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Journal of Pharmaceutical Analysis

Journal of Pharmaceutical Analysis 2013;3(5):367–375

2095-1779 & 2013 Xihttp://dx.doi.org/10.10

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ORIGINAL ARTICLE

In situ modified screen printed and carbon paste ionselective electrodes for potentiometric determination ofnaphazoline hydrochloride in its formulation

Gehad G. Mohamedn, F.A. Nour El-Dien, Eman Y.Z. Frag,Marwa El-Badry Mohamed

Chemistry Department, Faculty of Science, Cairo University, Gamaa Str., 12613 Giza, Egypt

Received 16 January 2013; accepted 26 March 2013Available online 10 April 2013

KEYWORDS

ISPE and ICPE;Potentiometry;Naphazoline hydrochlor-ide;Eye drops

’an Jiaotong Univer16/j.jpha.2013.03.00

or. Tel.: +20 35676enidy@hotmail.com

esponsibility of Xi'a

Abstract The construction and performance characteristics of new sensitive and selective in situmodified screen printed (ISPE) and carbon paste (ICPE) electrodes for determination of naphazolinehydrochloride (NPZ-HCl) have been developed. The electrodes under investigation show potentiometricresponse for NPZ-HCl in the concentration range from 7.0� 10–7 to 1.0� 10–2 M at 25 1C and theelectrode response is independent of pH in the range of 3.1–7.9. These sensors have slope values of59.770.6 and 59.270.2 mV decade−1 with detection limit values of 5.6� 10–7 and 5.9� 10–7 M NPZ-HCl using ISPE and ICPE, respectively. These electrodes show fast response time of 4–7 s and 5–8 s andexhibits lifetimes of 28 and 30 days for ISPE and ICPE, respectively. Selectivity for NPZ-HCl withrespect to a number of interfering materials was also investigated. It was found that there is no interferencefrom the investigated inorganic cations, anions, sugars and other pharmaceutical excipients. The proposedsensors were applied for the determination of NPZ-HCl in pharmaceutical formulation using the directpotentiometric method. It showed a mean average recovery of 100.2% and 102.6% for ISPE and ICPE,respectively. The obtained results using the proposed sensors were in good agreement with those obtainedusing the official method. The proposed sensors show significantly high selectivity, response time,accuracy, precision, limit of detection (LOD) and limit of quantification (LOQ) compared with otherproposed methods.

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1. Introduction

Naphazoline hydrochloride (NPZ-HCl; Fig. 1) is used for thetemporary relief of redness of the eye associated with minorirritations such as those caused by colds, pollen related allergies,smog, dust, wind, wearing contact lenses, or swimming. It acts

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NH

NHCl

Fig. 1 The structure of naphazoline hydrochloride.

G.G. Mohamed et al.368

on alpha adrenergic receptors in the arterioles of the conjunctiva tocause vasoconstriction, resulting in decreased conjunctival con-gestion [1]. Some analytical procedures have been reported for thedetermination of NPZ in both pure and pharmaceutical preparationsincluding high-performance liquid chromatographic (HPLC) [2],micellar electrokinetic chromatographic [3,4], spectrophotometric[5–9] and potentiometric methods [10,11].

Potentiometry with ion selective electrodes (ISE) is one of themost important analytical tools capable of determining bothorganic and inorganic substances in medico-biological practice[12]. There is a constant progress in the number of electrodescapable of selectively identifying various drugs such as carbonpaste electrodes (CPEs) [13–18]. Although the CPEs play animportant role in the electrochemical analysis, the prepared pastesare soft and non-compactable and have to be packed into a specialpiston shaped holder with definite shape and size. However,shapes and designs of such electrodes are not suitable for everypurpose as in the case of measurements in flowing streams or fieldmonitoring with portable analyzers where the respective detectionunits need electrodes of special constructions [19].

Screen printing technology has increasingly been used for themass production of inexpensive, reproducible and sensitivedisposable electrochemical sensors for the determination of tracelevels of compounds in pharmaceutical, biomedical, and environ-mental samples [20].

In this work, in situ modified screen printed electrodes (SPEs)and CPEs were characterized and optimized with regard to themain experimental parameters affecting the electrode responses,including the nature and content of modifier, pH, response time,temperature and interferences, which were investigated anddiscussed in detail, and were then applied for the determinationof NPZ-HCl in pure form and pharmaceutical preparation (eyedrop).

2. Experimental

2.1. Materials and reagents

Analytical grade chemicals and reagents were used. They includedNPZ-HCl provided by Misr Company for Pharmaceutical Industry,Egypt. Relative high molecular weight PVC and graphite powder(synthetic 1–2 mm) were supplied by Aldrich. Tricresylphosphate(TCP) was purchased from Alfa-Aesar. Ion pairing agents such assodium tetraphenylborate (NaTPB) and ammonium reineckate(RN; [NH4(Cr(NH3)2(SCN)4)H2O]) were supplied by Fluka.Phosphotungstic acid (PTA; H3[PW12O40]) and phosphomolybdicacid (PMA; H3[PMo12O40]), were purchased from BDH. Silico-tungstic acid (STA, H4SiW12O40) was purchased from Sigma.

Nitric acid was supplied by Merck. Acetonitrile (AR) wassupplied by Aldrich. Acetone, cyclohexanone and tetrahydrofuranwere supplied by El-Nasr Company, Egypt.

Neozoline (Eye/Nasal drops) was produced by Amoun Phar-maceutical Company, El-Obour City, Cairo, Egypt. Every 100 mLcontains 50 mg NPZ-HCl.

2.2. Solutions

Stock drug solution (1.0� 10–2 M) was prepared by dissolving theappropriate amount of NPZ-HCl in bidistilled water. Othersolutions (1.0� 10−3–7.0� 10–7 M) were prepared by serial dilu-tion from stock solution. NaTPB solution (1.0� 10–2 M) wasprepared by dissolving an accurately weighed amount of thesubstance in warm water, adjusted to pH 9 by adding sodiumhydroxide and made up to the desired volume with water. Theresulting solution was standardized potentiometrically againststandard thallium acetate solution (1.0� 10–2 M) [21]. Aqueoussolutions of phosphotungstic acid, phosphomolybdic acid, silico-tungstic acid and ammonium reineckate were prepared using theanalytical grade chemicals and the exact concentrations of thesesolutions were determined by the appropriate recommendedmethods [22–24] and lower concentrated solutions were preparedby the appropriate dilutions.

All solutions must be protected from light by keeping them indark colored quickfit bottles during the whole work. The bidistilledwater was used throughout the experiments.

2.3. Equipment

Laboratory potential measurements were performed using aHANNA 211 pH meter. Silver–silver chloride double-junctionreference electrode (Metrohm 6.0222.100) in conjugation withdifferent drug ion selective electrodes was used. pH measurementswere done using a Jenway 3505 pH meter. A digital multimeterconnected to a portable PC and Brand digital buret was used forthe measurement of the drug under investigation.

2.4. Procedures

2.4.1. Preparation of the working electrodes2.4.1.1. Preparation of the SPEs. A manual screen printer wasused to produce disposable SPEs. The used substrate whichwas not affected by the curing temperature or the ink solventand was easily cut by scissors is a polyvinyl chloride flexible sheet(0.2 mm). The working electrodes were prepared using 6–30 mgion pairing agent, PVC (8%, w/w), carbon powder (500 mg) andTCP as plasticizer. They were printed using homemade carbon inkand cured at 50 1C for 30 min. The prepared electrodes were storedin the refrigerator at 4 1C and can be used for measurementsdirectly.

2.4.1.2. Preparation of the CPEs. The CPEs were prepared bythoroughly mixing various amounts (3–20 mg) of ion pairingagents with carbon powder (250 mg) and TCP (100 μL) in themortar until homogenization of this mixture was achieved. Theresulting paste was then packed firmly into the hole of theelectrode body. The surface of the resulting CPE was polishedusing a filter paper to obtain new working surface.

2.4.2. Calibration of the sensorsThe calibration of the ISPE and ICPE sensors under investigationwas established by immersing the ISPE and ICPE working

In situ modified electrodes for potentiometric determination of naphazoline hydrochloride 369

electrodes in conjunction with Ag–AgCl reference electrode in50 mL beakers containing known aliquots of 1.0� 10−7–1.0� 10–2 M NPZ-HCl standard solution. The potential reading was plottedagainst the negative logarithmic value of NPZ-HCl concentrations.The established calibration graph was used for subsequentdetermination of unknown naphazoline concentrations.

2.4.3. Effect of foreign compounds on the electrode selectivityThe potentiometric selectivity coefficients (KP1tA, B) were deter-mined according to IUPAC guidelines using the separate solutions(SSM) and matched potential (MPM) methods [25,26].

2.4.4. Analytical applicationsThe proposed ISPE and ICPE sensors were found to be useful inthe potentiometric determination of NPZ-HCl in pure solution andin pharmaceutical preparation by using the calibration, standardaddition and potentiometric titration methods. Before the applica-tion of this potentiometric method, the neozoline eye drop solutionwas evaporated gently on water bath till dryness. The residue wasthen dissolved in methylene chloride depending on the fact thatNPZ-HCl is insoluble in methylene chloride while chlorophenra-mine maleate is soluble in methylene chloride, so the residue waswashed twice with methylene chloride to get rid of chlorophenra-mine maleate and the white residue containing NPZ-HCl wasdissolved in a definite volume of distilled water and transferredquantitatively to a 50 mL beaker.

In the calibration method the electromotive force (EMF) resultingfrom immersing the prepared electrodes in conjunction with Ag/AgClas the reference electrode in the prepared solutions was determined, andthen the concentration of NPZ-HCl was calculated from the calibrationgraph of the corresponding electrode.

In the standard addition method, known increments of NPZ-HCl standard solution were added to constant volume of samplesof different concentrations. The voltage was first measured in thepure sample, then the standard was added and the solutions weremixed well; a second reading was taken. From the change inpotential readings for each increment the concentration of theunknown sample was calculated. In the potentiometric titrationaliquots of the drug solution containing 0.5–6.0 mg were trans-ferred to a 25 mL beaker. A standard solution of NaTPB was usedas a titrant and the titration is monitored using ISPE and ICPE asindicator electrodes conjugated with Ag/AgCl as the referenceelectrode. The potential values were plotted against the volume ofthe titrant added and the end points were determined from the S-shaped curves using the first derivative plots. The obtained resultswere compared with that of British Pharmacopoeia.

3. Results and discussion

3.1. Electrode composition

The electroanalytical performance and electrode potential of anISE is dependent upon the selective extraction of the target ionwhich creates the electrochemical phase boundary potential due tothermodynamic equilibria at the sample/electrode interface. Usingof a suitable ion pairing agent in the electrode matrix followed bysoaking in the drug solution may lead to the formation of an ionexchanger at the electrode surface by adsorption of a counter ion(analyte) from the solution during the measurement itself and canbe extracted by the plasticizer into the electrode bulk. Using of a

suitable ion pairing agent in the electrode matrix has the advantageof reducing the time required for the electrode preparation wherethere is no need for ion pair (IP) preparation as well as expansionof the application of ion selective electrodes (ISEs) for thedetermination of drugs that cannot be precipitated as suitableIPs. Also the same prepared electrodes can be used in preparationof ISEs for many drugs by soaking these electrodes in the solutionof the drug under investigation.

In order to determine the suitable type and content of ionpairing agents (sodium tetraphenylborate, phosphotungstic acid,phosphomolybdic acid, silicotungstic acid and ammonium rein-eckate), several ISPEs and ICPEs were prepared containing 6–30and 3–20 mg of ion pairing agents. CPE was soaked in 10–2 M ofNPZ-HCl solution. It was found that the most suitable ion pairingagent was TPB ion and its most suitable amount was 6 mg thatgave the highest slope values of 59.770.6 and 59.270.2 mVdecade−1 for ISPE and ICPE, respectively, as shown in Table 1and Fig. 2. On the other hand, the lower concentration gaveelectrodes with lower slope while the higher content causedoversaturation and unsatisfactory performance due to steric hin-drance effects at the interface.

3.2. Electroanalytical performance characteristics of the sensors

3.2.1. Calibration plotsThe results presented in Table 2 and Fig. 2 showed that theprepared sensors can be successfully applied for the potentiometricdetermination of the drug under investigation with linear responsein the concentration range of 7.0� 10−7 to 1.0� 10–2 M withslope values of 59.770.6 and 59.270.2 mV decade−1 for ISPEand ICPE, respectively.

3.2.2. Effect of pHTo investigate pH effect on the electrode potential, the potentialreadings were recorded for two different concentrations (10–4 or10–2 M) of NPZ-HCl at different pH values (pH 2–10). Thepotential changes as a function of pH are shown in Fig. 3.This figure indicates that the electrodes response was independentof the pH within the range from 3.1 to 7.9 for ISPE and ICPE withNaTPB ion pairing agent, respectively. At lower pH values thedecrease in mV readings may be attributed to the interference fromhydronium ion, while at higher pH values the decrease in the mVreadings may be due to base precipitates in the test solution andconsequently, the concentration of unprotonated species graduallyincreased. As a result, lower EMF readings were recorded.

3.2.3. Effect of temperatureIn order to determine the isothermal coefficient of the electrodes(dE1/dt), the standard electrode potentials (E1) were determinedfrom the calibration graphs (the intercepts at p[NPZ]¼0)of ISPE and ICPE carried out at different temperatures (10, 20,30, 40, 50 and 60 1C) and plotted versus (t−25), where (t) is thetemperature of the experiment in degree centigrade and accordingto the following equation [27]:

E1cell ¼ E1cell ð25 1CÞ þ ½ðdE1Þcell=dt�ðt−25Þ

A straight line is obtained and the slope of this line representsthe isothermal coefficients of the electrodes which were found tobe 0.0022 and 0.0008 V/1C for ISPE and ICPE modified withTPB, respectively. The obtained values of the isothermal

Table 1 Effect of the nature and content of ion pairing agents on the performance of ISPE and ICPE sensors.

Electrodes Composition (mg)

ICPE ISPE

Ion pair (mg) Graphite (mg) Plasticizer (TCP) (mg) Slope (mV decade−1) Ion pair (mg) Slope (mV decade−1)

NaTPB 3 250 114.3 41.970.5 6 59.770.66 250 114.3 59.270.2 12 51.970.99 250 114.3 56.170.9 16 46.570.912 250 114.3 55.171.0 22 44.171.616 250 114.3 44.970.7 30 40.371.020 250 114.3 38.770.9

PMA 3 250 114.3 42.770.2 6 35.570.56 250 114.3 44.370.5 12 39.770.79 250 114.3 39.170.8 16 16.970.312 250 114.3 37.470.3 22 16.470.116 250 114.3 36.070.2 30 15.870.520 250 114.3 28.871.0

STA 3 250 114.3 47.970.4 6 14.970.26 250 114.3 38.970.3 12 20.170.39 250 114.3 36.771.0 16 36.170.812 250 114.3 33.170.5 22 31.170.616 250 114.3 23.870.2 30 30.670.820 250 114.3 21.671.0

PTA 3 250 114.3 36.470.2 6 28.770.96 250 114.3 40.670.4 12 38.470.49 250 114.3 39.070.5 16 47.570.512 250 114.3 26.470.6 22 35.570.916 250 114.3 25.271.0 30 34.670.520 250 114.3 25.070.6

RN 3 250 114.3 43.270.4 6 24.570.56 250 114.3 47.670.6 12 29.070.69 250 114.3 46.870.6 16 38.070.712 250 114.3 44.070.5 22 37.670.716 250 114.3 42.270.9 30 36.171.320 250 114.3 41.671.3

G.G. Mohamed et al.370

coefficient reveal that the electrodes under investigation had highthermal stability within the used range of temperature. Theinvestigated electrode can be used up to 60 1C without noticeabledeviation from the Nernstian behavior.

3.2.4. Interference studiesPotentiometric selectivity coefficient refers to the ability of the ISEto differentiate a particular (primary) ion from others (interferingions) [27–30]. The potentiometric selectivity coefficient (KPotA,B)was determined according to IUPAC recommendations using theSSM and MPM [25,26]. In the MPM the selectivity coefficient isdefined as the activity ratio of the primary ion and interfering ionwhich gives the same potential change in a reference solution. Thismethod does not require Nernstian responses to the activity(concentration) of either primary or interfering ions. It is a suitablemethod for determination of selectivity coefficients of the neutralcompounds.

As shown in Table 3 the results obtained show no significantinterference and this reflects a very high selectivity of the

electrodes under investigation toward NPZ-HCl. The results alsoindicate that there was no serious interference from glucose,sucrose, maltose, fructose, starch and lactose. The inorganiccations did not interfere due to the differences in their ionic sizeand hence their mobilities, polarities, and permeabilities ascompared to those of NPZ+ cation.

3.2.5. Response timeThe response time is an important factor which characterizes ISEs.It can be defined as the time taken by the electrodes to reach steadypotential values of 90% of the final equilibrium values, aftersuccessive immersions in a series of solutions, each having a 10-fold concentration difference [31–35]. The dynamic response timeof the sensors under study was determined for the concentrationrange from 1.0� 10–6 to 1.0� 10–3 M. Fig. 4 illustrates arepresentative plot of the potential changes versus time forelectrodes under study. The proposed sensors have very shortresponse time of 4–7 s and 5–8 s for ISPE and ICPE, respectively.

In situ modified electrodes for potentiometric determination of naphazoline hydrochloride 371

3.2.6. Effect of soaking timeAs shown in Fig. 5, it was found that the optimum soaking timefor the ICPE electrode was 15 min and longer soaking time affectsnegatively on the response of the electrode. This may be due to theleaching of the active ingredients (ion-pairing agent and plastici-zer) to the bathing solution [36–38].

3.2.7. LifetimeThe lifetime of the investigated sensors was studied by periodi-cally constructing the calibration graphs under optimum conditions

Fig. 2 Effect of NaTPB on the performance of (A) ISPE and (B) ICPEsensors.

Table 2 Response characteristics of ISPE and ICPE sensors.

Parameter ISPE

Slope (mV decade−1) 59.770.6Correlation coefficient, r 0.996Detection limit (M) 5.6� 10–7Quantification limit (M) 1.9� 10–6Working pH range 3.1–7.9Life time (days) 28Concentration range (M) 7.0� 10−7SD 0.023RSD (%) 0.5Response time (s) 4–7Isothermal coefficient (V/1C) 0.0022

on different days. For sensors under investigation it was found thatISPE has a life time of 28 days while ICPE has 30 days. A shinynew surface is obtained every time for measurement using ICPEby squeezing out a small portion of the paste and polishing it on afilter paper. After preparation of ISPEs, they were kept at 4 1C anddirectly used for potentiometric measurements.

3.2.8. Analytical applicationThe optimized sensors under investigation have been successfullyused for the potentiometric determination of NPZ-HCl by using

ICPE

59.2070.20.9745.9� 10–71.9� 10–63.1–7.930

–1.0� 10–2 7.0� 10−7–1.0� 10–20.0080.25–80.0008

Fig. 3 Effect of pH on (A) ISPE and (B) ICPE modified with NaTPBion pairing agent.

Table 3 Selectivity coefficients values for ISPE and ICPEsensors.

Interferingion

ICPE ISPEKpotD,B K

potD,B

MPM SSM MPM SSM

Glucose 3.00� 10–2 – 1.50� 10–2 –Lactose 1.65� 10–2 – 1.25� 10–2 –Maltose 1.60� 10–2 – 0.80� 10–2 –Starch 2.00� 10–2 – 3.00� 10–2 –Fructose 5.00� 10–3 – 9.71� 10–3 –Glycine 1.00� 10–2 – 8.75� 10–3 –NH4

+– 4.70� 10–4 – 1.48� 10–3

K+ – 1.29� 10–3 – 9.66� 10–4Na+ – 2.00� 10–4 – 5.01� 10–4Cu2+ – 6.83� 10–1 – 5.64� 10–2Ni2+ – 2.81� 10–2 – 3.69� 10–2Zn2+ – 9.67� 10–1 – 3.42� 10–2Ca2+ – 7.98� 10–6 – 1.36� 10–5Cd2+ – 7.10� 10–6 – 2.72� 10–5Cr3+ – 2.43� 10–1 – 8.57� 10–3

G.G. Mohamed et al.372

the standard addition, calibration and potentiometric titrationmethods, and the results are summarized in Table 4. In order toestimate the quality of the results, recovery values were alsodetermined and are presented in the same table. The obtainedresults in Table 4 using the proposed sensors were in goodagreement with those obtained using the official method [39].

Fig. 4 The dynamic response time of (A) ISPE and (B) ICPEpotentiometric sensors modified with NaTPB ion pairing agent.

3.3. Method validation

Electroanalytical method validation is the process used to confirmthat the determination procedure employed for a specific test issuitable for its intended use like other analytical methods [40].Accuracy, precision, linearity, specificity, limits of detection (LOD)and quantification (LOQ) were achieved using a standard NPZ-HClstock solution.

Fig. 5 Effect of soaking time on the ICPE.

3.3.1. AccuracyAccuracy is an important requirement of electroanalytical meth-ods. It can be defined as the closeness between the true or acceptedreference value and the obtained value [40]. The accuracy of theproposed method using in situ SPE and CPE was investigated bythe determination of NPZ-HCl in spiked samples prepared fromserial concentrations of NPZ-HCl reference standards. The resultssummarized in Table 5 show high accuracy of the proposedmethod, as indicated by the percentage recovery values. Thestatistical analysis of the results using student's t-test and F-testshowed no significant differences between them regarding accu-racy and precision (Table 4).

3.3.2. PrecisionPrecision is a measure of how close results are to one another.Precision is also expressed as the closeness of agreement betweenindependent test results obtained under stipulated conditions.Precision is usually expressed as standard or relative standarddeviations of the replicate analysis [40]. Hence the precision of theproposed potentiometric method using the sensors under

investigation was measured as percentage relative standard devia-tion (RSD %). Intra-day and inter-day precisions were assessedusing three concentrations and five replicates of each concentra-tion. The relative standard deviations were found to be very smallindicating reasonable repeatability and reproducibility of theproposed method as shown in Table 5.

3.3.3. LinearityLinearity of an electroanalytical technique can be defined as ameasure of how well the calibration plot of electroanalyticalresponse versus concentration approximates a straight line [40].

Table 4 Determination of NPZ-HCl in pharmaceutical formulation (Neozoline) using ISPE and ICPE.

Method Taken(mg)

ICPE ISPE British Pharmacopoeia

Found(mg)

Recovery(%)7SD

RSD(%)

Found(mg)

Recovery(%)7SD

RSD(%)

Found(mg)

Recovery(%)7SD

RSD(%)

Standard additionmethod

0.50 0.483 96.6070.23 0.238 0.489 97.8070.38 0.391 0.494 98.7570.35 0.3581.23 1.199 97.5070.71 0.725 1.200 97.5670.28 0.289 1.22 99.170.28 0.2863.00 2.921 97.3670.50 0.508 2.928 97.6070.57 0.580 2.96 98.6770.50 0.5026.00 5.849 97.4870.74 0.762 5.826 97.1070.28 0.291 5.91 98.5570.64 0.646

Potentiometric titrationmethod

0.50 0.488 97.6370.89 0.913 0.488 97.5570.33 0.3381.23 1.197 97.3270.26 0.270 1.184 96.2670.35 0.3673.00 2.921 97.3670.92 0.944 2.913 97.1070.28 0.2916.00 5.974 99.7570.35 0.644 5.805 96.7570.35 0.365

Calibration method 0.50 0.513 102.6070.53 0.528 0.501 100.2070.28 0.2751.23 1.235 100.4070.53 0.528 1.247 101.4070.18 0.1773.00 2.922 97.4070.57 0.581 2.996 99.8770.92 0.9216.00 5.805 96.7570.35 0.365 5.952 99.2070.42 0.428

Standard addition method: F-test: 0.422–6.241; t-test: 0.066–0.184 Standard addition method: F-test: 0.100–1.307; t-test: 0.029–0.664Potentiometric titration: F-test: 0.857–6.335; t-test: 0.015–0.417 Potentiometric titration: F-test: 0.310–1.565; t-test: 0.041–0.663Calibration method: F-test: 0.310–3.507; t-test: 0.063–0.663 Calibration method: F-test: 0.391–3.450; t-test: 0.057–0.341

Tabulated F value at 95% confidence level is 6.39 (n¼5).Tabulated t value at 95% confidence level is 2.776 (n¼5).

Table 5 Evaluation of accuracy and precision (intra- and inter-day) of the ISPE and ICPE sensors.

Drug Electrode type Taken (mg) Intra-day Inter-day

Found (mg) Recovery (%) SD RSD (%) Found (mg) Recovery (%) SD RSD (%)

NPZ-HCl ICPE 12.337 12.394 100.46 0.004 0.149 11.89 96.38 0.035 1.1631.2337 1.212 98.24 0.008 0.209 1.28 103.8 0.036 0.9150.1234 0.123 99.68 0.008 0.167 0.125 101.3 0.009 0.179

ISPE 12.337 12.74 103.3 0.009 0.3 12.17 98.65 0.026 0.8671.2337 1.245 100.9 0.057 1.422 1.245 100.92 0.005 0.1370.1234 0.126 102.1 0.023 0.47 0.125 101.3 0.009 0.179

Neozoline ICPE 5.5 5.5 100.0 0.008 0.167 5.482 99.67 0.021 0.4162.25 2.247 99.87 0.005 0.183 2.256 100.3 0.036 1.1850.5 0.482 96.40 0.011 0.566 0.476 95.20 0.033 1.618

ISPE 5.5 5.498 99.96 0.008 0.167 5.493 99.87 0.011 0.2282.25 2.239 99.51 0.009 0.3 2.253 100.1 0.027 0.8990.5 0.479 95.80 0.036 1.775 0.476 95.30 0.035 1.731

In situ modified electrodes for potentiometric determination of naphazoline hydrochloride 373

The standard calibration graph was established using five con-centrations of standard NPZ-HCl. It was found that a linearrelationship is present between the electrode potential (mV) andthe log[NPZ-HCl]. The regression data, correlation coefficients (r)and other statistical parameter are presented in Table 2.

3.3.4. SpecificityThe specificity of the method was examined by observing anyinterference caused by the common excipients of the pharmaceu-tical formulation. It was found that these compounds did notinterfere with the results of the proposed method as shown inTable 3.

3.3.5. LOD and LOQLOD is the lowest quantity of the investigated compound in asample that can be detected, but not necessarily quantified with anacceptable uncertainly. LOD of an electroanalytical method is animportant factor if quantitative measurements are to be made atconcentrations close to it. Especially, LOD is necessary for thetrace analysis of drug active components in pharmaceuticals and/orbiological samples [40]. The values of LOD that are presented inTable 2 indicate that the sensors under investigation are highlysensitive, selective and can be applied in determination of smallamounts of NPZ-HCl.

LOQ is the lowest amount of compound that can be measuredin the sample matrix at an acceptable level of accuracy and

G.G. Mohamed et al.374

precision. For many pharmaceutical and biological applications,the LOQ is generally a more useful factor than LOD. The LOQwas determined by establishing the least concentration that can bemeasured according to ICH Q2(R1) recommendations [41], belowwhich the calibration range is non-linear, and was found to be1.9� 10–6 and 1.9� 10–6 M for ISPE and ICPE, respectively.

4. Conclusion

The present work involves the preparation of screen printed andcarbon paste electrodes with in situ mode of modification for thedetermination of NPZ-HCl drug. The screen printed electrodeswere characterized and optimized with respect to the mainexperimental parameters affecting the electrode performance,including composition, pH, temperature, response time and inter-ferences in comparison to carbon paste electrodes. The electrodesshowed good selectivity for NPZ-HCl with respect to someinorganic cations, sugars and glycine. The developed electrodeshave been successfully used for the determination of NPZ-HCl inpharmaceutical preparation using the standard addition, potentio-metric titration and calibration methods. The obtained results werein good agreement with those obtained using the official method.According to the results obtained, the potentiometric sensors canbe successfully applied for the routine analysis of this drug.

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In situ modified screen printed and carbon paste ion selective electrodes for potentiometric determination of naphazoline...IntroductionExperimentalMaterials and reagentsSolutionsEquipmentProceduresPreparation of the working electrodesPreparation of the SPEsPreparation of the CPEs

Calibration of the sensorsEffect of foreign compounds on the electrode selectivityAnalytical applications

Results and discussionElectrode compositionElectroanalytical performance characteristics of the sensorsCalibration plotsEffect of pHEffect of temperatureInterference studiesResponse timeEffect of soaking timeLifetimeAnalytical application

Method validationAccuracyPrecisionLinearitySpecificityLOD and LOQ

ConclusionReferences