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Talanta 72 (2007) 699–710

reparation and characterization of a new organic–inorganic nano-compositepoly-o-toluidine Th(IV) phosphate: Its analytical applications as

cation-exchanger and in making ion-selective electrode

Asif Ali Khan ∗, Anish Khan, InamuddinAnalytical and Polymer Research Laboratory, Department of Applied Chemistry, Faculty of Engineering and Technology,

Aligarh Muslim University, Aligarh 202002, India

Received 16 October 2006; received in revised form 29 November 2006; accepted 29 November 2006Available online 20 February 2007

bstract

An organic–inorganic hybrid poly-o-toluidine Th(IV) phosphate was chemically synthesized by mixing ortho-tolidine into the gel of Th(IV)hosphate in different mixing volume ratios, concentration of inorganic reactant with a fixed mixing volume ratios of organic polymer. Thehysico-chemical characterization was carried out by elemental analysis, TEM, SEM, XRD, FTIR and simultaneous TGA–DTA studies. The ion-

xchange capacity, chemical stability, effect of eluant concentration, elution behavior and pH titration studies were also carried out to understandhe ion-exchange capabilities. The distribution studies revealed that the cation-exchange material is highly selective for Hg2+, which is an importantnvironmental pollutant. Due to selective nature of the cation-exchanger ion-selective membrane electrode was fabricated for the determination ofg(II) ions in solutions. The analytical utility of this electrode was established by employing it as an indicator electrode in electrometric titrations.2006 Elsevier B.V. All rights reserved.

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eywords: Organic–inorganic hybrid; Cation-exchanger; Poly-o-toluidine Th(I

. Introduction

Organic–inorganic hybrid, a new class of materials is attrac-ive for the creating high performance or high functionalolymeric behaviors that are expected to provide many pos-ibilities. Hybrid can be used to modify organic polymericaterial or to modify inorganic materials that exhibit very dif-

erent properties from their original component. The inorganicon-exchange materials besides other advantages are importantn being more stable to high temperature and radiation fieldhan the organic ones [1]. In order to obtain a combination ofhese advantages associated with polymeric and inorganic mate-ials as ion-exchangers, attempts have been made to developolymeric-inorganic composite ion-exchangers by incorpora-ion of organic monomers in the inorganic matrix [2]. Few such

xcellent ion-exchange materials have been developed in ouraboratory and successfully being used in chromatographic tech-iques [3–5]. An inorganic precipitate ion-exchanger based on

∗ Corresponding author. Tel.: +91 571 2720323.E-mail address: asifkhan42003@yahoo.com (A.A. Khan).

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039-9140/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.talanta.2006.11.044

osphate

rganic polymeric matrix must be an interesting material, ast should possess the mechanical stability due to the presencef organic polymeric species and the basic characteristics ofn inorganic ion-exchanger regarding its selectivity for somearticular metal ions [6–12]. It was therefore considered to syn-hesize such hybrid ion-exchangers with a good ion-exchangeapacity, high stability, reproducibility and selectivity for heavyetal ions, indicating its useful environmental application.Mercury is responsible for causing poisoning through water,

ood and smoking. Mercury contamination in drinking waternhibits the function of certain enzymes necessary for the for-

ation of haem in bone marrow, the pigment that combines withrotein to form haemoglobin. Its vapor, on inhalation, entershe brain through the blood stream and causes severe damageo the central nervous system. Inorganic mercuric compounds

ainly attack liver and kidney. Mercuric chloride is corro-ive and, when ingested, precipitates proteins of the mucousembrane causing ashen appearance of the mouth, pharynx

nd gastric mucus. Organic mercurial are the mist toxic sub-tances; the CH3Hg+ can pass through the placental barriernd enter the foetal tissues. Hg(II) is therefore a potentialollutant in the environment. Heavy metal ion removal from

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pnitrate (NaNO3) solutions of varying molar concentration werepassed through a column containing 1 g of the exchanger inthe H+-form with a flow rate of ∼0.5 ml min−1. The efflu-

00 A.A. Khan et al. / Ta

aters has been the subject of extensive technological research13].

The ion-exchange membranes obtained by embedding ion-xchangers as electroactive materials in a polymer binder, i.e.VC, has been extensively used as potentiometric sensors,

.e. ion sensors, chemical sensors or more commonly ion-elective electrodes. In our present studies attempt has beenade to obtain a new heterogeneous precipitate based mem-

rane electrode by using the poly-o-toluidine Th(IV) phosphate,nano-composite cation-exchanger as an electroactive mate-

ial for the determination of Hg(II) ion present in the sampleolution.

. Experimental

.1. Reagents and instruments

The main reagents used for the synthesis of the materialere obtained from CDH, Loba Chemie, E-Merck and Quali-ens (India Ltd., used as received). All other reagents andhemicals were of analytical grade. Following instruments weresed during present research work: a FTIR spectrophotometerPerkin-Elmer, USA, model Spectrum-BX); digital pH-meterElico Li-10, India); X-ray diffractometer—Phillips (Holland),odel PW 1148/89; UV/VIS spectrophotometer—Elico, modelI 301E; a thermal analyzer—V2.2A DuPont 9900; an elemen-

al analyzer—Elementary Vario EL III, Carlo-Erba, model 1108;digital potentiometer (Equiptronics EQ 609, India); accu-

acy ± 0.1 mV with a saturated calomel electrode as referencelectrode; an electronic balance (digital, Sartorius-21OS, Japan)nd an automatic temperature controlled water bath incubatorhaker—Elcon (India) were used.

.2. Preparation of reagents

Thorium nitrate Th(NO3)4·5H2O and phosphoric acid,3PO4 solutions of different molarities were prepared in 1 MNO3 and demineralised water (DMW), respectively.

.3. Preparation of poly-o-toluidine Th(IV) phosphateomposite

.3.1. Synthesis of poly-o-toluidineThe polymerization of the monomer ortho-tolidine was

nitiated by the addition of oxidizing agent, i.e. ammonium per-ulphate in 1:1 ratio under constant stirring at room temperatureor 2 h [14]. After 2 h a brown colored poly-o-toluidine polymeras obtained.

.3.2. Synthesis of thorium(IV) phosphateThe method of preparation of the inorganic precipitate of

h(IV) phosphate ion-exchanger was very similar to that oflberti and Constantino with slight modification [15] by mixing

solution of 0.1 M Th(NO3)4·5H2O prepared in 1 M HNO3 at

he flow rate at 0.5 ml min−1 to a solution of H3PO4 in differ-nt molarities. Constant string was done during mixing using aagnetic stirrer at a temperature of 85 ◦C, white gel type slurries

eNoF

72 (2007) 699–710

ere obtained. After digestion of the mixture for several hours,t was cooled to room temperature in each case.

.3.3. Synthesis of poly-o-toluidine Th(IV) phosphateThe composite cation-exchanger was prepared by the sol–gel

ixing of poly-o-toluidine an organic polymer into the inorganicrecipitate of thorium(IV) phosphate. In this process, when theels of poly-o-toluidine were added to the white inorganic pre-ipitate of thorium(IV) phosphate with a constant stirring, theesultant mixture was turned slowly into brown colored slurries.he resultant brown colored slurries were kept for 24 h at room

emperature.Now, the poly-o-toluidine based composite cation-exchanger

els were filtered off, washed thoroughly with DMW to removexcess acid and any adhering trace of ammonium persulphate.he washed gels were dried over P4O10 at 30 ◦C in an oven. Theried products were immersed in DMW to obtain small gran-les. They were converted to the H+ form by keeping it in 1 MNO3 solution for 24 h with occasionally shaking intermittently

eplacing the supernatant liquid. The excess acid was removedfter several washing with DMW. The material was finally driedt 40 ◦C and sieving to obtain particles of particular size range∼125 �m). Hence, a number of poly-o-toluidine thorium(IV)hosphate nano-composite cation-exchanger samples were pre-ared and on the basis of Na+ exchange capacity (i.e.c.), percentf yield and physical appearance, sample S-3 was selected forurther studies.

.4. Ion-exchange capacity (i.e.c.)

The ion-exchange capacity, which is generally taken as a mea-ure of the hydrogen ion liberated by neutral salt to flow throughhe composite cation-exchanger was determined by standard col-mn process. One gram of the dry cation-exchanger sample S-3n the H+-form was taken into a glass column having an internaliameter (i.d.) ∼1 cm and fitted with glass wool support at theottom. The bed length was approximately 1.5 cm long. Oneolar alkali and alkaline earth metal nitrates as eluants were

sed to elute the H+ ions completely from the cation-exchangeolumn, maintaining a very slow flow rate (∼0.5 ml min−1).he effluent was titrated against a standard 0.1 M NaOH usinghenolphthalein indicator.

.5. Effect of eluant concentration

To find out the optimum concentration of the eluant for com-lete elution of H+ ions, a fixed volume (250 ml) of sodium

nt was titrated against a standard alkali solution of 0.1 MaOH for the H+ ions eluted out. A maximum elution wasbserved with the concentration of 1.6 M NaNO3 as indicated inig. 1.

A.A. Khan et al. / Talanta 72 (2007) 699–710 701

2

w1oflwe

2

nm

Fe

Fc

otei

Fig. 1. Concentration plot of poly-o-toluidine Th(IV) phosphate.

.6. Elution behavior

Since an optimum concentration Fig. 2 for a complete elutionas observed to be 1.6 M for sample S-3, a column containingg of the exchanger in H+-form was eluted with NaNO3 solutionf this concentration in different 10 ml fractions with minimumow rate as described above and each fraction of 10 ml effluentas titrated against a standard alkali solution for the H+ ions

luted out.

.7. pH-titration

pH-titration studies of poly-o-toluidine Th(IV) phosphateano-composite cation-exchanger (S-3) was performed by theethod of Topp and Pepper [16]. A total of 500 mg portions

ig. 2. The elution behavior of poly-o-toluidine Th(IV) phosphate cation-xchange material.

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2

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ig. 3. pH-titration curves for poly-o-toluidine Th(IV) phosphate nano-omposite cation-exchanger.

f the cation-exchanger in the H+-form were placed in each ofhe several 250 ml conical flasks, followed by the addition ofquimolar solutions of alkali metal chlorides and their hydrox-des in equal volume ratio, the final volume is being 50 ml to

aintain the ionic strength constant. The pH of the solution wasecorded every 24 h until equilibrium was attained which neededdays. The pH at equilibrium was plotted against the m moles

f OH− ions added (Fig. 3)

.8. Chemical dissolution

Two hundred fifty milligrams portions of the compositeation-exchanger (poly-o-toluidine Th(IV) phosphate, S-3) in+-form were treated with 20 ml each of different acids such asCl, HNO3, H2SO4, HClO4, different bases such as NaOH,OH, NH4OH, salt (NaNO3) and organic solvents such asimethyl sulphoxide (DMSO), acetone, n-butyl alcohol, and alsoith double distilled water (DMW) for 24 h with occasional

haking.

.9. Chemical composition

The chemical composition of poly-o-toluidine Th(IV) phos-hate nano-composite cation-exchanger (sample S-3) wasetermined by using elemental analyzer, inductively coupled

lasma mass spectrophotometer and UV–vis spectrophotome-er for CHN, Th and phosphorous, respectively. The weightercent composition of C, H, N, Th, P and O in the materialas found to be 38.85, 4.579, 8.26, 36.62 and 1.88 and 9.81,

espectively.

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.10. Thermal studies

To study the effect of temperature on the i.e.c., 1 g samplesf the composite cation-exchange material (S-3) in the H+-formere heated at various temperatures in a muffle furnace for 1 h

nd the Na+ ion-exchange capacity was determined by columnrocess after cooling them at room temperature.

Simultaneous TGA and DTA studies of the composite cation-xchange material (poly-o-toluidine Th(IV) phosphate, S-3)n original form were carried out by an automatic thermoalance on heating the material from 10 to 1000 ◦C at a con-tant rate (10 ◦C min−1) in the air atmosphere (air flow rate of00 ml min−1).

.11. Fourier transform infra red (FTIR) studies

The FTIR spectrum of poly-o-toluidine (sample S-4); Th(IV)hosphate (sample S-5) and poly-o-toluidine Th(IV) phosphatesample S-3) in the original form dried at 40 ◦C were taken byBr disc method at room temperature.

.12. X-ray analysis

Powder X-ray diffraction (XRD) pattern was obtained in anluminum sample holder for the sample S-3 in the original formsing a PW 1148/89 based diffractometer with Cu K� radiations.

.13. Transmission electron microscopy (TEM) studies

TEM studies were carried out to know the particle size of theoly-o-toluidine Th(IV) phosphate composite cation-exchangeaterial.

.14. Scanning electron microscopy (SEM) studies

Microphotographs of the original form of poly-o-toluidineS-4); inorganic precipitate of Th(IV) phosphate (S-5); andrganic–inorganic composite material poly-o-toluidine tho-ium(IV) phosphate (S-3); were obtained by the scanninglectron microscope at various magnifications.

.15. Selectivity (sorption) studies

The distribution coefficient (Kd values) of various metal ionsn poly-o-toluidine Th(IV) phosphate nano-composite wereetermined by batch method in various solvent systems. Var-ous 200 mg of the composite cation-exchanger beads (S-3) inhe H+-form were taken in Erlenmeyer flasks with 20 ml of dif-erent metal nitrate solutions in the required medium and kept for4 h with continuous shaking for 6 h in a temperature controlledncubator shaker at 25 ± 2 ◦C to attain equilibrium. The initial

etal ion concentration was so adjusted that it did not exceed% of its total ion-exchange capacity. The metal ions in the solu-

ion before and after equilibrium were determined by titratinggainst standard 0.005 M solution of EDTA [17]. The alkalinearth metal ion [Ca2+] was determined by flame photometry andome heavy metal ions such as [Pb2+, Cd2+, Cu2+, Hg2+, Ni2+,

2

r

72 (2007) 699–710

n2+] were determined by atomic absorption spectrophotometryAAS). The distribution quantity is given by the ratio of amountf metal ion in the exchanger phase and in the solution phase.n other words, the distribution coefficient is the measure of aractional uptake of metal ions competing for H+ ions from aolution by an ion-exchange material and hence mathematicallyan be calculated using the formula given as:

d = m mol metal ions/g ion-exchanger

m mol metal ions/ml solution(ml g−1) (1)

.e.

d =[I − F

F

] [V

M

](ml g−1) (2)

here I is the initial amount of metal ion in the aqueous phase, Fhe final amount of metal ion in the aqueous phase, V the volumef the solution (ml) and M is the amount of cation-exchanger (g).

.16. Preparation of poly-o-toluidine Th(IV) phosphateation-exchange membrane

Ion-exchange membrane of poly-o-toluidine Th(IV) phos-hate was prepared as the method reported by Khan andnamuddin [18] in earlier studies. To find out the optimum mem-rane composition, different amounts of the composite materialas grounded to a fine powder and mixed thoroughly with a fixed

mount (200 mg) of PVC dissolved in 10 ml tetrahydrofuran.he resultant slurries were poured to cast in glass tube having0 cm in length 5 mm in diameter. These glass tubes were leftor slow evaporation for several hours. In this way, three sheetsf different thicknesses 0.15, 0.17 and 0.22 mm were obtained.fixed area of the membranes was cut using sharp edge blade.

.17. Characterization of membrane

The important factors to check the performance of the mem-rane, three parameters were determined as reported earlier19–22].

.17.1. Water content (% total wet weight)First the membranes were soaked in water to elute diffusible

alt, blotted quickly with Whatmann filter paper to remove sur-ace moisture and immediately weighted. These were furtherried to a constant weight in a vacuum over P2O5 for 24 h. Theater content (total wet weight) was calculated as:

Total wet weight = Ww − Wd

Ww100

here Ww is the weight of the soaked/wet membrane and Wd ishe weight of the dry membrane.

.17.2. PorosityPorosity (ε) was determined as the volume of water incorpo-

ated in the cavities per unit membrane volume from the water

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A.A. Khan et al. / Ta

ontent data:

= Ww − Wd

ALρw

here Ww is the weight of the soaked/wet membrane, Wd theeight of the dry membrane, A the area of the membrane, L the

hickness of the membrane and ρw is the density of water.

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

he average thickness of the membrane by using screw gauze.welling is measured as the difference between the average

hicknesses of the membrane equilibrated with 1 M NaCl for4 h and the dry membrane.

.18. Fabrication of ion-selective membrane electrode

The membrane sheet of 0.17 mm thickness as obtained byhe above procedure was cut in the shape of disc and mountedt the lower end of a Pyrex glass tube (o.d. 0.8 cm, i.d. 0.6)ith araldite. Finally, the assembly was allowed to dry in air

or 24 h. The glass tube was filled with 0.1 M mercuric nitrateolution. A saturated calomel electrode was inserted in the tubeor electrical contact and another saturated calomel electrodeas used as external reference electrode. The whole arrangement

an be shown as:

Internal reference electrode (SCE) | internal

electrolyte 0.1 M Hg2+ | membrane | sample solution |external reference electrode (SCE)

Following parameters were evaluated to study the character-stics of the electrode such as lower detection limit, electrodeesponse curve, response time and working pH range.

.19. Electrode response or membrane potential

To determine the electrode response, a series of standard solu-ions of varying concentrations ranging from 10−1 to 1−10 wererepared. External electrode and ion-selective membrane elec-rode are plugged in digital potentiometer and the potentials wereecorded.

For the determination of electrode potential the mem-rane of the electrode was conditioned by soaking in 0.1 Mg(NO3)2 solution for 5–7 days and for 1 h before use.hen electrode was not in use electrode must be kept in

.1 M Hg(NO3)2 solution. Potential measurement was plottedgainst selected concentration of the respective ion in aqueousolution.

.20. Effect of pH

pH solutions ranging from 1 to 13 were prepared at con-tant ion concentration, i.e. (1 × 10−2 M). The value of electrodeotential at each pH was recorded and plot of electrode potentialersus pH was plotted.

omcfi

72 (2007) 699–710 703

.21. The response time

The method of determining response time in the present works being outlined as follows.

The electrode is first dipped in a 1 × 10−3 M solution ofg(NO3)2 and 10-fold higher concentration. The potential of

he solution was read at 0 s; just after dipping of the electrode inhe second solution and subsequently recorded at the intervalsf 5 s. The potentials were then plotted versus time.

.22. Determination of Hg2+ by potentiometric titrationssing poly-o-toluidine Th(IV) phosphate nano-compositeembrane electrode

The practical utility of the proposed membrane sensor assem-ly was tested by its use as an indicator electrode in theotentiometric titration of Hg(II) with EDTA.

.23. Life span of the membrane

For this study, the electrode responses were noted forne or more weeks over weak and response curve is drawnor the data. The electrode response curve showed that theesponse remains constant over a period of time. After thiseriod the electrode starts behaving irregular, therefore can-ot be used for any measurements. This period in whichhe electrode response is constant can be termed as life oflectrode.

. Results and discussion

Various samples of a new and novel organic–inorganicomposite cation-exchange material have been developed byhe incorporation of electrically conducting polymer poly--toluidine into the inorganic matrices of fibrous Th(IV)hosphate. Due to the high percentage of yield, better ion-xchange capacity, reproducible behavior, chemical and thermaltabilities, sample S-3 (Table 1) was chosen for the details of theon-exchange studies.

The composite cation-exchange material possessed a bet-er Na+ ion-exchange capacity (1.90 meq g−1) as comparedo inorganic precipitate of fibrous type Th(IV) phosphate0.72 meq g−1) [15].

The ion-exchange capacity of the composite cation-xchanger for alkali metal ions and alkaline earth metal ionsncreased according to the decrease in their hydrated ionic radiiTable 2). The ion-exchange capacity of dried composite cation-xchanger sample (S-3) decreased as the temperature increased.he ion-exchange capacity of the material was found stable upto50 ◦C and it retained about 61% of the initial ion-exchangeapacity by heating upto 250 ◦C (Table 3).

It is evident from Fig. 1 that minimum molar concentrationf NaNO3 as eluant for sample (S-3) was found to be 1.6 M for

aximum release of H+ ions from 1 g of the cation-exchanger

olumn. The elution behavior indicates that the exchange is quiteast because only 250 ml of sodium nitrate solution (1.6 M)s enough to release the total H+ from 1 g sample of poly-

704 A.A. Khan et al. / Talanta 72 (2007) 699–710

Table 1Conditions of preparation and the ion-exchange capacity of poly-o-toluidine Th(IV) phosphate composite cation-exchange materials

Sample no. Mixing volume ratios (v/v) Heatingtime (h)

Heatingtemperature (◦C)

Mixing volume ratios (v/v) Appearance ofthe sample

Na+ ion-exchangecapacity (meq dry g−1)Th(NO3)4·5H2O

in 1 M HNO3

H3PO4 0.1 M (NH4)2S2O8

in 1 M HCl10% ortho-toluidinein 1 M HCl

S-1 1 (0.1 M) 1 (1 M) 2 85 1 1 Fibrous brown 1.26S-2 1 (0.1 M) 1 (1 M) 4 85 1 1 Fibrous brown 1.60S-3 1 (0.1 M) 2 (1 M) 4 85 1 1 Fibrous brown 1.90S-4 – – – – 1 1 Reddish 0.11S-5 5 (0.1 M) 2 (2 M) 4 85 – – White sheet 0.72S-6 2 (0.1 M) 1 (1 M) 4 85 1 1 Fibrous brown 1.15S-7 3 (0.1 M) 1 (1 M) 4 85 1 1 Fibrous brown 1.18S-8 3 (0.1 M) 1 (2 M) 4 85 1 1 Fibrous brown 1.24S-9 3 (0.1 M) 1 (2.4 M) 4 85 1 1 Fibrous brown 1.04S-10 5 (0.1 M) 1 (2.5 M) 4 85 1 1 Fibrous brown 0.95S-11 1 (0.2 M) 4 (1 M) 4 85 1 1 Fibrous brown 1.60S-12 1 (0.2 M) 2 (1 M) 4 85 1 1 Fibrous brown 1.52S-13 1 (0.2 M) 2 (2 M) 4 85 1 1 Fibrous brown 1.44

Table 2Ion-exchange capacity of various exchanging ions on poly-o-toluidine Th(IV) phosphate composite cation-exchanger

Exchanging ions pH of the metal solution Ionic radii Hydrated ionic radii Ion-exchange capacity (meq g−1)

Na+ 4.99 0.97 2.76 1.90K+ 6.50 1.33 2.32 1.76Li+ 3.30 0.68 3.40 0.40Mg2+ 4.87 0.78 7.00 0.97Ca2+ 6.20 1.06 6.30 0.83S 2+

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-toluidine thorium(IV) phosphate cation-exchange materialFig. 2).

The pH titration curves for poly-o-toluidine Th(IV) phos-hate (S-3) were obtained under equilibrium conditions withaOH/NaCl, KOH/KCl and LiOH/LiCl, systems indicatedifunctional behavior of the material as shown in Fig. 3. Forhe sample S-3, the rate of H+–Na+ exchange was faster thanhose of H+–K+ and H+–Li+ exchangers. The solubility experi-

ents showed that the material has good chemical stability, as

t is resistant to DMW, 1 M HCl, 0.5 M NaOH, 2 M NH4NO3,uffer pH 10 and 3.75. The chemical dissolution in 1.5 M HNO3,M tetrahydrofuran, 20% alcohol and 0.5 M NaOH is almost

2pbo

able 3ffect of temperature on ion-exchange capacity of poly-o-toluidine Th(IV) phosphate

erial no. Heating temperature (◦C) Na+ ion-exchange

1 50 1.902 100 1.903 150 1.904 200 1.165 250 0.926 300 0.847 350 0.528 400 0.209 500 0.080 600 0.00

– 1.405.90 1.70

egligible. The percent composition of C, H, N, Th, P and O inhe material was found to be 38.85, 4.579, 8.26, 36.62,1.88 and.81, respectively.

It is clear from the thermogravimetric analysis (TGA) curveFig. 4) of the material that upto 100 ◦C only 5.82% weightoss was observed, which may be due to the removal of exter-al H2O molecules present at the surface of the composite [23].urther weight loss of mass approximately 7% between 100 and

◦

50 C may be due to the slight conversion of inorganic phos-hate into pyrophosphate. Slow weight loss of mass about 5% inetween 250 and 600 ◦C may be due to the slight decompositionf organic part of the material. The thermo-gravimetric analysis

composite cation-exchanger on heating time for 1 h

capacity (meq dry g−1) % Retention of ion-exchange capacity

100100100

6361482710.52

4.020.00

A.A. Khan et al. / Talanta 72 (2007) 699–710 705

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oel

ttoor

Fa

Fn

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ig. 4. Simultaneous TGA–DTA curves of poly-o-toluidine Th(IV) phosphateas prepared).

f the composite exchanger indicates that the composite cation-xchanger is thermally stable because only 18% of mass wasost upto 1000 ◦C.

The peak values of the FTIR spectra (Fig. 5) of poly-o-oluidine Th(IV) phosphate composite material indicates that

he band centered at 3367 cm−1 results for the characteristicsf free N–H stretching vibration, suggest the presence of sec-ndary amino group (–NH–) [14]. The peak at 1640–1560 cm−1

epresents the free water molecule (water of crystallization) and

ig. 5. FTIR spectra of as prepared Th(IV) phosphate S-5, poly-o-toluidine S-4,nd poly-o-toluidine Th(IV) phosphate nano-composite cation-exchanger S-3.

io(e1npptwoop

(mfntCa

siHatagq

ig. 6. Power X-ray diffraction pattern of poly-o-toluidine Th(IV) phosphateano-composite.

trongly bonded –OH group in the matrix [24]. An assemblyf two peaks at 980–1072 cm−1 may represent the presence ofonic phosphate groups [25]. The I.R. spectrum of composite

aterial can be compared with the spectra of Th(IV) phosphateS-5) and poly-o-toluidine (S-4).

The X-ray diffraction pattern of this materials (S-3 as pre-ared) recorded in powdered sample exhibited some small peaksn the spectrum (Fig. 6) which suggest semi-crystalline naturef the composite material. From the TEM studies it is clearFig. 7) that the poly-o-toluidine Th(IV) phosphate cation-xchange material shows particles size in range of 11.0, 10.0,5.0, 3.0 and 18.0 nm. Thus, the material particle size shows theano-range. The scanning electron microphotograph (SEM) ofoly-o-toluidine, Th(IV) phosphate and poly-o-toluidine Th(IV)hosphate are represented in Fig. 8. It is clear from the pho-ographs that after binding of organic polymer poly-o-toluidineith inorganic precipitate of Th(IV) phosphate. The morphol-gy of the material has been changed with the formation ofrganic inorganic composite material poly-o-toluidine Th(IV)hosphate.

In order to explore the potential of the composite materialS-3) in the separation of metal ions, distribution studies for 15etal ions were performed in 15 solvent systems, it was observed

rom the data given in Table 4 that the Kd-values vary with theature of the contacting solvents. It was also observed fromhe (Kd) values that Hg2+ was strongly adsorbed Zn2+, Ba2+,a2+ and Al3+ are significantly adsorbed while the remainingre partially adsorbed on the surface of ion-exchange material.

The separation capability of the material has been demon-trated by achieving some important binary separationsnvolving Hg2+, viz. Hg2+–Cu2+, Hg2+–Zn2+, Hg2+–Ce4+,g2+–Ni2+, Hg2+–Mg2+, Hg2+–Co2+, Hg2+–Ag2+, Pb2+–Ni2+

nd Pb2+–Fe2+ (Table 5). The sequential elution of the ions fromhe column depends upon the stability of the metal-eluting lig-

nd (eluant). The order of elution and eluant used separation areiven in Fig. 9. The separations are quite sharp and recovery isuantitative and reproducible.

706 A.A. Khan et al. / Talanta

FT

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ig. 7. Transmission electron microphotographs (TEM) of poly-o-toluidineh(IV) phosphate showing different particle sizes.

In this study, organic–inorganic composite cation-exchanger

oly-o-toluidine Th(IV) phosphate was also used forhe preparation of heterogeneous ion-selective membranelectrode.

t[

able 4

d-values of some metal ions on poly-o-toluidine Th(IV) phosphate nano-composite

olvents Metal ions

Mg2+ Ba2+ Ca2+ Tl+ Pb2+ Hg2+ A

MW – 200 100 8 425 660−3 M HNO3 14 50 36 – 260 2000−2 M HNO3 – 15 13 20 150 1160−1 M HNO3 – 600 50 70 – –0−3 M HClO4 – 66 33 14 25 3000−2 M HClO4 14 27 70 40 113 1500−1 M HClO4 – 16 13 40 140 1400−3 M CH3COOH 100 67 60 400 260 4000−2 M CH3COOH 100 50 58 90 116 400 10−1 M CH3COOH – 116 50 40 180 87H3COOH + CH3COONa 134 180 200 100 375 2750% DMSO 56 700 57 200 300 2230% Acetone 70 45 50 – 200 4000% Formic acid – – – 160 166 –0−1 M H2SO4 20 17 160 75 400

72 (2007) 699–710

Sensitivity and selectivity of the ion-selective electrodesepend upon the nature of electro-active material, membraneomposition and physico-chemical properties of the membranesmployed. A number of samples of the poly-o-toluidine Th(IV)hosphate composite membrane were prepared with differentmounts of composite material and PVC and checked for theechanical stability, surface uniformity, materials distribution,

racks and thickness, etc. But the membranes obtained with 33%VC (w/w) were found to be good.

The results of thickness, swelling, porosity and waterontent capacity of poly-o-toluidine Th(IV) phosphate nano-omposite cation-exchanger membrane are summarized inable 6. The membrane sample M-2 (thickness 0.17 mm) waselected for further studies. Thus, low order of water content,welling and porosity with less thickness of this membraneuggests that interstices are negligible and diffusion acrosshe membrane would occur mainly through the exchangeites.

The heterogeneous precipitate Hg(II) ion-selective mem-rane electrode obtained from poly-o-toluidine Th(IV) phos-hate cation-exchanger material gives linear response in theange 1 × 10−1 and 5 × 10−5 M. Suitable concentrations werehosen for sloping portion of the linear curve. The limit ofetection determined from the intersection of the two extrap-lated segments of the calibration graph [26] was found to be× 10−5 M, and thus the working concentration range is found

o be 1 × 10−1 to 5 × 10−5 M (Fig. 10) for Hg2+ ions with aower Nerstian slope of 26.97 mV per decade change in Hg2+

on concentration, the slope value is blow to Nerstian value,9.6 mV per concentration decade for divalent cation [27].

pH effect on the potential response of the electrode wereeasured for a fixed (1 × 10−2 M) concentration of Hg2+ ions

n different pH values. It is clear that electrode potential remainsnchanged within the pH range 4.0–8.0 (Fig. 11), known as

Another important factor is the promptness of the response ofhe ion-selective electrode. The average response time is defined28] as the time required for the electrode to reach a stable poten-

cation-exchanger column in different solvent system

l2+ Ni2+ Fe2+ Ce4+ Co2+ Zn2+ Ag+ Cu2+ Cd2+

57 20 200 33 20 400 25 25 25100 140 300 26 50 10 45 43 43

25 33 – 90 25 50 66 25 25– – – 25 – 100 10 – –

80 11 57 25 25 – 44 43 439 20 – – 23 400 18 – –

500 600 66 100 43 150 33 – –43 40 40 80 – 50 116 50 50

000 16 – 50 50 275 120 63 63– 36 66 100 63 250 200 67 67

120 150 60 66 87 150 5 100 10043 30 100 80 50 366 18 – –60 16 66 130 60 233 80 – –

100 150 33 – – – – 20 20200 300 60 100 – – 7 50 50

A.A. Khan et al. / Talanta 72 (2007) 699–710 707

Fig. 8. Scanning electron microphotographs (SEM) of poly-o-toluidine at the magn(S-5) and poly-o-toluidine Th(IV) phosphate (S-3) at the magnification of 7.00k×.

Fig. 9. Binary separations of Hg(II) from Cu(II), Zn(II), Ce(IV), Ni(II) on poly-o-toluidine Th(IV) phosphate nano-composite columns.

ts

wsWr

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ifications of 7.00k× (S-4), Th(IV) phosphate at the magnification of 4.00k×

ial. It is clear (Fig. 12) that the response time of the membraneensor is found to be ∼25 s.

The membrane could be successfully used upto 7 monthsithout any notable drift in potential during which the potential

lope is reproducible within ±1 mV per concentration decade.henever a drift in the potential is observed, the membrane is

e-equilibrated with 0.1 M Hg(NO3)2 solutions for 3–4 days.The selectivity coefficients, K

potHg M of various differing

ations for the Hg(II) ion-selective poly-o-toluidine Th(IV)hosphate composite membrane electrode were determined byhe mixed solution method [29]. The selectivity coefficient indi-ates the extent to which a foreign ion (Mn+) interferes with

he response of the electrode towards its primary ions (Hg2+).y examine the selectivity coefficient data given in Table 7, it

s clear that the electrode is selective for Hg(II) in presence ofnterfering cations.

708 A.A. Khan et al. / Talanta 72 (2007) 699–710

Table 5Some binary separations of metal ions achieved on poly-o-toluidine Th(IV) phosphate composite column (A = 1 M HCl; C = 0.1 M HNO3; D = DMW; E = 0.01 MHClO4; F = 0.1 M HClO4)

Separation achieved Eluant used Volume of eluant (ml) Amount of metal loaded (�g) Amount of metal found (�g) Error (%)

Hg(II) A 40 4011.80 4011.80 0.00Cu(II) C 40 1271.00 1271.00 0.00Hg(II) A 40 4011.80 3961.65 −1.25Zn(II) D 40 1307.40 1307.40 0.00Hg(II) A 50 4011.80 3961.65 −1.25Ce(IV) C 40 2802.30 2802.30 0.00Hg(II) A 40 4011.80 3961.65 −1.25Ni(II) E 40 880.65 895.33 +1.66Hg(II) A 40 300.90 3018.87 −0.33Mg(II) E 40 486.10 492.18 +1.25Hg(II) A 50 4011.80 3961.65 −1.25Co(II) E 50 1001.80 1031.28 +2.94Hg(II) A 50 4011.80 3967.65 −1.25Ag(I) F 40 1921.62 1968.44 +1.38Hg(II) C 40 3315.20 3367.00 +1.56NI(II) F 50 1174.20 1174.20 0.00Hg(II) C 40 4144.00 4195.00 +1.25Fe(II) E 50 1117.00 1103.03 −1.25

Table 6Characterization of ion-exchange membrane

Poly-o-toluidine Th(IV) phosphatecomposite material

Thickness of themembrane (mm)

Water content as % weightof wet membrane

Porosity Swelling of % weightof wet membrane

M-1 0.15 0.02517 2.16 × 10−4 No swellingM-2 0.17 0.01957 1.85 × 10−4 No swellingM-3 0.22 0.13397 2.16 × 10−3 No swelling

Fig. 10. Calibration curve of poly-o-toluidine Th(IV) phosphate membraneelectrode in aqueous solutions of Hg(NO3)2.

Fig. 11. Effect of pH on the potential response of the poly-o-toluidine Th(IV)phosphate membrane electrode at 1 × 10−2 M Hg2+ concentration.

A.A. Khan et al. / Talanta

Fig. 12. Time response curve of poly-o-toluidine Th(IV) phosphate membraneelectrode.

Table 7Selectivity coefficient values for Hg2+ selective poly-o-toluidine Th(IV) phos-phate membrane electrode of mercury ions

Interfering ions (Mn+) Selectivity coefficient values

Zn2+ 0.025Cd2+ 0.055Cu2+ 0.040Mg2+ 0.035Co2+ 0.020Na+ 0.85K+ 0.9500Fe2+ 0.400Cr3+ 0.910Mn2+ 0.900

Fig. 13. Potentiometric titration of Hg(II) against EDTA solution using poly-o-toluidine Th(IV) phosphate PVC membrane electrode.

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72 (2007) 699–710 709

The practical utility of the proposed membrane sensor assem-ly was tested by its use as an indicator electrode in theotentiometric titration of Hg(II) with EDTA. The addition ofDTA causes a decrease in potential as a result of the decrease

n the free Hg(II) ions concentration due to its complexationith EDTA (Fig. 13). The amount of Hg(II) ions in solution cane accurately determined from the resulting neat titration curveroviding a sharp rise in the titration curve at the equivalenceoints.

. Conclusion

In the present study, a mercury selective composite cation-xchanger poly-o-toluidine Th(IV) phosphate having goodon-exchange capacity (1.90) as compared to Th(IV) phos-hate (0.72) have been prepared successfully. As shown inEM photograph the particle size of the composite materialre within the range of 11.00, 10.00, 15.00 and 18.00 nm. Thusaterial can be considered as nano-composite material. This

omposite material was also utilized as an electroactive com-onent for the preparation of ion-selective membrane electrodeor the determination of Hg(II) ions in aqueous solution. Theembrane electrode showed a working concentration range

0−1 – 5 × 10−5 M, response time 25 s, 4–8 pH range, and selec-ivity in presence of other metal ions. The practical utility wasetermined as potentiometric sensor for the titration of Hg(II)sing ethylenediaminetetra-acetic acid (EDTA) as a titrant.

cknowledgements

The authors are thankful to Department of Applied Chem-stry, Z.H. College of Engineering and Technology, A.M.U.Aligarh) for providing research facilities. Assistance providedy the R.S.I.C. Bombay, I.I.T. Delhi and I.I.T. Roorkee to carryut some instrumental analysis and financial assistance providedy the U.G.C. also acknowledged.

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