Voltammetric Ion-Selective Electrodes (VISE)

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Voltammetric Ion-Selective Electrodes (VISE) Dagmar Henn and Karl Cammann* Institute for Chemical and Biochemical Sensor Research, University of MuÈnster, Mendelstrasse 7, 48149 MuÈnster, Germany e-mail:[email protected] Received: March 8, 2000 Final version: June 26, 2000 Abstract

Voltammetric ion-selective electrodes (VISE) based on a traditional principle with internal electrolyte as well as solid-state devices in the form of electrochemical one-shot teststrip are developed. In the latter the interfering polarization at the solid-state backside contact can be avoided by increasing the contact area and thus decreasing the current density at this interface drastically as compared to the interface ISEmembrane=sample. The chemically modi®ed electrodes described here will hopefully replace the highly toxic mercury electrodes used in stripping analysis. In the development of highly reproducible screen-printed VISE, controlling the area of the active membrane surface is much easier. Therefore, it is possible to produce many VISE with absolutely identical features opening the route to precalibrated devices without any need for additional recalibration by the end user. Keywords: Voltammetric ion-selective electrodes (VISE), One-shot screen-printed VISE for potassium and lead

Dedicated to Professor G. A. Rechnitz on the Occasion of His 65th Birthday

1. Introduction Ion-selective potentiometry has become a widely used analytical technique. In clinical chemistry it has more or less completely replaced the traditional ¯ame photometric technique for the determination of the electrolyte ions (Na, K, Li, Ca, Mg, Cl). Main reason is the ease of use, its speed and the even more important fact that the corresponding ion activity in the actual biological ¯uid (urine or blood) is measured. With time, awareness has settled that it is the ion activity of the free and uncomplexed ion and not the total ion concentration which controls physiological processes. For measuring ion activity the ion-selective electrodes (ISE) are without any competition. However, the technique of potentiometry shows three inherent drawbacks: First it shows rather a logarithmic than a linear concentration dependence; second it becomes increasingly more inaccurate with increasing charge of the measured ion rendering this method inappropriate for ions with charges larger than two; third any temperature variation between calibration and sample measuring is dif®cult to compensate. Especially the latter is relevant for industrial applications under nonthermostatic conditions. In these cases an applicable temperature compensation implies the use of special electronic circuits different from the laboratory pH-meter method. The needed so-called ``isotherm crossing point'' has to be determined experimentally for the corresponding electrochemical cell in total and also under identical thermal conditions, since it also includes electrothermal gradients between the part of the electrodes in contact with the sample and the remaining part, most often at a different temperature. The isothermal crossing point is the point where three or more calibration plots EMF vs. ion activity for different temperatures intercept. Especially for unsymmetrical electrochemical cells (i.e., solid state electrodes without internal ®lling solutions) in combination with traditional reference half cells of the calomel or silver/silver chloride type the temperature dependent solubility product will have a marked in¯uence on the crossing point of different isotherms in the emf±plot diagram. Sometimes only a rather large range can be determined even with simple pH-glass electrodes, with ion-selective cells the crossing points are often scattered and outside the calibration range. In Electroanalysis 2000, 12, No. 16

these cases the measurements have to be performed under isothermal conditions. Additionally, with the exception of the Ross electrode, oversaturation effects can delay the establishment of the new solubility equilibrium causing a signi®cant increase in responding time for different temperatures. All these drawbacks could be overcome if the measuring ionselective electrode could be used under amperometric conditions. The temperature dependence of the diffusion coef®cients would lead to a variation of a few percent perÅ C in the more or less linear temperature dependent diffusion-limited current. This is much easier to control electronically. Moreover, higher valent ions are measurable with a similar accuracy then monovalent ones, just the steepness of the voltammetric step is increased. The article describes a new class of voltammetric ion-selective electrodes (VISE) with and without internal ®lling solutions for the analytes potassium, lead, and nitrate. They are based on the famous works of Koryta and co-workers [1] regarding the interface of two immissible electrolyte solutions (ITIES) back in the seventies and eighties. Based on the pioneering works of Gavach [2] and Guastalla [3] Koryta demonstrated that such interfaces can be treated theoretically like the interface: metal electrode=electrolyte solution if certain corrections for the nonmetallic character of the organic solvent are made. Based upon the obtained cyclovoltammograms the ionic species which is transported through the interface can be identi®ed by the required potential difference for this transfer. Koryta even demonstrated an analogous kind of polarography with rising droplets of an organic electrolyte with a lower speci®c density than water (replacing the toxic mercury). However, typical measurements with ITIES are not simple. It consists of a mechanically unstable interface: organic electrolyte=aqueous electrolyte which was held in a small glass tube, two current bearing platinum electrodes and two high ohmic reference electrodes were positioned very close to the phase boundary, the latter with Luggin-capillaries even closer to the corresponding phase boundary. The measuring apparatus was very prone to disturbances even by slight vibrations of the liquid phase boundary. For quite some time no reliable reference electrode for direct application in the organic phase was available. Thus, an additional liquid=liquid interface between the organic phase and

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D. Henn, K. Cammann

the reference element had to be used. A further problem, the rather high vapor pressure and the toxicity of the organic solvents (most commonly used were nitrobenzene or 1,2-dichoroethane). The mechanical fragility of the ITIES was improved by a technique also used in metal extraction: the use of membrane supported interfaces (like the ®rst potentiometric ion-selective liquid membrane electrodes: Orion Series 92 [4]. But still the electrochemical cell could not be considered transportable. Senda et al. [5] were among the ®rst who used a polymer-stabilized organic phase containing ionophores for voltammetric studies. In ®rst evaluation studies of the transfer of nitrate ions across the interface polymer membrane=aqueous electrolyte we used paper supported polymer membranes [6]. This article describes the development of electrochemical-teststrips based on ITIES. In this work we describe for the ®rst time a selective analyte enrichment and stripping technique of the sample analyte lead at an all solidstate VISE with no internal ®lling solution.

2. Theoretical Background The fundamentals concerning the ion transfer at ITIES can be found, e.g., in works of Parker [7, 8]. Most of the further discussion holds in assumption of dilute solutions. Applying a potential across the ITIES which is signi®cantly different from the equilibrium value with an experimental set-up shown in Figure 1 the thermodynamic equilibrium is disturbed and a redistribution of charged ions according to this potential difference (overpotential) will occur. The transfer of charged species is in fact equivalent to a directed electrical current across this interface. In most theoretical studies of ITIES [7±9] the charge transfer kinetics have been neglected and diffusion limitation was assumed. In the last years several models for the mathematical description of cyclic voltammograms obtained at metal electrodes have been developed [10±13]. Mostly known are the studies from Nicholson and Shain [14] and the continuation by Heinze [15, 16]. For a planar liquid=liquid interface a numerical solution of the theoretical current-voltage function at the peak current voltage leads to the Randles-Sevcik equation for reversible ion transfer: iP iEP k z3y2 AvDA 1y2 cA

1

with: k Randles-Sevcik-Konstante, z charge of the transferred species, A area of the interface, v scan rate of the potential difference, DA diffusion coef®cient of the transferred ion, cA bulk concentration of the transferred ion.

Fig. 1. Schematic drawing of the general experimental set-up of voltammetric studies at an interface of two immiscible electrolyte solutions (ITIES). w: water phase with added base electrolyte; o: organic solvent with added salt for ionic conductance; C1, C2: current bearing platinum electrodes; Ref1, Ref2: reference electrodes with Luggin capillaries nearest to the phase boundary: w w=o; I: sample ion being transferred from the aqueous into the organic phase. Electroanalysis 2000, 12, No. 16

Koryta has already demonstrated that ion transfer at ITIES can be treated analogously to redox reactions at metallic electrodes. In both cases the cyclic-voltammograms show typical current peaks as usual in the traditional cyclic voltammetry. In absence of kinetic limitations the anodic and cathodic current peaks have an equal height and the peak separation at 298 K is 59=z mV. At higher scan rates the charge transfer kinetics may be limiting and a larger separation of the forward and backward ion transfer peaks may occur. In addition, when a PVC polymer gel is used as a gel like PVC (membrane with lower diffusion coef®cients) the rising iR drop will cause additional deviations. Thus, an effective iR compensation is essential. In case of a hydrophilic supporting electrolyte like lithium sulfate one obtains a certain ``potential window'' in which the interface is polarized without any negligible current. Only at extreme overpotentials ions are transferred across the phase boundary. According to a suggestion of VanyÂsek [17] and the Prague school by convention the transport of cations from the aqueous phase into the organic phase is accompanied by a positive current; likewise the transport of anions from the organic phase into the aqueous phase. Thus, the transport from cations from the organic into the aqueous and anions from the aqueous into the organic phase is recorded as a negative current. If the aqueous supporting electrolyte contains any ion with a certain lipophilicity (higher than the ions of the ground electrolyte) it will be transported more easily if a suf®cient overpotential is applied and results in corresponding ion peaks within the CV diagram. Their transfer from the aqueous into the organic phase and vice versa is shown by the corresponding peak currents within the given potential window. If the experimental conditions are carefully controlled a strict linearity of both peak potentials with the corresponding bulk concentration is given. This dependence is analytically used with novel voltammetric ISE's developed in this study for ease of use. Concerning this striking similarity to metallic electrodes it should also be possible to perform some kind of preelectrolysis at an appropriate overpotential leading to a certain enrichment of the analyte in the volume of the organic phase. An electrochemical stripping technique should enhance the peak currents and thus the sensitivity of the set-up. This might be of interest as a promising alternative to the highly toxic Hg in order to replace it in those known powerful trace analytical techniques.

3. Experimental All measurements were performed using the 4-electrode potentiostat (Solartron Model 1280 B), controlled by a personal computer with the software Corrware and Corrview by Scribner Associates. Alternatively an ``Electrochemical IBM-HEKA'' instrument by HEKA electronics was used. As iR-compensation the interrupt method has been used. As reference electrode in the aqueous sample solution (output RE 1 of the potentiostat) an Orion model 90020 Ag=AgCl electrode with double liquid junction was used. As reference electrode for the organic phase a silver wire (output RE of the potentiostat) or a polymer conducting paste Auromal from DODUCO and a silver=silver chloride (3 : 2) ink from Acheson Colloiden in a self constructed Luggin-capillary contacting the backside of the ion-selective membrane has been employed. The two counter electrodes connected to output CE (analyte solution) and WE (organic phase) consisted of a 99.9 pure platinum wire . For the polymer

Voltammetric Ion-Selective Electrodes

stabilized ion-selective membrane PVC (high molecular form) Fluka Selectophore and ``blue ribbon'' ®lter paper type 589 of the company Schleicher&Schuell has been used. The scan rate most often used were 50 mV=s. iR compensation: off time 100; ratio on=off 10. A low pass ®lter of 8 Hz was applied. For the all solid state set-up the organic phase reference electrode (output RE 2 at the potentiostat) consisted of a ®lter paper (blue ribbon) with sputtered silver layer (see preparation details). In those cases a three electrode set-up was used (output RE2 and WE short circuited). Current bearing electrode was either a platinum wire or a platinum containing ink (R-474-DPM-80 from Ercon) for screen printing. The heat sealing ®lms were obtained from Team Codor. While recording the cyclic voltammograms the analyte solution was not stirred. All reagents used were analytical grade, selectophore, or purum and obtained from Fluka with the exception of the 99 % ascorbic acid and 99.9 % tetrahydrofuran from Aldrich. All were used without any further precautions or further cleaning. Deionized water was used (mho < 0.5 mS). The study started with ITIES in their typical glassware set-up with rather fragile liquid=liquid phase boundary. It was soon replaced by an organic phase solidi®ed by PVC (PVC and different plasticizers up to > 80 wt.%. like it is done with potentiometric ISE's. An interface like the one shown schematically in Figure 2 was obtained. Figure 2(top) shows the typical four-electrode arrangement and an internal ®lling solution containing the supporting electrolyte to carry the current. It mostly resembles the known ITIES. However, a convincing

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interpretation of the CV curves is only possible if the measuring solution is placed on both sides of the membrane. Then the effect on one side is just mirrored at the other site. By this, Cammann and Xie [18] were able to determine kinetic parameters at ISE membranes interfaces. In order to obtain a polarizable interface and a wide potential window in the cyclic voltammogram the electrolytes for each phase should be carefully chosen. Here, platinum wires were used as counter electrodes in an internal electrolyte of 0.04 M tetramethyammoniumhydrogensulfate and 0.2 M in NaCl. An Ag=AgCl wire in a Luggin capillary was used as internal reference electrode. The ISE membrane consisted of a ®lter paper as inert support for the different PVC=plasticizer membranes tested. In general, the studied membranes were also tested in an all solid-state version. In absence of the internal ®lling solution the internal working electrode and reference electrode were connected together resulting in a three electrode arrangement. To produce a large area internal backside contact of the organic membrane phase a ®lter paper sputtered with silver was used as a carrier. The basic layout of the VISE test strip is shown in Figure 3. Base material is a heat sealing folio (125 mm thick, 1.2 cm wide, and about 5 cm long). The ®lter paper with sputtered silver on the surface has a diameter of 8 mm; the spacer has inner and outer diameters of about 2 mm and 6 mm, respectively. The combined working and reference electrode is screen-printed with silver or silver=graphite ink on the backside of the upper heat sealing ®lm as shown in the ®gure. The two ®lms are then heat laminated together using the typical heated roller apparatus. The appropriate ISE membrane cocktail was cast into the well build by the opening of the upper folio and the spacer (365 mL cocktail of PVC=plasticizer organic salt=ionophore) with the help of an automatic dispenser and dried for one day at least. Before usage a minimum of 3 cyclic voltammograms were recorded to produce Ag ions which are needed for a layer of the organic silver salt (silver-tetrakis-(4chlorophenyl)-borate) to make the all solid state phase boundary Ag=PVC membrane less polarizable (reference electrode) than the interface under study. Typically, several dozen VISE's were produced in one batch leading to a very similar behavior of the sensors in that one batch.

3.1. VISE Membranes 3.1.1. Membranes Without Ionophores In case the analyte ion is more lipophilic than the supporting electrolyte ions (like nitrate or perchlorate) it can be transferred

Fig. 2. Top) Experimental set-up for the voltammetric studies performed with a four electrode arrangement using traditional VISE with internal ®lling solution (electrodes description see Fig. 1). Bottom) Experimental set-up for the voltammetric studies performed with a three electrode arrangement using planar screen printed all solid-state VISE (electrode description see top; Ref1 and C1 at the backside of the ISE membrane are short-circuited).

Fig. 3. Schematic layout of the planar screen printed all solid-state VISE with electrode descriptions analogous to Figures 1 and 2. Electroanalysis 2000, 12, No. 16

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Table 1. Membrane composition of the nitrate VISE.

Table 2. Membrane compositions of the studied three potassium VISE.

Composition

Weight %

PVC o-NPOE TDATCPB Solvent

5.45 85.66 8.89 THF

from the water phase into the organic membrane at suitable voltages within the potential window of the cyclic voltammogram without any facilitation by a speci®c ionophore. The addition of the TDTCCPB in the organic phase should mainly lower the bulk resistance and lead to a large potential window because of the hydrophilic character of both the cation and anion of this salt. The membrane studied concerning its voltammetric behavior with respect to the nitrate anion had the composition as shown in Table 1.

3.1.2. Membranes with Ionophores Potassium VISE. For more hydrophilic analytes selective ionophores have to be added to the VISE membranes in order to facilitate a selective ion transfer from the aqueous phase into the organic membrane phase and back within the potential-window

Component PVC (wt.%) TDATCPB (wt.%) o-NPOE (wt.%) Ionophore (mg) (wt.%)

K±ionophore I (type KI)

K±ionophore II (type KII)

K±ionophore III (type KIII)

5.18 8.45 81.44 5.0 4.93

5.17 8.43 81.28 5.2 5.12

5.16 8.4 81.04 5.5 5.39

available for a reliable evaluation. Unlike potentiometric ISE's in case of the VISE the used PVC content is lower and the ionophore concentration higher than usual. For a potassium VISE three different commercially available selective K ionophores (see Fig. 4) were compared: K±ionophore I: valinomycin, K±ionophore II: bis[(benzo[15]-crown-5)-40 -ylmethyl]pimelate, K±ionophore III: BME 44 (dodecyl-2-methyl-1,3-propandiyl-bis[N-(50 -nitro(benzo-[15]crown-5)-40 yl)]carbamate]. In case of the traditional VISE construction with internal electrolyte the membranes and the electrodes were prepared as described in [4]. In case of the screen-printed all solid-state teststrip they were cast (3 times 5 mL) with the corresponding tetrahydrofurane membrane cocktail by dropping in the opening and above the silver plated area forming the backside contact. The ®nal membrane composition after the evaporation of the THF is given in Table 2. Lead VISE. In the course of the development of a lead VISE three different commercially available selective Pb2 ionophores (see Fig. 5) were compared using base membrane composition (Table 3): Pb2±ionophore I: methylen-bis-(N,N-diisobutyldithiocarbamate) (MBDiBDTC), Pb2±ionophore II: N ; N ; N 0 ; N 0 -tetradodecyl-3,6-dioxaoctandithioamid (ETH 5435), Pb2±ionophore III: tert-butyl-calix[4]aren-tetrakis-(N,N-dimethylthioacetamid).

4. Results and Discussion

Fig. 4. a) Valinomycin (potassium ionophore I); b) bis[benzo-[15]crown-5)-40 -ylmethyl]pimelate (potassium ionophore II); c) 2-dodecyl-2methyl-1,3-propandiyl-bis[N-(50 -nitro(benzo-[15]-crown-5)-40 -yl)]-carbamate BME 44; (potassium ionophore III). Electroanalysis 2000, 12, No. 16

First step was to develop a VISE with a liquid inner ®lling solution. The membrane contained no ionophore, as target analyte nitrate was chosen. A four electrode set-up (see Fig. 2, top) was used in conjunction with a special two reference electrode potentiostat. Figure 6 shows the resulting voltammograms obtained by increasing the nitrate concentration. As expected a linear relationship between the nitrate concentration and the cathodic or anodic transfer peaks for the transfer of nitrate anions into the organic phase and vice versa was obtained as demonstrated in Figure 7. The evaluation data for Figures 6 and 7 are given in Table 4. As expected the current peak caused by the transfer of nitrate anions from the aqueous phase into the PVC membrane result in a greater sensitivity and thus better detection limit than the reverse transfer. However, the coef®cient of correlation is nearly similar and > 0.998. After this successful test a more simple to produce and to use test strip like VISE needing only an external classical reference electrode and an external platinum counter electrode described in Figure 3 above was developed and tested with the same membrane composition. Figure 8 shows the resulting cyclic


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4.1. Potassium VISE

Fig. 5. a) Methylen-bis-(N,N-diisobutyldithiocarbamate; MBDiBdTC (lead ionophore I); b) N,N,N 0 ,N 0 -tetradodecyl-3,6-dioaooctandithioamid; ETH 5435 (lead ionophore II); c) tert-butyl-calix[4]aren-tetrakis-(N,Ndimethylthioacetamid (lead ionophore III).

voltammograms. The two ion transfer peaks are well developed and the analytical parameters comparable to the traditional set-up with internal electrolyte. Thus, the easy-to-fabricate VISE can be used instead of the fragile ITIES. The evaluation data of the screen-printed VISE (see Fig. 8) is shown in Table 5. The linear range is nearly doubled and a standard deviation of about 5 % seems reasonable for such cheap sensors.

Also in this case the ®rst studies which should compare three different commercially available ionophores for a selective Kion transfer were performed in a four electrode set-up with internal electrolyte as shown in Figure 9. Without a potassiumselective ionophore no current peak due to a transfer of potassium ions across the interface solid state PVC membrane electrode=aqueous sample could be observed. However, with about 5 wt.% of ionophore concentration and an increased concentration of plasticizer compared to potentiometric ISE membranes (as indicated in Table 6) a facilitated potassium ion transfer from the aqueous sample solution into the organic membrane phase and backwards could be observed. Nitrate anions also cause two peaks in the voltammogram but well separated from the potassium ion transfers as shown in Figure 9. The general appearance of the obtained CV using KNO3 as analyte solution shown in Figure 9 is the same for the two other ionophores as indicated in Figure 10. The latter shows the superposition of CV for VISE using three studied potassium ionophores at two different concentrations of potassium in the sample solution. The analytical performance of the three VISE membranes using different ionophore types is presented in Table 6. Table 6 demonstrates that all three ionophores resulted in good LOD's and coef®cients of correlation, but further study was carried out using valinomycin. It was also found that a reduction of the PVC content in the membrane phase from 20 % to 5 % lead to higher diffusion coef®cients with the organic phase and thus to standard deviations less than 1 % relative. A potassium-selective membrane with 5.5 wt.% valinomycin and 20 wt.% PVC was also tested with the screen printed all solid state set-up. In order to evaluate the possible in¯uence of the corresponding anion of the analyte on the voltammetric curve, this parameter was modi®ed (from potassium chloride to potassium sulfate as shown in Figure 11). The evaluation data of this simple VISE test-strip-like sensor is shown in Table 7. At potentials > 1.0 V another transfer peak for potassium ions could be observed. This peak shows similar dependents on the K concentration with a somewhat wide linear range up to about 5 mmol=L. The nature of this potassium ion transfer at higher overpotentials is still unclear and might indicate a different

Fig. 6. Cyclic voltammograms with a traditional VISE with internal electrolyte and a composition mentioned in Table 1 at different LiNO3 concentrations; ground electrolyte: 0.005 M LiOH, scan rate: 50 MV=s. Electroanalysis 2000, 12, No. 16

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D. Henn, K. Cammann Table 3. Membrane composition of the studied three lead VISE. Pb ±ionophore I (type PHI)

Pb2±ionophore II (ETH 5435) (type PbII)

Pb2±ionophore III (type PBIII)

5.17 8.43 81.28 5.2 5.12

5.34 8.7 83.9 2 2.03

5.2 8.47 81.68 4.7 4.65

2

Component PVC TDATCPB o-NPOE Ionophore

(wt.%) (wt.%) (wt.%) (mg) (wt.%)

Table 4. Analytical characterization of the VISE with internal electrolyte. Linear calibration plot Slope b (mA L=mmol) Standard deviation (%) Coef®cient of correlation r Detection limit (mmol=L)

Cathodic peak current

Anodic peak currents

ÿ16.9 5.40 0.998 0.03

9.9 3.53 0.999 0.05

Table 5. Analytical characterization of the screen-printed all solid-state nitrate VISE. Linear calibration plot from Figure 8 Slope b (mA L=mmol) Standard deviation in (%) Coef®cient of correlation r Detection limit (mmol=L)

Cathodic peak currents N 6

Anodic peak currents N 6

ÿ17.3 4.9 0.998 0.03

8.2 5.9 0.998 0.07

4.2. Lead VISE The lead ionophore III resulted in voltammograms which were best in the evaluation and the analytical performance of the three studied ionophores. A typical voltammogram obtained with an all solid-state lead VISE is shown in Figure 12. One can distinguish two peaks corresponding to transfer of nitrite (left) and lead (right) ions. The comparison of the analytical performance data between the traditional construction with internal solution and the all solid-state set-up using the ionophore III is given in Table 8. It can be seen that the challenging simple construction of the all solid-state test strip leads to a higher slope of the calibration plot and slightly increased LOD compared with the forward lead transfer peak at VISE with internal solution. Different to potentiometric ISE the measurement reproducibility is not reduced by the double charged Pb2 ions. This is a further advantage of the amperometric system: Multivalent ions can be measured with a comparable sensitivity compared to monovalent ions.

4.3. Stripping Analysis at VISE mechanism [19]. Concerning the selectivity of the voltammetric electrodes against sodium ions the ratio of the corresponding sensitivities (sodium vs. potassium) lies in the range of 10ÿ3, slightly less than in the potentiometric case. However, if calibrated under physiological sodium concentrations the errors are less than 1 % relative.

Fig. 7. Calibration plot for nitrate constructed with the corresponding peak currents of Figure 6. Electroanalysis 2000, 12, No. 16

The encouraging results obtained with the all solid-state metal ion VISE led us to think about a further improvement of the LOD by an appropriate preelectrolysis step at this interface to combine the advantages of stripping analysis at metallic electrodes with the selectivity increase by using an analyte selective ionophore. In order to test this hypothesis a traditional Pb2±VISE with internal electrolyte was used. The experimental conditions were the same as in Figure 11 with the exception that the scan rate was increased to 80 mV=s here. A preelectrolysis at 0 V was performed with different duration between 4 and 100 s. According to the Cottrell equation the peak currents for the transfer back to the aqueous phase should be related to the enrichment time via a square root dependence. A plot of the peak current versus the square root of the preelectrolysis time is shown in Figure 13. The data show that the Cottrell equation is followed up to 36 s preelectrolysis time. The deviation can be assumed as being caused by further diffusion processes of the selectively transferred ion into the bulk of the membrane. Since thinner membranes are possible with the all solid-state construction they should allow longer preelectrolysis times and thus much higher enrichment factors with the corresponding decrease in the LOD. In the ®rst studies we obtained already an increase of the analytical signal by a factor of two just within the studied short time range. Another theoretical assumption was studied with the Pb2±VISE (ionophore III and internal solution). According to the Randles-Sevcik equation the peak currents should be linear to the square root of the potential scan rates. Figure 14 demonstrates the nice ®t of the experimental parameters with the theory.


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Table 6. Analytical performance of the three studied VISE membrane types; ft: forward transfer w ? o; bt: backward transfer o ? w. Parameters of the linear calibration plots for concentrations of up to 1.4 mmol=L Kÿ.

Slope (mA L=mmol) Standard deviation in (%) Coef®cient of correlation r Detection limit (mmol=L)

ft; KI

bt; KI

ft; KII

bt; KII

ft; KIII

bt; KIII

29.3 1.56 0.9998 0.005

ÿ7.88 5.58 ÿ0.997 0.016

24.6 1.78 0.997 0.005

ÿ5.10 6.11 ÿ0.997 0.022

22.7 3.16 0.9991 0.006

ÿ6.27 5.50 ÿ0.997 0.021

Table 7. Analytical performance of the all solid-state potassium VISE. Parameters of the linear calibration function Slope (mA L=mmol) Achsabschnitt a (mA) Standard deviation in % rel. Coef®cient of correlation LOD (mmol=L)

ft Peak 0±2 mmol=L 13.3 7.6 3.9 0.999 0.013

Figure 14 also demonstrates the importance of the preelectrolysis potential. Only if it is suf®ciently higher than the halfwave potential of the Pb2 transfer step w ?o good sensitivity can be expected. Applying a 100 s preelectrolysis time and a scan rate of 81 mV=s for the backward transfer the analytical sensitivity of the lead VISE could be increased from 0.24 mA L=mmol to 37 mA L=mmol. This factor of > 100 is also re¯ected in the corresponding LOD.

Fig. 8. Cyclic voltammograms of an all solid-state VISE described in Figure 3 with different LiNO3 concentrations; ground electrolyte GE: 0.1 M LiOH; scan rate: 50 mV=s.

Fig. 9. Cyclovoltammetric measurements at different concentrations for K±transfer at the polymeric membrane with potassium ionophore I. Electroanalysis 2000, 12, No. 16

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D. Henn, K. Cammann Table 8. Analytical performance of the different Pb2±VISE (with and without internal electrolyte) conditions like Figure 12.

Concentration range up to Slope (mA L=mmol) Standard deviation % rel. Coef®cient of correlation LOD (mmol=L)

ft Peak internal solution

bt Peak internal solution

ft Peak all solid-state

bt Peak all solid-state

0.12 mmol=L 26.9 2.97 0.9993 0.005

0.12 mmol=L ÿ10.6 13.9 0.985 0.012

0.1 mmol=L 32.1 4.91 0.9983 0.0083

0.1 mmol=L ÿ12.7 11.9 0.99 0.021

Fig. 10. Superposition of 3 voltammograms each of the VISE types I, II, and III at different KNO3 concentrations: - - - - : BE; - - - -: 0.6 mmol=L; : 1.4 mmol=L.

Fig. 11. All solid-state VISE with additions of different potassium salts. Base electrolyte: 0.01 M LiOH; scan rate: 50 mV=s

5. Conclusions Voltammetric ISE (VISE) both based on a traditional construction principle with internal electrolyte as well as all solid-state devices in the form of an electrochemical one-shot teststrip were developed. In the latter, the interfering polarization at the solid-state backside contact can be avoided by increasing this contact area an thus diminishing the current density drastically compared to the ISE-membrane=sample. The decrease of the PVC concentration from about 30 % of potentiometric ISE to about only 5 % at VISE was found to be advantageous to this end. Electroanalysis 2000, 12, No. 16

The chemically modi®ed electrodes described in this article will hopefully replace the highly toxic mercury electrodes used for common stripping analysis. It makes little sense to produce a toxic waste in the course of chemical analysis (which is sometimes more dangerous than the analyte). We currently pay a high environmental price for the information gained by a traditional electrochemical stripping analysis using either a mercury drop electrode or an in situ mercury ®lm electrode. We will proceed with the development of a highly reproducible production technology for screen-printed VISE. Controlling the area of the active membrane surface seems to be much easier than to control the Galvani-Potentials inside a traditional poten-


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Fig. 12. Cyclic voltammograms with an all solid-state Pb2±VISE (Ionophore III). Base electrolyte: 0.01 M ascorbic acid; addition of lead nitrate; scan rate: 20 mV=s.

Fig. 13. Dependence of the peak current on the square root of the preelectrolysis time.

tiometric ISE. It is therefore possible to produce a lot of VISE with absolutely identical features opening the route to precalibrated devices without any need of additional recalibration by the end user.

6. References [1] J. Koryta, Electrochim. Acta 1979, 24, 293; 1984, 29, 445; 1988, 33, 189. [2] C. Gavach, T. Mlodnicka, J. Guastalla, C. R. Acad. Sci. 1968, C266, 1196. [3] J. Guastalla, C. R. Acad. Sci. 1969, C269, 1360. [4] K. Cammann, Das Arbeiten mit ionenselektiven Elektroden, Springer, Berlin 1977. [5] M. Senda, T. Kakiuchi, T. Osakai, Electrochim. Acta 1991, 36, 253. [6] K. Cammann, B. Ahlers, D. Henn, C. Dumschat, A.A. Shulga, Sens. Actuators 1996, B35-36, 26. [7] H.H.J. Girault, D.J. Schiffrin, Electrochemistry of Liquid-Liquid Interfaces, in Electroanalytical Chemistry, Vol. 15 (Ed: A.J. Bard), Marcel Dekker, New York.

Fig. 14. Slopes of the calibration plots versus the square root of the scan rates for different preelectrolysis potentials: (j) 0.1 V; (d) 0.2 V; (m) 0.3 V; (.) 0.4 V; (r) 0.5 V. [8] M. Senda, T. Kakiuchi, T. Osakai, T. Kakutani, in The Interface Structure and Electrochemical Processes at the Boundary Between Two Immiscible Liquids (Ed: V.E. Kazarinov), Springer, Berlin 1987. [9] Z. Samec, V. Marecek, in The Interface Structure and Electrochemical Processes at the Boundary Between Two Immiscible Liquids (Ed: V.E. Kazarinov), Springer, Berlin 1987. [10] J.E.B. Randles, Trans. Faraday Soc. 1948, 44, 327. [11] A. Sevcik, Collect. Czech. Chem. Commun. 1948, 13, 349. [12] W.H. Reinmunth, J. Am. Chem. Soc. 1957, 79, 6358. [13] H. Matsuda, Y. Ayabe, Z. Elektrochem. 1955, 59, 494. [14] R.S. Nicholson, I. Shain, Anal. Chem. 1964, 36, 706. [15] J. Heinze, M. StoÈrzbach, J. Mortensen, J. Electroanal. Chem. 1984, 165, 61. [16] J. Heinze, Angew. Chem. 1984, 96, 823. [17] P. VanyÂsek, in Lecture Notes in Chemistry, Vol. 39 (Ed: G. Berthier), Springer, Berlin 1985, p. 24. [18] S.-L. Xie, K. Cammann, J. Electroanal. Chem. 1987, 229, 249. [19] P.D. Beattie, R.G. Wellington, H.H. Girault, J. Electroanal. Chem. 1995, 396, 317.

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