Ion exchange membrane, electrode

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... 

Figure 6.4. Calcium sensitive ion-exchange membrane electrode...

Potentiometry—the measurement of electric potentials in electrochemical cells—is probably one of the oldest methods of chemical analysis still in wide use. The early, essentially qualitative, work of Luigi Galvani (1737-1798) and Count Alessandro Volta (1745-1827) had its first fruit in the work of J. Willard Gibbs (1839-1903) and Walther Nernst (1864-1941), who laid the foundations for the treatment of electrochemical equilibria and electrode potentials. The early analytical applications of potentiometry were essentially to detect the endpoints of titrations. More extensive use of direct potentiometric methods came after Haber developed the glass electrode for pH measurements in 1909. In recent years, several new classes of ion-selective sensors have been introduced, beginning with glass electrodes more or less selectively responsive to other univalent cations (Na, NH ", etc.). Now, solid-state crystalline electrodes for ions such as F , Ag", and sulfide, and liquid ion-exchange membrane electrodes responsive to many simple and complex ions—Ca , BF4", CIO "—provide the chemist with electrochemical probes responsive to a wide variety of ionic species. 

G23. Gordievskii, A.V. E.L. Filippov. V.S. Sherman, A.S. Krivoshein, "Ion-exchange Membranes. III. Application of an ion-exchange membrane electrode in the study of complex-formation reactions . Russ. J. Phys. Chem., v42, 8, ppl050-1054 (1968)... 

The porous electrodes in PEFCs are bonded to the surface of the ion-exchange membranes which are 0.12- to 0.25-mm thick by pressure and at a temperature usually between the glass-transition temperature and the thermal degradation temperature of the membrane. These conditions provide the necessary environment to produce an intimate contact between the electrocatalyst and the membrane surface. The early PEFCs contained Nafton membranes and about 4 mg/cm of Pt black in both the cathode and anode. Such electrode/membrane combinations, using the appropriate current coUectors and supporting stmcture in PEFCs and water electrolysis ceUs, are capable of operating at pressures up to 20.7 MPa (3000 psi), differential pressures up to 3.5 MPa (500 psi), and current densities of 2000 m A/cm. ... 

Compact, ready-prepared calomel electrodes are available commercially and find wide application especially in conjunction with pH meters and ion-selective meters. A typical electrode is shown in Fig. 15.1(h). With time, the porous contact disc at the base of the electrode may become clogged, thus giving rise to a very high resistance. In some forms of the electrode the sintered disc may be removed and a new porous plate inserted, and in some modern electrodes an ion exchange membrane is incorporated in the lower part of the electrode which prevents any migration of mercury(I) ions to the sintered disc and thus... 

Commercial forms of the electrode are available and in general are similar to the calomel electrode depicted in Fig. 15.1(h) with the replacement of the mercury by a silver electrode, and calomel by silver chloride. The remarks concerning clogging of the sintered disc, and the use of ion exchange membranes and double junctions to reduce this are equally applicable to the silver-silver chloride electrode. 

To keep the consumption of the valuable platinum low, thin foils have been glued to a graphite support [34] or thin-layers of platinum have been sputtered on to a glass base [59]. Platinum can be used in a particle electrode by plating silica gel with platinum [60], or in a solid polymer electrolyte where platinum is incorporated into a nafion ion exchange membrane [61]. 

The search for models of biological membranes led to the formation of a separate branch of electrochemistry, i.e. membrane electrochemistry. The most important results obtained in this field include the theory and application of ion-exchanger membranes and the discovery of ion-selective electrodes (including glass electrodes) and bilayer lipid membranes. 

All ion-exchanger membranes with fixed ion-exchanger sites are porous to a certain degree (in contrast to liquid membranes and to membranes of ion-selective electrodes based on solid or glassy electrolytes, such as a single crystal of lanthanum fluoride). 

Consider the system shown in Fig. 6.3. The ion-exchanger membrane separates solutions of a single, completely dissociated, uni-univalent electrolyte. Two pistons can be employed to form a pressure difference between the two compartments. The two electrodes W and W2 are... 

Electrodialysis is a process for the separation of an electrolyte from the solvent and is used, for example, in desalination. This process occurs in a system with at least three compartments (in practice, a large number is often used). The terminal compartments contain the electrodes and the middle compartment is separated from the terminal compartments by ion-exchanger membranes, of which one membrane (1) is preferentially permeable for the cations and the other one (2) for the anions. Such a situation occurs when the concentration of the electrolyte in the compartments is less than the concentration of bonded ionic groups in the membrane. During current flow in the direction from membrane 1 to membrane 2, cations pass through membrane 1 in the same direction and anions pass through membrane 2 in the opposite direction. In order for the electrolyte to be accumulated in the central compartment, i.e. between membranes 1 and 2 (it is assumed for simplicity that a uni-univalent electrolyte is involved), the relative flux of the cations with respect to the flux of the solvent, /D +, and the relative flux of the anions with respect to... 

Ion-selective electrodes are membrane systems used as potentiometric sensors for various ions. In contrast to ion-exchanger membranes, they contain a compact (homogeneous or heterogeneous) membrane with either fixed (solid or glassy) or mobile (liquid) ion-exchanger sites. 

Electrodes with liquid ion-exchange membranes are typified by a calcium-sensitive electrode (Figure 6.4). The membrane-liquid consists of the calcium form of a di-alkyl phosphoric acid, [(RO)2POO ] 2Ca2+, which is prepared by repeated treatment of the acid with a calcium salt. The internal solution is calcium chloride and the membrane potential, which is determined by the extent of ion-exchange reactions at the interfaces between the membrane and the internal and sample solutions, is given by... 

Advances during the past 20 years in membrane, electrolyser, electrode, and brine purification technologies have substantially raised the performance levels and efficiency of chlor-alkali production by ion-exchange membrane electrolysis, bringing commercial operations with a unit power consumption of 2000-2050 kWh per ton of NaOH or lower at 4 kA m-2 current density with a membrane life of four years or longer. 

A separator is a porous membrane placed between electrodes of opposite polarity, permeable to ionic flow but preventing electric contact of the electrodes. A variety of separators have been used in batteries over the years. Starting with cedar shingles and sausage casing, separators have been manufactured from cellulosic papers and cellophane to nonwoven fabrics, foams, ion exchange membranes, and microporous flat sheet membranes made from polymeric materials. As batteries have become more sophisticated, separator function has also become more demanding and complex. 

Before the appearance of analytically useful ISEs with liquid membranes, Sollner and Shean [200] obtained a liquid ion-exchange membrane with marked selectivity for anions. The first ISE with a liquid membrane was the calcium electrode described by Ross [179] with Ca " -dialkylphenylphosphate in dioctylphenylphosphonate. 

Although there is no direct connection between the rules discussed in Chap. 4 for the monovalent-divalent ion selectivity of ligands in liquid membranes and the electrochemical selectivity behavior of these membranes (cf. Eqs. (22) and (31)), the effect of solvent on Na+/Ca2+ selectivity (Fig. 22) is remarkably similar to the calculated effect shown in Fig. 18 (actually for K+/Ba2+ preference). The discrimination against Mg2+ and especially HaO+ is considerably better for the electrode discussed here 117) than for liquid ion-exchange membranes responsive to Ca2+ (123). 

Equation (4.4.1b) expresses impermeability of the ideally cation-permselective interface under consideration for anions j is the unknown cationic flux (electric current density). Furthermore, (4.4.1d) asserts continuity of the electrochemical potential of cations at the interface, whereas (4.4. lg) states electro-neutrality of the interior of the interface, impenetrable for anions. Here N is a known positive constant, e.g., concentration of the fixed charges in an ion-exchanger (membrane), concentration of metal in an electrode, etc. E in (4.4.1h) is the equilibrium potential jump from the solution to the interior of the interface, given by the expression ... 

Ion-selective electrodes, including the glass pH electrode, respond mainly to one ion that is selectively bound to the ion-exchange membrane of the electrode. The potential difference across the membrane. E, depends on the activity, JAa, of the target ion in the external analyte solution. At 25°C, the relation is fV) = constant +... 

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