Thin Glass Membranes

Variable focusing lenses with glass diaphragms. (Source Ahn, S.H. and Y.K. Kim. 1999. Sensors and Actuators A, 78(1), 48-53. With permission.) 

The chamber and microchannels for fluid manipulation were formed on the silicon side by photolithography. 

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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. 

Poor adhesion of membrane to metal is the leading cause of failure in solid-state potentiometric sensors [116], For glass membranes, the mismatch of thermal coefficients of expansion between thin glass membrane and metal (mostly Pt) has been attributed to premature failure due to hairline crack formations in the glass layer [60], For polymer-based membranes, water vapor penetration was reported to compromise the membrane-metal interface, therefore affecting the sensor s performance. 

Haber and Klemenziewicz Zeit. Phys. Ohem, LXVil. 386, 1909) showed that if two solutions of different Ph, into each of which a calomel electrode dips, are separated by a thin glass membrane an RT... 

FIGURE 18.6 A glass electrode consists of a silver wire coated with silver chloride that dips into a reference solution of dilute hydrochloric acid. The hydrochloric acid is separated from the test solution of unknown pH by a thin glass membrane. When a glass electrode is immersed in the test solution, its electrical potential depends linearly on the difference in the pH of the solutions on the two sides of the membrane. 

In 1914 it was discovered that a thin glass membrane enclosing a solution of HC1 can produce a potential that varies with H+ in about the same way as that of the hydrogen electrode. Glass electrodes are manufactured in huge numbers for both laboratory and field measurements. They contain a built-in Ag-AgCl reference electrode in contact with the HC1 solution enclosed by the membrane. 

The glass electrode (see Fig. 11.10) contains a reference solution of dilute hydrochloric acid in contact with a thin glass membrane. The electrical poten-... 

In practice, commercial pH meters employing a glass electrode and a reference electrode are used to make pH measurements. The E value (potential) of the glass electrode does not result from an oxidation-reduction reaction but rather from the transfer of H ions through a thin glass membrane. The electrode assembly is shown in Figure 3-3. 

An ISE produces a potential that is proportional to the concentration of an analyte. Making measurements with an ISE is therefore a form of potentiometry. The most common ISE is the pH electrode, which contains a thin glass membrane that responds to the hydrogen ion concentration in a solution. The potential difference across an ion-sensitive membrane is as follows ... 

The first and third half-reactions have half-cell potentials (AgCl CH Ag) and (Hg2Cl2 cr Hg)that can be combined and called A ef because they make a constant contribution to the cell voltage. The second reaction is the source of a variable potential in the cell, corresponding to the free energy of dilution of H30 from a concentration of 1.0 M to an unknown and variable concentration, and its potential exists across the thin glass membrane of the glass electrode. The Nernst equation for the cell can therefore be written as... 

Because it is inconvenient to bubble H2 gas through a solution, a more sophisticated pH meter is used in standard laboratory practice. Dilute hydrochloric acid is used as the reference solution. The test solution is in contact with a thin glass membrane in which a silver wire coated with silver chloride is imbedded. This glass membrane is dipped into the test solution and the potential difference between the solutions is measured and interpreted by a computer, which displays the pH of the test solution. The same equation holds for both pH meters. 

Glass membrane electrodes are employed to measure pH and Na, and as an internal transducer for PCO2 sensors. The H" " response of thin glass membranes was first demonstrated in 1906 by Cremer. In the 1930s, practical appHcation of this... 

Glass electrode An electrode in which a potential develops across a thin glass membrane, which provides a measure of the pH of the solution in which the electrode is immersed. 

If a thin glass membrane separates two solutions a potential is developed, across the membrane. The magnitude of this membrane potential depends mainly on the pH of the solutions. If pH of one of the solution is kept constant and the other varied, then the electrode potential follows the relation, (refer article 1.1.4). 

The apparatus used in these measurements is shown in Fig. 3. The solution was contained in the vessel A, into which was inserted the tube E, open at the lower end, and containing the reference silver-silver chloride electrode. Three glass electrodes G, were also inserted into the vessel, the thin glass membranes being represented at the points m. The whole apparatus was gently rocked around the axis a to a and a slow stream of carbon dioxide was passed first over the solution in the saturator 5 and then over the solution, of the same concentration, in the vessel A. 

Figure 21.13 The laboratory measurement of pH. A, The glass electrode (left) is a self-contained Ag/AgCI half-cell immersed in an HCI solution of known concentration and enclosed by a thin glass membrane. It monitors the external [H ] in the solution relative to its fixed internal The saturated calomel electrode l/ight) acts as a reference. B, Most modern laboratories use a combination electrode, which houses both the glass and reference electrodes in one tube.

Figure 23-4, which is a schematic representation of the cell in Figure 23-3a, shows that this cell contains two reference electrodes (I) the external silver-silver chloride electrode (ref I) and (2) the internal silver-silver chloride electrode (ref 2). Although the internal reference electrode is a part of the glass electrode, it is noi ihc pH-sensing element. Instead, ii is the thin glass membrane at the lip of the electrode that responds to pH. 

The glass electrode (see Fig. 18.12) contains a reference solution of dilute hydrochloric acid in contact with a thin glass membrane. The electrical potential of the glass electrode depends on the difference in [Fl ] between the reference solution and the solution into which the electrode is dipped. Thus the electrical potential varies with the pFI of the solution being tested. 

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