Optimization of Polyaniline Supported Ti(IV) Arsenophosphate

Article pubs.acs.org/IECR

Optimization of Polyaniline Supported Ti(IV) Arsenophosphate Composite Cation Exchanger Based Ion-Selective Membrane Electrode for the Determination of Lead Rani Bushra,*,† Mohammad Shahadat,‡ Meraj A. Khan,‡ Inamuddin,§ Rohana Adnan,† and Mohd. Rafatullah∥ †

School of Chemical Sciences and ∥School of Industrial Technology, Universiti Sains Malaysia, 11800 Georgetown, Pulau Pinang, Malaysia ‡ Analytical Research Laboratory, Department of Chemistry, and §Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh−202002, India S Supporting Information *

ABSTRACT: Polyaniline supported Ti(IV) arsenophosphate [PANI Ti(IV) AsP] composite cation exchange material was synthesized by a simple chemical route. To explore the application of this composite material, a lead ion selective membrane electrode was fabricated. The composite membrane electrode demonstrated a Nernstian response of 29.3 mV per decade change in concentration of Pb2+ ions over a wide concentration range (1 × 10−1−1 × 10−6 M) with a detection limit of 1 × 10−6 M. Mixed solution method was used to determine the selectivity of Pb2+ ions in the presence of interfering cations. The practical utility of PANI Ti(IV) AsP membrane electrode has been fruitfully explored by using it as an indicator electrode for potentiometric determination of lead.

1.0. INTRODUCTION Nowadays, it is a pressing issue to protect the environment from the toxic effects of heavy metal pollutants. Lead has been used for thousands of years, and our reliance on lead has increased substantially since the industrial revolution. However, it is a well-known toxic pollutant to the human body and moves throughout ecosystems. Once it is ingested and absorbed into the body, it circulates in the blood, attached to specific proteins in red blood cells, and affects many body organs.1 Much of the lead ingested is excreted, mostly in feces and urine, although some of it is absorbed in soft tissues and some is incorporated into “hard” or calcified tissues such as bones and teeth. It was found that more than 90% of lead in the adult human body is found in bone; in children it is more than 70%. In China approximately 2000 children living near zinc and manganese smelters have been affected by lead poisoning.2 Lead is a potential pollutant in the environment, so its detection and determination from waters has been the subject of extensive technological research.3 Potentiometric detectors, based on ionselective electrodes, have become useful tools owing to their high selectivity, sensitivity, good precision, simplicity, and low cost.4,5 By using ion selective electrodes, direct monitoring of selected species has become feasible without any sample pretreatment. Precipitate based ion-selective membrane electrodes play a very important role in the determination of cations and anions.6−10 Ion-exchange membranes, which were fabricated by incorporating the composite material in a polymer binder have been substantially used as potentiometric sensors, i.e. ion sensors, chemical sensors, more commonly ion selective electrodes, etc., on a large scale. Recently, a number of composite ion exchange materials were successfully prepared and used as an ion selective membrane electrode.11−14 © XXXX American Chemical Society

Polyaniline (PANI) Ti(IV) arsenophosphate (AsP) composite material has been effectively utilized as an ion exchanger, photocatalyst, and microbial agent. To apply this material in the various fields, work was continued and it was observed that besides ion exchangers, photocatalysts, and microbial agents,15 PANI Ti(IV) AsP can also be effectively used as an ion selective electrode membrane for the detection of toxic heavy metal ions. The present paper reports the fabrication of polyaniline supported Ti(IV) arsenophosphate, [PANI Ti(IV) AsP] composite cation exchanger based ion selective membrane electrode and its application in the potentiometric titration of lead ions.

2.0. EXPERIMENTAL SECTION 2.1. Reagents, Solutions, and Instruments. Reagents used for the synthesis were titanium tetrachloride (TiCl4), carbon tetrachloride (CCl4), orthophosphoric acid (H3PO4) sodium hydrogen arsenate (NaH2AsO4), aniline (C6H5NH2), potassium persulfate (K2S2O8), and perchloric acid (HClO4) (E-Merck and Central Drug House, India). High molecular weight polyvinyl chloride powder (PVC) (average Mn ∼ 22 000, average Mw ∼ 43 000) and dioctyl phthalate (DOP) (molecular weight 390.56) were obtained from Aldrich, India. Tetrahydrofuran and salts of metal nitrate were procured from Fluka, India. The solution of TiCl4 was prepared in CCl4 while stock solution of 10% aniline and 0.10 M potassium persulfate solutions were prepared in 1.0 M HCl. The solutions of Received: September 3, 2014 Revised: November 17, 2014 Accepted: November 22, 2014

A

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thoroughly with demineralised water and the mixture of two metal ions (each with initial concentration of 0.1 M) was loaded on it. The mixture was allowed to pass through the column at a flow rate of 1.0 mL min−1 until the solution level was remain just above the surface of the composite material. The process was repeated 2−3 times to ensure the complete absorption of metal ions on the beads. The separation of metal ions was achieved by passing selected eluent and collected effluent was titrated against the standard solution of disodium salt of EDTA (0.01 M). 2.5. Fabrication of Ion-Selective Membrane of PANI Ti(IV) AsP. The ion selective membrane of PANI Ti(IV) AsP was fabricated as follows: electroactive material PANI Ti(IV) AsP in powdered form was mixed with 200 mg of polyvinyl chloride (PVC) dissolved in 10 mL tetrahydrofuran (THF), and 10 μL of plasticizer (dioctyl phthalate) was also added. The solution was cast onto a glass dish of 2 cm diameter.18 THF was evaporated slowly until a uniform, flexible membrane was obtained. On one end of the Pyrex glass tube stick a disc (10 mm) which was cut from the master membrane. After that, the electrode was conditioned by soaking into a solution of Pb(NO3)2 (0.1 M) for 12 h before use. Thus, by following the above method four membranes of PANI Ti(IV) AsP were fabricated. 2.7. Electromotive Force (emf) Measurements. The potential of the membrane electrode was measured at room temperature (25 ± 2 °C) using the digital potentiometer. The following cell assembly was set up for emf measurements:

orthophosphoric acid (0.10 M) and sodium arsenate (0.20 M) were prepared in demineralised water (DMW). A digital pH meter (Elico L 610) and potentiometer (EI 118) were used for the measurements of pH and potential, respectively. 2.2. Synthesis of Polyaniline Supported Ti(IV) Arsenophosphate [PANI Ti(IV) AsP] Composite Cation Exchanger. Synthesis of PANI gel was carried out using the same procedure as explained in a previously reported paper.16 The inorganic precipitate of Ti(IV) AsP was prepared by mixing the solutions of orthophosphoric acid (0.1M), sodium arsenate (0.1M), and titanium tetrachloride with continuous stirring for 1 h, whereby a white gel type slurry was obtained. The white precipitate so formed was kept for 24 h in the mother liquor for digestion. The composite material of PANI Ti(IV) AsP was prepared by the mixing of PANI gel and Ti(IV) AsP under varying experimental conditions with continuous stirring for 1 h at 25 ± 2 °C. The synthesized green gel was kept overnight at room temperature for digestion. Afterward, it was filtered and excess acid was washed with demineralized water (DMW) and dried in an oven at 45 ± 2 °C. The dried material was converted into H+ form by treating with 1.0 M nitric acid solution with occasional shaking, and finally dried at 45 ± 2 °C. In this way, a number of samples of PANI Ti(IV) AsP composite cation exchanger were synthesized. On the basis of high ion-exchange capacity and percentage yield, sample S-2 was selected for detailed studies (Supporting Information, Table S1). 2.3. Distribution (Sorption) Studies. A batch method was employed to determine the distribution coefficient (Kd values) of the metal ions in various solvent systems. In this method, a fixed amount (300 mg) of PANI Ti(IV) AsP composite material (in H+ form) was mixed with 30 mL of different metal nitrate solutions into a number of Erlenmeyer flasks. The mixture was shaken for 6 h at a temperature controlled shaker at 25 ± 2 °C to attain equilibrium. The metal ion concentrations before and after equilibrium were determined by ethylenediamine tetraacetic acid disodium salt (EDTA) titration.17 The distribution coefficient values (Kd) were calculated using the equation:

Ag, AgCl|KCl(satd) sample solution|membrane|0.1 M Pb2 +|Ag, AgCl

The calibration of the electrode was done with several standard solutions, and potential measurements were studied in the concentration range of 1 × 10−1−1 × 10−9 mol L−1. The potential measurements of the membrane electrode were plotted against various concentrations of the respective ion. 2.8. Response Time. The response time was measured by recording the emf of the electrode as a function of time when it was immersed in the solution to be studied. The electrodes was first dipped in 1 × 10−3 M solutions of the ion concerned and immediately shifted to another solutions (pH ≈ 3) of 1× 10−2 M ion concentrations of the same ion (10-fold higher concentration). The potential of the solution was read at zero second, that is, just after immediate dipping of the electrode in the second solution and subsequently recorded at the intervals of 5 s. The potentials were then plotted against time. 2.9. Effect of pH. The influence of pH of the test solution on the potential response of the ion selective membrane electrode was tested at 1 × 10−3 M concentrations of lead over the pH range 1−8. The pH variations were brought out by the addition of dilute acid (HCl) or dilute alkali (NaOH) solutions. The value of the electrode potential at each pH was recorded and was plotted against pH. 2.10. Selectivity Coefficients. Ideally, the response of the ion selective electrode should obey equation as given below:

Kd = amt of metal ion retained in 1 g of the exchanger phase (mg g −1) amt of metal ion in unit volume of the supernatant solution (mg mL−1)

Kd =

(I − F )/300 mg F /30 mL

where I is the volume of EDTA used before treatment of metal ion exchanger and F is the volume of EDTA consumed by metal ion left in solution phase. The sorption of metal ions involves the exchange of the H+ ions in the exchanger phase with that of metal ions in the solution phase as given below: 2R−H+ +

exchanger phase

M2 + ⇌

solution phase

R 2−M exchanger phase

+

2H+

solution phase

E = E° + (2.303RT /Zif ) log a i

R = PANI Ti(IV)AsP

2.4. Quantitative Separations of Metal Ions in Synthetic Binary Mixtures. Quantitative separations of metal ions were achieved by using columns of PANI Ti(IV) AsP. A fixed amount of exchanger (1.0 g in H+ form) was packed in a glass column (internal diameter 0.5 cm) with a glass wool support at the bottom. The column was washed

However, the above equation, is written on the assumption that the electrode responds only to the ion of interest, “i”. In practice, no electrode responds exclusively to the ion specified. The actual response of the electrode in a binary mixture of the primary and interfering ions (i and j respectively) is given by the Nikoloskii−Eisenman equation.19 B

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Z /Z

separations are quite sharp, and recovery was quantitative and reproducible. Thus, PANI Ti(IV) AsP shows selective behavior toward Pb2+ ion absorption and separation; therefore, the ionselective membrane of this material was fabricated for the detection of Pb2+ ions. To optimize the best performance of the material as ion selective membrane electrode, four different membranes of PANI Ti(IV) AsP were prepared (Table 3). It was observed that ion selective membrane electrode fabricated by using the membranes prepared with the 200, 250, and 350 mg of ion exchanger showed sub-Nernstian slope per decade change in the concentration of the Pb2+ ions. However, the membrane prepared with 200 mg of the composite cation exchanger (M-2) showed near-Nernstian slope and was selected for detailed analysis (Table 3). The potential response of the membrane electrode prepared by using membrane sample M-2 was measured in nine different concentrations of Pb2+ ion (Figure 1). Results revealed that the working concentration range of this electrode was found to be 1 × 10−1−1 × 10−6 M for Pb2+ ions due to its linear response in the region with a slope of 29.3 mV per decade change in Pb2+ ions concentration. The limit of detection which was determined from the intersection of the two extrapolated segments of the calibration graph was found to be 1.0 × 10−6 M.21 It is evident from Supporting Information Figure S-1 that the response time of composite membrane electrode for 1 × 10−2 M concentrations of lead was found to be 20 s, which was short enough. The stability of PANI Ti(IV) AsP membrane electrode was scrutinized for a period of 4 months before it was used. After a period of 4 months of use, it was found that the electrode potential response determined in the concentration range of 10−1−10−9 M was reproducible within ±2 mV without any deviation in linear range, detection limit, and Nernstian slope. This observation confirms the stable electrode performance of the PANI Ti(IV) AsP membrane electrode. The pH adjustment played a very important role in the determination of metal ions by using ion selective membrane electrode. To examine the effect of pH on the electrode response, the potential was measured at a constant concentration of the Pb2+ ions (1.0 × 10−3 M) in a wide pH range (1.0−8.0). Supporting Information Figure S-2 showed that the response of the electrode remain constant in the pH range 3−6. This means that Pb2+ selective membrane electrode can be used to measure a wide range of environmental and industrial water samples without pH adjustment. The selectivity is one of the most important aspects of an electrode which determines whether the reliable measurements in the sample are possible or not. By employing mixed solution method22 selectivity coefficients of various differing cations for Pb2+ ion-selective PANI Ti(IV) AsP composite membrane electrode were determined which demonstrates that these ions would not affect the selectivity of this electrode significantly (Table 4). Some interesting properties such as concentration range, response time, and slope of present Pb2+ selective electrodes were compared with other Pb2+ selective electrodes reported in the literature (Supporting Information, Table S-2) which shows that the performance of the PANI Ti(IV) AsP ion selective membrane electrode is comparatively better than other electrodes.23−29,18,30,31 The practical utility of the PANI Ti(IV) AsP membrane electrode was demonstrated as an indicator electrode in the potentiometric titration of Pb2+ ion against an EDTA solution.

E = E° + (2.303RT /Zif ) log(ai + K ijaj i j)

where E = potential of the electrode, E° = standard potential of the electrode, ai = activity of i ions, aj = activity of j ions, Zi = charge on the i ion, Zj = charge on the j ion, Kpot ij = selectivity coefficient of the electrode in the presence of j ions, which measures the relative affinity of ions i and j toward the ionselective membranes. However, no electrode has absolute selectivity for a particular ion. Thus, the selectivity of the electrode depends on selectivity coefficients. The lower the value of Kij, the more selective the electrode. For ideally selective electrodes, Kij would be zero. Selectivity coefficients can be measured by different methods which fall into two main groups, namely, separate solution techniques and mixedsolution techniques.20 In the mixed solution techniques, the electrode potentials are measured in solutions containing both the primary ion (i) and the interfering ion (j). In this method, a mixed solution having a fixed concentration of interfering ions (Mn+) (1 × 10−3 M) and varying concentrations (1 × 10−1−1 × 10−9 M) of the primary ions were taken in a beaker of constant volume and potential measurements were made using the membrane electrode assembly.

3.0. RESULT AND DISCUSSION To prepare the ion-selective membrane electrode, various samples of PANI Ti(IV) AsP were prepared by the mixing of polyaniline and Ti(IV) arsenophosphate under varying experimental conditions. On the basis of high ion exchange capacity of 1.13 mequiv g−1 for Na+ ions, sample S-2 was selected for further studies (Supporting Information, Table S1). The distribution coefficient values (Kd values) of different metal ions were examined by using PANI Ti(IV) AsP composite material to get an idea about the sorption and possible separation ability of metal ions. It was observed that adsorption of metal ions increases with the decrease in the concentration of the acidic solvent HClO4. On the basis of high Kd values, the material was found to be selective for Pb2+ ions (Table 1). However, adsorption of other metal ions was found to be comparatively lower than that of the lead. To examine the practical utility of this material, some quantitative separations were achieved on the columns of PANI Ti(IV) AsP. Table 2 summarizes the salient features of these separations. The Table 1. Distribution Coefficients (mL g−1, Kd Values) of Metal Ions on PANI Ti(IV) AsP metal ion

demineralized water (DMW)

HClO4 (0.1 M)

HClO4 (0.01 M)

HClO4 (0.001 M)

Mg2+ Zn2+ Sr2+ Ca2+ Ba2+ Pb2+ Cd2+ Cu2+ Ni2+ Al3+ Fe3+ Th4+ Ce4+

177.78 155.56 266.67 104.0 676.92 1700.0 285.71 200.0 187.18 384.21 458.82 1175.0 672.24

100.0 173.3 333.2 80.0 412.4 685.7 162.5 220.5 138.09 295.7 214.12 227.78 239.28

165.62 190.0 421.5 100.0 672.7 863.636 168.42 307.407 246.15 342.2 304.20 362.78 283.33

185.71 290.0 678.8 120.0 682.8 900.0 347.82 364.28 326.08 442.9 403.22 562.25 515.38 C

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Table 2. Quantitative Separation of Metal Ions from a Binary Mixture Using PANI Ti(IV) AsP Cation Exchange Material Column at Room Temperature sample

metal ions separation 2+

separation-1

Cu Pb2+ Zn2+ Pb2+ Cd2+ Pb2+ Mg2+ Pb2+

separation-2 separation-3 separation-4

amount loaded (mg)

amount found (mg)

% recovery

volume of eluent used (mL)

eluent used

6.538 20.72 6.538 6.355 11.241 20.72 2.431 20.72

6.34 19.48 6.47 6.16 11.00 20.00 2.38 19.78

97 94 98 97 97.8 96.5 98 95

90 80 70 70 50 70 60 80

demineralised water 0.01 M HClO4 demineralised water 0.1 M HClO4 demineralised water 0.1 M HClO4 demineralised water 0.1 M HClO4

Table 3. Fabrication of Membranes by Varying Amount of Electroactive Material (PANI Ti(IV) AsP) membrane sample

electroactive material (mg)

PVC (mg)

thickness (mm)

M-1 M-2 M-3 M-4

200 250 300 350

200 200 200 200

0.22 0.32 0.40 0.56

linear range (M) 1.0 1.0 1.0 1.0

× × × ×

10−1−1 × 10−7 10−1−1 × 10−6 10−1−1× 10−6 10−1−1× 10−5

response time (s)

slope (mV) per decade change in concentration

35 20 40 50

25.0 29.3 26.0 23.0

(≈3) was done before adding the titrant. As it is evident from Supporting Information Figure S-3, decreases in potential with continuous addition of EDTA were found to be due to decreases in free Pb2+ ions which were utilized in complex formation with EDTA. The amount of Pb2+ ions was accurately determined by the titration curve which provides a sharp end point.

4.0. CONCLUSIONS PANI Ti(IV) AsP composite cation exchanger was successfully used as a potentiometric sensor for the detection of Pb2+ ion which is a major environmental pollutant. The distribution coefficient results confirmed the selective behavior of composite toward Pb2+ ions. The fabricated composite membrane electrode of PANI Ti(IV) AsP demonstrated a Nernstian response of 29.3 mV per decade change in the concentration of Pb2+ over a wide range of concentration (1 × 10−1 −1 × 10−6 M) with a detection limit of 1.0 × 10−6 M. Thus, PANI Ti(IV) AsP can be fruitfully applied for the determination of Pb2+ ions.

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S Supporting Information *

Figure 1. Calibration curve for PANI Ti(IV) AsP membrane electrode in aqueous solution of Pb(NO3)2.

Conditions for the synthesis of PANI Ti(IV) AsP cation exchange material (Table S-1); response of PANI Ti(IV) AsP membrane electrode at different time intervals for 1 × 10−2 M concentrations of lead (Figure S-1); effect of pH of the test solution on the potential response of Pb2+ ion selective membrane electrode at 1 × 10−3 M concentration of lead (Figure S-2); characteristic properties of different lead selective electrodes (Table S-2); potentiometric titration curves of 5 mL of Pb(NO3)2 solutions at 1 × 10−2 M concentrations against 1 × 10−2 M EDTA solution (Figure S-3).This material is available free of charge via the Internet at http://pubs.acs.org/.

Table 4. Selectivity Coefficients of Various Interfering Ions (Mn+) interfering ions Cd2+ Cu2+ Hg2+ Mn2+ Al3+ Ca2+ Ba2+ Sr2+

selectivity coefficients 5.0 4.7 4.0 3.8 6.0 5.0 4.0 5.5

× × × × × × × ×

ASSOCIATED CONTENT

10−3 10−3 10−3 10−3 10−5 10−4 10−3 10−4

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Lead nitrate (0.01 M, 5 mL) solution was titrated against EDTA (0.01M), and the potential was recorded after addition of every 0.25 mL of EDTA solution. The adjustment of pH

Notes

The authors declare no competing financial interest. D

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial and technical support from Aligarh Muslim University and Universiti Sains Malaysia. One of the authors is grateful to the Universiti Sains Malaysia, for providing financial assistance through a USM RU grant (Grant number 1001/PKIMIA/815099) for this work. Inamuddin is also thankful to the University Grant Commission for funding vide grant F. No. 41-278/2012 (SR).

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