Ion-Exchange Sites

When thoria or zirconia is adsorbed on the silica surface, probably as polybasic cations or extremely small positively charged colloidal oxide particles, the surface becomes positively charged and can act as an anion exchanger. This surface in turn adsorbs phosphate ions irreversibly and these surface anions then can act as exchange sites for cations  

The actual surface structure is, of course, more complex than represented. A silica-based cation exchanger of this type could be used to separate metal ions such as Ag, Cu, Fe, Ca +, according to Kautsky and Wesslau (546). Ion-exchange surfaces of this type have been described by Yates (547), who prepared colloidal phosphates of Ti +, Zr +, Sn +, Hf, and Ce + and adsorbed them on the silica surface. The advantage of making an exchanger with a silica base rather than entirely of the polyvalent metal phosphate is that silica is easier to form into the wide-pored structure of high surface area needed for efficient performance. 

Silica is the base for an ammonium molybdophosphate exchanger which can be used for recovering cesium ions from acid solution (548, 549). Kaletka and Konecny also use silica gel as a support for insoluble nickel, copper, cadmium, or zinc hexa-cyanoferrates, which act as cation exchangers. They can remove traces of cesium ions even from strong nitric acid solution. Exchange capacities of the diflerent compounds were measured (550).  

It now appears that almost any type of ionic or chelating organic group can be attached to the surface of open-pored silica gel or powder to obtain useful exchange properties. Unger (6) has summarized the various ways in which Si-C bonds can be formed on the surface of porous silica. These are discussed further in Chapter 7. At this point consideration is given only to specific ion exchangers of this type. 

Egorov et al. (551) grafted styrene, acrylic acid, vinylpyridine, and vinylphos-phinic acid to the surface of silica by gas phase irradiation with X-rays and fast electrons, and then ionic groups were introduced by sulfonation, chloromethylation, and amination. The capacity was 1-2 meq g.  

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Structures of styrene, divinylbenzene, and a styrene-divinylbenzene co-polymer modified for use as an ion-exchange resin. The ion-exchange sites, indicated by R, are mostly in the para position and are not necessarily bound to all styrene units. 

Increasing the number of ion-exchange sites by the use of counter-current (counterflow) regeneration... 

Inhibition of anion transport in Nation was attributed to the inhomogeneous structure of the ion exchange sites in the polymer network (Gierke cluster network model). It was found that Nation contains (even in... 

Ion-exchanger membranes with fixed ion-exchanger sites contain ion conductive polymers (ionomers) the properties of which have already been described on p. 128. These membranes are either homogeneous, consisting only of a polyelectrolyte that may be chemically bonded to an un-ionized polymer matrix, and heterogeneous, where the grains of polyelectrolyte are incorporated into an un-ionized polymer membrane. The electrochemical behaviour of these two groups does not differ substantially. 

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). 

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. 

Ion-selective electrodes with fixed ion-exchanger sites... 

Solute ions X that compete weakly with mobile phase eluent ions Y for ion exchanger sites R will be retained only slightly on the column. Solute ions that interact strongly with the ion-exchanger elute later in the chromatogram. 

In this equation, the constant Kz, which can be easily inferred from the intercept, represents a system-specific constant that is related to the ion-exchange equilibrium constant K(Lmol ), the surface area 5 (in m the charge density on the surface, that is, the number of ion-exchange sites qx available for adsorption (in molm ), and the mobile phase volume Vo (in L) in the column as described by the following equation ... 

Polyphosphazene-based PEMs are potentially attractive materials for both hydrogen/air and direct methanol fuel cells because of their reported chemical and thermal stability and due to the ease of chemically attaching various side chains for ion exchange sites and polymer cross-linking onto the — P=N— polymer backbone. Polyphosphazenes were explored originally for use as elastomers and later as solvent-free solid polymer electrolytes in lithium batteries, and subsequently for proton exchange membranes. 

This group of membranes is also frequently called membranes with immobile ion-exchange sites. First systems in which a diffusion potential is formedin the membrane will be considered [10, 12, 18, 19, 25, 41, 54, 70]. It will be assumed that all sites are equivalent and that each has a single negative charge, while the membrane is permeable only for cations. It will be assumed also that two types of univalent cations are present in solution 1, J and K ... 

Plastic film membranes can also contain fixed ion-exchange groups. Jyo and coworkers [79] chloromethylated Amberlite XAD-2 (cross-linked styrene-divinylbenzene copolymer of the macroreticular type) and formed quaternary ammonium groups in the product by treatment with dimethyltetradecylamine. They converted the substance into the chloride, nitrate or perchlorate form and saturated it with nitrobenzene. The presence of hydrophobic ion-exchange sites... 

When the test component content in the samples varies over a wider interval, a calibration curve must be constructed. Calibration curves with ISEs are usually linear over several concentration orders (usually from about 10" m to about 10" m) and their slope is close to the theoretical Nernstian value. Both at high and at low concentrations with respect to the linear part, the caUbration dependence becomes curved and, eventually, independent of the test substance concentration (see fig. 5.1). The upper limit of the ISE response is mostly given by saturation of the active sites in the electrode membrane (for example ion-exchange sites), whereas the lower limit is determined by solubility of the... 

The group of ion-selective electrodes with fixed ion-exchange sites includes systems with various membrane structures. The membranes are either homogeneous (single crystals, pressed pellets, sintered materials) or heterogeneous, set in an inactive skeleton of various polymeric materials. Important electrode materials include silver halides, silver and divalent metal chalcogenides, lanthanum trifluoride and various glassy materials. Here, the latter will be surveyed only briefly, for the sake of completeness. 

Ionic Transport, a. Conductivity The specific conductance of the SPS (Na+ form) membranes is shown in Fig. 8, whose data are summarized in Table II, including values of an apparent energy of activation. An exponential increase in ionic conductance together with a decrease in an apparent energy of activation may be related to a decrease in a "jump" distance between ion-exchange sites as a function of lEC. 

Data for the electrical conductance of annealed samples shows that ionic transport is more restricted in comparison with those which are nonannealed [Table IV). Since the total number of ion-exchange sites is unaffected by the annealing process (cf. experimental) we may assume that the site to site distance has increased. 

For dynamic ion-exchange, it is proposed that the ion-pairing reagent, due to the hydrophobic portion, is adsorbed on the surface of the stationary phase originating in ion-exchange sites available for ion-exchange between its counterions and analytes [49,134-136]. 

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