Cent. Eur. J. Chem. • 12(3) • 2014 • 325-331
DOI: 10.2478/s11532-013-0383-4
Central European Journal of Chemistry
Determination of vitamin B6
(pyridoxine hydrochloride) based
on a novel BZ oscillating reaction
system catalyzed by a macrocyclic complex
Research Article
Qingling Zeng , Lulu Chen , Xianyi Song ,
Gang Hu1*, Lin Hu2
1
1
1
1
Department of Chemistry,
Anhui University,
Hefei 230601, P. R. China
2
Institute of Applied Chemistry,
East China Jiaotong University,
Nanchang 330013, P. R. China
Received 3 July 2013; Accepted 28 October 2013
Abstract: This
paper reports a new method for determination of VB6 (pyridoxine hydrochloride) by its perturbation effects on a novel BelousovZhabotinskii (BZ) oscillating system. This novel BZ system, in which malic acid serves as the substrate, contains an enzyme-like
complex, macrocyclic complex {[CuL](ClO4)2}, as catalyst. The ligand L in the complex is 5,7,7,12,14,14-hexamethyl-1,4,8,11tetraazacyclotetradeca-4,11-diene. Results show that the addition of pyridoxine hydrochloride can perturb the oscillation amplitude and
period, and the change of the oscillation amplitude is linearly proportional to the concentration of pyridoxine hydrochloride in the range of
5×10-7- 2.5×10-4 M. The obtained RSD with seven samples is 3.073%. An assay of pharmaceutical tablets of vitamin B6 was evaluated.
Some foreign ions were studied with respect to their possible influence on the determination of pyridoxine hydrochloride. The factors
which influence this reaction include the concentration of reactant, the temperature of the reaction, property of catalyst, etc. Furthermore,
the possible reaction mechanism has been proposed using the Field-Körös-Noyes (FKN) model.
Keywords: Determination • Oscillating reaction • Macrocyclic complex • Vitamin B6
© Versita Sp. z o.o.
1. Introduction
Metabolism, a set of life-sustaining chemical reactions,
involves enzymes or co-enzymes which catalyze
reactions allowing organisms to grow and reproduce [1].
Vitamin B6, which serves as an enzyme or co-enzyme,
plays an important role in amino acid metabolism,
gluconeogenesis and liquid metabolism. Vitamin B6 has
many medical applications such as treatment of vomiting
during pregnancy and radiation sickness, and it can also
help people preventing and recovering from anemia [2].
Thereby, a rapid and convenient determination method
is required.
An oscillatory chemical system, which is maintained
far-from equilibrium by keeping it open, has already
been used as a tool to analyze organic compounds
and inorganic ions. Among all oscillating systems,
the Belousov-Zhabotinskii (BZ) reaction, the catalytic
oxidation of organic compounds by the acidic bromate
[3,4], is the most widely studied for analyte determination
purposes and is modeled by the FKN mechanism [5].
The analyte pulse perturbation technique (APP) [6],
which is based on a pulse perturbation on the chemical
system (mainly the BZ system), has become a subject
in modern quantitatively analytic methods since it was
first proposed by Tichonova et al. in 1978 [7]. There
* E-mail: [email protected]
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Determination of vitamin B6 (pyridoxine hydrochloride)
based on a novel BZ oscillating reaction system
catalyzed by a macrocyclic complex
N
NH
Cu
N
Scheme 1.
2+
2ClO4-
HN
The structure of [CuL] (ClO4)2.
are many reports using this perturbation technique to
determine the quantity of tracing amounts of some metal
ions and species etc. [8-10], while the majority of the
catalysts being used in such BZ systems are Ce3+, Mn2+,
Fe(phen)32+, or Ru(bipy)32+ [11-14].
Compared to these catalysts, macrocyclic complexes
could be regarded as novel catalysts in the BZ system
because of the presence of the extended p-system in
the macrocyclic ligands, which ensure a high rate for
reactions involving electron transfer at individual steps
of the oscillating process [15]. This character favors their
application as catalysts in B-Z system for determination
of analytes [16-21], because such macrocyclic
complexes-catalyzed BZ systems have lower activation
energies, higher oscillating frequencies [20] and are
vulnerable to external perturbations. From the viewpoint
of analytical determination, the stronger the response is,
the higher the sensitivity is.
During the oscillation process, the macrocyclic
copper(III) complex [CuL]3+ gets an electron to form
macrocyclic copper (II) complex [CuL]2+ and back to this
state in a cycle. The reduced macrocyclic copper(II)
complex [CuL]2+ is red in color and the oxidized
macrocyclic copper(III) complex [CuL]3+ is orange.
During the oscillation, the system undergoes periodic
changes from red to orange then back to red due to
the changes in [CuL]2+ and [CuL]3+, and hence it can be
recorded by an electrochemical instrument grounded on
the potential changes over time. By using macrocyclic
copper(II) complex-catalyzed BZ system, we have
already measured catechol [16], alizarin red S [17],
pyrogallol [18], calcium pantothenate [19], Ag+ [20], and
paracetamol [21].
Apart from serving as the essential component of
two enzymes for amino acids metabolism, vitamin B6
also serves as a co-enzyme for many reactions and
can help facilitate decarboxylation, transamination,
racemization, elimination, replacement and beta-group
interconversion reactions [22]. So it is meaningful to
develop an efficient determination method to study its
physiological function and diagnosis in some diseases in
clinical medicine. Some methods for the determination
of pyridoxine hydrochloride have been reported
[23-25], and these methods need complex instruments
like Continuous stirred-tank reactor (CSTR) or their limits
of detection are not at μM level but at mM level. Here
we used macrocyclic copper (II) complex-catalyzed
BZ system, as a tool for determination of pyridoxine
hydrochloride. Our quantitative analytical method, with
a simple instrument, provides a new method for precise
determination of pyridoxine hydrochloride, which can be
detected in the range of 5×10- 7 M to 2.5×10- 4 M.
2. Experimental procedure
2.1. Reagents
The catalyst [CuL] (ClO4)2 (Scheme 1) was synthesized
according to literature [26] and was identified by its IR
spectrum and elemental analysis. The ligand L in the
catalyst [CuL] (ClO4)2 is 5,7,7,12,14,14-hexamethyl1,4,8,11-tetraazacyclotetradeca-4,11-diene. Pyridoxine
hydrochloride was obtained from Aladdin Chemistry
Co.Ltd; pharmaceutical vitamin B6 tablets (containing
pyridoxine hydrochloride) were purchased from Nanjing
Baijingyu Pharmaceutical Co., Ltd. and NaBrO3, malic
acid and sulfuric acid were obtained commercially.
All reagents were of analytical quality without further
purification. Solutions of 0.6 M NaBrO3, 2 M malic acid,
0.0221 M [CuL] (ClO4)2 were separately prepared in 1 M
sulfuric acid. Solutions of 0.1 M pyridoxine hydrochloride
and 0.1 g per 5 mL (0.09725 M) pharmaceutical Vitamin
B6 tablets were made immediately before the experiment.
Solutions with lower concentrations were prepared prior
to use. Double distilled water was used in all cases.
2.2. Apparatus
Oscillating experiments were carried out in a 50 mL vessel
thermostated at 18 ± 0.5ºC with a Model 79-3 magnetic
stirrer (Jiangsu, China) and stirring rate was kept at 500
rpm. A 213 type platinum electrode (Shanghai, China)
monitored the temporal oscillations, using a Model
217 saturated calomel electrode (Shanghai, China)
connected via a salt bridge containing 1 M Na2SO4 as
the reference electrode. Potentials of the electrode as
a function of time were recorded with a PHS-25B digital
voltmeter (Shanghai, China) connected with a Model
XWTD-204 Y-t recorder (Shanghai, China) to record
kinetic curves of the reaction.
3. Results and discussion
3.1. Behavior of the completely unperturbed
and perturbed oscillator
Unperturbed oscillator was prepared in the following
order: 28.6 mL of 1.0 M H2SO4, 1.3 mL of 0.6 M NaBO3,
3.6 mL of 2.0 M malic acid and 6.5 mL of 0.0221 M [CuL]
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Q. Zeng et al.
(ClO4)2 while perturbed BZ mixture had 0.6 mL of different
concentration of pyridoxine hydrochloride added into the
blank mixture using a pipette. Recordings of Potential
(Pt) vs. time in the absence (Fig. 1a) and presence
(Fig. 1b) of pyridoxine hydrochloride perturbations
is shown in Fig. 1. During the oscillation, the periodic
change of solution color (red–orange–red) was
observed. This can be explained by a one-electronic
transfer process:
[CuL] 2+ (red) ⇔ [CuL] 3+ (orange)
Several injection points were tested for the sake of the
experiment accuracy and repeatability. When pyridoxine
hydrochloride was injected at the minimum of the cycle,
the oscillation system went into a new oscillatory state
with the shorter period and reduced amplitude. The
changes in oscillation amplitude are proportional to the
concentration of pyridoxine hydrochloride, so a new
method could be expected to exploit this behavior for
determining pyridoxine hydrochloride.
3.2. Influence of experimental variables
The behavior of an oscillating chemical reaction is
easily influenced by the experimental variables in the
system. In order to obtain the optimum value for working
conditions, the influences of the experimental variables
on the proposed oscillating reaction were studied.
According to Jiménez-Prieto et al. [27], the optimum
value for working conditions were selected with three
criteria: (a) maximizing the stability of the oscillating
system over time, which enhances the reproducibility
of the results; (b) maximizing the oscillation amplitude,
which ensures maximal sensitivity for the determination
of the analyte; (c) ensuring that the oscillation period
allowed the effect of the analyte perturbation to be
accurately determined. Accordingly, we selected
changes in oscillation amplitude, ∆A, as one of the
measured parameters. ΔA = |A – A0|, where A0 and A are
the oscillation amplitude before and after the injection,
respectively. Other parameters like oscillation period are
also monitored.
The influence of changing the concentration of
sulfuric acid (0.8-1.3 M) on the changes in oscillation
amplitude of the system is shown in Fig. 2a. With
increasing initial concentration of sulfuric acid, the
changes in oscillation amplitude (∆A) decreased first
and increased again at concentration of 0.95 M. A
concentration of 1.0 M was chosen as the optimum point
because the system has a good profile in both perturbed
and unperturbed oscillation.
Changes in the catalyst [CuL] (ClO4)2 concentration
over the range 0.001 M to 0.0045 M had a significant
Figure 1.
Typical oscillation profiles for the proposed oscillation
system in the absence (a) and presence (b) of 2×10-5 M
pyridoxine hydrochloride perturbation using a platinum
electrode. Common conditions: [NaBrO3] =1.95×10-2 M,
[H2SO4] =1.0 M, [malic acid] =0.18 M, [CuL] (ClO4)2
=3.595×10-3 M. T=18.3ºC. (a) [pyridoxine hydrochloride]
= 0.000 M; (b) [pyridoxine hydrochloride] = 2×10-5 M.
effect on the behavior of the oscillation system. From
Fig. 2b, it could be easily noted that with increasing
[CuL] (ClO4)2 concentration, ∆A almost decreased till it
reached a minimum at 0.00359 M. A concentration of
0.00359 M was chosen according to the above three
criteria.
The effect of the sodium bromate was studied over the
range 0.01 to 0.028 M. The change of the amplitude with
increase in sodium bromate concentration is similar to
the [CuL] (ClO4)2 and the effects are illustrated in Fig. 2c.
As we can see, the changes in the oscillation amplitude
(∆A) decreased to a minimum at the concentration of
0.0195 M and then increased with the further increase
of sodium bromate concentration. A concentration of
0.0195 M was finally adopted as optimal as the system
oscillated uniformly.
We have studied the effects of malic acid
concentration from 0.15 to 0.21 M, and the results
are shown in Fig. 2d. As the concentration increased,
the curve of changes in oscillation amplitude (∆A) first
decreased to a minimum and then reached a maximum
slowly, after which it decreased again. A 0.18 M malic
acid concentration was finally selected as optimal
since it maximizes the stability of the oscillating system
over time, which enhances the reproducibility of the
results.
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Determination of vitamin B6 (pyridoxine hydrochloride)
based on a novel BZ oscillating reaction system
catalyzed by a macrocyclic complex
Figure 2.
Table 1.
Influence of the concentrations of (a) Sulfuric acid; (b) [CuL] (ClO4)2; (c) Sodium bromate; (d) malic acid on the pyridoxine
hydrochloride perturbed oscillation system. Common conditions: T = 18.3±0.5ºC. [VB6] = 2×10-5 M. (a) [NaBrO3] = 1.95×10-2 M,
[malic acid] = 0.18 M, [CuL] (ClO4)2 = 3.595×10-3 M. (b) [NaBrO3] = 1.95×10-2 M, [H2SO4] = 1.0 M, [malic acid] = 0.18 M. (c)
[H2SO4] = 1.0 M, [malic acid] = 0.18 M, [CuL] (ClO4)2 = 3.595×10-3 M. (d) [NaBrO3] = 1.95×10-2 M, [H2SO4] = 1.0 M, [CuL] (ClO4)2
= 3.595×10-3 M.
Influence of Foreign Ions and Species on the Determination of 2×10-5 M Pyridoxine Hydrochloride.
Foreign ions and species
Tolerated ratio
Al3+, NH4+, Li+, Na+
2000:1
Ni , Zn , Cu
2+
2+
2+
200:1
CO
100:1
23
glucose
10:1
Cl , Mn , C2O
-
1000:1
K+
2+
The oscillating system was perturbed with a sample,
which contained pyridoxine hydrochloride and variable
amounts of interferences. More than 10 foreign ions
causing an error of less than 5% in the determination
of 2×10–5 pyridoxine hydrochloride were studied
(Table 1). It can be concluded that some species such as
Ag+ and iodide has a strong effect on the determination
of analyte, but large amounts of Cu2+ and most common
ions have little effect on the determination.
24
Ag+, Fe2+, Fe3+, I-
1:1
0.1:1
The temperature also had a strong influence on the
oscillating system, as it was reported in the literature
by Körös et al. [28]. Temperature dependence and
temperature compensation were reported recently [29].
High temperature was found to decrease the amplitude
of the oscillation, the oscillation period and the life of
the oscillation. Moreover, the mixture turned muddy
and no oscillation happened at a higher temperature.
This could be explained by each reaction in the chain
having a different sensitivity to temperature. Increasing
the temperature accelerates the reaction process and
reduces the induction time. To obtain an exact and
recurrent oscillating system, the temperature was
maintained at 18°C.
The influences of foreign species and ions in
the perturbed B-Z mixture were also investigated.
3.3. Determination of pyridoxine hydrochloride
We performed perturbation experiments under the
optimal experimental conditions described above. The
response to the pyridoxine hydrochloride perturbation
was tested by employing changes in oscillation amplitude
(ΔA) for the cycle following the sample injection as the
measured parameter.
A fitted straight line was obtained by plotting
changes in oscillation amplitude (Δ A) vs. concentration
of pyridoxine hydrochloride, as it is shown in Fig. 3.
As can be seen from Figs. 1 and 3, the concentration
of pyridoxine hydrochloride in the mixture has to be at
μM level. (5×10- 7 M—2.5×10-4 M) The calibration data
obtained obey the following linear regression equation:
ΔA = 0.96874 + 173423.18683 [VB6] (r = 0.99929, n = 9)
The precision (%RSD, relative standard deviation),
calculated from seven perturbations of 2×10-5 M
pyridoxine hydrochloride, was 3.073%. The detection
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Q. Zeng et al.
mechanism in the B-Z reaction perturbed by pyridoxine
hydrochloride are the same as those perturbed by
calcium pantothenate.
Based on the well known Field-Körös-Noyes (FKN)
[5,30,31] mechanism, the overall reactions are simplified
into the following nine processes:
HOBr + Br– + H+
Br2 + H2O
(1)
HBrO2 + Br– + H+
2HOBr
(2)
BrO3–+ Br–+ 2H+
Figure 3.
Plotting of changes in amplitude (∆A) vs. concentration
of pyridoxine hydrochloride in the range 5×10-7
to 2.5×10-4 M. (Common conditions: [NaBrO3] =
1.95×10-2 M, [H2SO4] = 1.0 M, [malic acid] = 0.18 M,
[CuL] (ClO4)2 = 3.595×10-3 M. T = 18.3oC).
Table 2. Determination
Results and Recovery of Vitamin B6 Tablets in Water Sample.
Sample
Determined/
10–6mol dm–3
Found/
10 mol dm–3
Recovery
(%)
1
5.97
6.5
108.8
2
9.72
9.23
94.9
3
9.95
10.2
102.5
4
14.58
15.26
104.6
5
19.45
18.2
93.5
–6
2HBrO2
HOBr + HBrO2
BrO3– + HOBr + H+
HBrO2 + BrO3– + H+
Br2O4
Br2O4 + H2O
2BrO2·
(3)
(4)
(5)
(6)
Br2 + HOOCCHOHCH2COOH →
→ Br–+ H+ + HOOCCHOHBrCHCOOH
(7)
BrO2·+ [CuL] 2+ + H+ → [CuL] 3+ + HBrO2
(8)
HOOCCHOHBrCHCOOH + 6[CuL] 3+ + 3H2O →
→ 6[CuL] 2++ Br–+ 2HCOOH+ 2CO2 + 7H+
(9)
Pyridoxine hydrochloride could be oxidized to
4-pyridoxic acid, according to the literature [32,33]. So
it is reasonable for us to believe in the occurrence of
pyridoxine hydrochloride oxidation by oxidizing species
in such a system, which can be expressed as follows:
(10)
Scheme 2. Standard redox potentials of bromine.
limit obtained is 5×10 M. Such a precision is quite
acceptable.
In order to assay the suitability of the proposed
method, the same procedure with known amounts of
the analytical solution of the pharmaceutical vitamin B6
tablets was applied. The recovery experiments indicate
that the accuracy of the proposed method is in the range
93.5% to 108.8%. From above experimental results, it
is clear that this method is suitable for practical sample
analysis (Table 2).
-7
3.4. Mechanism of action of pyridoxine hydrochloride on the oscillating system
The shape of the oscillation amplitude (ΔA) vs.
concentration of pyridoxine hydrochloride is similar
to that perturbed by calcium pantothenate [19]. This
strongly suggests that the main features of the reaction
The oxidizing species in the system are either initial
reagents or intermediates that cannot be quantitatively
measured. These are BrO3–, HBrO2, HOBr, BrO2•, Br2O4,
or [CuL]3+ [30,31]. Pyridoxine hydrochloride could be
oxidized by one, two or more of those oxidizing species.
Standard redox potentials of bromine are shown in
Scheme 2, according to the literature [5]. We assumed
that HBrO2 is the most probable candidate that could
participate in the oxidation of pyridoxine hydrochloride,
for the potential of HBrO2/HOBr (1.74 V) is the highest
among the above potentials.
When pyridoxine hydrochloride is injected into the
oscillation system, the direct reaction of pyridoxine
hydrochloride with HBrO2 causes a reduction of HBrO2
concentration according to Reaction 10. According to
Reaction 5 Br2O4 concentration would decrease as a
result of the decrease of HBrO2 concentration. As the
Br2O4 concentration decreases, the BrO2· concentration
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Determination of vitamin B6 (pyridoxine hydrochloride)
based on a novel BZ oscillating reaction system
catalyzed by a macrocyclic complex
will decrease accordingly due to Reaction 6. The
decrease in BrO2·concentration causes the decrease
in [CuL] 3+ concentration according to Reaction 8; as
only little parts of the [CuL]2+ concentration could be
oxidized into [CuL]3+, [CuL]2+concentration would remain
unaffected. Therefore, the value of ln {[CuL]3+/[CuL]2+}
decreases accordingly. As a result, the decrease in the
maxim potential (corresponds to maxim concentration
of [CuL]3+) was obtained. As the [CuL]2+concentration
does not change, the minimum potential (corresponds
to maxim concentration of [CuL]2+) would not change
either. Therefore, a decrease in oscillation amplitude
(from minimum potential to maxim potential) could be
observed (Fig. 1b).
After the injection of pyridoxine hydrochloride,
the shorter period length observed may be ascribed
to an increased bromination reaction. According to
Reaction 10, the concentration of HBrO2 (oxidant) would
decrease because it is consumed when it reacts with
pyridoxine hydrochloride. Decrease in HBrO2 would
make the equilibrium of Reaction 3 shift to the right, and
such a shift causes an increase in the concentration of
HOBr. There will be an increase in Br2 concentration
as a result of the increase in the concentration of HOBr,
according to Reaction 1. Increase in Br2 concentration
would cause an increased bromination reaction,
according to Reaction 7. As a result, shorter period
length could be observed (Fig. 1b).
4. Conclusions
The results of our present paper illustrate that pyridoxine
hydrochloride can be detected using the macrocyclic
Cu(II) complex-catalyzed BZ reaction, composed of
malic acid/ BrO3- / [CuL](ClO4)2 / H+, as a tool. Moreover,
larger linear range ( ca. 5×10-7 M—2.5×10-4 M) and
lower detection limit (ca. 10-7 M) was found here. In
order to obtain the optimum working conditions for
quantitative analysis, the effects of concentrations of
sodium bromate, sulfuric acid, malic acid and [CuL]
(ClO4)2 were examined. A proper explanation was that
pyridoxine hydrochloride was oxidized into 4-pyridoxic
acid. Following from the experimental results presented
here, such a perturbation technique based on BZ
oscillating reaction is a suitable analytical method for
measuring vitamin B6.
Acknowledgment
The authors gratefully acknowledge funding of this
work by the National Science Foundation of China
(21171002).
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