Novel copper(II)-selective membrane electrode based on a new synthesized schiff base

Article abstract of DOI:10.1002/jccs.200700048

A PVC membrane electrode for copper(II) ion based on a recently synthesized Schiff base as a suitable ion carrier was constructed. The electrode exhibits a Nernstian slope of 28.3 ± 0.6 mV per decade of Cu²⁺ over a wide concentration range of 7

Full text of DOI:10.1002/jccs.200700048

Journal of the Chinese Chemical Society, 2007, 54, 331-337
331
Novel Copper(II)-Selective Membrane Electrode Based on a New
Synthesized Schiff Base
Morteza Akhond,a* Mohammad Bagher Najafi,a Hashem Sharghi,a
Javad Tashkhourianb and Hossein Naiemia
aDepartment of Chemistry, Shiraz University, Shiraz 71454, Iran
bDepartment of Chemistry, Faculty of Sciences, Persian Gulf University, Bushehr 75168, Iran
A PVC membrane electrode for copper(II) ion based on a recently synthesized Schiff base as a suit-
able ion carrier was constructed. The electrode exhibits a Nernstian slope of 28.3 ± 0.6 mV per decade of
Cu2+ over a wide concentration range of 7.0 ´ 10-6-2.6 ´ 10-2 M with a detection limit of 5.0 ´ 10-6 M in the
pH range of 4.2-5.8. The response time is about 10s and it can be used for at least 1 month without any con-
siderable divergence in potential. It was successfully applied as an indicator electrode in the potenti-
ometric titration of copper ions.
Keywords: Copper(II) ion-selective electrode; PVC membrane; Potentiometry;
2-(2-Mercaptophenylnitrilomethylidyne)-phenol; Schiff Base.
INTRODUCTION
Ion-selective electrodes (ISEs) are chemical sensors
poly(vinyl chloride) (PVC) membrane electrode. Optimal
experimental conditions, selectivity over interferents, and
sensing characteristics of poly(vinyl chloride) membranes
are evaluated for potentiometric analysis toward copper
ions.
with the longest history and probably with the most fre-
quent routine application. Polymeric membrane based ISEs
are the most versatile ones and have been described for
many different analytes.1-2 Copper is an essential element
and is also toxic at high concentration that its reactivity and
biological uptake are strongly influenced by its free ion
concentration. Potentiometric measurement with a copper
selective electrode allows direct determination of free cop-
per ion concentration in analytical samples. Due to the vital
importance of copper in industry,3 in many biological sys-
tems,4 and in medicinal and environmental samples, re-
searchers have attempted to develop sensors for copper de-
termination with high selectivity and sensitivity.5-10 For
construction of these copper selective electrodes a variety
of ion-carriers such as macrocyclic polyethers,11 lariat
crown ether,12 acyclic neutral ionophores with dithiocar-
bamate groups,13-14 dithioacetal,15 and Schiff bases16-17
have been used; however, most of these sensors suffer from
the interfering effect of such cations as Zn2+, Cd2+, Pb2+,
Hg2+, and Ag+.
RESULTS AND DISCUSSION
Ionophores for use in sensors should have rapid ex-
change kinetics and adequate complex formation constants
in the membrane. Also, they should be easily soluble in the
membrane matrix and have a sufficient lipophilicity to pre-
vent leaching from the membrane into the sample solu-
tion.18 In preliminary experiments, 2-(2-mercaptophenyl-
nitrilomethylidyne)-phenol was used as an ion carrier to
prepare a PVC membrane ion selective electrode for a wide
variety of metal ions, including alkali, alkaline earth, tran-
sition and heavy metal ions. The potential responses of var-
ious ISEs based on 2-(2-mercaptophenylnitrilomethyl-
idyne)-phenol were obtained separately for each ion and
the results are shown in Fig. 1. As seen among the different
cations, Cu2+ had the most sensitive response and so seems
to be suitably determined with the membrane electrode
based on 2-(2-mercaptophenylnitrilomethylidyne)-phenol.
It was reported that Schiff bases upon deprotonation form
In this paper, 2-(2-mercaptophenylnitrilomethyl-
idyne)-phenol, recently synthesized in our laboratories,
was used as an ion carrier for construction of a copper(II)-
* Corresponding author. Tel: +98-711-2284822; Fax: +98-711-2286008; E-mail: akhond@chem.susc.ac.ir

332 J. Chin. Chem. Soc., Vol. 54, No. 2, 2007
Akhond et al.
complex with Cu2+, which act as charge carriers in the
membrane matrix.17 The existence of a donating sulfur
atom as well as -N=C- group in this Schiff base was ex-
pected to both increase stability and selectivity of its cop-
per complex over other metal ions, especially alkali, and al-
kaline earth cations.17,19-20 The spectrophotometric studies
in acetonitrile solution and mol ratio plot revealed that
2-(2-mercaptophenylnitrilomethylidyne)-phenol as a che-
lating agent can form a 1:2 (metal/ligand) complex with a
Cu2+ ion. Fig. 2 shows the absorption spectra of Cu2+-2-(2-
mercaptophenylnitrilomethylidyne)-phenol complex as a
function of the ligand concentration. The spectrum reflects
that the absorbance at about 410 nm increased, while the
maximum at l = 347 nm decreased slightly until a 1:2
metal/ligand mole ratio was reached.
In fact, it has been demonstrated that the presence of
lipophilic negatively charged additives improve the poten-
tiometric behavior of certain cation-selective electrodes by
reducing the ohmic resistance and improving the response
behavior and selectivity,22 inducing permselectivity to
some PVC membrane selective electrodes23 and in some
cases by catalyzing the kinetics at the sample-membrane
interface.24-25 The use of ionic additives such as different
tetraphenylborate salts and their more lipophilic derivative,
tetrakis(p-chlorophenyl)borate (K-TCPB) and also fatty
acids such as oleic acid as lipophilic additives, have been
widely reported in the preparation of different ion selective
electrodes.26-28
The data given in Table 1 reveal that the nature and
amount of additive influences the performance characteris-
tics of the membrane sensor significantly. Addition of 8.3%
NaTPB will increase the sensitivity of the electrode re-
sponse considerably so that the selective electrode demon-
strates a Nernstian behavior (No. 10 of Table 1). The effect
of OA as an additive was examined on the potentiometric
response of the copper electrode. The results show that OA
is not a suitable lipophilic additive, in contrast to NaTPB.
Therefore, NaTPB was used as an additive for further stud-
ies.
It is well known that the sensitivity, linearity, and se-
lectivity obtained for a given ionophore depends signifi-
cantly on the membrane composition.18 The optimization
results of the membrane composition are shown in Table 1.
Since the nature of plasticizer influences the dielectric con-
stant of the membrane phase, the mobility of the ionophore
molecules, and the state of ligands,21 it was expected to
play a key role in determining the ion-selective characteris-
tics. It was found that o-NPOE acts superior with respect to
other common plasticizers such as DBP, DMS, and BA in
construction of proposed copper(II) ion-selective elec-
trodes. The electrode based on o-NPOE as a plasticizer ex-
hibits a nice Nernstian slope 28.3 mV per decade over a
wide concentration range of 7.0 ´ 10-6 - 2.6 ´ 10-2 M.
In general, the thickness and hardness of the mem-
brane depend upon the amount of PVC used. At higher
Fig. 2. UV-visible spectra of 1 ´ 10-4 mol/L of 2-(2-
mercaptophenylnitrilomethylidyne)-phenol in
acetonitrile in the presence of various concen-
trations of Cu2+ 1 ´ 10-3 mol/L, Inset: mol ratio
plot.
Fig. 1. Potential response of various ion-selective elec-
trodes based on 2-(2-mercaptophenylnitrilo-
methylidyne)-phenol: (a) Cu2+, (b) Zn2+, (c)
Cd2+, (d) Pb2+, (e) Ni2+, (f) Hg2+.

Copper(II) Selective Electrode
J. Chin. Chem. Soc., Vol. 54, No. 2, 2007 333
Table 1. Optimization of the membrane ingredients
No.
PVC
(mg)
o-NPOE
(mg)
Ionophore
(mg)
Additive
Slope
(mg)
(mV/decade)
1
33.0
32.2
32.1
29.8
29.3
28.9
30.2
30.0
29.7
29.4
31.5
30.8
29.6
29.3
29.3
30.5
30.5
66.0
64.4
64.1
59.6
58.7
57.7
60.5
59.9
59.6
58.9
63.0
61.5
59.2
58.7
58.7
61.2
60.9
1.0
3.4
--------
--------
10.0
13.3
11.0
20.0
23.0
22.0
19.5
21.9
23.1
28.3
14.2
16.4
23.2
22.0
18.0
4.0
2
3
3.8
--------
4
3.4
7.2 OA
5
3.4
8.6 OA
6
3.4
10.0 OA
7
1.0
8.3 NaTPB
8.3 NaTPB
8.3 NaTPB
8.3 NaTPB
2.2 NaTPB
4.3 NaTPB
7.8 NaTPB
8.6 NaTPB
6.0 NaTPB + 2.6 OA
8.3 NaTPB
8.6 OA
8
1.8
9
2.4
10
11
12
13
14
15
16
17
3.4
3.4
3.4
3.4
3.4
3.4
--------
--------
~0
PVC content, the membrane becomes too dense and results
in increased resistance. At lower PVC content, the mem-
brane becomes mechanically weak and swells up easily in
aqueous solution. The o-NPOE/PVC ratios of 1.5-2.5 were
examined. The membrane prepared with a o-NPOE/PVC
ratio of ~2 was found to have the highest sensitivity and the
widest linear range.
is 20 h. Afterward it then generates stable potentials when
placed in contact with Cu2+ solutions.
For analytical applications, the response time of a
membrane sensor is an important factor. The static re-
sponse time of the electrode, tested by measuring the aver-
age required to achieve a potential within ± 1 mV of the fi-
nal steady-state potential upon successive immersion of a
series of Cu2+ ion, each having a tenfold difference in con-
centration, was within 10s for Cu2+ concentrations < 1.0 ´
10-3 M. The potential stayed constant for about 3 min, after
which a very low divergence within the resolution of the
mV was observed. Reproducibility of the electrode was ex-
amined by using six similarly constructed electrodes under
the optimum conditions. The results showed good repro-
ducibility for the proposed electrode. For instance, the
slopes observed were 28.3 0.6 mV per decade. The long-
term stability of the electrode was studied by periodically
re-calibrating in standard solutions and calculating the re-
sponse slope. The slope of the electrode responses was re-
producible over a period of at least 1 month. Therefore the
proposed electrode can be used for 1 month without a con-
siderable change in its response characteristics towards
Cu2+.
The effect of amount of ionophore was investigated,
and 3.4 mg of ionophore was chosen as the optimum amount
of ionophore in the PVC membrane. It was reported that the
high amount of ionophore, however, resulted in some de-
creases in the response of the electrode, most probably due
to some inhomogeneities and possible saturation of the
membrane.29 The results that were obtained indicate that
the best sensitivity and linear range obtained for a mem-
brane (No. 10 of Table 1), with PVC/o-NPOE/Ionophore/
NaTPB of 29.4 mg:58.9 mg:3.4 mg:8.3 mg.
The proposed sensor was examined at various con-
centrations of inner reference Cu(NO3)2 solution in the
range of 1.0 ´ 10-1-1.0 ´ 10-3 M. The results showed that
variation of the concentration of the internal solution does
not cause any significant difference in the corresponding
potential response, except for an expected change in the in-
tercept of the resulting Nernstian plots. A 1.0 ´ 10-3 M con-
centration of the reference solution was found quite appro-
priate for smooth functioning of the electrode membrane.
The optimum conditioning time of the membrane electrode
The influence of pH for 1.0 ´ 10-4 M Cu2+ on the po-
tential response of the membrane sensor was tested in the
pH range 1-9, and the results are shown in Fig. 3. As seen,
the potential remained constant from pH 4.2-5.8. A sub-

334 J. Chin. Chem. Soc., Vol. 54, No. 2, 2007
Akhond et al.
stantial increasing trend in the potential at low pH may be
due to the interference from the hydrogen ions, which are
greater at low pH in comparison to the Cu2+ ion. At higher
pH, turbidity was observed and activity of the Cu2+ ion was
decreased due to the formation of some hydroxyl com-
plexes of Cu2+ ion in solution.
into consideration, and (2) Nernstian responses are not as-
sumed either to the primary or interfering ions. The charac-
teristics lead to the following advantages: (1) The power-
term problem for ions of unequal charge disappears, and
(2) This method is widely applicable, even to non-Nernstian
interfering ions.32 The resulting values are summarized in
Table 2. As can be seen, for most of the diverse ions used, the
selectivity coefficients are in the order of 10-3 or smaller,
which seems to indicate that these metal ions show negligi-
ble interference with this sensor and will not disturb the
functioning of the Cu2+ ion-selective membrane electrode
significantly. Moreover a comparison between the selectiv-
ity coefficients of the proposed electrode with those previ-
ously reported for the copper reveals that the proposed
electrode shows somewhat similar in some cases, and supe-
rior in most cases, selectivity behavior to foreign ions.
The applicability of the proposed sensor was checked
by its use as an indicator electrode for the titration of 20 mL
5.0 ´ 10-3 M Cu2+ solution with 0.01 M EDTA and the result
is shown in Fig. 5. A very good inflection point, showing
perfect stoichiometry, is observed in the titration plot. As
seen, the amount of copper ion in solution can be accurately
determined with the electrode. Also the performance of the
sensor system was investigated in partially non-aqueous
medium. The membrane was tested in 25% (V/V) content
of ethanol. In this condition the slope and working concen-
The potential response of the optimized electrode to
varying concentrations of Cu2+ ions was examined. The
calibration plot is shown in Fig. 4, which indicates a linear
range of 7.0 ´ 10-6-2.6 ´ 10-2 M with a Nernstian slope of
28.3 ± 0.6 mV per decade of Cu2+ activity. The practical
limit of detection was 5.0 ´ 10-6 M as determined from the
intersection of the two extrapolated segments of the cali-
bration graph based on the recommended procedure by
IUPAC.30
The most important characteristics of any ion sensi-
tive sensor is its response to the primary ion in the presence
of other ions present in solution, which is expressed in
terms of the potentiometric selectivity coefficient. Potenti-
ometric selectivity coefficients (KCu 2 + ,M ), describing the
preference by the membrane for an interfering ion Mn+ rela-
tive to Cu2+, were determined by the matched potential
method (MPM).31 In this method, the selectivity coefficient
is defined by the ratio of the activity of primary ion relative
to an interfering ion when they generate identical potentials
in the same reference solution. The characteristics of the
matched potential method are: (1) The charge number of
the primary and interfering ions does not need to be taken
Fig. 4. Calibration plot for the proposed Cu2+-selective
electrode. Membrane ingredients: 29.4% PVC,
58.9% o-NPOE, and 3.4% ionophore and 8.3
NaTPB.
Fig. 3. The effect of pH on the response of the pro-
posed Cu2+-selective membrane electrode.

Copper(II) Selective Electrode
J. Chin. Chem. Soc., Vol. 54, No. 2, 2007 335
sized and purified as described elsewhere;26 triply distilled
deionized water was used throughout.
Table 2. Selectivity coefficients of various interfering ions for
the proposed Cu2+ selective electrode
Cation
Log k
Ref. [11]
Ref. [15]
Ref. [17]
Pb2+
Ce3+
Co2+
Zn2+
Cd2+
Sr2+
-2.2
-2.5
-2.69
-2.44
-3.28
--------
-2.52
--------
-1.08
-1.10
-1.09
-1.09
-1.09
-0.35
---------
-1.12
-0.27
-1.07
-1.14
--------
--------
-1.25
-1.21
--------
-1.22
-1.20
-1.22
-2.35
-1.11
Synthesis of ionophore
The 2-aminothiophenol was reacted with salicylalde-
hyde in methanol to give Schiff base as crystals in 85-99%
yields (Scheme I). A solution of amine (0.01 mole) in meth-
anol (15 mL) was added to a stirring solution of salicylalde-
hyde (0.01 mole) in methanol (10 mL). The reaction mix-
ture was refluxed for about 1.5 hours. After the reaction
was completed the mixture was cooled and the resulting
precipitate was filtered off and washed with cold methanol.
The product was recrystallized from an appropriate solvent
for further purification. The structure of this compound
was confirmed by spectroscopic data.
< -4.0
-2.1
-3.42
-2.85
-2.88
-2.4
-3.39
-3.04
< -4.0
-2.3
-2.82
---------
-1.58
Hg2+
Mn2+
Ni2+
Ag+
-3.13
< -4.0
< -4.0
-2.0
--------
-3.18
-2.87
-2.61
-2.13
-0.10
Na+
< -4.0
-3.5
-3.55
-2.82
Fe3+
Al3+
-3.49
--------
--------
--------
-3.65
-3.7
-3.74
+
NH4
< -4.0
-2.9
--------
--------
-3.53
Ba2+
Mg2+
La3+
Cs+
Ca2+
K+
< -4.0
-3.0
-3.10
2-(2-Mercaptophenylnitrilomethylidyne)-phenol
Yellow solid; yield 90%, m.p = 123-125 °C, Rf = 0.52
(N-hexane-ethyl acetate/955); IR (KBr), 645 (m), 690 (m),
-1.62
--------
-2.41
< -4.0
-3.0
--------
--------
--------
--------
--------
-3.25
< -4.0
-2.7
-1.97
Li+
Tl+
-3.10
--------
--------
tration range of the electrode decreased and the results
show that the electrode can’t be used in a non-aqueous me-
dium.
EXPERIMENTAL
Reagents
Sodium tetraphenylborate (NaTPB), high relative
molecular weight poly(vinyl chloride) (PVC), tetrahydro-
furane (THF), o-nitrophenyloctyl ether (o-NPOE), dibutyl
phthalate (DBP), benzyl acetate (BA), dimethylsebacate
(DMS), and oleic acid (OA), were purchased from Merck
and used as received. The nitrate salts of all cations were
used (all from Merck). Schiff base (Scheme I) was synthe-
Fig. 5. Titration curve of 20 mL, 5.0 ´ 10-3 M of Cu2+
solution with 0.01 M EDTA using the proposed
sensor as an indicator electrode.
Scheme I

336 J. Chin. Chem. Soc., Vol. 54, No. 2, 2007
Akhond et al.
715 (s), 750 (vs), 780 (s), 818 (m), 845 (m), 910 (s), 975 (s),
1035 (s), 1050 (m), 1150 (s), 1180 (s), 1220 (s), 1280 (vs),
1365 (s), 1390 (br, m), 1460 (s), 1485 (s), 1560 (s), 1580
(m), 1615 (vs), 2990 (w), 3050 (m), 3410 (br, m) cm-1, UV
(CHCl3); lmax (e), 247 (22330), 271 (25600), 349 (20232)
nm.
is 4.2-5.8. The selectivity coefficients for all cations are
significantly low. The membrane showed poor response for
all cations as often observed with copper electrodes. The
proposed sensor exhibits a fast response time of 10s, and it
is easy to prepare and use. It was successfully applied as a
novel indicator electrode in potentiometric titration of cop-
per ion.
Preparation of PVC membrane
The general procedure for preparing the PVC mem-
brane was to mix 30.4 mg powdered PVC, 3.4 mg Schiff
base, and 8.3 mg NaTPB with 58 mg o-NPOE as solvent
mediator. The mixture was then thoroughly dissolved in 2
mL of THF. The resulting mixture was transferred into a
glass dish of 2 cm diameter. The solvent was slowly evapo-
rated until an oily concentrated mixture was obtained. A
polyethylene tube of 5 mm i.d. was dipped into the mixture
for about 5s so that a non-transparent membrane of about
0.3 mm thickness was formed. After removing the tube
from the mixture it was kept at room temperature for about
3 h. Then it was filled with an internal filling solution of 1.0
´ 10-3 M Cu(NO3)2. The electrode was finally conditioned
for 20 h by soaking in a 1.0 ´ 10-3 M copper nitrate. A sil-
ver/silver chloride coated wire was used as an internal ref-
erence electrode.
ACKNOWLEDGEMENT
We gratefully acknowledge the support of the Shiraz
University Research Council.
Received February 14, 2005.
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