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Analytica Chimica Acta 585 (2007) 171–178

Chromium(III) selective membrane sensors basedon Schiff bases as chelating ionophores

A.K. Singh ∗, V.K. Gupta, Barkha GuptaDepartment of Chemistry, Indian Institute of Technology-Roorkee, Roorkee 247667, India

Received 3 June 2006; received in revised form 8 September 2006; accepted 29 November 2006Available online 6 December 2006

bstract

The two chromium chelates of Schiff bases, N-(acetoacetanilide)-1,2-diaminoethane (L1) and N,N′-bis(acetoacetanilide)-triethylenetetraammineL2), have been synthesized and explored as neutral ionophores for preparing poly(vinylchloride) (PVC) based membrane sensors selective tor(III). The addition of lipophilic anion excluder (NaTPB) and various plasticizers viz. o-Nitrophenyloctyl ether (o-NPOE), dioctylpthalate (DOP),ibutylphthalate (DBP), tris(2-ethylhexyl)phosphate (TEHP), and benzyl acetate (BA) have found to improve the performance of the sensors. Theest performance was obtained for the membrane sensor having a composition of L1:PVC:DBP:NaTPB in the ratio 5:150:250:3 (w/w). The sensorxhibits Nernstian response in the concentration range 8.9 × 10−8 to 1.0 × 10−1 M Cr3+ with limit of detection 5.6 × 10−8 M. The proposed sensoranifest advantages of relatively fast response (10 s) and good selectivity over some alkali, alkaline earth, transition and heavy metal ions. The

electivity behavior of the proposed electrode revealed a considerable improvement as compared to the best previously PVC-membrane electrodeor chromium(III) ion. The potentiometric response of the proposed sensor was independent of pH of the test solution in the range of 2.0–7.0. The

ensor has found to work satisfactorily in partially non-aqueous media up to 20% (v/v) content of methanol, ethanol and acetonitrile and could besed for a period of 3 months. The proposed electrode was used as an indicator electrode in potentiometric titration of chromium ion with EDTAnd in direct determination in different water and food samples.

2006 Elsevier B.V. All rights reserved.

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cgf(Diasfldun

eywords: Chromium selective electrode; Poly(vinylchloride) membranes; Sch

. Introduction

The need of selective determination of heavy metal ions hasncreased immensely during the last few decades due to therowing environmental problems. Among heavy metals, theollution by chromium is of considerable concern, as it is highlyoxic and has used in chrome plating, pigment manufacturing,efractory industries, leather tanning, and wood treatment, mak-ng steel and other alloys. Chromium is also an essential tracelement in human nutrition. It is an essential element requiredor normal carbohydrate and fat metabolism by potentiatinghe action of insulin through glucose tolerance factor (GTF)

or activating certain enzymes and stabilization of proteinsnd nucleic acids. Insufficient dietary intake of chromiumeads to increases in risk factors associated with diabetes and

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

dianm[s

003-2670/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.aca.2006.11.074

se and potentiometric sensors

ardiovascular disease including elevated circulating insulin,lucose, triglycerides, total cholesterol and impaired immuneunction. The estimated safe and adequate daily dietary intakeESADDI) for chromium in adults is 50–200 �g per day [1].ue to vital importance of chromium in biological system and

ndustry, a narrow window of concentration between essentialitynd toxicity warrants the determination of chromium. Thoughophisticated analytical techniques viz. AAS, ICP-AES, X-rayuorescence, HPLC, DPP have been employed for trace leveletermination, they are disadvantageous in terms of cost andnsuitability for routine analysis. Therefore, there is criticaleed for the development of selective, portable, inexpensiveiagnostic tools for the determination of chromium. Recentlyon-selective electrodes have been proved promising alternatives these provide fast response time, linear dynamic range, and

on-destructive and ‘online’ analysis. Extensive efforts haveade to develop a good sensitive sensor for chromium ion

2–19] using different neutral ionophores. However, reportedensors exhibit narrow working concentration range [5,6,17],

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72 A.K. Singh et al. / Analytica C

on-Nernstian response [17] and high response time [9,14,17]nd showed significant interference from foreign ions [8,12,14].

In order to achieve wider applicability, these limitations needo be removed. An important requirement for the preparation ofselective ion sensor is that the electro active material, which

s used in the membranes, should exhibit high lipophilicity andtrong affinity for a particular metal ion to be determined andoor affinity for others. Sensors comprising Schiff bases as elec-ro active ingredient have been reported to exhibit excellentelectivity for specific metal ions [20,21]. Schiff base ligandsave been extensively studied in coordination chemistry mainlyue to their facile syntheses, easily tunable steric, electronicroperties, good solubility in common solvents [22]. They aretable under a variety of oxidative and reductive conditions,nd these imine ligands are borderline between hard and softewis bases [23,24]. They are becoming increasingly impor-

ant as biochemical, analytical and antimicrobial reagents. These of Schiff base as neutral carriers have been reported as ionelective electrodes for determination of cations such as cop-er(II) [25], mercury(II) [26], nickel(II) [27], silver(II) [28],ead(II) [29], cobalt(III) [30] gadolinium(III) [31], yttrium(III)32], dysprosium(III) [33].

Taking an account of the highly desirable attributes ofhis type of ligands, we synthesized two Schiff bases andxplored their poly(vinylchloride) (PVC)-based membranes asr3+ selective sensors and the results are reported in the presentommunication.

. Experimental

.1. Reagents

Reagent grade sodium tetraphenylborate (NaTPB),ibutylphthalate (DBP), tris(2-ethylhexyl)phosphate (TEHP),ioctylpthalate (DOP), o-nitrophenyloctyl ether (o-NPOE),enzyl acetate (BA), tetrahydrofuran (THF) and high moleculareight poly(vinylchloride) were purchased from Merck.riethylenetetraamine (SRL), acetoacetanilide (S.D. Finehem.) and ethylenediamine (Merck) sodiumdodecylsulphate

Lancaster), cetyl trimethylammonium bromide (Merck) havesed as obtained. The nitrate and chloride salts of all the cationssed were of analytical grade and used without any furtherurification. The solutions of metal salts were prepared inoubly distilled water and standardized whenever necessary.

.2. Synthesis of ionophore

.2.1. N-(Acetoacetanilide)-1,2-diaminoethane (L1)The ionophore was obtained by refluxing the acetoacetanilide

0.01 M) with ethylenediamine (0.01 M) in DCM with stirring.he mixture was refluxed for 10 h, cooled and concentratednder reduced pressure. The residue was extracted and recrys-allised from chloroform and dried in vacuo. White solid; yield

6%, mp 185 ◦C. 1H NMR (CDCl3): δ 7.01–7.06 (m, 5H, Ar-H),.62 (m, 1H-NH), 3.4 (s, 2H, NH2), 0.83–1.32 (m, 6H, CH2),.60 (s, 3H, COCH3). IR (KBr) 3420, 1715, 1600 cm−1. Elemen-al analysis % observed for C12H17N3O, C = 65.60; H = 7.60;

u0bb

ca Acta 585 (2007) 171–178

= 19.15 and calculated % was C = 65.75; H = 7.75; N = 19.17.he observed elemental analysis of the compound was consis-

ent with the theoretical data obtained based on the followingtructure.

.2.2. N,N′-bis(Acetoacetanilide)-triethylenetetraammineL2)

A mixture of acetoacetanilide (0.02 M), triethylenete-raamine (0.01 M) and catalytic amount of HCl was refluxed inCM for 8 h, cooled and concentrated under reduced pressure.he obtained residue was recrystallised from methanol and dried

n vacuo. Yellow solid; yield 78%, mp 70 ◦C. 1H NMR (DMSO):7.04–7.60 (m, 10H, Ar–H), 3.88 (bs, 4H, –NH), 1.84 (m,

H, CH2), 2.04–2.48 (m, 12H, CH2), 2.60 (s, 6H, COCH3). IRKBr): 3425, 1720, 1596 cm−1. Elemental analysis % observedor C26H37N6O2: C = 67.08; H = 8.01; N = 18.09 and calculated

was C = 67.0; H = 7.95; N = 18.06. The observed elementalnalysis of the compound was consistent with the theoreticalata obtained based on the following structure.

.3. Electrode preparation

The membranes were prepared as suggested by Craggs etl. [34]. It is known that the sensitivity, linearity and selectiv-ty obtained for a given ionophore depends significantly on the

embrane composition and nature of plasticizer used [35–37].he PVC-based membranes were prepared by dissolving appro-riate amounts of Schiff bases (L1 and L2), anion excluderNaTPB), and plasticizers TEHP, DBP, o-NPOE, BA, DOP andVC in THF (5 ml). The homogenous mixture was obtained afteromplete dissolution of all the components, concentrated byvaporating THF and it has poured into polyacrylate rings placedn a smooth glass plate. The viscosity of the solution and sol-ent evaporation was carefully controlled to obtain membranesith reproducible characteristics and uniform morphology and

hickness otherwise have shown a significant variation, which

ltimately affected the sensor response. The membranes of.4 mm thickness have glued to one end of a “Pyrex” glass tubey careful removal from the glass plate. Thus, several mem-ranes of varying compositions were prepared and investigated.

A.K. Singh et al. / Analytica Chimica Acta 585 (2007) 171–178 173

Table 1Formation constants of Schiff bases–metal complexes

L1 L2

Cation log Kf Cation log Kf

Ca2+ 2.06 ± 0.05 Ca2+ 2.02 ± 0.02Ni2+ 2.18 ± 0.12 Ni2+ 1.96 ± 0.15Cu2+ 2.38 ± 0.06 Cu2+ 2.13 ± 0.13Na+ 2.56 ± 0.16 Na+ 2.41 ± 0.05Al3+ 2.64 ± 0.08 Al3+ 2.53 ± 0.07Cd2+ 2.42 ± 0.15 Cd2+ 2.29 ± 0.14Zn2+ 2.15 ± 0.06 Zn2+ 2.11 ± 0.09Pb2+ 2.72 ± 0.11 Pb2+ 2.51 ± 0.13C 3+ 3+

CC

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lctibatAsThe emf responses obtained for all other cation-selective elec-trodes were much lower than that predicted by the Nernstequation. This is probably due to both the selective behaviorof the ionophore against Cr3+ in comparison to other metal ions

r 5.12 ± 0.05 Cr 4.52 ± 0.12e3+ 2.47 ± 0.12 Ce3+ 2.34 ± 0.02o2+ 2.0 ± 0.14 Co2+ 2.12 ± 0.04

he membranes with best performance characteristics and repro-ucible results have chosen for detailed studies. The activityoefficient γ , of metal ions have calculated from the modifiedorm of the Debye–Huckel equation:

og γ = −0.511Z2[

μ1/2

1 + 1.5μ1/2 − 0.2μ

]

here μ is the ionic strength and Z is the valency. All measure-ents were carried out at 25 ± 0.1 ◦C.

.4. Equilibration of membranes and potentialeasurements

The membranes were equilibrated for 3 days in 1.0 M CrCl3olutions. The potentials have been measured by varying theoncentration of CrCl3 in test solution in the range 1.0 × 10−8

o 1.0 × 10−1 M. The standard CrCl3 solutions have obtainedy gradual dilution of 0.1 M CrCl3 solution. The potential mea-urements were carried out at 25 ± 1 ◦C using saturated calomellectrodes (SCE) as reference electrodes with the following cellssembly:

g/Hg2Cl2|KCl (satd.)|0.1 MCrCl3||PVC membrane||test solution|Hg/Hg2Cl2|KCl (satd.)

. Result and discussion

.1. Conductance studies of Schiff bases complexation withetal ions

In preliminary experiments, the complexation of the Schiffases with a number of alkali, alkaline earth and transition metalons has been investigated conductometrically in acetonitrileolutions (1.0 × 10−4 M of cation solution and 1.0 × 10−4 M

f ligand) at 25 ± 1 ◦C [38,39]. The formation constants (Kf) ofhe resulting 1:1 complexes are summarized in Table 1. As cane seen, L1 formed most stable complex with Cr(III) ion hasxpected to act as a suitable ion-carrier for the fabrication of ad(II) ion-selective membrane sensor.

F(

ig. 1. Potential response of PVC membrane sensor based on ionophore (L1)or various metal ions.

.2. Response of the electrode based on Schiff bases tor(III) ions

The existence of nitrogen and oxygen as donor atoms in theigands with high lipophilic character, it seems to form strongomplexes with transition metal ions. In order to study the poten-ial response and selectivity of the Schiff bases for different metalons, L1 and L2 were used as neutral carriers to prepare PVC-ased membrane electrodes for a variety of metal ions includinglkali, alkaline earth, transition and heavy metal ions. The poten-ial responses obtained are shown in Figs. 1 and 2 respectively.s can be seen, chromium ion with the most sensitive response

eems to be suitably determined with the membrane electrodes.

ig. 2. Potentiometric response of PVC membrane sensor based on ionophoreL2) for various metal ions.

174 A.K. Singh et al. / Analytica Chimi

Fig. 3. Absorption spectra of 1.0 × 10−4 M Cr3+ (A), 1.0 × 10−5 M L1 (B),m(

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ixture of L1 with metal (C), 1.0 × 10−5 M L2 (D), mixture of L2 with metalE).

nd the rapid exchange kinetics of the resulting ligands L1 and2 with Cr3+ complexes.

.3. Spectroscopic studies of ionophores with Cr(III) ions

It is known that spectroscopic techniques can be used to probehe strength of ion-carrier interactions. It was found that ions thatause a large change in spectrum induce the largest potentio-etric responses and thus have better selectivity. In this study,V-spectroscopic experiments have carried out to investigate

he interaction between Schiff bases with Cr3+ ion. Our spec-rophotometric studies revealed that the Schiff bases (L1 and

2), form stable complexes with Cr3+ ion. The absorption spec-ra of L1 and L2 (1.0 × 10−5 M) and their mixtures with Cr3+

1.0 × 10−4 M) in DMSO solution are shown in Fig. 3. As can beeen, it is possible to distinguish the specific interaction betweenhe ligands and Cr3+ ion. As is obvious, while Cr3+ possessesegligible absorption in 260–370 nm spectral regions, L1 showswo distinct absorption maxima at 315.0, 260.2 nm while L2hows at 312.0 and 259.5 nm. The presence of Cr3+ ion resultedn the shift of two spectral absorption to 310.0, 251.20 nm (L1)nd 309.6, 258.2 nm (L2). The observed spectral shifts, togetherith the substantial increase in absorbance of L1, after the con-

act of the carrier solution with the chromium ion-containinghase, suggest the preferred coordination of the target cation byhe carrier.

.4. Effect of internal solution

In accordance with generally adopted response formalism ofon sensor, the influence of the concentration of internal solutionn the potential response of the polymeric membrane electrodesor Cr3+ ion based on L1 and L2 were studied. The concentrationas varied from (1.0 × 10−1 to 1.0 × 10−3 M) and the potential

esponse of the electrodes was obtained. It was found that theest results in terms of slope and working concentration rangeere obtained with internal solution of activity 1.0 × 10−1 M.hus, 1.0 × 10−1 M concentration of the reference solution was

laiw

ca Acta 585 (2007) 171–178

uite appropriate for the smooth functioning of the chromiumelective sensors.

.5. Optimization of membrane composition

It is known that the presence of lipophilic anionic addi-ives in cation selective membrane electrodes is necessary tontroduce perm selectivity, so that without such additives elec-rodes may fail to respond properly [40,41]. So tetra phenylorate was added to all the membranes to reduce the interfer-nce from sample anions and bulk membrane impedance [42].he further improvement in the performance has attemptedy the addition of appropriate amount of plasticizer to theembranes. The plasticizers to be used in membranes should

xhibit high lipophilicity, high molecular weight, low tendencyor exudation from the polymer matrix, low vapor pressurend high capacity to dissolve the substrate and other addi-ives present in the membrane and adequate dielectric constant43,44]. The addition of five plasticizers namely, DBP, o-NPOE,EHP, DOP and BA to the membranes of L1 (sensor no.–9) improved the working concentration range and the slopeTable 2). The best performance characteristics were obtainedith the membrane having DBP plasticizer (sensor no. 6). This

ensor exhibits the maximum working concentration range of.9 × 10−8 to 1.0 × 10−1 M with detection limit 5.6 × 10−8 Mnd a Nernstian slope of 19.8 mV/decade. Similar studies werelso performed with the membrane sensors of L2. In this case,he addition of o-NPOE exhibits best performance (sensor no.5) which showed the working concentration range 8.3 × 10−7

o 1.0 × 10−1 M with limit of detection 6.3 × 10−7 M and a slopef 19.5 mV/decade.

The response mechanism of neutral carrier-based sensorsepends mainly on the extraction equilibrium at the vicinityf the interface between the membrane and aqueous layer [45],s well as concentration of the ionophore complex in the PVCembrane. Therefore, the different amount of ionophores inVC membranes was tested. An ionophore content of 5 mgw/w) has been chosen as an optimum level because the sur-ace conditions of the PVC membranes were deteriorated onecreasing or increasing the amount of ionophore.

.6. Potentiometric selectivity

The influence of interfering ions on the response behaviorf ion-selective membrane electrode has usually described inerms of selectivity coefficient. The experimental selectivityoefficients depend on the activity and the method of theiretermination. Different methods of selectivity determinationave found in the literature. In the present study, the selectivityo-efficients were determined using the matched potentialethod (MPM) [46,47]. This method has an advantage of

emoving limitations imposed by Nicolsky–Eisenman equationhile calculating selectivity coefficient by other methods. These

imitations include non-Nernstian behavior of interfering ionnd problem of inequality of charges of primary and interferingons. In the matched potential method, the selectivity coefficientas determined by measuring the change in potential upon

A.K. Singh et al. / Analytica Chimica Acta 585 (2007) 171–178 175

Table 2Optimization of membrane composition

Membrane no. Composition of membrane (w/w) (mg) Working concentrationrange (M)

Detectionlimit (M)

Slope (mV/decade) Responsetime (s)

Ionophore Plasticizer Additive PVC

1 L1 (5) – 3 (NaTPB) 150 1.0 × 10−5 to 1.0 × 10−1 6.3 × 10−6 33.0 452 L1 (5) 150 (DOP) 3 (NaTPB) 150 6.6 × 10−6 to 1.0 × 10−1 3.9 × 10−6 32.0 323 L1 (5) 180 (BA) 3 (NaTPB) 150 2.5 × 10−6 to 1.0 × 10−1 1.4 × 10−6 28.0 304 L1 (5) 200 (NPOE) 3 (NaTPB) 150 1.0 × 10−6 to 1.0 × 10−1 7.0 × 10−7 25.0 205 L1 (5) 225 (TEHP) 3 (NaTPB) 150 2.8 × 10−7 to 1.0 × 10−1 1.7 × 10−7 23.0 156 L1 (5) 250 (DBP) 3 (NaTPB) 150 8.9 × 10−8 to 1.0 × 10−1 5.6 × 10−8 19.8 <107 L1 (4) 250 (DBP) 3 (NaTPB) 150 7.4 × 10−6 to 1.0 × 10−1 4.1 × 10−6 24 188 L1 (6) 250 (DBP) 3 (NaTPB) 150 4.7 × 10−7 to 1.0 × 10−1 3.0 × 10−7 22 239 L1 (7) 250 (DBP) 3 (NaTPB) 150 3.7 × 10−7 to 1.0 × 10−1 2.0 × 10−7 26 25

10 L2 (5) – 3 (NaTPB) 150 5.6 × 10−5 to 1.0 × 10−1 3.5 × 10−5 39.0 4211 L2 (5) 150 (DOP) 3 (NaTPB) 150 2.2 × 10−5 to 1.0 × 10−1 1.4 × 10−5 33.0 3512 L2 (5) 180 (BA) 3 (NaTPB) 150 7.9 × 10−6 to 1.0 × 10−1 5.0 × 10−6 30.0 3013 L2 (5) 200 (DBP) 3 (NaTPB) 150 3.9 × 10−6 to 1.0 × 10−1 2.2 × 10−6 26 2214 L2 (5) 225 (TEHP) 3 (NaTPB) 150 1.9 × 10−6 to 1.0 × 10−1 1.1 × 10−6 23.0 1815 L2 (5) 250 (NPOE) 3 (NaTPB) 150 8.3 × 10−7 to 1.0 × 10−1 6.3 × 10−7 19.2 1516 L2 (4) 250 (NPOE) 3 (NaTPB) 150 5.7 × 10−5 to 1.0 × 10−1 3.2 × 10−5 25 2717 L (6) 250 (NPOE) 3 (NaTPB) 150 6.6 × 10−5 to 1.0 × 10−1 4.7 × 10−6 23.0 331 1.8 × 10−5 to 1.0 × 10−1 1.2 × 10−5 26 40

ittb

K

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8 L2 (7) 250 (NPOE) 3 (NaTPB) 150

ncreasing the primary ion activity from an initial value of aAo a′

A and aB represents the activity of interfering ion added tohe reference solution of primary ion of activity aA which alsorings about same potential change. It has given by expression.

potA,B = a′

A − aA

aB

In the present studies aA and a′A were kept at 1.0 × 10−3 and

.0 × 10−3 M Cr3+ and aB was experimentally determined atH 3.0. The selectivity coefficient is expressed as the logarithmode and the negative value indicate a preference for the target

on relative to the interfering ion while positive value indicatehe preference of an electrode for the interfering ion. Of thewo sensors, selectivity of the sensor no. 6 was found to beetter and is thus compared with some good reported sensors inable 3. It has seen that the selectivity of the proposed sensor

oward chromium is better for most of the cations as comparedo all reported sensors and thus it is superior to all of them. Asensor no. 6 is better than the sensor no. 15 in terms of widerorking concentration range, low response time, high selectivity

nd Nernstian compliance, further studies have carried out witht only.

.7. Dynamic response and lifetime

It is known that the dynamic response time of a sensor is onef the most important factors in its evaluation. To measure theynamic response time of the proposed sensor the concentrationf the test solution was successively changed from 1.0 × 10−6 Mo 1.0 × 10−2 M. The resulting data depicted in Fig. 4, shows

hat the time needed to reach a potential with in ±1 mV of thenal equilibrium value after successive immersion of a seriesf Cr3+ ions, each having a tenfold difference in concentrations 10 s for L1 (sensor no. 6). The optimum conditioning time

tcei

ig. 4. Dynamic response of the membrane electrode based on ionophore (L1).

or the membrane electrodes in a 1.0 × 10−1 M CrCl3 solutionas 72 h after which it generated stable potentials with Cr3+ ion

olutions. The main factor responsible for the limited lifetime ofsensor is to be the loss of one or more of its components whileontacting with aqueous solutions. The lifetime of the sensoro. 6 was investigated by measuring potentials over a period of4 weeks. During this period, the sensor was used daily over anxtended period (1 h per day). The performances with respecto slope and detection limit were measured and results are sum-

arized in Table 4. The divergence in slope and detection limitrom 19.8 to 18.8 and 5.6 × 10−8 to 1.5 × 10−7 M) respectively,hus concluded that the membrane electrode was used practi-

ally for 3 months (sensor no. 6). However, it is important tomphasize that they were stored in 0.1 M Cr3+ solution when notn use.

176 A.K. Singh et al. / Analytica Chimica Acta 585 (2007) 171–178

Table 3Comparison of the Cr3+ selective electrodes with reported electrodes

Ionophore Working concentrationrange (M)

Detectionlimit (M)

Responsetime (s)

pH Selectivity coefficients Referenceno.

4-Amino-3-hydrazino-6-methyl-1,2,4-triazin-5-one

1.0 × 10−1 to 1.0 × 10−6 5.8 × 10−7 10 2.7–6.6 Fe3+ (−2.25), La3+ (−2.60), Ce3+ (−2.72),Cu2+ (−3.00), Cd2+ (−3.02), Co2+ (−3.14),Ni2+ (−3.07)

5

Glyoxal bis(2-hydroxyanil) 1.0 × 10−2 to 3.0 × 10−6 6.3 × 10−7 <20 2.7–6.5 Zn2+ (−2.16), Co2+ (−2.16), Pb2+ (−2.22),Na+ (−2.48), Sr2+ (−2.37), Ca2+ (−2.44)

12

Aurin TCA 1.0 × 10−1 to 7.0 × 10−6 N.M 10 3.5–6.5 Na+ (−1.21), Zn2+ (−1.88), Fe3+ (−0.649),Pb2+ (−1.88)

6

4-Dimethyl amino benzene 1.0 × 10−2 to 1.66 × 10−6 8.0 × 10−7 10 3.0–5.5 Cu2+ (−1.79), Ni2+ (−2.39), Fe3+ (−1.01),Ag+ (−1.00)

8

TTCT 1.0 × 10−1 to 1.0 × 10−6 7.0 × 10−7 15 3.0–5.5 Ca2+ (−2.60), Pb2+ (−3.09), Cu2+ (−3.00),Ag+ (−2.50), Co2+ (−2.80)

4

Tri-o-thymodite 1.0 × 10−1 to 4.0 × 10−6 2.0 × 10−7 15 2.8–5.1 Na+ (−0.65), Cd2+ (−0.65), Al3+ (−1.42),Pb2+ (−1.08), Cu2+ (−1.15), Zn2+ (−1.25),Mg2+ (−1.32)

11

Azamacrocycles 1.0 × 10−1 to 1.0 × 10−7 6.0 × 10−8 15 1.8–5.5 Na+ (−2.48), Sr2+ (−2.37), Mg2+ (−2.21),Pb2+ (−2.22), Co2+ (−2.16), Zn2+ (−2.16),Ca2+ (−2.44)

7

Oxalic acid bis(cyclohexylidenehydrazide)

1.0 × 10−2 to 1.0 × 10−7 6.3 × 10−8 <20 1.7–6.5 Na+ (−2.67), Cd2+ (−1.63), Ag+ (−2.39),Pb2+ (−3.22), Co2+ (−2.43), Mg2+ (−2.50),Ni2+ (−1.92), K+ (−2.50)

2

Schiff bases 8.0 × 10−3 to 1.5 × 10−6 1.0 × 10−6 <10 3.0–6.0 Hg2+ (−2.3), Al3+ (−2.6), Ag+ (−2.4), Cu2+

(−2.00), Ce3+ (−2.00)18

Tetraazacyclohexadecamacrocycle 1.0 × 10−1 to 1.6 × 10−6 N.M 18 3.0–6.5 Na+ (−0.75), Sr2+ (−1.12), Mg2+ (−1.62),Hg2+ (−2.15), Cu2+ (−1.57), Zn2+ (−0.647)

9

Proposed sensor (L1) 8. 9 × 10−8 to 1.0 × 10−1 5.6 × 10−8 10 2.0–7.0 Fe3+ (−3.41), La3+ (−3.97), Ce3+ (−3.56),Cu2+ (−3.91), Cd2+ (−3.68), Co2+ (−3.85),Ni2+ (−3.95), Na+ (−3.28), Pb2+ (−2.68),Hg2+ (−3.79), Sr2+ (−4.18), Ca2+ (−4.29),Al3+ (−2.92), Ag+ (−3.73), Zn2+ (−3.92),K+ (−3.98)

This work (L2) 8.3 × 10−7 to 1.0 × 10−1 6.3 × 10−7 22 2.0–5.5 Fe3+ (−2.85), La3+ (−3.88), Ce3+ (−3.40),Cu2+ (−3.95), Cd2+ (−2.08), Co2+ (−3.51),Ni2+ (−3.99), Na+ (−2.22), Pb2+ (−2.16),

3

o

TT

P

11111

.8. pH and non-aqueous effect

The pH dependence of the membrane sensor was testedver the pH range 1.0–8.0 at 1.0 × 10−2 M and 1.0 × 10−3 M

able 4he life time study of the Cr(III) selective membrane sensor no. 6

eriod (weeks) Slope (mV/decade) Detection limit (M)

1 19.8 5.6 × 10−8

2 19.8 5.6 × 10−8

3 19.8 5.6 × 10−8

4 19.8 5.6 × 10−8

5 19.6 5.9 × 10−8

6 19.7 6.1 × 10−8

7 19.5 6.7 × 10−8

8 19.3 7.5 × 10−8

9 19.4 7.9 × 10−8

0 19.2 8.5 × 10−8

1 19.0 1.0 × 10−7

2 18.8 1.5 × 10−7

3 17.7 6.9 × 10−6

4 17.2 1.8 × 10−6

oTisaoatpiaewHcr

3

o

Hg2+ (−3.85), Sr2+ (−4.24), Ca2+ (−4.16),Al3+ (−3.16), Ag+ (−3.75), Zn2+ (−3.62),K+ (−3.90)

f Cr(III) concentration, and the results are illustrated in Fig. 5.he response of all electrodes is independent from solution pH

n the range of about 2.0–7.0, which implies that the proposedensor can be used to measure a wide range of environmentalnd industrial water samples without pH adjustments. However,utside this range the electrode responses at pH < 2.0 seemsscribable to the competitive binding of proton to the ligand athe surface of the membrane electrode, while the diminishedotential at pH > 7.0 is to formation of chromium hydroxiden sample solution. The effect of non-aqueous medium haslso investigated by using the sensor no. 6 in methanol–water,thanol–water and acetonitrile–water mixtures. The sensororked satisfactorily up to 20% (v/v) of non-aqueous content.owever, above this non-aqueous content both working

oncentration range and slope decreased drastically and theesults were slightly irreproducible.

.9. Effect of surfactants

The presence of surfactants can result in the washing outf ionophore from the membrane to the aqueous phase, thus

A.K. Singh et al. / Analytica Chimica Acta 585 (2007) 171–178 177

sadTtt(tco

4

tcww

Fp

Fig. 7. Response of PVC-based membrane sensor no. 6 based on L1 in thepresence of SDS.

Table 5Quantification of chromium ion in different food samples using AAS and Cr3+

sensor

Samples ISEa (ppm) AASa (ppm)

Coffee 1.12 ± 0.01 0.98 ± 0.01Turmeric powder 1.58 ± 0.02 1.51 ± 0.04T

tdaa

Fig. 5. Effect of pH on cell potential of sensor no. 6 based on L1.

hortening the electrode lifetime significantly. As both ionicnd nonionic surfactants interact with the polymer membraneivided between the aqueous phase and the membrane phase.he performance of the electrode assembly has observed in solu-

ions contaminated with detergent matter. The concentration (upo 1.0 × 10−4 M) of cetyl trimethyl ammonium bromide (CTAB)Fig. 6) and sodium dodecyl sulphate (SDS) (Fig. 7) do not dis-urb the functioning of the membrane electrode, but at higheroncentration (5.0 × 10−4 M and above) they can be toleratedver reduced working concentration ranges.

. Analytical applications

The chromium selectivity exhibited by the membrane elec-rode (no. 6) makes it potentially useful for monitoring the

oncentration of Cr3+ in real samples. In this regard, experimentsere performed to measure Cr3+in different environmental (tap,aste, spring and well water) samples and food samples (coffee,

ig. 6. Response of PVC-based membrane sensor no. 6 based on L1 in theresence of CTAB.

HhcwbapAu

5

oCifsiitsf

ea leaves 0.94 ± 0.01 0.91 ± 0.02

a Results are based on triplicate measurements.

urmeric powder and tea leaves). The analysis of water samplesoes not require pretreatment except pH adjustment. The pH forll samples was adjusted at 4.0. For analysis of food samples,n amount of 1 g sample was accurately weighed and mixture ofNO3 and HClO4 (5:1) were added, followed by digestion on aot plate. After the addition of 5 mL of acid mixture, it was stirredontinuously until all the fumes ceased. An amount of 10 mL ofater was added and the solution was filtered to remove any tur-idity or suspended matter. The solution was made up to 50 mLnd analyzed by the proposed electrode. The results obtained areresented in Table 5 and compared with those obtained by usingAS. The comparison shows that the sensor can successfullysed to determine Cr(III) in real samples.

. Conclusion

The investigations on PVC-based membrane sensors basedn Schiff bases (L1 and L2) were used for quantification ofr3+ ions. The sensor developed using (L1) exhibit wide work-

ng concentration range, high sensitivity, long-term stability andast response over prolonged period as compared to (L2). Theelectivity of the membrane electrode (sensor no. 6) toward Cr3+

s quite good for most of the cations and the response character-stics of the proposed electrode are in a good comparison withhose previously reported electrodes. The proposed sensor wasuccessful in determination of chromium in different water andood samples.

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78 A.K. Singh et al. / Analytica C

cknowledgement

One of the authors Barkha Gupta is thankful to Ministry ofuman Resource Development (MHRD), New Delhi, India fornancial support.

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