Ion-selective glass

Most suitable would be the use of a perfectly NH4+ ion-selective glass electrode however, a disadvantage of this type of enzyme electrode is the time required for the establishment of equilibrium (several minutes) moreover, the normal Nernst response of 59 mV per decade (at 25° C) is practically never reached. Nevertheless, in biochemical investigations these electrodes offer special possibilities, especially because they can also be used in the reverse way as an enzyme-sensing electrode, i.e., by testing an enzyme with a substrate layer around the bulb of the glass electrode. 

It has been emphasized repeatedly that the individual activity coefficients cannot be measured experimentally. However, these values are required for a number of purposes, e.g. for calibration of ion-selective electrodes. Thus, a conventional scale of ionic activities must be defined on the basis of suitably selected standards. In addition, this definition must be consistent with the definition of the conventional activity scale for the oxonium ion, i.e. the definition of the practical pH scale. Similarly, the individual scales for the various ions must be mutually consistent, i.e. they must satisfy the relationship between the experimentally measurable mean activity of the electrolyte and the defined activities of the cation and anion in view of Eq. (1.1.11). Thus, by using galvanic cells without transport, e.g. a sodium-ion-selective glass electrode and a Cl -selective electrode in a NaCl solution, a series of (NaCl) is obtained from which the individual ion activity aNa+ is determined on the basis of the Bates-Guggenheim convention for acr (page 37). Table 6.1 lists three such standard solutions, where pNa = -logflNa+, etc. 

Illustrated in Fig. 18a.5 is a typical combination ion-selective glass electrode that houses both the reference and indicator electrode and is used to measure pH. Such glass electrodes have two main limitations. 

Figure 4. Free energies of transfer of sodium, lithium and cesium fluoride from water to hydrogen peroxide derived from measurements with ion-selective glass and fluoride electrodes (27)...

Potentiometric methods are easier to use and are nowadays the most frequently utilized to measure outside skin pH. They are also used to establish pH in deeper layers of the epidermis, by first exposing them, for example, by tape stripping.7 The most common potentiometric method is using the hydrogen ion-selective glass electrode with internal reference electrode, which is often called just glass electrode. 6 8 The electrode is often planar-shaped to make it more easily applied on the skin.9... 

One of the simplest ways to observe the electrostatic effect inherent in polyion systems is to measure the activity of the counterion of the polyelectrolyte solution in the absence of simple salt. Single ion activities of these sodium salt solutions, a a, measured electrochemically by use of a Na" ion selective glass electrode can be expressed by the foUowing equation ... 

In an ion-selective glass electrode the glass membrane is viewed as consisting of at least three distinct regions ... 

The extension to lonophore selectivity of a hypothesis based on analogy with the rigid matrices of Ion selective glasses (19)... 

J. R. Sandifer and R. P. Buck [1974] Impedance Characteristics of Ion Selective Glass Electrodes,... 

Figure 11.1 Dipped glass electrode. 1, a wire conductor, Au, Pt or Tl 2, etched wire section on top of which is an oxide layer containing small quantities of alkali metal ion SA 2 /zm thick, making a hemispherical tip 3, ion-selective glass 4, glass insulating jacket 5, plastic coating. (Redrawn from ref. 7.)...

Care of ion-selective glass-membrane electrodes is similar to that suggested in Chapter 5 for glass pH electrodes. One must be careful to maintain the correct amount of filling electrolyte. 

Ion-selective glass electrode (chalkogenide glasses) 4-6 Potentiometry Beverages mineral waters... 

Karlberg, B., Response-Time Properties of Some Hydrogen Ion-Selective Glass Electrodes in Nonaqueous Solutions, Anal. Chem. Acta 66, 93 (1973). 

FIGURE 16 Construction and hydration properties of an ion-selective glass electrode. 

We will pass over here the classic pH glass electrode. Those who wish to learn more about the topic of electrochemical pH measurements are referred to the monographs of M. Dole [5], L. Kratz [6], K. Schwabe [17] and G. Eisenman [7]. The last of these already describes the transition to ion-selective glass membrane electrodes. 

An ion-selective glass membrane electrode dipping in a solution of the corresponding measured ion, and complemented with a suitable reference electrode forms a cell (with or without liquid junction) which will exhibit an EMF according to the equation... 

The potential (E2) setup across ion-selective glass membrane due to the two... 

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