Anal. Methods Environ. Chem. J. 4 (2) (2021) 25-33
Research Article, Issue 2
Analytical Methods in Environmental Chemistry Journal
Journal home page:
A rapid cadmium determination based on ion selective
membrane potentiometric sensor by bis (salicylaldehydo)
ethylenediimine as carrier
Mahdiyeh Ghazizadeh
* and Hamideh Asadollahzadeh
Department of Chemistry, Kerman branch, Islamic Azad University, Kerman, Iran, P. O. Box 7635131-167
An ion selective potentiometric electrode (IPE) was prepared based
on salen material (bis(salicylaldehydo)ethylenediimine) as a suitable
carrier for determination of cadmium ions. An acceptable response
for cadmium ions obtained over a linear range 8 × 10
to 1.0 × 10
M with a slope of 29.8 ± 0.8 mv per decade of activity and a detection
limit of 3.2 × 10
M for Cd (II) ions in water and liquid samples.
It has a response time less than 10 s and can be used for at least
2.5 months without any measurable divergence in potential. The
ion selective electrode can be used based on potential and potential
changes in the pH range 3.5 to 6.5, so, the cadmium determination
was obtained at independent pH. Moreover, the selectivity of
proposed method in presence of interference ions was studied. The
results showed that the other cations do not interfere signicantly in
response electrode at optimized pH. This electrode was successfully
used for the determination of cadmium ions in aqueous samples. The
validation was obtained based on ICP analyzer and certied reference
material in water samples (CRM, NIST).
Ion selective electrode,
Membrane sensor,
Water samples
Received 24 Feb 2021
Revised form 29 Apr 2021
Accepted 17 May 2021
Available online 27 Jun 2021
*Corresponding Author: Mahdiyeh Ghazizadeh
1. Introduction
Due to recently reported by WHO, NIOSH
and ACGIHA organizations, the heavy metals
such as cadmium belong to harmful material
for animals and humans. Heavy metals exist in
different matrixes such as, food, tobacco smoke,
air and water samples and have toxic effect in
human cells. It may cause some serious diseases
in humans such as renal failure, pancreatic/ liver
cancers and accelerate tumor growth in human
body [1] and some organs such as liver, kidneys
or lungs may be hurt seriously [2]. So, the
removal, separation and determination of heavy
metals in blood serum, water and food samples is
necessary. Cadmium has carcinogenesis effect
in humans which was enter to human cells
by the inhalation, food, waters [3]. Cadmium
concentrations in natural waters are less than
1 µg L
. The tissues kidneys and livers can
be concentrated cadmium. Levels in fruit and
vegetables are below 10-1000 µg kg
in liver
and in kidney. Cadmium determined by atomic
absorption spectroscopy (ETAAS, FAAS) using
aspiration water samples into a flame or injection
to graphite tube of furnace spectrometric
technique (ET-AAS) [4,5]. The LOD is 10 µgL
with the GBC spectrometer of F-AAS and 0.1
µg L
with ET-AAS was obtained. Cadmium
can be determined by chemical vapor generation
atomic fluorescence spectrometry(CVG-AFS)
Anal. Methods Environ. Chem. J. 4 (2) (2021) 25-33
and carbon paste ion selective electrode(CP-IS)
[6,7].The potentiometric method and chemical
sensors are the reproducible and rapid methods
for cadmium determination in liquid phases.
In recent years, the design of chemical sensors
as interested method based on highly selective
carriers and ion-selective electrodes (ISEs)
was reported by researchers for determination
of various ionic species such as Pb, Cd, V
and Hg. The potentiometric sensors act based
on ion-selective electrodes (ISEs) and the
electrochemical response is usually controlled
by one ionic species presented in the solution.
Numerous applications of these potentiometric
selective electrodes have been reported.
Biochemical applications and environmental
applications are two important examples for
this propose [8]. The Schiff base ligands were
easily prepared from the condensation of a
ketone or an aldehyde compounds with an
amine groups. Ionophores include antibiotics
and use to shift ruminal fermentation patterns.
Many ionophores are lipid-soluble entities that
transport ions across the cell membrane. They
are suitable ionophore to form effective ion-
selective electrodes, because of affinity toward
metal ions [9, 10]. The Schiff bases are ligands
with mixed O,N-donor atoms which bonds
to some transition metal ions, such as Mn(II),
Fe(III), Cu(II), Pb(II), Cd(II). The Schiff base
ligands are so important for their various
applications in dye and plastic industries, liquid
crystal technology, biochemistry and physiology
[11]. They are also use in development of photonic
devices and have potential applications such as
metallomesogens [12]. However, despite extensive
applications of these ligands and many reports on
synthesis and characterization of the Schiff base
ligands and their complexes, there are a few reports
on their ion-selective studies [13-16]. Moreover,
it seems that Schiff base ligands are suitable
ionophores for preparation ion selective electrodes
and determination of many metal ions [17, 18].
Among Schiff bases, salens are tetradentate ligands
that derived from salicylaldehyde and could
be formed to stable complexes with transition
metal ions. For these reasons, we used the salen
compound as carrier in ion selective membrane for
determination of Cd (II).
2. Experimental
2.1. Materials
Ethanol, methanol, acetone, sodium
tetraphenylborate (NaTPB), dibutyl phthalate
(DBP) and tetrahydrofuran with high purify (99%)
were purchased from Merck chemical company.
High relative molecular weight polyvinyl chloride
(PVC) was purchased from sigma chemical
company. Reagent grade nitrate salts of the used
cations (all from Merck) were of the highest
purity available and were used without any further
purication. Aqueous solutions were prepared with
doubly distilled water. Sodium hydroxide (0.1 M)
and nitric acid (0.1 M) were used for pH control.
The target Schiff base was synthesized from
salicylaldehyde and ethylene diamine and puried
as described elsewhere.
2.2. Instruments
Control of pH was achieved by a digital pH
meter (inoLab 7110, Germany). The reference
calomel electrode (RCE) has reaction between
elemental mercury and mercury(I) chloride.
However, the calomel electrode has a reputation
of being more robust. The liquid phase in contact
with the Hg/ HgCl in a saturated solution of
KCl. The electrode is normally linked via a
porous frit to the solution as a salt bridge. Finally,
all of potentiometric measurements were made
with a pH/mV meter (Zag Chimi, Iran) using
calomel electrode (Azar electrode, Iran). The
electrode of system to act as a reference against
which potential measurements can be made and
potentiometric methods was obtained based on
measurements of the potential of electrochemical
cells in the absence of appreciable currents and
basic components such as reference electrode
(gives reference for potential measurement), the
indicator electrode and salt bridge and potential
measuring device were used as Schema 1.
Cadmium determination based on ISMP sensor Mahdiyeh Ghazizadeh et al
2.3. Electrode preparation
Preparation of the PVC membrane achieved by
mixing thoroughly 61 mg of powdered PVC,
134.6 mg of plasticizer DBP, 2 mg of NaTBP and
6.12 mg of ionophore (Fig. 1) in 10 mL of THF.
The mixture was transferred into a glass dish of
2 cm diameter. The solvent was evaporated in
room temperature during 5-6 hours until an oily
concentrated mixture was left. A Pyrex tube was
dipped into the oily mixture for a few seconds, so
that a non-transparent lm was formed. Then the
tube was pulled out from the mixture and kept at
room temperature for 48 hours to produce a dry
membrane. After that the tube was lled with an
internal 1.0 × 10
M solution of Cd(NO
The electrode was nally conditioned for 24
h by soaking in a solution of 1.0 × 10
Fig. 1. The synthesis and structure of salen as ionophore and complexation with cadmium
Schema 1. The potentiometric methods based on measurements of the potential of electrochemical cells
2.4. Emf measurements
All emf measurements were done with the
following assembly:
(sat’d), KCl (sat’d) | internal solution,
O (1.0×10
M) | PVC membrane |
sample solution | Hg/Hg
(sat’d), KCl (sat’d)
All emf measurements were carried out in a 50
mL of double walled glass cell with a constant
magnetic stirring of the test solution. Activities were
calculated according to Debye-Huckel procedure
about solutions of electrolytes and plasmas.
Debye-Huckel procedure provides a starting point
for modern treatments of non-ideality of electrolyte
solutions based on 2-dimensional section of
electrolyte solution. The ions have as spheres
with unit negative and positive charges. The EMF
of the cell for varying the concentrations of one
participating electrolyte (HCl) will be measured.
This measurement veries the Debye–Huckel
limiting Law and to determine the mean activity
coefcient of the electrolyte.
3. Results and Discussion
3.1. Potentiometric response of the prepared
At rst, membrane electrodes were prepared based
on PVC by using of salen as ionophore. Then
potentiometric responses of sensors to the different
metal ions were investigated under the same
conditions. It is obvious that the best response to
the metal ions belongs to cadmium ion (Fig. 2).
3.2. Effect of the membrane composition
It is obvious that the electrode response depends
on the nature and the amount of the electrode’s
components. The data clearly show that the
electrode response does not improve by increasing
of the ionophore’s amount. This divergence in
electrode response in higher concentration of the
ionophore caused the less selectivity and enhanced
interference of the lipophilic counter
ions of the
test solution as assumed in the phase boundary
potential model of carrier based ISEs. Plasticizer
also plays an important role in electrode responses
and inuences the detection limits [19, 20], the
sensitivity and selectivity [21] of the electrodes.
Moreover, the nature of the additive may have
signicant effect on the sensitivity and selectivity
[22-24]. Thus, cadmium selective electrodes
prepared with different amounts of ionophore and
the aspects of membrane preparation based on
salen for Cd
were optimized and the results are
reported in Table 1. It indicates that membrane no.
9 with an optimized composition of 66% DBP, 30%
Anal. Methods Environ. Chem. J. 4 (2) (2021) 25-33
Fig 2. The responses of different ion selective electrodes
PVC, 3% ionophore and 1% NaTBP due to the best
sensitivity. It has a Nernstian slope of 29.8 ± 0.8
mV decade
activity of Cd
ions with an extensive
dynamic range.
3.3. Effect of pH
The effect of pH on the responses of cadmium-selective
electrode was investigated by adjusting pH over a
range of 2.0–9.0. It was achieved by using small drops
of nitric acid (0.1 M) or sodium hydroxide (0.1 M) and
1.0 × 10
M Cd
solution. The Figure 3 showed that
potential is constant over a pH range of 3.5–6.5. Thus
pH was adjusted at 5 for all experiments. The observed
decreasing of electrode potential at higher pH values
could be due to the interference of hydroxide ion.
Acidic solutions can also cause the less potential at
low pH because of protonating of ionophore and
interference of H
Cadmium determination based on ISMP sensor Mahdiyeh Ghazizadeh et al
Table 1. Composition and optimization of the membranes ingredients.
Numbers ionophore(%) NaTBP)(%) DBP(%) PVC(%)
(mV decade
1 4.5 2.5 63 30 24.6±0.9
2 5 2 63 30 21.8±0.4
3 3.5 2.5 64 30 22.3±0.6
4 3 0 67 30 27.7±0.5
5 0 5 65 30 18.8±0.3
6 4 1 65 30 25.7±0.7
7 2 3 65 30 23.8±0.9
8 5.5 2.5 62 30 20.2±0.3
9 3 1 66 30 29.8±0.8
10 7 3 60 30 21.6±0.4
Fig. 3. Inuence of pH on the potential response of cadmium selective electrode (1.0 × 10
M Cd
3.4. Response time
An important factor for all ionic selective
electrodes is dynamic response time. In this
research, the practical response time was
measured by changing the Cd
from 1.0 × 10
to 1.0 × 10
M in solution. The
response time was about 10 s did not change
by varying Cd
concentration (Fig. 4). This
is most probably result in the fast exchanging
of complexation–decomplexation of Cd
with the ionophore (salen) at the test solution–
membrane interface.
3.5. Effect of internal solution
The effect of internal solution concentration on
the potentiometric responses of cadmium ion
selective electrode was investigated. There was
no considerable change in the potentiometric
responses by using different concentrations of
internal solution in the range of 1.0 × 10
to 1.0 ×
M. Therefore 1.0 × 10
M was chosen for the
concentration of internal solution.
3.6. Effect of non-aqueous solutions
The potentiometric responses of cadmium-selective
electrode were studied in non-aqueous solutions
by using several mixtures such as water/methanol,
Anal. Methods Environ. Chem. J. 4 (2) (2021) 25-33
Fig. 4. Response time of cadmium selective electrode
Table 2. Effect of non-aqueous solutions on the slope and linear range of the prepared electrode
Non-aqueous solution (V/V%) Linear range (M) Slope (mV decade
0 1.0 ×10
– 3.8 × 10
29.1 ± 0.4
Ethanol 10%
Ethanol 15%
Ethanol 20%
Ethanol 25%
1.0 ×10
– 3.0 × 10
1.0 ×10
– 3.0 × 10
1.0 ×10
– 1.1 × 10
1.0 ×10
– 1.3 × 10
29.0 ± 0.5
28.9 ±0.5
26.3 ± 0.6
24.5 ± 0.7
Methanol 10%
Methanol 15%
Methanol 20%
Methanol 25%
1.0 ×10
– 3.0 × 10
1.0 ×10
– 3.0 × 10
1.0 ×10
– 4.1 × 10
1.0 ×10
– 4.1 × 10
29.1 ± 0.6
29.0 ± 0.5
27.1 ± 0.6
25.4 ± 0.5
Acetone 10%
Acetone 15%
Acetone 20%
Acetone 25%
1.0 ×10
– 3.0 × 10
1.0 ×10
– 3.0 × 10
1.0 ×10
– 2.1 × 10
1.0 ×10
– 1.3 × 10
29.1 ± 0.6
28.5 ± 0.7
24.1 ± 0.7
19.0 ± 0.8
water/ethanol and water/acetone with 10, 15, 20 and
25 volume percent of non-aqueous part (Table 2).
The results indicated a slope decreasing when the
volume percent of non-aqueous solution was more
than 15%.
3.7. Potentiometric selectivity
The effect of interfering ions on the potentiometric
behavior of Cadmium ion selective electrode,
known as potentiometric selectivity coefcients,
was studied. The results showed the other ions do
not interfere in potentiometric responses of the
prepared electrode based on salen as ionophore
(Table 3). Therefore, Cd-selective electrode has
high selectivity for cadmium ions.
3.8. Calibration curve and statistical data
Optimized equilibrium time for membrane electrode
is 24 h. After that the potentiometric responses of
electrode was dened according to IUPAC. The
potential response of the electrode to different
concentration of Cd(II) ion in the range of 8 × 10
to 1.0 × 10
M at pH = 5 (Fig. 5) indicates a linear
response and the slope of calibration curve was 29.8
± 0.8 mV/decade of concentration of Cd
at room
temperature. The standard deviation of 5 replicate
measurements is ±0.5 mV. The detection limit as
determined from the crossing of the two extrapolated
parts of the calibration curve was 3.2 × 10
M. The
PVC membrane electrode could be applied for at
least 2.5 months without any measurable change.
Cadmium determination based on ISMP sensor Mahdiyeh Ghazizadeh et al
Table 3. Potentiometric selectivity coefcients
of various interfering cations.
Cation K
2.4 × 10
2.5 × 10
6.8 × 10
1.1 × 10
3.3 × 10
3.1 × 10
1.8 × 10
Fig. 5. Potentiometric response of the Cd
ion selective electrode to
cadmium concentration using the optimized membrane electrode.
3.9. Analytical applications at real sample
The prepared cadmium selective ion electrode based
on salen was successfully applied for determination
of cadmium ions concentration in the samples of
water. The results are shown in Table 4.
4. Conclusions
Rapid response, low detection limit and high
selectivity, make ion selective electrodes suitable
for measuring the concentration of metal ions.
Salen was easily synthesized and used as ionophore
in preparing an ion selective electrode for direct
determination of cadmium. The prepared electrode
showed high selectivity and low detection limit.
This electrode also applied successfully for Cd ions
determination in the samples of water.
5. Acknowledgement
The authors are gratefull to Islamic Azad University,
Kerman Branch, for nancial assistance of this work.
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--- 5.5×10
<LOD (5.6±0.06)×10
--- 101 97
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