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Sensors and Actuators B 121 (2007) 349–355

A PVC-based membrane electrode for nickel (II) ionsincorporating a tetraazamacrocycle as an ionophore

Ashok Kumar Singh ∗, Puja SaxenaDepartment of Chemistry, Indian Institute of Technology-Roorkee, Roorkee, Uttaranchal 247 667, India

Received 22 December 2005; received in revised form 29 March 2006; accepted 30 March 2006Available online 11 May 2006

bstract

Dibenzo[e,k]-2,3,8,9-tetraphenyl-1,4,7,10-tetraazacyclododeca-1,3,5,7,9,11-hexaene has been explored as an electroactive material for the fab-ication of a poly(vinyl chloride)-based membrane electrode for selective determination of Ni2+ ions. The optimized membranes incorporatingPh4Bzo2(12)hexaeneN4] as an ionophore, sodium tetraphenylborate (NaTPB) as an anion excluder and dioctylphthalate (DOP), dibutylphospho-ate (DBBP), tris(2-ethylhexyl)phosphate (TEHP) and 1-chloronaphthalene (CN) as solvent mediators were prepared and investigated for theiresponse towards Ni2+ ions. The best performance was observed with the membrane having 1.7% ligand, 34.8% PVC, 62.7% DOP and 0.8%aTPB, and it worked well in a wide concentration range of 3.98 × 10−6 to 1.00 × 10−1 M with a Nernstian slope of 29.5 mV per decade of activityetween pH 2.5 and 7.7. The electrode exhibited a detection limit of 2.98 × 10−6 M and a fast response time of 8 s, and was used over a period of

months with good reproducibility (S = 0.2 mV). The selectivity coefficient over mono-, di- and tri-valent cations indicated excellent selectivity

or Ni2+ ions over a large number of cations. Anions such as Cl− and SO42− did not interfere and the electrode worked satisfactorily in partially

on-aqueous media. It could be used successfully for determination of Ni2+ in real samples.2006 Elsevier B.V. All rights reserved.

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eywords: Tetraazamacrocycle; Ni(II)-selective electrode; PVC-based membra

. Introduction

During past many decades, a large number of neutral iono-hores have been used in the construction of ion-selective elec-rodes (ISEs) with high selectivity for specific metal ions. ISEsre known to be very useful tools for chemical, clinical and envi-onmental analysis as well as for process monitoring. Fabricationf new ion-specific ISEs with high selectivity and sensitivity, aide linear concentration range, long lifetime and good repro-ucibility, is always in need.

Over the years nickel has been regarded as a potentially tox-c element. The main source of nickel in aquatic systems isissociation of rocks and soil, biological cycles and especiallyndustrial processes and water disposal [1]. The maximum rec-

mmended concentration of Ni ions in drinking water for live-tock is 2.5 mg/ml [2]. Nickel is also present in food of bothnimal and vegetable origins such as red meat, cotton seed,

∗ Corresponding author.E-mail address: akscyfcy@iitr.ernet.in (A.K. Singh).

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925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2006.03.043

orn meal, unsaturated oils, chocolates, milk and milk products.ickel toxicity can cause acute pneumonitis, dermatitis, asthma,isorders of central nerve systems and cancer of the nasal cav-ty [3]. Oral nickel intake due to the use of nickel-containingitchen utensils or intake of food items with high nickel con-ent may exacerbate dermatitis. Availability of highly selectivelectroactive materials has opened up the possibility of devel-ping ISEs for determination of nickel in a variety of industrial,nvironmental and food samples.

A survey of literature reveals that various ISEs for Ni2+

ons have been reported previously [4–13] which showed poorelectivity, a small concentration range and non-Nernstianesponse. In recent years, Ni(II)-selective electrodes haveeen reported based on several neutral carriers viz. porphy-ins [14], 3,4:11,12-dibenzo-2,5,10,13-tetraoxa-1,6,9,14-tetra-zacyclohexadecane [15], 18-crown-6 derivative [16], tetraaza-nnulene (Me4Bzo2TAA) [17], benzoxazole [18] and thiopyran

19] derivative, dithizone [20], octaaza macrocycle [21], diben-ocyclamnickel(II) [22], benzyl bis(thiosemicarbazone) [23],,N′-bis-(4-dimethylamino-benzylidene)-benzene-1,2-diamine

24], mercapto compound [25], porphines [26], Schiff base

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50 A.K. Singh, P. Saxena / Sensors

helates [27] and t-octylcalix[6]arene derivative [28]. In theresent work, we report a Ni(II)-selective electrode based ontetraazamacrocycle [Ph4Bzo2(12)hexaeneN4] which exhibitswide concentration range and a Nernstian slope. It has also

een used successfully in the analysis of real samples.

. Experimental

.1. Reagents

All the reagents and chemicals used were of analytical grade.ickel (II) nitrate procured from Ranbaxy was used as theickel salt for membrane sensors. Double distilled water wassed for preparation of solutions of metal salts of different con-entrations by diluting stock standard solutions (0.1 M). Higholecular weight PVC obtained from Aldrich (USA), sodium

etraphenylborate (NaTPB) from BDH (UK), dibutylphospho-ate (DBBP) from Mobile (USA), dioctylphthalate (DOP)rom Reidel (India) and 1-chloronaphthalene (CN) and tris(2-thylhexyl) phosphate (TEHP) from BDH were used withouturification.

.2. Synthesis of [Ph4Bzo2(12)hexaeneN4] macrocycle

The macrocycle dibenzo[e,k]-2,3,8,9-tetraphenyl-1,4,7,10-etraazacyclododeca-1,3,5,7,9,11-hexaene was prepared aseported [29]. An ethanol solution of benzil (0.05 mol) wasefluxed with an ethanolic solution of o-phenylenediamine0.05 mol) for 4 h in the presence of few drops of conc. HCl.n cooling the contents at room temperature overnight, a lightrown crystalline compound separated out, which was filtered,ashed with ethanol and dried over P4O10. mp 126 ◦C; 1H NMR

CDCl3): δ 7.2–7.4 (m, 20H, Ph), δ 6.9 (d, 4H) and δ 7.1 (m, 4H)f o-phenylene moiety; IR (KBr pellet) in cm−1: 1610 (νC N),520, 1471, 1420 (νC C, ph), 760 (δC–H, ph). The calculated

nd observed elemental analysis data [found (%): C, 82.21; H,.92; N, 9.89] of the compound are consistent with the theoreti-al data [C40H28N4-calculated (%): C, 82.10; H, 4.96; N, 9.92]stimated on the basis of the structure given in Fig. 1.

ig. 1. Structure of the ionophore dibenzo-[e,k]-2,3,8,9-tetraphenyl-1,4,7,10-etraazacyclododeca-1,3,5,7,9,11-hexaene.

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ctuators B 121 (2007) 349–355

.3. Electrode preparation

A number of membranes incorporating the ionophorePh4Bzo2(12)hexaeneN4], anion excluders, and plasticizers inifferent ratios in the PVC matrix were fabricated [30]. Varyingmounts of ionophores (∼2–10 mg) and an appropriate amountf PVC (∼50–150 mg) were dissolved in approximately 20 mlHF. An anion excluder NaTPB (∼1–6 mg) and solvent medi-tors (∼100–200 mg) were also added to get membranes ofifferent compositions. The optimum composition of the mem-ranes was obtained after a good deal of experimentation. Afteromplete dissolution of all the components and thorough mix-ng the homogenous mixture was poured into polyacrylate ringslaced on a smooth glass plate. THF was allowed to evaporatet room temperature. After 48 h, transparent membranes withn average thickness of 0.5 mm were obtained. A disc of 6 mmiameter piece was cut out from the PVC membrane and gluedo an end of a ‘Pyrex’ glass tube with araldite. The membranesere further used for potential measurement studies.The membranes were also subjected to microscopic and elec-

rochemical examination for cracks and homogeneity of theurface and only those that had a smooth surface and gener-ted reproducible potentials were chosen for subsequent inves-igation. Membrane to membrane (and batch to batch) repro-ucibility was ensured by carefully controlling the conditionsf fabrication.

.4. Potential measurements

Proper equilibration of a membrane is essential to get sta-le and reproducible potentials to avoid long response time.t is necessary to optimize the concentration of the contact-ng solution and the time required for complete equilibration.hus all the membranes were immersed in Ni(NO3)2 solu-

ions of different concentrations for different time periods.he potential measurements were carried out at 25 ± 0.1 ◦Cy using the saturated calomel electrode (SCE) as a refer-nce electrode. The membranes were equilibrated for 6 daysn a 1.00 M Ni2+ solution. The following cell assembly waset up to measure the potentials: internal reference electrodeSCE)/internal solution/membrane/test solution/external refer-nce electrode (SCE).The potentials were measured by varyinghe concentration of the test solution in the range 1.0 × 10−7

o 1.0 × 10−1 M. An internal reference solution consisting of.1 M Ni(NO3)2 was used.

.5. Preparation of chocolate samples

A 10 g sample was ashed in a silica crucible for 4 h on a hotlate and the charred material was transferred to a Muffle furnaceor overnight heating at 350 ◦C. The residue was cooled, treatedith 2 ml concentrated nitric acid and again kept in a furnace forh at the same temperature so that no carbon traces are left. The

nal residue was treated with 0.5 ml concentrated hydrochloriccid. The solid residue was dissolved in water, filtered and madep to 25 ml keeping the overall pH ∼4.0. The solution was suit-bly diluted and its concentration was determined with an atomic

and Actuators B 121 (2007) 349–355 351

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A.K. Singh, P. Saxena / Sensors

bsorption spectrophotometer (AAS) (Perkin Elmer). Differentamples of chocolates were processed to obtain a representativealue.

. Results and discussion

The coordination chemistry of Ni2+ ions reveal that the ionsave a remarkable preference for formation of square planarmine complexes. This has been extensively exploited in syn-hesizing a variety of thermodynamically stable N4 macrocycleomplexes with varying ring size [31,32]. The 12-memberedetraphenyl substituted macrocycle was chosen as an ionophoreor Ni(II)-ISE as it has a flat and highly electron delocalizedtructure, and owing to its planar geometry and appropriateavity size it is expected to greatly influence the chemical andhysical properties of Ni2+ ions. The tetra phenyl groups add theipophilicity and thermodynamic stability for the rapid exchangef metal ions [33]. Therefore it was used as an ion-active phasen Ni(II)-ISE.

In preliminary investigations, [Ph4Bzo2(12)hexaeneN4] wassed as an ionophore in the construction of ISEs for various metalons. The potential response obtained is given in Fig. 2. As can beeen from the figure, the ligand gives the best potential responseo Ni2+ ions in comparison to other alkali, alkaline earth, transi-ion and heavy metal ions. Further, the response characteristics ofhe Ni2+-selective electrode based on [Ph4Bzo2(12)hexaeneN4]ere tested as a function of the membrane composition, nature of

he plasticizer and the amount of the ionophore used. The ratiof the ionophore, PVC, the plasticizer and the anion excluderNaTPB) was varied so as to obtain a composition that gavehe best performance with respect to the working concentrationange, slope and response time. It was found that the optimum

omposition of 1.7% ionophore, 34.8% PVC, 62.7% DOP and.8% NaTPB gave the best results.

To study the effect of various plasticizers, four plasticizersOP, DBBP, TEHP, 1-CN were used in the construction of the

Fig. 2. Potential response of the electrode to various metal ions.

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ig. 3. Variations in cell potential of membrane no. 1 without any plasticizernd nos. 2–5 with plasticizers DOP, DBBP, TEHP, and CN, respectively, withi2+ concentration.

i2+-selective electrode and the results are given in Table 1nd Fig. 3. It is seen that electrode no. 1 having a membraneithout any plasticizer exhibited a working concentration rangef 3.16 × 10−5 to 1.0 × 10−1 M with a slope of 31.0 mV perecade of concentration. However, on addition of a plasticizero the membrane (electrode nos. 2–5), the working concentrationange as well as the slope changed (Table 1). The membranesncorporating the plasticizers DBBP (electrode no. 3), TEHPno. 4) and 1-CN (no. 5) exhibited linearity in the concentrationanges 6.31 × 10−6 to 1.0 × 10−1, 2.51 × 10−5 to 1.0 × 10−1

nd 1.12 × 10−5 to 1.0 × 10−1 M, respectively. It can thus benferred that membrane no. 2, incorporating an ionophore, PVC,OP as a solvent mediator and NaTPB as an anion discrimina-

or in the ratio (w/w) of 10:24:12:5, gives the best response withegard to the working concentration range and slope as comparedo the rest of the membranes. It exhibited a wide concentrationange of 3.98 × 10−6 to 1.0 × 10−1 M and a Nernstian slope of9.5 mV per decade of concentration. Hence, membrane no. 2as chosen for further studies.An electrode having a membrane without any solvent medi-

tor gave a steady response time of 30 s, whereas after addinghe solvent mediators (DOP, DBBP, TEHP and 1-CN) the elec-rode achieved an equilibrium response within 8–25 s over thehole concentration range (Table 1). The potentials so obtained

emained constant for more than 5 min, after which a slow diver-ence was observed. Potentials were measured periodically andhe standard deviation of 20 identical measurements was 0.2 mV.

The experimental results show that the lifetime of the presentlectrode was about 4 months. During this time, the membranesould be used without any significant change in response time,lope and working concentration range. Thus the response char-

cteristics of the electrode remained almost constant. After thiseriod, the slight change in the slope and response time can bevercome by reequilibrating the membrane in a 1.0 M Ni2+ solu-ion for 2–3 days. This could prolong the lifetime of the electrode

352 A.K. Singh, P. Saxena / Sensors and Actuators B 121 (2007) 349–355

Table 1Composition of the PVC-based membrane of [Ph4Bzo2(12)hexaeneN4] and the performance characteristics of Ni(II)-selective electrodes

S. no. Composition of membrane (w/w) Working concentrationrange (mol l−1)

Slope (mV/decade ofconcentration)

Detection limit(mol l−1)

Responsetime (s)

Ligand PVC DOP DBBP TEHP 1-CN NaTPB

1 5 100 – – – – 2 3.16 × 10−5 to1.0 × 10−1

31.0 2.51 × 10−5 30

2 5 100 180 – – – 2 3.98 × 10−6 to1.0 × 10−1

29.5 2.98 × 10−6 8

3 5 100 – 180 – – 2 6.31 × 10−6 to1.0 × 10−1

28.5 5.01 × 10−6 15

4 5 100 – – 180 – 2 2.51 × 10−5 to1.0 × 10−1

33.5 1.99 × 10−5 25

5 1.12 −5

1.0 ×−6

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y 2 months. The electrode was kept in the 0.1 M solution whenot in use so as to avoid drying, cracking and poisoning.

The pH dependence of the electrode potential of membraneo. 2 was investigated at 1.0 × 10−2 and 1.0 × 10−3 M Ni2+

olutions by varying the pH from 1.0 to 9.0. The pH was adjustedith nitric acid or ammonia. The potentials remained constant

n the range of pH 2.5–7.7 (Fig. 4) and the same may be takens the working pH range of the sensor.

The functioning of the sensor was also tested in partiallyon-aqueous media using 10%, 20%, 30% and 35% methanol–,thanol– and acetone–water mixtures. As can be seen fromable 2, membrane no. 2 works satisfactorily up to a maximum0% (v/v) non-aqueous content. It appears that 30% methanol,thanol and acetone in water do not affect the properties of theembrane significantly to bring about the changes in poten-

ial, slope and working concentration range. In these mixtures,

he working concentration range and slope remain unaltered.bove 30% non-aqueous content, the media properties affected

he performance of the electrode, resulting in a decrease in slopend working concentration range. Also, the membranes were

ig. 4. Effect of pH on the cell potential for membrane no. 2 at 1.0 × 10−2 and.0 × 10−3 M Ni2+ concentration.

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estroyed due to leaching of the ionophore from the PVC matrix.ence, the electrode assembly can only be used in a non-aqueousedium when its content is not more than 30%.Selectivity coefficient values were determined by the

atched potential method [34]. The coefficients KpotA,B were cal-

ulated by

potA,B = a′

A − aA

aB

In this method, at first a known activity (a′A) of the primary

olution is added to a reference solution that contains primaryons of a fixed activity (aA), and the potential change is recorded.econdly, a solution of interfering ions (aB) is added to the pri-ary ion solution until the same potential change is observed.he change in potential produced at the constant backgroundf primary ions must be the same in both cases. The selectivity

oefficients of interfering ions were determined by membranelectrode no. 2, which showed wider concentration range thanther membranes (nos. 1 and 3–5).

able 2erformance of membrane no. 2 in partially non-aqueous media

on-aqueousontent (%, v/v)

Working concentration range (M) Slope (mV/decadeactivity)

0 3.98 × 10−6 to 1.0 × 10−1 29.5

ethanol10 3.98 × 10−6 to 1.0 × 10−1 29.520 3.98 × 10−6 to 1.0 × 10−1 29.530 4.18 × 10−6 to 1.0 × 10−1 28.635 8.23 × 10−6 to 1.0 × 10−1 19.2

thanol10 3.98 × 10−6 to 1.0 × 10−1 29.520 3.98 × 10−6 to 1.0 × 10−1 29.530 4.18 × 10−6 to 1.0 × 10−1 28.035 8.01 × 10−6 to 1.0 × 10−1 18.8

cetone10 3.98 × 10−6 to 1.0 × 10−1 29.520 3.98 × 10−6 to 1.0 × 10−1 29.530 4.96 × 10−6 to 1.0 × 10−1 27.835 1.56 × 10−5 to 1.0 × 10−1 17.6

A.K. Singh, P. Saxena / Sensors and Actuators B 121 (2007) 349–355 353

Table 3Selectivity coefficient values (KPot

Ni2+,B) of various interfering ions at

1.0 × 10−2 M with membrane no. 2

Interferent (B) Selectivity coefficient (KPotNi2+,B

),

matched potential method

Mg2+ 1.1 × 10−3

Sr2+ 5.5 × 10−3

Ba2+ 1.5 × 10−3

Hg2+ 9.1 × 10−3

Mn2+ 8.2 × 10−3

Na+ 1.6 × 10−2

Co2+ 3.8 × 10−1

Ca2+ 4.7 × 10−3

Cu2+ 2.1 × 10−2

Cd2+ 4.5 × 10−2

Zn2+ 1.9 × 10−3

Fe3+ 5.6 × 10−3

Cr3+ 2.2 × 10−3

Al3+ 1.8 × 10−4

Bi3+ 2.8 × 10−4

Ag+ 6.6 × 10−3

Table 4Analytical data of nickel content in various chocolate samples as determined byAAS and membrane sensor

S. no. Sample Average nickel concentrationobtained (mg/kg)a

Proposed ISE AAS

1 Milk chocolate 0.77 ± 0.02 0.75 ± 0.012 Five star 0.89 ± 0.01 0.89 ± 0.013 Perk 0.85 ± 0.02 0.84 ± 0.024 Dairy milk 0.92 ± 0.01 0.94 ± 0.015

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Nutties 0.69 ± 0.02 0.70 ± 0.01

a The average of three replicate measurements.

In the present study aA was 1.0 × 10−5 M, and a′A

.0 × 10−5 M, and aB was determined experimentally. The val-es of selectivity coefficients are given in Table 3. The valuesndicate that the electrode is selective to Ni2+ over a numberf cations. Hence, these are not expected to interfere even athe higher concentration levels (1.0 × 10−2 M) of the interfer-

ng ions. But, as can be seen, the selectivity coefficient is highor Co2+, and thus it is expected to cause interference. However,ven Co2+ would not interfere if present in small amounts, as theelectivity is concentration dependent. In order to establish the

ococ

able 5omparison of the proposed Ni2+-selective electrode with the recently reported elect

. no. Ionophore Working concentrationrange (M)

Sl(d

Tetraazaannulene derivative 7.9 × 10−6 to 1.0 × 10−1 30Dithizone 5.0 × 10−6 to 1.0 × 10−2 29Benzene-1,2-diamine derivative 2.0 × 10−7 to 1.0 × 10−2 30Schiff base 3.2 × 10−6 to 5.0 × 10−2 29t-Octyl-calix[6]arene derivative 5.0 × 10−6 to 1.0 × 10−1 29Ph4Bzo2(12)tetraeneN4 3.9 × 10−6 to 1.0 × 10−1 29

ig. 5. Variations in cell potential with Ni2+ concentrations at different concen-ration of Co2+ ions (M).

oncentration of Co2+ that can be tolerated in the determinationf Ni2+ ions, some mixed run studies were performed. It can beeen from Fig. 5 that the presence of Co2+ at a concentrationf 5.0 × 10−5 M caused no divergence in the potential versusi2+ concentration plot as obtained in pure Ni2+ solutions for

he proposed electrode. Hence, the electrode can tolerate Co2+

ons at concentrations ≤5.0 × 10−5 M over the whole workingoncentration range. Co2+ ions of higher concentrations causedivergence from the original potential versus Ni2+ concentra-

ion plot, and therefore they can be tolerated only in reducedoncentration ranges. It can be seen from Fig. 5 that Ni2+ can beetermined in the concentration ranges larger than 6.7 × 10−6,.8 × 10−5, 7.1 × 10−5 M in the presence of 1.0 × 10−4,.0 × 10−4 and 1.0 × 10−3 M Co2+ ions, respectively. Hencehe electrode could be successively used to determine Ni2+ ionsn the presence of equal or 10-fold higher concentration ofo2+ ions.

Further, to investigate the effect of anions, cell potentials were

btained using nickel chloride and nickel sulphate. No signifi-ant changes in the working concentration range and slope werebserved, indicating that these anions (SO4

2− and Cl−) do notause any interference.

rodes

ope mVecade activity)−1

pH range Responsetime (s)

Life time(months)

Reference

.0 2.7–7.6 15 6 [17]

.5 NM 45 2 [18]

.0 4.5–9.0 <10 1 [21]

.0 2.2–5.9 10 4 [24]

.75 3.5–7.5 <15 1 [25]

.5 2.5–7.7 8 4 Proposedelectrode

354 A.K. Singh, P. Saxena / Sensors and A

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ig. 6. Potentiometric titration plots of a 1.0 × 10−3 M Ni2+ solution (10 ml)ith 1.0 × 10−2 M EDTA (membrane no. 2).

. Analytical applications

.1. Application in real sample

The nickel content was determined in some Indian brands ofilk powder and chocolate samples using the membrane sensor.he results obtained by the electrode method are in satisfactorygreement with the values obtained by AAS (Table 4).

.2. Potentiometric titration

The practical utility of the proposed membrane sensor wasested by using it as an indicator electrode for the titration of0 ml of 1.0 × 10−3 M Ni2+ ions with a 1.0 × 10−2 M EDTAolution at pH 5, and the resulting titration curve is shown inig. 6. As can be seen, the amount of nickel ions in solution cane accurately determined with the electrode. Before the titrationndpoint, the measured potential shows an unusual logarithmichange with the amount (ml) of the titrant added, while the poten-ial response after the endpoint will remain almost constant, dueo the low concentration of free Ni2+ ions in solution. The plotn Fig. 6 is of standard sigmoid type, indicating sufficient selec-ivity of the proposed electrode for Ni2+ ions and the endpointorresponding to the 1:1 stoichiometry of Ni2+–EDTA complex,nd therefore, the electrode can be used as an indicator for deter-ining Ni2+ ions potentiometrically.

. Conclusion

The PVC-based membrane electrode of ligandPh4Bzo2(12)hexaeneN4] with a composition of 1.7%igand, 34.8% PVC, 62.7% DOP and 0.8% NaTPB exhibits the

est performance characteristics. The sensor exhibited goodeproducibility with a useful lifetime of 120 days. Further, itan be used in non-aqueous media up to 30% (v/v) content. Theroposed electrode was successfully applied in determining

[

ctuators B 121 (2007) 349–355

i2+ ions in real samples. A comparison of the performance ofhe given electrode with the recently reported Ni(II)-selectivelectrodes is given in Table 5 indicates that the proposed sensors superior to the reported ones.

cknowledgment

Authors are grateful to Ministry of Human Resource andevelopment, New Delhi, India for providing financial assis-

ance to undertake this work.

eferences

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19] M.R. Ganjali, M.R. Fathi, H. Rahmani, H. Pirelahi, Nickel (II) ion-selectiveelectrode based on 2-methyl-4-(4-methoxy phenyl)-2,6-diphenyl-2H-thiopyran, Electroanalysis 12 (2000) 1138–1142.

20] A. Abbaspour, A. Izadyar, A highly selective electrode for nickel (II) ionbased on 1,5-diphenylthiocarbazone (dithizone), Microchem. J. 69 (2001)7–11.

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22] V.K. Gupta, R. Prasad, A. Kumar, Dibenzocyclamnickel(II) as ionophorein PVC-matrix for Ni2+-selective sensor, Sensors 2 (2002) 384–396.

23] M.R. Ganjali, M. Hosseini, M. Salavati-Niasari, T. Poursaberi, M. Sham-sipur, M. Javanbakht, O.R. Hashemi, Nickel ion-selective coated graphitePVC-membrane electrode based on benzylbis(thiosemicarbazone), Elec-troanalysis 14 (2002) 526–531.

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iographies

shok Kumar Singh is a professor of organic chemistry at Indian Institute ofechnology-Roorkee, Roorkee, India. He received his PhD in organic chemistryrom Banaras Hindu University, Varanasi, India in 1977. His research work haseen mainly focused on the synthesis of macrocyclic ligand-complexes and theirpplications in analytical chemistry as chemical sensors.

uja Saxena obtained her MSc degree in chemistry from University of Roorkee,oorkee in 2000. She is currently working towards her PhD at the Indian Institutef Technology-Roorkee, Roorkee, India under the supervision of Prof. Ashokumar Singh.