Analytica Chimica Acta 459 (2002) 11–17

Chronocoulometric flow-injection analysis withsolvent polymeric membrane ion sensors

Concepción Sánchez-Pedreño∗, Joaquın A. Ortuño, Jorge HernándezDepartment of Analytical Chemistry, University of Murcia, 30071 Murcia, Spain

Received 29 June 2001; received in revised form 5 February 2002; accepted 8 February 2002

Abstract

A method to carry out chronocoulometric measurements with solvent polymeric membrane ion sensors in flow-injectionsystems has been developed. For this, a double potential step was synchronised to the passage of the sample plug throughthe detector cell. A four-electrode potentiostat with ohmic drop compensation and a new flow-through cell to incorporate thefour-electrode and the membrane were developed. A plasticized poly(vinyl chloride) (PVC) membrane containing TBATBPwas used and the procedure was applied to the determination of tetraethyl-ammonium. The effect of the electrochemical andflow-injection variables was studied. In the selected conditions, a linear relationship between the quantity of electricity andtetraethyl-ammonium concentration was obtained in the range 5× 10−7–5× 10−5M. The detection limit was 7× 10−8M.Good repeatability and between day reproducibility were obtained. The potential application to other quaternary ammoniumions including acetylcholine was also studied. © 2002 Elsevier Science B.V. All rights reserved.

Keywords:Chronocoulometry; Flow-injection analysis; Solvent polymeric membrane; Ion sensor; Tetraethyl-ammonium determination

1. Introduction

The electrochemistry at the interface between twoimmiscible electrolyte solutions (ITIES) has been thesubject of many theoretical and experimental studies[1–3]. However, analytical applications are difficultdue, among other reasons, to the mechanical insta-bility of the liquid/liquid interface. Stabilisation ofthe interface with a dialysis membrane [4] and, es-pecially, solidification of one side of the interfaceto become a polymer gel [5] or even a plasticizedpoly(vinyl chloride) (PVC) membrane [6,7] similarto those used in ion-selective electrodes (ISEs), sim-plified the experimental arrangements and extended

∗ Corresponding author. Tel.:+34-968-367400;fax: +34-968-364148.E-mail address:spedreno@um.es (C. Sanchez-Pedreño).

the possibility of analytical applications using amper-ometric and voltammetric transductions [8–10]. Anarray of micro-interfaces between an analyte solutionand a PVC gel electrolyte has been used for the am-perometric monitoring of ion transfer reactions [11].Flow-injection methodology has been used in somecases [4,5,8].The most striking advantage of the amperometric

transduction over classical potentiometric ion sensorsis that the ion selectivity of the former can be tunedby altering the magnitude of the applied potential[10]. Another advantage is that the amperometricISE, which provides a current response directly pro-portional to analyte concentration, is more suitablefor detecting small changes in analyte concentrationthan the usual potentiometric ISE, which provides apotential response proportional to the logarithm of theanalyte activity (concentration) [8]. These advantages

0003-2670/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0003-2670(02)00100-9

12 C. Sanchez-Pedreño et al. / Analytica Chimica Acta 459 (2002) 11–17

are shared by other controlled-potential electroanalyt-ical techniques, such as chronocoulometry, which toour knowledge has not used with solvent polymericmembrane ion sensors.The aim of this work was to develop a technique

for carrying out chronocoulometric measurements us-ing solvent polymeric membrane ion sensors in flow-injection systems in order to develop new analyticalmethods that share the advantages of flow-injectionmethodology. The method has been applied to the det-ermination of tetraethyl-ammonium and tested for thedetermination of other lipophilic ions. The determi-nation of lipophilic ions, such as organic quaternaryammonium ions, is of great interest since they includemany ions with biological, pharmacological and sur-factant activity. Several tetraalkyl-ammonium ionshave been tested as ionic markers in permeabilityassays of liposomal membranes, and tetraethyl-ammo-nium was seen to be the most suitable marker [12].

2. Experimental

2.1. Reagents and solutions

PVC high molecular mass, 2-nitrophenyl octylether (NPOE) and tetrahydrofuran (THF) were Selec-tophore products from Fluka. Tetrabutyl-ammoniumtetraphenylborate (TBATPB), tetraethyl- and tetra-butyl-ammonium chlorides (TEAC and TBAC) werepurchased from Sigma, tetramethyl-ammonium chlo-ride (TMAC), acetylcholine chloride (ACC) anddopamine hydrochloride (DPHC) were from Flukaand tetrapropyl-ammonium (TPAC) was from Aldrich.A volume of 0.01M TMAC, TEAC, TPAC, TBAC,ACC, DPHC solutions were prepared by dissolvingin water. Working solutions were prepared by dilut-ing with 0.01M LiCl. All other reagents used wereof analytical reagent grade and doubly distilled waterwas used throughout. A glass ring of 28mm inner di-ameter and 30mm height, glass plate, vial and punchwere purchased from Fluka for the construction ofthe membranes.

2.2. Membrane preparation

The membranes were prepared by dissolving200mg NPOE, 100mg PVC and 8.4mg TBATPB in

Fig. 1. Flow cell design. (1) Counter (A) electrode; (2) reference(A) electrode; (3) membrane; (4) agar gel; (5) outlet; (6) counter(B) electrode; (7) reference (B) electrode; (8) inlet.

3ml of THE. This solution was poured into the glassring resting on the glass plate and was left overnight toallow the solvent to evaporate slowly. A 7-mm diame-ter piece was cut out with the punch and incorporatedinto the flow-through cell described as follows.

2.3. Construction of the flow-through cell

The four-electrode flow cell is of the wall-jet typeand is made of Perspex (Fig. 1). Its body consists oftwo main parts. The upper part holds the reference (A)and counter (A) electrodes and the membrane, whilethe reference (B) and counter (B) electrodes and theinlet and outlet of the flowing solution are located inthe lower part. The area of membrane exposed to theflowing solution is about 20mm2. Ag/AgCl electrodesare used as reference electrodes and Pt electrodesare used as counter electrodes. The reference (A)

C. Sanchez-Pedreño et al. / Analytica Chimica Acta 459 (2002) 11–17 13

and counter (A) electrodes are immersed in 0.01MTBACl aqueous solution. The reference (B) electrodeis immersed in 0.01M LiCl, 3% agar–water solutionand the counter (B) electrode is immersed in the flow-ing solution. Thus, the electrochemical cell can beexpressed as

Ag|AgCl|0.01M||TBACl||0.05mol kg−1 TBATPB

||0.01MLiCl, TEA+ ×M|AgCl|AgE corresponds to the potential difference between theright and left hand terminals. Since the concentrationof TBA+ in the left hand solution is high, this half-cellcan be used as a practical reference electrode in asimilar way as that described in [6].E is maintainedat the pre-set value by means of a four-electrodepotentiostat (see later) that applies the necessary po-tential between the right and left counter electrodes.The electrical current flows between the counterelectrodes. A positive current corresponds to the

Fig. 2. Circuit diagram of the four-electrode instrumental system. MC: main control amplifier; F: voltage follower; CF: current follower;D: differential amplifier; FG: fixed gain amplifier; CE and RE: counter and reference electrodes; G: signal generator.

transfer of positive charge from the right side to theleft.

2.4. Apparatus

The diagram of the four-electrode potentiostat usedis shown in Fig. 2. TheiR compensation was set tothe nearest point before oscillation. A one-channelflow-injection assembly was used. The distance be-tween the injection valve and the cell was 20 cm. AGilson (Villiers le Bel, France) Minipuls 3 peristalticpump, Omnifit (Cambridge, UK) injection valve,connecting tubing of 0.5mm bore, PTFE tubing andvarious end fittings and connectors were used toconstruct the flow-injection system.

2.5. Chronocoulometric flow-injection procedure

Aliquots of 60�l of tetraethyl-ammonium samplesolutions (5× 10−7–5× 10−5M) were injected on a

14 C. Sanchez-Pedreño et al. / Analytica Chimica Acta 459 (2002) 11–17

0.01M LiCl carrier solution pumped at a flow rate of1mlmin−1, and at the same time a double potentialstep was applied. This consisted of an initial poten-tial of 0.280V for 7 s, after which the potential wasstepped to a forward potential of 0.475V for 2 s, thenback to 0.280V for 50 s. The current versus time re-sponse curve was registered and the correspondingquantity of electricity versus time was obtained by in-tegration. The quantity of electricity corresponding tothe peak maximum was recorded. A blank assay wasalso run to correct the values.

3. Results and discussion

The principle of the chronocoulometric flow-injection method developed is shown in Fig. 3. Ascan be seen, a double potential step was applied at the

Fig. 3. Principle of the method. Left: concentration profile (. . . ) and double potential step (—). Right: current and quantity of electricityvs. time.

same time that the sample was injected. At the initialpotential applied (Ei ), there was negligible ion transferthrough the water/membrane interfaces and thereforethe current response was almost null. At timet1, whenthe most concentrated portion of the sample plug hadarrived in the detector cell, a forward potential (Ef )capable of causing tetraethyl-ammonium to be takenup by the membrane was applied for a duration oftime τ , which produced a positive transient currentresponse. The potential was then back-stepped to theinitial potential, at which the tetraethyl-ammoniumin the membrane was back-extracted into the flowingsolution, producing a negative transient current res-ponse. This potential was held sufficient time to ensurecomplete renewal of the membrane composition,thus guaranteeing reproducible responses for suc-cessive injections. The corresponding processes tak-ing place at the inner solution/membrane interface

C. Sanchez-Pedreño et al. / Analytica Chimica Acta 459 (2002) 11–17 15

Fig. 4. Current (a) and quantity of electricity (b) vs. time curvesobtained for: (�) TEA (5× 10−5M); (�) blank.

involved the transfer of tetrabutyl-ammonium from themembrane to the internal solution and vice versa, thisinterface being reversible because of the compositionof the membrane and of the inner solution [6,7].Fig. 4 shows the experimental current (Fig. 4a)

and quantity of electricity (Fig. 4b) versus timecurves, obtained following the principle describedabove by injecting 60�l of a solution containing5 × 10−5M tetraethyl-ammonium and 0.01M LiClon a 0.01M LiCl carrier solution pumped at a flowrate of 1mlmin−1 and using the following values:Ei : 0.280V; t1: 7 s;Ef : 0.475V andτ : 2 s. The corre-sponding curves for the carrier solution (blank assay)are also included. The difference between the currentand quantity of electricity observed in the presenceand absence of tetraethyl-ammonium correspond toof its transfer processes through the interface.Armstrong and Marcos [6] have studied the

transfer of tetraalkyl-ammonium ions NR4+ at thepolarizable plasticized PVC membrane–water inter-

face by cyclic voltammetry. For these authors, thehalf-wave reversible potentialE1/2 can be written as

E1/2 = Eo +(

RT

F

)ln

(DW

DM

)1/2

(1)

whereDW andDM are the diffusion coefficients ofNR4+ in water and in the membrane, respectively.For PVC membranes usedDW/DM ∼= 103, and thusE1/2 = Eo +90mV. The current density “going into”the membrane can be expressed as

iW→M = nFAD1/2W

(CW − CsW)

(π1/2τ1/2)

= nFAD1/2M

CsM

(π1/2τ1/2)(2)

whereCsW andCs

M are the surface concentrations ofNR4+ at the water and membrane sides, respectively.

3.1. Influence of the FI and electrochemical variables

Different flow rates and sample volumes weretested and, for each, the timet1 was varied in order toachieve the maximum quantity of electricity, whichcorresponds to the best synchronisation between themost concentrated portion of the sample plug and theapplication of the forward potential. A flow rate of1mlmin−1 and a sample volume of 60�l and a t1of 7 s provided best results and were used through-out. Much higher flow rates and lower volumes ledto worse reproducibility, while much lower flowrates and higher sample volumes lengthened the timeneeded for each determination.The influence of the forward potentialEf on the

quantity of electricity was studied by fixing the initialpotentialEi at 0.280V, at which no current flows, andby varying the potentialEf . The duration of this stagewas 2 s and the concentration of TEA was 5×10−5M.The results are shown in Fig. 4. As can be seen, amaximum was obtained at a forward potentialEf of0.475V, which corresponds to step amplitude of about0.2V and this value was selected for further stud-ies. The influence of the duration of the applicationof Ef was studied by varying this between 0.5 and10 s (Fig. 5). The values of the remaining variableswere the same as above. The quantity of electricityincreased with the duration of the step. However,the blank values also increased considerably and this

16 C. Sanchez-Pedreño et al. / Analytica Chimica Acta 459 (2002) 11–17

Fig. 5. Influence of the forward potentialEf .

increased the detection limit. Taking all this into ac-count, a step of 2 s was selected. The influence ofthe duration (t3 − t2) of the reverse part of the stepwas also studied. A value of 50 s allowed completerenewal of the membrane, providing reproduciblesignals at all TEA concentrations assayed.

3.2. Features of the method

In the selected experimental conditions, a linearrelationship between the quantity of electricity andTEA+ concentration was obtained in the range 5×10−7–5× 10−5M (Fig. 6). The regression equationwasQ (�C) = 1.74× 104 [TEA+] (M) + 3× 10−4.The detection limit obtained from three times the stan-dard deviation of the blank was 7×10−8M. This valuewas 30 and 60 times lower than the detection limitfound by measuring the current at the initial and atthe end of the first step, respectively. The repeatabil-ity was evaluated by performing five consecutive de-terminations. The variation coefficient for 5×10−6Mwas±2.2%. The reproducibility between three con-secutive days was±3.9% (Fig. 7).

3.3. Potential application to other organic ions

The potential application of the method wastested by applying the procedure in the conditions

Fig. 6. Influence of the duration of the application ofEf .

described to other organic ions. Tetrapropyl- andtetrabutyl-ammonium gave a similar response toTEA+, acetylcholine and tetramethyl-ammoniumcations gave a linear response in the range 2.5 ×10−6–5× 10−5M with a slope about 5 times lowerthan that of TEA+ and dopamine cation gave no re-sponse up to 5× 10−5M. Common inorganic cationsgave no response up to a concentration of 10−3M

C. Sanchez-Pedreño et al. / Analytica Chimica Acta 459 (2002) 11–17 17

Fig. 7. Influence of the concentration of tetraethyl-ammonium.

for Na+, 2× 10−4M for K+ and Mg2+ and 10−4Mfor Ca2+. These finding are in agreement with thestandard potential differences and Gibbs energies oftransfer of ionic species from water to PVC–NPOEmembranes [6]. Data for more species are availablefor the transfer from water to nitrobenzene [13,14]and this information may also be useful for predic-tions in our system since the values reported for bothsystems are similar [6].A comparison of the chronocoulometric response,

we observed to different tetraalkyl-ammonium ionswith the corresponding potentiometric response re-ported with ion exchanger membrane electrodes[12,15] reveals some significant differences betweenboth techniques. The proposed chronocoulometricmethod for the determination of tetraethyl-ammoniumis much more tolerant of the higher homologues(tetrapropyl- and tetrabutyl-) but less tolerant of thelower homologue (tetramethyl-). The potentiomet-ric selectivity coefficients reported for the differenthomologues increase by about two logarithmic unitsbetween each other. This difference found betweenthe chronocoulometric and potentiometric techniquesmay be due to the fact that the chorocoulometric re-sponse towards the analyte and the higher homologues

is diffusion-limited (a process which is similar forall homologues), while the potentiometric responsedepends on the standard Gibbs energy values of iontransfer, which differ greatly between homologues. A7-fold lower detection limit was achieved with thechronocoulometric technique, although the linear re-sponse range was, as it was expected wider in thepotentiometric technique [12].A comparison of the characteristics of the chrono-

coulometric method proposed with previouslyreported amperometric methods [4,5,8] is not veryconclusive since the methods were used for the deter-mination of different analytes. Bearing this in mind,the detection limit and/or the lowest concentration ofthe linear response are about 10–1000-fold lower forthe chronocoulometric method described.

Acknowledgements

The authors are grateful to the Dirección Generalde Investigación, Spain (Project BQU2001-0414) forfinancial support.

References

[1] M. Senda, T. Kakiuchi, T. Osakai, Electrochim. Acta 36(1991) 253.

[2] J. Koryta, Electrochim. Acta 33 (1988) 189.[3] P. Vanýsek, Anal. Chem. 62 (1990) 827 A.[4] B. Hundhammer, S. Wilke, J. Electroanal. Chem. 266 (1989)

133.[5] V. Marecek, H. Janchenova, M.P. Colombini, P. Papofff, J.

Electroanal. Chem. 217 (1987) 213.[6] R.D. Armstrong, M.L. Marcos, Electrochim. Acta 37 (1992)

1021.[7] V. Horváth, G. Horvai, Anal. Chim. Acta 273 (1993) 145.[8] S. Sawada, H. Torii, T. Osakai, T. Kimoto, Anal. Chem. 70

(1998) 4286.[9] S. Jadhav, E. Bakker, Anal. Chem. 71 (1999) 3657.[10] S. Jadhav, E. Bakker, Anal. Chem. 73 (2001) 80.[11] H.J. Lee, P.D. Beattie, B.J. Seddon, M.D. Osborne, H.H.

Girault, J. Electroanal. Chem. 440 (1997) 73.[12] T. Katsu, Anal. Chem. 65 (1993) 176.[13] J. Koryta, Electrochim. Acta 29 (1984) 445.[14] V.S. Markin, A.G. Volkov, Electrochim. Acta 34 (1989)

93.[15] R. Scholer, W. Simon, Helv. Chim. Acta 55 (1972) 1801.