Electrocatalytic oxidation of hydrazine at overoxidized polypyrrole film modified glassy carbon electrode

Article abstract of DOI:10.1016/j.electacta.2007.04.019

Electrocatalytic oxidation of hydrazine (HZ) was studied on an overoxidized polypyrrole (OPPy) modified glassy carbon electrode using cyclic voltammetry and chronoamperometry techniques. The OPPy-modified glassy carbon electrode has very high catalytic ability for electrooxidation of HZ, which appeared as a reduced overpotential in a wide operational pH range of 5-10. The overall numbers of electrons involved in the catalytic oxidation of HZ, the number of electrons involved in the rate-determining and diffusion coefficient of HZ were estimated using cyclic voltammetry and chronoamperometry. It has been shown that using the OPPy-modified electrode, HZ can be determined by cyclic voltammetry and amperometry with limit of detection 36 and 3.7 μM, respectively. The results of the analysis suggest that the proposed method promises accurate results and could be employed for the routine determination of HZ.

Full text of DOI:10.1016/j.electacta.2007.04.019

Electrochimica Acta 52 (2007) 6248–6253
Electrocatalytic oxidation of hydrazine at overoxidized polypyrrole
film modified glassy carbon electrode
Mir Reza Majidia, Abolghasem Jouybanb, Karim Asadpour-Zeynalia,∗
a Department of Analytical Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz 51664, Iran
b Faculty of Pharmacy and Drug Applied Research Center, Tabriz University of Medical Sciences, Tabriz 51664, Iran
Received 4 January 2007; received in revised form 9 March 2007; accepted 3 April 2007
Available online 7 April 2007
Abstract
Electrocatalytic oxidation of hydrazine (HZ) was studied on an overoxidized polypyrrole (OPPy) modified glassy carbon electrode using cyclic
voltammetry and chronoamperometry techniques. The OPPy-modified glassy carbon electrode has very high catalytic ability for electrooxidation
of HZ, which appeared as a reduced overpotential in a wide operational pH range of 5–10. The overall numbers of electrons involved in the
catalytic oxidation of HZ, the number of electrons involved in the rate-determining and diffusion coefficient of HZ were estimated using cyclic
voltammetry and chronoamperometry. It has been shown that using the OPPy-modified electrode, HZ can be determined by cyclic voltammetry and
amperometry with limit of detection 36 and 3.7 ␮M, respectively. The results of the analysis suggest that the proposed method promises accurate
results and could be employed for the routine determination of HZ.
© 2007 Elsevier Ltd. All rights reserved.
Keywords: Hydrazine; Overoxidized polypyrrole; Voltammetric behavior
1. Introduction
properties, owing to the permeability of the polymer and to
the generation of anionic groups in the polymer backbone with
high electron density that act as a barrier against the diffusion
of anionic species into the film [8,9]. Overoxidized PPy films
acquire properties that resemble those of non-conducting poly-
mers, but offer an additional advantage of the control of the film
thickness during the electropolymerization process in contrast
with non-conducting polymers that exhibit self-limited growing
and afford only modest enzyme loads owing to the small film
thickness achieved [10].
Hydrazines (HZ) are important compounds of interest in
chemical and pharmaceutical industries. They are employed in
areas such as fuel cells, herbicides, catalysts, rocket propellants
and so on. There are several electrochemical methods for deter-
mination of hydrazine in the literature with different detection
limit, pH, linear range and the potential of hydrazine oxidation.
Table 1 summarized details of the methods from available refer-
ences along with their analytical parameters and compared them
with the proposed electrode in this work.
Conductingpolymerfilmshavebeenwidelystudiedforappli-
cations in chemical sensors and biosensors. Polypyrrole (PPy) is
one of the most extensively used conducting polymers for con-
structing of bioanalytical sensors and modified electrodes [1].
Applications of PPy to trace metal determinations by voltam-
metry have generally required an immobilization of a selective
chemical reagent onto/into the polymer film, such as PPy cova-
lently modified with N-carbodithionate ligating groups [2] or
incorporation of anionic reagents into the polycationic oxi-
dized PPy film during the pyrrole electropolymerisation process
[3–6].
The use of non-conducting polymers with permselec-
tive properties in enzyme entrapment applications is another
effective alternative for the construction of interference-free
biosensors [7]. In this sense, the irreversible overoxidation of
PPy films allows to obtain a non-conducting film with remark-
able permselectivity stemming from size- and anion-exclusion
Previously we have used the overoxidized polypyrrole–GC
electrode to elcrocatalysis and determination of isoniazid, a
derivative of HZ [27]. In this work, a glassy carbon electrode
coated with overoxidized polypyrrole (OPPy) film for electro-
∗
Corresponding author. Tel.: +98 411 3393111; fax: +98 411 3340191.
E-mail address: asadpour@tabrizu.ac.ir (K. Asadpour-Zeynali).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2007.04.019

M.R. Majidi et al. / Electrochimica Acta 52 (2007) 6248–6253
6249
Table 1
The reported electrodes from the literature and the proposed one for electrocatalytic determination of hydrazine, their common analytical parameters and the references
Electrode
Modifier
Electrochemical method
Ep
pH
Linear range (␮M)
LOD
References
1
2
3
4
Glassy carbon
Glassy carbon
Glassy carbon
Cylindrical carbon fiber
microelectrodes
Carbon nanotube powder
microelectrodes
Glassy carbon
Chlorogenic acid
Fe–tetraaminophthalocyanine Cyclic voltammetry
Pyrocatechol violet
Rhodium
Cyclic voltammetry
240
–
13
7.5
50–1000
1–10000
5–40
–
–
4.2
0.62
[11]
[12]
[13]
[14]
−300
Amperometry
240
Flow injection with
amperometric detection
Amperometric
0, 300, 500 7.5
5–1000
5
6
–
90
7
-
1–10000
0–12000
–
–
[15]
[16]
Cobalt
Cyclic voltammetry
500
pentacyanonitrosylferrate
Cobalt phthalocyanine
Hybrid copper–cobalt
hexacyanoferrate
Hydroquinone salophen
derivatives
7
8
Glassy carbon
Carbon paste
Cyclic voltammetry
Cyclic voltammetry
150
240
13.3
7
2000–20000
100–12000
–
–
[17]
[18]
9
Glassy carbon
Gold electrode
Glassy carbon
Glassy carbon
Cyclic voltammetry
212
350
200
203
–
7
9
7
10–400
0.13–0.92
5–600
1.6
5
[19]
[20]
[21]
[22]
10
11
12
Iron phthalocyanine
Osteryoung square wave
voltammetry
Linear sweep
voltammetry
Amperometric
Pyrogallol red
2
Hematoxylin on the
multi-wall carbon
nanotube
2–122.8
0.68
13
14
Glassy carbon
Carbon fiber
o-Aminophenol
Co and Cu
Amperometric
Potentiometric
110
550
9
7
2–20
1–1000
0.5
0.5
[23]
[24]
microelectrode
Carbon ceramic
composite electrode
Carbon
hexacyanoferrate
Chlorogenic acid
15
16
Amperometric
Flow injection
Amperometric
250
200
265
8
0.1–1000
2–100
0.02
0.27
3.6
[25]
Copper–palladium alloy
nanoparticle
Overoxidized polypyrrole
7.4
9
[26]
Glassy carbon
1.3–2000
This work
catalytic determination of HZ has been developed as a simple,
sensitive, rapid and new assay method for HZ.
potentiostatic method at 0.9 V for 100 s. This electrode was used
for preparation of modified electrode.
Overoxidized polypyrrole were prepared for characteriza-
tion in two polymerization/overoxidation steps. Polypyrrole was
first elctropolymerized on the GC by using cyclic voltammetry
method. In this procedure potential was scanned from −0.2 V to
0.8 V at the scan rate of 50 mVs−1 for four times. The polymeriz-
ing solution was 0.05 M pyrrole and 0.1 M LiClO4 in deionized
water. PPy was then overoxidized by scanning potential from
−0.3 V to 0.8 V versus SCE at the scan rate of 10 mVs−1 for
eight times in 0.5 M NaOH.
2. Experimental
2.1. Reagents and instrumentation
HZ and other reagents were of analytical grade supplied by
Merck. Pyrrole is purified by double distillation and stored in
a refrigerator and prevented from light. Deionized water was
used for the preparation of all solutions. The background elec-
trolyte solution was prepared from potassium chloride. The pH
of solutions was adjusted to 9 with ammonia buffer. A glassy
carbon disk electrode with 2 mm diameter was used as working
electrode. A platinum wire was employed as counter electrode
and a saturated calomel electrode (SCE) served as the reference
electrode and all potentials in the text refer to it (all electrodes
obtained from Azar Electrode Co., Urmia, Iran). Voltammo-
gramswereobtainedwithPGSTAT20Autolabpotentiostatfrom
ECO Chemie (the Netherlands).
3. Results and discussion
Fig. 1a shows the voltammograms recorded for the overox-
idation process of PPy. There is a prominent peak at
approximately 390 mV during the first cycle. During the sec-
ond and third cycles, however, featureless voltammograms
were recorded (Fig. 1b and c). This indicates that an irre-
versible electrochemical transformation of the polymer film
has been achieved during the first cycle. This transformation
is accomplished by a loss of conjugation and hence electronic
conductivity [10,13,27] and a net electronegative character is
imparted to the polymer film and it then undergoes permselec-
tive behavior resulting in overoxidized films [28]. Also at this
process, the surface of electrode is activated.
2.2. Preparation of modified electrode
The glassy carbon electrode was polished with 0.05 ␮m alu-
mina in water slurry using a polishing cloth and deionizied water.
The polished electrode was placed in a 0.5 M NaOH solution and
the electrochemical activation of the electrode was performed by

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M.R. Majidi et al. / Electrochimica Acta 52 (2007) 6248–6253
data at the bare GC electrode, an increase in peak current and a
decrease in the overpotential of HZ were observed at the OPPy
modified electrode.
The pH of the HZ solutions was changed using buffer
solutions ranging from pH 5 to 10 and its effect on the electro-
chemical signal was investigated. The peak potential (Ep) values
of HZ were dependent on pH and shifts toward a less positive
potential along with an increase in the pH of the buffer system.
The Ep changes with pH, and slope was 72 mV per pH unit at the
range 5–9. Such a discrepancy has also been observed by other
groups on electrocatalytic oxidation of hydrazine, and attributed
to have a kinetic rather than thermodynamic origin [13,29]. On
the other hand, the intensity of peaks decreases at pH > 10. From
these results, pH 9 was selected for further studies.
Fig. 1. Repetitive CVs for the overoxidation process of PPy doped with LiClO4
in 0.5 M NaOH. Scan rate: 100 mV s−1 (a) first cycle; (b) second cycle; (c) third
cycle.
The nature of the oxidation process was found to be diffusion-
controlled in the buffer system studied, as evidenced from the
linearplotsofthepeakcurrent(ip)versussquarerootofthescane
rate (v1/2) for HZ. Fig. 3A shows the cyclic voltammograms of
OPPyGC electrode in 0.1 M ammonia buffer (pH 9.0) containing
1 mM HZ at scan rates: 1, 2, 5, 10, 20, 50, 75, 100, 200, 300,
400 and 500 mVs−1. It can be noted from Fig. 3 that the anodic
currents increase and the peak potential shifts as the scan rate
increases. When peak current values were plotted against v1/2
(Fig. 3B), the following linear relationship was obtained:
Ip = 0.6888 + 1.3918 v1/2(mV1/2
s
−1/2), R2 = 0.999 (3)
Fig. 2. Cyclic voltammograms of 1 mM HZ at (a) modified and (b) bare glassy
carbon electrode in pH 9 NH3/NH4Cl buffer solution at scan rate 50 mVs−1
The voltammogram of c is the background.
.
This behavior suggests that the oxidation process is controlled
by diffusion.
Thecyclicvoltammogramsof1 mMHZataGC(b)andOPPy
modified GC (a) at pH 9 NH3/NH4Cl buffer solution at scan
rate 50 mVs−1, were shown in Fig. 2. At the bare GC (without
pretreatment, modification and activation) electrode, HZ shows
a featureless voltammogram (Fig. 2b). But at the OPPy modified
electrode, it appears at 265 mV (Fig. 2a). In comparison with the
For determination of the transfer coefficient (α) and the num-
ber of electrons involved in the rate-determining (nα), Tafel plot
was drawn (not shown hear) using the data from the rising part
of the current voltage curve at a scan rate of 1 mVs−1. The Tafel
slope was 164 mVdecade−1 and by considering that α equals
∼
0.64, theresultsindeedsuggestone-electron(n = 1.1 1)trans-
=
α
Fig. 3. (A) Cyclic voltammorams of OPPy–GC electrode in 0.1 M ammonia buffer (pH 9) containing 1 mM HZ at scan rate (a)1 mVs−1; (b) 2 mVs−1; (c) 5 mVs−1
;
(d) 10 mVs−1; (e) 20 mVs−1; (f) 50 mVs−1; (g) 75 mVs−1; (h) 100 mVs−1; (i) 200 mVs−1; (j) 300 mVs−1; (k) 400 mVs−1 and (l) 500 mVs−1. (B) Dependence of
the peak current with square root of the scan rate in 0.1 M ammonia buffer (pH 9) containing 1 mM HZ.

M.R. Majidi et al. / Electrochimica Acta 52 (2007) 6248–6253
6251
Fig. 4. The plot of Ep vs. Log v.
fer process in the rate-determining step for the electrocatalytic
oxidation of HZ.
The Tafel slope could be calculated by another method, i.e.
it can be determined from the following equation [30]:
b
Ep = Log(v) + k.
(4)
2
Fig. 6. Plot of i vs. t−1/2 obtained from chronoamprometric experiments for the
OPPyGC modified electrode in 0.1 M ammonium buffer solution (pH 9) con-
taining HZ concentrations of (1) 15 ␮M; (2) 43 ␮M; (3) 77 ␮M; (4) 112 ␮M; (5)
148 ␮M; (6) 188 ␮M; (7) 225 ␮M and (8) 310 ␮M. Inset shows the relationship
between the slope of the linear segments and the HZ concentration.
where b is the Tafel slope and k is a constant value. Fig. 4 shows
the plot of Ep versus Log(v). The slope of this plot is 68.79 mV,
therefore, the slope becomes 137.58 mVdecade−1. This result
is close to that obtained from the rising part of voltammogram
recorded at the 1 mVs−1. If α was assumed equal to 0.64, nα is 1.
Totalnumber of electrons(n)in oxidation process wascalculated
according to the following equation for a totally irreversible
Fig. 5. Chronoamprometric response of a OPPyGC electrode in 0.1 M ammo-
nium buffer solution (pH 9) containing different concentrations of HZ for a
potential step of 450 mV vs. SCE. The numbers 1–13 correspond to 15, 43, 77,
112, 148, 188, 225, 310, 515, 901, 1585, 2438 and 3338 ␮M of HZ, respectively.
Insets: Variation of chronoamperometric currents at t = 35 s vs. concentration of
HZ.
Fig. 7. Amperometric response of a OPPy modified glassy carbon electrode
kept in 450 mV in 0.1 M ammonium buffer solution pH 9 containing different
concentrations of HZ from 0.15 to 1.29 mM. Insets: (A) the same amperograms
in the range of 0.15 × 10−6 to 7.30 × 10−6 M; (B) calibration plot of HZ.

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M.R. Majidi et al. / Electrochimica Acta 52 (2007) 6248–6253
diffusive process [31]:
ship between current and time can be described by the Cottrell
equation [30]:
Ip = 3.01 × 105n[(1 − α)]nα]1/2ACD1/2v1/2
(5)
i = nFAD1/2C/π1/2 1/2
t
(6)
With considering [(1 − α)nα] = 0.46, D = 3.1 × 10−5 cm2 s−1
(see chronoamperometric studies) and A = 0.0314 cm2, it is esti-
where D and C are the diffusion coefficient (cm2 s−1) and the
bulk concentration (M), respectively. From the Cottrell equation,
it can be seen that the plot of i versus t−1/2 is linear, and from
the slope, the value of D can be obtained. Fig. 6 shows the
experimental plots for different concentrations of HZ employed.
The slopes of the resulting straight lines were then plotted versus
the concentration of HZ (Fig. 6, inset), from which we calculated
a diffusion coefficient of 3.1 × 10−5 cm2 s−1 for HZ.
mated that the total number of electrons involved in the anodic
∼
oxidation of HZ is n 4. Based on the above results the follow-
=
ing mechanism can be proposed for the oxidation of HZ:
N2H4 + H2O → N2H3 + H3O+ + e−
N2H3 + 3H2O → N2 + 3H3O+ + 3e−
(I)
(II)
The rate-determining step is a single electron transfer fol-
lowed by a three-electron process to give N2 as a final product.
The electrocatalytic oxidation of HZ at the OPPyGC
electrode was studied by chronoamperometry. The chronoam-
perograms obtained for a series of HZ solutions with various
concentrations as illustrated in Fig. 5. An increase in concentra-
tion of HZ was accompanied by an increase in anodic currents
obtained for a potential step of 450 mV versus SCE. In addi-
tion, the level of the Cottrell current, which was measured
for 35 s, increases with increasing concentration of HZ. The
response is linearly proportional to the concentration of HZ in
the range of 15–3338 ␮M with two linear segments (Fig. 5, inset
C). In chronoamperometric studies, the diffusion coefficient of
HZ for the modified electrode can be determined. The relation-
Fig. 7 shows amperograms, which were recorded from stirred
solutions under conditions where the OPPy modified electrode
potential, was kept at 450 mV during the successive addition of
HZ. Fig. 7 (inset B) clearly shows that the measured current
increases by increasing the HZ concentration in solution.
As Fig. 7 shows, an increase in concentration of HZ was
accompanied by an increase in anodic currents obtained for a
potential step of 450 mV versus SCE. In addition, the level of
currentsincreaseswithincreasingconcentrationofHZ. Theinset
(B) of Fig. 7 clearly shows that the plot of peak current versus
HZ concentration is constituted from two linear segments with
different slopes, corresponding to two different ranges of sub-
strate concentrations. We ascribe this to a change in catalytic
reaction conditions arising from the formation of nitrogen gas
Fig. 8. Cyclic voltammograms for increasing concentrations of HZ from (1) 1.3 × 10−6 to (27) 2.1 × 10−3 M in buffer solution (pH 9.0) on OPPyGC electrode. Scan
rate was 50 mVs−1
.

M.R. Majidi et al. / Electrochimica Acta 52 (2007) 6248–6253
6253
4. Conclusion
Glassy carbon electrode coated with overoxidized polypyr-
role film was used for electrocatalytic determination of HZ. It
was shown that HZ can be determined using the cyclic voltam-
metry and amperometry techniques on the basis of its oxidation
process at an overoxidation polypyrrole glassy carbon mod-
ified electrode. The overall number of electrons involved in
the oxidation of HZ, the number of electrons involved in the
rate-determining step and the diffusion coefficient of HZ were
calculated.
Acknowledgement
Fig. 9. Calibration plot for concentrations of HZ from cyclic voltammograms.
Financial support from the University of Tabriz is gratefully
acknowledged.
bubbles at the surface of the modifier. Indeed, at low substrate
concentrations, gas formation is negligible, and has no effect
on the diffusion of HZ towards the electrode surface. While, at
high concentrations of HZ, gas evolution at the electrode surface
slackened to some extent the normal diffusion of substrate.
Under the optimum conditions, peak currents of cyclic
voltammograms linearly depended on HZ concentrations. The
cyclic voltammograms at different concentrations of HZ are
shown in Fig. 8. A plot of the peak current values as a func-
tion of the concentration was drawn. The plot was linear in the
concentration range of 1.3 × 10−6 to 2.1 × 10−3 M HZ. For the
regression plot of the peak current versus HZ concentration,
the slope was 14.68 ␮A mM−1, the y-intercept was 0.0036 ␮A
and the correlation coefficient was R2 = 0.9992 (see Fig. 9). The
detection limit calculated from the calibration graph was 36 ␮M
HZ [32].
The stability and reproducibility of the OPPy–GC electrode
was studied by cyclic voltammetry and repetitive amperome-
try. Fig. 10 shows the repetitive amperogram of the OPPy–GC
electrode kept in 450 and 0 mV in 0.1 M ammonium buffer
solution (pH 9) in five levels of HZ concentrations. Also five
modified electrodes were prepared and their cyclic voltammo-
grams were recorded at 0.5 mM HZ (pH 9). The R.S.D. for ip
and Ep of voltammograms were less than 4%. Above results
showed that the modified electrodes have good stability and
reproducibility.
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Fig. 10. Repetitive amperogram of the OPPy–GC electrode kept in 450 and
0 mV in 0.1 M ammonium buffer solution pH 9 containing (a) background; (b)
1.71; (c) 3.88; (d) 6.70; (e) 17.81 and (f) 28.15 ␮M from HZ.