Membrane glass

When first developed, potentiometry was restricted to redox equilibria at metallic electrodes, limiting its application to a few ions. In 1906, Cremer discovered that a potential difference exists between the two sides of a thin glass membrane when opposite sides of the membrane are in contact with solutions containing different concentrations of H3O+. This discovery led to the development of the glass pH electrode in 1909. Other types of membranes also yield useful potentials. Kolthoff and Sanders, for example, showed in 1937 that pellets made from AgCl could be used to determine the concentration of Ag+. Electrodes based on membrane potentials are called ion-selective electrodes, and their continued development has extended potentiometry to a diverse array of analytes. 

If metallic electrodes were the only useful class of indicator electrodes, potentiometry would be of limited applicability. The discovery, in 1906, that a thin glass membrane develops a potential, called a membrane potential, when opposite sides of the membrane are in contact with solutions of different pH led to the eventual development of a whole new class of indicator electrodes called ion-selective electrodes (ISEs). following the discovery of the glass pH electrode, ion-selective electrodes have been developed for a wide range of ions. Membrane electrodes also have been developed that respond to the concentration of molecular analytes by using a chemical reaction to generate an ion that can be monitored with an ion-selective electrode. The development of new membrane electrodes continues to be an active area of research. 

An ion-selective electrode based on a glass membrane in which the potential develops from an ion-exchange reaction on the membrane s surface. 

Replacing Na20 and CaO with Li20 and BaO extends the useful pH range of glass membrane electrodes to pH levels greater than 12. 

Glass membrane pH electrodes are often available in a combination form that includes both the indicator and the reference electrode. The use of a single electrode greatly simplifies the measurement of pH. An example of a typical combination electrode is shown in Figure 11.12. 

V Representative Examples of Glass Membrane r Ion-Selective Electrodes ... 

Unlike ion-selective electrodes using glass membranes, crystalline solid-state ion-selective electrodes do not need to be conditioned before use and may be stored dry. The surface of the electrode is subject to poisoning, as described earlier for a Ck ISE in contact with an excessive concentration of Br. When this happens, the electrode can be returned to its original condition by sanding and polishing the crystalline membrane. 

A second complication in measuring pH results from uncertainties in the relationship between potential and activity. For a glass membrane electrode, the cell potential, Ex, for a solution of unknown pH is given as... 

The potential of a membrane electrode is determined by a difference in the composition of the solution on either side of the membrane. Electrodes using a glass membrane respond to ions... 

Y. Sajima, K. Sato, and H. Ukihashi, Recent Progress of Asahi Glass Membrane Chlor—Alkali Process, AICHE Symp. Ser. 82(248), 108 (1985). 

Because of the very large resistance of the glass membrane in a conventional pH electrode, an input amplifier of high impedance (usually 10 —10 Q) is required to avoid errors in the pH (or mV) readings. Most pH meters have field-effect transistor amplifiers that typically exhibit bias currents of only a pico-ampere (10 ampere), which, for an electrode resistance of 100 MQ, results in an emf error of only 0.1 mV (0.002 pH unit). 

LaFs crystals developed by J. W. Ross and M. S. Frant as the first non-glass membrane electrode... 

Finally some assumptions could not be verified as, e.g., the complete co-ion exclusion necessary for the treatment of the phase boundary potential as a Donnan potential, or the constant ion mobility through glass membranes with hydrated layers76). 

The explicit mathematical treatment for such stationary-state situations at certain ion-selective membranes was performed by Iljuschenko and Mirkin 106). As the publication is in Russian and in a not widely distributed journal, their work will be cited in the appendix. The authors obtain an equation (s. (34) on page 28) similar to the one developed by Eisenman et al. 6) for glass membranes using the three-segment potential approach. However, the mobilities used in the stationary-state treatment are those which describe the ion migration in an electric field through a diffusion layer at the phase boundary. A diffusion process through the entire membrane with constant ion mobilities does not have to be assumed. The non-Nernstian behavior of extremely thin layers (i.e., ISFET) can therefore also be described, as well as the role of an electron transfer at solid-state membranes. 

In the diffusion-controlled domain (preferable in situations with large overpotentials) a diffusion layer is formed. This layer is found on the solution side of solid-state membranes it is located with in the membrane surface of liquid and glass membranes. 

Amalgam electrodes, and liquid and glass membrane electrodes... 

As has been shown 82 85 88), the behavior of amalgam electrodes under conditions of cementation is very similar to that of liquid and glass membrane electrodes under stationary state conditions. Here, Eq. (2) should be written in the following way ... 

The conductivity of liquid and glass membranes is determined by ion-migration (absence of an excess of supporting electrolyte is assumed) in the diffusion layer. Equation (25) should then be written as ... 

Owing to the high resistance of the glass membrane, a simple potentiometer cannot be employed for measuring the cell e.m.f. and specialised instrumentation (Section 15.14) must be used. The e.m.f. of the cell may be expressed by the equation ... 

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