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Ion-Selective Electrodes

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Introduction: An ion-selective electrode (ISE) is an example of an electrochemical sensor utilizing the principle of potentiometry, or measurement of the cell potential (i.e., ISE against a standard reference electrode) at near-zero current. Under these conditions, the boundary potential at the ISE–solution interface is governed by the laws of electrochemical thermodynamics or is compliant with the famous Nernst equation. To ensure a compliance with the Nernst equation, the very best commercially available potentiometers utilize a high input impedance 1013 O, and an appropriate operational amplifier in the electrometer to handle the associated minute currents. . | Ion-Selective Electrodes R De Marco and G Clarke Curtin University of Technology Perth Western Australia Australia 2009 Elsevier B.V. All rights reserved. Introduction An ion-selective electrode ISE is an example of an electrochemical sensor utilizing the principle of potentiometry or measurement of the cell potential i.e. ISE against a standard reference electrode at near-zero current. Under these conditions the boundary potential at the ISE-solution interface is governed by the laws of electrochemical thermodynamics or is compliant with the famous Nernst equation. To ensure a compliance with the Nernst equation the very best commercially available potentiometers utilize a high input impedance 1013 O and an appropriate operational amplifier in the electrometer to handle the associated minute currents. The heart of the ISE is the permselective membrane that is capable of species recognition and Figure 1 presents schematic representations of conventional solidcontact left-hand side and liquid-contact right-hand side ISEs. Ion-selective electrode sensors have been classified according to the membrane type and include the following electrode classifications 1 homogeneous crystalline membrane electrodes e.g. LaF3 for F Ag2S for Ag or S2 AgCl Ag2S for Cl 2 heterogeneous crystalline membrane electrodes e.g. silicone rubbers comprising crystalline powders such as LaF3 Ag2S AgCl Ag2S 3 rigid noncrystalline membrane electrodes embodying silicate and chalcogenide glasses 4 nonrigid noncrystalline electrodes employing plasticized polymer membranes incorporating neutral ionophores as receptor molecules 5 sensitized electrodes such as carbon dioxide or amonia ISEs separating a filling solution housing an immersed pH indicator electrode from the sample solution by a gas diffusion membrane or enzyme-sensitized electrodes incorporating a pH indicator electrode encapsulated by a gel film of enzyme e.g. glucose oxidase for the selective detection of the analyte e.g. glucose . .