Solid state membranes

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. 

Mixed potential in case of a solid state membrane Dissolving without complexing agents... 

Insertion of Eqs. (18) in (15) results in an Equation for a dissolving solid state membrane electrode in the absence of complexing agents. In case aA = aB = 0 (e.g., pure H20) the stationary potential can be expressed as ... 

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. 

Glass membrane Crystalline solid-state membrane ... 

Considerable work has been devoted to the development of solid membranes that are selective primarily to anions. The solid-state membrane can be made of single... 

Gel of collodion, PVC, etc. Solution (with low ) of ion exchangers or complex in suitable solvent (apparently dry) "solid-state membrane electrode K, nh , Ca bf4, no3... 

Solid-state membrane electrodes. All members of this class are direct working. 

Huggins, R. A., Ionically conducting solid state membranes, AE, 10, 323 (1977). 

Walters [24] examined the effect of chloride on the use of bromide and iodide solid state membrane electrodes, and he calculated selectivity constants. Multiple linear regression analysis was used to determine the concentrations of bromide, fluorine, and iodide in geothermal brines, and indicated high interferences at high salt concentrations. The standard curve method was preferred to the multiple standard addition method because of ... 

The simplest solid-state membranes are designed to measure test ions, which are also the mobile ions of the crystal (first-order response) and are usually single-substance crystals (Figure 4.11). Alternatively, the test substance may be involved in one or two chemical reactions on the surface of the electrode which alter the activity of the mobile ion in the membrane (Figures 4.12 and 4.13). Such membranes, which are often mixtures of substances, are said to show second- and third-order responses. While only a limited number of ions can gain access to a particular membrane, a greater number of substances will be able to react at the surface of the membrane. As a result, the selectivity of electrodes showing second- and third-order responses is reduced. 

Apart from interference the greatest problem in the use of ion-selective electrodes is that of contamination. Any insoluble material deposited on the surface of the electrode will significantly reduce its sensitivity and oil films or protein deposits must be removed by frequent and thorough washing. It is possible to wipe membranes with soft tissue but they can be easily damaged. Solid-state membranes are more robust but they must not be used in any solution which might react with the membrane material. 

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... 

Solid state Membrane consists of compacted disc or single crystal of active material. F , cr, Br", I", S , Ag Cu2+, Pb2 Cd +... 

Other types of solid-state membranes include single crystals of sparsely soluble salts and are often called heterogeneous membranes, in which the insoluble salt is embedded in some inert polymer matrix. Obviously, in order for these membranes to be at equilibrium they should be in a saturated solution. In practice, these membranes are used in solutions that are below saturation. In that case, the insoluble salt slowly dissolves. 

In contrast, in most ion-selective membranes the charge conduction is done by ions. Thus, a mismatch between the charge-transfer carriers can exist at the noble metal/membrane interface. This is particularly true for polymer-based membranes, which are invariably ionic conductors. On the other hand, solid-state membranes that exhibit mixed ionic and electronic conductivity such as chalcogenide glasses, perovskites, and silver halides and conducting polymers (Lewenstam and Hulanicky, 1990) form good contact with noble metals. 

Traditionally, potentiometric sensors are distinguished by the membrane material. Glass electrodes are very well established especially in the detection of H+. However, fine-tuning of the potentiometric response of this type of membrane is chemically difficult. Solid-state membranes such as silver halides or metal sulphides are also well established for a number of cations and anions [25,26]. Their LOD is ideally a direct function of the solubility product of the materials [27], but it is often limited by dissolution of impurities [28-30]. Polymeric membrane-based ISEs are a group of the most versatile and widespread potentiometric sensors. Their versatility is based on the possibility of chemical tuning because the selectivity is based on the extraction of an ion into a polymer and its complexation with a receptor that can be chemically designed. Most research has been done on polymer-based ISEs and the remainder of this work will focus on this sensor type. 

Table 13.1. Examples of electrodes with solid state membranes...

Fig. 13.7. Forms of ion-selective electrodes with solid state membranes (a) with internal reference electrode (b) with ohmic contact (c) with ohmic contact and combined reference electrode.

In these sensors the technology developed for ISFET construction is used in conventional electrodes. Links between the membrane and internal reference are metallic (ohmic contact), by deposition of the metal on the membrane (solid state membranes), or by deposition of an ion-selective membrane on a metal. This latter is an integrated sensor. 

Subsequent to these early developments of alloy electrocatalysts in the PAFC technology, have been attempts to use the same in pefluorinated sulfonic acid fuel cells (solid state membranes such as Nation from Dupont, Dow, Asahi and others). Yeagei has reviewed the effect of different electrolytes on the ORR electrocatalysis. The summaiy of this work was that the solid state peifluorinated acid environment offered a significant advantage over phosphoric acid. These were... 

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