Membranes with dissolved ion-exchanger ions

A membrane can be either a liquid or a solid. Its electrical properties arise when it allows transport of an ion of one charge but not that of another. Membranes are usually sufficiently thick that one can distinguish an inside region and two outer boundary regions which are in contact with electrolyte solutions. Two types of membranes are considered here (1) membranes of solid and glassy materials (2) liquid membranes with dissolved ion-exchanging ions or neutral ion carriers (ionophores). In fact all of these membranes are involved in ion exchange. It is important to understand how this process affects the potentials which develop in the system at both sides of the membrane. 

Liquid membranes with dissolved ion-exchangers were used first by Sollner and Shean [91], but the first actual ion-selective electrode was constructed on this principle much later by Ross [87]. 

Ion-exchange membrane is a membrane comprising fixed ions in its polymer matrix. It preferentially adsorbs and dissolves ions of opposite charge but repels those of the same charge. Membranes with fixed ions or dipoles will preferentially adsorb and dissolve molecules with dipoles, for example, water, but will repel nonpolar molecules. 

Removal of brine contaminants accounts for a significant portion of overall chlor—alkali production cost, especially for the membrane process. Moreover, part or all of the depleted brine from mercury and membrane cells must first be dechlorinated to recover the dissolved chlorine and to prevent corrosion during further processing. In a typical membrane plant, HCl is added to Hberate chlorine, then a vacuum is appHed to recover it. A reducing agent such as sodium sulfite is added to remove the final traces because chlorine would adversely react with the ion-exchange resins used later in the process. Dechlorinated brine is then resaturated with soHd salt for further use. 

This chapter is concerned with processes that lead to formation of ISE membrane potentials. The membrane potentials of electrodes with liquid membranes containing a dissolved ion-exchanger ion or a dissolved ionophore (ion carrier), and of electrodes with solid or glassy membranes will be considered. More complicated systems, for example ISEs with a gas gap and enzyme electrodes, will be discussed in chapters 4 and 9. 

More recently cation-exchange membranes with good mechanical and chemical stability and well-controlled ion-exchange capacity are prepared by dissolving and casting a functionalized polymer such as sulfonated polysulfone, or sulfonated polyetheretherketone in an appropriate solvent, followed by casting the mixture into a film and then evaporation of the solvent [12]. 

Another design that is used in chlo-ralkali electrolysis, water electrolysis, and electro-organic synthesis [95-97] is the solid polymer electrolyte (SPE) cell, where an ion exchanger membrane, for example, Nafion , serves as the electrolyte, Fig. 9. The microporous catalytic reaction layers are pressed directly onto the membrane with porous current collectors allowing transport of dissolved reactants and gaseous products into and out of the reaction layer. 

One of the most important liquid-membrane electrodes is selective toward calcium ion in neutral media. The active ingredient in the membrane is a cation exchanger consisting of an aliphatic dicster of phosphoric acid dissolved in a polar solvent. The dicster contains a single acidic proton thus, two molecules react with the... 

A mixture of two liquid ion exchangers, respectively cation and anion, containing mobile ionogenic groups dissolved in a diluent, functions as a liquid membrane across which ions can move selectively. When a liquid membrane of this kind is interposed between two aqueous solutions, one of which contains the specimen while the other is a reference solution (see Fig. 20), and two identical reference electrodes are immersed, one in each solution, the magnitude of the electromotive force (EMF) between them can be correlated with the activities of the ionic species in the sample solution. 

Soil as an Electrolyte Most soils are conductive due to the presence of dissolved ions, such as calcium, magnesium, sodium, potassium, (bi) carbonate, some soluble fatty acids, nitrate, phosphate, sulfate, and chloride ions. Most seemingly, dry soils have more than 5% moisture, sufficient to provide a continuous path for these ions to move. This is essential for plants as the roots need access to these nutrients and ion transport across membranes is the means with which they extract them. The most signihcant feature of natural soils, with respect to their contamination and subsequent remediation, is the high ion exchange capacity (Table 33.1). 

A NF/IX water treatment system is very effective in removing virtually aU nitrates from drinking water [6]. In the NF/IX hybrid system, pre-filtered water flows through a NF membrane system, which removes all the multivalent ions and some of the monovalent nitrate ions. The NF permeate is then polished in an anion IX column where the nitrates are exchanged with chloride ions. NF pre-treatment increases the efficiency of the IX process since it removes dissolved organic carbon and divalent ions such as sulphate ions. 

A liquid membrane comprises a thin, porous inert support impregnated with an ion-exchanging material in the liquid phase. This material is often an organic species dissolved in some organic solvent, in which case use in non-aqueous solutions may be restricted. There are two types of liquid membranes (a) charged liquid membranes which usually contain the ion for which the electrode is sensitive (the operation here is very similar to the solid state case), and (b) neutral liquid membranes which usually do not contain the ion to be... 

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