New and novel organic–inorganic type crystalline ‘polypyrrolel/polyantimonic acid’ composite...

Analytica Chimica Acta 504 (2004) 253–264

New and novel organic–inorganic type crystalline‘polypyrrolel/polyantimonic acid’ composite system: preparation,characterization and analytical applications as a cation-exchange

material and Hg(II) ion-selective membrane electrode

Asif Ali Khan∗, M. Mezbaul Alam

Analytical and Polymer Research Laboratory, Department of Applied Chemistry, Faculty of Engineering and Technology,Aligarh Muslim University, Aligarh 202002, India

Received 24 July 2003; received in revised form 7 October 2003; accepted 21 October 2003

Abstract

A new phase of ‘organic–inorganic’ composite system, polypyrrole polyantimonic acid, prepared by mixing the inorganic precipitate ofhydrated antimony oxide with organic conducting polymer i.e., polypyrrole, providing a novel granular form hybrid cation-exchanger suitablefor column operation with better chemical and thermal stability, good ion-exchange capacity, reproducibility and selectivity for heavy metals.The physicochemical properties of this material were studied using elemental analyses, AAS, SEM, XRD, FTIR and simultaneous TGA-DTAstudies. Ion-exchange capacity, pH-titrations, elution and distribution behavior were also carried out to characterize the material. Distributionstudies revealed the cation-exchange material to be highly selective for Hg(II) and its selectivity was performed by achieving some importantbinary separations like Hg2+–Zn2+, Hg2+–Ni2+, Hg2+–Cu2+, Hg2+–Fe3+, Hg2+–Cd2+, Hg2+–Mg2+ etc. on its column. Using this electroactivecomposite material, a new heterogeneous precipitate based selective ion-sensitive membrane electrode was fabricated for the determinationof Hg(II) ions in solutions. The membrane electrode is mechanically stable, with a quick response time, and can be operated within a wide pHrange. The selectivity coefficients for different cations determined by mixed solution method were found to be less than unity The electrodewas also found to be satisfactory in electrometric titrations.© 2003 Elsevier B.V. All rights reserved.

Keywords: Organic–inorganic composite material; Hybrid cation-exchanger; Polypyrrole polyantimonic acid; Hg(II) ion-selective membrane electrode

1. Introduction

Composite materials formed by the combination of in-organic materials and organic polymers are attractive forthe purpose of creating high performance or high functionalpolymeric materials that are expected to provide many possi-bilities, termed as ‘organic–inorganic’ hybrid materials. Ac-cordingly, hybrids can be used to modify organic polymermaterials or to modify inorganic materials that exhibit verydifferent properties from their original components (organicpolymer and inorganic materials), especially in the case ofmolecular level hybrids[1–5]. These hybrid materials should

∗ Corresponding author. Tel.:+91-571-2720323.E-mail address: [email protected] (A.A. Khan).

be considered as next generation composite materials thatwill encompass a wide variety of applications. Some im-provements of the properties or modifications of these ma-terials have been explored from the viewpoint of industrialapplications.

In order to achieve stable materials with chromatographicproperties interest has been generated in ‘organic–inorganic’composite ion-exchange materials. An inorganic precipitateion-exchanger based on organic polymeric matrix must bean interesting material, as it should possess the mechanicalstability due to the presence of organic polymeric speciesand the basic characteristics of an inorganic ion-exchangerregarding its some selectivity for some particular metalions [6–16]. It was therefore considered to synthesize suchhybrid ion-exchangers with a good ion-exchange capac-ity, high stability, reproducibility and selectivity for heavy

0003-2670/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.aca.2003.10.054

254 A.A. Khan, M.M. Alam / Analytica Chimica Acta 504 (2004) 253–264

metal ions, indicating its useful environmental applica-tion. A number of organic–inorganic composite samples ofpolypyrrole polyantimonic acid have been synthesized inour laboratory that possessed such characteristics and highselectivity for mercury.

Mercury is considered as highly toxic element and re-sponsible for several cases of poisoning through water, foodand smoking. It finds extensive use in chlor-alkali industry(for the manufacture of Cl2 and NaOH), electrical and elec-tronic industries (in manufacture of mercury vapor lampsand fluorescent tubes, batteries, electric switchgears, etc.),plastics industry (in the manufacture of vinyl chloride), pa-per and pulp industry and pharmaceutical industries. Hencethe effluents from these industries pose environmental haz-ard. Organomercurials used as fungicides for seed dress-ing in agriculture are also a widespread source of pollution.Combustion of fossil fuels is the main source of air pollu-tion by mercury. This element is now mostly used as fillingmaterial for dental cavities as silver amalgam.

Mercury contamination in drinking water inhibits thefunction of certain enzymes necessary for the formationin bone marrow of haem, the pigment that combines withprotein to form haemoglobin. Its vapor, on inhalation, en-ters the brain through the blood stream and causes severedamage to the central nervous system. Inorganic mercuriccompounds mainly attack liver and kidney. Mercuric chlo-ride is corrosive and, when ingested, precipitates proteinsof the mucous membrane causing ashen appearance of themouth, pharynx and gastric mucosa. Organic mercurials arethe most toxic substances; the CH3Hg+ can pass throughthe placental barrier and enter the foetal tissues. This cancause irreversible damage to the central nervous systemin the babies born to mothers exposed to poisoning byCH3Hg+. This compound may even lead to chromosomalsegregation, chromosome disruption and inhibition of celldivision. Hg is therefore a potential pollutant in the envi-ronment. The utility of this composite cation-exchanger hasbeen explored for the quantitative separation of Hg2+ fromsome binary mixtures on its column.

Precipitate based ion-selective membrane electrodes arewell known as they are successfully employed for determi-nation of several anions and cations[17]. The ion-exchangemembranes obtained by embedding ion-exchangers as elec-troactive materials in a polymer binder, i.e. epoxy resin(Araldite) or polystyrene or PVC, have been extensivelystudied as potentiometric sensors, i.e. ion sensors, chem-ical sensors or more commonly ion-selective electrodes.An attempt has also been made to obtain a new heteroge-neous precipitate based membrane electrode by using thepolypyrrole polyantimonic acid composite cation-exchangeras electroactive material for the determination of Hg(II) ionspresent in the solutions. This paper presents the preparativeconditions, ion-exchange properties, physicochemical prop-erties and analytical applications of this composite materialused as a cation-exchanger and Hg(II) ion-selective mem-brane electrode.

2. Experimental

2.1. Reagents and instruments

The main reagents used for the synthesis of the mate-rial were obtained from CDH, Loba Chemie, E-merck andQualigens (India). All other reagents and chemicals were ofanalytical reagent grade. A digital pH-meter (Elico LI-10, In-dia), FTIR spectrophotometer (Nicolet 400D, USA), an au-tomatic thermal analyzer (V2.2A DuPont 9900), an elemen-tal analyzer (Carlo-Erba 1180), a double beam atomic ab-sorption spectrophotometer (GBC 902, Australia), an elec-tron microscope (LEO 435 VP, Australia) with attachedimaging device, a digital flame photometer (Elico CL 22D,India), a UV-Vis spectrophotometer (Elico EI 301E, India),a temperature controlled shaker and a digital potentiometer(Equiptronics EQ 609, India) with a saturated calomel elec-trode as reference electrode were used.

2.2. Preparation of inorganic precipitate ofpolyantimonic acid

The products of polyantimonic acid were obtained bydissolving 0.05 to 0.2 mol of potassium pyroantimonate in5.8 M HCl. The solutions were kept overnight at room tem-perature (25± 2 ◦C) and some solutions were refluxed for16 h and thereafter neutralized with conc. NH4OH until aresidual acidity of 0.75 M HCl was obtained. A white gelwas obtained in each case.

2.3. Preparation of polypyrrole polyantimonic acidcomposite cation-exchanger

Approximately 33.33% solutions of pyrrole (in CCl4) indifferent ratios were mixed thoroughly with the inorganicprecipitates of polyantimonic acid, to which 0.1 M FeCl3(ferric chloride) solutions prepared in demineralized water(DMW) were added drop wise. Continuous stirring was doneduring the addition of ferric chloride solutions, slowly thewhite inorganic precipitate gels turned first green and thenblack. The reaction mixtures were then kept for 24 h underambient conditions (25±2 ◦C). Now these polypyrrole basedcomposite gels were filtered off; washed with 0.75 M HCland then washed thoroughly with DMW to remove excessacids and any adhering traces of ferric chloride. After filtra-tion the gels were dried at 50◦C in an air oven for 48 h. Theproducts were again washed with acetone by soxhlation andfinally dried at 50◦C. The dried products were immersedin DMW to obtain small granules. These were converted tothe H+ form by placing them in 0.5 M HNO3 solution for24 h with occasional shaking intermittently, replacing thesupernatant liquid with a fresh acid. The excess acid wasremoved after several washings with DMW and again thematerials were dried at 50◦C and sieving to obtain particlesof particular size range (∼125�m) and kept in a desiccator.Hence, a number of composite cation-exchanger samples

A.A. Khan, M.M. Alam / Analytica Chimica Acta 504 (2004) 253–264 255

Table 1Synthesis of various samples of polypyrrole polyantimonic composite cation-exchanger

Samples Molarity ofKSb(OH)6 (M)

Mixing volume ratio (v/v) Appearance of thebeads after drying

Ion-exchange capacityfor Na+ (meq g−1)

KSb(OH)6 in5.8 M HClsolution

Residual acidity ofthe inorganicprecipitate (M HCl)

Pyrrole inCCl4(33.33%)

FeCl3(0.1 M)

S-1 0.1 1 0.75 0.015 0.3 Blackish granular 2.90S-2 0.1 1 0.75 0.030 0.4 Blackish granular 3.19S-3 0.1 1 0.75 0.045 0.5 Blackish granular 2.75S-4 0.05 1 0.75 0.030 0.3 Blackish granular 2.84S-5 0.2 1 0.75 0.030 0.5 Blackish granular 3.10aS-6 0.1 1 0.75 0.015 0.3 Blackish granular 2.43aS-7 0.1 1 0.75 0.030 0.4 Blackish granular 2.91aS-8 0.1 1 0.75 0.045 0.5 Blackish granular 2.54S-9 0.1 1 0.75 – – White granular 2.83S-10 – – – 1 10 Black powder 0.05

a Refluxed for 16 h.

of polypyrrole polyantimonic acid were prepared by chang-ing the mixing volume ratios of the reagents. On the basisof Na+ ion-exchange capacity and physical appearance ofbeads, sample S-2 (Table 1) was chosen for further studies.

2.4. Ion-exchange capacity (IEC)

One gram of the dry cation-exchanger, sample S-2 inH+-form, was taken into a glass column having an inter-nal diameter (i.d.)∼1 cm and fitted with glass wool supportat the bottom. The bed length was approximately 1.5 cmlong. Alkali and alkaline earth metal nitrates (0.8 M) as elu-ants were used to elute the H+ ions completely from thecation-exchange column, maintaining a very slow flow rate(∼0.5 ml min−1). The effluent was titrated against a standard(0.1 M) NaOH solution and the ion-exchange capacities inmeq g−1 were: Li+, 2.63; Na+, 3.19; K+, 3.25; Mg2+, 3.41;Ca2+, 3.79; Sr2+, 4.05 and Ba2+, 3.76.

2.5. pH-titration

pH-titrations were performed by the method of equi-librium process[9]. A total of 500 mg portions of thecation-exchanger were placed in each of the several 250 mlconical flasks, followed by equimolar solutions of alkalimetal chlorides and their hydroxides in different volumeratio, the final volume being 50 ml to maintain the ionicstrength constant. The pH of the solution was recorded every24 h until equilibrium was attained which needed∼7 days,and pH at equilibrium was plotted against the milliequiva-lents of OH− ions added. The results are shown inFig. 1.

2.6. Chemical dissolution

Two hundred and fifty milligrams portions of thecation-exchanger in H+-form were equilibrated with 25 mleach of different solvents for 24 h with occasional shak-ing. The supernatant liquid was analyzed for ‘antimony’by Vis spectrophotometric method[18]. The results aresummarized inTable 2.

2.7. Chemical composition

After dissolving in concentrated hydrochloric acid, thesample S-2 was analyzed for ‘antimony’ by Vis spectropho-tometric method. Carbon, hydrogen and nitrogen contentsof the material were determined by elemental analysis. Theweight percent composition of the material was: Sb, 54.46;C, 10.29; H, 2.58; N, 2.92 and O, 29.75.

2.8. Thermal studies

To study the effect of drying temperature on the IEC, 1 gsamples of the material in H+-form were heated at varioustemperatures in a muffle furnace for 1 h each and the Na+ion-exchange capacity was determined by column process

Fig. 1. pH-titration curves for polypyrrole polyantimonic acid compositecation-exchanger with various alkali metal hydroxides.


Table 2The solubility of polypyrrole polyantimonic acid composite in varioussolvent systems

Solvent (25 ml) Sb(mg/25 ml)

Solvent (25 ml) Sb(mg/25 ml)

2 M HNO3 6.32 1 M NH3 25.54 M HNO3 36.1 1 M NaNO3 1.312 M HCl 6.74 1 M CH3COOH 2.404 M HCl 34.7 1 M Citric acid 17.32 M H2SO4 4.38 1 M Oxalic acid 15.24 M H2SO4 24.0 1 M Formic acid n.d.2 M HClO4 4.13 Dimethyl sulphoxide

(DMSO)8.64

4 M HClO4 22.6 n-Butanol 14.00.1 M NaOH 28.3 Acetone n.d.0.1 M KOH 45.0 DMW 1.35

n.d. = not dissolved.

after cooling them at room temperature. The results are givenin Table 3.

Simultaneous TGA-DTA studies were also carried out onheating the sample material S-2 (as-prepared) up to 800◦Cat a constant rate (∼15◦C/min) in the air atmosphere.Fig. 2 shows the TGA-DTA curves of the cation-exchangematerial.

Table 3Thermal stability of polypyrrole polyantimonic acid after heating to various temperatures for 1 h

Drying temperature (◦C) Change in color Weight loss (%) Na+ ion-exchange capacity (meq dry g−1) Retention of IEC (%)

50 Black 0.0 3.19 100100 Black 1.24 3.17 99.37150 Black 3.83 3.14 98.43200 Black 5.82 3.09 96.87400 Light black 16.77 1.86 57.10500 Pale yellow 22.55 1.91 59.88600 Pale yellow 23.66 1.89 59.24

Fig. 2. Simultaneous TGA-DTA curves of polypyrrole polyantimonic acid (as-prepared).

2.9. FTIR studies

The FTIR spectrum of the sample S-2 (as-prepared) driedat 50◦C was taken by KBr disc method and is given inFig. 3.

2.10. X-ray studies

Powder X-ray diffraction pattern of the material S-2(as-prepared) was recorded by a PW 1710 based diffrac-tometer (Phillips, Holland) with Cu K� radiation and isshown inFig. 4.

2.11. SEM (scanning electron microscopy) studies

SEM was performed on ground materials by an electronmicroscope at various magnifications.Fig. 5 presents themicrophotographs of samples S-2 and S-9.

2.12. Sorption (selectivity) studies

The distribution coefficients (Kd-values) of metal ions onthe sample material (S-2) were determined by batch methodin various solvent systems. Various 200 mg portions of the


Fig. 3. FTIR spectrum of polypyrrole polyantimonic acid (as-prepared).

exchanger in the H+-form were taken in Erlenmeyer flaskswith 20 ml different metal nitrate solutions in the requiredmedium and kept for 24 h with intermittent shaking or con-tinuous shaking for 6 h in a shaker at 25±2 ◦C to attain equi-librium. The initial metal ion concentration was so adjustedthat it did not exceed 3% of its total ion-exchange capacity.The metal ions in the solutions before and after equilibriumwere determined by EDTA titration[19]. The alkali and al-kaline earth metal ions (K+, Na+, Ca2+) were determined by

Fig. 4. X-ray diffraction pattern of polypyrrole polyantimonic acid (as-prepared).

flame photometry and some heavy metal ions, such as Pb2+,Cd2+, Cu2+, Cr3+, Hg2+, Ni2+, Mn2+ and Zn2+ were de-termined by AAS. Distribution coefficients were calculatedusing the formula given as:

Kd = mmoles of metal ions/g of ion–exchanger

mmoles of metal ions/ml of solution(ml g−1)

i.e. Kd = I − F

F× V

M(ml g−1)


Fig. 5. Scanning electron microphotographs (SEM) of polyantimonic acid at the magnifications of 43× (a) and 2500× (a′), and polypyrrole polyantimonicacid at the magnifications of 100× (b) and 2500× (b′).

where I is the initial amount of metal ion in the aqueousphase,F is the final amount of metal ion in the aqueousphase,V is the volume of the solution (ml) andM is theamount of cation-exchanger (g).

2.13. Quantitative separation of metal ions

Quantitative separations of some important metal ionsof analytical utility were achieved on ‘polypyrrole polyan-timonic acid’ (sample S-2) column. Two grams of thecation-exchanger in H+-form was packed in an open glasscolumn (35 cm height and∼0.6 cm i.d.). After washing thecolumn thoroughly with DMW, the mixture of two metalions of 0.01 M each, was loaded on it, and was allowed topass gently (maintaining a flow rate of 2–3 drops/min) tillthe level was above the surface of the material. The processwas repeated two or three times to ensure complete adsorp-tion of the mixture on bead. The separation was achievedby passing a suitable solvent at a flow rate of 1 ml min−1

through the column as eluant. The metal ions in the ef-

fluent were determined quantitatively by AAS and EDTAtitration.

2.14. Fabrication of ion-selective electrode

The cation-exchanger (400 mg) was ground to fine pow-der, and was mixed thoroughly with PVC (E-Merck) so-lution (0.17 g PVC dissolves in 6 ml THF), and a mastermembrane of 0.31 mm thickness was prepared. A piece ofmembrane was cut out and fixed at one end of a Pyrexglass tube (o.d. 0.8 cm, i.d. 0.6 cm) with Araldite. The tubewas filled with 0.1 M mercuric nitrate. A saturated calomelelectrode was inserted in the tube for electrical contact andanother saturated calomel electrode was used as externalreference electrode. The whole arrangement can be shownas:

Internal reference electrode(SCE) | Internal electrolyte

(0.1 MHg2+) | Membrane| Sample solution| External

reference electrode(SCE)


In advance of measurements of the electrode poten-tial (at 25± 2 ◦C) for a series of standard solutions ofHg(NO3)2 (10−7 to 10−1 M), the membrane of the elec-trode was conditioned by soaking in 0.1 M Hg(NO3)2solution for five days and at least for 1 h before use. Inorder to study the characteristics of the electrode, the fol-lowing parameters were evaluated: lower detection limit,slope response curve, response time and working pHrange.

3. Results and discussion

In this work, incorporating polypyrrole into inorganic ma-trices of polyantimonic acid by changing the mixing vol-ume ratios of the reagents developed various samples of anew and novel polypyrrole based ‘organic–inorganic’ com-posite cation-exchange material, polypyrrole polyantimonicacid system. Among them, sample S-2 (Table 1) possessedgood yield, better ion-exchange capacity, reproducible be-havior and chemical and thermal stability.

The polymerization reaction for the synthesis of polypyr-role is a very complicated one. The initial oxidation step,in which a radical cation is formed, is followed by a cou-pling reaction, deprotonation, and one-electron oxidation inorder to regenerate the aromatic system[20], using FeCl3 inaqueous medium at room temperature (25± 2 ◦C) as givenin the following reactions:

The formation of inorganic precipitate polyantimonic acidwas carried out by adjusting the pH of the highly acidicpotassium pyroantimonate solution, and at pH∼0.125 theprecipitate material showed good percentage of yield and

high IEC. Since it was known that in slightly acidic solutionantimony salts are polymerized, the inorganic precipitate hasbeen termed as polyantimonic acid. When aqueous solutionof FeCl3 was added with the inorganic precipitate, Fe3+ mayconvert antimonic acid into a radical that can be shown as:

Sb–OH+ Fe3+ → Sb–O• + H+ + Fe2+

Hence, the binding of polypyrrole into the matrix ofpolyantimonic acid can be shown as:

However, sample S-2 of polypyrrole polyantimonic acidexhibited fair granulometric and mechanical properties,showing a good reproducible behavior as is evident from thefact that these materials obtained from various batches didnot show any appreciable deviation in their ion-exchange ca-pacities. It was also found that the values of H+-adsorptionand H+-liberation capacities are in close agreement. Thismaterial possessed a better Na+ ion-exchange capacity(3.19 meq g−1) as compared to simple polyantimonic acid(2.83 meq g−1) and some other similar materials i.e. sin-gle and double salts of tetravalent metals, prepared earlier(Table 4).

The column elution experiments indicated a dependenceof the concentration of the eluant on the rate of elution,which is a usual behavior for such materials. The mini-mum molar concentration of NaNO3 as eluant was 0.8 Mfor the maximum release of H+ ions from 1 g of thecation-exchanger. The elution was appreciably fast as only130 ml of the effluent was sufficient for almost completeelution of the H+ ions from its column within 4 h. The ef-fect of the size and charge of the exchanging ion on the IECwas also observed for this material. The IEC of the com-posite cation-exchanger for alkali metal ions and alkaline

earth metal ions (except Ba2+) increased according to thedecrease in their hydrated ionic radii. This is in agreementwith theoretical considerations.


Table 4Comparison of the preparation and properties of polypyrrole polyantimonic acid with those of other cation-exchangers

Ion-exchange materials Reagents Mixingratio

pH of the inorganicprecipitates

Na+-exchang capacity(meq dry g−1)

Polypyrrole polyantimonic acid(sample S-2)

0.1 M potassium pyroantimonate in 5.8 M HCl+ 33.33% pyrrole in CCl4 + 0.1 M FeCl3

1:0.03:0.5 0.125 3.19

Polyantimonic acid (sample S-9) 0.1 M potassium pyroantimonate in 5.8 M HCl+ conc. NH4OH

– 0.125 2.83

Antimonic(V) acid [21] 20% antimony metal in mixed solution of HCl+ HNO3 (4:1) + distilled water

1:13 0–0.6 1.30 (for K+)

Antimony phosphate[22] 0.1 M potassium pyroantimonate+ 0.05 Morthophosphoric acid

1:1 0.0–1.0 2.05

Sn(IV) antimonate[23] 0.05 M tin(IV)chloride+ 0.05 M potassiumpyroantimonate

1:2 0.0–1.0 2.40

Antimony(III) molybdate[24] 0.1 M sodium molybdate+ 0.1 M antimony(III)chloride

1:1 0.0 1.02

Antimony(III) arsenate[24] 0.05 M sodium arsenate+ 0.05 M antimony(III)chloride

1:1 0.85 0.70

Antimony silicate[25] 0.1 M antimony pentachloride in conc. HCl+ 0.1 M sodium silicate

1:2 0–1 1.60

Th(IV) antimonite[26] 0.1 M thorium(IV) nitrate in 0.1 M HNO3 + 0.1 Mantimony(V) chloride in 4 M HCl

1:1 0.0 2.00

Antimony(V) arsenophosphate[27]

0.05 M SbCl5 + 0.05 M sodium arsenate+ 0.05 Mtrisodium orthophosphate

3:1:1 0–1 2.20

Polyaniline Sn(IV)tungstoarsenate[6]

0.1 M tin(IV)chloride + 0.1 M sodium tungstate+ 0.1 M sod. arsenate+ 10% aniline+ 0.1 MK2S2O8

1:1:1:1:1 1.0 1.67

Polyaniline Sn(IV)arsenophosphate[9]

0.1 M tin(IV)chloride + 0.1 M sodium arsenate+ 0.1 M H3PO4 + 10% aniline+ 0.1 M(NH4)2S2O8

1:1:1:1:1 1.0 1.58

Polypyrroole (sample S-10) 33.33% pyrrole in CCl4 + 0.1 M FeCl3 1:10 – 0.03

The pH-titration curves obtained under equilibrium con-ditions for LiOH/LiCl, NaOH/NaCl and KOH/KCl systemsindicated bifunctional behavior of the material as shown inFig. 1. It appears to be a strong cation-exchanger as indicatedby a low pH (∼2.6) of the solution when no OH− ions wereadded to the system. The theoretical IEC for these ions wasfound to be∼4.25 meq g−1. The adsorption behavior for al-kali metals was observed to be in the order Na(I) > K(I) >

Li (I) in acidic pH. It is reversed to Li(I) > K(I) > Na(I) inthe basic media.

The solubility experiments showed that the material hasreasonably good chemical stability. As the results indicatedthat the material was resistant to 2 M HClO4 and 2 M H2SO4with higher solubility in NH3 and in alkaline media andslightly higher solubility in 2 M HNO3, 2 M HCl, citric acid,oxalic acid, n-butanol and DMSO. The chemical dissolu-tion in DMW, DMS, acetone, formic acid, CH3COOH andNaNO3 was almost negligible. This chemical stability maybe due to the presence of binding polymer, which can pre-vent the dissolution of heteropolyacid salt or leaching of anyconstituent elements into the solution. On heating at differenttemperatures for one hour, the ion-exchange capacity of thedried material decreased as the temperature increased. How-ever, the material was found to possess higher thermal sta-bility as it maintained about 59% of the initial ion-exchangecapacity by heating up to 600◦C. From a comparative studyof heating effect on Na+ ion-exchange capacity of polypyr-

role polyantimonic acid with those of other ion-exchangers,as shown inFig. 6, it is apparent that this compositecation-exchanger is more thermally stable than others.

Thermogravimetric analysis (TGA) curve (Fig. 2) of thematerial showed a continuous weight loss of mass (about8.0%) up to 250◦C, which may be due to the removal ofthe water of crystallization[28]. Further weight loss be-tween 250 and 530◦C may be due to complete decomposi-tion of the organic part of the material. At 530◦C onwards asmooth horizontal section which represents the complete for-mation of the oxide form of the material. These transforma-tions have also been supported by differential thermal anal-ysis (DTA). The DTA curve indicates two exothermic peakswith maxima at 240 and 508◦C that confirm the removalof external water molecules and structural transformations,respectively.

The FTIR studies (Fig. 3) revealed the presence of ex-ternal water molecules in addition to the –OH groups andthe metal oxides present internally in the material. Fre-quencies due to the Sb–O stretching vibrations have beenobserved in the region 1100–900 cm−1 [29]. The peaks at1640–1560 cm−1 represent the free water molecules (wa-ter of crystallization) and also being representative of thestrongly bonded –OH groups in the matrix[30]. A sharppeak at 1402 cm−1 can be ascribed to stretching vibra-tion of C-N [31]. This indicates that the material containsa considerable amount of pyrrole. The absorption band


Fig. 6. Comparison of heating effect upon ion-exchange capacity.

around 1200 cm−1 may be due to C–O stretching[32] thatcorresponds the binding of Sb with pyrrole.

Scanning electron microscope (SEM) photographs ofpolyantimonic acid and polypyrrole polyantimonic acidwere obtained at different magnifications (Fig. 5), indi-cating the binding of inorganic ion-exchange material byorganic polymer, i.e. polypyrrole. It has been revealed thatpolyantimonic acid shows a plate like morphology. After thebinding of polypyrrole with polyantimonic acid, the mor-phology has been changed. The X-ray powder diffractionpattern of this cation-exchanger (sample S-2, as-prepared)exhibited the presence of fourteen peaks (Fig. 4) indicatingits crystalline character.

The molar ratio of Sb, C, H, N and O in the material wasestimated to be 2.15:4.12:12.31:1.0:8.94, which can suggestthe following tentative formula of the material:

[(Sb2O5)(–C4H4NH–)] · nH2O

and its structure can be written as

Assuming that only the external water molecules are lostat 250◦C, the∼8.0% weight loss of mass represented by theTGA curve must be due to the loss ofnH2O from the abovestructure. The value of ‘n’, the external water molecules, canbe calculated using Alberti’s equation[33]:

18n = X(M + 18n)

100

whereX is the percent weight loss (∼8.0%) in the exchangerby heating up to 250◦C, and (M + 18n) is the molecularweight of the material. The calculation gives∼2 for the

external water molecules (n) per molecule of the compositecation-exchanger (sample S-2).

In order to explore the potentiality of the material inthe separation of metal ions, distribution studies for 15metal ions were performed in eight solvent systems. It isapparent from the data given inTable 5that theKd-valuescan vary with the composition and nature of the con-tacting solvents. It was observed from theKd-values inDMW that Hg2+ is strongly adsorbed; Pb2+, Fe3+, La3+,UO2+ are also significantly adsorbed while the remainingare partially adsorbed. The high uptake of certain metalions demonstrates not only the ion-exchange propertiesbut also the adsorption and ion-sieve characteristics of thecation-exchanger. The difference in adsorption behavior indifferent solvents media is largely explained on the basis ofdifferences in the stability constants of the metal-exchangercomplexes.

On the basis of distribution studies, the most promisingproperty of the material was found to be the high selectivitytowards Hg(II) (a major polluting metal in the environment),indicating its importance in environmental studies. Theseparation capability of the material has been demonstratedby achieving some important binary separations of differ-ent synthetic metal mixtures involving Hg(II), for example:Hg2+–Zn2+, Hg2+–Ni2+, Hg2+–Cu2+, Hg2+–Fe3+ andHg2+–Cd2+, Hg2+–Mg2+, Hg2+–Pb2+ and Hg2+–Al3+.Table 6summarizes the salient features of these separations.The sequential elution of ions from the column dependsupon the stability of metal-eluting ligand (eluent). The or-der of elution and eluents used for these separations arealso illustrated inFig. 7. the separations are quite sharp andrecovery is quantative and reproducible.

The heterogeneous precipitate Hg(II) ion-selective mem-brane electrode obtained from polypyrrole polyantimonicacid cation-exchange material gives linear response (Fig. 8)in the given range of 1× 10−1 to 1 × 10−5 M with aslope of 29 mV per decade change in Hg(II) ion concentra-tion and the slope value is very close to Nernstian value,


Table 5Kd-values of some metal ions on polypyrrole polyantimonic acid column in different solvent systems

Metal ions DMW 1× 10−2 MHNO3

1 × 10−1 MHNO3

1 M HNO3 1 × 10−2 MHClO4

1 × 10−1 MHClO4

1 M HClO4 0.2 M CH3COOH +0.2 M CH3COONa (1:2)

Cu2+ 400 300 100 45 340 82 30 213Ni2+ 400 260 233 122 350 111 38 300Pb2+ 1900 1800 1500 115 1800 850 240 2200Cd2+ 550 400 317 286 733 475 400 575Mn2+ 467 200 54 425 163 157 18 360Zn2+ 144 144 47 25 133 26 4 233Hg2+ 2300 2700 1350 329 1600 1200 500 3200Co2+ 600 150 93 35 243 200 110 567Al3+ 50 140 200 83 250 300 165 138Fe3+ 1100 243 143 22 133 88 16 300La3+ 833 314 155 76 480 400 155 767Zr4+ 168 800 300 80 900 600 120 200Ce4+ 133 600 285 60 600 325 135 166Tl+ 500 500 300 190 300 267 200 333UO2

2+ 600 340 184 67 400 350 98 1150

Table 6Some binary separations of metal ions achieved on polypyrrole polyantimonic acid column

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

Hg(II) A 50 3710.92 3670.80 −1.08Zn(II) B 40 1045.85 1007.01 +2.67Hg(II) A 50 3008.85 2978.76 −1.00Ni(II) C 40 997.79 983.12 −1.47Hg(II) A 40 2607.67 2568.44 −1.15Cu(II) C 40 953.19 927.49 −2.17Hg(II) A 50 3209.44 3249.56 +1.25Cd(II) E 40 1573.75 1545.65 −1.79Hg(II) A 40 2607.67 2607.67 0.0Mg(II) D 50 607.63 602.77 −0.80Hg(II) A 50 3811.21 3851.33 +1.05Pb(II) C 50 3315.20 3273.76 −1.25Hg(II) A 40 2607.67 2587.61 −0.77Al(III) E 50 593.56 597.63 +0.68Hg(II) A 50 3008.85 2968.73 −1.33Fe(III) D 40 949.40 960.57 +1.18

A = 1 M HCl; B = 0.01 M HNO3; C = 0.1 M HClO4; D = 0.01 M HClO4; E = 0.1 M HNO3.

Fig. 7. Binary separations of Hg(II) from Zn(II), Ni(II), Cu(II), Cd(II) on polypyrrole polyantimonic acid columns.


Fig. 8. Calibration curve for polypyrrole polyantimonic acid membraneelectrode in aqueous solutions of HgO3)2.

29.6 mV/concentration decade for divalent cations. Below1×10−5 M, a non-linear response was observed. A constantpotential was obtained after 30 s and it was also observedthat the electrode potential remained unchanged within thepH range 2.5–7.5.

The selectivity coefficients,KPOTHg.M of various interfering

cations for the Hg(II) ion-selective polypyrrole polyanti-monic acid electrode were deteimined by the mixed solu-tion method[34] and the following numerical values wereobtained:

Membrane electrode Selectivity coefficients (KPOTHg.M) for interfering cations (Mn+)

Na+ K+ Mg2+ Co2+ Ni+2 Cu2+ Mn2+ Zn2+ Pb2+ Cd2+ Al3+ Fe3+

Polypyrrolepolyantimonic acid

0.04 0.03 0.02 0.04 0.02 0.03 0.03 0.02 0.07 0.04 0.01 0.05

The selectivity coefficient values indicate the extent towhich a foreign ion (Mn+) interferes with the response ofthe electrode towards its primary ion (Hg2+). The resultsreveal that the membrane electrode is highly selective forHg(II) ions over a number of cations.

The practical utility of the proposed membrane sensorassembly was tested by its use as an indicator electrodein the potentiometric titration of Hg(II) with 1× 10−2 MEDTA. A 50 ml portion of 1× 10−3 M Hg(NO3)2 solu-tion, at pH of 4, buffered by acetate buffer (Fig. 9). Theaddition of EDTA causes a decrease in potential as a re-sult of the decrease in free Hg(II) ion concentration due toits complexation with EDTA. The amount of Hg(II) ionsin solution can be accurately determined from the result-ing neat titration curve providing a sharp end point. Thepotentiometric titration of Hg(II) was also successfully car-

Fig. 9. Precipitation titration of Hg(II) against EDTA solution.

ried out in the presence of 1× 10−5 M Zn(II) and Cu(II),hence demonstrating the usefulness of the sensor devel-oped for potentiometric determination of Hg(II) in mix-tures.

4. Conclusion

Polyantimonic acid precipitate modified by incorpora-tion of polypyrrole (a conducting polymer), was preparedin this study as a new and novel crystalline ‘polymeric-inorganic’ composite cation-exchange material, has better

ion-exchange capacity and is highly selective for mercury.This adsorption behavior of this exchanger is promising inthe field of pollution chemistry where an effective sepa-ration method is needed for Hg(II) from other pollutants.It is evident from the results that the quantitative and effi-cient separations of various metal ions such as Hg(II) fromZn(II), Ni(II), Cu(II), Cd(II) etc. are feasible on polypyr-role polyantimonic acid column. Hg(II) sensitive membraneelectrode was also developed which is chemically and me-chanically stable and gives reproducible results with a usefullifetime, exhibiting a nearly Nernstian slope within func-tional pH range of 2.5 to 7.5. Further, the electrode can beused to determine Hg(II) ions in aqueous and non-aqueousmedia by both direct potentiometry and titration, and cansuccessfully be used in determining Hg(II) ions in realsamples.


Acknowledgements

The authors are thankful to the Department of AppliedChemistry, Z. H. College of Engineering and Technology,Aligarh for providing research facilities, and Regional So-phisticated Instrumentation Centers (Nagpur UniversityCampus, Nagpur and Indian Institute of Technology, Bom-bay) and All India Institute of Medical Sciences, New Delhifor technical assistance.

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New and novel organic–inorganic type crystalline ‘polypyrrolel/polyantimonic acid’ composite...

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