of the activated complex.
Acknowledgement
The financial support of the research council at the United Arab Emirates Uni-
versity is gratefully acknowledged. I also acknowledge Ms. Eman Al-Ameri
and Ms. Shaikha Al-Neyadi for their technical assistance.
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