Journal
of Chemical
Technology
and Metallurgy,
52, 3,52,
2017,
526-531
Journal
of Chemical
Technology
and Metallurgy,
3, 2017
KINETICS OF THE REACTION BETWEEN MALACHITE GREEN
AND HYDROXYL ION IN THE PRESENCE OF REDUCING SUGARS
Latona Dayo Felix
Department of Chemical Sciences
Osun State University, Osogbo, Nigeria
E-mail: [email protected]
Received 07 November 2016
Accepted 15 January 2017
ABSTRACT
The reaction studied is found a first order one with respect to malachite green, sugar and hydroxyl ion concentrations.
However, the reaction is independent of the solution ionic strength and shows no salt effect dependence. This provides to
imply an outer sphere mechanism. No polymerization of the reaction mixture with acrylonitrile is observed, which indicates
an absence of radicals’ formation in the course of the reaction. The Michaelis-Menten plot suggests the participation of
a reaction intermediate in the rate determining step. The activation parameters of the reaction are calculated, while the
products are identified by FTIR spectroscopy. A reaction stoichiometry of 1:1 is found. A mechanism consistent with the
facts pointed above is advanced.
Keywords: Malachite green(MG), sodium hydroxide, glucose, fructose, xylose.
INTRODUCTION
Malachite green is a triphenylmethane dye which has
a wide range of industrial applications, most especially
in aquaculture production because of its antibacterial,
antifungal and antiparasitic properties [1]. However, it is
found to be mutagenic and carcinogenic [2]. Due to its
hazardous effect, several researches have been reported
to remove it from water or industrial effluents mainly
by biosorption [3 - 6]. It has been established that wood
fibre of phoenix tree (Firmiana Sinplex) is an effective
adsorbent of malachite green [7].
Fading of malachite green (MG) in the presence of
a hydroxyl ion has been attributed to the hydroxide ion
attack on the central C atom of the planar ring system
of malachite green, thereby destroying its conjugation
orientation as shown below. It has been found that sugars
such as dextrose, lactose and sucrose increase the rate of
fading of dyes in general [8]. Hitherto, there has been no
report on the kinetics and mechanism of alkaline malachite green fading in sugars presence. However, few or
scanty investigations have been reported on the kinetics
of alkaline malachite green fading in alcohols and in the
presence of surfactants [9,10]. Pressure and temperature
526
effects on the kinetics of alkaline fading of malachite
green in aqueous solution have been investigated [11].
Furthermore, the influence of selective salvation on
the kinetics of reaction between malachite green and
hydroxide ion has been reported [12].
Therefore, it is of great interest to investigate the
effect of some reducing sugars on the alkaline fading of
malachite green with the view to determine the mechanism via a vis the mechanism earlier proposed for the
oxidation of sugars in sodium hydroxide medium [13].
EXPERIMENTAL
Analar grade malachite green, NaOH, KOH, glucose, fructose and xylose were used and the solutions
were prepared with doubly distilled water.
CH3
CH3
N
N
CH3
H3C
CH3
CH3
N
N
CH3
H3C
OH
–
OH–
Malachite Green
Carbinol
The reactions were performed under pseudo-first
order conditions by maintaining a large excess (x10 or
Latona Dayo Felix
greater) of sugar in respect to malachite green. Appropriate quantities of solutions of sugar, potassium nitrate,
sodium hydroxide and malachite green were taken from
stock solutions prepared in advance. The latter were kept
in a water bath for 30 min prior to the kinetic runs. The
reaction investigated was started with the introduction of
the required volume of malachite green solution to the
reaction mixture. The kinetic data was obtained by monitoring the change in the absorbance of malachite green at
the absorption maximum observed as a function of time.
The absorbance (A) was followed using a UV-1800 Shimadzu spectrophotometer provided with a thermostated
cell interfaced to a computer. λmax was found equal to
617 nm. The pseudo- first order rate constant (kobs) values
were obtained from a plot of ln A vs. time. The reaction
products were characterized by FTIR and the reaction showed
no presence of radicals as there was no polymerization on
addition of acrylonitrile to the reaction mixture.
RESULTS AND DISCUSSION
Stoichiometry and products analysis
The stoichiometry of the reaction is determined
by spectroscopic titration. The absorbance of solutions
containing various concentrations of sugar within the
range from 1.00 x 10-2 M to 6.00 x 10-2 M, a constant
initial concentration of MG of 1.00 x 10-3 M and constant
initial concentrations of NaOH and KNO3 of 6.00 x 10-2
M and of 0.25 M, correspondingly, is followed. The
stoichiometry is evaluated from the plot of absorbance vs
sugar concentration ([s]) curve and was found to be 1:1.
Effect of concentration of Мalachite green on the
rate constant observed
The effect of malachite green concentration on the rate
constant observed is determined studying reaction mixtures
containing MG in concentrations ([MG]) ranging from 8.00
Fig. 1. Plot of the observed rate constant versus Мalachite
green concentration: ( Fructose Glucose
Xylose).
x 10-4 M to 4.00 x 10-3M, sugar in a concentration of 2.00 x
10-2M, OH- in a concentration of 6.00 x 10-2M . The ionic
strength is equal to 0.25M, while T = 298 K. A plot of ln
kobs vs ln [MG] gives a slope equal to 1, indicating a first
order dependence. The reaction shows an increase in rate
constant with c MG increase as shown in Fig. 1.
Effect of sugar concentration on the observed
rate constant
The effect of sugar concentration on the observed rate
constant is determined studying reaction mixtures containing MG in a concentration of 1.00 X 10-3 M, sugar in a concentration range from 1.00 x 10-2 M to 6.00 x 10-2 M, OH- in
a concentration of 6.00 x 10-2 M. The ionic strength I is
equal to 0.25M, while T = 298 K. The rate constants
obtained increase with sugar concentration increase as
shown in Table 1. The plot of ln kobs vs ln[S] shows that
the reaction increases with increase of sugar concentration. The slope of this plot is equal to unity, implying a
first order dependence on sugar concentration. The plot
Table 1. Effect of sugar concentration on the rate constant observed.
102[S]/M
103kobs/s-1
Glucose
Fructose
1.00
1.81
2.10
2.00
3.60
3.40
3.00
5.12
4.75
4.00
6.84
6.02
6.00
8.10
7.41
-3
-2
[MG]
1.00
x x1010-3
MM
; [OH
] 6.00
10 M
M ;; TT ==298
298KK
[MG]
1.00
; [OH-]
6.00Xx10-2
M;;II == 0.25
0.25 M
Xylose
1.73
2.81
3.62
4.80
6.51
527
Journal of Chemical Technology and Metallurgy, 52, 3, 2017
Fig. 2. A plot of 1/ k obs vs. 1/ [S] : (
cose,
Xylose.
Fructose
, Glu-
of 1/kobs vs 1/[s] has an intercept, revealing the presence
of an intermediate complex as shown in Fig. 2. The
second order rate constants are 0.152 M-1s-1(Glucose),
0.138 M-1s-1 (Fructose), 0.125M-1s-1 (Xylose).
Effect of hydroxyl ion concentration on the rate
constant observed
The effect of hydroxyl ion concentration on the rate
constant observed is determined studying reaction mixtures
containing [MG] of 1.00 ь 10-3 M, [sugar] of 2.00 x 10-2M,
and [OH-] ranging from 1.00 x 10-2 to 6.00 x 10-2 M. The
ionic strength is equal to 0.25 M, while T = 298 K. The
increase of [OH-] results in increase of the reaction rate as
shown in Table 2. The slope of a plot of ln kobs vs ln[OH-]
indicates that the reaction is a first order one.
Effect of the ionic strength on the rate constant
observed
The effect of the ionic strength on the rate constants
observed is determined studying reaction mixtures of
[MG] equal to1.00x10-3, [S] equal to 2.00x10-2 M, [OH-]
Fig. 3. Plot of lnKobs vs. 10 l1/2.
equal to 6.00x10-2 M. The ionic strength I ranges between
0.10 M and 0.90 M, while T = 298 K. The reaction rate is
independent of the ionic strength of the solution. A plot
of lnkobs vs I1/2 gives a straight line of a slope equal to
zero as shown in Fig. 3. This implies a neutral molecule
participation in the rate determining step.
Salt effect on the rate constant observed
The salt effect on the observed rate constants observed
is determined studying reaction mixtures containing [MG]
of 1.00x10-3, [sugar] of 2.00x10-2 M, [OH-] of 6.00x10-2 M,
[salt] ranging from 0.002 M to 0.2 M. The ionic strength is
equal to 0.25 M, while T = 298 K. The addition of KCl and
NaNO3 do not affect the reaction rate as illustrated in Fig.
4. This implies an outer sphere mechanism for the reaction.
Effect of temperature and determination of
activation parameters
The temperature effect on the rate constants observed is determined varying the temperature within
the range from 298 K to 308 K in case [MG] is equal
Table 2. Effect of hydroxyl ion concentration on the rate constant observed.
102[OH-]/M
103kobs/s-1
Glucose
Fructose
Xylose
1.00
1.98
1.81
2.20
2.00
3.14
2.30
3.00
3.00
4.22
3.82
4.21
4.00
5.05
4.85
5.30
5.00
6.70
6.25
6.00
6.00
8.10
7.41
6.91
-3
-3M [S] 2.00 X 10-2-2M I = 0.25 M T = 298 K
[MG]
1.00
X
10
[MG] 1.00 x 10 M [S] 2.00 x 10 M I = 0.25 M T = 298 K
528
Latona Dayo Felix
where k is the rate constant, T is the temperature, ΔH#
is the enthalpy of activation, ΔS# is the entropy of activation, ΔG# is the free Gibb’s energy of activation, R is
the gas constant, k/ is the Boltzmann’s constant, while
h is the Plank’s constant.
Fig. 4. Plot of the observed rate constant on the salt concentration used.
to 1.00x10-3 M, [sugar] is equal to 2.00x10-2 M, [OH-]
is equal to 6.00x10-2 M and I = 0.25 M. The rates constants observed at different temperatures are estimated.
The values of the activation parameters are calculated
using the Arrhenius equation (Eq.1). They are listed in
Table 3. It is evident from the high negative values of
entropies of activation that the reaction rate increases
with temperature increase. Moreover, the nearly constant
values of the Gibbs’ free energies of activation suggest
a similar mechanism in case of sugars studied.
log 𝑘𝑘 = log 𝐴𝐴 –
𝑘𝑘
ln � � =
𝑇𝑇
𝐸𝐸𝑎𝑎
2.303𝑅𝑅𝑅𝑅
(1)
−∆𝐻𝐻 #
𝑘𝑘 /
∆𝑆𝑆 #
+ ln � � + �
�
𝑅𝑅𝑅𝑅
ℎ
𝑅𝑅
Mechanism and rate law
The non-dependence of the oxidation reaction on
the ionic strength indicates the participation of a neutral
molecule in the rate determining step. The almost identical values of ΔG# show that the reaction mechanism
stays unchanged, while and negative values of ΔS# are
indicative of an associative mechanism, i.e. the reaction occurs between ions of similar charges. The FTIR
spectrum of the product (Fig. 5) shows very broad –OH
stretching at 3400 cm-1 - 2400 cm-1, C=O stretching at
1730 cm-1 - 1700 cm-1 and C-O stretching between 1320
cm-1 - 1210 cm-1, which can be attributed to the presence
of carboxylic acid. The Michaelis-Menten plot shows
the presence of an intermediate complex. The relatively
low values of ΔH# reveal the absence of high energy
free radicals.
In view of the results pointed above, the following
reaction mechanism is advanced. The alkaline fading of
malachite green in the presence of sugars proceeds via
the conversion of sugars to their corresponding enediols,
which is a fast step. Then the enediol reacts with malachite
green via an outer sphere mechanism to give a complex
(slow step), which in turn decomposes into products.
(2)
𝑘𝑘 /
ln �
ℎ
� = 23.76
ΔG#= ΔH# - TΔS#
(3) (4)
Table 3. Activation parameters.
Sugar
Ea(kJmol-1)
ΔH#(kJmol-1)
ΔS#(kJK-1mol-1)
ΔG#(kJmol-1)
Glucose
23.52
21.04
-0.182
75.28
Fructose
25.12
22.64
-0.177
75.38
Xylose
31.34
28.86
-0.156
75.35
-3
-2
-2
[MG] 1.00x10
M [OH-]
[OH-] 6.00x10
[MG]
1.00x10-3 M
M [sugar]
[sugar] 2.00x10
2.00x10-2M
6.00x10-2 M
M II == 0.25
0.25 M
MT
T=
= 298
298 K
K
529
Journal of Chemical Technology and Metallurgy, 52, 3, 2017
Fig. 5. FTIR spectrum of the product.
(5)
REFERENCE
(6)
From Eqs. (4) and (6)
−𝑑𝑑[𝑀𝑀𝑀𝑀]
𝑑𝑑𝑑𝑑
=
𝑘𝑘 2 𝑘𝑘 1 [𝑆𝑆][𝑂𝑂𝑂𝑂 − ][𝑀𝑀𝑀𝑀]
𝑘𝑘 −1 +𝑘𝑘 2 [𝑀𝑀𝑀𝑀]
(7)
Limiting condition
If 𝑘𝑘−1 is for greater than к2 [MG]. Then Eq. 7 becomes
−𝑑𝑑[𝑀𝑀𝑀𝑀]
𝑑𝑑𝑑𝑑
= 𝐾𝐾1 𝑘𝑘2 [S][O𝐻𝐻−][MG]
CONCLUSIONS
The research work shows that the mechanism of
alkaline malachite green fading in presence of reducing
sugar is similar to that of the oxidation of sugars in an
alkaline medium. The reaction product characterized by
FTIR spectrometry reveals the presence of a carboxylic
acid. This implies that sugars are oxidized to their corresponding acids by malachite green, while malachite green
is decolorized via the corresponding reduction process.
However, in absence of sugar, the hydroxyl ion decolorizes malachite green by attacking the central C atom of
its planar ring, thereby destroying its conjugation orientation, which could have resulted in production of carbinol.
530
Acknowledgements
I appreciate the technical support given by the technical staff of the Central Science Laboratory, Obafemi
Awolowo University, Ile-Ife, Nigeria.
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