Full Paper

Flow-Injection Amperometric Detection with Solvent PolymericMembrane Ion SensorsJoaquÌn A. Ortunƒo, Jorge Herna¬ndez, Concepcio¬n Sa¬nchez-Pedrenƒo*

Department of Analytical Chemistry, University of Murcia, 30071 Murcia, Spain*e-mail: spedreno@um.es

Received: February 26, 2003Final version: June 10, 2003

AbstractAn amperometric detector for hydrophobic ions based on a plasticized poly(vinyl) chloride (PVC) membraneincorporated in a flow-injection system was developed. A four-electrode potentiostat with ohmic drop compensationwas used, while a flow-through cell incorporated the four electrodes and the membrane, which containedtetrabutylammonium tetraphenylborate. When the influence of the applied potential and of the flow-injectionvariables on the determination of tetrabutylammonium was studied, a linear relationship was observed betweencurrent peak height and ion concentration over a range of 5� 10�6 ± 6� 10�5 M tetrabutylammonium. Goodrepeatability and between-day reproducibility and high sample frequency were obtained. The effect of other ions wasstudied. Two different amperometric methods, indirect and direct, were also developed for the determination ofdodecylsulfate in the concentration range 3� 10�5 ± 9� 10�4 M.

Keywords: Amperometry, Flow-injection analysis, Solvent polymeric membrane, Ion sensor, Tetrabutylammoniumdetermination, Dodecylsulfate determination

1. Introduction

Electrochemistry at the interface between two immiscibleelectrolyte solutions (ITIES) [1 ± 3], also called electro-chemistry at liquid-liquid interfaces, is a field in electro-chemistry which has recently found new applications insensor development. Some authors [4 ± 9] have developedflow voltammetric detectors based on ion transfer acrossITIES. These have the advantage over conventional am-perometric flow detectors in that they can be used to detecteven non-redox ions. The reported flow detectors of thistype either use the organic phase as a gel [4 ± 5, 8] or insert aporous membrane between the two liquid phases [6 ± 7] toovercome the mechanical instability of the liquid/liquidinterface, which is themain limitation of ITIES for practicalanalytical purposes. However, it is also possible to makevoltammetric measurements using plasticized poly(vinyl)chloride (PVC) membranes similar to those used in ion-selective electrodes [10 ± 13]. The properties of thesemembranes have been reviewed [14]. The aim of the presentpaper was to study their suitability for the construction offlow amperometric ion detectors in flow-injection analysis(FIA). The method developed was applied to the flow-injection amperometric determination of tetrabutylammo-nium and dodecylsulfate. Tetraalkylammonium ions havebeen used as ionic markers in permeability assays of lip-osomal membranes [15], while dodecylsulfate, commonlyknown as laurylsulfate, is widely used as a surfactant. Inmostcases surfactants are not electroactive, meaning that indirectmethods must be used for their amperometric determinationalthough potentiometricmethods are a good alternative [16].

2. Experimental

2.1. Apparatus

The four electrode potentiostat, four-electrode flow cell andflow-injection system were described in [13]. A Gilson(Villiers leBel, France)Minipuls 3 peristaltic pump,Omnifit(Cambridge, UK) injection valve, connecting tubing of0.5 mm bore, PTFE tubing and various end fittings andconnectors were used to construct the flow-injection system.A glass ring of 28 mm inner diameter and 30 mm height,glass plate, vial and punch were purchased from Fluka forthe construction of the membranes.

2.2. Reagents and Solutions

Poly(vinyl chloride) (PVC) high molecular mass, 2-nitro-phenyl octyl ether (NPOE) and tetrahydrofuran (THF)were Selectophore products from Fluka. Tetrabutylammo-nium tetraphenylborate (TBA�TPB�), tetraethyl- and tet-rabutyl- and tetradecyltrimethyl- ammonium chlorides(TEA�Cl�, TBA�Cl� and TDTMA�Cl�) and sodium dode-cylsulfate (SDS) were purchased from Sigma, tetramethy-lammonium chloride (TMA�Cl�) was from Fluka andtetrapropylammonium chloride (TPA�Cl�) was from Al-drich. Hexadecyltrimethylammonium chloride (HDTMA�

Cl�) was from Merck.A 1� 10�2 M TMA�, TEA�, TPA� and 1� 10�3 M DS�

solutions were prepared by dissolving the correspondingsalts in water. Working sample solutions were prepared by

827

Electroanalysis 2004, 16, No. 10 ¹ 2004 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim DOI: 10.1002/elan.200302886

diluting with 1� 10�2 M LiCl. All the other reagents usedwere of analytical reagent grade and doubly distilled waterwas used throughout.

2.3. Membrane Preparation

Themembranewas prepared fromNPOE,PVCTBA�TPB�

and was incorporated in a homemade flow-through cell asdescribed previously [13]. The electrochemical cell can beexpressed as

Ag �AgCl � 1� 10�2 M TBA�Cl� � 5� 10�2 mol kg�1 TBA�

TPB� � 1� 10�2 M LiCl, xM TBA�Cl� �AgCl �Ag

the applied potentialE is defined as the potential differencebetween the right and left hand terminals.E ismaintained atthe pre-set valuebymeans of the four-electrodepotentiostatthat applies the necessary potential between the right andleft counter electrodes. A positive current corresponds tothe transfer of positive charge from the right side to the left.

2.4. Amperometric Flow-Injection Procedures for theDetermination of Tetrabutylammonium andDodecylsulfate

For the determination of tetrabutylammonium, 160 �Laliquots of working sample solutions (5� 10�6 ± 6� 10�5 MTBA�) were injected into a 1� 10�2 M LiCl carrier solutionpumped at a flow rate of 1.5 mL min�1. The potentialdifference, E, was adjusted to 250 mV and the current wasrecorded. The peak height was plotted versus TBA�

concentration.For the determination of dodecylsulfate two methods

were used. In the indirect method, the working samplesolutions (0 ± 5� 10�4 M DS�) also contained TBA� (5�10�5 M), while in the direct method working samplesolutions of 1� 10�5 ± 1� 10�3 MDS�were injected directly.The other experimental conditions were those described in2.4. In bothmethods, the peakheightwas plotted versusDS�

concentration.

3. Results and Discussion

Figure 1 shows the dynamic detector response obtained fora 5� 10�5 M TBA� working solution following the proce-dure described in 2.4. As can be seen a well-defined positivecurrent peak was obtained corresponding to the transfer ofTBA� from the flowing solution to the plasticized PVCmembrane. The rapid return of the current to the baselinewhich was observed represents an advantage over flow-injection potentiometric detection methods, in which thecorresponding return is slow.

3.1. Influence of the Applied Potential

The influence of the applied potential on the backgroundcurrent corresponding to the carrier solution and on thecurrent peak height corresponding to TBA�was studied byvarying the applied potential in the range 100 ± 500 mV. Ateach potential value tested, the current baseline corre-sponding to a 1� 10�2 M LiCl carrier solution pumped at aflow rate of 1.5 mLmin�1 and the current peak correspond-ing to the injection of 160 �L of 3� 10�5 M TBA�workingsolution into the carrier were recorded. The results obtainedare shown in Figure 2.As can be seen, therewas awindow atabout 225 ± 425 mV where the background current re-mained low, while it was null at about 250 mV. Much higheror lower potential values than those of the potential windowlead to high positive and negative current values, respec-tively, due to the background current corresponding to thesupporting electrolytes of the membrane and flowingsolution The positive current peak obtained by the injectionof TBA� solution corresponds to the ingress transfer ofTBA� from the sample plug to the outer side of themembrane, accompanied by an equivalent transfer of TBA�

from the inner side of the membrane to the inner solution.As can be seen from Figure 2, the peak height obtainedincreased when the applied potential was increased,although this effect was not very pronounced. Taking allthis into account and the possible interfering effect of otherions at high potential values, an applied potential of 250 mVwas selected for further studies.

Fig. 1. Dynamic detector response obtained for a 5� 10�5 MTBA� working solution under the following experimental con-ditions: applied potential, 250 mV; sample volume, 160 �L; flowrate, 1.5 mL min�1.

828 J. A. Ortunƒo et al.

Electroanalysis 2004, 16, No. 10 ¹ 2004 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim

3.2. Influence of the Flow-Injection Variables

Theeffect of the flow rate on thedeterminationofTBA�wasstudied at two concentrations ofTBA�, 4� 10�5 and 5� 10�6

M, by injecting aliquots of 160 �L of the working samplesolution into a LiCl 10�2 M carrier solution and by varyingthe flow rate between 0.25 and 2.75 mL min�1. The resultsobtained are shown in Figure 3. As can be seen, an increasein peak height was obtained by increasing the flow rate.However, the reproducibility of the peak height obtainedwasworse at the higher flow rates assayed.As a compromisebetween sensitivity and reproducibility, a flow rate of 1.5 mLmin�1 was selected for further studies.The limiting diffusion current il corresponding to a wall-

jet arrangement follows the equation [17]

il� 1.38 n F D2/3 v �5/12 a�1/2 R3/4 q3/4 C (1)

Where v is the kinematic viscosity, a is the diameter of thenozzle, R is the radius of the electrode surface and q is thevolume flow rate. The other symbols have their usualmeanings. The FIA dispersion coefficient, P, which is ameasure of the degree of dilution of a particular part of thesample between injection and detection, is defined as

P�C0/C (2)

where C0 and C are the concentration before and aftertransport through a given FIA system. The relationshipbetween P and the flow rate follows the following equation[18]

P� kq1/2 (3)

By combining Equation 1 and 3

ii� k�q1/4 (4)

with k�� 1.38 n F D2/3 v �5/12 a�1/2 R3/4 k�1 C0

As can be seen in Figure 3b, when the experimentalcurrent values obtained for the two TBA� concentrationsstudied were plotted versus q1/4, a linear relationship withcorrelation coefficient values 0.9941 and 0.9957, respective-ly, was obtained.The influence of the sample volume was studied by

injecting different volumes of 5� 10�5 M TBA� workingsolution in the range 80 ± 350 �L. Almost constant peakheight values were obtained above 150 �L, and a samplevolume of 160 �L was selected for further studies.

3.3. Determination of Tetrabutylammonium

The calibration plot for TBA� concentration obtained in theexperimental conditions described above is shown in Fig-ure 4. As can be seen a linear relationship between the peakheight and TBA� concentration was obtained in the range5�10�6 ±6�10�5 M. The regression equation was H(�A)�9.13�103 [TBA�] (M)�5.1�10�3 with a regression coeffi-cient of 0.9999. The detection limit obtained from three timesthe standard deviation of the blank was 4�10�7 M.The repeatability of the proposedmethodwas evaluatedby

performing twenty-sevenconsecutive injectionsof5�10�5 MTBA� working solution. The variation coefficient obtainedfor the peak height was �2.2%. The between-day reprodu-cibility was studied by injecting the above solution on sixrandomdaysduringa twoweekperiod.During this period the

Fig. 2. Influence of the applied potential on the: (�) backgroundcurrent corresponding to a 1� 10�2 M LiCl carrier solutionpumped at a flow rate of 1.5 mL min�1 and (�) peak heightcorresponding to the injection of 160 �L of 3� 10�5 M TBA�

working solution.

Fig. 3. Plot of the peak height corresponding to the injection of160 �L of (�) 4� 10�5 M and (�) 5� 10�6 M TBA� workingsolutions into a LiCl 1� 10�2 M carrier solution, versus a) flowrate and b) (flow rate)1/4.

829FIA with Solvent Polymeric Membrane Ion Sensors

Electroanalysis 2004, 16, No. 10 ¹ 2004 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim

detector cell was stored filled with the supporting electrolytesolutions. The variation coefficient of the means correspond-ing to 3 injections each day was �1.5 %. The samplefrequency was 180 samples per hour, which is higher than theusual values reported using potentiometric detectors.

3.4. Effect of Other Ions

The effect of other ions on the flow-injection amperometricmethod for the determination of TBA� was studied byinjecting solutions of different inorganic and organic ions,using the same experimental conditions. Cations solutionswere prepared in the formof chlorides or sulfates and anionsin the formof sodium salts. In the case of the non-interferingions, Na�, K� and Cl�, the maximum concentration assayedwas 1� 10�2 M of the corresponding salt dissolved in water.For each of the other inorganic ions tested and for TMA�

and TEA�, a 1� 10�4 M working solution (prepared in 1�10�2 M LiCl) of the corresponding ion was used. For theorganic ions, TPA�, TDTMA�, HDTMA� and DS� (dode-cylsulfate), a 1� 10�5 M working solution was used. Thesensitivity of each ion was calculated from the quotientbetween peak height and ion concentration. The sensitivityratios of each ion with respect to that of TBA� (obtainedfrom the slope of the calibration plot) are shown in Table 1.As can be seen Na�, K�, Mn2�, Cl�, Br�, acetate and SO2�

4

gave no response. The rest of the metal ions showed asensitivity between 30 and 200 fold lower than that of TBA�.The sensitivity obtained for the organic cations, TMA� andTEA�, was respectively 50 and 30 fold lower, while thatobtained for the more lipophilic TPA�, TDTMA� andHDTMA�was similar to that of TBA�. With respect to theanion dodecylsulfate, a sensitivity about 5 fold lower andwith an opposite sign to that of TBA�was found.

3.5. Determination of Dodecylsulfate

Twodifferent approaches for the flow-injection amperometricdetermination of the anion dodecylsulfate were also studied,based on the interaction between DS� and TBA� to form theTBA�DS� ion associate. In the first approach, termed theindirect method, a fixed amount of TBA� was added to theDS� working sample solution and the positive currentcorresponding to the free TBA� was measured. As can beseen in Figure 5, the peak height obtained by this methoddecreased as the concentration ofDS� increased up to 5�10�4

M. The detection limit was 4�10�5 M. In the secondapproach, termed the direct method, the working sampleDS� solution (without TBA�) was injected directly. As can beseen in Figure 5, negative current peaks were obtained withthis method, peak height (absolute values) increasing as theDS� concentration was increased from 3�10�5 to 9�10�4 M.The detection limit was 1�10�5 M. In this case, the currentcorresponds to the transferofTBA� fromthemembrane to theflowing solution. Both approaches could probably be appliedto the determination of other anions that interact with TBA�.

Fig. 4. Calibration plot obtained for TBA� in the selectedexperimental conditions: applied potential, 250 mV; sample vol-ume, 160 �L; flow rate, 1.5 mL min�1.

Table 1. Sensitivity of the detector towards other ions.

Ion Sion/STBA�

Na�, K�, Mn2�, Cl�, Br�, Ac�, SO2�4 ±

Mg2� 5� 10�3

Ca2�, Ni2� 7� 10�3

Co2� 1� 10�2

Zn2�, TMA� 2� 10�2

Cu2�, TEA� 3� 10�2

H� 5� 10�2

OH� 5� 10�2 ( ± )DS� 2� 10�1 ( ± )TPA�, TDTMA�, HDTMA� 1

Fig. 5. Calibration plot obtained for DS�:(�; left) indirectmethod with added 5� 10�5 M TBA� and (�; right) directmethod.

830 J. A. Ortunƒo et al.

Electroanalysis 2004, 16, No. 10 ¹ 2004 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim

4. Conclusions

The detector developed, which is based on a plasticizedpoly(vinyl chloride) solvent polymeric membrane, permitsthe flow-injection amperometric determination of non-redox hydrophobic cations and anions. Good sensitivity andreproducibility and high sampling frequency were achieved.The detector could be used in ion chromatography.

5. Acknowledgement

The authors are grateful to the Direccio¬ n General deInvestigacio¬n, Spain (Project BQU2001-0414) for financialsupport.

6. References

[1] J. Koryta, Electrochim. Acta 1988, 33, 189.[2] M. Senda, T. Kakiuchi, T. Osakai, Electrochim. Acta 1991, 36,

253.[3] F. Reymond, D. FermÌn, H. J. Lee, H. H. Girault, Electro-

chim. Acta 2000, 45, 2647.

[4] V. Marecek, H. Janchenova, M. P. Colombini, P. Papofff, J.Electroanal. Chem. 1987, 217, 213.

[5] H. Li, E. Wang, Analyst 1988, 113, 1541.[6] B. Hundhammer, S. Wilke, J. Electroanal. Chem. 1989, 266,

133.[7] S. Wilke, H. Franzke, H. M¸ller, Anal. Chim. Acta 1992, 268,

285.[8] H. J. Lee, H. H. Girault, Anal. Chem. 1998, 70, 4280.[9] S. Sawada, H. Torii, T. Osakai, T. Kimoto, Anal. Chem. 1998,

70, 4286.[10] R. D. Armstrong, M. L. Marcos, Electrochim. Acta 1992, 37,

1021.[11] V. Horva¬th, G. Horvai, Anal. Chim. Acta 1993, 273, 145.[12] S. Jadhav, E. Bakker, Anal. Chem. 1999, 71, 3657.[13] C. Sa¬nchez-Pedrenƒo, J. A. Ortunƒo, J. Herna¬ndez, Anal. Chim.

Acta 2002, 459, 11.[14] R. D. Armstrong, G. Horvai, Electrochim. Acta 1990, 35, 1.[15] T. Katsu, Anal. Chem. 1993, 65, 176.[16] M. Gerlache, J. M. Kauffmann, G. Quarin, J. C. Vire, G. A.

Bryant, J. M. Talbot, Talanta 1996, 43, 507.[17] H. Gunasingham, B. Fleet, Anal. Chem. 1983, 55, 1409.[18] M. Valca¬rcel, M. D. Luque de Castro, Flow-Injection Anal-

ysis Principles and Applications, Ellis Horwood, Chichester1987, p. 76.

831FIA with Solvent Polymeric Membrane Ion Sensors

Electroanalysis 2004, 16, No. 10 ¹ 2004 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim