American Journal of Anal yt ical Chemistry, 2010, 1, 150-158
doi:10.4236/ajac.2010.13019 Published Online November 2010 (
Copyright © 2010 SciRes. AJAC
Square Wave Voltammetry for Analytical Determination
of Cadmium in Natural Water Using
Ca10(PO4)6(OH)2-Modified Platinum Electr ode
Moulay Abderrahim El Mhammedi1*, Mounia Achak2, Mina Bakasse3
1Equipe de Chimie Anlaytique et Modélisation Statistique, Faculté Polydisciplinaire,
Université Hassan 1er, Khouribga, Morocco
2Laboratoire dHydrobiologie et d Algologie, Faculté des Sciences Semlalia,
Université Cadi Ayyad, Marrakech, Morocco
3Equipe dAnalyse des Micro-Polluants Organiques, Faculté des Sciences,
Université Chouaib Doukkali, El Jadida, Morocco
Received June 29, 2010; revised August 2, 2010; accepted October 26, 2010
This paper reports on the development of a novel electrochemical assay for cadmium (II) in natural water,
which involves the use of disposable hydroxyapatite modified platinum electrode (HAP/Pt). Cadmium (II)
was preconcentrated on the surface of the modified electrode and adsorbed onto HAP and oxidized at E =
–680 mV. The HAP-modified platinum electrode exhibited superior performance in comparison to the plati-
num electrode and surprisingly, yielded a higher electrochemical response. The best defined anodic peak was
obtained with 0.2 mol L-1 KNO3 pH 5.0 after 25 min of accumulation time. Using these conditions, the cali-
bration plot was linear over the range 1 × 108 to 5 × 106 mol L-1 Cd2+. The precision was examined by car-
rying out eight replicate measurements at a concentration of 2.5 × 105 mol L-1; the coefficient of variation
was 2.9%. The method was applied to the determination of the analyte in river water samples. The interfere-
ence of other metal ions on the voltammetric response of Cd(II) was studied. The HAP films was clearly ob-
served in the SEM images and characterized by X-ray diffraction, IR spectroscopy and chemical analysis.
Keywords: Hydroxyapatite, Cadmium, Platinum Electrode, Square Wave Voltammetry
1. Introduction
Heavy metals such as lead, cadmium and mercury are
rapidly increasing continuously to an alarming level,
particularly in rivers and near shore waters where indus-
trial wastes are being discharged. They tend to concen-
trate in all matrices in the environment [1]. Mining,
pouring, casting, processing, and metals use have led to
their dispersion into the general environment. Ingestion
of food and beverages contaminated with heavy metals
can impair the health of the general population [2]. In
this context, heavy metals have been extensively inves-
tigated. Procedures related to atomic adsorption spec-
trometry (AAS) [3,4], atomic emission spectrometry
(AES) [5] and ion chromatography techniques [6,7] have
been published. However, these techniques have some
disadvantages, such as complicated operation, high cost
of maintenance, expensive apparatus requiring well-
controlled experimental conditions.
More specifically, the electrochemical detection of
heavy metals has been performed on different electrode
surfaces, including solid electrodes [8,9], Nafion-modified
electrodes, [10] and microeletrodes [11]. Several re-
searchers have reported the use of chemically modified
electrodes (CMEs) in electroanalysis, using pulse tech-
niques such as square-wave voltammetry, and different-
tial pulse voltammetry [12]. This electroanalytical tech-
nique is thus less sensitive to the effects of matrix inter-
ferences, a property which characterizes its success in
electroanalysis [13]. CMEs have several advantages such
as low background current, wide range of usable poten-
tial, rapid renewability and easy fabrication [14,15].
Electrochemical methods of analysis occupy one of the
leading places among other methods of analytical chem-
istry. This is favored by the well-developed theory and
modern instrumentation based on microprocessor and
computer facilities. Electrochemical methods allow re-
searchers to study and determine both macro and trace
amounts of inorganic and organic compounds in the as-
say of the basic substance and in the analysis of impuri-
ties respectively.
Mercury-based electrodes, especially mercury-film
electrodes, have been widely used in anodic stripping
voltammetry [16,17] for cadmium determination [18].
The formation of an amalgam enables the analyte to be
accumulated in the mercury film, thus providing the
stripping with high sensitivity and reproducibility. How-
ever, because of the toxicity of mercury, it is important
to develop mercury-free electrodes for voltammetric
stripping determination of cadmium.
For trace element concentrations, adsorption is the
most common method and research has focussed on
natural adsorbents, either organic or mineral [19,20].
Hydroxyapatite (Ca10(PO4)6(OH)2), the main component
of bones and teeth, attracts considerable interests in
many areas because of acid–base properties, ion-exchange
ability, and adsorption capacity [21].
Many studies have recognized the ability of hy-
droxyapatite (HAP) to bind divalent heavy metal ions.
Previous studies have shown that synthetic HAP has a
high removal capacity for Pb, Zn, Cu, Cd, Co, and Sb
from aqueous solutions [22-31].
In the work discussed in this paper, the advantages of
using HAP-modified platinum electrode combined with
square-wave voltammetry were explored to establish an
appropriate method for analytical determination of cad-
mium (II) in pure and natural water samples. The inter-
ference of some common heavy metal ions, such as
Cu(II), Zn(II) and Pb(II) was investigated. The perform-
ance of analytical method and the HAP/Cd(II) interaction
were investigated using cyclic voltammetry, square wave
voltammetry and electrochemical impedance spectros-
copy. Some experiment conditions were optimized and a
voltammetric method for determination of cadmium (II)
in natural water was proposed and the result was satis-
factory. This method was convenient and available be-
cause of its higher sensitivity, lower detection limit and
low costs.
2. Experimental
2.1. Reagents
All chemicals were reagent grade and were used as re-
ceived. Aqueous solutions were prepared by dissolving a
certain amount of chemicals into high-purity Bi-distilled
deionized (BDW) water (MilliQ water system). Cad-
mium nitrate and potassium nitrate were purchased from
Fisher Scientific. Carbon paste was supplied from (Car-
bone, Lorraine, ref 9900, French). Hydroxyapatite used
in this work was synthesized by an aqueous solution
route involving mixing calcium nitrate and ammonium
phosphate [32]. In each experiment, the total sample
volume in the electrochemical cell was 20 mL.
2.2. Apparatus
The coated specimen was examined using a scanning
electron microscope (SEM, Jeol JSM-5500). One coated
specimen was investigated by inductively coupled
plasma-atomic emission spectrometry (ICP-AES, Perkin-
Elmer DV 3300) analysis.
The powder produced in this study was analyzed with
an X-ray diffraction (XRD) diffractometer (Philips PW
1710 power X-ray diffractometer) using Cu K radiation
(K1 = 0.15406 nm; K2 = 0.1.5444 nm). The detector was
scanned between 10° and 70° angles with a step size of
0.02° s1 2θ. The identification of phases was achieved
by comparing the diffraction pattern obtained for the
powder and comparing their diffraction patterns with the
standard cards on the ICDD-JCPDS database (*09-0432*)
for hydroxyapatite [33]. The powdered sample was
tested after calcination. A Fourier transform infrared
(FTIR) spectrum of the synthesized HAP was obtained
using a 1700 FTIR spectrometer. Spectra were obtained
in the mid-infrared region (4000-400 cm-1).
Voltammetric measurements were performed with a
voltalab potentiostat (model PGSTAT 100, Eco Chemie
B.V., Utrecht, The Netherlands) driven by the general
purpose electrochemical systems data processing soft-
ware (voltalab master 4 software) connected to a Pen-
tium III computer run under windows 98. The electro-
chemical equipment consisted of glass cell containing a
HAP modified platinum electrode, a platinum counter
electrode and Ag/AgCl/3 M KCl reference electrode.
The pH-meter (Radiometer Copenhagen, PHM210, Ta-
cussel, French) was used for adjusting pH values.
2.3. Substrat Preparation and Electrodeposition
of HAP
The platinum plate was abraded with silicon carbide pa-
per in successive grades from 400, 600 to 1200 grit
(Leco Corporation, MI) and then ultrasonically cleaned
in bi-distilled water and dried, degreased with AR grade
ethanol and acetone, and rinsed with double distilled
water before use. The equipment for electrodeposition of
HAP consisted of a glass cell containing the HAP sus-
pension, a platinum substrate electrode and a platinum
counter electrode. The separation distance between elec-
trodes was 2 cm. Electrodeposition process was carried
out at a constant voltage of 14.5 V for 24h. After the
Copyright © 2010 SciRes. AJAC
electrodeposition process, the coated platinum substrate
was removed from the suspension and was dried at room
2.4. Physico-chemical Characterization of
Scanning electron microscopy (SEM) was used to ob-
serve the morphology of samples. The coating formed on
platinum support was characterized by X-ray diffraction
(XRD) and Fourier transform infrared spectroscopy
(FTIR). ICP-AES permitted also the quantitative deter-
mination of Ca and P.
2.5. Analytical Procedure
The electrode was first immersed in a preconcentration
solution containing the cadmium ions, where the accu-
mulation of cadmium ions was achieved chemically by
binding to hydroxyapatite at open circuit. The electrode
was then removed from the accumulation cell, rinsed
with water, and transferred to the separate voltammetric
cell containing only a supporting electrolyte (0.2 mol L-1
KNO3). The same procedure was carried out in sample
analysis and all electrochemical experiments were car-
ried out at room temperature.
The HAP modified platinum electrode (HAP/Pt) was
characterized by impedance spectroscopy (EIS). This
measurement was carried out with the same electro-
chemical system described above before and after im-
mersion in solution containing cadmium (II) ions. The
frequencies between 100 kHz and 10 MHz can be mea-
sured with AC amplitude of 5-10 mV at 0 mV. Computer
programs automatically controlled the measurements
performed at rest potentials after 30 min of exposure.
The impedance diagrams are given in the Nyquist repre-
sentation. The charge-transfer resistance (Rt) values are
calculated from the difference in impedance at lower and
higher frequencies, as suggested by Tsurus et al. [34].
The double-layer capacitance (Cdl) and the frequency at
which the imaginary component of the impedance is
maximal (Zmax) are found as represented in the equa-
Cdl = 1/Rt were = 2fmax (1)
After accumulation step, the electrodeposit layer was
analyzed at carbon paste electrode (CPE) by cyclic
voltammetry. The carbon paste was prepared by hand
mixing of high purity graphite powder with HAP in
weight ratio 1:1.
Experiments were conducted using the square-wave-
voltammetry technique by deposition of the target cad-
mium (II) at open circuit. The accumulated metal was
then anodically oxidised at the potential ranging from
–1.0 to –0.4 V using a step potential of 25 mV; ampli-
tude 5 mV and duration 5 sec at scan rate 1 mV s-1.
After the optimization of the voltammetric parameters,
analytical curves were obtained in pure electrolyte by the
standard addition method for cadmium (II). The standard
deviation of the mean current (
) was measured at the
oxidation potential of cadmium for seven voltammo-
grams of the blank solution in pure electrolytes for the
determination of the quantification limit (QL, 3
) and
the detection limit (DL, 10
) together with the slope of
the straight line of the analytical curves. The method of
standard additions was used for analysis of real samples,
by spiking with appropriate amounts of standard Cd(II)
solution. All measurements were taken at room tempera-
3. Results and Discussion
3.1. Electrode Synthesis
Figure 1 shows SEM micrograph of the calcium phos-
phate coating obtained at a potential of E = 14.5V. It was
observed that the powder layer exhibited a porous micro-
structure with micropores which were relatively well
separated and homogeneously distributed over the sur-
The Ca/P ratio deduced from ICP-AES analysis is:
Ca/P = 1.67. This Ca/P ratio value corresponds to a sto-
chiometrc Ca-hydroxyapatite. This was confirmed by the
XRD pattern (Figure 2) which shows the typical diffrac-
tion peaks at 25.8°~31.6°~32.1°~32.7° and 34° corre-
sponding to the Ca-HAP as indicated in Joint Committee
on Powder Diffraction Standards card no. 09-0432. It
should be noted that the development of refined HAP
was not the only prime objective of the present work but
Figure 1. Scanning electron micrograph of apatite/plati-
um. n
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Figure 2. IR spectra of the apatite deposed on the platinum surfaces after analysis at 900°C for 3 h.
to develop an adsorbent calcium phosphate coating with
high activity.
The IR spectra of the investigated HAP powder (Fig-
ure 3) shows the typical absorption bands related to the
modes of phosphate (470 cm1; 565 cm1; 960 cm1;
1035 cm1), hydroxyl (3570 cm1) and water (1635 cm1;
3410 cm1).
3.2. Voltammetric Characteristics
Figure 4 shows the uptake of cadmium (II) by the
HAP-modified platinum electrode (HAP/Pt). The volt
ammograms were obtained by using cyclic voltammetry
(CV) at an effective scan rate of 100 mV s-1. The base
curve, which shows no peak, was obtained with HAP/Pt
before preconcentration step in Cd(II) solution. As it can
be seen from Figure 4(b), the peak current appeared
after the preconcentration step, manifesting the uptake of
Cd(II) by HAP-modified platinum electrode from bulk
3.3. Electrochemical Impedance Spectroscopy
Figure 5 shows the impedance diagram for HAP-modified
platinum electrode (HAP/Pt) before and after preconcen-
tration step in 3 10-4 mol L-1 cadmium (II) solution.
Similar Nyquist diagrams were obtained in 0.2 mol L-1
KNO3 electrolyte. The parameters associated with the
diagrams impedance are given in Table 1, as it can be
seen from the figure, the Nyquist plots contain depressed
semi-circles with the centre under the real axis; this kind
of phenomenon is known as dispersing effect. It appears
clearly from these data that the capacitance at the inter-
face increases when the HAP/Pt is exposed to cadmium
(II). This observation is in agreement with the literature
in the case of adsorption phenomenon at the electrode
surface. Our result gives another evidence for Cd(II) ad-
sorption on the HAP layer as part of an integrated proc-
ess leading to the electrolytic reduction of the heavy
metal at the modified surface. The observed decrease of
the charge-transfer resistance means also that the modi-
fied electrode becomes more conductive, which can be
explained by the presence of cadmium on the electrode
Figure 3. XRD of hydroxyapatite layer on platinum sur-
faces, calcined at 900°C for 3 h.
Figure 4. Cyclic voltammogram of HAP modified platinum
electrode: (a) before any contact with cadmium (II), (b)
after incubation with cadmium (II) species during 30 min;
supporting electrolyte is 0.2 mol L-1 KNO3, pH 5.6; the scan
rate was 100 mV s-1. [Cd(II)] = 310-4 mol L-1.
Figure 5. Impedance spectra at 0 V of HAP modified plati-
num electrode (a) before any contact with cadmium (b)
after preconcentration in cadmium (II) solutions. Condi-
tions are as described in Figure 4.
Table 1. Electrical parameters calculated from the imped-
ance spectra in 0.2 mol L-1 KNO3 for the HAP/Pt and
HAP/Pt-Cd(II) solution interfaces.
e (Kohm.cm²) Rct (kohm.cm²) Cdl (pF/cm²)
HAP/Pt 2.831 30.94 205.7
Cd-HAP/Pt 0.9 21.27 236.3
3.4. Voltammetric Analysis of HAP Layer
Before and after the preconcentration step in cadmium
solution, the deposit powder on platinum surfaces was
analyzed at carbon paste electrode (CPE) by cyclic
voltammetry (Figure 6). No peaks were observed for
HAP layer in the potential range -0.4 to -/1.0 V (vs.
Ag/AgCl/3M KCl) in 1.0 mol L-1 HClO4 electrolyte be-
fore the accumulation step (curve a). As shown in Figure
6(b), a pair of stable and well-defined redox peaks was
observed for HAP/Cd(II). This result confirms the pres-
ence of Cd(II) into or/and into apatite coating platinum
electrode after preconcentration in cadmium solution.
The result demonstrates that the HAP plays an important
role in the accumulation process of Cd(II) on the plati-
num electrode surface suggesting that the anodic surface
processes could be exploited systematically by square
wave voltammetry with some advantages in sentivity.
3.5. Optimization of the Experimental
EIS can give information on the impedance changes of
the electrode surface with different immersion time. The
HAP-modified platinum electrode was dipped into cad-
mium (II) solution (2.0×103 mol L1), and then was
washed with double distilled water, then put into 0.2 mol
L-1 KNO3 to perform the EIS experiments. The immer-
sion time of the HAP/Pt in Cd(II) solution had great ef-
fect on the surface state and the electron-transfer imped-
ance. Figure 7 shows the complex plan diagram of EIS
after the modified electrode soaked in Cd(II) solution at
various times, which indicated that the adsorption had an
important effect on electron transfer impedance. It can be
observed that the diameter of the semicircle decreased to
higher frequencies with the increase of adsorption time.
When the soaking time in the cadmium solution was over
20 min, there was no drastic change of the interfacial
electron-transfer resistance, which meant that the adsorp-
tion of Cd(II) reached a state of saturation. Hence for all
subsequent measurements preconcentration time (tp) of
25 min was employed.
The variation of anodic peak current of cadmium ver-
sus pH was investigated (Figure 8). The peak current
versus pH behaviour indicated a variation in sensitivity
over the 2.0-10.0 pH range for cadmium (II) studied, and
the highest response was achieved at pH 5.6
According to Wu et al. [35] the main interactions re-
sponsible for the surface properties of HAP in aqueous
solutions are:
PO + H+ POH0 (2)
CaOH2+ CaOH0 + H+ (3)
The positively charged CaOH2+ and neutral POH0,
POxH2 and POxH sites must prevail on HAP surface in
acidic solutions [36,37] making surface charge in this pH
region positive and at higher pH values negative. Cad-
mium exists in such a solution only in the form of Cd2+
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Figure 6. Cyclic voltammogram in HClO4 (0.1 M) for HAP
layer at carbon paste electrode (CPE) (a): before precon-
centration step (b): after preconcentration in 4.3 10-3 mol
L-1 Cb(II), pH 5.6.Vb = 100 mV s-1, between –0.4 and –1.0 V.
Figure 7. Influence of preconcentration time on charge
transfer of 3 10-4 mol L-1 cadmium (II) in 0.2 mol L-1
KNO3, pH 5.6 at HAP/Pt.
Figure 8. Effect of pH on SWV oxidation peak for 1.3
10-4 mol L-1 cadmium (II) in 0.2 mol L-1 KNO3 at HAP/Pt,
tp = 25 min.
ions, since no hydrolysis takes place at pH values lower
than 6.0 [38]. Consequently, in acidic solutions where
apatite surface is positively charged, Cd2+ could be
bound due to ion exchange in accordance with the molar
ratio of Cd2+ bound by apatite to Ca2+ released from apa-
tite [39,40].
3.7. Calibration
The calibration plot generated using the optimum condi-
tions determined above (pH 6.5, 0.2 mol L-1 potassium
nitrate, 25 min of accumulation time) is shown in Figure
9. The HAP-modified platinum electrode response was
linear up to 20 mol L-1 with a sensitivity (slope) of
0.0301 µA per µmol L-1 cadmium (II) (R² = 0.9969). The
lower detection limit (DL, 3
) and the quantification
limit (QL, 3
) (defined as five times the standard devia-
tion of the response obtained for a blank) were 8.85
10-8 mol L-1 and 9.35 10-7 mol L-1 respectively.
Such results are shown to be very appropriate for the
determination of ultratraces of cadmium in water sam-
ples, where the recommended maximum residue stipu-
lated is 10µg/Kg [41], indicating that the method could
be employed to analyze cadmium (II) in natural water
Figure 9. Square-wave voltammograms in 0.2 mol L-1
KNO3, pH 5.6, tp = 25 min, at HAP/Pt of cadmium (II); a)
0.2 10-6, b) 2.2 10-6, c ) 4 10-6, d) 6 10-6, e) 910-6, f) 11
10-6 mol L-1.
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3.9. Application of the Proposed Method in
Environmental Samples
It should, however, be pointed out that even though
the DL and the QL obtained in the present study are
comparable to the values obtained in the literature [42].
However, it was higher than those obtained with bismuth
film [43]. The detection limit can be improved by opti-
mizing the Ca/P ratio of hydroxyapatite. Particularly, the
detection limit can be improved significantly by increas-
ing the surface area of the modifier.
To verify the accuracy of the method, the developed
method was applied to the determination of Cd(II) in
natural water samples collected from Oum Er Rbia river,
Tadla-Azilal region, Morocco. Apart from the element of
interest, the natural waters analyzed contained many
other elements (Table 2). The relative standard deviation of eight successive
scans was 2.9% for 2.5 × 105 mol L1 cadmium, indi-
cating excellent reproducibility of the modified electrode.
In addition, the stability of the modified electrode was
investigated. The peak current was daily tested in the pH
6.5 of 2.5 × 105 mol L1 cadmium (II). No apparent de-
crease in the current response was found over 3 days, and
10 % decrease after 6 days and kept almost constant af-
The water samples were used to prepare the electro-
lytes employed for construction of the new analytical
curves and recovery curves by applying the proposed
procedure. In this way, the influence of the total compo-
nents present in natural water samples in the responses
obtained for the modified electrode could be evaluated.
Recovery experiments were also carried out in order to
evaluate the interference of matrix effects of the natural
water samples on the oxidation of cadmium on HAP/Pt.
Recovery curves for the samples spiked with 2.0×105
mol L1 Cd(II) were then obtained by the standard addi-
tion method. Moreover, the results obtained with HAP/Pt
were compared with those obtained with ICP-MS
method for spiked river water. The quantification of
these samples for both techniques was carried out by the
standard addition method. The results of the present
study are shown in Table 3. As shown, cadmium con-
centrations determined by the two techniques are in a
very good agreement and no significant differences at the
94% confidence level were found. The percentage re-
covered obtained presents satisfactory values for the
proposed electroanalytical methods, indicating the suit-
ability of the proposed method for use in natural water
3.8. Effects of Other Ions
The presence of other metal ions could interfere with
cadmium (II) determination if they compete for adsorp-
tion at the HAP sites. Large amounts of alkaline and al-
kaline earth metal ions have no interference with the mi-
croextraction of Cd(II) under the selected conditions be-
cause of their very low stability in the formation of
metal-HAP complexes (Figure 10). When the developed
procedure was employed, for the determination of 6.3 ×
10-6 mol L-1 Cd(II) with an accumulation time of 25 min,
no interference was found for additions of 6.3 × 10-6 mol
L-1 each of Cu2+, Fe3+, Zn2+, Ag+, Pb2+ and Hg2+. How-
ever Pb2+ interfere significantly by decreasing the Cd(II)
signal, because it can be rapidly adsorbed at HAP surface
and prevent the complex formation and accumulation of
Cd(II) at the electrode surface. This is explained by the
capacity of adsorption of HAP that varies according to
the scale of affinity decreasing: Pb > Cd.
The Cd sorption can be decomposed in two steps: in
the first step a high rate of sorption occurred; in the sec-
ond step, the sorption was slower before reaching the
equilibrium. In the mixture of Cd2+ of Pb2+, competitive
uptake among the heavy metals affected retention of
Cd2+ by HAP. Competition among the two heavy metals
determined reduction of the amount of sorbed Cd(II) by
15% compared to the single-metal system. This result
showed that the uptake behavior of Cd(II) was altered by
the competitive effects among the metals in agreement
with Chen et al. [44].
Figure 10. Cyclic voltammogram after exposure to a solu-
tion containing Cd(II), Ag(I), Cu(II), Hg(II), and Pb(II) of
concentration 6.3 10-6 mol L-1.
Table 2. Physicochemical parameters of used natural water.
pH MES (mg L-1) O2 (dissous) (mg L-1) NH4+ (mg L-1)Mn2+ (mg L-1)NO2-(mg L-1)NO3-(mg L-1) Nitrogen total (mg L-1)
8.85 9.5 11 0.26 0.03 0.18 5.6 0.7
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Table 3. Results obtained from the linear regression curves
for the determination of Cd(II) at HAP/Pt in natural wa-
Parameters Peak
Equation Y = 0.0213X + 0.425
R² 0.992
Slope A (mol L1) 0.503
Standard deviation () (×109 A) 16.3
Relative standard deviation (%) 3.07
Recovery (%) (SWV) 94.9
Recovery (%) (ICP-MS) 93.6
4. Conclusions
It was shown that one well-defined anodic peak could be
obtained at a potential of -0.74 V versus Ag/AgCl, using
cyclic and square wave voltammetry. The oxidation of
Cd deposed on HAP/Pt electrode occurs in a reversible
system. Optimization of the experimental conditions for
square wave voltammetry yielded a detection limit for
cadmium (II) of 8.85 10-8 mol L1 much better than
that described in the literature and this sensor shows a
wide linear response range, good sensitivity and repro-
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