Other Internal Reference Electrodes

Other internal reference electrodes have been devised for use in high srrbcritical and in supercritical aqueous solutions. For example, Dobson et al., Macdonald and coworkers, and Nagy et 

Insiide Rnd Chloridi2 d Silver Wire (Ae/AaCl) Teflnn Insuletins 

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One barrel-tip contains the organic membrane phase and an internal reference electrode the other constitutes a second reference electrode. A four-barrel configuration with a 1-pm tip in which three barrels are liquid membrane electrodes for Na, Ca and and the fourth is a reference electrode has been reported Some representative applications of ion-selective electrodes for intracellular measurements are shown in Table 3. 

An inner filling solution and internal reference electrode are used in macro ISEs due to a very good stability of the potential at the inner membrane-solution interface in such a setup (see Fig. 4.4). However, the presence of a solution inside a sensor could be a serious limitation for development of microelectrodes and may be undesired for a variety of other reasons, including ionic fluxes in the membrane and limited temperature range of sensor operation. There are several requirements for such an inner contact. First of all, a reversible change of electricity carriers ions-electrons must take place at the membrane-substrate interface. The potential of the electrochemical reaction, ensuring this transfer, has to be constant, stable, and must not depend on the sample composition. At last, the substrate must not influence the membrane analytical performance. 

Ion-selective electrodes are systems containing a membrane consisting basically either of a layer of solid electrolyte or of an electrolyte solution whose solvent is immiscible with water. The membrane is in contact with an aqueous electrolyte solution on both sides (or sometimes only on one). The ion-selective electrode frequently contains an internal reference electrode, sometimes only a metallic contact, or, for an ion-selective field-effect transistor (ISFET), an insulating and a semiconducting layer. In order to understand what takes place at the boundary between the membrane and the other phases with which it is in contact, various types of electric potential or of potential difference formed in these membrane systems must first be defined. 

Potentiometric measurements are based on the determination of a voltage difference between two electrodes plunged into a sample solution under null current conditions. Each of these electrodes constitutes a half-cell. The external reference electrode (ERE) is the electrochemical reference half-cell, which has a constant potential relative to that of the solution. The other electrode is the ion selective electrode (ISE) which is used for measurement (Fig. 18.1). The ISE is composed of an internal reference electrode (IRE) bathed in a reference solution that is physically separated from the sample by a membrane. The ion selective electrode can be represented in the following way ... 

The other group comprises silver-silver halide electrodes, mercury pools, metal-metal-ion electrodes, and others normally prepared In the solvent used for the compound being studied (and often, indeed, employed as internal "reference" electrodes). For such an electrode, the abbreviation alone signifies that the solvent was the same throughout the cell, while the symbol "(w)" for ("water") following the abbreviation signifies that the reference electrode was prepared with water and used as an external reference electrode. 

Ion-selective membranes can be used in two basic configurations. If the solution is placed on either side of the membrane, the arrangement (e.g., Fig. 6.16a) is symmetrical. It is found in conventional ion-selective electrodes in which the internal contact is realized by the solution in which the internal reference electrode is immersed. In the nonsymmetrical arrangement (Fig. 6.16b), one side of the membrane is contacted by the sample (usually aqueous), and the other side is interfaced with some solid material. Examples of this type are coated wire electrodes and Ion-Sensitive Field-Effect Transistors (ISFETs). 

Fortunately, glass electrodes have wide applicability in solvents other than water. Even in a solvent as different from water as acetonitrile, glass electrodes respond reversibly to changes in hydrogen ion activity, in agreement with the Nernst equation. In setting up pH values for reference buffers, it is not always possible to use internal reference electrodes without liquid junction. For example, Ag-AgX electrodes are unstable in acetonitrile owing to slow formation of species of the type Ag X +j. When external reference electrodes must be substituted, the reliability of the measurements is reduced because uncertainties in the liquid junction are always present. 

Commercial silver/silver-chloride reference electrodes are available in a variety of styles and sizes. They are often used as the internal reference electrodes in glass pH and other ion-selective electrodes. Ag/AgCl microelectrodes formed from very thin silver wire have found extensive use, for example, in biomedical applications such as in vivo studies of biological fluids and intracellular measurements, because of the miniaturization possible with these electrodes. 

Fundamentals. Based on the functional principles of the scanning electrochemical microscope, other scanning probe methods used to determine localized surface properties of the electrode under investigation or of the solution phase adjacent to this surface have been developed utilizing suitable microelectrodes. A pH-sensitive microelectrode based on a glass capillary filled with a pH-constant buffer solution and containing an internal reference electrode that has a tip filled with a proton-selective ionophor cocktail is scanned across the surface. The potential of the internal reference electrode with respect to an external reference electrode is directly correlated to the local pH value. A schematic cross section of this microelectrode is shown in Fig. 7.18. 

Glass electrode for pH measurement A bulb or other form of hydrogen ion-sensitive glass is attached to the end of a glass tube made from high-resistance glass. Commercially it is usually obtained outfitted with internal filling solution and an internal reference electrode. For special applications, e.g., blood pH measurements, other forms, such as capillaries, are more convenient. 

Glass pH electrodes and most other practical ion-selective electrodes (ISEs) are membrane devices. The term electrode is, strictly, a misnomer because the active ion-selective membrane has on one side an analyte (the test solution) and on the other an internal (reference) solution containing an internal reference electrode the internal solution and electrode together provide the electrical contact to the membrane. A solid-state ion-selective electrode is one in which the internal components are replaced by a direct back contact to a metal or semiconductor, i.e., an electronic conductor. This is not a trivial matter, because most ion-selective materials are ionic conductors, and... 

Samples that contain suspended matter are among the most difficult types from which to obtain accurate pH readings because of the so-called suspension effect, ie, the suspended particles produce abnormal Hquid-junction potentials at the reference electrode (16). This effect is especially noticeable with soil slurries, pastes, and other types of colloidal suspensions. In the case of a slurry that separates into two layers, pH differences of several units may result, depending on the placement of the electrodes in the layers. Internal consistency is achieved by pH measurement using carefully prescribed measurement protocols, as has been used in the determination of soil pH (17). 

The membrane phase m is a solution of hydrophobic anion Ax (ion-exchanger ion) and cation Bx+ in an organic solvent that is immiscible with water. Solution 1 (the test aqueous solution) contains the salt of cation Bx+ with the hydrophilic anion A2. The Gibbs transfer energy of anions Ax and A2 is such that transport of these anions into the second phase is negligible. Solution 2 (the internal solution of the ion-selective electrode) contains the salt of cation B with anion A2 (or some other similar hydrophilic anion). The reference electrodes are identical and the liquid junction potentials A0L(1) and A0L(2) will be neglected. 

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