American Journal of Anal yt ical Chemistry, 2011, 2, 731-738
doi:10.4236/ajac.2011.26084 Published Online October 2011 (http://www.SciRP.org/journal/ajac)
Thiocyanate Ion Selective Solid Contact Electrode Based
on Mn Complex of
N,N'-BIs-(4-Phenylazosalicylidene)-O-Phenylene Diamine
Ionophore
Won-Sik Han1, Tae-Kee Hong2, Young-Hoon Lee2*
1The Research Center of Conservation Science for Cultural Heritage, Hanseo University,
Seosan, Choong-Nam, Korea
2Depsrtment of Chemistry, Hanseo University, Seosan, Choong-Nam, Ko rea
Received May 3, 2011; revised June 18, 2011; accepted June 30, 2011
Abstract
A thiocyanate ion selective poly(aniline) solid contact electrode based on manganese complex of
N,N’-bis-(4-phenylazosalicylidene)-o-phenylene diamine ionophore was successfully developed. The elec-
trode exhibits a good linear response of 58.1 mV/decade (at 20˚C ± 0.2˚C, r2 = 0.998) with in the concentra-
tion range of 1 × 10–1.0 ~ 1 × 10–5.8 M thiocyanate solution. The composition of this electrode was: ionophore
0.040, polyvinylchloride 0.300, dibutylphthalate 0.660 (mass). This dibutylphthalate plasticizer provides the
best response characteristics. The electrode shows good selectivity for thiocyanate ion in comparison with
any other anions and is suitable for use with aqueous solutions of pH 4.0 ~ 6.0. The standard deviations of
the measured emf difference were ±1.70 and ±2.01 mV for thiocyanate sample solutions of 1.0 × 10–2 M and
1.0 × 10–3 M, respectively. The stabilization time was less than 170 sec. and response time was less than 17
sec.
Keywords: Thiocyanate Ion, SCEs, ISEs, Mn Complexed Ionophore, Schiff Base, Ion Sensor
1. Introduction
The thiocyanate ion is usually present in low concentra-
tions in human serum, saliva, and urine as a result of the
digestion of some vegetables of the genus Brassica con-
taining glucosinolates (cabbage, turnip, kale) or by intake
of thiocyanate-containing foods such as milk and cheese.
Higher concentration of this ion, which is a metabolic
product of cyanide, arises from tobacco smoke [1,2]. In
this respect, the concentration level of thiocyanate is
considered to be a good probe to distinguish between
smokers and non-smokers. It has been found that there is
a correlation among the blood cyanide, the plasma thio-
cyanate, and the salivary thiocyanate [3]. Therefore, an
accurate, simple, and rapid method for the determination
of thiocyanate is significant in medicine and in the life
sciences [4]. Several methods for the determination of
thiocyanate ion such as spectrophotometry [5-9], chro-
matography [10-15], polarography [16], capillary zone
electrophoresis [17], amperometry [18], potentiometric
methods [19-38] etc., have been reported previously.
Among the various methods, the thiocyanate ion selec-
tive electrodes (ISEs) are useful since it provides high
sensitivity and a wide dynamic range. Especially, solid
contact type electrode (SCEs) have a electro-conductive
polymer layer, such as poly(aniline), to conduct ions and
electrons, and which are reported to strengthen the bonds
between metal substrates and reaction membranes and to
have stable and highly selective electrodes as they move
electrons to the metal substrate and the ions to the outer
PVC layer that provides new advantages. Simplicity of
design, lower costs, mechanical flexibility and the possi-
bility of miniaturization and micro-fabrication have
widened the applications for these type electrodes, espe-
cially in the fields of medicine and biotechnology.
In this work, we report the anion response behavior of
manganese chelate of a new Schiff base, N,N’-bis-(4-
phenylazosalicylidene)-o-phenylene diamine. The results

W.-S. HAN ET AL. 732
show that the Mn(II) chelate-based electrodes demon-
strate a highly specific response towards thiocyanate ion.
This paper describes the construction, potentiometric
characterization, and analytical application of a thiocy-
anate ion selective poly(aniline) solid contact electrode
based on the use of a manganese chelated N,N’-bis-(4-
phenylazo salicylidene)-o-phenylene diamine ionophore.
2. Experimental
2.1. Reagents
Aniline and tetrahydrofuran (THF) from Aldrich were
purified by vacuum distillation. salicylaldehyde, ethylene
diamine and manganese acetate (Mn(OAc)2) from Al-
drich was used without further purification. For all ex-
periments, analytical grade chemicals were used and all
aqueous solutions were prepared in doubly distilled wa-
ter obtained from a Millipore Milli-Q water purification
system. The membrane matrix high molecular weight
poly(vinylchloride) (PVC, n = 1,100), the plasticizers
2-nitrophenyloctylether (o-NPOE), tris(ethylhexyl)pho-
sphate (TEHP), bis(2-ethylhexyl)adipate (DOA), dioc-
tylphthalate (DOP), bis(2-ethylhexyl)sebacate (DOS),
dibutylphthalate (DBP) were from Aldrich.
2.2. Synthesis of
N,N’-Bis-(4-Phenylazosalicylidene)-
O-Phenylene Diamine (Mn-PASPD)
Ionophore
To a solution of 4.43 mL of freshly distilled aniline (0.05
mol) in 18.0 mL of concentrated hydrochloride and 20.0
mL of water, was added 4.00 g of sodium nitrite (0.10
mol) in 20 mL of water, the reaction was carried out for
1 h. below 5˚C. The reaction mixture was then added to a
solution of 18.0 g of sodium carbonate (0.17 mol) and
5.24 mL of salicylaldehyde in 150 mL of water, the reac-
tion temperature was remained within 0˚C ~ 5˚C. The
yellow precipitate was filtered off after 1 h. and recrys-
tallized from ethanol. The above product (2.26 g, 0.01
mol) refuxed with ethylene diamine (0.30 g, 5.0 mmol)
in 50 mL of absolute alcohol for 2 h. with stirring to give
a golden yellow solid which was purified by recrystalli-
zation from alcohol. Melting point is about 225˚C. 5.0
mmol of this product was mixed with Mn(OAc)2 (5.5
mmol) in absolute ethanol. The reaction mixture was
refluxed for 2 h. After cooled to the room temperature
and stand overnight, the manganese chelates obtained
were filtered, washed with absolute alcohol and dried in
vacuum. Melting point of this final product is about
345˚C.
2.3. Polymerization
Electrochemical experiments were performed in a con-
ventional cell with three electrodes. A saturated calomel
electrode was used as the reference electrode and all po-
tentials were recorded and reported with respect to this
electrode. Platinum wires (1 mm in diameter, 50 mm in
length) were used as the working and counter electrodes.
Electro-polymerization was carried out at the one end of
a platinum wire by cyclic voltammetry in 3.0 × 10–2 M
aniline and 6.0 × 10–2 M HCl solution. Cyclic voltam-
mograms were recorded using a potentiostat (EG & G
273A). For electrochemical polymerization of aniline,
the potential was swept between 0.0 V and 1.0 V at a
scan rate of 100 mV/s. The potential cycling was re-
peated up to 30 cycles and stopped at 1.0 V. After elec-
trodeposition, the poly(aniline) was washed with distilled
water and then dried for 24 h. in an 80˚C oven. Then the
metal part of the Pt-poly(aniline) electrode was covered
with a thermo-contractive insulation tube.
2.4. Preparation of Cocktail Solutions and
Fabrication of Solid Contact Electrode
Typical cocktail solution consists of ionophore 2.0% -
5.0% : PVC 30.0% : plasticizer 65.0% - 68.0%. All com-
ponents were dissolved in THF. The solid contact elec-
trodes were produced by dipping the Pt-poly(aniline)
electrode directly into the cocktail solution to coat it with
a thin film. The resulting solid contact electrode contains
three layers of Pt/electro-conductive polymer/PVC film
with an ionophore with a thickness of 2.5 ± 0.1 mm.
2.5. Measurements of Stabilization Time and
Response Time
The stabilization time was obtained as follows. First, the
dry electrode was deposited in a 1.0 × 10–3 M pH 5.5
Tris buffer NaSCN solution. Then the time until the po-
tential stabilized to lower than ±0.1 mV/min was meas-
ured. The response time was obtained as follows. After
the potential of the electrode had stabilized, 10.00 mL of
1.0 × 10–2 M NaSCN - pH 5.5 Tris buffer solution was
added to the 1.0 × 10–3 M pH 5.5 tris buffer NaSCN so-
lution while they were vigorously stirred. Then the time
until the potential stabilized to lower than ±0.1 mV/min
was measured.
2.6. Standard Thiocyanate Sample Solution
A stock solution of NaSCN (1.0 × 10–1 M) was prepared
using pH 5.5 Tris buffer solution. Dilute solutions (1.0 ×
10–1 to 1.0 × 10–7 M) of NaSCN were freshly prepared by

W.-S. HAN ET AL. 733
2.8. E.M.f. Measurements diluting the stock solution with doubly distilled water
and Tris buffer solution of pH 5.5.
The emf values were measured at 20˚C ± 0.2˚C using a
model 355 Ion-analyzer (Mettler-Toledo Ltd., England).
In all experiments, the pH measurements of the sample
solutions were determined with a Mettler-Toledo InLab
412 glass electrode. The external reference electrode was
a double-junction calomel electrode Orion 90-20-00
(Orion Research, USA.). The standard deviation arising
from this equipment was <0.1 mV for a single determi-
nation. Before use, the electrodes were conditioned in
distilled water for at least 1 h.
2.7. Selectivity of the Developed Sensors
The thiocyanate sensor was immersed in the 1.0 × 10–3.0
M NaSCN solution that had been adjusted to pH 5.5 with
Tris buffer and the potential was measured. The poten-
tials of solution adjusted to pH 5.5 with an interferent
concentration of 1.0 × 10–3.0 M were measured. The se-
lectivity coefficients , were determined by em-
ploying the separate solution method (SSM) with the
following generalized equation:
pot
SCN X
K
3. Result and Discussion

,12
log1 1log
pot
SCN X
K
EEZ a 
For six solid contact electrodes with different plasticizers
based on Mn-PASPD ionophore (Figure 1), there was no
tendency in the dynamic range and response slope as
dielectric constant of plasticizers was increased (Table
1). The electrodes based on Mn-PASPD ionophore
where E1 is the potential measured in 1.0 × 10–3.0 M
NaSCN, E2 the potential measured in 1.0 × 10–3.0 M of
the interfering compound, z is the charges of interfering
species X respectively, and S is slope of the electrode
calibration plot.
N
Mn
N
O
CH
HC
O
N
NN N
Figure 1. The scheme of Mn complex of N,N’-bis-(4-phenylazosalicylidene)-o-phenylene diamine ionophore.
Table 1. The response characteristics of thiocyanate ion selective solid contact electrode based on Mn-PASPD ionophore with
various plasticizers and composition.
Mn-PASPD PVC Plasticizers Response slope Dynamic range r2
3.0 30.0 DBP 67.0 56.4 mV/decade 1×10–1.0 M ~ 1×10–5.0 M 0.996
4.0 30.0 DBP 66.0 58.1 mV/decade 1×10–1.0 M ~ 1×10–5.8 M 0.998
3.0 30.0 DOS 67.0 50.2 mV/decade 1×10–1.0 M ~ 1×10–5.0 M 0.997
4.0 30.0 DOS 66.0 49.8 mV/decade 1×10–1.0 M ~ 1×10–5.2 M 0.977
3.0 30.0 DOA 67.0 50.4 mV/decade 1×10–1.0 M ~ 1×10–4.8 M 0.996
4.0 30.0 DOA 66.0 52.1 mV/decade 1×10–1.0 M ~ 1×10–4.7 M 0.977
3.0 30.0 TEHP 67.0 40.2 mV/decade 1×10–1.0 M ~ 1×10–4.6 M 0.976
4.0 30.0 TEHP 66.0 40.2 mV/decade 1×10–1.0 M ~ 1×10–4.7 M 0.995
3.0 30.0 NPOE 67.0 40.2 mV/decade 1×10–1.0 M ~ 1×10–4.9 M 0.989
4.0 30.0 NPOE 66.0 40.5 mV/decade 1×10–1.0 M ~ 1×10–4.8 M 0.978
3.0 30.0 DOP 67.0 58.0 mV/decade 1×10–1.0 M ~ 1x10–5.5 M 0.997
4.0 30.0 DOP 66.0 54.9 mV/decade 1×10–1.0 M ~ 1×10–5.2 M 0.997

W.-S. HAN ET AL. 734
which used DBP and DOP plasticizers showed better
response slope, dynamic range and correlation coeffi-
cient than other plasticizers such as DOA, DOS, NPOE,
and TEHP. For DBP, the linear dynamic range of the
solid contact electrode based on Mn-PASPD ionophore
was 1 × 10–1.0 ~ 1.0 × 10–5.8 M and the Nernstian slopes
showed 58.1 mV/decade (r2 = 0.998). For DOP plasti-
cizer, it was 1 × 10–1.0 ~ 1 × 10–5.5 M dynamic range and
-58.0 mV/decade response slope (r2 = 0.997). The com-
position of these electrodes were as follows: ionophore
4.0: PVC 30.0 : DBP 66.0 and ionophore 3.0: PVC 30.0:
DOP 67.0, respectively (Figure 2).
The selectivity factors for solid contact electrodes
based on Mn-PASPD ionophore with DBP and DOP
plasticizers, as determined with the separate solution
method, are represented in Figure 3. Solid contact elec-
trode containing DBP plasticizer was by far most selec-
tive towards thiocyanate ion in the presence of various
anions. It showed low interference from 2
4
CrO

, I–,
, , 3 and almost no interference
from the rest of anions in concentrations 100 - 10,000
times higher than thiocyanate ion. Solid contact electrode
containing DOP plasticizer was also selective in the pres-
ence of various anions. It showed low interference from
, , I–, , 3, S2–,
2
4
ClO 
2
4
CrO 
2
27
Cr O
2
4
ClO 
ClO
Cr 2
27
OClO2
3
SO

and
almost no interference from the rest of anions present in
concentrations 100 - 10,000 times higher than thiocy-
anate ion. But this result shows that the selectivity of
solid contact electrode based on DBP plasticizer is better
than that with DOP.
However, as it shown Table 2, the other characteris-
tics of the electrode based on DBP plasticizer are similar
to those of DOP. For the solid contact electrode based on
Mn-PASPD ionophore, the stabilization time of emf was
measured in 1.0 × 10–3 M thiocyanate in pH 5.5 Tris
buffered solution. First 10 seconds, the measured emf
value increased rapidly, and after about 170 seconds, it
stabilized the change of the emf being under 0.1 mV. So,
before using this electrode, we had to condition all elec-
trodes in distilled water or 1.0 × 10–3 M pH 5.5 Tris buff-
ered thiocyanate standard solution for at least 5 min
(Figure 4). The response time of the electrodes obtained
by injection of 10 mL of 1.0 × 10–2 M thiocyanate solu-
tion into pH 5.5 Tris buffered 1.0 × 10–3 M thiocyanate
standard solution was less than 17 sec. The reproducibility
of emf measurements with this electrode was checked by
alternating measurements (1 min each) on two pH 5.5
Tris-buffered thiocyanate standard solutions of 1.0 × 10–2
M and 1.0 × 10–3 M (20˚C ± 1˚C). The standard deviation in
Figure 2. The response characteristics of the poly(aniline)
solid contact electrode based on M n-PASPD ionophor e w ith
DBP plasticizer and DOP plasticizer.
Figure 3. Selectivity coefficients of the poly(aniline) solid
contact electrode based on Mn-PASPD with DBP plasticizer
and DOP plasticizer in solution of various anions.
Table 2. The response characteristics of solid contact electrode based on Mn-PASPD ionophore with DBP and DOP plasti-
cizers.
Standard deviation, mV
Composition of PVC Response timeStabilization time
1 × 10–2 M 1 × 10–3 M
pH range
Mn-PASPD 4.0:PVC 30.0:DBP 66.0 17 sec 170 sec. ±1.70 ±2.01 4.0 ~ 6.0
Mn-PASPD 3.0:PVC 30.0:DOP 67.0 20 sec 210 sec. ±1.65 ±1.93 4.0 ~ 6.0

W.-S. HAN ET AL. 735
Figure 4. The stabilization and response time of the poly
(aniline) solid contact electrode based on Mn-PASPD iono-
phore with DBP plasticizer.
the measured emf differences was ± 1.70 mV (n = 10) in
pH 5.5 Tris-buffered 1.0 × 10–2 M thiocyanate standard
solution and ±2.01 mV in pH 5.5 Tris-buffered 1.0 × 10–3
M thiocyanate standard solution. In the pH range of 4.0 ~
6.0, the potential remained stable regardless of the hy-
drogen ion concentrations (Figure 5). The effect of pH is
negligible at pH (4.0 ~ 6.0) and a near-Nernst response
towards thiocyanate is observed. As it showed in Figure
6, outside of pH stabilization range, at lower pH 4.0, the
response slope of this electrode remains the same but the
response range was reduced (dynamic range was 1 ×
10–1.0 M ~ 1 × 10–5.5 M). And the increasing interference
from hydroxide at higher pH (>6.0) results in a narrower
linear response range (dynamic range was 1 × 10–1.0 M ~
1 × 10–5.1 M) with a concomitant decrease in the response
slope (54.0 mV/decade). This effect of higher pH on the
response characteristics can be explained by coordination
competition between thiocyanate and hydroxide.
The preferential response towards thiocyanate is be-
lieved to be associated with the coordination of thiocy-
anate with the central metal of the carriers. In order to
investigate the interactions between thiocyanate ions and
the central metals, UV/vis spectra of the chloroform so-
lutions containing the carriers are compared with that of
the same solutions treated with 0.1 M NaSCN solution
for 2 h (Figure 7). In spectra of Mn-PASPD, show a max.
absorption band at 354 nm in the chloroform solutions
containing the carriers, while the spectrum of Mn-PASPD
with addition of NaSCN absorbs at 365 nm. This wave-
length shift in the UV/vis spectra, and calculated molar
absorptivity were 2.42/mol and 1.81/mol, respectively.
Obvious changes are observed in UV/vis spectra of the
Figure 5. pH stabilization range of the poly(aniline) solid
contact electrode based on Mn-PASPD ionophore with DBP
plasticizer.
Figure 6. Potential response characteristics of SCE based
on Mn-PASPD ionophore with DBP plasticizer at different
various pH buffered thiocyanate sample solution.
Figure 7. UV-VIS absorption spectra of chloroform solution
of Mn-PASPD and treated with 0.1 M NaSCN solution.

W.-S. HAN ET AL. 736
chloroform solution containing Mn-PASPD after treated
with 0.1 M NaSCN solution, it can be due to the unique
interactions between the central metal manganese in
Mn-PASPD and thiocyanate ion.
As it is to be expected from the selectivity data given
above, there is no interference from electrolytes in artifi-
cial serum in the physiologically relevant various ions.
(Figure 8) In artificial serum, their Nernstian slope of
these electrodes showed 57.9 mV/decade (20˚C ± 0.5˚C)
but their dynamic range of these electrodes showed mi-
nor decrease to ~ 1.0 × 10–5.1 M. Measured results in
artificial serum are the almost same values as the one’s
which were measured in acetate buffered (57.9 mV/
decade of Nernstian slope, ~1 × 10–5.8 M of dynamic
range) and tris buffered thiocyanide sample solution. The
result of decreasing Nernstian slope is considered to be
the lower interference affect of inorganic cations as like
Ca2+ or Mg2+. Thus it doesn’t seem to be affected by
abundant interference ions existing in human whole
blood.
Solid contact electrode continuously contacted with
Tris 5.5 buffered thiocyanate solutions and distilled wa-
ter for one month did not lose its performance. Also,
these electrodes, dried daily after daily measurements in
the Tris buffered thiocyanate solution, maintained a sta-
ble electrode potential for more than 6 months. Therefore
it seems that the electrodes can be used for over 6
months.
4. Conclusions
A highly selective electrode for thiocyanate ion based on
Mn-PASPD as an excellent ionophore is described. The
poly(aniline) solid contact electrode based on Mn-PASPD
Figure 8. The response characteristics of solid contact elec-
trode based on Mn-PASPD ionophore in acetate buffer
solution and artificial serum.
ionophore may provide an alternative for the direct de-
termination of thiocyanate. This electrode and ionophore
are very easy to prepare, show high selectivity and sensi-
tivity, wide dynamic range, low detection limit and very
fast response time. These properties make this electrode
suitable for measuring the concentration of thiocyanate
in a wide variety of samples without the need for pre-
concentration or pre-treatment steps, and without sig-
nificant interferences from other anionic species present
in the sample.
5. Acknowledgements
This work was supported by the research fund of Hanseo
University made in program year 2011
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