Sensors and Actuators B 106 (2005) 779–783

Novel potentiometric sensor for monitoring barium(II) based on2,3,4-pyridine-1,3,5,7,12-pentaazacyclopentadeca-3-ene

Ashok Kumar Singh∗, Rupam Singh, R.P. Singh, Puja SaxenaDepartment of Chemistry, Indian Institute of Technology-Roorkee, Roorkee 247667, India

Received 12 April 2004; received in revised form 15 September 2004; accepted 28 September 2004Available online 28 October 2004

Abstract

A poly (vinyl chloride)-based membrane of 2,3,4-pyridine-1,3,5,7,12-pentaazacyclopentadeca-3-ene with sodium tetraphenylbo-rate(NaTPB) as an anion excluder and dioctyl phthalate (DOP) as plasticizer was prepared and investigated as a Ba(II)-selective electrode.The best performance was observed with the membrane having the ligand–PVC–DOP–NaTPB composition 1:4:1:1, which worked well overa wide concentration range (1.41× 10−6–1.0× 10−1 mol L−1) with a Nernstian slope of 30.0 mV per decade of activity between pH 2.5 and7.0. This electrode showed a fast response time of 18 s and was used over a period of 3 months with good reproducibility (S= 0.4 mV). Thes lectrow lectrode inp©

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icmtcmot

ptya

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ode-resinda-18-

hthyl

tentio-

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electivity coefficient for mono-, di- and trivalent cations indicates excellent selectivity for Ba(II) ions over a large number of cations. Edeorks satisfactorily in methanolic and ethanolic content up to a maximum of 30%. The sensor has been used as an indicator eotentiometric titration of Ba(II) with EDTA.2004 Published by Elsevier B.V.

eywords:Macrocyclic compound; Barium-selective electrode; PVC-based membrane; Potentiometry

. Introduction

Barium compounds are heavily used by the oil and gasndustries to serve as a weighing agent in drilling mud. Dis-harge from medical test and X-ray applications and fromanufacturing of automotive paints, bricks, jet fuels, elec-

ronic components, dyes, fire-works and ceramic/glass alsoontribute to the release of barium compounds to the environ-ent. Apart from the anthropogenic sources, natural erosionf soil can also contribute significantly to barium contamina-

ion.High levels of barium may lead to a series of medical

roblems in human beings, some of which include acute gas-roenteritis, loss of deep reflexes with onset of muscular paral-sis, respiratory failure, rhabdomylosis, hypophosphatemiand baritosis from inhaled barium.

∗ Corresponding author. Tel.: +91 332285798; fax: +91 1332273560.E-mail address:akscyfcy@iitr.ernet.in (A.K. Singh).

Literature survey reveals that very little work hbeen done on the development of ISEs for Ba(II) ioGuggi et al.[1] prepared a barium ion-selective electrbased on the neutral carrierN,N,N′,N′-tetraphenyl-1,3,6,9trioxaundecanediamide. The membranes of an epoxy-binder were found suitable for Ba2+-sensors by Lal ancoworkers [2]. Nakamura and Rechnitz reported a B2+

selective electrode based on immobilized dibenzocrown-6 ionophore[3] while Bouklouze et al.[4] haveused ethylene/vinylacetate-based membrane of binappolyethers as Ba2+ sensors. Moutet et al.[5] reported anew ferrocenyl crown ether as ionophore for Ba2+-selectiveelectrode. Recently, Saleh[6] reported neutral bidentaorganophosphorus compounds as ionophores for potemetric membrane sensors for Ba(II) while Hassan et a[7]have tested a DDB liver drug as an electroactive matfor Ba(II) membrane sensor. In this work we are repora highly selective and sensitive barium membrane elecfor fast monitoring of Ba2+ ions based on 2,3,4-pyridin1,3,5,7,12-pentaazacyclopentadeca-3-ene.

925-4005/$ – see front matter © 2004 Published by Elsevier B.V.oi:10.1016/j.snb.2004.09.030

780 A.K. Singh et al. / Sensors and Actuators B 106 (2005) 779–783

Table 1Composition of PVC-based membranes of 2,3,4-pyridine-1,3,5,7,12-pentaazacyclopenta-deca-3-ene and their performance as Ba(II)-selective electrodes

Membrane no. Composition of membrane (w/w) Working concentrationrange (mol L−1)

Slope (mV/decadeof concentration)

Response time (s)

Ionophore PVC STB DOP

1 1 2 1 – 1.00× 10−5–1.00× 10−1 32.0 282 1 4 1 1 1.41× 10−6–1.00× 10−1 30.0 183 1 6 1 1 3.16× 10−6–1.00× 10−1 30.0 224 1 8 1 1 2.82× 10−5–1.00× 10−1 42.0 205 1 10 1 1 7.90× 10−6–1.00× 10−1 39.0 23

2. Experimental

2.1. Reagents

All the reagents and chemicals used were of analyticalgrade. Barium(II) nitrate was used as barium salt for the studyof membrane sensor. Doubly distilled water was used for thepreparation of solution of metal salts of different concentra-tions by diluting stock standard solution (0.1 M). High molec-ular mass PVC was obtained from Aldrich (USA), sodiumtetraphenylborate (NaTPB) from BDH (UK), and dioctyl ph-thalate (DOP) from Merck (Germany).

2.2. Synthesis of macrocycle

The macrocycle 2,3,4-pyridine-1,3,5,7,12-pentaaza-cyclopentadeca-3-ene was prepared by the followingprocedure:

Equimolar amounts ofN,N′-bis(3-aminopropyl) ethyl-

enediamine and 2,6-dichloropyridine were taken. 2,6-Dichloropyridine was dissolved in minimum quantity ofmethanol and to this solutionN,N′-bis(3-aminopropyl)ethylenediamine was added drop wise and the reaction mix-ture was stirred for 8 h with slow heating and then mixturewas concentrated to half of its original volume on a waterbath, cooled and then kept aside for 1 day. The shiny whitep fromm :C .5].T or thel

2

aredb s re-

ported earlier[8]. The membrane components outlined inTable 1were mixed and dissolved in diluent tetrahydrofuran.The resulting homogeneous syrup was vigorously stirred andafter removing all air bubbles it was poured into glass castingrings resting on a smooth glass plate. The solvent was madeto evaporate at room temperature. After 48 h, a transparentmembrane of 0.5 mm thickness was obtained from which a6 mm diameter piece was cut away from the inner edge ofthe ring and glued to one end of the Pyrex glass tube witharaldite.

The membranes were also subjected to microscopic andelectrochemical examination for cracks and homogeneity ofthe surface and only those, which had smooth surface andgenerated reproducible potentials, were chosen for subse-quent investigation. Membrane to membrane (and batch tobatch) reproducibility was ensured by carefully controllingthe condition of fabrication.

2.4. Potential measurement

The membranes were equilibrated with 1.00 mol L−1

Ba(NO3)2 solution for 6 days and the potential acrossthe membranes were measured with ECIL (Hyderabad, In-dia) digital pH meter potentiometer model pH 5662, andCentury (Chandigarh, India) CBM 301 micro-voltmeter inc ref-e seda adeap tion[

solu-t ure-m werc takent twom ationf readm

3

theB andm

roduct thus obtained was filtered and recrystallisedethanol and dried in vacuo. [C13H23N5 – calculated (%), 62.6; H, 9.2; N, 28.1; found (%): C, 63.0; H, 9.3; N, 27he calculated and observed elemental analysis data f

igand are in good agreement with the structure.

.3. Preparation of membrane

Homogeneous membranes of the ligand were prepy using PVC as binder and NaTPB (anion excluder) a

onjunction with saturated calomel electrodes (SCE) asrence electrodes. Ba(II) nitrate solution (0.1 M) was us an internal solution. All the measurements were mt a constant temperature of 25± 0.1◦C. Debye Huckelrocedure was adopted for activity concentration rela

9].Response time was determined after potential of one

ion of Ba (II) nitrate became constant, and similar measents were carried out in another solution of 100-fold lo

oncentration. The response time is defined as the timeo reach a potential of 90% of the potential difference ineasurements. Reproducibility was defined by the devi

rom the average potential value in the same four dip toeasurements.

. Results and discussion

The optimum conditions for the best performance ofa(II)-selective electrode based on a macrocyclic ligembrane were investigated systematically.

A.K. Singh et al. / Sensors and Actuators B 106 (2005) 779–783 781

Fig. 1. Variation of potential of the membranes of the macrocyclic ligandwith the varying concentration of Ba(II) ions.

The response time was recorded according to IUPAC rec-ommendation[10]. An electrode having a membrane withoutsolvent mediator gave a steady response time of 28 s, whereasafter adding the solvent mediator (DOP), the electrodeachieved an equilibrium response within 18–23 s over a wholeconcentration range of 1.41× 10−6–1.00× 10−1 mol L−1.The potentials so obtained remained constant for more than5 min, after which a slow divergence was observed. Poten-tials were measured periodically and the standard deviationof 20 identical measurements was 0.4 mV.

The lifetime of electrodes based on ionophores in sol-vent polymeric membranes depends on the distribution coef-ficient of the ionophore and plasticizer between the aqueousand membrane phases[9]. Hence the lifetime of electrodesmust depend on the components of the solution and the mea-sured specimens with electrodes. The experimental resultsshow that the lifetime of the present electrode was about 100days. During this time, the detection limit and the slope ofthe electrode remained almost constant. The electrochemi-cal behavior of the electrode gradually deteriorated after 100days, which can be attributed to aging of the polymer (PVC),plasticizer and ionophore (macrocyclic ligand)[11].

The working concentration range and slope for allthe membrane electrodes (No. 1–5) are given inTable 1(Fig. 1). The macrocycle [2,3,4-pyridine-1,3,5,7,12-penta-azacyclopentadeca-3-ene] based membrane shows lin-e1 n-c . 1).

However, with the addition of a solvent mediator (DOP),it was observed that addition of DOP leads to signifi-cant improvement in the working concentration range to1.41× 10−6–1.00× 10−1 mol L−1 with a Nernstian slopeof 30 mV per decade of concentration (Membrane No.2) with ionophore–PVC–NaTPB–DOP in the ratio 1:4:1:1(Table 1). Hence this particular membrane was stud-ied in detail as a Ba(II)-selective electrode and all fur-ther investigations were carried out with this particularmembrane.

Incorporation of salts consisting of a hydrophobiccation and a lipophilic anion, e.g. sodium tetraphenylborate(NaTPB), in cation-selective membranes based on macro-cyclic ligand has proved to be beneficial in several respects[12,13]. The additives reduce interference by lipophilic an-ions in the sample by bringing about significant changes inselectivity and boosting the cation selectivity in the case ofcarriers with poor extraction capability and lowering the elec-trical membrane resistance considerably[13,14]. The elec-trode prepared without NaTPB is less sensitive and selectivefor Ba(II) ions, owing to high resistance or a response to an-ions in the test solution. NaTPB behaves as an ion exchanger[15]; hence use of NaTPB leads to drastic changes in theslope of the electrode response and in membrane selectivity.

The effect of the content of the PVC on the response andselectivity of the sensor was also studied. Construction oft lesst d tob aina thef tiono

wast1 dw asi nb mbly.

sti-g 40%( elec-t ousm ) int loper 30%m de-c

ec-o1 cet iva-l wass ingi in-d ys

arity in the concentration range 1.00× 10−5–1.00×0−1 mol L−1 with a slope of 32 mV per decade of coentration without any solvent mediator (Membrane No

he membrane was difficult when the PVC content washan 40% m/m. The optimum content of PVC was foune 40%. This implies that the chlorine on the PVC chre solvating the Na(I) ions in NaTPB; hence preventing

ormation of the contract in pairs and favoring the formaf solvent-separated ion pairs[16].

The pH dependence of electrode potentialested over the pH range 1.0–7.0 for 1.00× 10−2 and.00× 10−3 mol L−1 of Ba(II) ions. The pH was adjusteith nitric acid or ammonia solution. The potential w

ndependent in the range of 2.5–7.0 (Fig. 2) and the same cae taken as the working pH range of the electrode asse

The practical utility of the proposed sensor was inveated in partially non-aqueous media using 20, 30 andv/v) water–methanol and water–ethanol mixtures. Therode assembly works satisfactorily in partially non-aqueedia with methanol or ethanol content up to 30% (v/v

hese mixtures. The working concentration range and semains almost constant in these media. However aboveethanolic or ethanolic content, the slope appreciably

reases.Selectivity coefficients were determined by IUPAC r

mmended matched potential method (MPM)[17] at a.0× 10−2 mol L−1 concentration of interfering ions. Sin

he selectivity coefficients for monovalent, bivalent and trent ions are considerably less than unity, the electrodeelective to Ba2+ ions in the presence of these interferons (Table 2). Thus, these selectivity coefficient valuesicate that the ions listed inTable 2, would not cause anignificant interference in the estimation of Ba2+ ions by the

782 A.K. Singh et al. / Sensors and Actuators B 106 (2005) 779–783

Fig. 2. Effect of the pH of the test solution on the potential response of theBa2+ ion-selective electrode, at 1.0× 10−2 and 1.0× 10−3 mol L−1 concen-tration of Ba(II) ions.

electrode unless present in large amounts. The proposed sen-sor exhibited much better potentiometric selectivity for theBa2+ over many common and related metal ions viz Na+

(3.28× 10−5), K+ (1.0× 10−5), Mg2+ (6.78× 10−4), Ca2+

(2.44× 10−3) and Sr2+ (4.02× 10−3) when compared to pre-viously reported sensors[1–7].

The proposed sensor was used as an indicator elec-trode in the potentiometric titration of Ba(II) with EDTA. A25 mL of 1.0× 10−3 mol L−1 of Ba(II) was titrated against1.0× 10−2 mol L−1 EDTA solution. The resulting titrationcurve is shown inFig. 3. As can be seen from the figure,the electrode can accurately determine Ba2+ in the solution.After the end point, the concentration of Ba2+ decreases andthe electrode does not respond to the decrease in Ba2+ con-centration and remains constant.

Table 2Selectivity coefficient values (KPot

Ba2+,B) for Ba(II)-selective membrane

electrode based on 2,3,4-pyridine-1,3,5,7,12-pentaazacyclopentadeca-3-enebarium(II) macrocyclic complex

Interferent (B) Selectivity coefficient (KPotBa2+,B

)matched potential method

Li+ 2.82× 10−5

K+ 1.00× 10−5

Na+ 3.28× 10−5

NH4+ 5.00× 10−5

AMCSCNCZPCFA

Fig. 3. Potentiometric titration curve of 25 mL of 1.00× 10−3 mol L−1 ofBa(II) with 1.00× 10−2 mol L−1 EDTA using the proposed membrane sen-sor as an indicator electrode.

4. Conclusion

The membrane electrode incorporating 2,3,4-pyridine-1,3,5,7,12-pentaazacyclopentadeca-3-ene as carrier can beused in the development of a barium ion-selective elec-trode. The results show that dioctyl phthalate (DOP) provesto be a good solvent mediator for electrode construction.The proposed electrode shows high selectivity and sensitiv-ity to Ba2+ ions, fast response time with a Nernstian Slopeof 30.0 mV/decade to Ba2+ over a wide concentration rangeof 1.41× 10−6–1.0× 10−1 mol L−1. The propose electroderevealed excellent selectivity towards Ba2+ with respect tomany common and related metal ions viz Na+, K+, Mg2+,Ca2+ and Sr2+. The electrode was found to work well underlaboratory conditions. It was used as an indicator electrodein potentiometric titration with EDTA.

Acknowledgement

Authors are grateful to Council of Scientific and Industrialresearch (CSIR), India for providing financial assistance toundertake this work.

References

lec--

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elec-

de993)

enebe-

g+ 3.28× 10−5

g2+ 6.78× 10−4

a2+ 2.44× 10−3

r2+ 4.02× 10−3

o2+ 6.50× 10−5

i2+ 4.02× 10−5

u2+ 5.05× 10−5

n2+ 6.02× 10−5

b2+ 4.26× 10−4

d2+ 8.24× 10−4

e3+ 4.62× 10−4

l 3+ 3.00× 10−3

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Biographies

Ashok Kumar Singh is a professor of organic chemistry at Indian In-stitute of Technology Roorkee, Roorkee, India. He received his Ph.D.in Organic Chemistryfrom Banaras Hindu University, Varanasi, India in1977. His research work has been mainly focused on the synthesis ofmacrocyclic ligand-complexes and their applications in analytical chem-istry as chemical sensors.

Rupam Singh received her Ph.D. degree in 2003 from the Indian Instituteof Technology Roorkee, Roorkee, India under the supervision of Prof.Ashok Kumar Singh.

R.P. Singh received his Ph.D. degree in 2003 under the supervision ofProf. Ashok Kumar Singh.

Puja Saxenaobtained her M.Sc. degree in Chemistry from University ofRoorkee, Roorkee in 2000. She is currently working towards her Ph.D.at the Indian Institute of Technology Roorkee, Roorkee, India under thesupervision of Prof. Ashok Kumar Singh.