Crystalline membranes

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. 

Crystallization. Acidified aluminum sulfate solutions can be supercooled 10 °C or more below the saturation point. However, once nucleation begins, the crystallization rate is rapid and the supersaturated solution sets up. The onset of nucleation in a gentiy stirred supersaturated solution is marked by the appearance of silky, curling streamers of microscopic nuclei resulting from orientation effects of hydraulic currents on the thin, platelike crystals. Without agitation, nucleation in an acidified solution, in glass tubes, can yield extended crystalline membranes of such thinness to exhibit colors resulting from optical interference. 

The pressed disc (or pellet) type of crystalline membrane electrode is illustrated by silver sulphide, in which substance silver ions can migrate. The pellet is sealed into the base of a plastic container as in the case of the lanthanum fluoride electrode, and contact is made by means of a silver wire with its lower end embedded in the pellet this wire establishes equilibrium with silver ions in the pellet and thus functions as an internal reference electrode. Placed in a solution containing silver ions the electrode acquires a potential which is dictated by the activity of the silver ions in the test solution. Placed in a solution containing sulphide ions, the electrode acquires a potential which is governed by the silver ion activity in the solution, and this is itself dictated by the activity of the sulphide ions in the test solution and the solubility product of silver sulphide — i.e. it is an electrode of the second kind (Section 15.1). 

The concept of the pH electrode has been extended to include other ions as well. Considerable research has gone into the development of these ion-selective electrodes over the years, especially in studying the composition of the membrane that separates the internal solution from the analyte solution. The internal solution must contain a constant concentration of the analyte ion, as with the pH electrode. Today we utilize electrodes with 1) glass membranes of varying compositions, 2) crystalline membranes, 3) liquid membranes, and 4) gas-permeable membranes. In each case, the interior of the electrode has a silver-silver chloride wire immersed in a solution of the analyte ion. 

With crystalline membranes, the membrane material is most often an insoluble ionic crystal cut to a round, flat shape and having a thickness of 1 or 2 mm and a diameter of about 10 mm. This flat disk is... 

Crystalline membrane electrodes, and (iv) Gas-sensing electrodes, which will be described below briefly ... 

The crystalline membrane electrodes have a very close similarity to those of glass-membrane electrodes (see Section 16.3.1.2.2.1 ) except that glass has been replaced with crystalline membrane. In fact, these electrodes offer a means to devise responsive to anions by making use of a membrane containing specific anionic sites. 

Table 16.2 records the characteristics of certain selected crystalline-membrane electrodes. 

Table 16.2 Characteristics of Certain Selected Crystalline Membrane Electrodes...

Figure 4.11 A solid-state electrode showing a first-order response. An electrode designed to measure the activity of silver ions uses a crystalline membrane of silver sulphide. An equilibrium between the mobile silver ions of the membrane and the silver ions in the solutions results in the development of a potential difference across the membrane.

In another model, Harland and Peppas [159] considered the diffusion of solutes through semicrystalline hydrogel membranes. These types of membranes were assumed to consist of a crosslinked, swollen (amorphous) phase through which solute diffusion occurred and an impermeable, crystalline phase. A simplified form of the model assumes uniform amorphous regions. With this assumption, the diffusion coefficient through a semi-crystalline membrane, Dc, was written as... 

Critical temperature The temperature above which a substance can no longer exist in the liquid state, regardless of pressure. Cross-linked stationary phase A polymer stationary phase in a chromatographic column in which covalent bonds link different strands of the polymer, thus creating a more stable phase. Crystalline membrane electrode Electrode in which the sensing element is a crystalline solid that responds selectively to the activity of an ionic analyte. 

Amorphous silica has also been mentioned as a starting metal oxide material for the preparation of particulate mesoporous membranes. These membranes were prepared from commercial sols, Ludox (DuPont) or Cecasol (Sobret), and coated on a macroporous a-alumina support [35]. In contrast to crystalline membrane materials such as alumina, titania or zirconia, the evolution of pore size with temperature of amorphous silica membranes was revealed to be more sensitive to drying conditions than to firing temperature (Table 7.1). When heat-treated for several hours at 800°C the silica top layer transformed from an amorphous state to cristobalite. 

Fig, 6. Graphical representation of two types of membrane-protein crystals. Type 1 crystal consists of stacks of two-dimensional crystalline membranes ordered in the third dimension to form the "membrane crystal. Type 11 crystals are formed by membrane proteins with the hydrocarbon tails of detergent molecules bound to their hydrophobic surfaces. The hydrophilic surfaces of the protein are indicated by dotted boundaries. Figure source Michel (1983) Crystallization of membrane proteins. Trends Biochem... 

Fig. 9.6 Illustration of the structures of three ion selective electrodes (left to right) a glass electrode an electrode with a crystalline membrane and a liquid membrane electrode.

The construction of an ISE with a crystalline membrane is shown in fig. 9.6(b). The membrane is located at the bottom of the electrode, where it comes into contact with the test solution. The solution in the reference compartment inside the electrode contains both the CE ion, which establishes the potential of the Ag AgCl reference electrode, and the ion to which the membrane is responding. For example, in the case of the F ISE, it also contains the fluoride anion. 

Y. Alifragis, G. Konstantinidis, A. Georgakilas, and N. Chaniotakis, Anion selective potentiometric sensor based on gallium nitride crystalline membrane, Electroanalysis 17, 527-531 (2005). 

Figure 3.14 The two basic types of membrane protein ciystals. Type I stacks of membranes containing two-dimensionally crystalline membrane proteins, which are then ordered in the third dimension. Type II a membrane protein crystallised with detergents bound to its hydrophobic surface. The polar surface part of the membrane proteins is indicated by broken lines. The symbols for liquids and detergents are the same as in figure 3.13. From Deisenhofer and Michel (1989) with the permission of the authors, EMBO J, Oxford University Press and copyright The Nobel Foundation (1989).

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