Novel selective thiocyanate PVC membrane electrode based on new Schiff base complex of 2.2-[(1,3-dimethyl-1,3-propanediylidene)dinitrilo]bis- benzenethiolato cadmium(II)

Article abstract of DOI:10.1039/b400681j

An ion selective electrode for thiocyanate based on a complex as ionophore is described. The influence of membrane composition, pH and interference anions were investigated. The sensor responds to thiocyanate in the linear range from 1 × 10^-6-1 × 10^-1 M with a slope -58.9 ± 0.5 mV decade^-1. The limit of detection is 5 × 10^-7 M SCN^-. Selectivity coefficients determined with the fixed interference method (FIM) indicate a good discriminating ability towards SCN^- ion in comparison to other anions. The proposed sensor has a fast response time of about 5-20 s and can be used for at least 2 months without any considerable divergence in potential. It was applied as indicator electrode in the titration of thiocyanate with Ag⁺ and in the potentiometric determination of thiocyanate in saliva and urine samples.

Full text of DOI:10.1039/b400681j

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P A P E R
Novel selective thiocyanate PVC membrane electrode based
on new Schiff base complex of 2.2-[(1,3-dimethyl-1,3-
propanediylidene)dinitrilo]bis-benzenethiolato cadmium(II)
Mohammad Mazloum Ardakani,* Masoud Salavati-Niassari and Azam Sadeghi
Department of Chemistry, Faculty of Science, Kashan University, Kashan, Iran.
E-mail: mazloum@kashanu.ac.ir; Fax: 0098361552930
Received (in Durham, UK) 14th January 2004, Accepted 3rd March 2004
First published as an Advance Article on the web 26th March 2004
An ion selective electrode for thiocyanate based on a complex as ionophore is described. The influence of
membrane composition, pH and interference anions were investigated. The sensor responds to thiocyanate in
the linear range from 1 ꢀ 10ꢁ6–1 ꢀ 10ꢁ1 M with a slope ꢁ58.9 ꢂ 0.5 mV decadeꢁ1. The limit of detection is
5 ꢀ 10ꢁ7 M SCNꢁ. Selectivity coefficients determined with the fixed interference method (FIM) indicate a good
discriminating ability towards SCNꢁ ion in comparison to other anions. The proposed sensor has a fast
response time of about 5–20 s and can be used for at least 2 months without any considerable divergence
in potential. It was applied as indicator electrode in the titration of thiocyanate with Agþ and in the
potentiometric determination of thiocyanate in saliva and urine samples.
chemicals were of analytical reagent grade from Merck. Dou-
Introduction
bly distilled water was used for preparing all solutions. Adjust-
ments of pH were made with dilute nitric acid or sodium
hydroxide.
It has been well documented that the selective complexation of
anions by synthetic ionophores can be used to design anion
selective electrodes. Solid-state thiocyanate-selective electrodes
are commercially available but show a large interference from
Iꢁ and S2ꢁ 1
. Most of these electrodes are based on vitamin
B12 derivatives,2–3 metalloporphyrins,4–6 organomercury com-
pounds,7 Schiff base complexes of metal ions,8 phthalocya-
nines9–11 and organometallic complexes.12–14 In all these
cases, ligation of the primary anion to the central metal ion
is responsible for the observed selectivity.
Preparation of the new thio Schiff-base complex
The thio Schiff-base ligand, 2,2-[(1,3-dimethyl-1,3propanediy-
lidene)dinitrilo]bis-benzenethiol was prepared by the reaction
of the 2-aminothiophenol (2.50 g, 0.02 mol) with acetyl acet-
one (1.01 g, 0.01 mol) in ethanol.17 The thio Schiff-base
ligand (3 g, 9.54 mmol) was dissolved in 100 ml of ethanol,
and the solution was brought to reflux. To this hot solution
was added cadmium acetate (2.54 g, 9.54 mmol) dissolved in
100 ml of ethanol. Refluxing was continued for 8 h. Upon
cooling the solution, a red crystalline solid formed which
was filtered, washed with ethanol, and dried in vacuo and
purified by recrystallization from chloroform. Elemental ana-
lysis showed for C17H16N2S2Cd: C, 48.0; H, 3.8; N, 6.6; S,
15.09% (theoretical C, 47.86; H, 3.74; N, 6.71; S, 14.86%).
For a truly anion-selective electrode, a strong interaction
between the ionophore and the anion is required in order to
complex anion in a selective fashion. The potentiometric
response of the membranes doped with these complexes is
believed to be based on the coordination of analyte anion axial
ligand to the metal center of the carrier molecule.15–16 Most
of the thiocyanate potentiometric sensors suffer from serious
interfering affects from other anions such as Iꢁ, ClO3ꢁ, Clꢁ,
Brꢁ and IO4
.
ꢁ
The purpose of the present work has been the development
of a thiocyanate selective electrode based on poly(vinyl chlo-
ride) (PVC) membrane impregnated with a new cadmium Schiff
base. This sensor is potentially useful for detection of thiocya-
nate in biological samples (e.g., urine), where evaluated levels
of thiocyanate correlate with excessive cigarette smoking.
The proposed sensor displays a low detection limit and high
selectivity and sensitivity to thiocyanate determination in real
samples.
Also infrared spectra of complex showed: nC
(1617
(1514
=
C
cmꢁ1,s); nC–H (2868 cmꢁ1, m: 2736 cmꢁ1, m), nC
=
N
cmꢁ1, m); nC–N (1162 cmꢁ1, s), nC–S (1002 cmꢁ1, s) and ring
vibration (1391 cmꢁ1, s; 1103 cmꢁ1, w; 768 cmꢁ1, w). Chemi-
cal structure of complex 2,2-[(1,3-dimethyl-1,3-propane-
diylidene)dinitrilo]bis-benzenethiolato cadmium(II), [CdL] is
shown in Fig. 1.
Experimental
Reagents
Reagent grade dibutylphthalate (DBP), methyltrioctyl ammo-
nium chloride (MTOAC), dioctylphthalate (DOP), 2-nitrophe-
nyloctylether (NPOE) and tetrahydrofuran (THF) were
obtained from Merck and were used as received, except
THF, which was distilled before use. PVC of high relative
molecular weight was purchased from Aldrich. All other
Fig. 1 Structure of complex, 2,2-[(1,3-dimethyl-1,3-propanediylidene)-
dinitrilo]bis-benzenethiolato cadmium(II), [CdL] used as carrier for
SCNꢁ-ISE.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 5 9 5 – 5 9 9
595

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Electrode preparation
Results and discussion
The membrane ion-selective electrodes were prepared accord-
ing to a previously reported method.18 A mixture of PVC,
plasticizer (DBP), and the membrane additive (MTOAC),
total mass 200 mg was dissolved in approximately 10 ml
freshly distilled THF. To this mixture was added the electro-
active material [CdL], and the solution was mixed well. The
resulting mixture was poured into a small flat bottom dish,
covered with a filter paper and the solvent was allowed to eva-
porate at room temperature. The resulting membrane (ca. 0.2
mm thick) was then sectioned with a cork borer and mounted
across the opening of a PVC tube of about 7 mm i.d. and 1.5
cm length using a glue of PVC in THF. The PVC tube with
the membrane was then incorporated into a silver–silver
chloride wire electrode. The electrode was then filled with
an internal solution of 0.10 M KSCN. The filled electrode
was conditioned by soaking in 0.10 M KSCN. The first con-
ditioning time was approximately 5 h and then 30–40 min for
successive uses.
The plasticized PVC-based membrane electrode containing the
ionophore, generates a stable potential response in solution
containing thiocyanate. The response of the electrode was
improved by addition of lipophilic salts such as MTOAC to
the membrane. This not only reduces the membrane resistance
but also enhances the response behavior and selectivity and
reduces interference from lipophilic sample anions.20–21 The
membrane without the ionophore displayed insignificant
selectivity toward thiocyanate whereas, in the presence of the
ionophore, the membrane showed remarkable selectivity for
thiocyanate over most common inorganic and organic anions.
The preferential response toward SCNꢁ is believed to be asso-
ciated with the coordination of thiocyanate with the central
metal ion of the carrier. It is well-known that the sensitivity
and selectivity obtained for a given ionophore depends signifi-
cantly on the membrane condition.22–25 Several membrane
compositions were investigated by varying the proportions of
PVC, DBP (or DOP), and membrane active material, [CdL]
and MTOAC. Irrespective of ionophore concentration the
slope was relatively larger when the DBP/PVC weight ratio
was approximately 2.0. It was also observed that the potentio-
metric response of the electrode toward thiocyanate ion
depended on the concentration of the ionophore incorporated
within the membrane. Increasing the amount of [CdL] up to
5% resulted in membranes for which slopes were larger and
the linear range wider.
Potential measurement and calibration
The electrochemical system for this electrode can be repre-
sented as follows:
Ag j AgCl j internal solution ð0:10 M KSCNÞj
PVC membrane j test solution jj SCE
The potentiometric response of the membrane was greatly
improved by the presence of the lipophilic cationic additive,
MTOAC. Better response characteristics, i.e. Nernstian
response and improved selectivity, were usually observed with
an ionophore/MTOAC weight ratio of approximately 2.5
which corresponds to a mole ratio of approximately 2.0. The
presence of lipophilic ionic sites is beneficial for both neutral
carrier and charged carrier-based ion-selective electrodes.26–27
Among the different compositions studied (Table 1)
responses were best for the membrane incorporating 32%
PVC, 61% DBP, 2% additive and 5% [CdL]. As is obvious
from Table 1, among two different plasticizer used, DBP is a
more effective solvent mediator in preparing the thiocyanate
ion-selective electrode. The composition was, therefore, used
to study the performance of the electrode, viz, working concen-
tration range, sensitivity, selectivity, life time, response time,
and effect of pH. The characteristic properties of the optimized
membrane are summarized in Table 2.
All potentials were measured at 25 ꢂ 1 ꢃC using a Metrohm
Model 691 pH/mV meter. A saturated calomel electrode
(SCE, Metrohm) with a fiber junction was used as the external
reference electrode. Activities were calculated according to the
Debye–Huckel procedure,19 for the calibration curve, concen-
¨
tration instead of activity was used. The pH of the sample solu-
tion was monitored simultaneously with a conventional glass
pH electrode (Metrohm).
Before starting the measurements, the electrode was precon-
ditioned in stirred water until a steady potential was obtained.
The performance of the electrode was investigated by measur-
ing its potential in potassium thiocyanate solutions prepared
in the concentration range 1 ꢀ 10ꢁ7–1 ꢀ 10ꢁ1 M by serial dilu-
tion at constant pH ¼ 5. All solutions were freshly prepared
by dilution from the stock standard solution, 1 ꢀ 10ꢁ1 M,
with doubly distilled water. The solutions were stirred and
potential readings recorded when they became stable. The
data were plotted as observed potential vs. the logarithm of
the SCNꢁ concentration. Potentiometric titration of 10 ml
1 ꢀ 10ꢁ3 M KSCN solution was carried out with 0.01 M
AgNO3 solution using the thiocyanate selective electrode as
the indicator electrode in conjunction with a fiber function
SCE electrode.
The influence of the concentration of the internal solution
on the potential response of the thiocyanate-selective electrode
was studied and the results showed the concentration of the
internal solution does not cause any significant difference in
the potential response of the electrodes, except for an expected
change in the intercept of the resulting Nernstian plots.
Table 1 Optimization of membrane ingredients
Percent (w/v) of various components
No
PVC
DBP
Ionophore
DOP
MTOAC
Slope/mV decadeꢁ1
Linear range/M
Detection limit/M
1
2
31
31
31.5
30
32
32
32
32
32
32
—
60.5
62
61
61
—
61
—
63
—
7
7
6
7
6
6
5
5
5
5
60.5
—
—
—
—
61
—
61
—
63
1.5
1.5
0.5
2
ꢁ49.0
ꢁ53.6
ꢁ62.2
ꢁ54.8
ꢁ65.4
ꢁ55.0
ꢁ58.9
ꢁ62.0
42.3ꢁ
40.3ꢁ
1 ꢀ 10ꢁ5–1 ꢀ 10ꢁ1
1 ꢀ 10ꢁ5–1 ꢀ 10ꢁ1
1 ꢀ 10ꢁ5–1 ꢀ 10ꢁ1
1 ꢀ 10ꢁ5–1 ꢀ 10ꢁ1
1 ꢀ 10ꢁ5–1 ꢀ 10ꢁ1
1 ꢀ 10ꢁ4–1 ꢀ 10ꢁ1
1 ꢀ 10ꢁ6–1 ꢀ 10ꢁ1
1 ꢀ 10ꢁ5–1 ꢀ 10ꢁ1
5 ꢀ 10ꢁ5–1 ꢀ 10ꢁ1
5 ꢀ 10ꢁ4–1 ꢀ 10ꢁ1
1 ꢀ 10ꢁ5
1 ꢀ 10ꢁ5
5 ꢀ 10ꢁ6
1 ꢀ 10ꢁ5
5 ꢀ 10ꢁ6
5 ꢀ 10ꢁ5
5 ꢀ 10ꢁ7
1 ꢀ 10ꢁ5
5 ꢀ 10ꢁ5
1 ꢀ 10ꢁ4
3
4
5
1
6
1
7
2
8
2
9
10
0.0
0.0
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
596
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 5 9 5 – 5 9 9

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The response time of the electrode was measured after suc-
Table 2 Characteristics of optimized SCNꢁ-ISE
cessive immersion of the electrode in a series of SCNꢁ solu-
tions, in each of which the SCNꢁ concentration increased
tenfold, from 1 ꢀ 10ꢁ6 to 1 ꢀ 10ꢁ1 M. The static response time
thus obtained was 5 s for 1 ꢀ 10ꢁ1M SCNꢁ concentration. At
lower concentrations, however, the response time was longer
and reached 20 s for a SCNꢁ concentration of 1 ꢀ 10ꢁ5 M.
The actual potential vs. time traces are shown in Fig. 3. The
potentials remained constant for approximately 5 min, after
which a very slow change within the resolution of the meter
was recorded. The sensing behavior of the membrane electrode
did not depend on whether the potentials were recorded from
low to high concentrations or vice versa.
Linear range/M
Slope/mV decadeꢁ1
pH range
1 ꢀ 10ꢁ6–1 ꢀ 10ꢁ1
ꢁ58.9
M
2.5–8.0.
Precision
At concentrations of 1 ꢀ 10ꢁ2
M
and 1 ꢀ 10ꢁ3 M SCNꢁ standard
deviations were of ꢂ0.5 and ꢂ0.7 mV
respectively.
Detection limit/M
Life time/month
Response time/s
5 ꢀ 10ꢁ7
2
5–20
The electrode was tested over a period of two months to
investigate stability. During this period the electrode was in
daily use and was stored in 1 ꢀ 10ꢁ4 M SCNꢁ solution when
not in use. Before measurements the electrode was conditioned
in 0.1 M KSCN for approximately 30 min. No significant
change in the performance of electrode (slope, linear range)
was observed during this period. Storage of the electrode in
solution for extended periods, however, especially in stirred
solution, resulted in a slight gradual decrease in the slope, as
is usual for many plasticized PVC membranes, probably as
a result of leaching of the ionophore from the membrane.
The most important characteristics of any ion-sensitive sen-
sor its response to the primary ion in the presence of other ions
in solution, which is expressed in terms of the potentiometric
selectivity coefficient. Potentiometric selectivity coefficients
The effect of the pH of the test solution (1 ꢀ 10ꢁ2, 1 ꢀ 10ꢁ3
and 1 ꢀ 10ꢁ4 M) on the response of the membrane electrode
was examined at three SCNꢁ concentrations. pH was adjusted
with dilute nitric acid and sodium hydroxide as required. As
illustrated in Fig. 2 the potentials remain constant within a
pH range of approximately 2.5–8.0. Variation of the potential
at pH < 2.5 could be related to protonation of [CdL] in the
membrane phase, which results in a loss of its ability to inter-
act with SCNꢁ ions. At higher pH > 7.5, the potential drop
(negative slope) may be because of interference from hydroxide
ions. In high pH media, hydroxide ion will compete with
thiocyanate ion for the cation site in the membrane.
The optimum equilibration time for the membrane electrode
in the presence of 1 ꢀ 10ꢁ1 M KSCN was 5 h, after which it
would generate stable potentials in contact with thiocyanate
solution. The potentiometric response of the membrane elec-
trode to different concentrations of SCNꢁ was examined using
the optimized membrane composition and conditions
described above. In the traditional procedure, the membrane
is conditioned in a solution that contains a relatively high con-
centration of the primary ion. This ensures stable and reprodu-
cible electrode behavior and is recommended for practical use
of the sensor. However, the presence of these primary ions is
often the reason for the non-Nernstian response toward highly
discriminated ions.28 The calibration plot shows the linear
range to be from 1 ꢀ 10ꢁ6–1 ꢀ 10ꢁ1 M; the near-Nernstian
slope is ꢁ58.9 ꢂ 0.5 mV decadeꢁ1 of SCNꢁ concentration.
The practical limit of detection, as determined from the inter-
section of the two extrapolated segments of the calibration
graph, was 5 ꢀ 10ꢁ7 M.
(Kpot
ꢁ) describing the preference of the membrane for
ꢁ
,
A
SCN
an interfering ion Aꢁ relative to SCNꢁ were determined by
The stability and reproducibility of the electrode was also
tested. The standard deviation of 20 replicate measurements
at three thiocyanate concentration of 1 ꢀ 10ꢁ2, 1 ꢀ 10ꢁ3 and
1 ꢀ 10ꢁ4 M were ꢂ0.5, ꢂ0.7 and ꢂ0.8 mV respectively.
Fig. 3 Response time of the optimized SCNꢁ-ISE for step changes in
concentration of SCNꢁ: a) 1 ꢀ 10ꢁ5 M, b) 1 ꢀ 10ꢁ4M, c) 1 ꢀ 10ꢁ3M, d)
1 ꢀ 10ꢁ2 M, e) 1 ꢀ 10ꢁ1 M.
Table 3 Selectivity of coefficients, determined by use of the fixed
interference method, for the thiocyanate-selective electrode
ꢁ
ꢁ
ꢁ ꢁ
,
A
Interfering ion log Kpot
,
Interfering ion log Kpot
SCN
A
SCN
ꢁ
2ꢁ
ꢁ
ClO4
CIꢁ
Brꢁ
Iꢁ
ꢁ2.4
ꢁ4.1
ꢁ2.5
ꢁ2.8
ꢁ3.1
ꢁ4
CO3
MnO4
ꢁ3.6
ꢁ2.2
ꢁ2.8
ꢁ3.5
ꢁ3.2
ꢁ3.5
ꢁ4
ꢁ
NO3
2ꢁ
SO4
S2O3
ꢁ
2ꢁ
NO2
CNꢁ
OAcꢁ
Sala
H2PO4
C2O4
HPO4
ꢁ
2ꢁ
ꢁ4.1
ꢁ4.2
ꢁ3.7
2ꢁ
ꢁ3.7
ꢁ2.5
2ꢁ
2ꢁ
Cr2O7
CrO4
a
Fig. 2 The influence of pH on the potential response of the optimized
SCNꢁ-ISE: (a) 1 ꢀ 10ꢁ2 M, (b) 1 ꢀ 10ꢁ3 M, (c)1 ꢀ 10ꢁ4 M.
Salicylate.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 5 9 5 – 5 9 9
597

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Table 4 Comparison of the potentiometric parameters of the proposed SCNꢁ-selective electrode with the other SCNꢁ-selective electrodes
The proposed
SCNꢁ-ISE
Aꢁ
Ref. 30
Ref. 31a
Ref. 32b
Ref. 33
Nernstian slope,
mV decadeꢁ1
ꢁ58.9 ꢂ 0.5
ꢁ57.8
Ionophores (I,II,III,
respectively) ꢁ56.8,
ꢁ58.3, ꢁ55.6
Ionophores
(I,II, respectively)
ꢁ60.6 ꢂ 0.8,
ꢁ57.5 ꢂ 1.2
Ionophore
ꢁ58.1
Linear range, M
1 ꢀ 10ꢁ6–1 ꢀ 10ꢁ1
1 ꢀ 10ꢁ7–1 ꢀ 10ꢁ1
Ionophores (I,II,III,
respectively)
5 ꢀ 10ꢁ5–1 ꢀ 10ꢁ2
(I,II)
1 ꢀ 10ꢁ6–1 ꢀ 10ꢁ1
1 ꢀ 10ꢁ5–1 ꢀ 10ꢁ2
9 ꢀ 10ꢁ6–1 ꢀ 10ꢁ2
1 ꢀ 10ꢁ5–1 ꢀ 10ꢁ2
5 ꢀ 10ꢁ6
Limit of
detection, M
Response
time, s
5 ꢀ 10ꢁ7
5–20
—
4.8 ꢀ 10ꢁ8
15–120
—
6 ꢀ 10ꢁ7
4 ꢀ 10ꢁ5
<10
5
5
Interferent
ions
Iꢁ
NO3ꢁ, NO2ꢁ, Iꢁ,
Clꢁ, ClO4ꢁ, Brꢁ
ꢁ
IO4
a
b
Ionophores I,II,III, were di-,tetra-,and hexa-imidepyridine derivatives respectively. Ionophores of I, II, were bis(2-mercaptobenzoxazolato)-
mercury(II) and bis(2-pyridinethiolato)mercury(II) respectively.
the fixed interference method (FIM)29. The selectivity coeffi-
cient for various anions were evaluated by the mixed solution
Table 5 Determination of thiocyanate in different samples
Spectrophotometric
method (SCNꢁ/mM)a
method with a fixed concentration of interference (0.10 M),
Sample
ISE (SCNꢁ/mM)a
and varying amounts of SCNꢁ concentrations. Table 3 lists
the potentiometric selectivity coefficient data of the sensor
for several anions relative to SCNꢁ. As is evident from the
data in Table 3 the electrode based on [CdL] has relatively high
selectivity toward SCNꢁ relative to anions such as salicy-
late, oxalate, and several common anions. The compound of
[CdL] seems to show a stronger interaction with the SCNꢁ ion
than with any of the other anions tested. It is interesting to
note that the observed selectivity pattern for the SCNꢁ-ISE
significantly differs from the so-called Hofmeister selectivity
sequence. From the data given in Table 3, it is immediately
obvious that the SCNꢁ-ISE is highly selective with respect to
other inorganic and organic anions. This is most probably
due to the weak interaction between these anions and the
ionophore.
Urine of smoker
Urine of non-smoker
0.77 ꢂ 0.07
0.32 ꢂ 0.04
0.79 ꢂ 0.06
0.31 ꢂ 0.04
0.74 ꢂ 0.05
1.71 ꢂ 0.06
Saliva of non-smoker 0.73 ꢂ 0.06
Saliva of smoker
1.73 ꢂ 0.08
a
Mean value ꢂ standard deviation (five determinations).
The proposed electrode was also successfully used an indica-
tor electrode in conjunction with SCE in the potentiometric
titration of KSCN solution with AgNO3 a suitable titrant.
Typical results for the titration for 10.0 ml of 0.001 M KSCN
with 0.01 M Ag NO3 show that the amount of KSCN ion in
solution can be accurately determined with the electrode.
Therefore, the end point can be obtained by extrapolation of
the linear portions of the titration plot.
Table 4 lists the linear range, detection limit, slope, response
time and selectivity coefficients of some of the other thiocya-
nate-selective electrodes against the proposed thiocyanate-
selective electrode for comparative purposes.30–33 As can be
seen from the Table 4, the selectivity coefficients obtained for
the proposed electrode are superior to those reported for other
SCNꢁ-selective electrodes listed. It is noteworthy that the limit
of detection, linear range, slope and response time of the pro-
posed electrode are also considerably improved with respect to
those of the previously reported SCNꢁ-selective electrodes.
Conclusions
The new electrode incorporating the 2,2-[(1,3-dimethyl-1,3-
propanediylidene)dinitrilo]bis-benzenethiolato cadmium(II) as
an ionophore showed fast response time, wide linear dynamic
range and low limit detection. The high degree of thiocyanate
selectivity by the electrode makes it potentially useful for
monitoring concentration levels of thiocyanate in biological
samples.
Analytical applications
The high degree of thiocyanate selectivity exhibited by the elec-
trode based on [CdL] carrier makes it potentially useful for
monitoring concentration levels of thiocyanate in biological
samples. To assess the applicability of the membrane electrode
to real samples an attempt was made to determine SCNꢁ in
smoker and non-smoker urine and saliva. Quantification of
salivary thiocyanate provides a reliable prediction of cyanide
exposure such as occurs in tobacco smoking and as a suitable
index for distinguishing smokers from non-smokers. Measure-
ments were carried out on different samples, taken from a
cigarette smoker and a non-smoker by the standard addition
method. The samples were diluted by a factor of 10 with phos-
phate buffer of pH ¼ 5. The results are compared with the
standard spectrophotometric method.34 The results, presented
in Table 5, indicate good agreement between the potentio-
metric and spectrophotometric procedures.
Acknowledgements
The authors thank the Kashan University Research Council
for the support of this work.
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T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 5 9 5 – 5 9 9
599