PVC Membrane and Coated Graphite Potentiometric Sensors Based on Et 4todit for Selective Determination of Samarium(III)

Article abstract of DOI:10.1021/ac0205659

Solution studies on the binding properties of 4,5,6,7-tetrathiocino[1,2-b:3,4-b′]diimidazolyl-1,3,8,10-tetraethyl-2, 9-dithione (Et₄todit) toward a number of cationic species including some lanthanide ions revealed the occurrence of a selective 1:1 complexation of the ligand with Sm³⁺ ion. Consequently, Et₄todit was used as a suitable neutral ionophore for the preparation of novel polymeric membrane (PME) and coated graphite (CGE) Sm³⁺-selective electrodes. The electrodes exhibit a Nernstian behavior for Sm³⁺ ions over wide concentration ranges (1.0 × 10^-5-1.0 × 10^-1 M for PME and 1.0 × 10^-7-1.0 × 10^-1 M for CGE) and very low limits of detection (8.0 × 10^-6 M for PME and 1.6 × 10^-8 M for CGE). The proposed potentiometric sensors manifest advantages of relatively fast response, and, most importantly, good selectivities relative to wide variety of other cations, including other lanthanide ions. The selectivity behavior of the proposed Sm ³⁺-selective electrodes revealed a great improvement compared to the best previously reported electrode for samarium(III) ion. The potentiometric responses of the electrodes are independent of the pH of the test solution in the pH range 4.0-6.5. The electrodes were successfully applied to the recovery of Sm³⁺ ion from tap water samples and also, as an indicator electrode, in potentiometric titration of samarium(III) ions.

Full text of DOI:10.1021/ac0205659

Anal. Chem. 2003, 75, 5680-5686
P VC Me m b ra n e a n d Co a t e d Gra p h it e
P o t e n t io m e t ric S e n s o rs Ba s e d o n Et t o d it fo r
4
S e le c t ive De t e rm in a t io n o f S a m a riu m (III)
Mojta ba Sha m s ipur,*,† Morte za Hos s e ini, Ka m a l Aliza de h, Za hra Ta le bpour, Mir Fa zlolla h Mous a vi,
‡
‡
‡
‡
§
|
|
Moha m m a d Re za Ga nja li, Ma s s im ilia no Arc a , a nd Vito Lippolis
Departments of Chemistry, Razi University, Kermanshah, Iran, Tarbiat Modarres University, Tehran, Iran, Tehran University,
Tehran, Iran, and Universit a` degli Studi di Cagliari, Cagliari, Italy
Solution studies on the binding properties of 4 ,5 ,6 ,7 -
tetrathiocino[1,2-b:3,4-b′]diimidazolyl-1,3,8,10-tetraethyl-
development of new ion-selective electrodes for lanthanide ions
is still a challenging task.
2
,9 -dithione (Et
4
todit) toward a number of cationic spe-
It is well known that the sulfur-containing ligands can selec-
tively coordinate different transition and heavy metal ions.1
1-13
In
cies including some lanthanide ions revealed the occur-
rence of a selective 1 :1 complexation of the ligand with
recent years, we have successfully used some thia-substituted
crown ethers as neutral ionophores in construction of ion-selective
Sm3 ion. Consequently, Et
+
todit was used as a suitable
4
membrane electrodes for Hg2 and Cu2+ ions.17
, Ag , Cd ,
+ 14
+ 15
2+ 16
neutral ionophore for the preparation of novel polymeric
3
+
membrane (P ME) and coated graphite (CGE) Sm
-
In 1998, Masuda et al. reported the successful extraction and
separation of landanide ions with a synergistic extraction system
consisting of lauric acid and a thiacrown ether (1,4,10,13-tetrathia-
7,16-diazacyclooctadecane).18 Recently, we have also used 1,3,5-
trithiane (as ionophore) in conjunction with oleic acid (as additive)
for the preparation of a highly selective PVC membrane sensor
for Ce3 and La3+ ions.8,10
selective electrodes. The electrodes exhibit a Nernstian
3
+
behavior for Sm
ions over wide concentration ranges
-
5
-1
-7
(
1
1 .0 × 1 0 -1 .0 × 1 0
M for P ME and 1 .0 × 1 0
-
-
1
.0 × 1 0 M for CGE) and very low limits of detection
-
6
-8
(
8 .0 × 1 0 M for P ME and 1 .6 × 1 0 M for CGE). The
+
proposed potentiometric sensors manifest advantages of
relatively fast response, and, most importantly, good
selectivities relative to wide variety of other cations,
including other lanthanide ions. The selectivity behavior
of the proposed Sm3 -selective electrodes revealed a great
improvement compared to the best previously reported
electrode for samarium(III) ion. The potentiometric re-
sponses of the electrodes are independent of the pH of
the test solution in the pH range 4 .0 -6 .5 . The electrodes
were successfully applied to the recovery of Sm3 ion from
tap water samples and also, as an indicator electrode, in
potentiometric titration of samarium(III) ions.
In continuation of our work on the use of S-containing neutral
ligands in PVC membrane electrode studies, in this paper we
employed 4,5,6,7-tetrathiocino[1,2-b:3,4-b′]diimidazolyl-1,3,8,10-tet-
+
4
raethyl-2,9-dithione (Et todit) as a very suitable neutral ionophore
for the preparation of polymeric membrane (PME) and coated
graphite electrodes (CGE) for selective and sensitive determina-
tion of Sm3 ion. Et
+
todit belongs to an interesting class of
4
polyfunctional ligands containing two equivalent thiocarbonyl
groups located in positions far apart in the molecule, so that they
may act as bridging ligands in the preparation of polynuclear metal
+
complexes.1
9-23
It has been clearly shown that the resulting
todit with Cu(II)19 and Cu(I)20 are of 1:1 stoichi-
4
complexes of Et
Despite the exciting advantages of potentiometric methods
based on ion-selective electrodes (ISEs), including simple design
and operation, wide linear dynamic range, relatively fast response,
(
9) Amarchand, S.; Menon, S. K.; Agrawal, Y. K. Electroanalysis 2 0 0 0 , 12, 522.
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4, 5538.
(
(
7
11) Cooper, S. R. Acc. Chem. Res. 1 9 8 8 , 21, 141.
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(13) Housecroft, C. E. Coord. Chem. Rev. 1 9 9 2 , 92, 141.
14) Fakhari, A. R.; Ganjali, M. R.; Shamsipur, M. Anal. Chem. 1 9 9 7 , 69, 3693.
reasonable selectivity, and low cost,1 there have been very
-3
limited studies on the ISEs for lanthanide ions.4
-10
Thus, the
(
*
Corresponding author. E-mail: mshamsipur@yahoo.com.
Razi University.
Tarbiat Modarres University.
Tehran University.
(15) Mashhadizadeh, M. H.; Shamsipur, M. Anal. Chim. Acta 1 9 9 9 , 381, 111.
(16) Shamsipur, M.; Mashhadizadeh, M. H. Talanta 2 0 0 1 , 53, 1065.
(17) Shamsipur, M.; Javanbakht, M.; Mousavi, M. F.; Ganjali, M. R.; Lippolis,
V.; Garau, A.; Tei, L. Talanta 2 0 0 1 , 55, 1047.
(18) Masuda, Y.; Zhang, Y.; Yan, C.; Li, B. Talanta 1 9 9 8 , 46, 203.
(19) Bigoli, F.; Pellinghelli, M. A.; Deplano, P.; Trogu, E. F.; Sabatini, A.; Vacca,
A. Inorg. Chim. Acta 1 9 9 1 , 180, 201.
†
‡
§
|
Universit a` degli Studi di Cagliari.
(
1) Umerzawa, Y., Ed. CRC Handbook of Ion-Selective Electrodes: Selectivity
Coefficients; CRC Press: Boca Raton, FL, 1990.
(
(
(
(
(
(
(
2) Bakker, E.; B u¨ hlmann, Pretsch, E. Chem. Rev. 1 9 9 7 , 97, 3083.
3) B u¨ hlmann, P.; Pretsch, E.; Bakker, E. Chem. Rev. 1 9 9 8 , 98, 1593.
4) Harrell, J. B.; Jones, A. D.; Choppin, G. R. Anal. Chem. 1 9 6 9 , 41, 1459.
5) Takasaka, Y.; Suzuki, Y. Bull. Chem. Soc. Jpn.1 9 7 9 , 52, 3455.
6) Zhang, Y.; Wu, J.; Wang, E. Electroanalysis 1 9 9 3 , 5, 868.
(20) Bigoli, F.; Pellinghelli, M. A.; Deplano, P.; Trogu, E. F.. Inorg. Chim. Acta
1 9 9 1 , 182, 33.
(21) Bigoli, F.; Deplano, P.; Devillanova, F. A.; Lippolis, V.; Lukes, P. J.; Mercuri,
M. L.; Pellinghelli, M. A.; Trogu, E. F. J. Chem. Soc., Chem. Commun. 1 9 9 5 ,
371.
7) Chowdhury, D. A.; Ogata, T.; Kamata, S. Anal. Chem. 1 9 9 6 , 68, 366.
8) Shamsipur, M.; Yousefi, M.; Ganjali, M. R. Anal. Chem. 2 0 0 0 , 72, 2391.
(22) Bogoli, F.; Deplano, P.; Mercuri, M. L.; Pellinghelli, M. A.; Sabatini, A.; Trogu,
E. F.; Vacca, A. Can. J. Chem. 1 9 9 5 , 73, 380.
5680 Analytical Chemistry, Vol. 75, No. 21, November 1, 2003
10.1021/ac0205659 CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/03/2003

-
3
ometry with a crystalline structure packing of [Cu(Et
4
todit)Cl
2
]
n
‚
h by soaking in a 1.0 × 10 M samarium nitrate solution. A silver/
silver chloride electrode was used as the internal reference
electrode.
nTHF, in which the coordination around the metal involves two
Cl- ions and two S-thioamide atoms of two different ligand
molecules related by a screw axis.
To prepare the coated graphite electrodes, spectroscopic grade
graphite rods 10 mm long and 3 mm in diameter were used. A
shielded copper wire was glued to one end of the graphite rod,
and the electrode was sealed into the end of a PVC tube of about
the same diameter with epoxy resin. The working surface of the
electrode was polished with fine alumina slurries on a polishing
cloth, sonicated in distilled water, and dried in air. The polished
graphite electrode was dipped into the membrane solution
mentioned above, and the solvent was evaporated. A membrane
was formed on the graphite surface, and the electrode was allowed
to stabilize overnight. The electrode was finally conditioned
EXPERIMENTAL SECTION
Reagents. Reagent grade dibutyl phthalate (DBP), benzyl
-
3
soaking in a 1.0 × 10 M solution Sm(NO
3 3
) for 48 h.
acetate (BA), 2-nitrophenyl octyl ether (NPOE), oleic acid (OA),
sodium tetraphenylborate (NaTPB), tetrahydrofuran (THF), and
high relative molecular weight PVC were purchased from Merck
and used as received. Reagent grade nitrate salts of all cations
used (all from Merck) were of the highest purity available and
used without any further purification except for vacuum-drying.
Reagent grade acetonitrile (Merck) was purified and dried as
described elsewhere.24 All other reagents needed were purchased
from Merck and used as received.
Emf Measurements. The emf measurements with the poly-
meric membrane electrodes and the coated graphite electrodes
were carried out with the following cell assemblies:
-3
Ag-AgCl || KCl (3 M) | internal solution 1.0 × 10
M
Sm(NO ) | PVC membrane| sample solution ||
3
3
Hg-Hg Cl , KCl (satd) (PME)
2
2
todit.2 3 A mixture of N,N′-diethylimidazoli-
graphite surface | PVC membrane | sample solution ||
Hg-Hg Cl , KCl (satd) (CGE)
Synthesis of Et
4
-
3
dine-2-thione-4,5-dione (1.19 g, 6.4 × 10 mol) and Lawesson’s
2
2
-3
reagent (2.60 g, 6.4 × 10 mol) in toluene (100 mL) was refluxed
for 3 h. The solvent was removed under reduced pressure and
the crude product crystallized from dichloromethane (36% yield).
The emf observations were made relative to a double-junction
saturated calomel electrode (SCE, Philips) with the chamber filled
with an ammonium nitrate solution. A double-junction silver/ silver
chloride electrode (Metrohm) containing a 3 M solution of KCl
was used as the internal reference electrode. Activity coefficients
were calculated according to the Debye-Huckel procedure, using
following equation:25
20 4 6
Mp: 238 °C. Elemental analysis, Found (calcd for C14H N S ):
C, 38.57 (38.51); H, 4.83 (4.62); N, 12.82 (12.83); S, 43.50 (44.05).
IR bands (KBr pellets, cm-1): 2945 m, 1623 w, 1430 msh, 1399
vs, 1367 ssh, 1328 m, 1259 s, 1145 m, 1092 m, 1047 w, 987 w, 955
w.
P reparation of [Sm.Et
43.6 mg, 0.1 mmol dissolved in 2 mL of CHCl
33.6 mg, 0.1 mmol dissolved in 2 mL of acetonitrile) were stirred
4
todit][NO
3
]
3
. A mixture of Et
4
todit
(
(
3
) and Sm(NO )
3 3
2
1/ 2
1/ 2
log γ ) -0.511z [µ / (1 + 1.5µ ) - 0.2 µ]
(1)
together for 2 h, after adding 6 mL of methanol. The complex
Sm.Et todit][NO was obtained as pale yellow crystals by slow
evaporation of the solvent (70% yield). Mp: >400 °C. Elemental
analysis, Found (calcd for SmC14 ): Sm, 18.48 (19.43);
C, 21.22 (21.76); H, 2.51 (2.59); O, 18.42 (18.65); N, 12.44 (12.69);
S, 24.50 (24.87). IR bands (KBr pellets, cm-1): 2945 m, 1618 mbr,
[
4
3 3
]
where µ is the ionic strength and z the valency.
P rocedures. Conductivity measurements were carried out
with a Metrohm 660 conductivity meter. A dip-type conductivity
cell, made of platinum black with a cell constant of 0.8310 cm-1,
was thermostated at the 25.00 ( 0.05 °C using a Phywe immersion
thermostat.
20 9 7 6
H O N S
1
1
432 msh, 1397 vs, 1372 vs, 1328 ssh, 1260 s, 1145 m, 1092 m,
046 w, 979 w, 953 w.
In a typical run, 15 mL of a metal nitrate solution in acetonitrile
P reparation of Electrodes. Membrane solutions were pre-
-
5
(
8.0 × 10 M) was placed in a water-jacketed cell equipped with
4
pared by thoroughly dissolving 3 mg of ionophore Et todit, 30
a magnetic stirrer and connected to the thermostat circulating
water at the desired temperature. Then, a known amount of the
macrocyclic solution was added in a stepwise manner using a
calibrated micropipet. The conductance of the solution was
measured after each addition. Addition of the ligand was continued
until the desired ligand-to-cation mole ratio was achieved.
mg of powdered PVC, 57 mg of plasticizer BA, and 10 mg of
additive OA in 3 mL of freshly distilled THF. The resulting clear
mixture was evaporated slowly at ambient temperature until an
oily concentrated mixture was obtained. A Pyrex tube (5-mm i.d.)
was dipped into the mixture for ∼10 s so that a nontransparent
membrane of ∼0.3-mm thickness was formed. The tube was then
pulled out from the mixture and kept at room temperature for
The 1:1 binding of the metal ions (Mn+) with Et
todit (L) can
4
be expressed by the following equilibrium:
∼
1 h. The tube was then filled with an internal solution (1.0 ×
-3
1
0
3 3
M Sm(NO ) ). The electrode was finally conditioned for 24
Kf
Mn+ + L y
\z ML
n+
(2)
(
(
23) Aragoni, M. C.; Arca, M.; Demartin, F.; Devillanova, F. A.; Garau, A.; Isaia,
F.; Lelj, F.; Lippolis, V.; Verani, G. J. Am. Chem. Soc. 1 9 9 9 , 121, 7098.
24) Greenberg, M. S.; Popov, A. I. Spectrochim. Acta, Part A 1 9 7 5 , 31, 697.
(25) Kamata, S.; Bhale, A.; Fukunaga, Y.; Nurata, A. Anal. Chem. 1 9 8 8 , 60, 2464.
Analytical Chemistry, Vol. 75, No. 21, November 1, 2003 5681

versus pSm3 plot for each electrode was constructed in a pSm3+
+
f
The corresponding equilibrium constant, K , is given by
range of 2-5.
K ) ([ML ]/ [Mn+][L]) (γ(Mn+)/ γ(Mn+)γ(L)) (3)
n+
f
To determine the selectivity coefficients of different interfering
ions for the PME and CGE, specified amounts of samarium ion
-
2
-5
(
1
(
A) (in the range of 1.0 × 10 -1.0 × 10 M for PME and 1.0 ×
where [MLn+], [Mn+], [L], and γ represent the equilibrium molar
concentration of complex, free cation, and free ligand and the
activity coefficients of the species indicated, respectively. Under
the dilute conditions used, the activity coefficient of uncharghed
-3
-7
0
-1.0 × 10 M for CGE) were added to a reference solution
-6
-8
1.0 × 10 M for PME and 1.0 × 10 M for CGE) and the
potential was measured. In a separate experiment, the interfering
-2
-3
ions (B) (in the range of 1.0 × 10 -1.0 × 10 M for PME and
2
6-29
ligand, γ(L), can be reasonably assumed as unity.
The use of
-2
-4
1
.0 × 10 -1.0 × 10 M for CGE) were successively added to
3
0
the Debye-Huckel limiting law leads to the conclusion that
γ(M ) ∼ γ(MLn+), so the activity coefficients in eq 3 cancel out.
Thus, the complex formation constant in terms of molar
conductance, Λ, can be expressed as31,32
an identical reference solution until the measured potential
n+
matched that obtained before by adding the samarium ions. The
matched potential method selectivity coefficient,
given by the resulting primary ion to interfering ion activity
MPM
A,B
K
, is then
MPM
(
concentration) ratio,
K
A,B
A B
) a / a .
n+
n+
K ) [ML ]/ [M ][L] ) (ΛM - Λobs)/ (L)
(4)
f
RESULTS AND DISCUSSION
P reliminary Studies. A solid-state 1:1 complex [Sm.Et
NO was prepared and characterized by elemental analysis, as
described in the Experimental Section. The molecular structures
of the uncomplexed Et todit and its complex with the lanthanide
4
todit]-
where
[
3 3
]
[
L] ) C - (C (Λ - Λobs)/ (ΛM - ΛML))
L M M
(5)
4
ion were built with the Hyperchem program. The structure of free
ligand was optimized using the 6-31G* basis set at the restricted
Hartree-Fock (RHF) level of theory. The optimized structure of
the ligand was then used to find out the initial structure of its
lanthanide complex. Finally, the structure of the resulting 1:1
complex was optimized using Lanl2mb basis set at the RHF level
of theory. No molecular symmetry constraint was applied; rather,
full optimization of all band lengths, bond angles, and torsion
angles was carried out using the Gaussian 98 program. The
optimized structures are shown in Chart 1.
is the molar conductance of the metal ion Mn+ before
addition of ligand, ΛML the molar conductance of the complexed
metal ion, Λobs the molar conductance of the solution during
Here, Λ
M
titration, C
the analytical concentration of the metal nitrate. The complex
formation constant, K , and the molar conductance of the complex,
ML, were obtained by computer fitting of eqs 4 and 5 to the molar
conductance-mole ratio data using a nonlinear least-squares
L
the analytical concentration of the ligand added, and
C
M
f
Λ
3
3
program KINFIT.
The stoichiometry of the Sm3+-Etbtodit complex was inves-
As is obvious from Chart 1, in the resulting 1:1 complex, the
two CdS groups and two adjacent nitrogen atoms of the two
thioimidazole rings of the ligand are nicely involved in bond
formation with the central lanthanide ion, while the four sulfur
atoms of the upper cycle of the ligand will remain unattached. It
is noteworthy that such an involvement of the two S atoms of the
thionic group of the ligand molecule in bond formation with the
central metal ion has already been observed in the crystalline
tigated by the absorbance measurements at three wavelengths
of the spectra of solutions in which varying concentrations of the
-
2
metal ion (1.0 × 10 M) were added to a fixed concentration of
-5
the ligand (5.0 × 10 M) until a desired metal-to-ligand mole ratio
was achieved. Attainment of equilibrium was checked by the
observation of no further change in the spectra after several hours.
The influence of the pH of a test solution on the potential
response of the PME was studied as follows. A 25.0-mL portion
structures of 1:1 complexes of Cu(I) and Cu(II) with Et
4
todit.19,20
-
4
In preliminary experiments, the complexation of Et todit with
of a 1.0 × 10 M solution of samarium nitrate was taken, and its
pH was adjusted by dropwise addition of a 0.1 M solution of either
HCl or NaOH, and the emf of the electrode was measured at each
pH value, in a pH range of 2.5-10.0.
4
La3+, Cd3+, Ce3+, and Sm3+ ions and, for comparison purposes,
with Ca2+, Cu2+, and Cd2+ was studied conductometrically in
acetonitrile solution at 25.00 ( 0.04 °C,2
6-28
in order to obtain a
clue about the stoichiometry and stability of the resulting
The influence of the concentration of internal solution of the
PME was studied as follows. Four similar PMEs were prepared
under optimal membrane composition, and each electrode was
filled with an internal solution of varying samarium nitrate
4
complexes. The resulting molar conductance, Λ, versus [Et todit]/
n+
[
M ] mole ratio plots are shown in Figure 1. As seen, in all cases,
addition of the ligand to the cation solution caused a continuous
increase in the molar conductance, indicating the higher mobility
of the complexed cations compared to the solvated ones. The
increased molar conductance of the metal nitrates in acetonitrile
solution upon addition of the ligand can also be related to some
extent to the dissociation of some ion-paired species usually
present in acetonitrile, as a solvent of intermediate dielectric
constant and solvating ability, as a result of the metal ion
-
1
-2
-3
concentrations of 1.0 × 10 , 1.0 × 10 , 1.0 × 10 , and 1.0 ×
-4
10
M. The electrodes were then conditioned for 24 h by soaking
in a 1.0 × 10 M samarium nitrate solution. Finally, the emf
-
3
(
(
(
(
(
(
(
26) Hasani, M.; Shamsipur, M. J. Inclusion Phenom. 1 9 9 3 , 16, 123.
27) Hasani, M.; Shamsipur, M. J. Solution Chem. 1 9 9 4 , 23, 721.
28) Tawarah, K. M.; Mizyed, S. A. J. Solution Chem. 1 9 8 9 , 18, 387.
29) Smetana, A. J.; Popov, A. I. J. Chem. Thermodyn. 1 9 7 9 , 11, 1145.
30) Debye, P.; Huckel, H. Phys. Z. 1 9 2 8 , 24, 305.
31) Takeda, Y. Bull. Chem. Soc. Jpn. 1 9 8 3 , 56, 3600.
32) Zollinger, D. P., Bulten, E.; Christenhuse, A.; Bos, M.; van der Linden, W.
E. Anal. Chim Acta 1 9 8 7 , 198, 207.
todit.34 The conditional complex formation
complexation with Et
4
constant of the resulting 1:1 complexes were evaluated by
(
33) Nicely, V. A.; Dye, J. L. J. Chem. Educ. 1 9 7 1 , 48, 443.
(34) Amini, M. K.; Shamsipur, M. Inorg. Chim. Acta 1 9 9 1 , 183, 65.
5682 Analytical Chemistry, Vol. 75, No. 21, November 1, 2003

Ch a rt 1 . Op t im ize d S t ru c t u re s o f Fre e (A) a n d
Co m p le x e d (B) Et t o d it
4
Fig u re 1 . Molar conductance vs [Et4todit]/[Mn+] for different cations
in acetonitrile solution at 25 °C. Inset shows the computer fit of the
molar conductance-mole ratio data for the resulting samarium
complex.
Ta b le 1 . Fo rm a t io n Co n s t a n t s o f 1 :1 Co m p le x e s o f
Et 4 t o d it w it h Diffe re n t Ca t io n s in Ac e t o n it rile S o lu t io n
a t 2 5 °C
cation
log Kf
cation
log Kf
3
+
2+
Sm
Gd
La
Ce
4.60 ( 0.02
3.63 ( 0.03
3.59 ( 0.03
3.51 ( 0.4
Cu
Cd
Ca
Tl
3.18 ( 0.05
3.02 ( 0.03
2.80 ( 0.05
2.78 ( 0.03
3+
2+
computer fitting of the conductance-mole ratio data to appropriate
equations, as given in Experimental Section, and the results are
summarized in Table 1. A sample computer fit of the molar
conductance-mole ratio data for the resulting samarium complex
is shown in the inset of Figure 1. It is interesting to note that
3+
2
3+
+
are significantly changed upon addition of the metal ion at metal-
to-ligand mole ratios of >0 until a molar ratio of 1 is reached.
Further addition of Sm3 resulted in no significant change in the
CD spectra of the system, clearly indicating the formation of a
1:1 complex in solution.
4
the formation of Et todit complexes of 1:1 stoichiometry with
+
copper ions in crystalline state have also been reported in the
literature.19,20
The formation of a 1:1 Sm3+-Et
4
todit complex in acetonitrile
solution was further supported by the resulting absorbance-mole
ratio plots obtained at three different wavelengths of the ligand
spectrum, during its titration with Sm3 ions (Figure 2). It should
From the data given in Table 1, it is immediately obvious that
todit may act as a selective ionophore for Sm3 ion in a PVC
+
Et
4
+
membrane electrode. Thus, in preliminary experiments it was
found that, while the use of an ionophore-free PVC membrane
resulted in no measurable response with respect to Sm(III), the
4
be noted that the UV spectrum of Et todit in acetonitrile possesses
two absorbance maximums at 267.0 and 330.3 nm, the molar
absorptivity of the first absorption being some 4 times greater
than the second one. The absorbance bands are most probably
due to the two substituted thioimidazole presented in the ligand’s
structure.
4
addition of Et todit shows a near-Nernstian response for the cation
-
1
-5
in the range of 1.0 × 10 -1.0 × 10 M.
Membrane Composition. The performance characteristics
reported for a given ionophore-incorporated PVC membrane may
vary depending on electrode composition and the nature of
To obtain further information about the stoichiometry of the
Sm3+-Et
todit complex in acetonitrile solution, the circular
solution (e.g., ionic strength, pH) to which the electrodes are
4
dichroism (CD) spectra of the ligand (with five asymmetric centers
in its molecular structure) in the absence and presence of Sm3
exposed.1
8,14-17,35
Thus, several membranes of varying nature and
+
ratios of plasticizer/ PVC/ ionophore/ additive were prepared for
the systematic investigation of the membrane composition, and
the results are summarized in Table 2. As expected, the amount
having varying [Sm3+]/ [Et
4
todit] were obtained using a Jasco J-715
spectropolarimeter, and the results are shown in Figure 3. As is
obvious from Figure 3, the well resolved CD spectra of the ligand
of ionophore Et
4
todit was also found to affect the PVC membrane
Analytical Chemistry, Vol. 75, No. 21, November 1, 2003 5683

Ta b le 2 . Op t im iza t io n o f Me m b ra n e In g re d ie n t s
composition (%)
slope
no. PVC plasticizer
additive ionophorea (mV decade-1)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
DBP, 70
DBP, 69
DBP, 68
DBP, 67
DBP, 66
DBP, 64
DBP, 62
DBP, 57
DBP, 52
DBP, 66
DBP, 65
DBP, 64
BA, 62
BA, 57
BA, 52
BA, 54
BA, 66
BA, 65
BA, 64
1
2
3
4
6.2 ( 0.3
12.4 ( 0.2
14.4 ( 0.1
13.8 ( 0.2
13.7 ( 0.3
15.1 ( 0.3
17.2 ( 0.3
17.2 ( 0.3
15.9 ( 0.2
16.8 ( 0.2
16.8 ( 0.3
18.1 ( 0.2
19.6 ( 0.3
19.6 ( 0.3
16.4 ( 0.2
16.1 ( 0.3
16.2 ( 0.3
16.2 ( 0.3
17.8 ( 0.2
19.5 ( 0.2
17.3 ( 0.2
19.6 ( 0.3
18.7 ( 0.3
6
OA, 5
3 (0.39)
3 (0.19)
3 (0.13)
3 (2.4)
3 (1.2)
3 (0.80)
3 (0.39)
3 (0.19)
3 (0.13)
6 (0.39)
3 (2.4)
3 (1.2)
3 (0.8)
3 (0.39)
3 (0.19)
3 (0.13)
3 (1.2)
3 (0.8)
OA, 10
OA, 15
TPB, 1
TPB, 2
TPB, 3
OA, 5
OA, 10
OA, 15
OA, 10
TPB, 1
TPB, 2
TPB, 3
OA, 5
0
1
2
3
4
5
Fig u re 2 . Absorbance vs [Sm3+]/[Et4todit] in acetonitrile solution
at 25 °C: (A) 267.0, (B) 268.5, and (C) 264.5 nm. The dash lines
represent the extrapolation of linear segments of the mole ratio plots.
16
1
1
1
2
2
2
2
2
7
8
9
0
1
2
3
4
NPOE, 62
NPOE, 57
NPOE, 52
NPOE, 65
NPOE, 64
OA, 10
TPB, 1
TPB, 2
TPB, 3
a
Values in parentheses are the ionophore-to-additive molar ratios.
1
4 and 21, respectively). It is noteworthy that the nature of the
plasticizer influences both the dielectric constant of the membrane
and the mobility of ionophore and its complex.2,35,37 We have
recently shown that a change in the nature of the plasticizer will
even result in altering the selectivity of the same membrane
system toward Ce3 and La3+ ions.8,10 In this work, we also found
that, despite the proper slope and linear range of the PVC
membrane containing NPOE for samarium ion, the use of this
plasticizer resulted in increased interfering effects of some other
cations of higher charge density such as Cu2 and Gd3+. However,
the use of 57% BA in conjunction with 10% OA resulted in improved
response characteristics and selectivity of the resulting samarium
ion-selective electrodes. It should be noted that, in both mem-
branes 14 and 21, 10% OA as a polar additive is present which, in
turn, can also modify the polarity of the PVC membranes prepared,
so that both membranes response well to the concentration of
Sm3 in solution.
+
+
+
Fig u re 3 . CD patterns of Et4todit in acetonitrile solution in the
3+
3+
The data given in Table 2 clearly demonstrate the important
role of lipophilic additives in the improvement of the response
behavior of the proposed Sm3+-selective membrane sensor. In this
table, the carrier-to-additive molar ratio in each membrane is also
given in parentheses. As is obvious, the presence of 10% OA or
2% NaTPB as suitable lipophilic additives will improve the
sensitivity (and the selectivity) of the resulting Sm3 sensors. It
is well known that the presence of lipophilic anionic sites improves
the potentiometric behavior of certain cation-selective electrodes
presence of varying amounts of Sm . [Sm ]/[Et4todit]: (A) 0.00, (B)
.27, (C) 0.70, (D) 1.00, (E) 1.21, and (F) 1.70.
0
sensitivity (nos. 2-6). The calibration slope increased with
increasing ionophore content until a value of 3% was reached.
+
4
However, further addition of Et todit resulted in a diminished
response slope of the electrode, most probably due to some
inhomogenities and possible saturation of the membrane.36
As seen in Table 2, among the three different plasticizers used,
BA and NPOE were found to be the most effective solvent
mediators in preparing the Sm3 ion-selective electrodes (i.e., nos.
by reducing the ohmic resistance and improving the response
behavior and selectivity3
8-40
and, in some cases, by catalyzing the
+
exchange kinetics at the sample-membrane interface.41 As can
(
(
35) Yang, X.; Kumar, N.; Chi, H.; Hibbert, D. D.; Alexander, P. N. W.
Electroanalysis 1 9 9 7 , 9, 549.
36) Ammann, D.; Pretsch, E.; Simon, W.; Lindner, E.; Bezegh, A.; Pungor, E.
Anal. Chim. Acta 1 9 8 5 , 171, 119.
(37) Norov, S. K.; Gulamova, M. T.; Zhukov, A. F.; Borova, G. M.; Mamediva, Y.
G. Zh. Anal. Khim. 1 9 8 8 , 43, 777.
(38) Eugster, R.; Gehrig, T. M.; Morf, W. E.; Spichiger, U. E.; Simon, W. Anal.
Chem. 1 9 9 1 , 63, 2285.
5684 Analytical Chemistry, Vol. 75, No. 21, November 1, 2003

Ta b le 3 . Re s p o n s e Ch a ra c t e ris t ic s o f t h e S m 3
Io n -S e le c t ive Ele c t ro d e s
+
limit of
response
time
slope (mV
linear range
(M)
detection
-
1
electrode decade
)
(M)
(s)
-
5
-1
-1
-6
-8
PME
CGE
19.6 ( 0.5 1.0 × 10 -1.0 × 10
8.0 × 10
1.6 × 10
20
18
-
7
19.3 ( 0.3 1.0 × 10 -1.0 × 10
-
3
technology, where an internal 1.0 × 10 M Sm(NO
3
)
3
solution,
in the case of PME, has been replaced by a copper wire, in the
case of CGE. It is well known that the higher limit of detection of
PME compared to that of the CGE is mainly due to some leakage
of internal solution into the test solution via the polymeric
2
membrane. Meanwhile, the much higher electrical conductivity
of copper wire (in CGE) than that of the internal solution (in PME)
is expected to result in lower response time of the CGE in
comparison with the PME. It is noteworthy that, in the case of
CGE, the potential is not expected to be stabilized at the graphite/
membrane interface if only the exchange of Sm3 is considered.
However, samarium is known to exist in two valences Sm3+/ Sm2+
that may form a suitable determining couple and stabilize the
potential at this interface. This was verified by obtaining a quasi-
Fig u re 4 . Calibration graphs for the PME and CGE at pH 5.0. The
PVC membrane composition used is that given for membrane 14,
Table 2.
+
4
be seen in Table 2, membrane 14 with a PVC/ BA/ Et todit/ OA
percent ratio of 30:57:3:10 resulted in Nernstian behavior of the
membrane electrode over a very wide concentration range.
Effect of Internal Solution. In accord with generally adopted
-
3
reversible cyclic voltammogram for a 1.0 × 10 M solution of
Sm(NO at the surface of a bare graphite electrode.
3 3
)
2
ISE response formalism, the internal solution may affect the ISE
The average time required for the electrodes to reach a
potential response within (1 mV of the final equilibrium value
after successive immersion in a series of Sm3 solutions, each
having a 10-fold difference in concentration, was investigated. The
resulting response times of the electrodes for concentrations of
response when the membrane internal diffusion potential is
appreciable. Thus, the influence of the concentration of the internal
solution on the potential response of the polymeric membrane
+
3 3
electrode was also studied. The Sm(NO ) concentration was
changed from 1.0 × 10 to 1.0 × 10 M, and the emf-pSm3+
plots were obtained. It was found that the concentration of the
internal solution does not affect the potential response of the
electrode, except for an expected change in the intercept of the
-
4
-1
-
3
e1.0 × 10 M are also included in Table 3.
A comparison of the data given in Table 3 revealed that, while
both electrodes show a Nernstian behavior with relatively fast
response, the linear range and limit of detection of the coated
graphite electrode are greatly improved relative to those of the
polymeric membrane electrode. Furthermore, the membrane
electrodes prepared could be used for at least 2 months without
any measurable divergence, the lifetime of the CGE being longer
than that of the PME. After this period of time, the calibration
slope of the proposed electrodes was found to diminish by at most
-
3
resulting plots. A 1.0 × 10 M concentration of the reference
solution is quite appropriate for a smooth Nernstian function of
the polymeric membrane system. It should be noted that a change
in concentration of the internal solution is expected to change
the potential difference on the internal solution/ membrane
interface and, consequently, the overall emf of the cell assembly.
Response Characteristics of P ME and CGE. The critical
response characteristics of the proposed PME and CGE were
assessed according to IUPAC recommendations.42 The emf
response of the polymeric membrane and coated graphite elec-
trodes (Figure 4) indicates their Nernstian behavior over a wide
concentration range. The slopes and linear ranges of the resulting
emf-pSm3 graphs are given in Table 3. The limits of detection,
defined as the concentration of samarium ion obtained when the
linear regions of the calibration graphs are extrapolated to the
baseline potentials, are also included in Table 3.
5
%, while the linear range remained almost unchanged.
The time of contact and concentration of equilibrating solution
were optimized so that the sensors generated stable and repro-
ducible potentials at relatively short response times. The optimum
-
3
equilibration time in a 1.0 × 10 M Sm(NO
3 3
) for the PME and
CGE was 24 and 48 h, respectively.
+
Effect of pH of the Test Solution. The influence of pH of
the test solution on the potential response of the membrane sensor
was tested in the pH range 3-10, and the results are shown in
Figure 5. As seen, the potential remained constant from pH 4.0
to 6.5, beyond which the potential changed considerably. The
observed large decrease in potential at higher pH values could
be due to the formation of some hydroxyl complexes of Sm3 in
solution. At low pH, the potentials increased, indicating that the
membrane sensor responds to hydrogen ions.
The improved performance characteristics of the CGE over
those of the PME presumably originate from coated graphite
+
(
(
(
(
39) Rosatzin, T.; Bakker, E.; Suzuki, K.; Simon, W. Anal. Chim. Acta 1 9 9 3 ,
80, 197.
40) Schaller, U.; Bakker, E.; Spichiger, U. E.; Pretsch, E. Anal. Chem. 1 9 9 4 ,
6, 391.
41) Gehrig, P. M.; Morf, W. E.; Welti, M.; Prersch, E.; Simon, W. Helv. Chim.
Acta 1 9 9 0 , 73, 203.
42) IUPAC Analytical Chemistry Division. Commission on Analytical Nomen-
clature. Recommendations for Namenclature of Ion-Selective Electrodes.
Pure Appl. Chem. 1 9 7 6 , 48, 127.
2
6
Selectivity Coefficients. Potentiometric selectivity coefficients,
describing the preference of the ion-selective electrodes for an
interfering ion, B, relative to Sm3 ion, A, were determined by
the matched potential method, which is recommended43 to
+
Analytical Chemistry, Vol. 75, No. 21, November 1, 2003 5685

Fig u re 5 . Effect of pH of test solution on the potential response of
-4
Fig u re 6 . Potentiometric titration curve of 15.0 mL of 1.0 × 10
M
the PME at a Sm3 concentration of 1.0 × 10 M. The PVC
+
-4
3+
-2
solution of Sm with 1.0 × 10 M EDTA at pH 6.5.
membrane composition used is that given for membrane 14, Table
2.
Ta b le 5 . Re c o ve ry o f S m 3 + Io n s fro m Ta p Wa t e r
S a m p le s b y t h e P ro p o s e d P ME a n d CGE
Ta b le 4 . S e le c t ivit y Co e ffic ie n t s o f Va rio u s In t e rfe rin g
Io n s
Sm3 (M)
+
a
electrode
PME
added
found
% recovery
MPM
A,B
MPM
K
K
A,B
-
-
-
-
3
4
3
5
-3
5.0 × 10
(5.1 ( 0.2) ×10
102.0
100.0
98.0
Mn+
PME
CGE
Mn+
PME
CGE
-4
-3
-5
6
.6 × 10
(6.6 ( 0.2) × 10
(4.9 ( 0.2) × 10
(6.7 ( 0.3) × 10
3
+
-2
-2
-2
-3
-2
-3
-3
-3
-3
-4
-4
-4
-4
-3
-4
-4
-4
-4
2+
-2
-3
-3
-3
-3
-4
-4
-4
-4
-3
-4
-4
-4
-4
-5
-5
-5
-5
CGE
5.0 × 10
Ce
La
8.1 × 10
5.4 × 10
8.6 × 10
3.1 × 10
9.2 × 10
7.3 × 10
1.9 × 10
4.2 × 10
5.2 × 10
7.6 × 10
6.2 × 10
7.2 × 10
4.2 × 10
6.1 × 10
1.2 × 10
2.1 × 10
4.0 × 10
5.0 × 10
Hg
Ba
Ca
8.9 × 10
2.2 × 10
1.2 × 10
1.9 × 10
3.9 × 10
6.1 × 10
4.1 × 10
3.2 × 10
4.1 × 10
6.2 × 10
3.4 × 10
3.8 × 10
2.6 × 10
4.2 × 10
5.2 × 10
2.7 × 10
8.2 × 10
1.5 × 10
3
+
2+
2+
6.6 × 10
101.5
3
+
Gd
Cu
2
+
2+
Mg
2
+
+
Pb
Zn
Cd
Ni
Co
Ag
seem to indicate that these cations have negligible impact on the
functionality of the Sm3 sensors. Meanwhile, the data given in
Table 4 revealed that, in all cases, the selectivity coefficients
obtained for the coated graphite electrode are lower than those
for the polymeric membrane electrode, emphasizing the superior-
ity of the CGE in this respect as well. It is interesting to note that
a comparison of the selectivity coefficients obtained with the
proposed PME and, especially, CGE with those reported before
by Chowdhury et al. for their proposed coated graphite Sm ion-
selective electrodes based on bis(thialkylxanthato) alkenes (which
are actually the most selective samarium sensors available) clearly
indicated a large enhancement in the selectivity behavior of the
proposed electrodes for Sm3 ion.
Applications. The proposed membrane sensors were found
to work well under laboratory conditions. The CGE was used as
an indicator electrode in the successful titration of a Sm ion
solution (1.0 × 10 M) with EDTA (1.0 × 10 M) at pH 6.5.
The resulting titration curve is shown in Figure 6, indicating that
the amount of Sm3 can be accurately determined with the
electrode.
2
+
+
Tl
+
2
+
+
K
2
+
+
Na
2
+
+
Li
a
-6
-5
Conditions: reference solution, 1.0 × 10 M for PME and 1.0 ×
-
-
8
3
3+
-2
10
10
M for CGE; Sm , 1.0 × 10 -1.0 × 10 M for PME and 1.0 ×
-7 n -2 -3
-1.0 × 10 M for CGE; M +, 1.0 × 10 -1.0 × 10 M for PME
-
2
-4
and 1.0 × 10 -1.0 × 10 M for CGE.
7
3+
overcome the difficulties associated with the methods based on
the Nicolsky-Eisenman equation.43,44 According to this method,
the specified activity (concentration) of the primary ion (A) is
added to a reference solution and the potential is measured. In a
separate experiment, the interfering ions (B) are successively
added to an identical reference solution until the measured
potential matched that obtained before by adding the primary ions.
The matched potential method selectivity coefficient,
then given by the resulting primary ion to interfering ion activity
concentration) ratio,
+
3
+
-
4
-2
MPM
K
, is
A,B
+
MPM
(
K
A B
) a / a . The resulting values for
A,B
both samarium(III) ion-selective electrodes are listed in Table 4.
From Table 4, it is immediately obvious that the proposed
samarium(III) ion-selective electrodes are highly selective with
respect to other common cations, including members of the
lanthanide family other than Sm3+. The selectivity coefficients are
in the order of 10- (for PME) and 10-3 (for CGE) or lower, which
Both the PME and CGE were also used for the recovery of
Sm3 ion from tap water samples, and the results are summarized
in Table 5. As is obvious, in the case of both electrodes,
quantitative recovery of Sm3 ions from tap water is achievable.
+
+
2
Received for review September 11, 2002. Accepted August
1
4, 2003.
(
(
43) Umezawa, Y.; Umezawa, K.; Sato, H. Pure Appl. Chem. 1 9 9 5 , 67, 507.
44) Bakker, E. Electroanalysis 1 9 9 7 , 9, 7.
AC0205659
5686 Analytical Chemistry, Vol. 75, No. 21, November 1, 2003