Antimony film electrode for direct cathodic measurement of sulfasalazine

Article abstract of DOI:10.1016/j.electacta.2011.09.087

The antimony film electrode (SbFE) is presented for the first time for direct cathodic voltammetric measurement of an organic compound, i.e. sulfasalazine, which has been chosen due to its aptness for electrochemical reduction and its great importance as a pharmaceutical product. The SbFE was prepared ex situ on the surface of a glassy carbon supporting electrode and several important parameters were studied and optimized, such as preparation of the SbFE, stripping voltammetric settings, pH of the measurement solution, etc. In addition, the electroanalytical performance of the SbFE was compared to its bismuth counterpart and bare glassy carbon electrode. The SbFE exhibited excellent linear dependence in the examined concentration range of 3 × 10^-6-2.5 × 10^-4 M together with the detection limit of 7.8 × 10^-7 M and good reproducibility with the RSD of ±0.7%. Finally, the applicability of the SbFE was successfully demonstrated through convenient measurements of sulfasalazine in its dosage forms of sulfasalazine delayed-release tablets.

Full text of DOI:10.1016/j.electacta.2011.09.087

Electrochimica Acta 58 (2011) 523–527
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Antimony film electrode for direct cathodic measurement of sulfasalazine
Biljana Nigovic´ a,∗, Samo B. Hocevarb
a University of Zagreb, Faculty of Pharmacy and Biochemistry, A. Kovacica 1, 10000 Zagreb, Croatia
b Analytical Chemistry Laboratory, National Institute of Chemistry, Hajdrihova 19, Sl-1000, Ljubljana, Slovenia
a r t i c l e i n f o
a b s t r a c t
Article history:
The antimony film electrode (SbFE) is presented for the first time for direct cathodic voltammetric mea-
surement of an organic compound, i.e. sulfasalazine, which has been chosen due to its aptness for
electrochemical reduction and its great importance as a pharmaceutical product. The SbFE was pre-
pared ex situ on the surface of a glassy carbon supporting electrode and several important parameters
were studied and optimized, such as preparation of the SbFE, stripping voltammetric settings, pH of the
measurement solution, etc. In addition, the electroanalytical performance of the SbFE was compared to
its bismuth counterpart and bare glassy carbon electrode. The SbFE exhibited excellent linear depen-
dence in the examined concentration range of 3 × 10−6–2.5 × 10−4 M together with the detection limit
of 7.8 × 10−7 M and good reproducibility with the RSD of 0.7%. Finally, the applicability of the SbFE
was successfully demonstrated through convenient measurements of sulfasalazine in its dosage forms
of sulfasalazine delayed-release tablets.
Received 8 August 2011
Received in revised form
23 September 2011
Accepted 30 September 2011
Available online 6 October 2011
Keywords:
Antimony film electrode
Voltammetry
Sulfasalazine
Pharmaceutical analysis
© 2011 Elsevier Ltd. All rights reserved.
1. Introduction
configurations and with respect to other analytes which might
be measured using bismuth electrodes [13]. Also the antimony
As advanced electrochemical techniques still allure consider-
able attention in modern analytical chemistry, the development
of novel electrode surfaces, sensors and associated sensing
approaches is still one of the most captivating topics in many
electroanalytical laboratories. Different electrode materials offer
almost unlimited possibilities for tailoring various supporting elec-
trodes with diverse geometries and sizes, and for their further
modification to address the growing needs for enhanced sensitiv-
ity, selectivity, durability, etc. [1]. For lengthy decades, mercury has
been one of the most commonly used electrode materials, partic-
ularly for its application in trace metal analysis and for measuring
selected organic compounds [2,3]. In a search for a mercury-free
electrode, which can be used for the aforementioned applications,
bismuth film and antimony film electrodes were introduced in 2000
and 2007, respectively, and by now, their convenient operation has
been demonstrated and proven mainly in combination with anodic
and cathodic stripping voltammetric measurements of trace metal
ions [4–7]. Besides, bismuth electrodes have been successfully
employed also in direct voltammetric and stripping voltammet-
ric mode for measuring several organic compounds [8–12]. After
one decade from its first introduction, the bismuth electrode
has been accepted in electroanalytical laboratories worldwide,
although there are still many important issues which need to
be adequately addressed, particularly in a view of their different
electrode, which exhibits attractive electroanalytical performance
comparing favorably to its bismuth and mercury counterparts, has
been investigated in different configurations, e.g. in situ and ex situ
prepared antimony film electrode, deposited onto different sup-
porting electrodes such as glassy carbon electrode, carbon paste
electrode, boron-doped diamond electrode and carbon fiber micro-
electrode, and as macroporous antimony film electrode [5,14–18].
Furthermore, Economou reported on the preparation of a sput-
tered antimony film electrode and its application for adsorptive
stripping analysis [19]. Among other features, the antimony elec-
trode revealed very good performance in more acidic media, e.g.
with pH ≤ 2. It has become obvious that antimony, as an electrode
material, exhibits extremely interesting electroanalytical proper-
ties and holds great promise in modern electroanalysis, particularly
for measuring trace metal ions. Nevertheless, first reports about
the application of antimony as an electrode material go back to
the beginning of the 20th century, however, associated only with
potentiometric measurements of pH in different environments
exploiting a well-known Sb/Sb2O3 equilibrium [20,21]. Moreover,
the field of organic electroanalysis based on the antimony elec-
trode has still remained completely unexplored and certainly needs
a particular attention.
Sulfasalazine belongs to the important class of medications
called anti-inflammatories (Scheme 1). It is used to treat inflamma-
tory bowel disease such as ulcerative colitis and Crohn’s disease. It is
employed in clinical practice as 5-aminosalicylic acid (5-ASA) pre-
cursor. The active part, 5-ASA, is produced in the colon by bacterial
reduction of azo-bridge [22]. 5-ASA may reduce inflammation by
∗
Corresponding author. Tel.: +385 1 6394 453; fax: +385 1 4856 201.
E-mail address: biljana@pharma.hr (B. Nigovic´).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.09.087

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B. Nigovi´c, S.B. Hocevar / Electrochimica Acta 58 (2011) 523–527
COOH
COOH
H
2e-, 2H+
H
H
N
N
N
HO
HO
N
S
S
N
N
N
N
H
O
O
O
O
Scheme 1. Molecular structure of sulfasalazine and the mechanism of its electro-reduction at the SbFE.
blocking the activity of cyclooxygenase and lipoxygenase thereby
reducing the production of prostaglandins. Sulfasalazine is also
known as a disease-modifying anti-rheumatic drug. It can reduce
the symptoms and slow-down the progress of rheumatoid arthritis,
ankylosing spondylitis, psoriatic arthritis, and juvenile rheumatoid
arthritis [23].
Britton–Robinson buffer solution (pH 4) covering the concentration
range of 3.0 × 10−6–2.5 × 10−4 mol L−1. The content of sulfasalazine
in the pharmaceutical preparation was measured and assessed via
calibration equation.
2.3. Preparation and renewal of the SbFE/GCE
In the present work, we have demonstrated the first successful
application of the ex situ prepared antimony film electrode oper-
ating under direct voltammetric mode for measuring sulfasalazine
as an example of organic analyte, thus expanding the scope of the
antimony electrodes towards organic electroanalysis.
A glassy carbon electrode (GCE) was polished thoroughly with
aqueous slurry of 0.05 ␮m alumina powder on a polishing pad and
subsequently rinsed with purified water. The electrode was then
transferred into the plating solution containing 0.01 M HCl and usu-
ally 20 mg L−1 Sb(III). The antimony film was deposited ex situ onto
the supporting GCE surface in the presence of dissolved oxygen by
applying a constant potential of −0.7 V for 60 s while stirring the
solution. Finally, the SbFE was rinsed with 0.01 M HCl and ready
to use. To remove the antimony film, an electrochemical cleaning
step was introduced by applying a potential of +0.5 V for 15 s in an
acidic solution of 0.01 M HCl, after which the deposition of a fresh
film was carried out. For comparison, the bismuth film electrode
was prepared ex situ according to the same procedure described
earlier [12]. Polishing was carried out prior to each electrochem-
ical measurement in the case when a bare GCE was used as the
working electrode and occasionally only, when GCE was used as
the substrate/supporting electrode for the preparation of SbFE.
2. Experimental
2.1. Apparatus
All voltammetric measurements were carried out using a
␮-Autolab potentiostat (Eco Chemie, Utrecht, The Netherlands)
controlled by GPES 4.9 software. A three-electrode configuration
was used with the working antimony (or bismuth) film mod-
ified glassy carbon electrode or a bare glassy carbon electrode
(GCE, 2-mm diameter, Metrohm, Switzerland), Ag/AgCl/3 M KCl
(Metrohm) reference electrode, and a graphite rod as counter elec-
trode. The test solutions were deoxygenated prior voltammetric
measurements by purging with purified nitrogen for 4 min. All mea-
surements were performed at room temperature (23 2 ◦C) in a
20 mL electrochemical cell. When required, stirring was applied
using a computer-controlled stirrer at ca. 300 rpm.
3. Results and discussion
The electroanalytical performance of the ex situ prepared SbFE
for direct voltammetric measurement of organic analyte was exam-
ined using a model drug molecule sulfasalazine with already
well-established cathodic current response studied at bare glassy
carbon electrode (GCE) and bismuth film electrode (BiFE) [12]. The
electrochemical reduction of sulfasalazine at the SbFE was inves-
tigated initially using square-wave voltammetric (SWV) mode in
Britton–Robinson buffer solutions in the pH range of 3.6–5.0 as
depicted in Fig. 1. Studying the pH effect of the model solution upon
the electrode response unveiled a signal increase up to pH 4 and
2.2. Reagents and solutions
Sulfasalazine and its commercial gastro-resistant Sulfasalazine
Krka EN® (Krka d.d., Novo mesto, Slovenia) tablets containing
500 mg of the active ingredient were kindly supplied by Agency
for Medicinal Products and Medical Devices (Zagreb, Croatia). Anti-
mony chloride and bismuth nitrate pentahydrate were purchased
from Sigma–Aldrich (Steinheim, Germany). All other chemicals
were of analytical grade quality. Ultra-pure water used for standard
and buffer solutions was prepared by a Milli-Q system (Millipore,
Bradford, USA).
-12
-0.52
2
A stock solution of sulfasalazine (0.005 M) was prepared by dis-
solving its adequate amount in purified water with addition of a
drop of 2 M NaOH to facilitate dissolution. Standard solutions were
prepared daily by diluting the stock solution with a supporting elec-
trolyte just before use. Britton–Robinson buffer solutions (0.04 M in
each of acetic, phosphoric and boric acids) adjusted to the desired
pH with addition of a 0.2 M NaOH were used as supporting elec-
trolytes.
Ten sulfasalazine delayed-release tablets were weighted and
crushed to a fine powder. An adequate amount of this pow-
der equivalent to 20 mg of sulfasalazine was transferred into the
10 mL volumetric flask. To dissolve the active ingredient, 5 mL of
0.2 M NaOH was added. The mixture was sonicated for 10 min
and then completed to volume with the purified water. The
tablet solution was filtered through a 0.2 ␮m Acrodisc GHP fil-
ter (Gelman, Ann Arbor, USA). An aliquot of the filtrate was then
transferred into a calibrated flask and dilution was made with a
-0.48
-0.44
3
-8
-4
0
1
-0.40
3.6
4.4 4.8 5.2
4.0pH
4
-0.2
-0.4
-0.6
-0.8
-1.0
E vs (Ag/AgCl)/V
Fig. 1. Square-wave voltammograms of 5 × 10−4 M sulfasalazine obtained at the
SbFE in Britton–Robinson buffer solutions with pH 3.6 (1), 4.0 (2), 4.5 (3) and 5.0 (4).
SWV settings: frequency of 100 Hz, amplitude of 50 mV and potential step of 2 mV.
Inset shows a dependence of peak potential upon the pH of measurements solution.

B. Nigovi´c, S.B. Hocevar / Electrochimica Acta 58 (2011) 523–527
525
-6
-4
-2
0
Evidently, the electroanalytical performance of the SbFE for mea-
suring sulfasalazine is advantageous over that observed at the BiFE
and, particularly, over that of bare GCE. This behavior suggests that
the electrochemical reduction of the examined drug molecule at
the selected electrodes, under identical conditions, must be the
same process, however, the electron transfer kinetics is obviously
higher in the case of SbFE. Furthermore, preliminary CV studies
revealed the improved electroanalytical performance of the SbFE
due to its favorable surface stability providing great possibilities for
convenient repetitive voltammetric measurements without time-
consuming surface cleaning step as in the case of GCE (inset of
Fig. 2). Namely, strong adsorption of sulfasalazine onto the sur-
face of bare GCE induced significant decrease of the corresponding
voltammetric signal after each successive scan.
1
2
-12
-8
-4
0
-0.2
-0.6 -0.8 -1.0
E vs-0(A.4g/AgCl)/V
3
-0.2
-0.4
-0.6
-0.8
-1.0
E vs (Ag/AgCl)/V
Fig. 2. Comparison of voltammetric signals for 5 × 10−4 M sulfasalazine obtained
at the antimony film (1), bismuth film (2) and bare glassy carbon electrode (3) in
Britton–Robinson buffer solution with pH 4.0. SWV settings are as in Fig. 1. Inset
depicts successive cyclic voltammograms of 5 × 10−4 M sulfasalazine recorded at the
Further electroanalytical characteristics of the SbFE were eval-
uated with respect to the dependence of a SWV response for
2 × 10−4 M sulfasalazine on the parameters such as concentra-
tion of the antimony ions in the plating solution, deposition time
and deposition potential associated with the antimony film prepa-
ration (Fig. 3). These experiments were conducted to find the
optimum conditions for the antimony film deposition via study-
ing the reproducibility of cathodic current response of sulfasalazine
for ten successive measurements for each prepared electrode. On
the basis of this study, the optimal concentration of antimony ions
for the ex situ film deposition was 20 mg L−1. The voltammetric
signal increased up to 40 mg L−1 of antimony ions in the plat-
ing solution (Fig. 3A), however, lower concentration of antimony
resulted in a more stable film. The effect of deposition time was
also investigated as another important parameter influencing the
quality of antimony coating. As demonstrated in Fig. 3B, the sig-
nal increased up to 90 s of deposition time, however, at longer
deposition times the reproducibility of current response decreased,
possibly as a consequence of thicker antimony film which, as antic-
ipated, became less stable. Finally, the deposition potential was
examined in the potential window of −0.5 V to −1.1 V in combina-
tion with the deposition time of 60 s (Fig. 3C). Evidently, the SWV
signal increased while decreasing the deposition potential from
−0.5 V to −0.7 V, whereas at more negative deposition potentials
the signal attenuated. Furthermore, the current response started to
decrease significantly after applying more negative potentials than
−0.9 V. The antimony film with optimal properties was attained
using the deposition potential of −0.7 V for 60 s in the plating
solution containing 20 mg L−1 antimony(III). The SbFE prepared
ex situ according to these optimized conditions exhibited favor-
able electroanalytical performance for measuring drug molecule
with a well-developed signal and an excellent reproducibility. The
relative standard deviation (RSD) of the peak current was 0.7%
(mean ip = 6.18 ␮A) and the RSD of corresponding peak potential
was found to be 0.5% (mean Ep = −0.475 V). The stability of SbFE
was further evaluated by monitoring a voltammetric response dur-
ing increased number of consecutive measurements at the same
antimony film. These recordings indicated that the signal was fairly
constant up to 40 repetitions in combination with 10 s stirring step
between each measurement with calculated RSD of the peak cur-
rent of 0.9%. The signals started to decrease slightly thereafter
indicating the need for re-plating a new antimony film onto the
supporting electrode surface.
SbFE in Britton–Robinson buffer solution (pH 5.0) using a scan rate of 100 mV s−1
.
then gradual decrease with increasing pH value due to involvement
of a proton transfer in the examined reduction process. Considering
the pH dependence of current response for sufasalazine at the SbFE,
a Britton–Robinson buffer solution with pH 4 was chosen for further
work. A linear shift of the peak potential towards more negative
values was also observed while increasing the pH. The slope value
obtained was 59.4 mV pH−1 indicating a 2e−/2H+ electro-reduction
process, which can be attributed, in accordance with previous stud-
ies, to the reduction of an azo-bond of the examined drug molecule
to corresponding hydrazo-species (Scheme 1) [12]. Also successive
cyclic voltammograms (CV) showed one well-defined irreversible
reduction peak, as depicted in the inset of Fig. 2. Undistorted signals
obtained with subsequent potential scans suggested the possibility
of achieving highly reproducible response at the antimony film. CV
behavior was examined further as a function of a scan rate in order
to obtain additional insights into the redox reaction under inves-
tigation. Variation of the scan rate in the range of 20–400 mV s−1
resulted in a linear relationship between the cathodic peak current
and the square-root of a scan rate. Additionally, the peak potential
was shifted towards more negative potential values with increasing
the scan rate. These observations imply on a diffusion-controlled
irreversible reduction process at the SbFE.
To improve the sensitivity, SWV was employed for measur-
ing sulfasalazine due to the most favorable response compared to
other voltammetric modes such as differential pulse voltamme-
try and linear-sweep voltammetry. Variation of the peak current
for sulfasalazine recorded at the SbFE was monitored while chang-
ing instrumental parameters, i.e. frequency, pulse amplitude and
potential step. The peak current was observed to increase linearly
upon raising the frequency in the range of 20–120 Hz, and at fre-
quencies higher than 100 Hz, the signal was obscured by a large
background current. When the pulse amplitude was varied in the
range of 10–90 mV the voltammetric signal was optimal for the
amplitude of 50 mV. The influence of potential step was investi-
gated in the interval from 1 to 6 mV. Finally, the most favorable
response was achieved using a combination with a frequency of
100 Hz, a pulse amplitude of 50 mV and a potential step of 2 mV.
A comparative study of the SbFE, BiFE and bare glassy car-
bon electrode for measuring 5 × 10−4 M sulfasalazine as a model
organic molecule is presented in Fig. 2. The newly introduced SbFE
exhibited a very similar behavior to that of the BiFE considering
position of the cathodic voltammetric signal, however the SbFE
provided approximately 35% higher current response compared to
the signal obtained at the BiFE. In comparison with bare GCE, the
peak potential of sulfasalazine obtained at the SbFE was ca. 50 mV
less negative and the voltammetric signal was higher for ca. 40%.
Using optimal conditions, the electroanalytical performance
of the SbFE was monitored with respect to the increas-
ing concentration levels of the analyte. An excellent linear
response in the concentration range of 3 × 10−6–2.5 × 10−4 M was
obtained, as shown in the inset of Fig. 4. The calibration plot
is described by the following regression/calibration curve: ipa
(␮A) = 2.16 × 104C − 0.133 (r = 0.9991), whereas the standard devi-
ations for the slope and the intercept of the calibration curve
were 3.99 × 102 and 0.0056, respectively. For comparison, the BiFE

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B. Nigovi´c, S.B. Hocevar / Electrochimica Acta 58 (2011) 523–527
6
5
4
3
A
B
5
4
3
0
5
10 15 20 25 30 35 40 45
0
15
30
45
t/s
60
75
90
-1
Conc. of Sb(III) / mg L
5
C
4
3
2
-0.4
-0.6
-0.8
-1.0
-1.2
E vs (Ag/AgCl)/V
Fig. 3. Effect of the antimony(III) concentration (A), deposition time (B) and deposition potential (C) upon the voltammetric response of 2 × 10−4 M sulfasalazine at SbFE in
Britton–Robinson buffer solution with pH 4. SWV settings are as in Fig. 1.
exhibited a similar linear range, but the improved sensitivity was
obtained in the case of SbFE (e.g. slope of the calibration plot for BiFE
was 1.56 × 104) [12]. The quantification limit calculated from the
calibration curve using the criteria of 10s/m, where s is the standard
deviation of the intercept and m denotes the slope of the calibration
curve, was 2.6 × 10−6 M and correspondingly the detection limit
defined as 3s/m was estimated to 7.8 × 10−7 M for SbFE, whereas
the detection limits for BiFE and bare GCE reported previously
were 1.4 × 10−6 and 6.2 × 10−6, respectively [12]. One of the most
important attributes of the SbFE is its remarkably improved repro-
ducibility without the need for additional and time-consuming
mechanical polishing of the electrode surface before each measure-
ment as, e.g. in the case of GCE. Additionally, the RSD value of peak
current for inter-day precision evaluated by six replicate measure-
ments of 1 × 10−4 M sulfasalazine over three days, each time at a
freshly prepared electrode surface, was 1.5%. These results con-
firm that the SbFE in combination with the proposed analytical
method is reliable and extremely convenient tool for measuring
conveniently low concentration levels of sulfasalazine.
In order to assess the practical use, the SbFE was employed for
direct voltammetric measurement of sulfasalazine in its pharma-
ceutical dosage form. Sulfasalazine was measured in commercial
delayed-release tablet form at different concentration levels
(1.5 × 10−5–1.8 × 10−4), as shown in Fig. 4. The amount of active
ingredient present in a tablet solution was calculated from the cor-
responding calibration equation/plot from the inset of Fig. 4. The
analysis of the compound in its pharmaceutical formulation exhib-
ited the mean recovery of 99.2% with the relative standard deviation
of 1.6% indicating the adequate accuracy and precision of the SbFE.
Finally, the effect of excipients upon the voltammetric response
for the examined drug was studied by adding different amounts of
standard to the formulation solution samples. The mean recovery
of 98.7% indicated that excipients did not interfere with the assay,
thus corroborating the suitability of the SbFE for this purpose.
4. Conclusions
-6
-6
In the previous sections, the first application of the antimony
film electrode (SbFE) in pharmaceutical analysis is demonstrated
when choosing sulfasalazine as the model compound. The anti-
mony film electrode was prepared ex situ and used in a direct
voltammetric mode. The SbFE revealed favorable electroanalyti-
cal characteristics and when compared to its bismuth and bare
glassy carbon counterpart, it exhibited superior performance. Fur-
thermore, the real applicability of the SbFE was demonstrated via
measuring sulfasalazine in its pharmaceutical dosage form. These
results pave the way towards organic electroanalysis in combina-
tion with the antimony-based electrodes – a broad research field
which certainly needs to be further investigated.
-4
-2
-4
0
1x10-4 2x10-4
0
c/M
-2
0
-0.2
-0.4
-0.6
-0.8
-1.0
E vs (Ag/AgCl)/V
Acknowledgements
Fig. 4. Voltammetric measurements of sulfasalazine tablet solutions for increas-
ing drug concentrations from 1.5 × 10−5 to 1.8 × 10−4 M in Britton–Robinson buffer
solution with pH 4 together with background response obtained at the SbFE. Inset
shows the calibration plot obtained using the pure substance in the range of
3 × 10−6–2.5 × 10−4 M. SWV settings are as in Fig. 1.
This work was supported through a grant (Investigation of new
methods in analysis of drugs and bioactive substances) from the
Ministry of Science, Education and Sports of the Republic of Croatia.

B. Nigovi´c, S.B. Hocevar / Electrochimica Acta 58 (2011) 523–527
527
Financial support from the Slovenian Research Agency (P1-0034) is
gratefully acknowledged.
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