Electrodes crystalline membrane

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). 

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...

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. 

Lanthanum fluoride, Lal- u is a nearly ideal substance for the preparation of a crystalline membrane electrode for the determination of fluoride ion. Although this compound is a natural conductor, its conductivity can be enhanced by doping with europium fluoride, LuF,. Membranes are prepared by cutting disks from a single crystal of the doped compound. 

Crystalline membrane electrodes, constructed either as the glass electrode, or with a direct contact as shown in Figure 2(c), have an outer crystal surface which responds to particular ions. The fluoride electrode has a crystal of LaFs, treated with Eu(II) to increase conductivity, which responds selectively to the adsorption of free ion on its surface. The selectivity is very good, due to the abihty of the small R on to fit to the LaFj crystal lattice to the exclusion of larger ions. However, OH ions are also small and can interfere, and F" may also form undissociated hydrofluoric acid. Therefore, it is necessary to use this electrode in a buffer solution at about pH 6. 

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. 

The glass membrane of the electrodes discussed above may be replaced by other materials such as a single crystal or a disc pressed from finely divided crystalline material it may be advantageous to incorporate the crystalline material into an inert carrier such as a suitable polymer thus producing a heterogeneous-membrane electrode. 

The physical concept of a single electrode potential has been also discussed in terms of the energy levels of ions in electrode systems. This concept may be usefirl in the cases where the system has no electronic energy levels in a range of practical interest, such as in ionic solid crystalline and electronically nonconductive membrane electrodes. "... 

It can be concluded that in principle the heterogeneous precipitate membrane electrodes act in the same way as the corresponding homogeneous electrodes, but often they are slower in response in practice however, they still offer manufacturing possibilities where suitable pellets of the pure crystalline material cannot be obtained. 

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. 

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.

For electrodes which have no electron energy levels in the energy range of general interest, such as ionic crystalline sohd electrodes and membrane electrodes, only the concept of ionic electrode potential can be of practical significance. 

They are classified by membrane material into glass membrane electrodes, crystalline (or solid-state) membrane electrodes, and liquid membrane electrodes. Liquid membrane electrodes are further classified into liquid ion-exchange membrane electrodes and neutral carrier-based liquid membrane electrodes. Some examples are shown in Fig. 5.36 and Table 5.3. If the membrane is sensitive to ion i of charge Z and the activities of i in the sample and internal solutions are equal to (i) and a2(i), respectively, the membrane potential, m, which is developed across the membrane, is... 

Table 21-2 lists some liquid-membrane electrodes available from commercial sources. The anion-sensitive electrodes shown make use of a solution containing an anion-exchange resin in an organic solvent. Liquid-membrane electrodes in which the exchange liquid is held in a polyvinyl chloride gel have been developed for Ca-, K", NOj, and BF4. These have the appearance of crystalline electrodes, which are considered in the following section. A homemade liquid-membrane ion-selective electrode is described in Feature 21-1. 

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. 

The most important type of crystalline membranes is manufactured from an ionic compound or a homogeneous mixture of ionic compounds. In some instances the membrane is cut from a single crystal in others, disks are formed from the finely ground crystalline solid by high pressures or by casting from a melt. Typical membranes have a diameter of about 10 mni and a thickness of 1 or 2 nirn. lb form an electrode, a membrane is sealed to the end of a tube made from a chemically inert plaslic such as Teflon or polyvinyl chloride (PVC). 

Conductivity of Crystalline Membranes. Most ionic crystals are insulators and do not have sufficient electrical conductivity at room temperature to be used as membrane electrodes. Those that are con-... 

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