Oxidation of toluidine blue by chlorite in acid and mechanisms of the uncatalyzed and ru(III)-catalyzed reactions: A kinetic approach

Article abstract of DOI:10.1021/jp1062644

The complex mechanism of the uncatalyzed and Ru(III)-catalyzed oxidation of toluidine blue [(7-amino-8-methylphenothiazin-3-ylidene)dimethyl ammonium chloride, TB⁺Cl^-] (λ _max = 626 nm) by acidic chlorite is elucidated by a kinetic approach. Both the uncatalyzed and catalyzed reactions had a first-order dependence on the initial ClO ₂^- and H⁺ concentrations ([ClO₂ ^-]₀ and [H⁺]₀, respectively). The catalyzed reaction had a first-order dependence on the initial Ru(III) concentration ([Ru(III)]₀). The overall reaction of toluidine blue and chlorite ion was as follows: TB⁺ + 5ClO₂^- + H⁺ = P + 2ClO₂ + 2HCOOH + 3Cl^- + H ₂O, where P is (7-amino-8-methyl-5-sulfoxophenothiazin-3-ylidene) amine. Consistent with the experimental results, the pertinent reaction mechanisms are proposed.

Full text of DOI:10.1021/jp1062644

12162
J. Phys. Chem. A 2010, 114, 12162–12167
Oxidation of Toluidine Blue by Chlorite in Acid and Mechanisms of the Uncatalyzed and
Ru(III)-Catalyzed Reactions: A Kinetic Approach
Sreekanth B. Jonnalagadda* and Brijesh K. Pare
School of Chemistry, UniVersity of KwaZulu-Natal, WestVille Campus, Chiltern Hills,
P Bag X54001, Durban 4000, South Africa
ReceiVed: July 7, 2010; ReVised Manuscript ReceiVed: October 8, 2010
The complex mechanism of the uncatalyzed and Ru(III)-catalyzed oxidation of toluidine blue [(7-amino-8-
methylphenothiazin-3-ylidene)dimethyl ammonium chloride, TB+Cl-] (λ max ) 626 nm) by acidic chlorite is
elucidated by a kinetic approach. Both the uncatalyzed and catalyzed reactions had a first-order dependence
-
-
on the initial ClO2 and H+ concentrations ([ClO2 ]0 and [H+]0, respectively). The catalyzed reaction had a
first-order dependence on the initial Ru(III) concentration ([Ru(III)]0). The overall reaction of toluidine blue
-
and chlorite ion was as follows: TB+ + 5ClO2 + H+ ) P + 2ClO2 + 2HCOOH + 3Cl- + H2O, where P
is (7-amino-8-methyl-5-sulfoxophenothiazin-3-ylidene)amine. Consistent with the experimental results, the
pertinent reaction mechanisms are proposed.
Introduction
peroxidise in the decolorization of methylene blue19 and on
the microbial degradation of toluidine blue by BreVibacillus
Reactions with chlorite ion have been of extensive interest
in the past two decades, because of the bistability of this
ion, leading to exotic temporal behavior, nonlinear kinetics,
and spatial patterns exhibited by reactions in both batch
reactors and continuous stirred-tank reactors (CSTRs).1-4
Because of its disinfectant properties, chlorite is also widely
used in water treatment.5,6 Phenothiazine compounds such
as methylene blue (MB+) are used as antimalarial drugs from
ancient times.7 Toluidine blue and related phenothiazine dyes
are water-soluble and have useful insecticidal and antithelm-
inite properties.8 Other synonyms of toluidine blue are
toluidine blue O, CI Basic Blue 17, and tolonium chloride.
Toluidine blue (TB+Cl-, referred to as TB+) is used for
dyeing cotton, staining tissue, and counterstaining tubercle
and lepra bacilli mammalian tissue, as well as staining of
DNA, RNA, and plant material. TB+ exhibits a broad
absorption peak in visible region (λmax ) 626 nm) and
experiences no peak shift due to pH changes. It is used as
an internal indicator in the nonaqueous titrimetric determi-
nation of food preservatives and non-nutritive sweeteners.
Although toluidine blue is nontoxic, many such heterocyclic
dyes are toxic, water-soluble, and stable. Because of their
stabilities, the presence of toxic dyes with long residence
times is hazardous to aquatic systems.9,10 Thus, information
about the degradation dynamics of water-soluble dyes has
much bearing toward sustaining the environment. A literature
survey shows the kinetics of reduction of heterocyclic dyes
with possible reversal to oxidized compounds.11,12 Limited
information is available in the literature on the oxidation
reactions of phenothiazine dyes in general and toluidine blue
in particular.8,13 Some results have been reported on the
photocatalyzed mineralization of dyes, in particular, methyl-
ene blue.14-17 During oxidative degradation of MB+ using
H2O2 on NiO2 catalyst, Oliveira et al. observed the hydroxy-
lation of the outer rings of MB, prior to cleavage of the
central hetero ring, finally leading to mineralization.18 In the
recent past, reports were published on the efficiency of lignin
sp. to CO2 and H2O, but no intermediates of the process were
identified.20
Unlike other reported chlorite reactions, which exhibit
nonlinear behavior,3,4 the TB+-acidic chlorite reaction exhibits
relatively simple kinetics. This departure from the nonlinear
behavior and the high observed sensitivity and selectivity to
the presence of Ru(III) has generated interest in the detailed
kinetic study of TB+-acidic chlorite reaction. In the present
article, the detailed kinetics of both the uncatalyzed and Ru(III)-
catalyzed reactions of chlorite with TB+ and probable mecha-
nisms are reported.
Experimental Section
Reagents. Sodium chlorite (BDH) was recrystallized before
use.21 Toluidine blue (Aldrich) was used as received. All other
reagents used were of Analar grade or high purity. All
solutions were made in deionized distilled water. Ru(III) stock
solution (0.02 M) was prepared by dissolving 0.207 g of
ruthenium(III) chloride trihydrate (Aldrich) in 50 mL of 0.10
M sulfuric acid.22
Kinetic Measurements. In all experiments, pseudo-first-order
kinetics with respect to toluidine blue was monitored at 626
nm (ε ) 1.9 × 104 M-1 cm-1) using a Cary II -Varian UV-vis
double-beam spectrophotometer (Varian, Mulgrave, Victoria,
Australia) interfaced for data storage and processing facilities.
Kinetic data were analyzed with Microsoft Excel software.
Beer’s law is valid for the measurements under the experimental
conditions considered. No interference from the reagents,
intermediates, or products was observed at 626 nm. The total
initial volume of the reaction mixture was always kept at 10
mL, and the temerature was held at 25.0 ( 0.1 °C. Requisite
volumes of reagent solutions were mixed in the following order:
toluidine blue, sulfuric acid, water plus catalyst or other reagents,
where necessary. A separately thermostatted solution of sodium
chlorite was added to commence the reaction. After vigorous
mixing, the reaction mixture was transferred to the thermostatted
spectrophotometer cell. In all experiments, the reactions were
followed for two half-lives.
* Corresponding author. E-mail: jonnalgaddas@ukzn.ac.za.
10.1021/jp1062644  2010 American Chemical Society
Published on Web 11/03/2010

Oxidation of Toluidine Blue by Chlorite in Acid
J. Phys. Chem. A, Vol. 114, No. 46, 2010 12163
The depletion kinetics of toluidine blue with HOCl was
studied at 25 ( 0.1 °C and constant ionic strength (0.1 mol
dm-3), using a Hi-Tech SF-61 double-mixing stopped-flow
apparatus (Salisbury, England) equipped with photomultiplier/
diode array spectrometer. Requisite volumes of TB+, salt, and
HOCl and acid solutions were separately thermostatted and
transferred to the syringes of the stopped-flow equipment. The
change in the absorbance due to depletion of TB+ was monitored
at 626 nm. Factor analysis and global fitting were performed to
obtain rate constants, using the computer program SPECFIT
from Spectrum Software Associates (TgK Scientific Limited,
Brasford on Avon, U.K.). Best fits were obtained with equations
for a simple first-order reaction, with a single-exponential
function [y ) -A exp(-kx) + C], which gave the computed
pseudo-first-order rate coefficients.
FT-IR Spectroscopy. FT-IR spectra of the major organic
product were recorded on a Nicolet Impact 400 spectropho-
tometer and a Nicolet Impact model-420 spectrometer with 4
cm-1 resolution and 128 scans in the mid-IR (400-4000 cm-1)
region using the KBr pellet technique.
Gas Chromatography-Mass Spectrometry (GC-MS). For
analysis of organics, an Agilent 6890 gas chromatograph
equipped with a quadruple Agilent 5973n mass-selective detector
was used. The column specifications are as follows: J&W
DB5MS, 30-m length, 250-µm diameter, and 0.25-µm film
thickness. The GC-MS analysis was carried out in electron
impact (EI) mode, and the spectra were recorded in the interval
35-500 amu.
A survey of the literature also shows that phenothiazines are
oxidized to their sulfoxides in the presence of sodium ethoxide27
and to the sulfones under stronger oxidizing conditions such as
the presence of fuming nitric acid and hydrogen peroxide.8 The
oxidation of phenothiazines with permanganate and hydrogen
peroxide also reportedly gives sulfoxides and occasionally
sulfones.8 The loss of color of the phenothiazines upon oxidation
is attributed to the nonplanar nature of the structure of product
at sulfur, upon oxidation to sulfoxide.28 The existence of
protonated CsS+(sOH)sC species as an intermediate is
reported during the oxidation of methylene blue by aqueous
bromine.29 In the current studies, with strongly acidic media,
the species remains potentially as CsS+(sOH)sC. During the
photodegradation of MB+, Houas et al. observed that an
electrophilic attack on the free doublet of heteroatom S, making
its oxidation state pass from 2- to 0, is a probable step of the
oxidative process. The passage from CsS+dC to CsS(dO)sC
requires the conservation of the double bond conjugation, which
induces the opening of the central ring containing both the hetero
atoms.25 In the current study, no cleavage in the heterocyclic
ring was observed. Gnaser et al., in similar studies on methylene
blue oxidation, reported that the ring-opening is necessitated
only when the attack takes place on the MB+ cation, with the
charge concentrated on the S atom. However, if neutral S is
attacked or if the cationic S is coordinated to a catalyst surface,
no such ring-opening is required.26 Thus, sulfoxide formation
without central ring rupture is possible. Yogi et al. reported the
demethylation of methylene blue during photocatalyzed oxida-
tion on titania.30
Results and Discussion
Product Analysis and Stoichiometry. For the product
analysis, toluidine blue (250 mg of TB+ in 250 mL of water),
5.0 M sulfuric acid (100 mL), and (500 mg/150 mL) chlorite
were mixed to make 500 mL. After 4 h of reaction, the reaction
mixture was filtered, and the filtrate was neutralized with sodium
bicarbonate, producing a precipitate that was filtered and
collected. The completion of the reaction was confirmed with
thin-layer chromatography (TLC) plates against the starting
material using 7:3 ratio of hexane to ethyl acetate as the eluent.
The same solvent system was used to separate the products using
a column packed with silica gel 60 (0.063-0.2, Merck). The
analysis of the product mixture by TLC using formic acid as
the standard reference confirmed the presence of formic acid
as one of the products.23
The stoichiometry of the reaction was determined using a
1:10 molar concentration ratio of TB+ to chlorite with excess
of H+. After about 60 min of incubation, the residual TB+ was
determined by measuring the absorbance at 626 nm, and the
residual amount of chlorite was estimated iodometrically using
sodium thiosulfate as a titrant and starch as an indicator.31 In
agreement with the major reaction product, the overall reaction
between toluidine blue and chlorite ion is
-
TB+ + 5ClO2 + H+ ) P + 2ClO2 + 2HCOOH +
The IR spectra of the main product showed a sharp band
at 1005-1070 cm-1, suggesting the formation of sulfoxide.
The strong peaks in the ranges of 3000-3300 and 2700-3000
cm-1 indicated the presence of intact primary and secondary
amino groups on the aromatic heterocyclic ring.24 Any strong
absorption bands indicative of sulfone in the 1300-1135
cm-1 region were absent, and no intense peak in the range
of 1300-1200 cm-1 indicative of N-O stretching was
observed, which excludes the formation sulfone and nitrogen
oxides in the heterocyclic ring. Further, the mass spectrometry
data of the main product showed m/z 268 (60%) as the main
peak, and the other significant peaks observed included m/z
252 (loss of oxygen atom from sulfoxide), m/z 235 (loss of
amino group), and m/z 157 (fragmented peak of heterocyclic
ring). The pattern of the fragmentation peaks resembles the
mass spectra of methlylene blue oxidation products reported
in the literature.25,26 Based the MS and IR data, the main
oxidation product was identified as 2-amino-3-methyl-7-
amino phenothiazine sulfoxide.
3Cl- + H2O
where P is (7-amino-8-methyl-5-sulfoxophenothiazin-3-ylide-
ne)amine. For a fixed initial concentration of TB+ ([TB+]0), the
stoichiometry and amount of chlorine dioxide formed varied
and was observed to be primarily dependent on the initial
concentration of ClO2- ([ClO2-]0) and marginally on the initial
concentration of H+ ([H+]0). Chlorite ion in acidic conditions
is known to decompose under catalysis by metal ions such as
Fe(II).21
Reaction Kinetics. The kinetics of all of the reaction runs
was conducted with excess concentrations of acid and chlorite
and a low concentration of toluidine blue. Further, the initial
concentration of acid was always kept equal or higher than the
-
concentration of ClO2
.
(a) Uncatalyzed Reaction and Dependence of Reaction Rate
on ClO2- and H+ Concentrations. Figure 1 shows typical kinetic
curves for the depletion of toluidine blue at varied initial

12164 J. Phys. Chem. A, Vol. 114, No. 46, 2010
Jonnalagadda and Pare
Figure 1. Kinetic curves showing the effect of the variation in intial
Figure 2. Repeated spectra (300-700 nm) showing the depletion of
toluidine blue and formation of chlorine dioxide for the Ru(III)-
catalyzed reaction. Conditions: [TB+] ) 5.0 × 10-5 M, [H+] ) 5.0 ×
10-2 M, [ClO2-]0 ) 2.0 × 10-2 M, and [Ru(III)] ) 1.0 × 10-7 M.
ClO2 concentration ([ClO2-]0) on TB+ depletion. Conditions: [TB+]
-
) 5.0 × 10-5 M; [H+] ) 0.05 M; and [ClO2-]0 ) (a) 0.02, (b) 0.03,
(c) 0.04, (d) 0.05, and (e) 0.06. Inset: Plot of ln(absorbance) versus
time.
TABLE 2: Toludine Blue Oxidation with Acidic Chloride:
Effects of Chlorite and Hydrogen Ion Variations on the
Reaction Rate in the Presence of a Fixed Concentration of
Ru(III)a
TABLE 1: Toludine Blue Oxidation with Acidic Chloride:
Effect of Chlorite and Hydrogen Ion Variation on the
Reaction Ratea
[H+] (M)
[ClO2-] (M)
k′0 (10-3 s-1
)
k0 (M-2 s-1
c
)
b
[H+] (M)
[ClO2-] (M)
k′′ b (10-3 s-1
)
kc (s-1
)
0.05
0.05
0.05
0.05
0.05
0.02
0.03
0.04
0.05
0.06
0.08
0.10
0.015
0.02
0.03
0.04
0.05
0.02
0.02
0.02
0.02
0.02
0.02
0.02
1.33
1.71
2.47
3.47
4.38
0.67
1.07
1.31
1.71
1.95
2.73
3.33
1.77
1.71
1.65
1.73
1.75
1.66
1.78
1.63
1.71
1.62
1.71
1.67
0.05
0.05
0.05
0.05
0.05
0.05
0.02
0.03
0.05
0.06
0.08
0.1
0.008
0.01
0.02
0.03
0.04
0.05
0.02
0.02
0.02
0.02
0.02
0.02
1.45
1.95
3.80
5.43
7.89
10.58
1.50
2.29
3.80
4.45
6.23
7.61
3.64
3.90
3.80
3.62
3.94
4.23
3.74
3.81
3.80
3.71
3.89
3.80
a Conditions: [TB+] ) 5.0 × 10-5 M, ionic strength ) 0.17 M,
temperature ) 25 ( 0.1 °C. b Means of duplicate experiments with
< 5% standard deviation. c Third-order rate constant, k0 ) k′0/
a Conditions: [TB+] ) 5.0 ×10-5 M, [Ru(III)] ) 1.0 × 10-7 M,
ionic strength ) 0.17 M, temperature ) 25 ( 0.1 °C. b Means of
duplicate experiments with < 5% standard deviation. c Third-order
rate constant, k ) k′′/([H+][ClO2-]). Mean third-order constant )
([H+][ClO2-]). Mean third-order constant ) 1.70 ( 0.05 M-2 s-1
.
3.82 ( 0.16 s-1
.
concentrations of chlorite, while other parameters were kept
fixed. Plots of the natural logarithm of the absorbance versus
time data gave good straight lines with R2 g 0.99 (see inset in
Figure 1), suggesting that the reaction follows pseudo-first-order
kinetics and that the order with respect to the TB+ concentration
([TB+]) is unity.
and [Ru(III)]0 ) 6.0 × 10-8 M in the range of 300-700 nm.
With the depletion of toluidine blue, the progressive increase
in the absorption peak at 360 nm and the characteristic wavelike
spectra of ClO2 confirm the accumulation of chlorine dioxide
(ε ) 1.26 × 103 M-1 cm-1) as the product of reductive
decomposition of chlorite.19 Chlorine dioxide accumulation
continued even after the decolorization of the dye blue,
suggesting the continued decomposition and disproportionation
of residual chlorite under acidic conditions.
With fixed initial concentrations of catalyst and other
reactants, in separate sets of experiments, the initial concentra-
tions of chlorite or acid were varied, and the TB+ depletion
kinetics was monitored. Table 2 summarizes the kinetic data at
different concentrations of acid and chlorite. A perusal of the
data shows that the k′′ values increased with increasing initial
concentration of chlorite or acid, while other parameters were
held constant. The plots of the natural logarithm of the
absorbance versus time were linear (R2 g 0.99). Furthermore,
the ln k′′ versus ln([ClO2-]) and ln k′′ versus [H+] graphs were
straight lines (R2 g 0.99), with gradients of 1.04 and 1.01,
respectively. The third-order rate constant for a fixed concentra-
Table 1 summarizes the k′0 values obtained at constant ionic
strength with varied concentrations of chlorite and H+ ions,
while the other parameters wer held constant. Plots of ln k′0
versus ln([chlorite]) and ln([H+]) versus ln k′0 gave straight lines
with slopes of 0.99 each and R2 g 0.99 in both cases. This
suggests that the reaction has first-order dependence on both
the chlorite and H+ concentrations. The mean third-order rate
constant for the uncatalyzed reaction was 1.70 ( 0.05 M-2 s-1
.
(b) Ru(III)-Catalyzed Reaction and Orders with Respect to
ClO2- and H+ Ions. Whereas the presence of many cations had
a marginal influence on the reaction, preliminary experiments
distinctly recorded significant increases in the reaction rates in
the presence of catalytic amounts of Ru(III). Hence, the kinetics
of the reaction was investigated in detail. Figure 2 shows
repeated absorption spectra of the reaction with [H+]0 ) 5.0 ×
10-2 M, [chlorite]0 ) 2.0 × 10-2 M, [TB+]0 ) 5.0 × 10-5 M,

Oxidation of Toluidine Blue by Chlorite in Acid
J. Phys. Chem. A, Vol. 114, No. 46, 2010 12165
TABLE 3: Catalyzed Oxidation of Toluidine Blue with
Acidic Chloride: Effect of Ru(III) Variationa
[Ru(III)] (10-7 M)
k′0 (10-2 s-1
b
)
0
0.17
0.20
0.27
0.31
0.38
0.57
0.80
0.99
1.17
1.30
0.40
0.60
0.80
1.00
2.00
3.00
4.00
5.00
6.00
a Conditions: [TB+] ) 5.0 × 10-5 M, [H+] ) 0.05 M, [ClO2-] )
0.02 M, ionic strength ) 0.17 M, temperature ) 25 ( 0.1 °C.
b Means of duplicate experiments with < 5% standard deviation.
Figure 3. Typical curves showing the effect of the initial HOCl
concentration ([HOCl]0) on the toluidine blue depletion kinetics.
Conditions: [TB+] ) 5.0 × 10-5 M; [H+] ) 0.05 M; and [HOCl]0 )
(a) 0.16, (b) 0.22, (c) 0.28, (d) 0.34, and (e) 0.40 × 10-2 M. Inset: Plot
of ln absorbance versus time.
tion of catalyst (1.0 × 10-7 M) was 3.8 ( 0.2 M-2 s-1. This
confirms that the catalyzed reaction also has a first-order
dependence on both chlorite and H+ ion concentrations.
(c) Kinetic Salt Effect. With reactant concentrations of [TB+]
) 5.0 × 10-5 M, [H+] ) 5.0 × 10-2 M, and [ClO2-] ) 2.0 ×
10-2 M, the initial ionic strength of the reaction mixture was
0.17 M. An increase in ionic strength from 0.17 to 0.32 resulted
in the decrease of k′0 from 1.70 × 10-3 to 1.20 × 10-3 s-1 for
the uncatalyzed reaction and a decrease of k′′ from 3.80 × 10-3
to 2.72 × 10-3 s-1 for the catalyzed reaction. The plots of the
logarithm of the rate constant versus the square root of the ionic
strength for the uncatalyzed and catalyzed reactions were linear
curves with negative slopes of -2.14 and -2.19, respectively
(R2 g 0.98), suggesting that oppositely charged species are
involved in the rate-limiting step.
(d) Order with Respect to Catalyst. The catalytic effect of
Ru(III) ion on the reaction was investigated using varied initial
concentrations of the metal ion (Table 3). The k′′ versus [Ru(III)]
plot gave a straight line (R2 ) 0.99) with a catalytic constant,
k′C, with a high value of 1.98 × 104 M-3 s-1. This confirms
that Ru(III) exhibits good catalytic efficiency toward the reaction
and that the reaction order with respect to the catalyst is unity.
The y intercept agreed well with the mean rate constant for the
uncatalyzed reaction.
TABLE 4: Effect of HOCl Variation on the Oxidation of
Toluidine Bluea
[HOCl] (M)
k′b (s-1
)
kc (s-1
)
0.0016
0.0022
0.0028
0.0034
0.0040
0.078
0.094
0.128
0.156
0.181
48.8
42.7
45.7
45.9
45.25
a Conditions: [TB+]
)
5.0
×
10-5 M, [H+]
) 0.05 M,
temperature ) 25 ( 0.1 °C. b Means of five replicate experiments
with < 4% standard deviation. c Mean second-order rate constant, k
) 45.8 ( 2.1 M-2 s-1
.
-
TB+ + ClO2 + H+ f TBO+ + HOCl
(R1)
(R2)
(R3)
(R4)
(R5)
-
H+ + ClO2 T HClO2
HClO2 + Cl- + H+ T 2HOCl
(e) Kinetics of Reaction of TB+ with HOCl. Considering that
hypochlorous acid is the stable reactive intermediate generated
during the decomposition of chlorite under acidic conditions,
the kinetics of TB+ with HOCl was also investigated. From the
preliminary studies, the reaction was found to be fast and hence
was studied using the stopped-flow technique. Figure 3 illustrates
the kinetic curves depicting the effect of variations in the HOCl
concentration on the depletion of toluidine blue. The plots the
natural logarithm of the absorbance versus time data were
straight lines (see inset in Figure 3), suggesting that the reaction
follows pseudo-first-order kinetics and that the order with respect
to TB+ is unity. Further, the plot of k versus [HOCl] was linear,
confirming that the reaction has first-order dependence on HOCl
(Table 4). The second-order rate constant was estimated to be
HOCl + Cl- + H+ T Cl2 + H2O
-
Cl2 + 2ClO2 f 2ClO2 + 2Cl-
-
Cl2O2 + 2ClO2 + 2H+ T 2ClO2 + 2HOCl
(R6)
(R7)
-
HOCl + ClO2 + H+ T Cl2O2 + H2O
45.8 ( 2.1 M-2 s-1
.
The other possible reactions to be considered are the reactions
of various chloro- and oxychlorine species with TB+ and the
other reaction intermediates to give the final products. Those
reactions can be expressed as follows
Mechanism. Uncatalyzed Reaction. The observed negative
salt effect suggests that the rate-determining step is the reaction
involving oppositely charge species, possibly TB+, H+, and
-
ClO2 ions. The rapid accumulation of ClO2 explains its
inertness toward the reaction (Figure 1b). Under acidic condi-
tions, the reaction of the following oxychlorine reactions are
important to explain the chemistry of title reaction.4,32-34
TB+ + HOCl f TBO+ + H+ + Cl-
(R8)

12166 J. Phys. Chem. A, Vol. 114, No. 46, 2010
TB+ + Cl2 + H2O f TBO+ + 2H+ + 2Cl-
Jonnalagadda and Pare
electron configuration, which is in contrast to the narrow scope
of oxidation states and simple square-planar structure of
palladium.36a As reviewed by Nioata et al., for ruthenium as a
catalyst, organic syntheses involve a variety of reactions
including reduction, oxidation, isomerization, carbon-carbon
bond formation, and miscellaneous nucleophilic and electrophilic
reactions.36a Ruthenium(III) trichloride was reported to catalyze
the oxidation of primary amines to nitriles by oxygen. A new
preparation method for the synthesis of iodylarenes using the
RuCl3-catalyzed oxidation of iodoarenes with peracetic acid has
also been reported.36b
(R9)
TB+ + Cl2O2 + H2O f TBO+ + 2HOCl (R10)
TBO+ + HOCl f I + HCHO + 2H+ + Cl-
(R11)
-
TBO+ + H+ + ClO2 f I + HCHO + HOCl + H+
In the absence of acid and/or oxidant, Ru(III) was found to
have no reactivity with TB+, confirming that, during the catalytic
cycle, Ru(III) does not get reduced to a lower oxidation state.
Ruthenium ion and its complexes are known to oxidize a variety
of compounds. Lee et al. also reported the formation of Ru(V)
during the RuCl3-catalyzed reactions of alkanes by peracetic
acid.37,38 Although many stable Ru(V)-oxo complexes have
been reported in the literature, some have been isolated and
characterized.39 Thus, based on the observed reaction orders with
respect to various reagents, for the Ru(III)-catalyzed oxidation,
formation of a complex between TB+ and Ru(III) is envisaged,
which readily combine with acid and chlorite ions or HOCl in
a rapid reaction forming an activated complex. The decomposi-
tion of the activated complex is proposed. During this process,
Ru(III) possibly gets oxidized to Ru(V) through an inner-sphere
electron-transfer mechanism and, in turn, abstracts electrons
from the substrate, reducing itself again to Ru(III), completing
the catalytic cycle.37,38 It is possible that ruthenium ion acts as
an electron mediator only within the activated complex.40-42
(R12)
TBO+ + Cl2 + H2O f I + HCHO + 3H+ + 2Cl-
(R13)
TBO+ + Cl2O2 + H2O f I + HCHO + 2HOCl + H+
(R14)
I + HOCl f P + HCHO + H+ + Cl-
(R15)
-
I + ClO2 + H+ f P + HCHO + HOCl (R16)
I + Cl2 + H2O f P + HCHO + 2H+ + 2Cl-
(R17)
HCHO + HOCl f HCOOH + H+ + Cl-
(R18)
Ru(III) + TB+ f [binary complex]
(R19)
Because of the presence of strong oxidizing species such as
Cl2, Cl2O2, and HOCl, the further oxidation and mineralization
of some formic acid formed to CO2, achieving a complete
mineralization, cannot be ruled out.34b
-
[binary complex] + ClO2 + H+ f
[activated complex]
slow (R20)
The oxidative demethylation step from TBO+ to the inter-
mediate (I, see Scheme 1) possibly involves more elementary
steps. Upon oxidative attack by acidic chlorite, the initial
abstraction of H atom from the amino group and hydroxylation,
followed by the shift of electrons results in demethylation and
formation of H+ and formaldehyde, as reported in the literature
for the cytochrome P450 2B1 catalyzed oxidative demethylation
of alkyl amines.35
[activated complex] f Ru(III) + TBO+ + HOCl
(R21)
Ru(V) abstracts electrons from TB+ with the formation of
sulfoxide intermediate (I+). Thus, within the activated complex,
ruthenium ion probably acts as an electron mediator only.40,41
The chemistry of the remaining reaction scheme remains the
same as for the uncatalyzed reaction.
Rate Laws. The rate equation for the uncatalyzed reaction
between TB+ and acidic chlorite can be represented by the
equation
Ru(III)-Catalyzed Reaction. Because ruthenium has a 4d75s1
electron configuration, it has the widest scope of oxidation states
2-
[from 2- in Ru(CO)4 to 8+ in RuO4] of all elements of the
periodic table and various coordination geometries in each
-
rate ) -d[TB+]/dt ) k0[ClO2 ][H+][TB+]
(1)
SCHEME 1: Substrate, Product, and Possible
Intermediates
Under excess concentration conditions of chlorite and acid, the
equation reduces to
r ) k'0[TB+]
(2)
where
-
k'0 ) k0[ClO2 ][H+]
(3)
and k′0 is the pseudo-first-order rate constant for uncatalyzed
reaction.

Oxidation of Toluidine Blue by Chlorite in Acid
J. Phys. Chem. A, Vol. 114, No. 46, 2010 12167
References and Notes
In the presence of the catalyst, the oxidation reaction proceeds
through both uncatalyzed and catalyzed pathways.
(1) Orban, M.; Epstein, I. R. J. Phys. Chem. 1982, 86, 3907.
(2) Nagypal, I.; Bazsa, Gy.; Epstein, I. R. J. Am. Chem. Soc. 1986,
108, 3635.
(3) Nagypal, I.; Epstein, I. R. J. Phys. Chem. 1986, 90, 6285.
(4) Jones, J. B.; Chinake, C. R.; Simoyi, R. H. J. Phys. Chem. 1992,
96, 5852.
(5) Clapper, T. W.; Gale, W. A. In Encyclopedia of Chemical
Technology, 2nd ed.; Kirk, R. E., Othmer, D. F., Eds.; Wiley: New York,
1964; Vol. 5.
(6) Gordon, G.; Cooper, W. J.; Rice, R. G.; Pacey, G. E. Disinfectant
Residual Measurement Methods; AWWA Research Report; American Water
Works Research Foundation: Denver, CO, 1987, and references therein.
(7) Wainwright, M.; Amaral, L. Trop. Med. Int. Health 2005, 10 (6),
501.
-d[TB+]/dt ) {k0[ClO-2 ][H+] +
kC[ClO-2 ][H+][Ru(III)]}[TB+] (4)
Therefore, the following represents the rate of depletion of TB+
in the presence of catalyst under excess chlorite and acid
concentrations
-d[TB+]/dt ) {k'0 + k'C[Ru(III)]}[TB+]
(5)
(8) Elderfield, R. C. Heterocyclic Compounds; John Wiley & Sons:
New York, 1957; Vol. 6.
(9) Santus, R.; Kohen, C.; Kohen, E.; Reyftmann, R. P.; Morliere, P.;
Dubertret, L.; Tocci, P. M. Photochem. Photobiol. 1984, 38, 71.
(10) Cizek, Z.; Studlova, V. Talanta 1984, 31, 547.
(11) Jonnalagadda, S. B.; Tshabalala, D. Int. J. Chem. Kinet. 1992, 24,
499.
(12) Jonnalagadda, S. B.; Dumba, M. Fresenius’ J. Anal. Chem. 1993,
345, 673.
where
where
k'C ) kC[ClO2 ][H+] ) k″[TB+]
(6)
(7)
-
(13) Jonnalagadda, S. B.; Musengiwa, N. Int. J. Chem. Kinet. 1998, 30,
111.
(14) Mills, A.; Wang, J. J. Photochem. Photobiol. A 1999, 127, 123.
(15) Salem, I. A.; El-Maazawi, M. S. Chemosphere 2000, 41, 1173.
(16) Lachheb, H.; Puzenat, E.; Houas, A.; Ksibi, M.; Elaloui, E.;
Guillard, C.; Herrmann, J. M. Appl. Catal. B 2002, 39, 75.
(17) Hsing, C.; Chern, J. M. Ind. Eng. Chem. Res. 2006, 45, 6450.
(18) Oliveira, L. C. A.; Ramalho, T. C.; Goncalves, M.; Cereda, F.;
Carvalho, K. T.; Nazzarro, M. S.; Sapag, K. Chem. Phys. Lett. 2007, 446,
133.
(19) Leitao, V. S. F.; Carvalho, M. L. A.; Bon, E. P. S. Dyes Pigm.
2007, 74, 230.
(20) Alhassani, H. A.; Rauf, M. A.; Ashraf, S. S. Dyes Pigm. 2007, 75,
395.
(21) Fabian, I.; Gordon, G. Inorg. Chem. 1991, 30, 3994.
(22) Connick, R. E.; Fine, D. A. J. Am. Chem. Soc. 1960, 82, 4187.
(23) Takizawa, T.; Watanabe, T.; Honda, K. J. Phys. Chem. 1978, 82,
1381.
(24) Silverstein, R. M.; Bassler, C. G.; Morrill, T. C. Spectrometric
Identification of Organic Compounds, 3rd ed.; John Wiley & Sons: New
York, 1974.
k″ ) k'0 + k'C[Ru(III)]
where k′′ represents the pseudo-first-order constant in the
presence of catalyst. If eq 5 holds well, then a plot of the
observed pseudo-first-order rate constant in the presence of
catalyst, k′′ versus [Ru(III)], should give a straight line with
intercept equal to k′0. (The plot is provided in the Supporting
Information.) The intercept of the plot (Table 3) indeed has a
value comparable to the experimental pseudo-first-order rate
constant for the uncatalyzed reaction, which validates the
assumption. For the chosen reaction conditions, the k′C value
for Ru(III) was (1.95 ( 0.3) × 104 M-1 s-1 and the catalytic
constant, kC ) k′C/([ClO-2][H+]), was equivalent to 1.95 × 107
(25) Houas, A.; Lachheb, H.; Ksibi, M.; Elaloui, E.; Guilard, C.;
Herrmann, J. M. Appl. Catal. B 2001, 31, 145.
M-3 s-1
.
(26) Gnaser, H.; Savina, M. R.; Calaway, W. F.; Tripa, C. E.; Veryovkin,
I. V.; Pellin, M. J. Int. J. Mass Spectrom. 2005, 245, 61.
(27) Gilchrist, T. L. Heterocyclic Chemistry; Wiley: New York, 1982;
p 338.
(28) Armarego, W. L. F. Stereochemistry of Heterocyclic Compounds;
Taylor, E. C., Weissberger, A., Eds.; John Wiley & Sons: New York, 1977;
Part 2, p 311.
Conclusions
Many aromatic and heterocyclic organic substrates exhibit
nonlinear dynamics in reactions with chlorite, but interestingly,
toluidine blue exhibits exponential decay during its oxidation
process. Further, this study confirms that, for the oxidation of
toluidine blue with acidic chlorite, Ru(III) is a good and selective
catalyst, and both uncatalyzed and catalyzed conditions result
in effective oxidation of toluidine blue to a demethylated
sulfoxide, although the heterocyclic structure was retained. This
reaction process also leads to disproportion of chlorite and
accumulation of chlorine dioxide, and ClO2 has a negligible
reactivity under acidic conditions.
(29) Peter, M. J. ArkiVoc 2003, 1, 37.
(30) Yogi, C.; Kojima, K.; Wada, N.; Tokumoto, H.; Takai, T.;
Mizoguchi, T.; Tamiaki, H. Thin Solid Films 2008, 516, 5881.
(31) Vogel, A. I. A Text Book of QuantitatiVe Inorganic Analysis, 3rd
ed.; Longmans: London, 1961; p 343.
(32) Lengyel, I.; Rabai, Gy.; Epstein, I. R. J. Am. Chem. Soc. 1990,
112, 9104.
(33) Fabian, I.; Gordon, G. Inorg. Chem. 1991, 30, 3785.
(34) (a) Chinake, C. R.; Mambo, E.; Simoyi, R. H. J. Phys. Chem. 1994,
98, 2908. (b) Chinake, C. R.; Olojo, O.; Simoyi, R. H. J. Phys. Chem.
1998, 102, 606.
(35) Guengerich, F. P.; Yun, C.-H.; Macdonaldi, T. L. J. Biol. Chem.
1996, 271 (44), 27321.
Acknowledgment. The authors thank the University of
KwaZulu-Natal (UKZN), Durban, South Africa, and the Na-
tional Research Foundation, Pretoria, South Africa, for financial
assistance and Mr. S. Naidoo and Prof. F. O. Shode, UKZN,
for their help in product characterization. Dr. Pare, who was a
postdoctoral fellow from the Madhav Science College, Vikram
University, Ujjain, India, thanks his institution for approval of
the sabbatical leave.
(36) (a) Naota, T.; Takaya, H.; Murahashi, S. Chem. ReV. 1998, 98,
2599. (b) Koposov, A. Y.; Karimov, R. R.; Pronin, A. A.; Skrupskaya, T.;
Yusubov, M. S.; Zhdankin, V. V. J. Org. Chem. 2006, 71 (26), 9912.
(37) Lee, W. A.; Bruice, T. C. J. Am. Chem. Soc. 1985, 107, 513.
(38) Kumiya, N.; Nuji, S.; Murahashi, S. Chem. Commun. 2001, 65.
(39) Griffith, W. P. Chem. Soc. ReV. 1992, 21, 179, and references
therein.
(40) Avtokratova, T. D. Analytical Chemistry of Ruthenium; Humphrey
Science Publishers: London, 1969; p 163.
(41) Subba Rao, P. V.; Vijayshree, M.; Saradamba, G. V.; Ramakrishna,
K. J. Indian Chem. Soc. 1986, 63, 743.
Supporting Information Available: Plot of k′′ versus
[Ru(III)] and an explanation for Figure 3. This information is
available free of charge via the Internet at http://pubs.acs.org.
(42) Brahmaiah, A.; Manikyamba, P. Indian J. Chem. 1995, 34A, 900.
JP1062644