American Journal of Analyt ical Chemistry, 2011, 2, 56-65
doi:10.4236/ajac.2011.21006 Published Online February 2011 (http://www.SciRP.org/journal/ajac)
A New Potentiometric Sensor for Determination of
Pethidine Hydrochloride in Ampoules and Urine
Hazem M. Abu-Shawish1, Nasser Abu Ghalwa2, Ghada I. Khraish1, Jehad Hammad3
1Faculty of Sc ience, Research Center, Al-Aqsa University, Gaza, Palestine
2Chemistry Department, Al-Azhar University, Gaza, Palestine
3Pharmacology Department, Faculty of Medicine, Islamic University, Gaza, Palestine
Received August 28, 2010; revised November 10, 2010; accepted November 17, 2010
Abstract
A simple, precise, rapid and low-cost potentiometric method for pethidine determination in pharmaceuticals
and urine is proposed. A chemically modified carbon paste electrode (CMCPE) for pethidine hydrochloride
(PDCl) based on pethidine-phosphotungstate (PD-PT) as ion-pair complex was prepared and fully character-
ized in terms of composition, usable pH range, response time and temperature. The pethidine electrode
showed Nernstain responses in the concentration range 2.1 × 10–6 - 1.0 × 10–2 M with a detection limit of 7.3
× 10–7 and usable within the pH range 3.5 - 6.6. This sensor exhibited a fast response time (about 5 - 8 s),
good stability. The value (dE/dt) of the electrode was found to be 0.00071 V/˚C, which indicates fairly high
thermal stability. Selectivity coefficients determined by matched potential method (MPM) and separate solu-
tion method (SSM) showed high selectivity for PDCl with respect to a large number of inorganic cations,
organic cations, sugars and some common drug excipients. The sensor could be used successfully in the es-
timation of PDCl in ampoules and in spiked urine samples.
Keywords: Potentiometry, Carbon Paste Electrode, Pethidine, Ion-Selective Electrode
1. Introduction
Pethidine (Meperidine hydrochloride, Dolantin) (Ethyl,
1-methyl-4-phenylpiperidine-4-carboxylate hydrochloride),
(Figure 1) is a potent opiate analgesics, which has been
employed in the treatment of a variety of medical condi-
tions [1]. Pethidine hydrochloride is also used as an illicit
drug and therefore it is placed on the schedule II con-
trolled substances list (drugs that have acceptable medi-
cal use and have high potential for abuse) in the United
States and many other countries. Pethidine is also pre-
scribed as a substitute for heroin [2], and often used
medically as postoperative analgesia. In sports, athletes
often take far higher doses of drugs than have been given
for therapeutic use or in clinical studies to excel in com-
petition. They have been barred to use by the Interna-
tional Olympic Committee and other sports organizations
[3]. Therefore, determination of pethidine has important
practical meanings. Many Several sophisticated analyti-
cal methods were reported to determine the pethidine
such as high performance liquid chromatography (HPLC)
[4,5], gas chromatography [6], gas chromatography in
combination with mass spectrometry (GC-MS) [7,8],
spectrophotometry [9] and potentiometric analysis [10-14].
Many of these methods involve several time-consuming
manipulations, extraction steps, derivatization reactions
that are liable to various interferences, and are not appli-
cable to colored and turbid solutions either.
Potentiometric methods with ion-selective electrodes
(ISE’s) have proved to be effective for the analysis of
pharmaceutical formulations and biological samples, be-
cause these sensors offer the advantages of simple design,
construction, and manipulation, reasonable selectivity,
fast response time, applicability to colored and turbid
solutions and possible interfacing with automated and
computerized systems. [15-17]. A number of ISEs based
on PVC membrane [10-14], for determination of pethi-
dine have been reported. However, these electrodes still
have certain inherent limitations. Cleary, there is an ur-
gent need for a recipe that would make the fabrication
and optimization of potentiometric sensors with low de-
tection limit, high stability and reproducibility a straight-
forward process.
The advantages of carbon paste electrodes, such as che-

H. M. ABU-SHAWISH ET AL.
57
Figure 1. Chemical structure of pethidine hydrochloride.
mical inertness, robustness, low cost, renewability, very
low background current, stable response, low ohmic re-
sistance and no need for internal solution over conven-
tional polymeric membrane electrodes has attracted the
attention of researchers in recent years [15-18]. After
careful review of literature, there is no report on deter-
mination of pethidine hydrochloride using a carbon paste
electrode.
This paper describes the construction, potentiometric
characterization, and analytical application of a pethidine-
chemically modified carbon paste electrode (D-CMCPE)
based on the use of pethidine-phosphotungstate (PD-PT)
as electroactive materials and 2-nitrophenyl octyl ether
(2-NPOE) as plasticizer. The results presented in this
paper show that the sensor constructed for pethidine ion
has a wide concentration range, low limit of detection,
good Nernstain slope, and high selectivity over a wide
variety of other cations and compared with electrodes
previously reported [10-14].
2. Experimental
2.1. Reagents and Solutions
Pethidine hydrochloride PDCl was obtained from general
administration of pharmacy, ministry of health (Gaza-
Palestine). Graphite powder, 2-nitrophenyl octyl ether
(2-NPOE), dioctyl phthalate (DOP), dibutyl phthalate
(DBP), tris(2-ethylhexyl) phosphate (DOPh), dioctyl
sebacate (DOS), tributyl phosphate (TPh), tricresyl phos-
phate (TCP) and dibutyl butyl phosphonate (DDBP), as
well as metal salts were purchased from Aldrich and
used as received. Silicotungstic acid (STA), phospho-
tungstic acid (PTA), phosphomolybdic acid (PMA), sili-
comolybdic acid (SMA) and sodium tetraphenyl borate
(Na-TPB) were obtained from Sigma.
2.2. Apparatus
Potentiometric measurements were carried out with a
digital millivoltmeter (SR-MUL-3800). pH measurements
were made on a digital pH meter (Wissenscha-ftlich-Te-
chnische WerkstattenGmbH (WTW)-Germany) at room
temperature (25.0˚C ± 1.0˚C).
2.3. Preparation of Ion-Exchanger Complex
An ion-exchanger complex was made from pethidine
hydrochloride (PD) and one of the following substances:
silicotungstic acid (STA), silicomolybdic acid (SMA)
phosphotungstic acid (PTA), phosphomolybdic acid
(PMA), or sodium tetraphenyl borate (Na-TPB) accord-
ing to a previously reported method [19]. This precipi-
tated ion-pair was washed, dried and used as the active
substances for preparing the chemically modified carbon
paste electrodes of pethidine hydrochloride.
2.4. Preparation of the Electrode
A Teflon holder (12 cm length) with a hole at one end (7
mm diameter, 3.5 mm deep) for the carbon paste filling
served as the electrode body. Electrical contact was made
with a stainless steel rod through the centre of the holder.
This rod moved up and down by screw movement to
press the paste down when renewal of the electrode sur-
face was needed. Modified carbon paste was prepared by
mixing weighed amounts of PD-PT and high purity
graphite. The mixture was homogenized, and then the
impregnated carbon powder was added to a weighed
amount of 2-nitrophenyl octyl ether (2-NPOE). A very
high degree of homogenization is then achieved by care-
ful mixing with a glass rod in an agate mortar and sub-
sequent thorough crushing with a pestle. The ready-pre-
pared paste was then packed into the hole of the elec-
trode body. The carbon paste was smoothed onto paper
until it had a shiny appearance and was used directly for
potentiometric measurements without preconditioning
requirements.
2.5. Sample Analysis
The standard addition method in which small increments
(10 - 100 µl) of (0.1 M) TDCl solution were added to 50
mL aliquot-samples of various concentrations (5.0 × 10–6
to 1.0 × 10–5 M) PDCl was applied. The potential after
each increment was recorded at 25˚C ± 0.1˚C and used to
calculate the concentration of TDCl in the drug samples.
The potentiometric titration of different volumes of
1.0 × 10–3 M and 1.0 × 10–2 M PDCl solution: 5 - 10 mL
equivalent to 1.42 - 28.4 mg, were transferred to a 25 mL
beaker, and titrated with a standard solutions of Na-TPB
and PTA using the prepared PD-electrode as indicator
electrode. The end points were determined from the
S-shaped curve.
In the calibration graph method, different amounts of

58 H. M. ABU-SHAWISH ET AL.
PDCl were added to 50 mL of water comprising a con-
centration range from 1.0 × 10–7 to 1.0 × 10–2 M and the
measured potential was recorded using the present elec-
trode. Data were plotted as potential versus logarithm of
the PD+ activity and the resulting graph was used for
subsequent determination of unknown drug concentra-
tion [20].
2.6. Analysis of Spiked Urine Samples
The samples (5ml of urine) were spiked with pethidine
hydrochloride and left stirred for 5 min, transferred to a
25 mL volumetric flask and completed to the mark with
distilled water to give 5.0 × 10−6 to 1.0 × 10−5 M PDCl.
These solutions were subjected to the standard additions
method or the calibration graph method for PDCl deter-
mination [21].
3. Result and Discussion
3.1. Characteristics of the Electrode
The general characteristics of sensor are outlined by de-
termining its sensitivity, detection limit, linear range and
selectivity coefficients. Some important features of car-
bon paste electrode, such as the properties of the plasti-
cizer, the graphite (G)/plasticizer (P) ratio, the nature and
amount of the ion-exchanger complex, are reported to
significantly influence the sensitivity and selectivity of
the sensor.
3.1.1. Effect of Ion-Exchanger
Ion-exchanger complex used in ISEs should have rapid
exchange kinetics and adequate formation constants in
the paste. In addition, they should have good solubility in
the paste matrix and sufficient lipophilicity to prevent
leaching from the paste into the sample solution [22,23].
The ion-exchangers of PD-ST, PD-SM, PD-PT, PD-PM
and PD-TPB were prepared and tested as modifiers for
the present electrode. The influences of the amount of
the different pethidine ion-exchangers in the carbon paste
were investigated and the corresponding results are
summarized in Table 1. As can be seen, the electrode
containing zero percentage of ion-exchanger complexes
(sensor No. 1) showed a negligible response towards
pethidine cations, whereas in the presence of the
Ion-exchanger complexes the sensor displayed remark-
able selectivity for pethidine cations, as found for elec-
trode (composition #2) that contains PD-PT and pro-
duced the widest concentration range, the lowest detec-
tion limit and shortest response time with Nernstain
slope towards PD+ cations. The other ion-exchangers
gave inferior response. Further increase in the ratio of the
ion-exchanger complexes (sensors No. 3 and 4), however,
resulted in a little decrease in the response of the elec-
trode, most probably due to some inhomogenieties and
possible saturation of the paste [24].
3.1.2. Effect of Graphite/Plasticizer (G/P) Ratio
Different graphite/plasticizer ratios that is 0.85, 0.95,
1.05, 1.15 and 1.30 using 2-NPOE as plasticizer while
keeping the amount of ion-exchanger complex constant
(i.e. 0.5%) were examined while optimizing the mem-
brane composition as shown in Table 1. The paste with
(G/P) ratio of 0.95 showed the optimum physical proper-
ties and ensured high enough mobilities of their con-
stituents [25] and was found to give the best reproduci-
bility and sensitivity of 54.2 mV per decade over the
widest linear concentration range of 2.1 × 10−6 - 1.0 ×
10−2 M in comparison to the other ratios tested. Pastes
with G/P more than 1.30 produced “crumbly” pastes and
those with ratio smaller than 0.85 had a consistency re-
sembling that of “peanut butter”, i.e. , not workable.
3.1.3. E ffect of Plasticizers
Two parameters are of importance when manufacturing a
carbon paste: 1) its mechanical stability and 2) its active
surface area. Mechanical stability can be interpreted as
the ability of the carbon paste to avoid erosion in solu-
tion. The use of plasticizers will give some permeable
properties to the paste and will improve its mechanical
stability by promoting binding between grains [26]. In
addition, the solvent mediator, in particular, has a dual
function: it acts as a liquifying agent, enabling homoge-
nous solubilization and modifying the distribution con-
stant of the ion-exchanger complex used. The proportion
of solvent mediator must be optimized in order to mini-
mize the electrical asymmetry of the paste, to keep the
sensor as clean as possible, and to stop leaching to the
aqueous phase [27]. In exploration for a suitable plasti-
cizer for constructing this electrode, we used seven plas-
ticizers, with the values of dielectric constants, lipo-
philicity and molecular weight respectively listed in
parantheses, namely, 2-NPOE (εr = 23.6, PTLC = 5.9,
M.wt. = 251), DOP (εr = 5.1, PTLC = 7.0, M.wt. = 391),
DBP (εr = 6.4, PTLC = 4.5, M.wt. = 278), DOS (εr = 3.9,
PTLC = 10.1, M.wt. = 427), DOPh (εr = 4.8, PTLC = 10.2,
M.wt. = 434), TBP (εr = 8.0, PTLC = 4.0, M.wt. = 266),
TCP (εr = 6.9, M.wt. = 368) and DBBP (εr = 4.6, M.wt =
250) in sample electrodes to figure out the plasticizer
with the best response. The sensor with 2-NPOE as a
solvent mediator produced the best response, as shown in
Figure 2, likely due to better dielectric characteristics of
2-NPOE comparing to other solvents, and the ability of
2-NPOE to extract pethidine ions from the aqueous solu-
tion to the organic paste phase [28].

H. M. ABU-SHAWISH ET AL.
59
Table 1. Optimization of paste ingredients of the pethidine sensor.
Electrode response Composition (%)
R(s) LOD C.R. S P (2-NPOE) G I-E Sensor
PD-PT
20 - 25 8.3 × 10¯5 1.7 × 10−4 - 1.0 × 10−2 40.8 48.5 51.5 -- 1-
5 - 8 7.3 × 10¯7 2.1 × 10−6 - 1.0 × 10−2 54.2 51.0 48.5 0.5 2-
8 - 12 3.0 × 10¯6 5.3 × 10−6 - 1.0 × 10−2 50.9 50.8 48.2 1.0 3-
10 - 12 4.2 × 10¯6 5.7 × 10−6 - 1.0 × 10−2 48.2 50.2 47.8 2.0 4-
PD-PM
13 - 15 2.3 × 10−5 6.6 × 10−5 - 1.0 × 10−2 48.3 51.0 48.5 0.5 5-
20 - 23 3.9 × 10¯5 7.9 × 10−5 - 1.0 × 10−2 48.5 50.8 48.2 1.0 6-
17 - 20 5.3 × 10¯5 9.2 × 10−5 - 1.0 × 10−2 47.1 50.2 47.8 2.0 7-
PD-ST
10 - 13 3.3 × 10¯6 5.2 × 10−6 - 1.0 × 10−2 47.1 51.0 48.5 0.5 8-
12 - 15 5.1 × 10¯6 8.5 × 10−6 - 1.0 × 10−2 46.5 50.8 48.2 1.0 9-
15 - 18 5.5 × 10¯6 9.4 × 10−5 - 1.0 × 10−2 46.2 50.2 47.8 2.0 10-
PD-SM
12 - 15 2.3 × 10−5 3.6 × 10−5 - 1.0 × 10−2 51.5 51.0 48.5 0.5 11-
15 - 20 3.7 × 10¯5 5.9 × 10−5 - 1.0 × 10−2 49.6 50.8 48.2 1.0 12-
20 - 23 4.3 × 10¯5 7.8 × 10−5 - 1.0 × 10−2 49.0 50.2 47.8 2.0 13-
PD-TPB
15 - 20 1.3 × 10-5 2.6 × 10−5 - 1.0 × 10−2 49.2 51.0 48.5 0.5 14-
12 - 15 1.7 × 10¯5 3.2 × 10−5 - 1.0 × 10−2 47.1 50.8 48.2 1.0 15-
15 - 18 2.0 × 10¯5 3.8 × 10−5 - 1.0 × 10−2 47.7 50.2 47.8 2.0 16-
PD-PT with different plasticizers
plasticizers
5 - 8 7.3 × 10¯7 2.1 × 10−6 - 1.0 × 10−2 54.2 51.0 48.5 0.5 (2-NPOE)
12 - 15 5.5 × 10¯6 8.1 × 10−5 - 1.0 × 10−2 46.5 51.0 48.5 0.5 (DOP)
15 - 18 1.0 × 10¯5 3.2 × 10−5 - 1.0 × 10−2 50.6 51.0 48.5 0.5 (DBP)
13 - 17 3.2 × 10¯5 5.5 × 10−5 - 1.0 × 10−2 42.6 51.0 48.5 0.5 (DOS)
15 - 20 6.5 × 10¯6 8.3 × 10−6 - 1.0 × 10−2 51.2 51.0 48.5 0.5 (DOPh)
10 - 12 2.5 × 10¯6 5.3 × 10−6 - 1.0 × 10−2 52.6 51.0 48.5 0.5 (TCP)
20 - 23 1.7 × 10¯4 2.1 × 10−4 - 1.0 × 10−2 41.3 51.0 48.5 0.5 (TBP)
20 - 25 1.0 × 10¯4 1.8 × 10−4 - 1.0 × 10−2 40.9 51.0 48.5 0.5 (DBBP)
PD-PT with different g/p ratios
g/p ratios
8 - 10 8.7 × 10¯7 2.5 × 10−6 - 1.0 × 10−2 52.8 53.8 (2-NPOE)45.7 0.5 0.85
5 - 8 7.3 ×10¯7 2.1 × 10−6 - 1.0 × 10−2 54.2 51.0 (2-NPOE)48.5 0.5 0.95
10 - 12 8.1 × 10¯7 2.7 × 10−6 - 1.0 × 10−2 53.8 48.5(2-NPOE)51.0 0.5 1.05
10 - 12 1.3 × 10¯6 3.2 × 10−6 - 1.0 × 10−2 51.8 46.3 (2-NPOE)53.2 0.5 1.15
12 - 15 4.4 × 10¯6 7.3 × 10−6 - 1.0 × 10−2 48.9 43.3 (2-NPOE)56.2 0.5 1.30
I. E: ion-exchanger complex, G: graphite, P: plasticizers, S: slope (mV/decade), C.R.: concentration range (M), LOD: limit of detection, R (s): response time (s).

H. M. ABU-SHAWISH ET AL.
60
Figure 2. Effect of different plasticizers on the response of
PD-CMCPE.
Among the different compositions studied, a paste
containing ion-exchanger complex 0.5 wt% PD-PT , 48.5
wt% graphite, 51.0 wt% 2-NPOE exhibited the best re-
sponse characteristics and the lowest detection limit.
Therefore, this composition was used to study various
operation parameters of the electrodes. The electro-
chemical performance characteristics of this electrode
were systematically evaluated according to the IUPAC
recommendations [29].
3.2. Surface-Renewal and Reproducibility of the
Electrode
The main attraction of using the modified electrode is
that the electrode surface can be renewed after every use.
The bulk modified electrode can be renewed by squeez-
ing a little carbon paste out of the tube and a fresh sur-
face is smoothed on a piece of weighing paper whenever
needed [30]. Accordingly, a paste of optimum composi-
tion and suitable weight (1.5 g) can be used for several
months without any deterioration or change in the re-
sponse of the electrode. The slope of the calibration
graph was found to decrease slightly from 54.4 to 50.8
mV/decade after three times of use. This decrease may
be attributed to memory effect due to the surface con-
tamination [31]. In this case, the electrode surface should
be renewed. The repeatability of the potential reading of
the electrode was examined by subsequent measurements
of 1 × 10−4 M PDCl solution immediately after measur-
ing the first set of solutions at 1.0 × 10−3 M PDCl. The
standard deviation of measuring emf for 5 replicate
measurements was found to be 1.127 for 1.0 × 10−4 M
solution and 0.747 for 1.0 × 10−3 M solution.
3.3. Response Time and Reversibility of the
Electrode
For analytical applications, dynamic response time is a
significant parameter for any sensor. The response time
of the electrode is defined as the time between addition
of the analyte to the sample solution and the time when a
limiting potential has been reached [29].
In this work, the response time of the electrode was
measured by varying the pethidine concentration over the
range 1.0 × 10−5 to 1.0 × 10−2 M. The electrode reaches
equilibrium in about 5 - 8 s. As shown in Figure 3. To
evaluate the reversibility of the electrode, the potential
measurements were performed in the sequence high-to-
low (from 1.0 × 10−2 to 1.0 × 10−5 M) sample concentra-
tions. Figure 3 showed that the response of the electrode
was reversible; although the time needed to reach equi-
librium values (12 s) was longer than that for low-to-high
sample concentrations.
3.4. Effect of Diverse Ions
The separate solution method (SSM) is recommended by
IUPAC to determine the selectivity coefficient of the ISE
[32]. SSM is based on Nickolsky-Eisenman equation.
However, it has been shown that this method suffers
some limitations in terms of the values for ions of un-
equal charges, a non-Nernstain behavior of interfering
ions [33]. Therefore another method named the “matched
potential method (MPM)” was recommended [32] espe-
cially when the primary ion and/or the interfering ion
dissatisfy with the Nernst response or when the involved
ions have unequal charges [34]. The resulting values,
presented in Table 2, show that the electrode display
significantly high selectivity for pethidine over many
common organic and inorganic compounds, drugs, sug-
ars, amino acids as well as some anions. In pharmaceuti-
cal analysis, it is important to test the selectivity towards

H. M. ABU-SHAWISH ET AL.
61
Figure 3. Dynamic response of sensor A for step changes in
concentration of pethidine hydrochloride (from low to high
and vice versa).
Table 2. Selectivity coefficients of various interfering ions
for PD-CMCPE.
MPM SSM Interfering ions
8.41 × 10¯5 1.41 × 10¯4 NH+
1.18 × 10¯5 9.52 × 10¯5 Na+
1.20 × 10¯4 2.71 × 10¯4 K+
1.66 × 10¯4
4.34 × 10¯4 Li+
8.34 × 10¯6 5.69 × 10¯4 Cd2+
7.81 × 10¯6
1.94 × 10¯4 Mg2+
2.10 × 10¯5
1.13 × 10¯4 Ca2+
2.02 × 10¯5
8.02 × 10¯4 Ba2+
5.11 × 10¯5
2.73 × 10¯4 Zn2+
3.85 × 10¯5 1.19 × 10¯4 Co2+
7.73 × 10¯5
6.98 × 10¯4 Al3+
4.30 × 10¯5 1.86 × 10¯4 Ce3+
4.86 × 10¯2 2.53 × 10¯2 Diclofenac sodium
4.86 × 10¯2 3.41 × 10¯2 Spiramycine
2.47 × 10¯2 2.93 × 10¯2
Spectinomycine
Hcl
2.45 × 10¯2 6.76 × 10¯2 Captopril
8.89 × 10¯³ 1.27 × 10¯2 Ephidrine
4.86 × 10¯2 1.27 × 10¯1 Lidocaine
9.14 × 10¯7 - D-Fractose
1.35 × 10¯6 - D- Galactose
5.54 × 10¯6 - Maltose
8.04 × 10¯6 - Sucrose
2.47 × 10¯6 - Glucose
the excipients such as such as lactose, glucose, sucrose,
starch, stearic acid, magnesium stearate and microcrys-
talline cellulose and the fillers added to the pharmaceu-
tical preparations. The interference of some of these ex-
cipients was explored and measured. It is found that they
cause minor effect on the function of the electrode as
shown in Table 2.
3.5. Effect of Temperature
To study the thermal stability of the sensor, calibration
graphs were constructed at different test solution tem-
peratures 20, 30, 40, 50 and 60˚C. From these graphs the
electrode potentials at pPD = 0 were obtained and plotted
versus (t−25), where t is the temperature of the experi-
ment. A straight line plot was obtained for each electrode
with slope represents the thermal coefficient value (dE/dt)
of the electrode [35]. The value (dE/dt) of the electrode
was found to be 0.00071 V/˚C. This indicates fairly high
thermal stability of the electrode within the investigated
temperature range and shows no deviation from the
theoretical Nernstian behavior.
3.6. Effect of pH
The pH dependence of the potentials of the proposed
electrode was tested over the pH range 2.2 - 10.0 for 1.0
× 10¯5 and 1.0 × 10¯4 M PDCl solutions. The acidity was
adjusted by adding small volumes of (1.0 M HCl or
NaOH) to the test solutions and the variation in potential
was followed. As it can be seen in Figure 4, the potential
response remains almost constant over the pH range 2.7 -
6.8 which can be taken as the working pH range of the
electrode. However, there is a slight deviation at pH val-
ues lower than 2.7 which may be due to H+ interference.
On the other hand, the potential decreases gradually at
pH values higher than 6.8. This drop may be attributed to
formation of free pethidine base in the test solution.
4. Applications
4.1. Titration of Pethidine Solution with a
Standard PTA and Na-TPB Solution
The PD-CMCPE was successfully used as an indicator
electrode in the potentiometric titration of 5 ml of 0.01 M
(14.2 mg) of pethidine hydrochloride with a standard
solution of Na-TPB and PTA solution. The method for
pethidine ion (PD ) titration is based on the decrease of
(PD ) concentration by precipitation with PTA and
Na-TPB standard solution. As is obvious from Figure 5,

62 H. M. ABU-SHAWISH ET AL.
Figure 4. Effect of pH of the test solution on the potential
response of PD-CMCPE.
Figure 5. Potentiometric titration of 14.2 mg PDCl with
Na-TPB (a) and PTA (b) as titrants using PD-CMCPE.
the amount of pethidine can be accurately determined
from the end point of the titration curve.
4.2. Determination of Pethidine in Ampoules
The standard additions and calibration methods were
applied for determination PDCl content in ampoule sam-
ples as can be seen in Table 3, the recovery of PDCl is
almost quantitative.
4.3. Recovery and Determination of Pethidine
Ions in Urine
Recovery experiments were conducted by spiking urine
samples with appropriate amounts of pethidine ions, and
determined by this electrode using the standard addition
method and calibration curve. The results shown in Ta-
ble 3. Recoveries and R.S.D. values range between
96.6% and 103.0% of pethidine, and 0.27 to 1.33, re-
spectively. It is noted that accurate and reproducible re-
sults Thus the sensor can be employed for quantification
of pethidine in urine samples.
5. Comparison of the Pethidine Selective
Electrodes
The performance characteristics of the proposed elec-
trode and those of some reported carbon paste electrodes
are presented in Table 4 for comparison. It is clear that
the proposed electrode is comparable with most of the
reported electrodes with regard to working concentration
range, response time and low detection limit. Overall
evaluation indicates this electrode is more useful in such
applications.
6. Conclusions
The proposed chemically modified carbon paste elec-
trode based on pethidine phosphotungstate as an electro-
active ion exchanger complex might be a useful analyti-
cal tool and interesting alternative for the determination
of PD ions in pharmaceutical preparations and urine
samples. The electrode shows high sensitivity, reason-
able selectivity, fast static response, long term stability
and applicability over a wide concentration range with
minimal sample pretreatment. The electrode developed is
superior as compared with the pethidine selective elec-
trode described in the literature [10-14].
7. Acknowledgements
The authors would like to thank Dr Muneer A. Alboursh
and Dr Amal Zendah (general administration of phar-
macy, Ministry of Health, Gaza-Palestine), Dr. Aleksan-
dar Radu, Adaptive Sensors Group, National Centre for
Sensor Research, School of Chemical Sciences, Dublin
City University, Dublin 9, Ireland, Prof. Yousry M. Issa
and Prof. Hosny Ibrahim (Cairo University-Egypt)

H. M. ABU-SHAWISH ET AL.
63
Table 3. Results for determination of PDCl in ampoules and urine samples using PD-CMCPE.
Standard error RSDb % Recovery % PD/mga Method* Sample
Ampoule
0.025 0.85 96.8 0.0248 SDM
0.032 0.48 97.8 0.248
0.054 0.35 101.5 0.0355 CCM
0.033 0.27 100.8 0.355
Urine
0.021 0.74 96.6 0.0248 SDM
0.017 1.02 97.8 0.248
0.051 1.33 101.5 0.0355 CCM
0.041 1.08 100.8 0.355
PTM using Na-TPB and PTA as a titrant
0.026 0.72 100.5 1.42 Na-TPB
0.022 0.29 100.0 14.2
0.019 0.36 101.2 1.42 PTA
0.047 0.44 103.0 14.2
aMilligrams taken of PDCl in ampoules or spiked to urine samples, bAverage of three determinations, Methods* (used in analytical appli-
cations) SDM: standard additions method, CCM: calibration curve method, PTM: potentiometric titration method.
Table 4. Comparison of the proposed pethidine sensor wi th published sensors.
R(s) LOD C. R S Reference
10 3.3 × 10−6 1.0 × 10−5 - 1.0 × 10−2 55.3 [10]
< 30 9.9 × 10−7 1.0 × 10−5 - 1.0 × 10−2 53.7 [11]
< 30 2.2 × 10−6 1.0 × 10−5 - 1.0 × 10−2 51.8 [12]
< 30 8.2 × 10−7 5.0 × 10−6 - 1.0 × 10−2 56.7 [13]
30 4.4 × 10−6 1.0 × 10−5 - 1.0 × 10−3 53.5 [14]
5 - 8 7.3 × 10−7 2.1 × 10−6 - 1.0 × 10−2 54.2 Present work
C.R.: concentration range (M), LOD: limit of detection, S: slope (mV/decade), R(s): response time.
for encouragement, support and providing facilities for
research.
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