RuCl3 Catalyzed and Uncatalyzed Oxidative Decolorization of Acid Orange 7 Dye with Chloramine-B in Acid Medium: Spectrophotometric, Kinetic and Mechanistic Study

Article abstract of DOI:10.1007/s10562-015-1526-3

Acid orange 7, chemically known as sodium 4-[(2E)-2-(2-oxonaphthalen-1-ylidene)hydrazinyl]benzenesulfonate, is extensively used for dyeing textiles, paper and leather. The discharge of wastewater containing this dye, causes environmental and health related problems. Therefore, in the present research, we have developed optimum conditions for the facile oxidative decolorization of this dye with sodium N-chlorobenzenesulfonamide or chloramine-B (CAB). The kinetics and mechanism of oxidative decolorization of acid orange 7 dye with CAB in acidic medium have also been studied spectrophotometrically at 303 K in the presence and absence of RuCl₃ catalyst. Under similar experimental conditions, the reaction exhibits a first-order dependence of rate each on [CAB]_o and [dye]_o, and an inverse-fractional-order dependence on [H⁺] for both the RuCl₃ catalyzed and uncatalyzed reactions. The order with respect to RuCl₃ is fractional. Activation parameters have been computed. Dielectric effect is negative in both the cases. Oxidation products of the acid orange 7 dye are identified as 1,2-naphthoquinone and benzenesulfonic acid by GC-MS data. The RuCl₃ catalyzed reaction is about four fold faster than the uncatalyzed reaction. The chemical oxygen demand value of the dye was determined. The mechanistic pathways and kinetic modelings have been computed based on experimental results. The developed oxidative decolorization method is expected to be helpful to treat acid orange 7 dye present in wastewater after suitable modifications. (Chemical Equation Presented).

Full text of DOI:10.1007/s10562-015-1526-3

Catal Lett
DOI 10.1007/s10562-015-1526-3
RuCl3 Catalyzed and Uncatalyzed Oxidative Decolorization
of Acid Orange 7 Dye with Chloramine-B in Acid Medium:
Spectrophotometric, Kinetic and Mechanistic Study
Adalagere Somashekar Manjunatha1 Puttaswamy1
•
Received: 8 January 2015 / Accepted: 6 April 2015
Ó Springer Science+Business Media New York 2015
Abstract Acid orange 7, chemically known as sodium
4-[(2E)-2-(2-oxonaphthalen-1-ylidene)hydrazinyl]benzene-
sulfonate, is extensively used for dyeing textiles, paper and
leather. The discharge of wastewater containing this dye,
causes environmental and health related problems. There-
fore, in the present research, we have developed optimum
conditions for the facile oxidative decolorization of this dye
with sodium N-chlorobenzenesulfonamide or chloramine-B
(CAB). The kinetics and mechanism of oxidative decol-
orization of acid orange 7 dye with CAB in acidic medium
have also been studied spectrophotometrically at 303 K in
the presence and absence of RuCl3 catalyst. Under similar
experimental conditions, the reaction exhibits a first-order
dependence of rate each on [CAB]o and [dye]o, and an in-
verse-fractional-order dependence on [H?] for both the
RuCl3 catalyzed and uncatalyzed reactions. The order with
respect to RuCl3 is fractional. Activation parameters have
beencomputed. Dielectric effect is negative in both thecases.
Oxidation products of the acid orange 7 dye are identified as
1,2-naphthoquinone and benzenesulfonic acid by GC–MS
data. The RuCl3 catalyzed reaction is about four fold faster
than the uncatalyzed reaction. The chemical oxygen demand
value of the dye was determined. The mechanistic pathways
and kinetic modelings have been computed based on ex-
perimental results. The developed oxidative decolorization
method is expected to be helpful to treat acid orange 7 dye
present in wastewater after suitable modifications.
Graphical Abstract The stoichiometry of the reaction is
in the mole ratio of 1:1(AO7:CAB) in both the cases as
shown given below
OH
O
Cl
N
SO3H
N
S
N
+
H2O
Na
O
Chloramine-B
Acid Orange 7
H+ / RuCl3
O
O
S
O
+
SO3H
Benzenesulfonic acid
NH2 + NaCl
+
N2
+
O
Benzenesulfonamide
1,2 napthoquinone
Keywords Chloramine-B Á Acid orange 7 Á
RuCl3-catalysis Á Oxidative-decolorization Á Kinetics-
mechanism
1 Introduction
Acid orange 7 is chemically known as sodium 4-[(2E)-2-
(2-oxonaphthalen-1-ylidene)hydrazinyl]benzenesulfonate
and it is widely used for dyeing textiles, paper and leather [1,
2]. The industrial wastewater bearing this dye causes envi-
ronment and health problems. Oxidation decolorization is
considered a simple and economic technology for the re-
moval of dyestuffs from wastewater. Though there are many
reports in literature on the oxidative decolorization of this
dye [3–5], similar studies have not yet been carried out by
keeping its kinetic and mechanistic aspects. The chemistry
of organic N-haloamines is of great interest due to their
diverse behavior [6]. The important chloramine compounds
of this class are sodium N-chloro-p-toluenesulfonamide or
& Puttaswamy
pswamy_chem@yahoo.com
1
Department of Chemistry, Bangalore University, Central
College Campus, Bangalore 560 001, India
123

A. S. Manjunatha, Puttaswamy
chloramine-T (CAT) and sodium N-chlorobenzenesulfon-
amide or chloramine-B (CAB). The mechanistic aspects of
many of these reactions have been documented [7–16]. The
decolorization of acid orange 7 dye was tested with both
CAT and CAB in acid medium. It was observed that the
decolorization of this dye is most effective with CAB.
To the best of our knowledge, the impact of platinum
group metal ions on the oxidation of acid orange 7 dye has
not yet been investigated. Among platinum group metal ions,
ruthenium (III) chloride (RuCl3) has been extensively em-
ployed as a homogeneous catalyst in redox reactions [17–19]
and some of these systems have proved suitable for kinetic
analysis [20–23]. Our preliminary experimental results
indicated that a micro-quantity of RuCl3 effectively catalyzes
the oxidation of acid orange 7 dye with CAB in acid medium.
Therefore, in the present work, we have explored the kinetics
and mechanism of the acid orange 7—CAB redox system in
acid medium in the absence and presence of RuCl3 catalyst.
The other objective of the present study is to optimize the
reaction conditions for the efficient decolorization of acid
orange 7 dye with CAB in acid medium.
amount of acid present in the catalyst solutions while preparing
reaction mixtures for kinetic runs. All the chemicals used were
of analytical grade and used as such. All the solutions were
prepared using double distilled water.
2.2 Kinetic Measurements
Kinetic measurements were carried out using a UV–Vis
spectrophotometer. (SL 159. Elico Ltd). Kinetic runs were
performed under pseudo-first-order conditions by keeping an
excess of CAB over dye in HClO4 medium with and without
RuCl3 at 303 K and the detailed procedure which was fol-
lowed is similar to that reported earlier [25]. Absorbance
measurements were made at 484 nm (kmax for the dye) for
more than two half-lives. The absorbance readings at t = 0
and t = t are D0 and Dt and plots of log Do/Dt versus time
were made to evaluate the pseudo-first-order rate constants
(k0 s-1). All the kinetic runs were carried out twice to check
the reproducibility, and was found to be reproducible within
5 % error. Calculations of the regression coefficients (R2)
were carried out using fx—100 W scientific calculator.
2 Experimental and Methods
2.1 Materials and Reagents
3 Results
Chloramine-B was obtained from Sigma and purified by the
method of Morris et al. [24]. An aqueous solution of CAB was
prepared afresh whenever required, standardized by iodometric
method and stored in brown bottles until further use to prevent
its any photochemical deterioration. Acid Orange 7 (SD Fine
Chem. Ltd., India) was used as received and an aqueous solu-
tion of the desired strength of the dye was prepared afresh each
time. A solution of RuCl3 (Merck) was prepared in 20 mM HCl
and employed as a catalyst. Allowance was made for the
3.1 Reaction Stoichiometry and Product Analysis
The stoichiometry of the reaction was determined by
equilibrating varying ratios of [CAB]o and [dye]o in pres-
ence of 3.0 9 10-3 mol dm-3 HClO4 (2.0 9 10-5
mol dm-3 RuCl3 for the catalyzed reaction) at 303 K for
24 h Determination of the residual oxidant showed that the
stoichiometry of the reaction is in the mole ratio of 1:1 in
both the cases as shown in Eq. 1.
OH
O
Cl
N
SO3H
N
S
N
+
H2O
Na
O
Chloramine-B
Acid Orange 7
H+ / RuCl3
ð1Þ
O
O
S
O
SO3H
Benzenesulfonic acid
+
NH2 + NaCl +
N2
+
O
Benzenesulfonamide
1,2-napthoquinone
123

RuCl3 Catalyzed and Uncatalyzed Oxidative Decolorization of Acid Orange 7 Dye with…
The reaction mixture in the stoichiometric ratio was al-
lowed to progressfor 24 h inthepresenceofHClO4 (RuCl3 for
thecatalyzed reaction) at 303 K under stirred conditions. After
completion of the reaction (monitored by TLC), the reaction
products were neutralized with dilute NaOH and extracted
with ethyl acetate. Separation of these products was achieved
using silica gel (60–100 mesh) column chromatography with
hexane/ethyl acetate (8:6, v/v) as mobile phase. The oxidation
products were identified as 1,2-naphthoquinone and ben-
zenesulfonic acid for the RuCl3 catalyzed and uncatalyzed
reactions. These are confirmed by GC–MS analysis. GC–MS
data were obtained on a 17A Shimadzu gas chromatograph
with a QP-5050A Shimadzu mass spectrometer. The mass
spectra showed a molecular ion peak at 158 and 158 amu,
clearly conforming 1,2-naphthoquinone and benzenesulfonic
acid, respectively. It was also noticed that there was no further
reaction of these oxidation products under the present set of
experimental conditions. Benzenesulfonamide (BSA or
PhSO2NH2), the reduction product of CAB, was detected [26]
by TLC using light petroleum-chloroform-butan-1-ol (2:2:1
v/v/v)as the solvent and iodinefordetection. The Rf value was
found to be 0.88, which is in good agreement with the literature
value [26]. Nitrogen was detected by the conventional test.
investigated spectrophotometrically at several initial con-
centrations of the reactants in HClO4 medium with and
without RuCl3 catalyst. The standard experimental condi-
tions established for the facile oxidative decolorization of
the dye are: [CAB]o = 3.0 9 10-3 mol dm-3, [dye]o =
2.0 9 10-4 mol dm-3, [HClO4] = 3.0 9 10-3 mol dm-3
,
and [RuCl3] = 2.0 9 10-5 mol dm-3 (in case of catalyzed
reaction) at T = 303 K. Under these conditions, CAB de-
colorizes the dye completely within 25 and 90 min in the
presence and absence of RuCl3 catalyst. The pseudo-first
order rate constants obtained for standard run in the pres-
ence and absence of RuCl3 catalyst are 16.2 9 10-4 and
3.80 9 10-4 s-1, respectively. Hence, it can be said that
RuCl3 acts as an effective catalyst for the facile oxidative
decolorization of acid orange-7 by CAB in acid medium.
Unless otherwise noted, the other attributes of the reactions
were unchanged by the presence of RuCl3.
The kinetic runs were performed under pseudo-first-order
conditions of [CAB]O ꢀ [dye]O in both the cases. At con-
stant [CAB]O, [HClO4], RuCl3 (in case of catalyzed reaction)
and temperature, plots of log(absorbance) versus time were
linear (R2 [ 0.9884), indicating a first-order dependence of
rate on [dye]o for both RuCl3 catalyzed and uncatalyzed re-
actions. The values of pseudo-first order rate constants
(k0 s-1) were found to be independent of [dye]o, confirming
the first-order dependence of rate on [dye]o. The values are
given in Table 1. The rate increased with the increase in
[CAB]o (Table 1) in both the cases and log–log plots of rate
3.2 Kinetic Orders
The kinetics of oxidative decolorization of acid orange-7
(hereafter abbreviated as dye) with CAB has been
Table 1 Effect of varying
[CAB]o 9 103
[AO7]o 9 104
[HClO4] 9 103
[RuCl3] 9 105
(mol dm-3
)
k0 9 104 (s-1
)
[CAB]0, [Dye]0, [HClO4] and
[RuCl3] on the rate of reaction
at 303 K
(mol dm-3
)
(mol dm-3
)
(mol dm-3
)
Uncatalyzed
RuCl3
catalyzed
0.5
1.0
3.0
6.0
10.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
2.0
2.0
2.0
2.0
2.0
0.5
1.0
2.0
4.0
8.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
0.5
1.0
3.0
6.0
12.0
3.0
3.0
3.0
3.0
3.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
0.5
1.0
2.0
4.0
8.0
0.52
1.64
3.80
11.2
16.5
3.79
3.81
3.80
3.82
3.81
18.6
14.8
3.80
2.43
1.32
–
2.52
5.75
16.2
32.0
53.3
16.2
16.1
16.2
16.3
16.1
31.4
25.2
16.2
10.2
7.81
8.06
11.3
16.2
23.2
30.7
–
–
–
–
123

A. S. Manjunatha, Puttaswamy
3.0
2.5
2.0
1.5
1.0
0.5
1.5
1.4
1.3
1.2
1.1
1.0
0.9
Uncatalyzed
Catalyzed
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
6+log[RuCl3]
Fig. 3 Plots of log k0 versus log [RuCl3]
4+log[CAB]
Fig. 1 Plots of log k0 versus log [CAB]0
Table 2 Effect of varying dielectric constant of the medium on the
rate of reaction at 303 K
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Uncatalyzed
Catalyzed
% MeOH (v/v)
D
k09 104 (s-1
)
Uncatalyzed
RuCl3 catalyzed
0
76.73
72.37
67.48
62.71
3.80
2.69
1.65
0.93
16.2
14.8
12.3
10.1
10
20
30
Experimental
conditions:
[CAB]o = 3.0 9 10-3 mol dm-3
;
;
[dye]o = 2.0 9 10-4 mol dm-3; [HClO4] = 3.0 9 10-3 mol dm-3
and in catalyzed reaction [RuCl3] = 2.0 9 10-5 mol dm-3
slope of log k0 versus log [RuCl3] plot (Fig. 3; R2 = 0.9990)
was found to be 0.48, indicating a fractional-order depen-
dence of rate on [RuCl3]. The dielectric constant of the
medium was varied using different amounts of methanol and
H2O (0–30 % v/v). The reaction rate decreases with an in-
crease in methanol content in both the cases (Table 2). Plots
of log k0 versus 1/D were found to be linear (Fig. 4;
R2 [ 0.9960) with negative slopes. Values of dielectric
constant of methanol–water mixture reported in literature
were employed [27]. Blank experiments performed showed
that MeOH was not oxidized significantly by CAB under the
prevailing experimental conditions (without substrate). No
attempt was made to keep the ionic strength of the system
fixed for both the reactions, since the pseudo-first-order rate
constants remains unaltered in presence of 0.2 mol dm-3
NaClO4 solution.
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
4+log [HClO4]
Fig. 2 Plots of log k0 versus log [HClO4]
versus [CAB] were linear (Fig. 1; R2 [ 0.9926) with a slope
of unity. This ensures that the kinetics exhibits a first-order
dependence on [CAB]o. Further, plots of k0 versus [CAB]
were linear (R2 [ 0.9939) passed through the origin, con-
firming the first-order dependence of rate on [CAB]o and also
showing the transient nature of the intermediate formed with
the dye.
The reaction rate decreased with increase in [HClO4]
(Table 1) and plots of log k0 versus log [HClO4] were linear
(Fig. 2; R2 [ 0.9904) with fractional slopes of -0.50 and
-0.86 for RuCl3 catalyzed and uncatalyzed reactions,
respectively. This clearly indicates a negative fractional-
order dependence of rate on [HClO4] in both the cases. The
rate increased with an increase in [RuCl3] (Table 1) and the
Addition of 5.0 9 10-3 mol dm-3 benzenesulfonamide
(BSA or PhSO2NH2), the reduction product of CAB, to the
reaction mixture showed negligible influence on the rate of
the reaction in both the cases. It signifies that the BSA is not
123

RuCl3 Catalyzed and Uncatalyzed Oxidative Decolorization of Acid Orange 7 Dye with…
pronounced effect on the rate of the reaction, indicating that
Uncatalyzed
Catalyzed
2.2
2.0
1.8
1.6
1.4
1.2
1.0
the Cl- ion plays no role in the reaction sequence. Acti-
vation parameters (Ea, DH=, DG=, DS= and log A) for the
reaction have been evaluated from Arrhenius plots of log k0
versus 1/T (Fig. 5; R2 [ 0.9939) by studying the reactions
at different temperatures (293–313 K). These results are
summarized in Table 3. On adding a small amount of the
reaction mixture to acrylonitrile, there was no polymeriza-
tion indicating the absence of free radical species in the
reaction sequence.
4 Discussion
0.0130 0.0135 0.0140 0.0145 0.0150 0.0155 0.0160
As CAT and CAB exhibit similar chemical properties, it is
expected that identical equilibria exist in aqueous solutions
of these compounds [28, 29]. In general, CAB undergoes a
two electron change in its reactions [30]. The redox po-
tential of CAB/BSA couple is pH dependent and decreases
with increase in pH of the medium [30].
1/D
Fig. 4 Plots of log k0 versus 1/D
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Uncatalyzed
Catalyzed
Catalytic constant
(b)
4.1 Reactive Species of CAB
Chloramine-B (PhSO2NClNa) behaves as a strong elec-
trolyte [28] and depending on the pH of the medium, it
furnishes different equilibria in aqueous solutions [24, 28,
29]. The possible oxidizing species in acidified CAB so-
lutions are PhSO2NHCl, PhSO2NCl2, HOCl and possibly
H2O?Cl, and in alkaline solutions they are PhSO2NHCl,
PhSO2NCl-, HOCl and OCl-. The present redox system
was carried out in acid medium and hence from the four
possibilities, the reactive species of CAB can be decided
based on the observed kinetic data.
(a)
(a)
3.15
3.20
3.25
3.30
103/T
3.35
3.40
3.45
Fig. 5 Plots of (a) log k0 versus 1/T and (b) log KC versus 1/T
4.2 Tautomerism of Acid Orange 7 Dye
involved in any step prior to the rate determining steps (rds)
in the reaction schemes. Similarly, addition of Cl- ion in the
form of NaCl (2.0 9 10-3 mol dm-3) does not have any
Azo dyes, such as Acid Orange 7, containing hydroxyl
groups conjugated to azo group [31] exhibit azo-hydrazone
tautomerism as shown below:
O
H
N
OH
N
SO3H
N
N
SO3H
ð2Þ
(Azo)
(Hydrazone)
123

A. S. Manjunatha, Puttaswamy
Kc
Table 3 Temperature
Temperature (K)
k0 9 104 (s-1
)
dependence and activation
parameters for the oxidative
decolorization of AO 7 by CAB
in acid medium in the presence
and absence of RuCl3 catalyst
Uncatalyzed
RuCl3-catalyzed
293
298
303
308
313
1.79
8.02
0.14
2.35
11.8
0.21
3.80
16.1
0.27
6.14
27.1
0.47
7.92
35.2
0.61
Ea (kJ mol-1
)
70.1
55.3
55.0
DH= (kJ mol-1
DG= (kJ mol-1
DS= (JK-1 mol-1
)
)
67.8 ( 0.06)
94.1 ( 0.20)
52.8 ( 0.01)
90.4 ( 0.15)
-124 ( 0.09)
11.5 ( 0.01)
52.4 ( 0.01)
52.5 ( 0.01)
-82.6 ( 0.01)
13.7 ( 0.02)
)
-87.6 ( 0.04)
Log A
12.0 ( 0.27)
Experimental conditions: [CAB]o = 3.0 9 10-3 mol dm-3; [dye]o = 2.0 9 10-4 mol dm-3; [HClO4] =
3.0 9 10-3 mol dm-3; and in catalyzed reaction [RuCl3] = 2.0 9 10-5 mol dm-3
In the present studies, the azo form of acid orange 7 is
involved in the reaction.
K1
PhSO2N+H2Cl
PhSO2NHCl
+
PhSO NHCl
+ H
(i)
2
fast
k2
(ii)
+ dye
X
slow & rds
fast
4.3 Reactive Species of RuCl3
k3
(iii)
X
Products
RuCl3 catalysis in redox reactions involves different degrees
of complexicity, due to formation of different intermediate
complexes, free radicals and its variable oxidation states.
Cady and Connick [32], and Connick and Fine [33] have in-
vestigated aqueous RuCl3 complex species using the ion ex-
change resins and UV-spectral studies. They found that the
octahedral complex species [RuCl5(H2O)]2-, [RuCl3(H2O)3],
[RuCl2(H2O)4]? and [RuCl(H2O)5]2? may not exist in aque-
ous solution of RuCl3. Other studies [17, 34, 35] have shown
the existence of following equations for RuCl3 in acidic
solutions:
Scheme 1 A general reaction Scheme for the oxidative decoloriza-
tion of acid orange 7 with CAB in acid medium
solutions of comparable molarities and have shown that
PhSO2NHCl is the likely oxidizing species in acid medium.
Narayanan and Rao [36] and, Subhashini et al. [37] have
reported that CAB can further be protonated at pH \ 2 to
give PhSO2N?H2Cl. In the present case, protonated oxi-
dant (PhSO2N?H2Cl) generates the free conjugate acid
(PhSO2NHCl) in the acid retarding step and hence
PhSO2NHCl is the active oxidizing species. In view of
these points, Scheme 1 can be formulated for the oxidation
of acid orange 7 dye with CAB in acidic medium.
RuCl3xH2O þ 3HCl ! ½RuCl6Š3À þ xH2O þ 3Hþ
½RuCl6Š3À þ H2O ꢀ ½RuCl5ðH2Oފ2À þ ClÀ
ð3Þ
ð4Þ
Here X is the complex intermediate species whose
structure is shown in Scheme 2. In Scheme 2, in an initial
equilibrium [step(i)], deprotonation of PhSO2N?H2Cl
generates the conjugate free acid PhSO2NHCl. In the next
slow and rds [step(ii)], the azo form of the dye reacts with
the conjugate acid of the oxidant to form the dye-CAB
complex (X) with the elimination of PhSO2NH2. The
complex X in the presence of a molecule of water through
the several fast steps [step(iii)] yields the ultimate products
viz., 1,2-naphthoquinone and benzenesulfonic acid with the
elimination of a molecule each of HCl and N2.
In the present study, the absence of chloride ion on the
rate indicates that the equilibrium (4) does not play any
role in the reaction. Hence, the complex ion, [RuCl5
(H2O)]2-, is assumed to be the reactive catalyst species.
4.4 Reaction Scheme and Rate Law for Uncatalyzed
Reaction
The probable reactive species in acidic solutions of CAB
are PhSO2NHCl, PhSO2NCl2, HOCl and possibly H2O?Cl.
The first-order dependence of rate on [CAB]o and the ad-
dition of PhSO2NH2 (benzenesulfonamide) having no ef-
fect on the reaction rate both indicate that PhSO2NCl2 and
HOCl may not be the reactive species. Hardy and Johnston
[29], who have studied the pH-dependent relative con-
centrations of the species present in acidified CAB
If [CAB]t is the total effective concentration of CAB,
then based on Scheme 1, we can write
½CABŠt ¼ ½PhSO2NþH2ClŠ þ ½PhSO2NHClŠ
From step (i) of Scheme 1,
ð5Þ
½PhSO2NHClŠ½HþŠ
½PhSO2NþH2ClŠ ¼
ð6Þ
K1
123

RuCl3 Catalyzed and Uncatalyzed Oxidative Decolorization of Acid Orange 7 Dye with…
Scheme 2 A detailed
K1
fast
PhSO2NHCl + H+
i) PhSO2N+H2Cl
ii) PhSO2NHCl
mechanistic interpretation for
the oxidative decolorization of
acid orange 7 with CAB in acid
medium
OH
N=N
O
Cl
k2
SO3H
N=N
SO3H
+
slow & rds
+
PhSO2NH2
(X)
O
Cl
O
Cl
N
k3
iii)
SO3H
N
N=N
SO3H
fast
(X)
..
H2O
O
O
O
Cl
N
H
N
H
SO3H
O
N=N
SO3H
+
H
-HCl
1,2 napthoquinone
-N2
SO3H
Benzenesulfonic acid
On substituting the value of PhSO2N?H2Cl from Eq. 6 into
Eq. 5, and solving for PhSO2NHCl, we get
K4
PhSO2N+H2Cl
PhSO2NHCl
+
PhSO NHCl
+ H
(i)
2
fast
K5
X/
(ii) fast
+ RuCl3
k6
K1½CABŠt
½PhSO2NHClŠ ¼
ð7Þ
½HþŠ þ K1
From slow/rds of Scheme 1,
(iii) slow & rds
(iv) fast
X/
X//
+ dye
X//
k7
Products
Rate ¼ k2½PhSO2NHClŠ½dyeŠ
On substituting for [PhSO2NHCl] from Eq. 7 into Eq. 8,
the rate of reaction is given as
ð8Þ
Scheme 3 A general reaction Scheme for the RuCl3 catalyzed
oxidative decolorization of acid orange 7 dye with CAB in acid
medium
order dependence of rate on [H?]. The proposed mechan-
ism for RuCl3 catalyzed oxidation of acid orange 7 by CAB
in acidic medium is presented in Scheme 3.
K1k2½CABŠt½dyeŠ
Rate ¼
ð9Þ
½HþŠ þ K1
Rate law 9 holds good agreement with the experimental
results, wherein a first-order dependence of rate each on
[CAB]o and [dye]o, and an inverse-fractional order on [H?]
was noted.
Here X0 and X00 represent the complex intermediate
species whose structures are shown in Scheme 4. In
Scheme 4, in the initial equilibrium [step(i)] deprotona-
tion of PhSO2NH2Cl? generates the conjugate free acid
PhSO2NHCl. In the next fast pre-equilibrium step [ste-
p(ii], the donor nitrogen atom of the oxidizing species
coordinates to the metal centre of the active catalyst
species and gives an intermediate complex X0. In the next
slow and rds step [step (iii)], another intermediate com-
plex X00 is formed from the reaction between X0 and dye.
4.5 Reaction Scheme and Rate Law for RuCl3
Catalyzed Reaction
The reactive oxidizing species in this case also PhSO2
NHCl, which accounts for the observed inverse-fractional-
123

A. S. Manjunatha, Puttaswamy
Scheme 4 A detailed
O
mechanistic interpretation for
RuCl3 catalyzed oxidative
decolorization of acid orange 7
dye with CAB in acid medium
O
SO3H
+
[RuCl6]3- + H2O
-Cl-
PhSO2N+H2Cl
(i)
Benzenesulfonic acid
-N2
(fast)
K4
(fast)
1,2 napthoquinone
K5
[RuCl5(H2O)]-2
PhSO2NHCl + H+
-HCl
PhSO2NH2
PhSO2NHCl
(ii)
k7
[RuCl5(H2O)]-2
(X/)
H2O
k6 (slow & rds)
(iii)
AO 7 dye
O
SO3H
N
N
(X/)
(X//)
Finally, the complex X00 undergoes decomposition
through several fast steps in the presence of H2O, yielding
the final products 1,2-naphthoquinone and benzenesul-
fonic acid with the elimination of PhSO2NH2, RuCl3, HCl
and N2.
Rate law 13 is in accordance with the experimental find-
ings. The proposed schemes and the derived rate laws in
both the cases are also substantiated by the experimental
observations discussed below:
Evidence for the formation of complex between oxidant
and catalyst is obtained from the UV–Visible spectra of
CAB, RuCl3 and the mixture of both. Absorption maxima
appear at 224 nm for CAT, 215 nm for RuCl3 in aqueous
acidic medium, and 230 nm for their mixture. A bath-
ochromic shift of 15 nm from 215 to 230 nm of RuCl3
suggests that complexation occurs between CAB and
RuCl3 in the present case.
4.6 Effect of Dielectric Constant
In order to find out the nature of reactive species, the di-
electric constant (D) of the medium was varied by adding
different amounts of methanol (0–30 % v/v) to the reaction
mixture. The rate decreased with increasing methanol
content (Table 2). Several approaches [38–41] have been
made to explain quantitatively the effect of dielectric con-
stant of the medium on the rates of reactions in solutions.
Amis and Jaffe [38] predicted a linear relation between log
k0 versus 1/D. The slope of such a plot should be negative
for a reaction between a negative ion and a dipole or be-
tween two dipoles, while a positive slope obtained for
positive ion–dipole reactions. In the present investigations,
plots of log k0 versus 1/D were linear with negative slopes in
both cases, thus supporting the participation of the two
dipoles in the rate-determining steps (Schemes 2, 4).
Based on the above Scheme 3, the total CAB concen-
tration is
½CABŠ ¼ ½PhSO2NþH2ClŠ þ ½PhSO2NHClŠ þ ½X0Š
ð10Þ
t
By substituting for [PhSO2N?H2Cl] and [PhSO2NHCl] from
steps (i) and (ii) of Scheme 3 and solving for X0, we get
K4K5½CABŠt½RuCl3Š
½X0Š ¼
ð11Þ
½HþŠ þ K4 þ K4K5½RuCl3Š
From slow/rds of Scheme 3,
Rate ¼ k6½dyeŠ½X0Š
ð12Þ
4.7 Effect of Ionic Strength
By substituting for [X0] from Eq. 11 into Eq. 12 we get
The proposed reaction mechanisms are also evinced by the
observed zero effect of ionic strength on the rate of the re-
action. The primary salt effect on the reaction rates has been
K4K5k6½CABŠt½dyeŠ½RuCl3Š
Rate ¼
ð13Þ
½HþŠ þ K4 þ K4K5½RuCl3Š
123

RuCl3 Catalyzed and Uncatalyzed Oxidative Decolorization of Acid Orange 7 Dye with…
4.10 Activation Parameters
described by Bronsted and Bjerrum theory [42]. In the pre-
sent investigations, in both the cases, neutral molecules are
involved in the rate determining steps of Schemes 2 and 4.
Hence, variation of the ionic strength of the medium does not
alter the rate in both the cases clearly conform to the above
theory [42].
The variation of rate constants with temperatures and val-
ues of activation parameters are shown in Table 3. Values
of DH= indicates that the reactions are enthalpy controlled.
The more positive values of DG= points out a highly
solvated transition state. The large negative values of DS=
signifies that the transition state is more rigid than the
initial state with less degrees of freedom. The values of
frequency factor (log A) specify the frequency of collisions
and orientation of the reaction molecules.
4.8 Catalytic Activity of RuCl3
The general equation relating for uncatalyzed and cat-
alyzed reactions have been derived from Moelwyn –
Hughes [43] and can be correlated as:
x
4.11 Determination of Chemical Oxygen Demand
Value
k1 ¼ ko þ KC½catalystŠ
ð14Þ
Here k1 is the observed pseudo-first-order rate constant
obtained in the presence of RuCl3 catalyst, ko is that for the
uncatalyzed reaction, KC is the catalytic constant and x is
the order of the reaction with respect to RuCl3. In the
present case, x value for the standard run was found to be
0.48 for RuCl3 catalyst. Then the Kc has been evaluated
using the equation:
In the present research, an effort has been made to determine
the chemical oxygen demand (COD) for acid orange 7 dye.
COD is a measure of oxidizable matter in dye stuff. The
COD of acid orange 7 dye was determined using the stan-
dard dichromate method. The procedure followed to deter-
mine COD value was according to a literature procedure
[44]. Under the prevailing experimental conditions, COD of
acid orange 7 dye sample was found to be 1168.10 mg/lit.
0:48
KC ¼ ðk1ÀkoÞ=½RuCl3Š
ð15Þ
The values of KC have been evaluated at different tem-
peratures (293, 298, 303, 308 and 313 K), and Kc was
found to vary with temperature. Further, a plot of log KC
versus 1/T was linear (Fig. 5; R2 = 0.9972) and values of
energy of activation and other activation parameters with
respect to RuCl3 catalyst were evaluated. All these results
are tabulated in Table 3.
5 Conclusion
Different kinetic conditions were experimented and better
optimum conditions were established, for the facile ox-
idative decolorization of acid orange 7 dye with CAB. The
rate laws obtained are: -d[CAB]/dt = k[CAB]o [dye]o
[H?]-0.50 [RuCl3]0.48 for RuCl3 catalyzed reaction and
-d[CAB]/dt = k[CAB]o [dye]o [H?]-0.86 for uncatalyzed
reaction. Activation parameters have been evaluated. 1,2-
naphthoquinone and benzenesulfonic acid were identified
as the oxidation products of acid orange 7 dye. RuCl3
catalyzed reaction is about four-fold faster than the un-
catalyzed reaction. Based on the kinetic results, reaction
mechanisms and rate laws have been worked out. The COD
value of acid orange 7 dye was also determined. The pre-
sent redox-system can be adopted for treating acid orange 7
dye present in industrial wastewater with suitable modifi-
cations to minimize the toxicity caused by it.
4.9 Comparison of RuCl3 Catalyzed
and Uncatalyzed Reactions
RuCl3 catalyzed oxidative decolorization of the dye by
CAB with that of uncatalyzed reaction (without RuCl3
catalyst) under an identical set of experimental conditions
was compared. The observed rates of oxidative decol-
orization of the dye in the presence of RuCl3 catalyst re-
vealed that the reactions are about four-fold faster than the
uncatalyzed reactions (Table 3). This was also confirmed
by the calculated activation energies. The difference in
activation energies for the catalyzed and uncatalyzed re-
actions explained the catalytic effect on the rate of the
reaction. This may be attributed to the formation of the
intermediate complex X0 between RuCl3 and the oxidant,
which increases the oxidizing property of the oxidant than
without RuCl3 catalyst. Further, RuCl3 favorably modifies
the reaction path by stabilizing the transition state, which in
turn provides an alternative pathway having lower activa-
tion energy for the reaction. Consequently, it can be said
that RuCl3 is an efficient catalyst for the present redox
system.
Acknowledgments The authors greatly acknowledge the University
Grant Commission, New Delhi for the award of UGC-Major Research
Project [F. No. 39-721/2010 (SR)]. We thank Prof. M. A. Pasha of this
department for his valuable suggestions regarding the reaction schemes.
References
1. Zollinger H (1981) Color chemistry: synthesis, properties and
applications of organic dyes and pigments. VCH, New York
123

A. S. Manjunatha, Puttaswamy
2. Silva JP, Sousa S, Rodrigues J, Antunes H, Porter JJ, Goncalves I,
Ferreira-Dias S (2004) Sep Purify Technol 40:309–315 and ref-
erences therein
23. Jagadeesh RV, Puttaswamy (2008) J Phys Org Chem 21:844–858
24. Morris JC, Salazar JA, Wineman MA (1948) J Am Chem Soc
70:2036–2041
3. Li G, Wang N, Liu B, Zhang X (2009) Desalination 249:936–941
4. Yang S, Yang X, Shao X, Niu R, Wang L (2011) J Hazard Mater
186(1):659–666
5. Chen X, Qiao X, Wang D, Lin J, Chen J (2007) Chemosphere
67(4):802–808
6. Campbell MM, Johnson G (1978) Chem Rev 78:65–79
7. Banerji KK, Jayaram B, Mahadevappa DS (1987) J Sci Ind Res
46:65–76
25. Puttaswamy, Shubha JP, Jagadeesh RV (2007) Trans Met Chem
32:991–999
26. Venkatesha BM, Ananda S, Mahadevappa DS (1992) J Phys Org
Chem 5:373–381
27. Akerloff G (1932) J Chem Soc 54:4125–4139
28. Bishop E, Jennings VJ (1958) Talanta 1:197–199
29. Hardy FF, Johnston JP (1973) J Chem Soc Perkin Trans
2:742–745
8. Armesto XL, Canle L, Garia MV, Santaballa JA (1998) Chem
Soc Rev 27:453–460
30. Murthy ARV, Rao BS (1952) Proc Ind Acad Sci 35:69–72
31. Oakes J, Gratton P (1998) J Chem Soc Perkin Trans 2:2201 and
reference therein
32. Cady HH, Connick RE (1958) J Am Chem Soc 80:2646–2652
33. Connick RE, Fine DA (1960) J Am Chem Soc 82:4187–4192
34. Backhouse JR, Dwyer FD, Shales N (1950) Proc R Soc
83:146–155
9. Agnihotri G (2005) Synlett 18:2857–2858
10. Kolaveri E, Ghorbeni-Choghamarani A, Salehi P, Shirini F,
Zolfigol MA (2007) J Iran Chem Soc 4:126–174
11. Vinod KN, Puttaswamy, Gowda KNN (2009) Inorg Chim Acta
362:2044–2051
12. Puttaswamy, Sukhdev A, Shubha JP (2012) Prog React Kinet
Mech 37:42–58
35. Davfokratova T (1963) Analytical chemistry of ruthenium.
Academy of Sciences, USSR
13. Puttaswamy, Jagadeesh RV (2006) Int J Chem Kinet 38:48–56
14. Puttaswamy, Jagadeesh RV (2005) Eur J Chem 3:482–501
15. Puttaswamy, Shubha JP (2008) Prog React Kinet Mech
33:313–330
16. Puttaswamy, Sukhdev A (2009) Indian J Chem 48:339–345
17. Griffith WP (1967) The chemistry of rare platinum metals. In-
terscience, New York
18. Cotton FA, Wilkinson G, Murillo CA, Bochmann M (1999)
Advanced inorganic chemistry, 6th edn. Wiley, New York
19. Mallesh RT, Bellakki MB, Nandibewoor ST (2004) Catal Lett
97:91–98
36. Narayanan SS, Rao VRS (1983) Radiochem Acta 32:211–212
37. Subhashini M, Subramanian M, Rao VRS (1985) Talanta
32:1082–1085
38. Amis ES, Jaffe G (1942) J Chem Phys 10:598–604
39. Laidler KJ, Landskroener PA (1956) Trans Faraday Soc
52:200–210
40. Tanford C, Kirkwood JG (1957) J Am Chem Soc 79:5333–5339
41. Reihardt C (2003) Solvent and solvent effects in organic chem-
istry, 3rd edn. Wiley, New York
42. Laidler KJ (1995) Chemical kinetics, 2nd edn. Tata Mc-Graw
Hill, New Delhi
20. Bhat KR, Jyothi K, Gowda BT (2002) Oxid Commun 25:117–141
and references therein
43. Moelwyn-Hughes EA (1947) Kinetics of reactions in solutions.
Oxford University Press, London
21. Rashmi R, Sushma G, Upadyay SK (1990) Indian J Chem
29A:847–851
44. Gomati Devi L, Mohan Reddy
256:3116–3122
K (2010) Appl Surf Sci
22. Mulla RM, Hiremath GC, Nandibewoor ST (2004) Monatshefte
fur Chemie 135:1489–1502
123