A Schiff base complex of Zn(II) as a neutral carrier for highly selective PVC membrane sensors for the sulfate ion

Article abstract of DOI:10.1021/ac001449d

Novel polymeric membrane (PME) and coated graphite (CGE) sulfate-selective electrodes based on a recently synthesized Schiff base complex of Zn(II) were prepared. The electrodes reveal a Nernstian behavior over wide SO₄^2- ion concent

Full text of DOI:10.1021/ac001449d

Anal. Chem. 2001, 73, 2869-2874
A S c h iff Ba s e Co m p le x o f Zn (II) a s a Ne u t ra l
Ca rrie r fo r Hig h ly S e le c t ive P VC Me m b ra n e
S e n s o rs fo r t h e S u lfa t e Io n
,
†
‡
‡
‡
Mojta ba Sha m s ipur,* Moha m m a d Yous e fi, Morte za Hos s e ini, Moha m m a d Re za Ga nja li,
§
§
Ha s he m Sha rghi, a nd Hos s e in Na e im i
Departments of Chemistry, Razi University, Kermanshah, Iran, Tehran University, Tehran, Iran, and
Shiraz University, Shiraz, Iran
Novel polymeric membrane (P ME) and coated graphite
CGE) sulfate-selective electrodes based on a recently
synthesized Schiff base complex of Zn(II) were prepared.
The electrodes reveal a Nernstian behavior over wide
Anion-selective electrodes based on conventional ion exchang-
ers, such as quaternary ammonium salts, respond to anions in
(
-
-
-
-
> Br- > NO
-
the order ClO
4
> SCN > salicylate > NO
3
2
=
-
-
-
-
2-
Cl > HCO
3
> H
2
PO
4
=F = SO
4
, and this behavior reflects
2
-
-5
-1
9,10
SO
4
ion concentration ranges (5 .0 × 1 0 -1 .0 × 1 0
the order of decreasing hydrophobicity (Hofmeister selectivity).
Nevertheless, the search for anion carriers with a selectivity
sequence different from the Hofmeister pattern has been the
subject of a series of recent investigations.5,6,11 Among different
-
7
-1
M for P ME and 1 .0 × 1 0 -1 .0 × 1 0 M for CGE) and
-
5
very low detection limits (2 .8 × 1 0 M for P ME and 8 .5
1 0 M for CGE). The potentiometric response is
independent of the pH of the solution in the pH range
.0 -7 .0 . The electrodes manifest advantages of low
-
8
×
anion carriers investigated for this purpose,5,6 the use of
metalloporphyrins1
2-18
and Schiff base complexes of the transition
3
metal ions1
9-25
as ionophores in solvent polymeric membrane
resistance, very fast response, and, most importantly,
good selectivities relative to a wide variety of other anions.
electrodes is well known to induce potentiometric anion selectivity
sequences that differ significantly from the classical Hofmeister
pattern. The observed selectivity has been ascribed to the selective
axial ligation between the metal centers coordinated to given
porphyrins and Schiff bases and certain anions.
2
-
In fact, the selectivity behavior of the proposed SO
4
ion-
selective electrodes shows a great improvement compared
to the previously reported electrodes for sulfate ion. The
electrodes can be used for at least 3 months without any
appreciable divergence in potentials. The electrodes were
used as an indicator electrode in the potentiometric
titration of sulfate and barium ions and in the determi-
nation of iron in ferrous sulfate tablets.
Due to its very low lipophilic character, sulfate ion recognition
by potentiometric methods was found to be quite difficult.3,5 Thus,
despite the urgent need for selective potentiometric determination
of trace amounts of the SO 2
-
ion in many chemical, pharmaceuti-
4
(
9) Hofmeister, P. Arch. Exp. Pathol. Pharmakol. 1 8 8 8 , 24, 247.
(
(
10) Marcus, Y. Ion Solvation; Wiley: New York, 1985; pp 107-109.
11) Chang, Q.; Park, S. B.; Kilza, D.; Cha, G. S.; Kim, H.; Meyerhoff, M. E. Am.
Biotechnol. Lab. 1 9 9 0 , 8, 10.
The quick determination of trace quantities of anionic species
by simple methods is of critical importance in chemical, clinical,
and environmental analyses. Ion-selective electrodes based on
solvent polymeric membranes with incorporation of ion carriers
have been shown to be very useful tools for these purposes, due
to their ease of preparation, simple operation, reasonable selectiv-
ity, relatively fast response, wide linear dynamic range, and low
cost.1 However, anion complexation and the design of sensing
(12) Gao, D.; Li, J.-Z.; Yu, R.-Q.; Zheng, G.-D. Anal. Chem. 1 9 9 4 , 66, 2245.
(
(
(
13) Gao, D.; Gu, J.; Yu, R.-Q.; Zheng, G.-D. Analyst 1 9 9 5 , 12, 499.
14) Malinowska, E.; Meyerhoff, M. E. Anal. Chim. Acta 1 9 9 5 , 300, 33.
15) Yorn, I. H.; Shin, J. H.; Paeng, I. R.; Nam, H.; Cha, G. S.; Paeng, K.-J. Anal.
Chim. Acta 1 9 9 8 , 367, 175.
16) Oh, K.-C.; Kim, K.-A.; Paeng, I. R.; Baek, D.-J.; Paeng, K.-J. J. Electroanal.
Chem. 1 9 9 9 , 468, 98.
(
(
-6
17) Gupta, V. K.; Jain, A. K.; Singh, L. P.; Khurana, U.; Kumar, P. Anal. Chim.
Acta 1 9 9 9 , 379, 201.
elements for anions is far less developed than the field of cation
receptors.7,8
(18) Shamsipur, M.; Khayatian, G.; Tangestaninejad, S. Electroanalysis 1 9 9 9 ,
1, 1340.
1
(
19) Yuan, R.; Chai, Y.-Q.; Liu, D.; Gao, D.; Li J.-Z.; Yu, R.-Q. Anal. Chem. 1 9 9 3 ,
65, 2572.
†
Razi University.
Tehran University.
Shiraz University.
‡
(20) Worblewski, W.; Brzozka, Z.; Rudkevich, D. M.; Reinhoudt, D. N. Sens.
Actuators B 1 9 9 6 , 37, 151.
§
(
(
1) Meyerhoff, M. E.; Opdyche, M. N. Adv. Clin. Chem. 1 9 8 6 , 25, 1.
2) Moody, G. J.; Saad, B. B.; Thomas, J. D. R. Sel. Electrode Rev. 1 9 8 8 , 10,
(21) Li, Z.-Q.; Yuan, R.; Ying, M.; Song, Y.-Q.; Shen, G.-L.; Yu, R.-Q. Anal. Lett.
1 9 9 7 , 30, 1455.
7
1.
(22) Antonisse, M. M. G.; Snellink-Ruel, B. H. M.; Engbersen, J. F. J.; Reinhoudt,
D. N. J. Org. Chem. 1 9 9 8 , 63, 9776.
(23) Antonisse, M. M. G.; Snellink-Ruel, B. H. M.; Engbersen, J. F. J.; Reinhoudt,
D. N. Sens. Actuators B 1 9 9 8 , 47, 9.
(24) Shahrokhian, S.; Amini, M. K.; Kia, R.; Tangestaninejad, S. Anal. Chem.
2 0 0 0 , 72, 956.
(25) Shamsipur, M.; Sadeghi, S.; Naeimi, H.; Sharghi, H. Pol. J. Chem. 2 0 0 0 ,
74, 231.
(
3) Umezawa, Y., Ed. CRC Handbook of Ion-Selective Electrodes: Selectivity
Coefficients; CRC Press: Boca Raton, FL, 1990.
(
(
(
(
(
4) Janata, J.; Jasowicz, M.; Devaney, D. M. Anal. Chem. 1 9 9 4 , 66, 207.
5) B u¨ hlmann, P.; Pretsch, E.; Bakker, E. Chem. Rev. 1 9 9 8 , 98, 1593.
6) Antonisse, M. M. G.; Reinhoudt, D. N. Electroanalysis 1 9 9 9 , 11, 1035.
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8) Antonisse, M. M. G.; Reinhoudt, D. N. Chem. Commun. 1 9 9 8 , 443.
1
0.1021/ac001449d CCC: $20.00 © 2001 American Chemical Society
Analytical Chemistry, Vol. 73, No. 13, July 1, 2001 2869
Published on Web 05/24/2001

cal, environmental, and industrial samples, only a few reports
high relative molecular weight PVC were purchased from Merck
and used as received. Reagent grade sodium salts of all anions
used (all from Aldrich) were of the highest purity available, and
they were used without any further purification except for vacuum-
dealing with sulfate ion-selective electrodes are available in the
literature.2
6-29
None of the solid-state and ionophore-free ion-
exchange sulfate electrodes that have been reported are satisfac-
tory, due to the lack of adequate selectivities.26 Recently, new
sulfate ion-selective electrodes based on a bis(thiourea) iono-
phore,27 a derivative of imodazole,28 and a zwitterionic bis-
2 5
drying over P O . All reagents needed were purchased from
Merck and used as received.
Synthesis of Schiff Base L. A solution of 0.01 mol of diamine
(
guanidinum) ionophore carrier29 with a Nernstian behavior over
2 6 4 2 6 4 2
NH C H CH C H NH dissolved in 10 mL of methanol was added
a relatively wide concentration range have been reported. Com-
to a stirring solution of salicylaldehyde (0.02 mol in 10 mL of
methanol). The reaction mixture was refluxed with stirring for 3
h. The resulting precipitate was filtered off and washed with cold
methanol. The product was finally recrystallized from methanol
pared to ionophore-free anion-exchange electrodes,26 the interfer-
-
-, SO
2-, HCO
-,
ence effect of anions such as Cl , HSO
3
3
3
-
-
CH
3
COO , and H
2
PO
4
for these new electrodes is significantly
reduced. However, other anions, such as NO
-
-
-
-,
to get a yellow solid with a yield of 98%, mp ) 123-124 °C: 1
NMR (CDCl , 250 MHz) δH (ppm) 4.04 (s, 2H), 6.98 (q, 4H, J )
7.57 Hz), 7.24 (d, 8H, J ) 3.78 Hz), 7.36 (dt, 4H, J, ) 13.22 Hz, J
) 4.82 Hz), 8.62 (s, 2H), 13.31 (s, 2H); 13C NMR (CDCl
, 62.9
MHz) δ (ppm) 40.98, 96.11, 117.24, 119.05, 121.23, 129.73, 135.14,
, Br , I , NO
2
H
3
-
-
ClO
4
, and SCN , show considerable interference to the potential
3
response of the electrodes.2
7-29
2
Potentiometric sensors prepared by coating polymeric films
containing electroactive species on metallic or graphite conduc-
tors,30,31 with no internal electrolyte solution, are known to be very
effective for a variety of cations and anions.24,32-35 Electrodes of
this type are advantageous in terms of simplicity, durability, high
mechanical durability, and low cost, and they are capable of
reliable response over a wide concentration range.
3
C
141.19, 148.24, 161.37, 163.22; IR (KBr) ν (cm-1) 638 (s), 710 (w),
735 (w), 750 (vs), 758 (s), 775 (m), 790 (s), 820 (s), 845 (s), 865
(m), 910 (s), 980 (w), 1030 (m), 1115 (m), 1148 (s), 1158 (s), 1185
(br, s) 1282 (vs), 1368 (s), 1410 (m), 1455 (br, m), 1495 (s), 1505
(s), 1568 (vs), 159 5 (s), 1615 (vs), 2920 (s) 294 (w) 3020 (s) 3050
We have recently introduced some PVC-based membrane
3
(s), 3445 (br, s); UV (CHCl ) λmax (nm) 245, 271, 323, 348.
sensors for SCN-,18 I ,
- 25
- 36
3
I ,
and Br- ions37 using (octabromo-
Synthesis of a Zn-L Complex. A solution of 0.1 mol of Schiff
base L dissolved in 20 mL of methanol was added to a stirring
solution of zinc acetate (0.1 mol in 20 mL of methanol). The
mixture was heated to ∼50 °C for 5 h under a nitrogen
atmosphere. The reaction mixture was then cooled, and the
resulting dark yellow precipitate was filtered off and washed with
cold methanol. The precipitate was recrystallized from dichloro-
methane (yield, 91%). The structure of the pure complex Zn-L
was confirmed by elemental analysis.
Apparatus. NMR spectra were recorded on a Bruaker
Advance DPX-250 spectrometer. All chemical shifts are reported
as ppm downfield from the TMS. The IR spectra were obtained
on a Perkin-Elmer 781 spectrometer. The UV spectra were
recorded on a Philips PU 8750 spectrophotometer. A Corning ion
analyzer 250 pH/ mV meter was used for the potential measure-
ments at 25.0 ( 0.1 °C.
tetraphenylporphyrinato)manganese(III) chloride, a recently syn-
thesized salen-Mn(II) complex, a charge-transfer complex of
iodine with 2,4,6,8-tetraphenyl-2,4,6,8-tetraazabicyclo[3.3.0]octane,
and a benzo derivative of xanthenium bromide as suitable ion
carriers, respectively. Due to the vital importance of sulfate
determination in chemical, pharmaceutical, environmental, and
industrial analyses, we became interested in preparing a new PVC-
based membrane sensor for the selective monitoring of the sulfate
ion in solution. In this paper, we report highly selective solvent
polymeric membrane (PME) and membrane-coated graphite
2
-
electrodes (CGE) for the SO
4
ion based on the Zn(II)-2,2′-[4,4′-
diphenylmethane bis(nitrilomethylidyne)]-bis(phenol) complex
38
(
ZnL) recently synthesized in our laboratories that manifests the
properties of an excellent neutral sulfate ion carrier.
P reparation of Electrodes. Membrane solutions were pre-
pared by thoroughly dissolving 2 mg of the Zn-L ionophore, 37
mg of powdered PVC, and 61 mg of plasticizer BA in 3 mL of
THF. The resulting clear mixture was evaporated slowly until an
oily concentrated mixture was obtained. A Pyrex tube (5-mm o.d.)
was dipped into the mixture for ∼10 s so that a nontransparent
membrane 0.3 mm thick was formed. The tube was then pulled
out from the mixture and kept at room temperature for ∼1 h.
-
3
EXPERIMENTAL SECTION
The tube was then filled with an internal solution (1.0 × 10
M
Reagents. Reagent grade dibutyl phthalate (DBP), aceto-
phenone (AP), benzyl acetate (BA), tetrahydrofuran (THF), and
Na SO ). The electrode was finally conditioned for 48 h by soaking
2
4
-2
in a 1.0 × 10 M sodium sulfate solution. A silver/ silver chloride
(
(
26) Midgley, D. Ion-Sel. Electrode Rev. 1 9 8 6 , 8, 3.
27) Nishizawa, S.; B u¨ hhnan, P., Xiao, K. P.; Umezawa, Y. Anal. Chim. Acta 1998,
(32) B u¨ hlmann, P. B.; Yajima, S.; Tohda, K.; Umezawa, K.; Nishizawa, S.;
Umezawa, Y. Electroanalysis 1 9 9 5 , 7, 811.
3
58, 35.
28) Li, Z.-Q.; Liu, G.-D.; Duan, L.-M.; Shen, G.-L. Yu, R.-Q. Anal. Chim. Acta
9 9 9 , 382, 165.
29) Fibbioli, M.; Berger, M.; Schmidtchen, F. P.; Pretsch, E. Anal. Chem. 2 0 0 0 ,
2, 156.
30) Lee, Y. K.; Park, J. T.; Kim, C. K.; Whang, K. J. Anal. Chem. 1 9 8 6 , 58,
101.
31) Freiser, H. J. Chem. Soc., Faraday Trans. 1 1 9 8 6 , 82, 1217.
(33) Schnierle, P.; Kappes, T.; Hauser, P. C. Anal. Chem. 1 9 9 8 , 70, 3585.
(34) Amini, M. K.; Shahrokhian, S.; Tangestaninejad, S. Analyst 1 9 9 9 , 124, 1319.
(35) Amini, M. K.; Shahrokhian, S.; Tangestaninejad, S. Anal. Chim. Acta 1 9 9 9 ,
402, 137.
(36) Rouhollahi, A.; Shamsipur, M. Anal. Chem. 1 9 9 9 , 71, 1350.
(37) Shamsipur, M.; Rouhani, S.; Mohajeri, A.; Ganjali, M. R.; Rashidi-Ranjbar,
P. Anal. Chim. Acta 2 0 0 0 , 418, 197.
(
(
(
(
1
7
2
(38) Sharghi, H.; Naeimi, H. Bull. Chem. Soc. Jpn. 1 9 9 9 , 72, 1525.
2870 Analytical Chemistry, Vol. 73, No. 13, July 1, 2001

electrode or a silver/ silver chloride-coated wire was used as the
internal reference electrode.
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 by
-
3
soaking in a 1.0 × 10 M solution Na
2 4
SO .
EMF Measurements. The EMF measurements with the
polymeric membrane electrodes and the coated graphite elec-
trodes were carried out with the following cell assemblies:
Ag-AgCl | KCl (3 M) | internal solution,
-
3
1
.0 × 10 M Na SO | PVC membrane | test solution |
2
4
Hg-Hg Cl , KCl (satd) (PME)
2
2
-
4
Fig u re 1 . Absorption spectra of 1.0 × 10 M Zn-L in acetonitrile
in the absence (1) and presence of an equimolar concentration of
the sulfate ion (2).
Hg Cl , KCl (satd) | sample solution |
2
2
membrane | graphite surface (CGE)
Meanwhile, the influences of other anions on the spectrum of
Zn-L were also investigated, and almost no detectable spectral
change was observed. Thus, the results obtained clearly revealed
the specific interaction of the sulfate ion with Zn-L.
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. It should be noted
that a silver/ silver chloride-coated wire electrode, in conjunction
with an internal filling solution containing 3 M solutions of both
It is well understood that the sensitivity, linearity, and selectiv-
ity obtained for a given ionophore depend significantly on the
membrane composition and nature of plasticizer used.6,24,25,39,40
Thus, several membranes of varying plasticizer/ PVC/ carrier ratios
were tested. The optimum membrane ingredient showing the
most sensitive, reproducible, and stable results was obtained with
a plasticizer/ PVC ratio of ∼1.6 together with 2% of the ionophore.
A study of the influence of the plasticizer on the potentiometric
response characteristics was conducted by using BA, AP, and
DBP, and the results are given in Figure 2. That is, while BA as
plasticizer implies the Nernstian behavior of the electrode over a
wide concentration range, the use of AP and DBP resulted in the
pronounced reduction in both sensitivity and linear dynamic range
of the electrode. Thus, a membrane with the optimized ingredient
composition of BA/ PVC/ Zn-L, in the ratio of 61:37:2, was selected
to prepare both the polymeric membrane and the coated graphite
electrodes for the sulfate ion. It has been shown that the presence
of lipophilic ionic additives in the ion-selective membrane sensors
is necessary to introduce permselectivity.40 However, the selective
and Nernstian response of the proposed electrodes in the absence
of an additional lipophilic anion exchanger may be caused by
possible ionic impurities within the used PVC.
Na
2 4
SO and NaCl, can also be used as a proper reference
electrode. Activities were calculated according to the Debye-
3
9
H u¨ ckel procedure.
RESULTS AND DISCUSSION
In preliminary experiments, we found that the plasicized PVC-
based membranes containing Zn-L as a neutral ionophore,
generated stable potentials in solutions containing the sulfate ion,
-
2
after proper conditioning in a 1.0 × 10 M solution of Na
2
SO
relative
4
.
2-
The membranes revealed remarkable selectivity for SO
4
to most common inorganic and organic anions. Such anti-
Hofmeister selectivity is believed to originate from the selective
axial ligation between the metal ion centers and certain anions.12-14
With the aid of UV-visible spectra, as illustrated in Figure 1, it
was possible to distinguish the specific interaction between the
central metal and sulfate ion. That is, the neutral Zn-L (1.0 ×
-
4
1
0
M) shows two maximums at 254.5 and 287.3 nm, but in the
presence of an equimolar concentration of the sulfate ion, the two
spectral maximums are shifted to 247.2 and 296.0 nm, respectively.
The observed spectral shifts, together with the substantial increase
The influence of the concentration of the internal solution on
the potential response of the polymeric membrane electrode was
in absorbance, after contact of the carrier solution with the SO 2
-
also studied. The Na
2
SO
4
concentration was changed from 1.0 ×
4
4
-1
2-
1
0- to 1.0 × 10 M and the EMF-pSO
4
plots were obtained
ion-containing phase, suggests that the absorbing species had
increased in size, and axial coordination was thought to take place.
(
Figure 3). As is obvious from Figure 3, the concentration of the
internal solution has a negligible effect on the potential response
(
39) Kamata, S.; Bhale, A.; Fukunaga, Y.; Murata, A. Anal. Chem. 1 9 8 8 , 60,
464.
2
(40) Bakker, E.; B u¨ hlmann, P.; Pretsch, E. Chem. Rev. 1 9 9 7 , 97, 3087.
Analytical Chemistry, Vol. 73, No. 13, July 1, 2001 2871

Ta b le 1 . Re s p o n s e Ch a ra c t e ris t ic s o f t h e S u lfa t e Io n -S e le c t ive Ele c t ro d e s
slope
electrode
(mV decade-1)
linear range (M)
limit of detection (M)
response time (s)
-
5
-1
-1
-5
PME
CGE
-29.7 ( 0.9
-29.3 ( 0.7
5.0 × 10 -1.0 × 10
2.8 × 10
<15
<15
-
7
-8
1.0 × 10 -1.0 × 10
8.5 × 10
Fig u re 2 . Effect of plasticizer on potential response of the CGE.
Fig u re 4 . Calibration graphs for the CGE and PME at pH 5.5.
trodes (Figure 4) indicates their Nernstian behavior over a wide
concentration range. The slopes and linear ranges of the resulting
EMF-pSO 2
-
graphs are given in Table 1. The limits of detection,
4
defined as the concentration of sulfate obtained when the linear
regions of the calibration graphs are extrapolated to the baseline
potentials, are also included in Table 1. The improved performance
characteristics of the CGE over those of the PME presumably
originate from the coated graphite technology, where an internal
-
3
1
.0 × 10 M Na
2 4
SO solution, in the case of PME, has been
replaced by a copper wire of much higher electrical conductivity,
in the case of CGE.
The average time required for the electrodes to reach a
potential response within (1 mV of the final equilibrium value
2-
after successive immersion in a series of SO
4
solutions, each
having a 10-fold difference in concentration, was investigated.
Thus, the static response time of the electrodes obtained for
-
3
concentrations of e1.0 × 10 M are also listed in Table 1.
A comparison of the data given in Table 1 revealed that, while
both electrodes show a nice Nernstian behavior with 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 3 months without any
measurable divergence, the lifetime of the CGE being longer than
that of the PME.
Fig u re 3 . Effect of internal filling solution on the potential response
of the PME: 1, 1.0 × 10 M; 2, 1.0 × 10-2 M; 3, 1.0 × 10-3 M; 4,
-1
-
4
1
.0 × 10 M.
of the electrode, except for an expected change in the intercept
-
3
of the resulting plots. Obviously, a 1.0 × 10 M concentration of
the reference solution is quite appropriate for a smooth Nernstian
function of the polymeric membrane system.
The potentiometric response of the Zn-L-based electrodes was
found to be sensitive to pH changes. Thus, the pH dependence
of the electrodes was tested by measuring the potential response
The critical response characteristics of the proposed electrodes
were assessed according to IUPAC recommemdations.41 The EMF
response of the polymeric membrane and coated graphite elec-
-
3
of solutions containing 1.0 × 10 M of the sulfate ion in the pH
range 1.0-9.5, and the results are shown in Figure 5. A similar
pH dependence has been reported before,3
6,42,43
although the
(
41) IUPAC Analytical Chemistry Division. Commission on Analytical Nomen-
clature. Recommendations for Nomenclature of Ion-Selective Electrodes.
Pure Appl. Chem. 1 9 7 6 , 48, 127.
(42) Dong, S.; Sun, Z.; Lu, Z. Analyst 1 9 8 8 , 113, 1525.
2872 Analytical Chemistry, Vol. 73, No. 13, July 1, 2001

Ta b le 2 . 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 An io n s
selectivity coefficient
SCN-
F-
Cl-
Br-
I-
CN-
NO
-
NO
-
ClO
-
CH
COO-
SO
2-
CO
2-
3
ref
2
3
4
3
3
CGE 1.0 × 10- 9.5 × 10
2
-3
-3
9.1 × 10
4.0 × 10
7.9 × 10
1.1 × 10
-3
-3
-1
8.5 × 10
3.2 × 10
1.3 × 10
7.9
-3
-3
1
3.2 × 10
1.6 × 10
-3
-2
8.3 × 10
1.5 × 10
-3
-2
1.3 × 10
1.0 × 10
4.0
-3
-2
1.7 × 10
6.2 × 10
4.0 × 10
1.2 × 10
2.0 × 10
-3
-3
1
6.2 × 10
-3
-2
1.6 × 10
6.3 × 10
3.2 × 10
1.0 × 10
2.0 × 10
-3
5.1 × 10
1.2 × 10
5.0 × 10
-3
-3
-1
1.8 × 10
6.3 × 10
-3
-3
-2
-3
-2
-3
-2
PME 7.9 × 10
1.5 × 10
3.1 × 10
3
2
2
2
7
8
9
7.9 × 10
-
-
-
-1
2
2.0
6.3 × 10-
1
2.5 × 10
2
2.5 × 10
7
4
2.5 × 10
10
7.9
Fig u re 5 . Effect of pH of the test solution on the potential response
of the CGE and PME, at a sulfate concentration of 1.0 × 10-3 M.
doping and sensing anions are different. That is, for both
electrodes, the potential response remains almost constant over
-
3
2-
the pH range 3.0-7.0 in a 1.0 × 10 M SO
4
solution. The
significant change in potential response observed at a pH below
3
.0 can be reasonably related to the simultaneous response of the
O+ and SO
2-
ions; the
electrode to the oppositely charged H
3
4
contribution of H
3
O+ to potential response counteracts that of
ion. On the other hand, the observed potential drift at high
pH values could be due to competition of OH- ion with SO 2-
ion
2
-
SO
4
4
-
3
Fig u re 6 . Potentiometric titration curves of 1.0 × 10 M solutions
for the sensing species. Therefore, the best performance for the
sulfate ion electrodes based on Zn-L should be achieved in the
pH range 3.0-7.0.
2-
2+
-1
2+
of SO4 (1) and Ba (2) with 1.0 × 10 M solutions of Ba (1)
2-
and SO4 (2), using the CGE as an indicator electrode.
Potentiometric selectivity coefficients, describing the prefer-
ence of the Zn-L-based membrane for an interfering ion, B,
relative to sulfate ion, A, were determined by the matched potential
method (MPM), which is recommended by IUPAC44 to overcome
the difficulties associated with the methods based on the Nicol-
sky-Eisenman equation.44,45 According to this method, the speci-
fied activity (concentration) of the primary ion (A) is added to a
reference solution, and the potential is measured. In a separate
experiment, interfering ions (B) are successively added to an
identical reference solution until the measured potential matched
of 10-3, which seems to indicate that these anions have negligible
impact on the functionality of the SO 2
-
sensors. Meanwhile, the
4
data given in Table 2 revealed that, in most cases, the selectivity
coefficients obtained for the coated graphite electrode are lower
than those for the polymeric membrane electrode, emphasizing
the superiority of the former electrode in this respect as well.3
It is interesting to note that a comparison of the selectivity
2-35
coefficients obtained with the proposed sensors along with those
reported before2
7-29
clearly indicated a tremendous enhancement
in the selectivity behavior of the proposed electrodes for the SO42
-
that obtained before by adding the primary ions. The matched
MPM
A,B
potential method selectivity coefficient,
K
, is then given by
ion. That is, despite the fact that the previously reported electrodes
for the SO 2 ion27-29 show a close Nernstian EMF-concentration
-
the resulting primary ion to interfering ion activity (concentration)
4
MPM
A,B
ratio,
K
) a
A
/ a
B
. The resulting values for both sulfate ion-
behavior, they could actually be considered good selective
electrodes for thiocyanate,27 nitrate,28 and perchlorate29 ions (rather
than for sulfate ion), as is quite obvious from the selectivity
coefficient data given in Table 2. Therefore, on the basis of these
data, we may conclude that the membrane sensors proposed in
this work are the most selective sulfate electrodes prepared to
date.
The proposed membrane electrodes were found to work well
under laboratory conditions. The coated graphite electrode was
used as an indicator electrode in the successful titration of a sulfate
selective electrodes are listed in Table 2. The selectivity coef-
ficients for the previously reported sulfate ion-selective electrodes
based on a bis(thiourea) ionophore,27 a derivative of imidazole,28
and a zwitterionic bis(guadinium) ionophore,29 are also included
in Table 2 for comparison.
From the data given in Table 2, it is immediately obvious that
the proposed sulfate sensors are highly selective with respect to
other common anions. The selectivity coefficients are on the order
(
(
(
43) Sun, B.; Fitch, P. Electroanalysis 1 9 9 7 , 9, 494.
44) Umezawa, Y.; Umezawa, K.; Sato, H. Pure Appl. Chem. 1 9 9 5 , 67, 507.
45) Bakker, E. Electroanalysis 1 9 9 7 , 9, 7.
ion solution (1.0 × 10 M) with the Ba2+ ion (1.0 × 10 M) and
-
3
-1
vice versa. The results of both titrations are shown in Figure 6,
Analytical Chemistry, Vol. 73, No. 13, July 1, 2001 2873

indicating that the amount of SO 2-
determined with the electrode.
The CGE was also used for the determination of iron in a
ferrous sulfate tablet (from Rose Daru Pharmaceutical Co., Tehran,
Iran). Each tablet was dissolved in water in a 50-mL volumetric
flask, and 0.5 mL of 3 M nitric acid was added and diluted to the
mark with water. The resulting solution was then titrated with a
or Ba2+ ions can be accurately
as an indicator electrode. The iron content of the drug calculated
from four replicate measurements (50.4 ( 1.0 mg/ tablet) was
found to be in satisfactory agreement with the declared amount
of 50 mg/ tablet.
4
Received for review December 8, 2000. Accepted March 1,
2001.
-
1
1
.0 × 10 M standard BaCl
2
solution, using the proposed sensor
AC001449D
2874 Analytical Chemistry, Vol. 73, No. 13, July 1, 2001