DSpace at VNU: A conductive polypyrrole based ammonium ion selective electrode

DSpace at VNU: A conductive polypyrrole based ammonium ion selective electrode tài liệu, giáo án, bài giảng , luận văn,...

SELECTIVE ELECTRODE

DO PHUC QUAN, CHU XUAN QUANG, LE THE DUAN and PHAM HUNG VIET ∗

Centre of Environmental Chemistry, Vietnam National University, Hanoi, Vietnam

(∗author for correspondence, e-mail: cec@fpt.vn)

Abstract In view of the development of miniaturized sensor arrays, a solid-contact ammonium

ion selective electrode has been investigated A conductive polypyrrole film was electrochemically deposited on a glassy carbon surface and used as an internal solid contact layer between the sensing membrane and solid electrode surface A systematic evaluation of the important parameters affecting the electromotive force (emf) response is presented The performances of this solid-contact sensor were verified using a batch-mode measurement setup and a wall-jet flow cell system The designed sensor exhibited excellent selectivity for the primary ion and a linear response over the pNH +

4 range 1–5 with a slope of 56.3 mV decade −1 The sensor has a fast response and is relatively robustness,

and was also used to determine ammonium concentrations in natural waters, with promising results.

Keywords: ammonium, conductive polypyrrole, ion selective electrode, solid-contact sensor

1 Introduction

The presence of ammonium ions in environmental samples can indicate the extent

of pollution and the eutrophication of natural water (Nigel, 1994; Gerald et al.,

1999), and over recent years, the growing importance of controlling the levels of environmental pollutants has increased interest in the development of novel sensors

for the detection of these ions (Erkang et al., 1997; Magalhaes et al., 1997; Deviteri and Diamond, 1994; Peter et al., 1997) Ion selective electrodes (ISES) offer a

simple and useful method for the direct detection of inorganic ammonium ions They offer great advantages, which include, speed and ease of preparation, simpli-fied procedures, relatively fast response, reasonable selectivity, and a wide linear dynamic range, at a relatively low cost Furthermore, ISE developments offer the possibility of sensor miniaturization, based on solid-state ion sensors, which use solvent polymeric membranes that allow the sensing liquid membrane to be cast

on the solid electrode surfaces and eliminates the need for an internal solution (Henry, 1987; Lemke and Cammann, 1989)

However, instability is a problem frequently encountered with solid-state ion selective electrodes It is generally agreed that this is caused by the lack of a stable internal reference potential at the boundary between the sensing membrane and the inner reference element Attempts have been made to overcome this problem, not-ably by the development of solid contact membrane sensors, in which, the transfer

Environmental Monitoring and Assessment 70: 153–165, 2001.

© 2001 Kluwer Academic Publishers Printed in the Netherlands.

from the oxidized to the reduced state (Erkang and Anhua, 1991; Omowunmi and

Wallace, 1994; Barisci et al., 1997) Further application of conducting polymers

in potentiometric sensors involve the incorporation of an organic solvent soluble ionophore in the polymer, which then allows the fabrication of wholly solid-state

potentiometric sensors (Pia et al., 1999).

In this study, it was found that adding the polypyrrole film by electropolymer-ization, as solid contact layer, significantly improves the potentiometric stability

of solid-state potentiometric sensors, which is believed to be the result of a better defined interfacial potential between the sensing membrane and the solid electrode contact We report upon the determination of ammonium levels in a range of natural waters using such a solid-contact ammonium ion selective electrode

2 Materials and Methods

2.1 MATERIALS

All reagents used were of analytical reagent grade Standards and buffer solutions were prepared with Milli-Q water Pyrrole purchased from Fluka was redistilled under vacuum prior to use and covered with aluminium foil in the refrigerator to prevent UV degradation

Nonactin and monactin were kindly donated by Dr Beat Muller of EAWAG, Switzerland 2-nitrophenyl-octyl ether (2-NPOE), bis(1-butylpentyl)adipate (BBPA), high molecular weight polyvinyl chloride (PVC), potassium tetrakis(4-chlorophenyl)borate (KT4ClPB) and tetrahydrofuran (THF) were obtained from Fluka (Buchs, Switzerland)

2.2 ELECTRODE PREPARATION

2.2.1 Preparation of a Conventional Internal Electrolyte Electrode

The membrane components in Table I (200 mg in total) were dissolved in 6 mL of fresh the distilled THF This solution was placed in a glass ring of 24 mm i.d rest-ing on a glass plate After solvent evaporation overnight, the resultrest-ing membrane was peeled off the glass mould and discs of 7 mm i.d were cut out, and mounted

TABLE I Composition of the ammonium solvent polymeric membrane mixtures

prepared in this study

Membrane composition Membranes

AM1 AM2 AM3 AM4

(w./w %)

2-Nitrophenyl-octyl ether 67 – – –

Bis(1-butylpentyl) adipate – – 67 67

Potassium tetrakis(4-chlorophenyl) – – – 1

borate in a 70% molar to the inophore

High molecular weight PVC 30 30 30 30

in Philips IS 561 electrode bodies (Eindhoven, The Netherlands) for electromotive force (EMF) measurements in batch-mode setup A 10 mM solution of NH4Cl was used as the internal filling solution

2.2.2 Fabrication of the Solid-Contact Ion Selective Electrode

Polypyrrole (PPy) synthesis and characterization were performed in a conventional three electrode system comprising, the 2 mm i.d glassy carbon disc (GC) working electrode (6.1204.110 GC, Metrohm, Switzerland), a platinum wire gauze auxili-ary electrode and an Ag/AgCl (3 M NaCl) reference electrode, against which all potentials were measured A laboratory-made microgalvanostat was used for the electropolymerization Chronopotentiograms were recorded using this instrument with data acquisition system support Cyclic voltammetric measurements of con-ductive polymer were performed using a PC-controlled system for Voltammetry (Model 757 VA Computrace, Metrohm, Switzerland)

Before polymerization, the surface of the glassy carbon working electrode was

polished on a polishing cloth with alumina slurry (0.05 µm) and then cleaned with

double distilled water and finally in a water-filled ultrasonic bath for 30 sec The polypyrrole film was prepared by anodic galvanostatic electropolymerization of the pyrrole monomer from aqueous solution (0.5 M) onto the electrode surface The counterion solution used for polymerization contained 1 M of chloride Solution was deoxygenated with nitrogen for 5 min to remove any trace of oxygen from the solution, prior to the polymer synthesis A current density of 2 mA cm−2 for

150 sec was used to achieve electropolymerisation

Scheme 1.

The polymeric membrane-coated modified electrode was a solution of a mixture

of ammonium membrane AM4 solution (30 µL) was applied directly on the top of

the polypyrrole film, then dried for 2 hr under a gently nitrogen atmosphere, and conditioned overnight in 10 mM of the selected primary ion solution prior to any measurement

2.3 POTENTIOMETRIC MEASUREMENTS

Batch-mode potentiometric measurements were made while stirring at a constant rate and with the electrodes immersed to the same depth in the solution The calib-ration curve was obtained by a standard addition method, involving the additions

of 10−6to 10−1 M of the primary ion

The potentials were measured against a double junction Ag/AgCl reference electrode (Orion 90-02-00) using a 692 pH/Ions meter (Metrohm, Switzerland), the accuracy of the potential measurements were +0.1 mV

Flow injection potentiometric measurement was performed with a simple flow manifold including a four channel peristaltic pump (Ismatec - Switzerland), a low

pressure six way injection valve (5020 Rheodyne) with 100 µL sample loop and a

wall-jet flow cell (Metrohm, Switzerland) The carrier eluent in the flow injection experiments contained 3 mM of sodium acetate and 2 mM of sodium chloride

including 1 µM of ammonium, to provide base line stability.

All measurements were carried out at room temperature

3 Results and Discussion

3.1 CYCLIC VOLTAMMETRY MEASUREMENTS

A polypyrrole film was deposited on electrode surface by oxidation of monomer

Figure 1 Cyclic voltammetry of a PPy/Cl electrode in 0.1 M of different cation solutions (a) NH+

4 ; (b) K +; (c) Na+; (d) Li+at scan rate 100 mV sec−1.

from an aqueous solution containing the appropriate counterion as supporting elec-trolyte Chronopotentiograms were recorded during film growth Relatively con-stant potential was observed throughout the polymerization, indicating the forma-tion of a conductive polymer layer Since the polypyrrole film plays the role of the solid contact layer in the ion selective electrode, the investigation of the incorpor-ation process of ions of interest into the polymer film is of great importance to the development process

Counterion injection and release to accompany the redox cycling of electro-polymerized polypyrrole films in electrolytes is well known Recent studies (Lien

et al., 1991; John and Wallace, 1993) of the doping-dedoping process occurring at

polypyrrole have reported that, at least in some instances, two distinct processes occur, as shown in Scheme 1, one involving anion transport and the other cation transport The degree to which each of these processes occur depends upon the nature of the polymer materials used and the mobilities of the corresponding anion and cation in solution and through the polymer

In the present study, cyclic voltammograms recorded after the electropolymer-ization of the polypyrrole indicated that polymer PPy/Cl was conductive in the

Figure 2 Cyclic voltammetry of a PPy/Cl electrode in 0.1 M of different cation solutions (a) NH+

4 ; (b) Ba2+; (c) Ca2 +; (d) Mg2 +at scan rate 100 mV sec−1.

counterion solution In order to investigate the incorporation of cation into the polypyrrole film, cyclic voltammogram measurements of the PPy/Cl electrode in solutions of different cations were taken The most striking feature of these voltam-mograms was the fact that the second reduction wave which occurs at a more negative potential was present in the univalent cation solutions (Figure 1) Figure 1 also indicates that the second reduction response moved in the positive direction with a decrease in the solvated cation size according to Li+ > Na+> K+> NH+4 The absence of a second reduction response in the divalent cation solutions (Fig-ure 2) suggests that polymer was not reduced to the extent that occurred in the univalent cation solutions This was confirmed by the fact that the subsequent oxidation peaks were smaller in solutions containing the divalent cations

3.2 POTENTIOMETRIC RESPONSE CHARACTERISTICS OF ELECTRODES Principle electrode characteristics, such as a Nerntian slope, dynamic linear range, detection limit and selectivity, depend on the composition of the ion selective membrane One of the first tasks in a development of this type involves the

determ-TABLE II Determination of ammonium in natural water samples using the designed

solid-contact ammonium selective electrode as compared with the

ammo-nia gas electrode Real samples were prefiltered through filter membrane of

0.45 µm pore size

Sample place Solid-contact NH +

4 NH3gas electrode selective liquid (Orion Inc., U.S.A.) membrane electrode

Ammonium (NH +

4 ) in groundwater samples, mg L −1(SRD)

Phap Van 13.9 (0.88%) 13.4 (0.95%)

Ha Dinh 10.2 (0.87%) 9.5 (0.92%)

Yen Phu 4.7 (0.79%) 4.4 (0.86%)

Tuong Mai 6.9 (0.82%) 6.4 (0.89%)

Ammonium (NH +

4 ) in river and ponds water samples, mg L −1(SRD)

Thanh Tri fish pond 1 3.8 (1.45%) 3.2 (1.49%)

Thanh Tri fish pond 2 5.1 (1.46%) 4.6 (1.52%)

Kim Nguu River 5.5 (1.53%) 5.0 (1.62%)

To Lich River 5.2 (1.51%) 4.8 (1.58%)

Lu River 5.0 (1.48%) 4.3 (1.53%)

ination of the best composition for the preparation of a solid-contact ammonium ion selective electrode The optimal composition of the ion selective membrane for the ammonium was determined in several experiments by varying the nature and the percentage of various liquid membrane plasticizers, results are summarized in Table II

Three ammonium membrane electrodes employing three different plasticizers were prepared The electrode responses observed for the AM1, AM2 and AM3 ISEs were both Nernstian in character over range between 0.01 and 100 mM in pure ammonium chloride solutions However, the AM2 and AM3 ISE gave super-ior response characteristics with a Nernstian slope of 57.6 and 58.3 mV dec−1, respectively Furthermore, this study also indicated that anion interference at con-centrations >100 mM, for the liquid membrane – based ISE, are more evident for a polar plasticizer, such as NPOE The severity of the anion interference is also de-pendent on the lipophilicity of the anion, that is, the greater the lipophilicity of the anion, the greater the penetration of the anion into the liquid membrane, and con-sequently, the greater the interference exerted upon the cation selective membrane

at higher concentrations Therefore, in order to reduce anion interference at higher concentrations, a lipophilic salt, such as potassium tetrakis(4-chlorophenyl) borate,

Figure 3 Charge transfer process occurring in (A) a conventional internal electrolyte selective

electrode; and (B) in a solid-contact conducting polymer based electrode.

was added to the membrane In the following section, we describe a solid-contact sensor that was fabricated with the optimal AM4 mixture composition

Figure 3 compares the charge transfer process occurring in a conventional in-ternal electrolyte selective electrode with that in a solid-contact conducting poly-mer based electrode Comparisons of the charge transfer between the sensing mem-brane and inner element of both ammonium electrodes showed similar trends The potential response of the solid-contact electrode was very stable in the meas-ured ammonium concentration range Calibration curves for a conventional internal electrolyte electrode and the solid-contact electrode in ammonium solutions, are shown in Figure 4 This result demonstrates that the response was linear over the investigated range of 10−5 to 10−1 M L−1 of ammonium with a slope of 56.3 mV dec−1

Response time was also investigated, because it is a very important factor in terms of the practical use of solid-contact sensors The time taken for the both electrodes to attain 90% of the steady-state response was typically a few seconds, suggesting that this electrode is ideal for flow injection measurement

Figure 4 Comparison of ammonium ion concentration calibration curves for the internal

electro-lyte ammonium ISE and the solid-contact ammonium electrode in 2 mM NaCl, to adjust the ionic strength.

The selectivity of solvent polymeric membrane electrodes in the presence of in-terfering cations were determined by the separateg solution method (SSM) (Morf, 1981) and calculated using Equation (1)

log K i,jpot= (E j − E)

1− z i

z j



where K i,jpot is the selectivity coefficient, i is the primary ion (NH+4), j is the in-terfering ion, E is the measured potential (mV), S is the Nernstian slope factor (mV/decade), z is the electrical charge of the ion, and a is the activity calculated

from the activity coefficients The selectivity coefficient values obtained by the separate solution method are given in Figure 5 and are compared to the selectivity

coefficients reported for ammonium electrodes in previous studies (Thomas et al.,

1988) Comparison of the selectivity coefficients of interfering ions showed that the potassium ion is the most notable interference ion of the ions tested

The solid-contact ammonium ion selective electrode AM4SCS was mounted in the FIA system described in the Experimental Section Typical FIA signals

ob-Figure 5 Selectivity coefficients, logKpotNH

4,j for the ammonium selective electrodes made during this work, compared to a previously reported ammonium ISE Results were obtained by the separate solu-tion methods in Tris buffered solusolu-tions of 0.1 M chloride salts at pH 7.1 AM (1–4): four convensolu-tional internal electrolyte ammonium selective electrodes; ISE ∗: ammonium ion selective microelectrode

(Thomas et al., 1988); and AM4SCS: solid-contact ammonium ion selective electrode.

tained for series injection of different ammonium solutions are shown in Figure 6, which demonstrates the high reproducibility of the observed peak heights The potentiometric response was linear over the range investigated i.e., between 10−5 and 10−1M L−1of ammonium

3.3 WATER ANALYSIS

Finally the analytical performance of the solid-contact ammonium ion selective electrode was tested in terms of the determination of ammonium in natural water The samples of natural water used were representative complexes of real matrices containing high concentrations of inorganic and organic substrates Moreover, the concentration of ammonium in these samples represented an important quality level determining factor In order to verify the analytical results obtained using this