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Multi-flow-through sensor for simultaneous cation determination
Electrochemistry Communications 10 (2008) 1355–1359
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Multi-flow-through sensor for simultaneous cation determination W. Vonau *, U. Enseleit, F. Gerlach Kurt-Schwabe-Institut für Mess- und Sensortechnik e.V. Meinsberg, Kurt-Schwabe-Straße 4, D-04720 Ziegra-Knobelsdorf, Germany
a r t i c l e
i n f o
Article history: Received 13 June 2008 Received in revised form 3 July 2008 Accepted 4 July 2008 Available online 11 July 2008 Keywords: Multi-flow-through sensor Glass electrode Simultaneous cation determination All-solid-state Reversible phase interface
a b s t r a c t The activity of several metal ions can be detected potentiometrically using glass electrode measuring chains, which have normally a cylindrical design. To analyse different cations simultaneously, on one hand several single indicator electrodes can be integrated in a prefabricated flow-through equipment, where they can be measured versus electrochemical reference electrodes. On the other hand, by principle it is possible to construct tubular flow-through electrodes. In the present case, glass electrodes are double-walled, so that the liquid internal reference system consisting of reference solution and reference element can be placed. In the case of multi-analysis, for constructional reasons it would be necessary to combine several of such flow-through electrodes by complicated connecting elements. In this paper a multi-flow-through electrode is described, which can be fabricated in one piece by substitution of the liquid by a solid reference half cell. The contact of ion-selective glasses with semiconducting zinc oxide leads to a reversible phase interface, so that a high stability of the individual half cell potentials is achieved. Nearly all electrode properties are in a good agreement with those of traditional cation selective electrodes. The pH electrodes, for example, exhibit almost linear response in a range of pH 1–10 with a slope of 57–59 mV/ pH at h = 25 °C. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction There are a lot of glasses, which are suitable as membrane materials for potentiometric indicator electrodes to determine metal ions [1]. Depending on their linear thermal coefficient of expansion, usually they are joined by casting on compatible shaft glasses, resulting in cylindrical electrodes. If a simultaneous cation determination is planned by means of such sensors, they have to be integrated in flow-through armatures. In case no single-rod measuring cells are used, separate reference electrodes can also be integrated [2] (see also Fig. 1A). When additional armatures are not used, it is possible to invert the conventional dipping type glass electrode. According to Fig. 1B, in this case the liquid internal reference half cell must be arranged on the shell of a shaft glass tube in which a segment of ion-selective glass is fused-in. This approach requires a glass double wall vessel. From constructive point of view, it is nearly impossible to translate this conception into a multi-flow-through sensor. In particular strong stresses in the glass lead to formation of cracks at the fusion regions during the use of such devices. In the present paper an electrode glass based multi-flowthrough sensor is introduced, containing a solid reference half cell [3]. It does not require additional double wall vessels, which re-
* Corresponding author. Tel.: +49 34327 6080; fax: +49 34327 608131. E-mail address: [email protected] (W. Vonau). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.07.009
duce the mechanical stability. Contrary to known solutions of solid-state flow-through sensors [4] there is reversibility at the phase interface ‘electrode glass/reference element’, because of the choice of a semiconducting metal oxide as contact material to the ionic conducting selective glass.
2. Experimental Based on earlier results [5], where the fundamental possibility for the application of zinc oxide as contact material for glass electrodes is demonstrated, a multi-flow-through sensor for simultaneous determination of several cations was fabricated, according to the schematical drawing in Fig. 2A. For this, in a tube of soda-lime–silica glass [AR-Glass, EMGO (Belgium)] with a length of 12 cm and an outer diameter of 6 mm, three tube sections (consisting of hydrogen, potassium and sodium ion-selective glasses of the same diameter, 15 mm length and 0.2 mm wall thickness in each case) were fused-in. The distance between the single segments of the electrode glasses was 2 cm. It is necessary that all connection regions do not possess a large bulge. Table 1 includes compositions and thermal coefficients of expansion of all glasses used. All selective glasses were molten in a muffle furnace at h = 1300–1450 °C in a covered platinum cup (from oxides and carbonates). Fabrication of the cylindrical membranes and their fusion with the shaft glass were carried out with a universal glass turning
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W. Vonau et al. / Electrochemistry Communications 10 (2008) 1355–1359
Fig. 1. Arrangement for measurement in flowing media with glass electrodes. (A) Module to determine cations simultaneously using several cylindrical glass electrodes, integrated in a flow-through armature. (B) Flow-through glass electrode in conventional setup with a double wall vessel, filled with liquid internal reference electrolyte to determine one component.
Fig. 2. All-solid-state multi-sensor with electrode glass membranes for simultaneous cation determination. (A) Flow-through sensor. (B) One side sealed flow-through sensor for test purposes.
W. Vonau et al. / Electrochemistry Communications 10 (2008) 1355–1359 Table 1 Compositions and expansion coefficients (a) of all glasses, used for the fabrication of the multi-flow-through sensor for a simultaneous determination of H3O+, K+ and Na+ ions Glass type
Composition (wt%)
a (K 1)
pH glass pK glass pNa glass Shaft glass
72SiO2, 22Na2O, 6CaO 65.2SiO2, 26.7Na2O, 8.1Al2O3 60.06SiO2, 19.06Na2O, 4.7Al2O3, 11.78B2O3, 4.4UO3 Soda-lime–silica glass (AR-Glas, EMGO Belgium)
10.65 10 6 11.30 10 6 9.38 10 6 9.0 10 6
centre [type 1060, Arnold (Germany)] using rods with a diameter of 5–10 mm which were pulled from the glass melt by a glass blower beforehand. After completion of the multi-sensor body approximately 50 nm thick zinc oxide layers have been formed by spin coating – each layer entirely overlapping the single sensors inclusive a small surrounding edge of the laboratory glass. This process was followed by an activation of the surface by immersing in an acidic PdCl2 solution (pH 2.5, b = 0.2 g/L) for 10 s. The electroless NiP plating was carried out in an alkaline bath (pH 9.5) consisting of 15 g/L
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nickel(II) sulfate hexahydrate, 10 g/L citric acid, 2 g/L sodium tetraborate and 20 g/L sodium hypophosphate in distilled water at 55 °C for 3 min. In this process NiP layers with a thickness of about 200 nm and a phosphorus content of 6–14 wt% could be achieved. Details for the forming of the functional layers on the ion-selective glass electrode basic bodies are given in [6], where the preparation of all-solid-state single electrodes for pH monitoring is described. In addition all NiP layers of the multi-sensor were covered with a gold layer of arbitrary thickness by chemical deposition using the commercial electrolyte Aurocor K24Ò (without brightener) [Atotech (Germany)], where an electrical contact of the noble metal layers among themselves was safely prevented. The galvanostatic gold deposition was carried out over a period of 10 min in an electrochemical cell at a current density, i = 2 mA/cm2, a temperature of h = 55 °C and a pH value of pH 6.5–7. The two-component conductive laquer coat CW 2400 [ITW Chemtronics (Georgia USA)] was used to glue silver wires on the gold layers. Afterwards, the entire sensor was insulated with epoxy resin. Now the multi-electrode was integrated in a flow-through system by hose fittings or one side sealed for test purposes (see Fig.
Fig. 3. Electrode functions (S) of all-solid-state multi-glass electrodes based on hydrogen, potassium and sodium selective cylindrical membranes, coated with zinc oxide and of corresponding conventional glass electrodes (two electrodes of each type), in each case using an Ag/AgCl, Clsat: -reference electrode at h = 25 °C. (A) pH-Determination; liquid internal reference system LIRS); inner electrolyte: Wilke buffer pH 7.0; (1) S = 58 mV/pH (1.68–9.18), zero point of the chain (ZPC) = 7.0; (2) S = 58 mV/pH (1.68–9.18), ZPC = 6.9. Solid-state internal reference system (SIRS) (ZnO/NiP/Au); (3) S = 59 mV/pH (1.68–9.18), ZPC = 6.9 4 S = 59 mV/pH (1.30–9.04), ZPC = 6.4. (B) pK-Determination; LIRS; inner electrolyte: triethanolamine + 0.1 M KCl; (1) S = 58 mV/pK; (2) S = 58 mV/pK. SIRS (ZnO/NiP/Au); (3) S = 57 mV/pK; (4) S = 57 mV/pK. (C) pNa-Determination; LIRS; inner electrolyte: triethanolamine + 0.1 M NaCl; (1) S = 55 mV/pNa; (2) S = 54 mV/pNa. SIRS (ZnO/NiP/Au) ; (3) S = 55 mV/pNa; (4) S = 54 mV/pNa).
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Fig. 4. Electrode function (S) of a pH flow-through electrode (glass/ZnO/Au) and conventional pH glass electrode depending on time, measured vs. Ag/AgCl, Clsat: reference electrode at h = 25 °C. Liquid internal reference system: (1) S = 58 mV/pH, zero point of the chain (ZPC) = 7.0/1st day; (2) S = 58 mV/pH, ZPC = 6.8/6 weeks; (3) S = 58 mV/pH, ZPC = 6.9/10 weeks. Solid-state internal reference system (ZnO/NiP/ Au): (4) S = 57 mV/pH, ZPC = 7.5/10 weeks; (5) S = 58 mV/pH, ZPC = 7.4/1st day; (6) S = 59 mV/pH, ZPC = 6.0/6 weeks.
2B). In the hollow body which was formed in this way, test solutions were filled in as shown below and measured versus a miniaturised reference electrode (Ag/AgCl, Clsat: ), which was immersed in these media. For the ion determination, a microprocessor pH/ ION meter pMX 3000 (WTW, Germany) was used. All solutions were freshly prepared, using chemicals of the p.a. quality from Fluka. Test solutions: 1. pH Measurement: Tris buffer (pH 7.32; pH 9.04), sat. Ca(OH)2 (pH 12.3), HCl solutions (pH 1.3; pH 2.9), NBS buffer (pH 1.68; 4.01; 6.86; 9.18). 2. Na+ ions: 10 4–1 mol/L NaCl solutions. 3. K+ ions: 10 4–1 mol/L KCl solutions.
3. Results and discussion In Fig. 3 the electrode functions of all coated ion-selective glass segments are shown, compared with results of measurements with conventional liquid filled ion-selective electrodes (two identically prepared electrodes of each type). Due to the completely different characteristics of the internal reference systems for both electrode types, different zero points of the chains (ZPC) are expected. However, in the case of pH glass electrodes the data obtained between solid-state electrodes and conventional ones with an internal buffer of pH 7.0 at random (see Fig. 3A), was found to be nearly constant. In other respects, all curve progressions are in agreement, whereby an application of the tubular multi-electrodes is demonstrated in principle. Fig. 4 for an all-solid-state glass pH electrode exemplifies the relative high stability of electrode functions over a period of several weeks. As is known, this is not the case with indicator electrodes having irreversible phase interfaces [7]. For comparison, the behaviour of a conventionally configured probe is also presented in this figure. The cross-sensitivity, which in particular is often described for potassium and sodium selective glass electrodes in the literature [8] and which is calculable by the Nikolskij equation [9], as
Fig. 5. Cross-sensitivity of sodium (A) and potassium function (B) by the other cation (composition of the electrode glasses – see Table 1).
expected also appears similar to liquid filled glass electrodes. It was determined by the fabricated multi-flow-through electrodes. Results are given in Fig. 5. The cross-sensitivity is caused by the material composition of the selective membrane and not by the
W. Vonau et al. / Electrochemistry Communications 10 (2008) 1355–1359
internal reference system. Therefore it only can be influenced by improved electrode glasses. 4. Conclusions An all-solid-state multi-flow-through sensor for simultaneous cation determination, based on a stream tube of chemical inert glass with segments of ion-selective glasses, insulated from each other, is described. The solid contact is based on the semiconductor zinc oxide that is coated with NiP (and optional with an additional gold film). The introduced concept in principle allows the combination of an arbitrary number of identical or different tubular glass membranes by fusion of ion-selective glasses with a shaft glass tube with similar thermal coefficients of expansion. Thus, no additional connecting elements are necessary to bring the single electrodes together. So a new quality for multi-parameter determination by chemosensors is achieved, because conventional sensors require additional flow-through armatures. Tube electrodes according to conventional glass electrode configuration (inverse glass electrode) from technological point of view hardly permit fusion among themselves, because of their double vessel construction.
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The measured values with the newly developed sensors correspond with those, obtained by traditional cation selective glass electrodes. This means that also for this kind of electrodes cross sensitivities can be detected in the same dimension. Therefore, a selectivity improvement of the electrode glasses is still a current research objective. References [1] G. Eisenman, Glass Electrodes for Hydrogen and Other Cations, Marcel Dekker, New York, 1967. [2] S. Herrmann, W. Vonau, F. Gerlach, H. Kaden, Fresen. J. Anal. Chem. 362 (1998) 215. [3] W. Vonau, F. Gerlach, U. Enseleit, T. Bachmainn, Mehrparameter-Elektrode, German Patent P 10 2008 016 985.4, 2008. [4] H. Kaden, W. Vonau, Messen–Steuern–Regeln 33 (7) (1990) 312. [5] W. Vonau, F. Gerlach, U. Enseleit, T. Bachmann, J. Solid State Electrochem., in press, doi:10.1007/s10008-008-0573-8. [6] W. Vonau, U. Enseleit, F. Gerlach, T. Bachmann, J. Spindler, Durchflussmessfühler zur Bestimmung von Ionenaktivitäten und Verfahren zu dessen Anwendung. German Patent P 10 2007 042 476.2, 2007. [7] F. Oehme, Ionenselektive Elektroden, Hüthig, Heidelberg, 1986. p. 1. [8] K. Schwabe, pH-Messtechnik, Verlag Theodor Steinkopff, Dresden, 1976. [9] B.P. Nicolskij, M.M. Schulz, A.A. Belijustin, A.N. Lev, in: Glass Electrodes for Hydrogen and Other Cations, Marcel Dekker, New York, pp. 174–223.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Multi-flow-through sensor for simultaneous cation determination W. Vonau *, U. Enseleit, F. Gerlach Kurt-Schwabe-Institut für Mess- und Sensortechnik e.V. Meinsberg, Kurt-Schwabe-Straße 4, D-04720 Ziegra-Knobelsdorf, Germany
a r t i c l e
i n f o
Article history: Received 13 June 2008 Received in revised form 3 July 2008 Accepted 4 July 2008 Available online 11 July 2008 Keywords: Multi-flow-through sensor Glass electrode Simultaneous cation determination All-solid-state Reversible phase interface
a b s t r a c t The activity of several metal ions can be detected potentiometrically using glass electrode measuring chains, which have normally a cylindrical design. To analyse different cations simultaneously, on one hand several single indicator electrodes can be integrated in a prefabricated flow-through equipment, where they can be measured versus electrochemical reference electrodes. On the other hand, by principle it is possible to construct tubular flow-through electrodes. In the present case, glass electrodes are double-walled, so that the liquid internal reference system consisting of reference solution and reference element can be placed. In the case of multi-analysis, for constructional reasons it would be necessary to combine several of such flow-through electrodes by complicated connecting elements. In this paper a multi-flow-through electrode is described, which can be fabricated in one piece by substitution of the liquid by a solid reference half cell. The contact of ion-selective glasses with semiconducting zinc oxide leads to a reversible phase interface, so that a high stability of the individual half cell potentials is achieved. Nearly all electrode properties are in a good agreement with those of traditional cation selective electrodes. The pH electrodes, for example, exhibit almost linear response in a range of pH 1–10 with a slope of 57–59 mV/ pH at h = 25 °C. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction There are a lot of glasses, which are suitable as membrane materials for potentiometric indicator electrodes to determine metal ions [1]. Depending on their linear thermal coefficient of expansion, usually they are joined by casting on compatible shaft glasses, resulting in cylindrical electrodes. If a simultaneous cation determination is planned by means of such sensors, they have to be integrated in flow-through armatures. In case no single-rod measuring cells are used, separate reference electrodes can also be integrated [2] (see also Fig. 1A). When additional armatures are not used, it is possible to invert the conventional dipping type glass electrode. According to Fig. 1B, in this case the liquid internal reference half cell must be arranged on the shell of a shaft glass tube in which a segment of ion-selective glass is fused-in. This approach requires a glass double wall vessel. From constructive point of view, it is nearly impossible to translate this conception into a multi-flow-through sensor. In particular strong stresses in the glass lead to formation of cracks at the fusion regions during the use of such devices. In the present paper an electrode glass based multi-flowthrough sensor is introduced, containing a solid reference half cell [3]. It does not require additional double wall vessels, which re-
* Corresponding author. Tel.: +49 34327 6080; fax: +49 34327 608131. E-mail address: [email protected] (W. Vonau). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.07.009
duce the mechanical stability. Contrary to known solutions of solid-state flow-through sensors [4] there is reversibility at the phase interface ‘electrode glass/reference element’, because of the choice of a semiconducting metal oxide as contact material to the ionic conducting selective glass.
2. Experimental Based on earlier results [5], where the fundamental possibility for the application of zinc oxide as contact material for glass electrodes is demonstrated, a multi-flow-through sensor for simultaneous determination of several cations was fabricated, according to the schematical drawing in Fig. 2A. For this, in a tube of soda-lime–silica glass [AR-Glass, EMGO (Belgium)] with a length of 12 cm and an outer diameter of 6 mm, three tube sections (consisting of hydrogen, potassium and sodium ion-selective glasses of the same diameter, 15 mm length and 0.2 mm wall thickness in each case) were fused-in. The distance between the single segments of the electrode glasses was 2 cm. It is necessary that all connection regions do not possess a large bulge. Table 1 includes compositions and thermal coefficients of expansion of all glasses used. All selective glasses were molten in a muffle furnace at h = 1300–1450 °C in a covered platinum cup (from oxides and carbonates). Fabrication of the cylindrical membranes and their fusion with the shaft glass were carried out with a universal glass turning
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W. Vonau et al. / Electrochemistry Communications 10 (2008) 1355–1359
Fig. 1. Arrangement for measurement in flowing media with glass electrodes. (A) Module to determine cations simultaneously using several cylindrical glass electrodes, integrated in a flow-through armature. (B) Flow-through glass electrode in conventional setup with a double wall vessel, filled with liquid internal reference electrolyte to determine one component.
Fig. 2. All-solid-state multi-sensor with electrode glass membranes for simultaneous cation determination. (A) Flow-through sensor. (B) One side sealed flow-through sensor for test purposes.
W. Vonau et al. / Electrochemistry Communications 10 (2008) 1355–1359 Table 1 Compositions and expansion coefficients (a) of all glasses, used for the fabrication of the multi-flow-through sensor for a simultaneous determination of H3O+, K+ and Na+ ions Glass type
Composition (wt%)
a (K 1)
pH glass pK glass pNa glass Shaft glass
72SiO2, 22Na2O, 6CaO 65.2SiO2, 26.7Na2O, 8.1Al2O3 60.06SiO2, 19.06Na2O, 4.7Al2O3, 11.78B2O3, 4.4UO3 Soda-lime–silica glass (AR-Glas, EMGO Belgium)
10.65 10 6 11.30 10 6 9.38 10 6 9.0 10 6
centre [type 1060, Arnold (Germany)] using rods with a diameter of 5–10 mm which were pulled from the glass melt by a glass blower beforehand. After completion of the multi-sensor body approximately 50 nm thick zinc oxide layers have been formed by spin coating – each layer entirely overlapping the single sensors inclusive a small surrounding edge of the laboratory glass. This process was followed by an activation of the surface by immersing in an acidic PdCl2 solution (pH 2.5, b = 0.2 g/L) for 10 s. The electroless NiP plating was carried out in an alkaline bath (pH 9.5) consisting of 15 g/L
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nickel(II) sulfate hexahydrate, 10 g/L citric acid, 2 g/L sodium tetraborate and 20 g/L sodium hypophosphate in distilled water at 55 °C for 3 min. In this process NiP layers with a thickness of about 200 nm and a phosphorus content of 6–14 wt% could be achieved. Details for the forming of the functional layers on the ion-selective glass electrode basic bodies are given in [6], where the preparation of all-solid-state single electrodes for pH monitoring is described. In addition all NiP layers of the multi-sensor were covered with a gold layer of arbitrary thickness by chemical deposition using the commercial electrolyte Aurocor K24Ò (without brightener) [Atotech (Germany)], where an electrical contact of the noble metal layers among themselves was safely prevented. The galvanostatic gold deposition was carried out over a period of 10 min in an electrochemical cell at a current density, i = 2 mA/cm2, a temperature of h = 55 °C and a pH value of pH 6.5–7. The two-component conductive laquer coat CW 2400 [ITW Chemtronics (Georgia USA)] was used to glue silver wires on the gold layers. Afterwards, the entire sensor was insulated with epoxy resin. Now the multi-electrode was integrated in a flow-through system by hose fittings or one side sealed for test purposes (see Fig.
Fig. 3. Electrode functions (S) of all-solid-state multi-glass electrodes based on hydrogen, potassium and sodium selective cylindrical membranes, coated with zinc oxide and of corresponding conventional glass electrodes (two electrodes of each type), in each case using an Ag/AgCl, Clsat: -reference electrode at h = 25 °C. (A) pH-Determination; liquid internal reference system LIRS); inner electrolyte: Wilke buffer pH 7.0; (1) S = 58 mV/pH (1.68–9.18), zero point of the chain (ZPC) = 7.0; (2) S = 58 mV/pH (1.68–9.18), ZPC = 6.9. Solid-state internal reference system (SIRS) (ZnO/NiP/Au); (3) S = 59 mV/pH (1.68–9.18), ZPC = 6.9 4 S = 59 mV/pH (1.30–9.04), ZPC = 6.4. (B) pK-Determination; LIRS; inner electrolyte: triethanolamine + 0.1 M KCl; (1) S = 58 mV/pK; (2) S = 58 mV/pK. SIRS (ZnO/NiP/Au); (3) S = 57 mV/pK; (4) S = 57 mV/pK. (C) pNa-Determination; LIRS; inner electrolyte: triethanolamine + 0.1 M NaCl; (1) S = 55 mV/pNa; (2) S = 54 mV/pNa. SIRS (ZnO/NiP/Au) ; (3) S = 55 mV/pNa; (4) S = 54 mV/pNa).
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Fig. 4. Electrode function (S) of a pH flow-through electrode (glass/ZnO/Au) and conventional pH glass electrode depending on time, measured vs. Ag/AgCl, Clsat: reference electrode at h = 25 °C. Liquid internal reference system: (1) S = 58 mV/pH, zero point of the chain (ZPC) = 7.0/1st day; (2) S = 58 mV/pH, ZPC = 6.8/6 weeks; (3) S = 58 mV/pH, ZPC = 6.9/10 weeks. Solid-state internal reference system (ZnO/NiP/ Au): (4) S = 57 mV/pH, ZPC = 7.5/10 weeks; (5) S = 58 mV/pH, ZPC = 7.4/1st day; (6) S = 59 mV/pH, ZPC = 6.0/6 weeks.
2B). In the hollow body which was formed in this way, test solutions were filled in as shown below and measured versus a miniaturised reference electrode (Ag/AgCl, Clsat: ), which was immersed in these media. For the ion determination, a microprocessor pH/ ION meter pMX 3000 (WTW, Germany) was used. All solutions were freshly prepared, using chemicals of the p.a. quality from Fluka. Test solutions: 1. pH Measurement: Tris buffer (pH 7.32; pH 9.04), sat. Ca(OH)2 (pH 12.3), HCl solutions (pH 1.3; pH 2.9), NBS buffer (pH 1.68; 4.01; 6.86; 9.18). 2. Na+ ions: 10 4–1 mol/L NaCl solutions. 3. K+ ions: 10 4–1 mol/L KCl solutions.
3. Results and discussion In Fig. 3 the electrode functions of all coated ion-selective glass segments are shown, compared with results of measurements with conventional liquid filled ion-selective electrodes (two identically prepared electrodes of each type). Due to the completely different characteristics of the internal reference systems for both electrode types, different zero points of the chains (ZPC) are expected. However, in the case of pH glass electrodes the data obtained between solid-state electrodes and conventional ones with an internal buffer of pH 7.0 at random (see Fig. 3A), was found to be nearly constant. In other respects, all curve progressions are in agreement, whereby an application of the tubular multi-electrodes is demonstrated in principle. Fig. 4 for an all-solid-state glass pH electrode exemplifies the relative high stability of electrode functions over a period of several weeks. As is known, this is not the case with indicator electrodes having irreversible phase interfaces [7]. For comparison, the behaviour of a conventionally configured probe is also presented in this figure. The cross-sensitivity, which in particular is often described for potassium and sodium selective glass electrodes in the literature [8] and which is calculable by the Nikolskij equation [9], as
Fig. 5. Cross-sensitivity of sodium (A) and potassium function (B) by the other cation (composition of the electrode glasses – see Table 1).
expected also appears similar to liquid filled glass electrodes. It was determined by the fabricated multi-flow-through electrodes. Results are given in Fig. 5. The cross-sensitivity is caused by the material composition of the selective membrane and not by the
W. Vonau et al. / Electrochemistry Communications 10 (2008) 1355–1359
internal reference system. Therefore it only can be influenced by improved electrode glasses. 4. Conclusions An all-solid-state multi-flow-through sensor for simultaneous cation determination, based on a stream tube of chemical inert glass with segments of ion-selective glasses, insulated from each other, is described. The solid contact is based on the semiconductor zinc oxide that is coated with NiP (and optional with an additional gold film). The introduced concept in principle allows the combination of an arbitrary number of identical or different tubular glass membranes by fusion of ion-selective glasses with a shaft glass tube with similar thermal coefficients of expansion. Thus, no additional connecting elements are necessary to bring the single electrodes together. So a new quality for multi-parameter determination by chemosensors is achieved, because conventional sensors require additional flow-through armatures. Tube electrodes according to conventional glass electrode configuration (inverse glass electrode) from technological point of view hardly permit fusion among themselves, because of their double vessel construction.
1359
The measured values with the newly developed sensors correspond with those, obtained by traditional cation selective glass electrodes. This means that also for this kind of electrodes cross sensitivities can be detected in the same dimension. Therefore, a selectivity improvement of the electrode glasses is still a current research objective. References [1] G. Eisenman, Glass Electrodes for Hydrogen and Other Cations, Marcel Dekker, New York, 1967. [2] S. Herrmann, W. Vonau, F. Gerlach, H. Kaden, Fresen. J. Anal. Chem. 362 (1998) 215. [3] W. Vonau, F. Gerlach, U. Enseleit, T. Bachmainn, Mehrparameter-Elektrode, German Patent P 10 2008 016 985.4, 2008. [4] H. Kaden, W. Vonau, Messen–Steuern–Regeln 33 (7) (1990) 312. [5] W. Vonau, F. Gerlach, U. Enseleit, T. Bachmann, J. Solid State Electrochem., in press, doi:10.1007/s10008-008-0573-8. [6] W. Vonau, U. Enseleit, F. Gerlach, T. Bachmann, J. Spindler, Durchflussmessfühler zur Bestimmung von Ionenaktivitäten und Verfahren zu dessen Anwendung. German Patent P 10 2007 042 476.2, 2007. [7] F. Oehme, Ionenselektive Elektroden, Hüthig, Heidelberg, 1986. p. 1. [8] K. Schwabe, pH-Messtechnik, Verlag Theodor Steinkopff, Dresden, 1976. [9] B.P. Nicolskij, M.M. Schulz, A.A. Belijustin, A.N. Lev, in: Glass Electrodes for Hydrogen and Other Cations, Marcel Dekker, New York, pp. 174–223.