Determination of iron in water samples by adsorptive
stripping voltammetry with a bismuth film electrode
in the presence of 1-(2-piridylazo)-2-naphthol
Rodrigo Seguraa,∗, Mar´ ıa In´ es Torala, Ver´ onica Arancibiab
aDepartamento de Qu´ ımica Anal´ ıtica, Facultad de Qu´ ımica, Universidad de Chile,
Las Palmeras 3425, Casilla 653, Santiago, Chile
bDepartamento de Qu´ ımica Inorg´ anica, Facultad de Qu´ ımica, Pontificia Universidad Cat´ olica de Chile,
Vicu˜ na Mackenna 4860, Santiago 22, Chile
of a complex of iron with 1-(2-piridylazo)-2-naphthol (PAN) on a bismuth-coated glassy carbon electrode (BiFE). Factors affecting the stripping
performance, such as pH, PAN concentration (CPAN), potential, accumulation time (Eads, tads), and interference by other ions were also studied.
The optimum conditions were obtained in a 0.1molL−1acetate buffer at pH 4.0, CPAN5.0?molL−1, tads60s, Eads−400mV, pulse height 4.0mV,
pulse amplitude 25mV, and frequency 15Hz. The detection limit was found to be 0.1?gL−1when a tadsof 60s was used, and the linear range
was from 0.4 to 60.0?gL−1. The proposed procedure was validated by determining of Fe(III) in CRM-MFD, QCS-19 and CRM-SW certified
reference materials and applied in seawater samples with satisfactory results.
Keywords: Adsorptive stripping voltammetry; Iron; 1-(2-Piridylazo)-2-naphthol (PAN); Seawater samples; Bismuth film electrode
Total dissolved iron in surface waters of oceanic regimes
can range from less than 0.05 to greater than 10nmolL−1
[1–3]. However, in some beaches near populated or indus-
trial areas iron concentration can be higher. The analysis of
iron in seawater is difficult due to both the low concentrations
and the seawater matrix. Therefore, shipboard determination
of iron in seawater requires a sensitive analytical technique
and trace-metal clean sample handling to obtain meaningful,
oceanographically consistent results. The presence of iron in
research vessels, laboratories and many manufactured materi-
als poses a risk of contamination during sampling, filtration,
storage and analysis. The first large-scale international inter-
comparison of analytical methods for the determination of
dissolved iron in seawater was carried out between October
∗Corresponding author. Tel.: +56 29787262.
E-mail address: email@example.com (R. Segura).
2000 and December 2002. The exercise was conducted as a
rigorously “blind” comparison of seven analytical techniques
by 24 international laboratories. For the complete dataset of 45
results (after excluding three outliers not passing the screen-
ing criteria), the mean concentration of dissolved iron in the
ironages samples was 0.59±0.21nmolL−1, with a coefficient
of variation of 36% . Higher values were found in acidi-
fied samples from Monterey Bay by a flow injection method
combined with inductively coupled plasma sector field mass
spectrometry (ICP-SFMS) using the NTA superflow resin in
the preconcentration step (average 2.89nmolL−1) . Iron can
be determined by several methods such as inductively coupled
plasma mass spectrometry (ICP-MS) , electrothermal atom-
ization atomic absorption spectrometry (ETAAS) , cathodic
stripping voltammetry (CSV) [8–10], luminescence , and
available are expensive to be used in routine analysis (ICP-MS
and ETAAS). Electroanalytical techniques like anodic stripping
voltammetry (ASV), cathodic stripping voltammetry (CSV),
and adsorptive stripping voltammetry (AdSV) have important
advantages including high sensitivity, accuracy and precision,
as well as the low cost of instrumentation. AdSV is based on
by potential controlled adsorption and subsequent electrochem-
ical oxidation or reduction of the preconcentrated species. For
decades, due to several electrochemical advantages, mercury
electrodes have been widely used in stripping analysis. How-
ever, the well-known toxicity and handling inconveniences of
mercury have recently declined considerably the popularity of
mercury electrodes. The bismuth film electrode (BiFE) was
introduced as an extremely promising alternative, based in that
no present toxic character in relation to with those mercury
electrodes [13–22]. BiFE has been used principally in anodic
Ni ; Mo ; Cr [31,32]; V ) and over the last years
(2003–2007), a few selected application of AdSV on the BiFE
have also been reported with promising results (Cr with DTPA
; Co-DMG [35–37]; Ni-DMG ; Co and Ni with DMG
[39–41]; U-Cupferron ; Al-Cupferron ).
The aim of this study was to optimize the determination of
iron using the bismuth film electrode. The metal was accumu-
This ligand has been widely used as chomophore reagent in
spectroscopic techniques for the determination of several metal
ions at trace level, but there are no reports on adsorptive strip-
ping voltammetry with BiFE. The method was validated using
certified reference material (CRM-MFD mixed food diet and
QCS-19 standard solution) and was applied to the analysis of
(SWAdSV) were obtained with a CV50W Voltammetric
analyzer (Bioanalytical Systems, Inc., BAS, USA). A 10mL
capacity cell was equipped with Ag/AgCl/KCl 3molL−1
reference electrode, a glassy carbon working electrode (3-mm
diameter, BAS, USA) and auxiliary platinum electrode. A
mechanical mini-stirrer, and a capillary to supply an inert gas
were also used. An Orion pH meter was used to determine the
pH of the solutions.
lipore Milli-Q system (Milford, MA, USA). Bismuth and iron
standard solutions (1000mgL−1) were obtained from Merck
(Darmstadt, Germany). Acetic acid buffers (pH 3.0–6.0) were
prepared by mixing 5.7mL of acid and diluting to 1L with
water. The pH was adjusted with sodium hydroxide solution.
A 1mmolL−1solution of PAN (Sigma) was prepared by dis-
solving 0.2493g of solid compound in 100mL of ethanol.
ASTM D 665 synthetic seawater was obtained from Aldrich.
Certified reference material of seawater (CRM-SW), trace met-
als in mixed food diet (CRM-MFD) reference materials, and
quality control standards (QCS-19) obtained from high-purity
standards (Charleston, SC, USA) were used for validation
2.3. Preparation of BiFE electrode
alumina powder, then, washed with deionized water in an ultra-
sonic bath. Bismuth was deposited on the GCE from 10.0mL of
a 100mgL−1Bi(III) solution containing 0.1molL−1of acetate
for 5min with stirring. The modified electrode was rinsed with
water and was ready for use.
All bottles and containers used for standards and samples
were thoroughly cleaned with 5% nitric acid before use. Fil-
tration was done through 0.45-?m membrane filters. Seawater
samples were obtained from five different beaches of Vi˜ na del
Mar (Chile) in a highly populated zone and near copper and oil
All voltammetric measurements were carried out in
0.10molL−1acetate buffer solution (pH 4.0) at room temper-
ature (23±2◦C) containing 5.0?molL−1PAN as complexing
agent. The solution was purged with nitrogen for at least 5min.
A deposition potential of −400mV vs. Ag/AgCl was applied to
the working electrode. During the deposition step, the solution
was stirred, and after an equilibration period of 10s the voltam-
mogram was recorded by applying a negative-going potential
scan between −300 and −1100mV. Square wave voltammo-
grams were obtained with an amplitude of 25mV, a frequency
of 15Hz, and a potential step of 4mV.
3. Results and discussion
3.1. Cyclic voltammetry
Two successive cyclic voltammograms of a solution contain-
ing PAN in the presence and absence of Fe(III) are shown in
Fig. 1 (scan between −300 and −1250mV). In the absence of
Fe(III) a cathodic peak was obtained at −470mV (solid line
in Fig. 1), attributed to the reduction of free PAN. In the pres-
−670mV (dotted line in Fig. 1). The second peak is attributed
to the reduction of the Fe(III)–PAN complex. In the back scan
no peaks were observed, suggesting that the reduction of the
free PAN and the reduction of the complex are irreversible pro-
3.2. Effect of pH
The formation of the complexes and their stability are
strongly dependent on the pH of the solution. The influence
of pH on the peak current of the Fe(III)–PAN complex was
Fig. 1. Cyclic voltammograms of a solution containing 10.0?molL−1PAN
(solid line) plus 20.0?gL−1Fe(III) (dotted line) in 0.1molL−1acetate buffer,
pH 4.0, with Eads−400mV, tads60s, and a scan rate of 100mVs−1.
studied in the range of pH 3.0–6.0 in acetate buffer media
(Fig. 2). It was found that at pH 4.0 the peak current of
the Fe(III)–PAN complex was maximum. At higher pH val-
ues the peak current decreases and then remains constant.
This profile indicates that about pH 4.0 offers the most favor-
able performance, and this value was used in all succeeding
3.3. Effect of adsorptive potential
The adsorption of a complex on BiFE depends strongly not
only on the potential at which the accumulation process is car-
ried out, but also on both the complex and the electrode charge.
Complexes with positive charge will be adsorbed strongly on
surfaces with a negative charge. The effect of adsorptive poten-
in the range between −300 and −1100mV (Fig. 3). The peak
current due to the Fe–PAN complex increased from −300 to
−400mV and then decreased to zero. The peak current was
obtained at about −400mV, and this value was used in all later
Fig. 2. Effect of pH on the peak current of the Fe–PAN complex. Conditions:
Fe(III), 10.0?gL−1; PAN, 5.0?molL−1; supporting electrolyte, 0.1molL−1
potential 4mV, and stirring speed in the accumulation step 700rpm.
Fig. 3. Effect of accumulation potential on the peak current of the Fe–PAN
complex. Conditions: Fe(III) 10.0?gL−1; PAN 5.0?molL−1; supporting elec-
15Hz; step potential 4mV, and stirring speed in the accumulation step 700rpm.
3.4. Effect of accumulation time
The effect of accumulation time on the Fe(III)–PAN com-
plex peak current was studied in the 0–400s range in solutions
containing 0.5, 0.9 and 10.0?gL−1of Fe, as illustrated in
Fig. 4. It is seen that the peak current of the Fe(III)–PAN com-
plex increases linearly as accumulation time increases, up to
80s (10.0?gL−1), 120s (0.9?gL−1), and 200s (0.5?gL−1).
At longer times the peak current for higher concentration
(10.0?gL−1) decreased notoriously and for 0.5 and 0.9?gL−1
concentration became almost constant, probably due to sat-
uration of the film electrode. For succeeding studies an
accumulation time of 60s was chosen.
3.5. Effect of PAN concentration
PAN concentration affects greatly the voltammetric peak
ied from 1.0 to 17.0?molL−1. The peak current of the complex
was maximum between 3.8 and 5.0?molL−1of ligand concen-
tration; for higher values a significant decrease was seen due to
Conditions: Fe(III) 0.5, 0.9, and 10.0?gL−1; PAN 5.0?molL−1; supporting
frequency 15Hz; step potential 4mV, and stirring speed in accumulation step
Fig. 5. Effect of PAN concentration on the peak current of 10.0?gL−1Fe(III).
Conditions: supporting electrolyte 0.1molL−1acetate buffer, pH 4.0; Eads
−400mV; tads60s; amplitude 25mV; frequency 15Hz; step potential 4mV
and stirring rate in accumulation step 700rpm.
competitive adsorption between free PAN and the Fe(III)–PAN
was used in all succeeding measurements.
3.6. Construction of calibration curves and determination
of detection limits and linear range
For the evaluation of the analytical parameters, a study of
the influence of the concentration of the Fe(III)–PAN complex
was made in aqueous solution under the optimal conditions
mentioned above. Measurements were made with successive
additions of aliquots of Fe(III) solution, with increments of
about 0.9?gL−1. An accumulation time of 60s and an accu-
mulation potential of −400mV were applied. Fig. 6(A) shows
the voltammograms and Fig. 6(B) calibrate curve obtained. The
peak current increased linearly with metal concentration in the
detection limit was 0.10?gL−1as Fe(III) . A series of
repetitive measurements with 20.0?gL−1of Fe(III) solution
produced a very stable response with a relative standard devi-
Fig. 6. (A) Adsorptive voltammograms of PAN solution in the presence of
increasing amounts of Fe(III). Conditions: Eads−400mV; tads60s; amplitude
25mV; frequency 15Hz; step potential 4mV, and stirring speed in accumu-
lation step 700rpm (a) supporting electrolyte 0.1molL−1acetate buffer, pH
4.0; (b) PAN 5?molL−1; (c–i) Fe(III) 10.0, 20.0, 29.9, 39.8, 49.8, 59.6 and
material and in seawater samples
Sample Fe found (?gL−1) Fe certified (?gL−1)
CRM-MFD mixed food
QCS-19 quality controla
cValues obtained with ICP-MS.
ation of 3.8% (tads60s). These results were obtained without
an electrochemical cleaning period, using the same bismuth
electrode surface, indicative of total desorption of the complex.
High sensitivity and reproducibility are coupled with high
selectivity. The possible interference of various trace metals
was investigated to test for selectivity. When a solution con-
taining Ag(I), Al(III), As(III), Bi(III), Cu(II), Cd(II), Cr(III),
Mo(VI), Ni(II) and Zn(II) at 100?gL−1concentration contains
20.0?gL−1of Fe(III) in the presence of 5.0?molL−1of PAN
(pH 4.0), the peak current of the Fe(III)–PAN complex was not
affected. This agrees with literature reports, because these met-
als form complexes with PAN at pH higher than 4.0, and their
reduction peaks were not observed in this potential zone.
3.8. Validation of the methodology
ining the analysis of Fe(III) in CRM-MFD mixed food diet,
seawater CRM-SW certified reference material, and QCS-19
quality control standards using an ex situ plated bismuth film
electrode. A standard addition method was used for Fe(III)
quantitation. Three replicate analyses were carried out for each
sample. The results are given in Table 1, indicating that the
proposed method is applicable to the analysis of seawater sam-
ples containing more than 0.1?gL−1of Fe(III). The proposed
method was successfully applied to the determination of iron
in synthetic seawater (ASTM D665) spiked with 10.0 and
3.9. Application of the proposed method
Direct measurements of the samples were not possible due
to lack of reproducibility. For that reason, 10.0mL aliquots of
the samples were previously digested with concentrated nitric
acid and warmed on a hot plate almost to dryness. The pH was
then adjusted to 4.0, the volume was made up to 10.0mL with
Fig. 7. (A) Typical voltammograms for the determination of Fe(III) contents in Download full-text
a seawater sample by the standard addition method. Conditions: Eads−400mV;
tads120s; amplitude 25mV; frequency 15Hz; step potential 4mV, and stirring
speed in accumulation step 700rpm (a) sample in 0.1molL−1acetate buffer,
pH 4.0; (b) (a) plus 9.1; (c) (a) plus 18.1; (d) (a) plus 27.2; (e) (a) plus 36.2 and
(f) (a) plus 45.2?gL−1of Fe(III), respectively. (B) Dependence of peak current
of the Fe(III)–PAN complex in a seawater sample in the presence of increasing
amounts of Fe(III).
was obtained using the standard addition method. Adsorptive
voltammograms of a digested seawater sample are shown in
Fig. 7(A) and calibrate curve in Fig. 7(B). The data obtained
by inductively coupled plasma spectrometry (ICP) in a service
laboratory. The results obtained by both methods were com-
The optimized method has been successfully applied to the
and precision. The proposed method is inexpensive and fast.
The detection limit of 0.10?gL−1can be lowered further by
This work was supported by a research grant from FONDE-
CYT (Chile) under project numbers 3060070 and 11070046.
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