Desalination 272 (2011) 278–285

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Desalination

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Analysis of mercury ions in effluents using potentiometric sensor based onnanocomposite cation exchanger Polyaniline–zirconium titanium phosphate

Asif Ali Khan ⁎, Leena PaquizaAnalytical and Polymer Research Laboratory, Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh 202002, India

⁎ Corresponding author. Tel.: +91 571 2720323.E-mail addresses: asifkhan42003@yahoo.com (A.A. K

(L. Paquiza).

0011-9164/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.desal.2011.01.039

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 November 2010Received in revised form 8 January 2011Accepted 12 January 2011Available online 23 February 2011

Keywords:Mercury-selective electrodePotentiometryPoly(vinyl chloride) membraneHybrid compositeZirconium titanium phosphatePolyaniline

The objective of the present research was to synthesize, characterize and to investigate the efficiency of anadvanced class of hybrid nano-composite cation exchanger for the detection of mercury(II) ion in aqueoussolution. In the present study the cation exchanger ployaniline–zirconium titanium phosphate (PANI–ZTP)nanocomposite, was synthesized via sol–gel mixing of organic polymer polyaniline (PANI) into the matrices ofthe inorganic precipitate of zirconium titanium phosphate (ZTP), having extraordinary ion exchange capacity,thus used as electroactive component for the construction of an ion-selectivemembrane electrode. The proposedelectrode shows fairly good discrimination of mercury ion over several other inorganic ions. The membraneelectrode was mechanically stable, having wide dynamic range of 10−10 mol/m3 to 10−1 mol/m3 of Hg2+ ions,with quick response time and could be operated for at least 4 monthswithout any considerable divergence in thepotential response characteristics. The electrode was successfully applied for direct determination of mercuryions in some real sample with satisfactory results and acts as indicator electrode in complexation titrations.

han), leenaz1@rediffmail.com

ll rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Heavy metals are natural components of the Earth's crust. They aredangerous because they tend to bioaccumulate and cannot be degradedor destroyed. The most pollutant heavy metals are Lead, Cadmium,Copper, Chromium, Selenium and Mercury. Normally Mercury is a toxicsubstance which has no known function in human biochemistry; itsexposure is still anoccupational hazard for people inmany industries andin environment. Over the last two decades, the need for highly sensitiveand selective mercury determination arises from its wide application inindustry and its long term toxicity towards human beings which causesmany harmful effects [1,2]. Various common methods are used todetermine mercury such as complexometry [3], spectrophotometry,flame and atomic absorption spectrometry (AAS), inductively coupledplasma (ICP), fluorimetry, X-ray fluorescence, voltammetry and poten-tiometry. The available method for low level determination of mercuryand other heavy metals in solution is AAS, but it involves expensiveinstrumentation and sample pretreatment, which is time consuming.

Among several available methods, ion exchange appeared to beinteresting because it exhibits a high efficiency of sorption fromgaseousand liquid media [4] and its application is relatively simple as mildoperating conditions are required. Hybrid ion-exchangemembranes areeither homogeneous or heterogeneous. Homogeneous ion-exchangemembranes are coherent ion-exchanger gels in the shape of disks,

ribbons, etc. The heterogeneous precipitate ion-exchange membranesconsist of suitable colloidal ion-exchanger particles as electroactivematerials embedded in a polymer (inert) binder, i.e., poly(vinylchloride) (PVC), epoxy resin (Araldite), or polystyrene, polyethylene,nylon etc., have been extensively studied as potentiometric sensors andused for the measurement of a wide variety of different ions, directly incomplex biological and environmental samples [5,6] as it offers greatadvantages such as speed, ease of preparation and procedure, nondestructive analysis, wide dynamic range and low cost. Ion sensorsemploying inorganic ion exchangers have been reviewed by Arnold [7]and Buck [8]. Recently, composite materials are used for the fabricationof various electrometric sensors for analytical purposes [9,10].

In this paper we report the electroanalytical applicability of highlyadvanced nano-composite cation exchanger polyaniline–zirconium tita-nium phosphate (PANI–ZTP), bimetallic tetravalent metal compositehaving extraordinary ion exchange capacity, in construction of mercuryion-sensor electrode. Three component inorganic ion-exchangers werefound to show relatively increased ion-exchange capacity and selectivity[11–15]. The proposed sensor exhibited significantly high sensitivity,stability, and selectivity for Hg(II) ions over many common ions and wassuccessfully used for determining Hg(II) ions in various matrices.

2. Experimental

2.1. Reagents and instruments

The main reagents used for the synthesis of the material wereobtained from CDH, Loba Chemie, E-merck and Qualigens (India Ltd.,

279A.A. Khan, L. Paquiza / Desalination 272 (2011) 278–285

used as received). All other reagents and chemicals were of analyticalgrade. Following instruments were used during present research work:A FTIR spectrophotometer (Perkin Elmer, USA, model Spectrum-BX);digital pH-meter (Elico Li-10, India); X-ray diffractometer ― Phillips(Holland), model PW 1148/89; UV/VIS spectrophotometer ― Elico(India),model EI 301E; a thermal analyzer―V2.2ADuPont9900; Carlo-Erba, model 1108; a digital potentiometer (Equiptronics EQ 609, India);accuracy±0.1 mV with a saturated calomel electrode as referenceelectrode; an electronic balance (digital, Sartorius-21OS, Japan) and anautomatic temperature controlledwater bath incubator shaker― Elcon(India).

2.2. Synthesis of composite cation-exchanger Polyaniline–Zirconiumtitanium phosphate (PANI–ZTP)

The composite cation-exchanger was prepared by sol–gel mixing ofpolyaniline (an organic polymer) into the inorganic precipitate ofzirconium titanium phosphate (ZTP) with varying mixing ratio asindicated in Table 1. The preparation method for inorganic precipitate ofzirconium titaniumphosphate (ZTP) is same as reported earlier [16]. Darkgreen colored polyaniline gel was prepared by oxidative coupling usingammonium persulphate in acidic aqueous medium. The precipitate ofpolyaniline was added into the white inorganic gel of ZTP with constantstirring. Black coloredprecipitate obtainedwasallowed to settleovernightwas filtered off and then washed thoroughly with demineralized water(DMW) to remove excess acid and any adhering ions (chloride andsulphate).The washed gel was dried over P4O10 at 30 °C in an oven andsieved to obtain small shiny black granules of PANI–ZTP. Thematerialwasconverted into H+-form by treating it with 1 mol/m3 HNO3 for 24 hwithoccasional shaking and intermittently changing the acid. The excess acidwaswashedwithDMWandthenfinallydriedat40 °C.On thebasisofNa+

exchange capacity (I.E.C.), sample PA-7 was selected for further studies.

2.3. Ion-exchange capacity

The ion-exchange capacity of various samples was determined bycolumn operation. Exchanger in the H+-form was placed in a columncontaining a glass wool support. Sodium nitrate solution (1.0 mol/m3)was used as an eluent and about 450 ml of it was passed through the ionexchanger columncontaining1 gof the exchanger at a rate of 5–10 drops/min. Hydrogen ions eluted from the column were determined titrime-trically against standard solution of sodium hydroxide.

2.4. Regeneration of ion exchanger

Exhausted exchanger was regenerated by keeping it overnight inhydrochloric acid (0.1 mol/m3). It was then washed with DMW, till it

Table 1Conditions of preparation and the ion-exchange capacity of various sample of PANI–ZTP co

Sample Mixing volume ratio (v/v) (inorganic) Mixing volu

0.1 mol/m3 TiCl4in H2SO4

0.1 mol/m3 ZrOCl2in H2SO4

Na2HPO4 InDMW

0.1 mol/m3

in 1 mol/m3

PA-1 1 (1 mol/m3) 1 (1 mol/m3) 1 (2 mol/m3) 1PA-2 1 (0.1 mol/m3) 1 (4 mol/m3) 2 (1 mol/m3) 2PA-3 1 (0.1 mol/m3) 1 (2 mol/m3) 2 (0.2 mol/m3) 1PA-4 1 (1 mol/m3) 1 (1 mol/m3) 2 (1 mol/m3) 1PA-5 1 (1 mol/m3) 1 (1 mol/m3) 1 (2 mol/m3) 1PA-6 1 (1 mol/m3) 1 (2 mol/m3) 2.5 (2 mol/m3) 1PA-7 1 (0.2 mol/m3) 1 (0.1 mol/m3) 2 (0.2 mol/m3) 1PA-8 1 (0.2 mol/m3) 1(0.1 mol/m3) 2 (.2 mol/m3) 0.25PA-9 1 (0.1 mol/m3) 1 (0.1 mol/m3) 2 (0.2 mol/m3) 1PA-10 1 (0.1 mol/m3) 1 (0.1 mol/m3) 2 (1 mol/m3) 1PA-11 1 (0.2 mol/m3) 1 (0.1 mol/m3) 2 (0.2 mol/m3) 1PA-12 1 (0.2 mol/m3) 1 (0.1 mol/m3) 2 (0.2 mol/m3) –

PA-13 – – – 1

became neutral. The exchange capacity was determined and thisprocedure was repeated five times. It was seen that after five regenera-tions, ion exchanger loses 35% of its original capacity.

2.5. Characterization of hybrid composite material

2.5.1. Scanning electron microscopyScanning electron microscope images of the samples were obtained

by dispersing the powder on a double face conducting tape fixed on abrass studs.

2.5.2. Transmission electron microscopyThe particle size of the prepared composite material was studied

by TEM.

2.6. Sorption studies

The distribution behavior of metal ions plays an important role inthe determination of the material's selectivity. In certain practicalapplications, equilibrium is most conveniently expressed in terms ofthe distribution coefficients of the counter ions. The determination ofmetal ions before and after equilibriumwas carried out volumetricallyusing EDTA as the titrant [17]. 200 mg of the dry exchanger in H+

form were equilibrated with 20 ml of different metal ion solutions inthe requiredmedium andwas kept for 24 hwith intermittent shaking.The initial metal ion concentration (0.01 mol/m3) was so adjustedthat it did not exceed 3% of the total ion exchange capacity of thematerial.

The distribution coefficient (Kd) values were calculated by usingthe formula given below:

Kd = I � Fð Þ=F½ �×V=M ml g�1� �

ð1Þ

where ‘I’ is the initial amount of metal ion in the aqueous phase, F isthe final amount of metal ion in the aqueous phase, V is the volume ofthe solution (ml) and M is the amount of cation-exchanger (g).

2.7. Preparation and characterization of composite membrane

The method used by [18] was employed in the preparation of ionexchange membranes of PAZTP. A number of membranes wereprepared using varying amounts of different binding materials such asaraldite and PVC. The preparation method was same as reported [19]earlier. Those membranes which exhibited good surface qualities, likeporosity, thickness, swelling etc. as described elsewhere [20] wereselected for further investigations.

mposite cation exchanger.

me ratio (v/v) (organic) Appearance of thebeads after drying

Na+ ion exchangecapacity (meq/g)

AnilineHCI

(NH4)2S2O8 in1 mol/m3 HCl

1 Black shiny granules 3.71 Black shiny granules 1.122 Dark blackish purple granules 1.61 Greenish granules 0.721 Dull purple crystal 1.721 Greenish granules 0.5122 Black shiny granules 4.522 Black shiny granules 4.22 Blackish purple crystal 3.011 Dark green crystal 2.81 Greenish granules 2.5– White Granules 3.361 Dark Green Granules 0.20

280 A.A. Khan, L. Paquiza / Desalination 272 (2011) 278–285

2.7.1. Conditioning of the membraneThe membranes were conditioned by equilibrating with 1 mol/m3

sodium chloride; about 1 ml of sodium acetate was also added toadjust the pH 5–6.5 (to neutralize the excess acid present in the film).

2.7.2. Water content (% total wet weight)The conditioned membranes were first soaked in water to elute

diffusible salts, blotted quickly with Whatman filter paper to removesurface moisture and immediately weighed. These were further driedand weighed. The water content (% total wet weight) was calculatedusing the given formula:

% Total wet weight=Ww−Wd

Ww×100 ð2Þ

where Wd = weight of the dry membrane and Ww = weight of thesoaked/wet membrane.

2.7.3. PorosityPorosity (ε) was determined as the volume of water incorporated

in the cavities per unit membrane volume from the water contentdata using formula:

ε=Ww−Wd

AL ρwð3Þ

where Ww = weight of the soaked/wet membrane, Wd = weight ofthe dry membrane, A = area of the membrane, L = thickness of themembrane and ρw = density of water.

2.7.4. Thickness and swellingThe thickness of the membrane was measured by taking the

average thickness of the membrane by using screw gauze. Swelling ismeasured as the difference between the average thicknesses of themembrane equilibrated with 1 mol/m3 NaCl for 24 h and the drymembrane.

2.7.5. EMF measurementsThe use of ion-selective electrodes depends on the determination of

potentials. The potentials cannot be determined directly but can beeasily derived from theEMF (Electrodepotential) E, in electrochemistry,according to an IUPACdefinition, is the electromotive force of a cell builtof two electrodes. The electromotive force (EMF) is the maximumpotential differences between two electrodes. It is the potentialdifference when no current is flowing and can be measured bypotentiometer.

The membrane potential measurement is carried out using a cellset out of the following types:

Solution 2

Solution 1

External saturatedcalomel electrode(SCE)

Test orexternalsolution

Membrane

Internalsolution

Internal saturatedcalomel electrode(SCE)

EL (2)

EL (1)

SCE is saturated calomel electrode which acts as a referenceelectrode based on the reaction between elemental mercury andmercury(I) chloride. The aqueous phase in contact with the mercuryand the mercury(I) chloride (Hg2Cl2, “calomel”) is a saturatedsolution of potassium chloride in water. This electrode and thesilver/silver chloride reference electrode work in the same way. Inboth electrodes, the activity of the metal ion is fixed by the solubilityof the metal salt. The SCE is used in pH measurement, cyclicvoltammetry and general aqueous electrochemistry.

In general practice, the concentration of one of the solutions (say1) is kept constant (usually 0.1 mol/m3) and this solution is referred

as an internal or reference solution and a SCE is dipped in this internalsolution as an internal reference electrode. The membrane togetherwith internal solution and internal reference electrode is one compactunit, which as whole is called as membrane electrode. This membraneelectrode is then immersed in solution 2, usually referred as externalsolution or test solution, having an external reference electrode.

The EMF of this potentiometric cell is given by the followingexpression:

Ecell=ESCE+EL 2ð Þ+Em+EL 1ð Þ–ESCE ð4Þ

where ESCE, EL and Em refer to Calomel Electrode, junction andmembrane potentials, respectively.

Membranes were fixed to one end of a Pyrex glass tube (o.d.1.6 cm, i.d. 0.8 cm) using PVC/araldite as adhesive. These were thenequilibrated with Hg2+ solution (0.1 mol/m3) for 5–7 days. The tubewas filled 3/4th with Hg(NO3)2 solution (0.1 mol/m3) and thenimmersed in a beaker containing the test solution of varyingconcentration of Hg2+ion, keeping the level of inner filling solutionhigher than the level of the test solution to avoid any reverse diffusionof the electrolyte. All the EMF measurements were carried out usingthe following cell assembly:

SCE j0:1mol =m3Hg2+ jj Membrane jj 0:1mol=m3Hg2+ test solutionð ÞjSCE

Potentiometric measurements were observed for a series ofstandard solutions of Hg(NO3)2 (10−10–10−1 mol/m3), prepared bygradual dilution of the stock solution, as described by IUPACCommission for Analytical Nomenclature [21]. Potential measure-ments were made in unbuffered solutions to avoid interference fromany foreign ion. In order to study the characteristics of the electrode,the following parameters were evaluated: lower detection limit, sloperesponse curve, response time and working pH range. The calibrationgraphs were plotted three times to check the reproducibility of thesystem.

2.7.6. Effect of pHA series of pH solution ranging from 1–13 were prepared for

1×10−4 mol/m3 and 1×10−3 mol/m3 ion concentrations. The pHvariations were brought about by the addition of dilute acid (HCl)and alkali (NaOH) solution. The value of electrode potential at eachpH was recorded and was plotted against the pH.

2.7.7. The response timeThe response time was measured by recording the EMF of the

electrode as a function of timewhen it was immersed in the solution tobe studied. The initial potential of the solution was read at zero secondwhen the electrodewasdipped into 1×10−2 mol/m3 test solutionofHg(NO3)2 and subsequently recorded at the intervals of 5 s.

2.7.8. Selectivity coefficient of interfering cationsOne of the most important characteristics of a membrane sensor is

its response for the primary ion in the presence of other foreign ions,which is measured in terms of the potentiometric selectivitycoefficient (KAB

pot). In the present work we used the fixed interferencemethod, which is one of the mixed solution methods [22]. Theselectivity coefficient was calculated using the equation given below:

KpotAB =aA= aBð ÞzA=zB ð9Þ

where aA and aB activities of primary and interfering ion and zA and zBare charges on the ions.

2.7.9. Storage of electrodesThe polyaniline-based composite electrode was stored in distilled

water when not in use for more than one day. It was activated with

281A.A. Khan, L. Paquiza / Desalination 272 (2011) 278–285

(0.1 mol/m3) Hg(II) solution by keeping immersed in it for 2 h, beforeuse, to compensate for any loss of metal ions in the membrane phasethat might have taken place due to a long storage in distilled water.Electrode was then washed thoroughly with DMW before use.

2.8. Analytical application of the electrode

The analytical utility of this membrane electrode has beenestablished by employing it as an indicator electrode in thepotentiometric titration of a 0.01 mol/m3 Hg(NO3)2 solution against0.005 mol/m3 EDTA solution and 0.005 mol/m3 Hg(NO3)2 was titratedwith 0.01 mol/m3 oxalic acid. Potential values are plotted against thevolume of EDTA/Oxalic acid.

Fig. 1. TEM of PANI–ZTP showing different particle size.

Fig. 2. (a) Calibration curve for PANI–ZTP membrane (M-3) electrode in aqueoussolution of Hg(NO3)2, (b) Nerstian value of linear working range of calibration plot.

3. Results and discussion

A number of samples of ‘organic–inorganic’ electrically conductingcomposite cation-exchanger polyaniline–zirconium titanium phos-phate (PANI–ZTP) were prepared by the sol–gel mixing of electricallyconducting polymer polyaniline with tetravalent bimetallic inorganicion-exchanger zirconium titanium phosphate in different mixingvolume ratios. The Na+ ion-exchange capacity of sample PA-7(Table 1)was observed to be4.52 meq dry g−1. This compositematerialPA-7 possessed a better Na+ ion-exchange capacity as compared toinorganic ion-exchanger, zirconium titanium phosphate (3.36 meq -dry g−1) [23]. In this study, polyaniline gel was prepared by oxidativecoupling using (NH4)2S2O8 in acidic aqueous medium as given below[24]:

The effect of temperature on the reaction seems to be verypronounced. Aniline underwent oxidative coupling only at below10 °C very effectively, leading to a good yield of polyaniline. The bindingof polyaniline into the matrices of zirconium titanium phosphate (ZTP)can be represented as:

However, PANI–ZTP (PA-7) exhibited granulometric propertieswitha good reproducible behavior as is evident from the fact that thesematerials obtained from various batches did not show appreciabledeviation in their percentage of yield and ion-exchange capacity. Theaverages and deviation of the ion exchange capacity and yield werefound to be 4.52 meq g−1 and ±0.03%, respectively. It can beregenerated andused over and over again. Even afterfive regenerations,the ion exchanger loses only 35% of its original capacity. The ionexchange behavior, characterization and its application in separation oftoxic metal ions from synthetic mixture has been already explored inour communicated paper [25].

A scanning electron microscopy (SEM) study was performed toexamine the difference in surface morphology between the parentmaterials and their composites. SEM photographs of the PANI (PA-10),ZTP (PA-9) and PANI–ZTP (PA-7) obtained at different magnifications(Fig. 1), indicates the binding of inorganic material with organic polymer,i.e. polyaniline. The SEM pictures showed that the surface morphology ofcomposite material is totally different from their individual inorganic andorganic components. The morphology of the composite material is

essentially different due to the binding of polyaniline with zirconiumtitanium phosphate.

From TEM micrograph, it is clear that the particle size of thecomposite polymer PANI–ZTP is in the range of 21 nm–77 nm whichlies in nano-range (Fig. 2) thus, the proposed cation exchanger is anano-composite material.

Table2

Kdva

lues

ofsomemetal

ions

onPA

NI–ZT

Pcation

exch

ange

rco

lumnin

differen

tsolven

tsystem

.

Metal

ions

DMW

10−

1mol/m

3

HNO3

10−

2mol/m

3

HNO3

10−

3mol/m

3

HNO3

10−

1mol/m

3

H2SO4

10−

2mol/m

3

H2SO

4

10−

1mol/m

3

HCl

10−

2mol/m

3

HCl

10−

3mol/m

3

HCl

pH5.75

10%

HCO

OH

10−

2mol/m

3

HClO4

10%

C 2H5O

H20

%Acetone

Buffe

r10

Ba2+

150

3964

8486

9584

6225

0–

3216

345

9182

Mg2

+12

765

2554

4626

3345

1715

611

1646

4440

0Cu

2+

162

9711

613

635

101

3044

155

226

8024

136

136

144

Cd2+

159

2979

2933

15–

9342

172

7011

652

526

358

0Co

2+

115

TA27

8710

781

5010

914

679

5–

7626

123

–

Sr2+

7080

3848

312

130

–16

110

189

4753

22–

110

Ca2+

1175

9012

849

149

100

8416

226

711

9215

7–

290

Mn2

+67

––

26–

38–

–74

132

120

–84

4841

9Pb

2+

710

194

600

750

263

345

6423

043

475

620

477

568

7TA

–

Hg2

+17

3435

510

00TA

387

728

195

178

800

1200

7918

58TA

2000

700

Al3+

1012

315

815

032

18–

105

616

732

26–

309

308

Ni2+

6311

749

219

140

3890

––

136

–58

79–

26Cr

3+

8627

3–

200

7375

240

160

110

490

220

7595

–20

0Th

4+

63–

124

234

–66

717

189

108

–23

470

187

215

–

Zn2+

4245

288

48–

2638

7690

109

118

2711

812

4Fe

2+

–27

0–

112

255

3380

106

–56

––

127

229

146

282 A.A. Khan, L. Paquiza / Desalination 272 (2011) 278–285

The results of sorption studies (Table 2) indicated that Kd valuesvaried with the nature and composition of contacting solvents. It wasalso observed from the distribution studies (Kd values) that the Hg2+

was highly adsorbed in all solvents, while remaining metal ions werepoorly adsorbed. The high uptake of mercury ions in all solventsdemonstrates not only the ion-exchange properties but also theadsorption and ion-selective characteristics of the cation-exchanger.Thus, we can say that this composite cation exchanger is highlyselective for mercury ions and can be very well utilized for thedetermination and separation of mercury ions from waste effluents.

On this basis, PANI–ZTP has been used as an electro-activecomponent in the preparation of the heterogeneous solid-stateelectrode sensitive to Hg2+ ions. Ion-selective electrodes work onthe principle of measurements at zero current. The membranes werefixed in the electrode assembly and all measurements are made in aconcentration cell. The concentration of the electrolyte on the innerside of the membrane was fixed at 0.1 mol/m3 of Hg2+ ions whileouter solution varied from 10−12 mol/m3 to 10−1 mol/m3. When ionspenetrated the boundary between the two phases leading to theattainment of electrochemical equilibrium, the potentials developed.

3.1. Optimization of membrane ingredients

It is well known that the sensitivity, linear dynamic range, andselectivity of the ISEs depend not only on the nature of the carrierused, but also significantly on the membrane composition, propertiesof the additives employed as well as the adhesive/plasticizer ratioused [26]. It should be noted that plasticizer acts as membrane solventaffecting membrane selectivity and also provides mobility of themembrane constituents within the membrane phase. In the presentstudy, dioctylpthalate was used as plasticizer.

Influence of the membrane composition and possible interferingions was investigated on the response properties of the electrode. Theslope of the calibration curve, the measurement range and responsetime were noted for a number of electrodes having different composi-tions of binders and the exchanger. The results are shown in Table 3.Among the variousmembranesprepared using the ion-active phase, theplasticized PVC-based membrane electrode was found to give the bestsensitivity and widest linear range. Hence, membrane electrode no. 3(M-3) was chosen for further electroanalytical studies.

3.2. Calibration range

Using the optimized membrane composition described above, thepotentiometric response of the sensor was studied for Hg2+ in theconcentration range of 10−12 mol/m3 to 10−1 mol/m3 at 25 °C asshown in Fig. 3. The results showed a Nernstian response of 29.8 mV/decade of Hg2+ concentration, and the wide linear range within theconcentration ranges from 10−10 mol/m3 to 10−1 mol/m3 of Hg2+

ions. Experiments were conducted a number of times to check thereproducibility of the results. EMFs were plotted against log ofactivities of mercury ions and calibration curves were drawn for fivesets of experiments and a standard deviation of ±0.3 mV wasobserved. The detection limit of sensor was determined according toIUPAC recommendations from the intersection of two extrapolatedlinear portions of the curve [27] and was found to be 5×10−10.Toevaluate the reversibility of the electrode, a similar procedure at theopposite direction was adopted. This time, measurements wereperformed in the sequence of high-to-low sample concentrationsand very similar results were obtained.

3.3. Response, reproducibility and lifetime

The influence of the pH of the test solution on the potentialresponse of the membrane sensor was investigated over a pH range of1–12. The pH was adjusted with dilute hydrochloric acid and sodium

Table 3Characterization of ion-exchanger membranes of PANI–ZTP.

S.no.

Membrane composition Characterization of membrane Slope(mV/decade)

Working range(M)

Responsetime (s)

PAZTP (%) Binder (%) Plasticizer (drops) Thickness (mm) total wet weight (%) Porosity (−) Swelling (%)

M1. 10 PVC(20) 10 0.30 1.5385 0.00259 No swelling 26.37 1×10−9–1×10−2 14M2. 20 PVC(20) 10 0.35 1.0128 0.0045 1.0038 27.12 5×10−9–1×10−1 15M3. 20 PVC(30) 10 0.28 0.6529 0.00173 0.0547 28.58 1×10−10–1×10−1 10M4. 25 PVC(30) 10 0.59 1.8915 0.0029 1.6412 26.3 1×10−8–1×10−1 20M5. 10 Araldite(10) – 0.34 0.7889 0.00154 0.5581 27.61 6×10−9–1×10−1 15M6. 20 Araldite(10) – 0.44 1.1221 0.00321 1.1265 26.83 1×10−8–1×10−2 18M7. 30 Araldite(10) – 0.78 1.4367 0.00232 2.0122 25.74 5×10−6–1×10−1 22

283A.A. Khan, L. Paquiza / Desalination 272 (2011) 278–285

hydroxide solutions (as required). The influence of pH on the responseof the membrane sensor is shown in Fig. 4. The results showed that thepotential is independent of pH in the range of 2.5 to 6.0, beyond whichthe potential changes considerably. The dependency of the potential tothe pH values out of this range can be attributed to the formation of

Fig. 3. The influence of pH on the potential response of the membrane electrode at1×10−3 and 1×10−4 M Hg2+ ion.

Fig. 4. Dynamic response time of PANI–ZTP electrode for different Hg2+ concentration:(A) 1×10−10 M (B) 1×10−9 M (C) 1×10−8 M (D) 1×10−7 M (E) 1×10−6 M(F)1×10−5 M (G) 1×10−4 M (H) 1×10−3 M (I) 1×10−2 M and (J) 1×10−1 M.

some hydroxyl complexes of mercury(II) ion [28] at higher pH valuesand to the hydrogen ion response at lower pH values.

For analytical applications, the response time of a sensor is animportant factor. The response time for the Hg2+-selective electrode toattain a response that is within ±1 mV of steady state potential aftersuccessive immersion of the electrodes in a series of mercury solutions,each having a 10-fold difference in concentration from10−10 mol/m3 to10−1 mol/m3 was investigated. The electrode showed reasonably fastand stable potential within 10 s and no change was normally observedup to 5 min after which it started deviating (Fig. 5).

The long-term stability was worked out by performing calibrationsperiodically with standard solutions and calculating the slopes overthe concentration ranges of 10−10 mol/m3 to 10−1 mol/m3 of Hg(NO3)2 solutions over a period of 180 days. During this period, theelectrodes were in daily use over an extended period of time (1 h/day), and the results are provided in Table 4. The experimental resultsshowed that the electrode response was quite reproducible over thelifetime of 120 days (Table 4) after that a very slight gradual decreasein the slopes and working range was observed. Subsequently, theelectrochemical behavior of the sensor gradually deteriorated whichmay be due to aging of the adhesive (PVC), the plasticizers, and theelectroactive material [29]. A decrease in slope was a symptom of lossof the normal characteristics of the electrodes.

3.4. Potentiometric selectivity

The selectivity behavior is obviously one of the importantcharacteristics of the ion-selective electrodes, determining whetherreliable measurement in the target sample is possible or not. In orderto assess the selectivity preference of the membrane for an interfering

Fig. 5. Application of the electrode based on PANI–ZTP for potentiometric titration of(A) 0.01 M Hg(NO3)2 with 0.005 M EDTA and (B) 0.005 M Hg(NO3)2 with 0.01 M oxalicacid.

Table 4The stability of the Hg2+ selective PANI–ZTP membrane electrode.

Time (Day) Slope (mV decade−1) Working range(mol/m3)

1 28.58±0.3 1×10−10–1×10−1

5 28.58±0.3 1×10−10–1×10−1

10 28.58±0.2 1×10−10–1×10−1

15 28.58±0.2 1×10−10–1×10−1

30 28.55±0.2 1×10−10–1×10−1

50 28.38±0.05 1×10−10–1×10−1

70 28.22±0.04 1×10−10–1×10−1

90 28.14±0.03 1×10−10–1×10−1

120 28±0.03 1×10−10–1×10−1

150 27.91±0.2 5×10−9–1×10−1

180 27.75±0.3 1×10−9–1×10−1

Table 5The selectivity coefficient of various interfering cations for Hg2+ selective PANI–ZTPmembrane electrode.

Interfering ion (Mn+) Selectivity coefficients (KMSM)

K+ 5×10−6

Na+ 5×10−6

Mg2+ 5×10−6

Cu2+ 3.8×10−4

Pb2+ 3.8×10−4

Ca2+ 5.5×10−7

Al3+ 1×10−5

Sr2+ 2.7×10−5

Mn2+ 2.7×10−5

Fe3+ 1×10−5

Ni2+ 8×10−7

Zn2+ 8×10−7

Cd2+ 7×10−4

Table 7Potentiometric determination of Hg2+ ion in different real sample.

Sample Added(mol/m3)

Found(mol/m3)

Recovery(%)

AAS(mol/m3)

Tap water 0 bLOD – bLOD1.0×10−6 1.0×10−6

(±0.03)100 1.2×10−6

(± 0.03)Waste water 0 5×10−7

(± 0.06)– 6.8×10−7

(± 0.03)5.0×10−6 4.8×10−6

(± 0.04)97.0 4.95×10−6

(± 0.06)Synthetic mixture Pb2+,Cu2+,Ca2+, Cd2+ and Hg2+ (3:2)

– 1.9×10−6

(± 0.06)98.0 2.11×10−6

(± 0.05)

aThe values based on five replicate analysis.

284 A.A. Khan, L. Paquiza / Desalination 272 (2011) 278–285

ion relative to Hg(II) was determined by the mixed solution method(MSM). It is evident from Table 5 that most of the interfering ionsshowed low values of selectivity coefficient, indicating no interferencein the performance of the membrane electrode assembly. Suchremarkable selectivity of the proposed ion-selective electrode overother ions reflects the high affinity of the membrane toward themercury ions.

Table 6 compares the working concentration range, response time,life time, pH range and potentiometric selectivity of the proposedelectrode with other reported mercury ion-selective electrode [30–38].The results clearly indicated the superiority of the proposed electrode interms of linear range, pH and response behavior.

4. Analytical applications

The proposed membrane sensor was found to work well underlaboratory conditions.

Table 6Comparison of the response characteristics of different Hg2+ sensors with the proposedsensor.

Reference Linear range (mol/m3) Life time(months)

Responsetime(s)

pHrange

30 1.0×10−1–1.0×10−7 1 40 4.5–7.031 1.0×10−1–1.0×10−6 3 1 2.0–11.532 1.0×10−1–1.0×10−6 3 40 4.0–9.033 8.0×10−2–6.2×10−7 2 30 1.0–4.034 1.0×10−2–1.0×10−5 Not mentioned 20 1.5–7.535 1.0×10−1–5.0×10−6 Not mentioned 20–25 1–236 1.0×10−1–5.0×10−6 5 20 2.5–4.037 1.0×10−1–1.0×10−6 2 30 1.0–3.038 1.0×10−3–1.0×10−6 Not mentioned 30–40 2.0–4.5Proposedassembly

1.0×10−1–1.0×10−10 4 10 2.5–6.0

The newmercury-selective electrode was satisfactorily applied forthe determination of mercury in various samples from differentsources. The samples were collected by a routine technique, preservedby acidification with HNO3 and analyzed within 24 h of collection.Each sample was analyzed in triplicate, using the sensor by standardaddition method. The results given in Table 7, shows that the amountof mercury recovered with the help of the sensor are in goodagreement with that determined by atomic absorption spectrometry(AAS), thereby reflecting the utility of the proposed method.

The sensor was also successfully applied as indicator electrodes inthe potentiometric titration of Hg2+ ion solutionwith EDTA and oxalicacid. The addition of titrants causes a decrease in potential as a resultof the decrease in free Hg(II) ion concentration due to formation of acomplex with EDTA/oxalic acid (Fig. 6). The titration curves showedgood inflection point at the equivalence point, showing perfectstoichiometry. Titration curve ‘A’ represents the decreasing trend ofpotential response, on addition of lower volume of the titrant to thehigher volume of it, and the curve ‘B’ represents vice versa.

5. Conclusion

The proposed potentiometric sensor of nano-composite cationexchanger, PANI–ZTP, has good operating characteristics includingNernstian response, reasonable detection limit, relatively high selectivity,wide dynamic range and fast response. These characteristics and thetypical applications presented in this paper make the sensor suitable formeasuringHg(II) content in real sampleswithout a significant interactionfrom cationic or anionic species. A comparison between the responsecharacteristics of the proposed potentiometric sensor and those ofpreviously reported mercury ion-selective electrode indicated that thepresent sensor is invariably superior.

Acknowledgment

The authors are thankful to Department of Applied Chemistry, Z.H.College of Engineering and Technology, A.M.U. (Aligarh) for providingresearch facilities andUniversity Grant Commission (India) for financialassistance.

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