Other ion selective electrodes

About 20 ISEs are used and classified according to the nature of the membrane. They serve either in direct ionometry, or as indicator electrodes for measurements involving both titrimetry and complexometry performed with the aid of automatic titrators. 

1 Single crystal and solid-state membrane electrodes 

S/Sample solution ( onoc stal) (contains fluoride ion)  

Reference electrode Glass electrode Internal solution 

A measurable PD will appear if the concentrations of the calcium ions are different on the two sides of the membrane. 

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Especially sensitive and selective potassium and some other ion-selective electrodes employ special complexing agents in their membranes, termed ionophores (discussed in detail on page 445). These substances, which often have cyclic structures, bind alkali metal ions and some other cations in complexes with widely varying stability constants. The membrane of an ion-selective electrode contains the salt of the determined cation with a hydrophobic anion (usually tetraphenylborate) and excess ionophore, so that the cation is mostly bound in the complex in the membrane. It can readily be demonstrated that the membrane potential obeys Eq. (6.3.3). In the presence of interferents, the selectivity coefficient is given approximately by the ratio of the stability constants of the complexes of the two ions with the ionophore. For the determination of potassium ions in the presence of interfering sodium ions, where the ionophore is the cyclic depsipeptide, valinomycin, the selectivity coefficient is Na+ 10"4, so that this electrode can be used to determine potassium ions in the presence of a 104-fold excess of sodium ions. 

The membranes of the other ion-selective electrodes can be either homogeneous (a single crystal, a pressed polycrystalline pellet) or heterogeneous, where the crystalline substance is incorporated in the matrix of a suitable polymer (e.g. silicon rubber or Teflon). The equation controlling the potential is analogous to Eq. (6.3.9). 

These and other ion-selective electrodes are discussed in Chapter 5. A large selection of such electrodes is commercially available, and the biennial reviews of Analytical Chemistry include a section that describes new types of electrodes and their applications. 

Before using the pH electrode, it should be calibrated using two (or more) buffers of known pH. Many standard buffers are commercially available, with an accuracy of 0.01 pH unit. Calibration must be performed at the same temperature at which the measurement will be made care must be taken to match the temperature of samples and standards. The exact procedure depends on the model of pH meter used. Modern pH meters, such as the one shown in Figure 5.8, are microcomputer-controlled, and allow double-point calibration, slope calculation, temperature adjustment, and accuracy to +0.001 pH unit, all with few basic steps. The electrode must be stored in an aqueous solution when not in use, so that the hydrated gel layer of the glass does not dry out. A highly stable response can thus be obtained over long time periods. As with other ion-selective electrodes, the operator should consult the manufacturer s instructions for proper use. Commercial glass electrodes are remarkably... 

By using different membranes, it is possible to obtain potentiometric sensors for gases such as sulfur dioxide or nitrogen dioxide. Such sensors employ similar (acid-base) or other equilibrium processes. These devices, along with their equilibrium processes and internal electrodes, are summarized in Table 6.2. Membrane coverage of other ion-selective electrodes (e.g., chloride) can be used for the sensing of other gases (e.g., chlorine). 

Asymmetry potential — In case of any membrane it happens that the potential drop between the solution and either inner side of the - membrane is not completely identical so that a nonzero net potential drop arises across the entire membrane. This is best known for - glass electrodes and other - ion-selective electrodes. The reasons of asymmetry potentials are chemical or physical differences between each side of a membrane, in particular an inhomogeneous membrane structure resulting from fabrication conditions and/or curvature. Asymmetry p. can change in the course of membrane ageing. To measure asymmetry p. one should use a symmetrical cell with identical solutions and -> reference electrodes on each side of the membrane. 

In -> glass electrodes and other -> ion-selective electrodes two reference electrodes are used, the one which is placed inside the bulb-shaped glass or other membrane is called internal or inner reference electrode. It is usually an Ag AgCl HCl(aq) system. 

Oxidation and Reduction Equilibria and Microbial Mediation Other Ion-Selective Electrodes... 

Pungor has presented evidence that the establishment of an electrode potential is caused by charge separation, due to chemisorption of the primary ion (H ) from the solution phase onto the electrode surface, that is, a surface chemical reaction. Counter ions of the opposite charge accumulate in the solution phase, and this charge separation represents a potential. A similar mechanism applies to other ion-selective electrodes (below). [See E. Pungor, The New Theory of Ion-Selective Electrodes, Sensors, 1 (2001) 1-12 (this is an electronic journal www. mdpi.net/sensors). ... 

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. 

The pH electrode may be used to make measurements of the hydrogen ion activity by directly immersing the electrodes in the water flow. In this case, it is not possible to buffer the solution and since the pH measurement is normally made in this way there is no problem for pH measurement. Other ion-selective electrodes tend not to be used in this way as activity measurements are not normally used for other ions. Special cells are required for pH measurements, which are carried out in situ. These are of two types cells for use in pipework where the cell is part of the pipe that carries the water, and dip cells that are used to dip the electrodes into ponds, lakes, reservoirs, and other water supplies. 

In addition to the glass electrode responsive to hydrogen ions, other ion selective electrodes are now available. These electrodes were first developed around 1964, and E. Pungor was associated with some of the early developments. A great many ions can now be estimated in this way, and among the more widely used electrodes are those responsive to potassium, calcium, fluoride and nitrate ions. 

One possible way to avoid some of the problems described above would be to use an electrode pair without a liquid junction, i.e., a cell without transference. In this way, uncertainties due to the liquid junction, such as alteration of the sample solution by electrolyte diffusion, streaming potentials, suspension effect, and the liquid junction potential itself, may be eliminated by using a pH or other ion-selective electrode as the reference electrode. The difficulty in this approach arises because, in order to assign an accurate emf value to the reference electrode, the activity of the reference ion in the sample solution must be accurately known and remain constant. Once again we are confronted by the necessity of a bootstrap operation. There is no way, at the present state-of-the-art, to accurately calculate the activity of an ion in such a complex mixture as a biologic fluid. If an activity is arbitrarily assigned to the reference ion and if it remains constant, then such an electrode system can be used for precise measurements of relative ion activities, but little can be said about the absolute activities. 

Surely, the difference in the two calculated E values is not a large difference in voltages. But it is an easily measurable one, and for precise measurements the difference can have a big impact on the predicted properties of the ionic solution. For example, it is necessary to consider activity factors when using pH and other ion-selective electrodes, because the exact voltage of the electrochemical cell that is made in the course of the measurement is dependent on the activity of the ions involved, not their concentration. Activity, like fugacity, is a more realistic measure of how real chemical species behave. For precise calculations, activity must be used for ionic solutions, not concentration. 

The use of electrodes, particularly the glass electrode for pH measurements and the wide range of other ion selective electrodes (ISE) described in Topic C3, enables titrations to be studied throughout the addition of titrant, so that small changes may be detected. It also allows automation of the titration. 

Chemical methods may involve many of the techniques described in Section C. Any acidic or alkaline gas can be detected by absorption and titration, or potentiometry. Gas-sensing membrane electrodes and other ion-selective electrodes allow analysis of halide and sulfide ions. 

With other ion-selective electrodes, the liquid junction potential can be estimated and corrected with the equations given below, the calibration can be performed in a similar electrolyte background, or the electrolyte background can be adjusted by adding a buffer electrolyte (TISAB). In some cases, a liquid junction is not necessary, such as with potentiometric gas sensing probes and in some dynamic electrochemistry approaches. 

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