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Reactive & Functional Polymers 55 (2003) 277–290

www.elsevier.com/locate/react

Synthesis, characterization and analytical applications of a newand novel ‘organic–inorganic’ composite material as a cation

exchanger and Cd(II) ion-selective membrane electrode:polyaniline Sn(IV) tungstoarsenate

*Asif Ali Khan , M. Mezbaul Alam Analytical and Polymer Research Laboratory, Department of Applied Chemistry, Faculty of Engineering and Technology,

Aligarh Muslim University, Aligarh 202 002, India

Received 29 July 2002; received in revised form 18 October 2002; accepted 19 February 2003

Abstract

Composite materials formed by the combination of inorganic ion exchangers of multivalent metal acid salts and organic

conducting polymers (polyaniline, polypyrrole, polythiophene, etc.), providing a new class of ‘organic–inorganic’ hybrid ion

exchangers with better mechanical and granulometric properties, good ion-exchange capacity, higher stability, reproducibility

and selectivity for heavy metals. Such a type of ion exchanger ‘polyaniline Sn(IV) tungstoarsenate’ was developed by

mixing polyaniline into inorganic precipitate of Sn(IV) tungstoarsenate. This material was characterized using atomicabsorption spectrometry, elemental analysis, Fourier transform infrared spectroscopy, simultaneous thermogravimetry–

differential thermogravimetry, X-ray and scanning electron microscopy studies. Ion-exchange capacity, pH-titrations, elution

and distribution behavior were also carried out to characterize the material. On the basis of distribution studies, the material

was found to be highly selective for Cd(II) and its selectivity was tested by achieving some important binary separations like

Cd(II)–Zn(II), Cd(II)–Pb(II), Cd(II)–Hg(II), Cd(II)–Cu(II), etc., on its column. Using this composite cation exchanger as

electroactive material, a new heterogeneous precipitate based selective ion-sensitive membrane electrode was developed for

the determination of Cd(II) ions in solutions. The membrane electrode is mechanically stable, with a quick response time,

and can be operated within a wide pH range. The selectivity coefficients for different cations determined by mixed solution

method were found to be less than unity. The electrode was also found to be satisfactory in electrometric titrations.

© 2003 Elsevier Science B.V. All rights reserved.

Keywords: Organic–inorganic composite material; Cation exchanger; Polyaniline Sn(IV) tungstoarsenate; Cd(II) ion-selective membrane

electrode

1. Introduction in diverse fields such as purification of nuclearreactor cooling water at high temperature and

Exploration of inorganic ion exchangers is pressure, development of ion-selective elec-always of interest because of their applications trodes, construction of ion-exchange membranes

and their applications to electrodialysis, extrac-tion of uranium from sea water and separation*Corresponding author. Fax: 191-571-270-1260.

E -mail address: [email protected] (A.A. Khan). of metal ions, etc. Advancement in inorganic

1381-5148/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S1381-5148(03)00018-X

mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]

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278 A. A. Khan, M . M . Alam / Reactive & Functional Polymers 55 (2003) 277–290

ion exchangers is not only due to their high from smelters and factories processing Cd andthermal stability and resistivity towards radia- also from the incineration or disposal of cad-tion fields but also for their unusual selectivity mium-bearing products. Cadmium enters naturalfor ionic species and versatility in separation. water through industrial discharges mainly fromInorganic ion exchangers of double salts, based electroplating industry and nickel–cadmiumon tetravalent metal acid (TMA) salts often battery industry or the deterioration of galvan-exhibit much better ion-exchange behavior as ized water pipes. Cd is therefore a potentialcompared with single salts [1]. Derivatization of pollutant in the environment. The utility of thisinorganic ion exchangers by organic molecules composite ion exchanger has been explored for

2 1depends on the nature of the inorganic matrix. the quantitative separation of Cd from someTMA salts can be derivatized by organic moi- binary mixtures on its column.eties bearing inorganic groups such as –OH, Precipitate based ion-selective membrane–COOH, –SO H, –NH , etc., which also act as electrodes are well known as they are success-

3 2

ion exchangers, and are known as organo–inor- fully employed for determination of severalganic ion exchangers or as derivatized tetraval- anions and cations [14]. The ion-exchange

ent metal acid (DTMA) salts. Many organic– membranes obtained by embedding ion ex-inorganic composite ion exchangers have been changers as electroactive materials in a polymerdeveloped earlier by incorporation of organic binder, i.e., epoxy resin (Araldite) or poly-monomers in the inorganic matrix, by way of styrene or poly(vinyl chloride) (PVC), havepillaring or non-pillaring methods [2–13]. been extensively studied as potentiometric sen-

Efforts have been made to improve the sors, i.e., ion sensors, chemical sensors or morechemical, thermal and mechanical stability of commonly ion selective electrodes. Besides theion exchanger and to make them highly selec- electrodes that respond directly to the change intive for certain metal ions. An inorganic ion concentration of particular ion for which it isexchanger based on organic polymeric matrix made, a number of electrodes are used formust be an interesting material, as it should indirect determination of certain ions. An at-possess the mechanical stability due to the tempt has also been made to obtain a newpresence of organic polymeric species and the heterogeneous precipitate based membrane elec-basic characteristics of an inorganic ion ex- trode by using the polyaniline Sn(IV) tung-changer regarding its some selectivity for some stoarsenate composite cation exchanger as elec-specific metal ions. It was therefore considered troactive material for the determination of to synthesize such hybrid ion exchangers with a Cd(II) ions present in the solutions. This papergood ion-exchange capacity, high stability, re- presents the preparative conditions, ion-ex-producibility and selectivity for heavy metal change properties, physicochemical propertiesions, indicating its useful environmental appli- and analytical applications of this organic–inor-cation. A number of organic–inorganic compo- ganic composite material used as a cation

site samples of polyaniline Sn(IV) tungstoarse- exchanger and Cd(II) ion-selective membranenate have been synthesized in our laboratory. electrode.

Cadmium is considered as highly toxic ele-ment and responsible for several cases of poisoning through water, food and smoking. 2. Experimental

21When excessive amounts of Cd are ingested,

2 1it replaces Zn at key enzymatic sites, causing 2.1. Reagents and instrumentsmetabolic disorders, kidney damage, renaldysfunction, anemia, hypertension, bone mar- The main reagents used for the synthesis of row disorders, cancer and toxicity to aquatic the material were obtained from CDH, Loba

biota. Cadmium is released into the atmosphere Chemie, E-Merck and Qualigens (India). All

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A. A. Khan, M . M . Alam / Reactive & Functional Polymers 55 (2003) 277–290 279

other reagents and chemicals were of analytical- adding the solution of 0.1 M stannic chloride toreagent grade. A digital pH-meter (Elico LI-10, an aqueous mixture of 0.1 M sodium arsenateIndia), Fourier transform infrared (FT-IR) spec- and 0.1 M sodium tungstate in different volumetrophotometer (Nicolet 400D, USA), an auto- ratios and at different pH values. The whitematic thermal analyzer (V2.2A DuPont 9900), precipitates were obtained when the pH of thean elemental analyzer (Carlo-Erba 1180), a mixture was adjusted by adding ammonia waterdouble beam atomic absorption spec- with constant stirring.trophotometer (GBC 902, Australia), an electron After this, the gel of polyaniline was added tomicroscope (LEO 435 VP) with attached imag- the inorganic precipitate of Sn(IV) tungstoarse-ing device, a digital flame photometer (Elico CL nate and mixed thoroughly with constant stir-22D, India), a UV–Vis spectrophotometer (Elico ring. The resultant green colored gel was al-EI 301E, India), a temperature controlled shaker lowed to settle overnight at room temperatureand a digital potentiometer (Equiptronics EQ (2562 8C). The supernatant liquid was decanted609, India) with a saturated calomel electrode as and the gel was filtered under suction andreference electrode were used. washed with 1 M HNO to remove excess

3

reagent. The excess acid was removed bywashing with DMW and again washed with2.2. Preparation of the reagent solutionsacetone by soxhlation. The materials were dried

0.1 M Sodium arsenate (Na HAsO ? 7H O) in an air oven at 50 8C for 48 h. The dry2 4 2

and sodium tungstate (Na WO ? 2H O) were products were crushed into small granules of 2 4 2

prepared in demineralized water (DMW) while uniform size suitable for column separations0.1 M stannic chloride (SnCl ? 5H O) was when immersed in DMW. They were then

4 2

prepared in 4 M HCl. Solutions of 10% (v/ v) treated with large excess of 1 M HNO for 24 h3

doubly distilled aniline (C H NH ) and 0.1 M at room temperature with occasional shaking,6 5 2

potassium persulphate (K S O ) were prepared intermittently replacing the supernatant liquid2 2 8

in 1 M HCl. with a fresh acid to ensure the complete conver-1

sion to the H -form. The excess acid wasremoved after several washing with DMW. The2.3. Preparation of polyaniline Sn( IV ) materials were finally dried at 50 8C in the oven,tungstoarsenatesieving to obtain particles of a particular size

At first, green colored polyaniline gels were range (| 125 mm) and kept in a desiccator.obtained by mixing the acidic solutions of 10% Hence, a number of samples of polyanilineaniline (C H NH ) and 0.1 M potassium per- Sn(IV) tungstoarsenate were prepared by chang-

6 5 2

sulfate (K S O ) in different volume ratios with ing the mixing volume ratios of the reagents. On2 2 8

continuous stirring by a magnetic stirrer below the basis of appearance, percentage of yield,1

10 8C for an hour. Inorganic precipitate ion Na ion-exchange capacity and reproducibility,

exchanger gels of Sn(IV) tungstoarsenate were following sample was chosen for furtherprepared at room temperature (2562 8C) by studies:

1

Sample Mixing volume ratio Appearance Na

of beads ion-exchange

Sn W As pH of the K S O Aniline after drying capacity2 2 8

21(0.1 M) (0.1 M) (0.1 M) inorganic (0.1 M) (10%) (mequiv. g )

precipitate

S-1 1 1 1 1.0 1 1 Greenish 1.67

granular

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2.4. Ion-exchange capacity followed by equimolar solutions of alkali metalchlorides and their hydroxides in different vol-

A 1-g amount of the dry cation exchanger, ume ratio, the final volume being 50 ml to1

sample S-1 in the H -form was placed in a maintain the ionic strength constant. The pH of glass column having an internal diameter (I.D.) the solution was recorded every 24 h until| 1 cm and fitted with glass wool support at the equilibrium was attained which needed | 5bottom. The bed length was approximately 1.5 days, and pH at equilibrium was plotted against

2cm long. 1 M alkali and alkaline earth metal the milliequivalents of OH ions added. The1

nitrates as eluents were used to elute the H results are shown in Fig. 1.ions completely from the cation-exchange col-umn, maintaining a very slow flow rate ( | 0.5

21 2.6. Chemical dissolutionml min ). The effluent was titrated against astandard (0.1 M) NaOH solution and the ion-

Portions (250 mg) of the cation exchanger in21exchange capacities in mequiv. g were as 1

the H -form were equilibrated with 25 ml each1 1 1 2 1follows: Li , 1.46; Na , 1.67; K , 1.54; Mg ,

of different acids (such as HCl, HNO , H SO ,21 2 1 21 3 2 41.73; Ca , 1.78; Sr , 1.86 and Ba , 2.03. acetic acid, formic acid, oxalic acid and citric

acid); bases (such as NaOH, KOH); organic2.5. pH -titration solvents (such as n-butanol, acetone, dimethyl

sulfide) and also NH and DMW for 24 h with3

pH-titrations were performed by the method occasional shaking. The supernatant liquid wasof Topp and Pepper [15]. A total of 500 mg analyzed for ‘tin’ and ‘arsenic’ by atomicportions of the cation exchanger was placed in absorption spectrophotometry (AAS), whileeach of the several 250-ml conical flasks, ‘tungsten’ was analyzed by the Vis spectro-

photometric method [16]. The results are sum-marized in Table 1.

2.7. Thermal stability

To study the effect of drying temperature onthe ion-exchange capacity, 1-g samples of the

1

material in the H -form were heated at varioustemperatures in a muffle furnace for 1 h each

1

and the Na ion-exchange capacity was de-termined by column process after cooling them

21at room temperature. The results in mequiv. g

are given below: 1.67 (50 8C); 1.43 (100 8C);1.27 (200 8C); 0.92 (400 8C); 0.78 (500 8C);0.59 (600 8C).

2.8. Chemical composition

After dissolving in hot concentrated hydro-chloric acid, the sample S-1 was analyzed for‘tin’ and ‘arsenic’ by AAS and ‘tungsten’ by aFig. 1. pH-titration curves for polyaniline Sn(IV) tungstoarsenate

composite cation exchanger with various alkali metal hydroxides. standard spectrophotometric method. Carbon,

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Table 1

The solubility of polyaniline Sn(IV) tungstoarsenate in various solvent systems

Solvent Sn W As

(20 ml) (mg/20 ml) (mg/20 ml) (mg/20 ml)

4 M HNO 2.13 0.25 1.273

4 M HCl 5.99 1.03 3.524 M H SO 2.03 0.65 1.31

2 4

0.1 M NaOH 16.31 13.30 22.07

0.1 M KOH 24.59 35.86 36.41

1 M NH 13.84 1.15 6.873

1 M NH NO 0.13 0.53 1.804 3

1 M CH COOH 0.69 1.28 1.213

1 M CH COONa 0.51 1.13 2.203

1 M Citric acid 7.02 1.10 0.94

1 M Oxalic acid 13.96 4.40 8.31

1 M Formic Acid 0.21 0.86 3.64

Dimethyl sulfide (DMS) 0.23 0.08 1.48

n-Butanol 0.48 0.19 2.17

Acetone 0.07 0.63 0.51

DMW 0.05 0.15 0.45

hydrogen and nitrogen contents of the materialmmol of metal ions/g of ion exchanger]]]]]]]]]]]]were determined by elemental analysis. The K 5

d mmol of metal ions/ ml of solutionpercent composition of the material was: Sn,

21(ml g )7.60; W, 11.85; As, 37.14; C, 9.00; H, 1.79; N,

211.41. i.e., K 5( I 2F ) / F 3V / M (ml g ), where I isd

the initial amount of metal ion in the aqueous2.9. Selectivity studies phase, F is the final amount of metal ion in the

aqueous phase, V is the volume of the solutionThe distribution coefficients (K -values) of d (ml) and M is the amount of cation exchanger

metal ions on the sample material (S-1) were (g).determined by the batch method in varioussolvent systems.Various 200-mg portions of the 2.10. Quantitative separation of metal ions

1

exchanger in the H -form were taken in Erlen-meyer flasks with 20 ml different metal nitrate Quantitative separations of some importantsolutions in the required medium and kept for metal ions of analytical utility were achieved on24 h with intermittent shaking or continuous ‘polyaniline Sn(IV) tungstoarsenate’ (sample S-shaking for 6 h in a shaker at 2562 8C to attain 1) column. A 2-g amount of the cation ex-

1equilibrium. The initial metal ion concentration changer in the H -form was packed in an open

was so adjusted that it did not exceed 3% of its glass column (35 cm height and |0.6 cm I.D.).total ion-exchange capacity. The metal ions in After washing the column thoroughly withthe solutions before and after equilibrium were DMW, the mixture of two metal ions of 0.01 Mdetermined by EDTA titration [17]. The alkali each, was loaded on it, and was allowed to pass

1 1 2 1and alkaline earth metal ions (K , Na , Ca ) gently (maintaining a flow rate of 2–3 drops/ were determined by flame photometry and some min) until the level was above the surface of the

21 21 2 1heavy metal ions, such as Pb , Cd , Cu , material. The process was repeated two or three

3 1 21 21 2 1 21Cr , Hg , Ni , Mn and Zn were de- times to ensure complete adsorption of thetermined by AAS. Distribution coefficients were mixture on bead. The separation was achievedcalculated using the formula given as: by passing a suitable solvent at a flow rate of 1

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21ml min through the column as eluent. The reproducible behavior and chemical and thermalmetal ions in the effluent were determined stability.quantitatively by AAS and EDTA titration. Polyaniline gel was prepared by oxidative

coupling using K S O in acidic aqueous2 2 8

medium as given in the following reaction:2.11. Fabrication of ion selective electrode

The cation exchanger (100 mg) was groundto a fine powder, and was mixed thoroughlywith Araldite (Ciba-Geigy) (100 mg) on What-man’s filter paper No. 42, and a master mem-brane of 0.35 mm thickness was prepared. Apiece of membrane was cut out and fixed at oneend of a pyrex glass tube (0.8 cm O.D.30.6 cmI.D.) with Araldite. The tube was filled with 0.1

The effect of temperature on the reactionM cadmium nitrate. A saturated calomel elec-

seems to be very pronounced. Aniline undertrode was inserted in the tube for electricalwent oxidative coupling only at below 10 8Ccontact and another saturated calomel electrodevery efficiently, leading to a good quantity of was used as external reference electrode. Thepolyaniline with fairly good yield. The forma-whole arrangement can be shown as:tion of inorganic precipitate Sn(IV) tungstoarse-

Internal reference Internal electrolyte nate was significantly affected by the pH of theU U21electrode (SCE) (0.1 M Cd ) mixture, and the most favorable pH of the

mixture was |1.0. The preparation of theExternal referenceMembrane Sample solutionU U inorganic precipitate at pH lower or higher thanelectrode (SCE)

1.0 lead to decrease in yield and ion-exchangecapacity of the material. The mixing volume

In advance of measurements of the electrode ratio of the mixture is also critical. The bindingpotential (at 2562 8C) for a series of standard27 21 of polyaniline into the matrix of Sn(IV) tung-solutions of Cd(NO ) (10 M–10 M), the

3 2

stoarsenate can be shown as:membrane of the electrode was conditioned bysoaking in 0.1 M Cd(NO ) solution for 7 days

3 2

and for 1 h at least before use. In order to studythe characteristics of the electrode, the follow-ing parameters were evaluated: lower detectionlimit, slope response curve, response time andworking pH range.

3. Results and discussion

However, sample S-1 of polyaniline Sn(IV)In this work, various samples of new andtungstoarsenate exhibited high granulometricnovel polyaniline based ‘organic–inorganic’and mechanical properties, showing a goodcomposite ion-exchange materials were de-reproducible behavior as is evident from the factveloped by incorporating polyaniline into inor-that these materials obtained from various bat-ganic matrices of Sn(IV) tungstoarsenate.ches did not show any appreciable deviation inAmong them, sample S-1 possessed high per-their ion-exchange capacities. It was also foundcentage of yield, better ion-exchange capacity,

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1 1

that the values of the H -adsorption and H - for 1 h, the ion-exchange capacity of the driedliberation capacities are in close agreement. The material decreased as the temperature increased.ion-exchange capacity of the composite cation However, the material was found to possess

1

exchanger for alkali metal ions (except Na ) higher thermal stability as it maintained aboutand alkaline earth metal ions increased accord- 55% of the initial ion-exchange capacity bying to the decrease in their hydrated ionic radii. heating up to 400 8C. From a comparative study

1

The column elution experiments indicated a of heating effect on Na ion-exchange capacitydependence of the concentration of the eluent of polyaniline Sn(IV) tungstoarsenate withon the rate of elution, which is a usual behavior those of other ion exchangers, as shown in Fig.for such materials. The minimum molar con- 2, it is apparent that this composite cationcentration of NaNO as eluent was 1 M for the exchanger is more thermally stable than others.

31

maximum release of H ions from 1 g of the Scanning electron microscopy (SEM) photo-cation exchanger. The elution was appreciably graphs of Sn(IV) tungstoarsenate and polyani-fast as only 140 ml of the effluent was sufficient line Sn(IV) tungstoarsenate obtained at different

1

for almost complete elution of the H ions from magnifications (Fig. 3), indicating the binding

its column within 5 h. This material possessed a of inorganic ion-exchange material by organic1

better Na ion-exchange capacity (1.67 mequiv. polymer, i.e., polyaniline. It has been revealed21

g ) as compared to simple Sn(IV) tungstoarse- that Sn(IV) tungstoarsenate shows a plate like21

nate (1.12 mequiv. g ) and some other similar morphology. After the binding of polyanilinematerials, i.e., double salts of tetravalent metals, with Sn(IV) tungstoarsenate, the morphologyprepared earlier (Table 2). has been changed. The detail analysis of these

The pH-titration curves obtained under SEM photographs informs that its particle sizeequilibrium conditions for LiOH/LiCl, NaOH/ may be about 3.0 mm. The X-ray powderNaCl and KOH/ KCl systems indicated bifunc- diffraction pattern of this cation exchanger intional behavior of the material as shown in Fig. original form (sample S-1) clearly exhibited the1. It appears to be a strong cation exchanger as presence of two sharp peaks with d -values 3.31

˚ ˚ indicated by a low pH (|2.5) of the solution A and 1.66 A at angles (2u ) 26.915 and2

when no OH ions were added to the system. 55.2108, respectively, that suggesting a1 1

The rate of H –Na exchange was faster than semicrystalline nature of the material.1 1 1 1

those of H –K and H –Li exchanges. The The thermogravimetry (TGA) analysis curvetheoretical ion-exchange capacity for these ions (Fig. 4) of the material showed a continuous

21was found to be |3.2 mequiv. g . weight loss of mass (about 6.0%) up to 198 8C,

The solubility experiments showed that the which may be due to the removal of the externalmaterial has reasonably good chemical stability. water molecules [20]. An inflection point ob-As the results indicated that the material was served at 99.01 8C may be due to the formationresistant to 4 M HNO and 4 M H SO with of As O by removal of water molecules from

3 2 4 2 5

higher solubility in NH and in alkaline media initial composition As O ?nH O. Slow weight3 2 5 2and slightly higher solubility in HCl, citric acid loss observed between 200 8C and 271 8C mayand oxalic acid. The chemical dissolution in be due to a slow decomposition of the material.DMW, acetone, DMS, n-butanol, formic acid, Further weight loss between 275 8C and 672 8CCH COOH, CH COONa, NH NO was almost may be due to complete decomposition of the

3 3 4 3

negligible. This chemical stability may be due organic part of the material. At 675 8C onwardsto the presence of binding polymer, which can a smooth horizontal section which representsprevent the dissolution of heteropolyacid salt or the complete formation of the oxide form of theleaching of any constituent elements into the material. These transformations have also beensolution. On heating at different temperatures supported by differential thermal analysis

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Table 2

Comparison of the preparation and properties of polyaniline Sn(IV) tungstoarsenate with those of other cation exchangers

Ion-exchange materials Reagents Mixing ratio

Polyanil ine Sn(I V) tungstoarsenate 0 .1 M Tin( IV)chloride10.1 M sodium tungstate10.1 M sodium 1:1:1:1:1

(sample S-1) arsenate110% aniline10.1 M K S O2 2 8

Sn(IV) tungstoarsenate 0.25 M Tin(IV)chloride10.25 M sodium tungstate10.25 M sodium arsenate 2:1:1

[18]

Sn(IV) tungstoarsenate 0.1 M Tin(IV)chloride1

0.1 M sodium tungstate1

0.1 M sodium arsenate 1:1:1 (as prepared)

Polyaniline Sn( IV ) ars enophosphate 0.1 M Tin( IV )chloride10.1 M sodium arsenate10.1 M H PO 1 1:1:1:1:1 3 4

[13] 10% aniline10.1 M (NH ) S O4 2 2 8

Polyani line Zr( IV) tungstophosphate 0 .1 M Zirconium oxychloride10.1 M sodium tungstate10.1 M ammonium 2:1:2:1:2

[19] sodium hydrogen phosphate1aniline10.4 M (NH ) S O4 2 2 8

Polyaniline Sn(IV) tungstate (as prepared) 0.1 M Tin(IV)chloride10.1 M sodium tungstate110% aniline10.1 M K S O 1:1:1:1 2 2 8

Polyaniline Sn(IV) arsenate 0.1 M Tin(IV)chloride10.1 M sodium arsenate110%aniline10.1 M K S O 1:1:1:1 2 2 8

(as prepared)

Polyaniline 10% Aniline10.1 M K S O 1:1 2 2 8

(as prepared)

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Fig. 2. Comparison of heating effect upon ion-exchange capacity.

(DTA). The DTA curve indicates two exother- 1:1:1.75:11.71:27.74:1.57, which can suggestmic peaks with maxima at 84.46 and 480.20 8C the following tentative formula of the material:that confirm the removal of external water

2

[(SnO )(WO )(As O ) ( –NH ) ] ?2 3 2 5 4 2molecules and structural transformations, re-

nH Ospectively. 2The FT-IR spectrum of the exchanger in the

and its structure can be written as:1H -form indicated the presence of externalwater molecules in addition to the –OH groupsand metal oxides present internally in the ma-terial. In the spectrum two weak broad bands

21around 3500 cm are found, which can beattributed to O–H stretching frequency. A

21medium band around 1600 cm can be attribu-ted to H–O–H bending band, being also repre-sentative of the strongly bonded –OH groups inthe matrix [21]. The O–H stretching bandsmerge together and are shifted to lower fre-quency in the spectrum of the exchanger. This isdue to the possibility of H-bonding. The sharp

21

peak around 1300 cm may represent the32

presence of [AsO ] group in the material. The4

21additional band at about 1400 cm can be

Assuming that only the external water mole-ascribed to stretching vibration of C–N [22].

cules are lost, at 198 8C the |7.0% weight lossThis indicates that the material contains a

of mass represented by the TGA curve must beconsiderable amount of aniline. It was also

due to the loss of nH O from the above2observed that there is no difference in the FT-IR

structure. The value of ‘n’, the external water1 1spectra between the H -form, Na -form and

molecules, can be calculated using Alberti’soriginal form of the sample S-1 dried at 50 8C.

equation [23]:The molar ratio of Sn, W, As, C, H and N in

18n 5 X ( M 1 18n)/100the material was estimated to be

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Fig. 3. Scanning electron microphotographs of chemically prepared Sn(IV) tungstoarsenate at the magnifications of 1003 and 5

tungstoarsenate at the magnifications of 1003 and 25003 (b, b9).

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al. The calculation gives |5 for the externalwater molecules (n) per molecule of the cationexchanger (sample S-1).

In order to explore the potentiality of thematerial in the separation of metal ions, dis-tribution studies for 24 metal ions were per-formed in seven solvent systems. It is apparentfrom the data given in Table 3 that the K -

d

values can vary with the composition and natureof the contacting solvents. On the basis of distribution studies, the most promising propertyof the material was found to be the high

Fig. 4. Simultaneous TGA–DTA curve of polyaniline Sn(IV) selectivity towards Cd(II), which is a majortungstoarsenate (as prepared).

polluting metal in the environment. The sepa-ration capability of the material has been dem-

onstrated by achieving some important binary2 1 2 1 21 21where, X is the percent weight loss (|6.0%) in separations such as Cd –Zn , Cd –Pb ,

2 1 2 1 21 21 21 31 21the exchanger by heating up to 198 8C, and Cd –Hg , Cd –Mg , Cd –Cr , Cd –

2 1 2 1 21( M 118n) is the molecular weight of the materi- Cu and Mg –Ba . Table 4 summarizes the

Table 3

K -values of some metal ions on polyaniline Sn(IV) tungstoarsenate in different solvent systemsd

23 22 21Metal DMW 1310 M 1310 M 1310 M 0.1 M CH COOH1 0.1 M CH COOH1 0.1 M HNO 13 3 3

ion HNO , HNO , HNO , 0.1 M CH COONa 0.1 M CH COONa 0.1 M NH NO3 3 3 3 3 4 3

pH 3 pH 2 pH 1 (1:2) (2:1) (1:1)

1Na 36 33 100 21 9 12 201

K 446 525 485 295 148 192 3521

Mg 33 35 35 10 84 105 521

Ca 117 123 38 32 238 217 11421

Sr 198 140 39 38 767 600 1421

Ba 440 333 180 50 2500 2600 2521

Cu 180 180 170 56 240 600 821

Ni 400 233 180 25 300 700 3321

Pb 203 600 100 64 900 500 21021

Cd 2920 4228 1400 450 544 810 2721

Mn 233 267 141 71 650 700 5021

Zn 263 350 281 275 400 2067 12021

Hg 133 367 350 300 833 1500 24321Co 300 275 44 30 550 600 83

21Bi 1000 2200 1100 1000 138 600 667

31Cr 767 475 191 40 38 200 36

31Al 86 71 40 44 250 440 140

31Fe 100 83 43 22 300 400 69

31La 225 170 155 63 550 767 13

41Th 340 360 300 266 130 210 92

41Ce 122 114 75 40 133 300 7

1Ag 67 92 114 74 44 86 18

21UO 75 85 107 130 160 245 562

1Tl 155 100 117 86 100 130 30

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Table 4

Some binary separations of metal ions achieved on the polyaniline Sn(IV) tungstoarsenate column

Separations Amount loaded Amount found % Error Eluent Volume of

achieved (mg) (mg) used eluent (ml)

Cd(II) 1686.0 1663.5 21.4 A 40

Zn(II) 980.85 993.9 11.3 B 50

Cd(II) 1967.0 1967.0 0.0 A 50

Pb(II) 2591.3 2621.1 21.2 C 50

Cd(II) 1742.2 1719.7 21.3 A 40

Hg(II) 1905.7 1935.8 11.6 B 50

Cd(II) 1124.0 1107.1 21.5 A 40

Mg(II) 364.7 370.7 11.7 B 60

Cd(II) 1686.0 1674.7 20.7 A 40

Cr(III) 780.0 790.2 11.3 D 60

Cd(II) 1124.0 1135.2 11.0 A 40Cu(II) 635.5 627.9 21.2 C 50

Mg(II) 243.1 243.1 0.0 B 40

Ba(II) 1373.0 1359.3 21.0 C 50

A50.1 M HNO ; B5DMW; C50.1 M HNO 10.1 M NH NO (1:1); D50.1 M CH COOH10.1 M CH COONa (1:2).3 3 4 3 3 3

salient features of these separations. It was also The heterogeneous precipitate Cd(II) ion-selective membrane electrode obtained fromobserved that Cd(II) retained strongly on thepolyaniline Sn(IV) tungstoarsenate exchangercation exchanger column. The weakly retainedmaterial gives linear response (Fig. 5) in themetal ions appear out of the column faster than

21 24Cd(II) and Cd(II) was eluted after by HNO given range of 1310 –1310 M with a

3

solution. slope of 27 mV per decade change in Cd(II) ionconcentration and the slope value is nearly closeto Nernstian value, 29.6 mV/concentration de-

24 cade for divalent cations. Below 1310 M, anon linear response was observed but the cali-bration curve could be utilized for the determi-

24nation of Cd(II) down to 1310 M. A con-stant potential was obtained after 25 s and it

was also observed that the electrode potentialremained unchanged within the pH range 3.0–8.0.

PO TThe selectivity coefficients, K of various

Cd.M

interfering cations for the Cd(II) ion-selectivepolyaniline Sn(IV) tungstoarsenate electrodewere determined by the mixed solution method[24] and the following numerical values wereFig. 5. Calibration curve for polyaniline Sn(IV) tungstoarsenate

membrane electrode in aqueous solutions of Cd(NO ) . obtained:3 2

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POT n1Electrode Selectivity coefficients (K ) for interfering cations (M )

Cd.M

1 1 21 21 2 1 21 21 21 21 21 31 31Na K Mg Ca Sr Cu Mn Zn Pb Hg Al Fe

Polyaniline Sn(IV) 0.02 0.03 0.02 0.06 0.08 0.09 0.03 0.04 0.18 0.07 0.04 0.05

tungstoarsenate

Thus the selectivity coefficient indicates the Applied Chemistry, Z.H. College of Engineer-n1

extent to which a foreign ion (M ) interferes ing and Technology, Aligarh for providingwith the response of the electrode towards its research facilities, and Central Drug Research

21primary ion (Cd ). The results reveal that the Institute (Lucknow) and Regional Sophisticatedelectrode is selective for Cd(II) in presence of Instrumentation Center (Nagpur) for technicalinterfering cations. assistance.

The analytical utility of this membrane elec-trode has been established by employing it as anindicator electrode in the titration of 0.01 M References

Cd(NO ) solution against 0.01 M EDTA solu-3 2tion. It was observed that a sharp rise in the [1] A. Clearfield (Ed.), Inorganic Ion Exchange Materials, CRC

Press, Boca Raton, FL, 1982.titration curve occurs at the equivalence point[2] C.Y. Yang, A. Clearfield, React. Polym. 5 (1987) 13.(Fig. 6). It has been used in the determination of [3] M.B. Dines, P.D. Giacomo, K.P. Callahan, P.C. Griffith, R.H.21

Cd ions in some synthetic samples containing Lane, R.E. Cooksey, Chemically Modified Surfaces inCatalysis and Electrocatalysis, ACS Symposium Series 192,a number of different metal ions as well asACS, Washington, DC, 1982.samples obtained from electroplating units and

[4] A. Clearfield, in: Proceedings of the International Conferencethe error has been found as 64–9%. on Ion Exchange, ICIE ‘91, Tokyo, New Developments in

Ion Exchange, 1991, p. 121.

[5] U. Costantino, R.Vivani, in: Proceedings of the International

Conference on Ion Exchange, ICIE ‘91, Tokyo, New De-

Acknowledgements velopments in Ion Exchange, 1991, p. 205.[6] S. Tandon, B. Pandit, U. Chudasma, Transition Met. Chem.

21 (1996) 7.The authors are thankful to the Department of [7] B. Zhang, D.M. Poojary, A. Clearfield, G. Peng, Chem.

Mater. 8 (1996) 1333.

[8] G. Alberti, U. Costantino, R. Millini, R.Vivani, J. Solid State

Chem. 113 (1994) 289.

[9] G. Alberti, M. Casciola, C. Dionigi, R. Vivani, in: Proceed-

ings of the International Conference on Ion Exchange, ICIE

‘95, Takamatsu, 1995.

[10] K.G. Varshney, N. Tayal, A.A. Khan, R. Niwas, Coll. Surf.

181 (2001) 123.

[11] J.P. Rawat, M. Igbal, Ann. Chim. (1981) 431.

[12] D.K. Singh, A. Darbari, Bull. Chem. Soc. Jpn. 61 (1988)

1369.

[13] R. Niwas, A.A. Khan, K.G.Varshney, Coll. Surf. 150 (1999)

7.

[14] E. Pungor, K. Toth, in: H. Freiser (Ed.), Ion Selective

Electrodes in Analytical Chemistry, Vol. 1, Plenum Press,

New York, 1978, p. 143, Chapter 2.

[15] N.E. Topp, K.W. Pepper, J. Chem. Soc. (1949) 3299.

[16] N.H. Furman, in: 6th Edition, Standard Methods of Chemical

Analysis, Vol. 1, Van Nastrand, Princeton, NJ, 1963, p. 631.

[17] C.N. Reiliy, R.W. Schmidt, F.S. Sadek, J. Chem. Educ. 36

Fig. 6. Precipitation titration of Cd(II) against EDTA solution. (1959) 555.

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[18] M. Qureshi, R. Kumar, V. Sharma, T. Khan, J. Chromatogr. [22] Reference 21, p. 250.

118 (1976) 175. [23] G. Alberti, E. Torracca, A. Conte, J. Inorg. Nucl. Chem. 28

[19] S. Ikram, Ph.D. Thesis, D.C.E., Delhi, 2000. (1966) 607.

[20] C. Duval, in: Inorganic Thermogravimetric Analysis, [24] G.J. Moody, J.R.D. Thomas, Selective Ion Sensitive Elec-

Elsevier, Amsterdam, 1963, p. 315. trode, Marrow, Watford, 1971.

[21] C.N.R. Rao, in: Chemical Applications of Infrared Spec-

troscopy, Academic Press, New York, 1963, p. 355.

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33. Pani Snwaso4