Ion-Selective Interface

This interface is also known as the perm-selective interface (Fig. 6.1a). It is found in ion-selective sensors, such as ion-selective electrodes and ion-selective field-effect transistors. It is the site of the Nernst potential, which we now derive from the thermodynamic point of view. Because the zero-current axis in Fig. 5.1 represents the electrochemical cell at equilibrium, the partitioning of charged species between the two phases is described by the Gibbs equation (A.20), from which it follows that the electrochemical potential of the species i in the sample phase (S) and in the electrode phase (m) must be equal. 

The electrochemical potential of the charged species i in a given phase contains terms originating from both the electrical and the chemical work. Thus, for the electrochemical potential of charged species i in the electrode we can write 

Each phase is charged and has electrostatic potential cp, which is called the Gal-vani potential. The profiles of the ion activity, of the Galvani potential, and of the electrochemical potential across the sample/electrode interface are shown in Fig. 6.2. 

Let us assume that at time t = 0 there is some initial activity of o of the exchanging ion in the solution and that the electrostatic potentials of the two phases are equal. When the equilibrium is established, a small number of cations are transferred from the solution to the electrode phase. This changes the electrostatic potential difference between the two phases, with the electrode becoming more positive with respect to the sample. The interfacial potential it is obtained from 

The first term on the right-hand side of (6.4) is the standard potential it0. 

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Finally, some simple estimates will be presented for the three-dimensional electrolyte concentration and electric potential fields resulting from concentration polarization in a diffusion layer adjacent to a spatially inhomogeneous ion-selective interface (membrane). It will be shown that the appropriate fields are incompatible with mechanical equilibrium in an ionic fluid, so that a related (nongravitational) convection is expected to arise at an inhomogeneous ion-exchange membrane upon the passage of an electric current. 

Many ion-selective interfaces have been studied, and several different types of electrodes have been marketed commercially. We will examine the basic strategies for introducing selectivity by considering a few of them here. The glass membrane is our starting point because it offers a fairly complete view of the fundamentals as well as the usual complications found in practical devices. 

With the latest interests on sodium ion, potassium ion and other types of batteries, it is optimistic to anticipate development of new types of ion-selective interfaces that will function as barrier to pH gradient. In addition, there is room for optimising operation parameters using present interfaces for the various acid-alkaline devices. 

Each porin molecule has three channels Ion channels combine ion selectivity with high levels of ion conductance The K+ channel is a tetrameric molecule with one ion pore in the interface between the four subunits... 

Koryta, J. Electrolysis at the interface of two Immiscible Electrolyte Solutions and its Analytical Aspects, in Ion-Selective Electrodes. 3rd. Symposium held at Matrafured, Hungary 1980, ed. Pundor, E., Elsevier, Amsterdam—Oxford—New York 1981... 

An ion-selective electrode contains a semipermeable membrane in contact with a reference solution on one side and a sample solution on the other side. The membrane will be permeable to either cations or anions and the transport of counter ions will be restricted by the membrane, and thus a separation of charge occurs at the interface. This is the Donnan potential (Fig. 5 a) and contains the analytically useful information. A concentration gradient will promote diffusion of ions within the membrane. If the ionic mobilities vary greatly, a charge separation occurs (Fig. 5 b) giving rise to what is called a diffusion potential. 

Interfaces of the type in Scheme 8 are used as liquid ion-selective electrodes. It is apparent that they constitute a special case of distribution systems reversible in regard to two or more ions. Here, Le Hung s equation, (16) and (17), allows quantitative evaluation of the influence of the presence of other ions on the selectivity of these systems. 

Other studies involved the measurements of the SHG response from ion selective electrodes (ISE) [105,106] but one of the difficulties lies in the reabsorption of the SH signal generated at the interface in the bulk of one phase as the active species transfer. 

A group of techniques employing differential selection of solute ions relies on nebulisation and ionisation of the eluent, with some discrimination of ion selection in favour of the solute. Main representatives are APCI [544] and thermospray [545]. In a thermospray interface a supersonic jet of vapour and small droplets is generated out of a heated vaporiser tube. Controlled, partial vaporisation of the HPLC solvent occurs before it enters the ion source. Ionisation of nonvolatile analytes takes place by means of solvent-mediated Cl reactions and ion evaporation processes. Most thermospray sources are fitted with a discharge electrode. When this is used, the technique is called plasmaspray (PSP) or discharge-assisted thermospray. In practice, many... 

Potential differences at the interface between two immiscible electrolyte solutions (ITIES) are typical Galvani potential differences and cannot be measured directly. However, their existence follows from the properties of the electrical double layer at the ITIES (Section 4.5.3) and from the kinetics of charge transfer across the ITIES (Section 5.3.2). By means of potential differences at the ITIES or at the aqueous electrolyte-solid electrolyte phase boundary (Eq. 3.1.23), the phenomena occurring at the membranes of ion-selective electrodes (Section 6.3) can be explained. 

This type of sensor often does not have a membrane it instead utilizes the properties of a water-oil interface, a boundary between an aqueous and a non-aqueous (organic) phase. Traditionally, sensors based on non-equilibrium ion-selective transport phenomena were distinguished as a separate group and considered as the electrochemistry of the ion transfer between two immiscible electrolyte solutions (IT1ES). Here, we will not distinguish polymeric membrane electrodes and ITIES-based electrodes due to the similarity in the theoretical consideration. 

Z. Samec, E. Samcova, and H.H. Girault, Ion amperometry at the interface between two immiscible electrolyte solutions in view of realizing the amperometric ion-selective electrode. Talcmta 63, 21—32 (2004). 

Yoon el al. [112] reported an all-solid-state sensor for blood analysis. The sensor consists of a set of ion-selective membranes for the measurement of H+, K+, Na+, Ca2+, and Cl. The metal electrodes were patterned on a ceramic substrate and covered with a layer of solvent-processible polyurethane (PU) membrane. However, the pH measurement was reported to suffer severe unstable drift due to the permeation of water vapor and carbon dioxide through the membrane to the membrane-electrode interface. For conducting polymer-modified electrodes, the adhesion of conducting polymer to the membrane has been improved by introducing an adhesion layer. For example, polypyrrole (PPy) to membrane adhesion is improved by using an adhesion layer, such as Nafion [60] or a composite of PPy and Nafion [117],... 

The ion-selective field-effect transistor (ISFET) represents a remarkable new construction principle [7, 63], Inverse potentiometry with ion-selective electrodes is the electrolysis at the interface between two immiscible electrolyte solutions (ITIES) [28, 55],... 

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