Selectivity ion-exchange

In the membrane-cell process, highly selective ion-exchange membranes of Du Font s Nation type are used which allow only the sodium ions to pass. Thus, in the anode compartment an alkali solution of high purity is produced. The introduction of Nafion-type membranes in chlor-alkali electrolyzers led to a significant improvement in their efficiency. Today, most new chlor-alkafi installations use the membrane technology. Unfortunately, the cost of Nafion-type membranes is still very high. 

Highly Selective Ion Exchange in Clay Minerals and Zeolites... 

Highly selective ion exchange reactions described here in clay minerals and zeolites are reversible and occur on the constant charge fraction of these minerals. Interactions with a siloxane surface are therefore involved in contrast to the so-called specific adsorption effects occuring on hydroxyl bearing surfaces. 

Various processes separate rare earths from other metal salts. These processes also separate rare earths into specific subgroups. The methods are based on fractional precipitation, selective extraction by nonaqueous solvents, or selective ion exchange. Separation of individual rare earths is the most important step in recovery. Separation may be achieved by ion exchange and solvent extraction techniques. Also, ytterbium may be separated from a mixture of heavy rare earths by reduction with sodium amalgam. In this method, a buffered acidic solution of trivalent heavy rare earths is treated with molten sodium mercury alloy. Ybs+ is reduced and dissolved in the molten alloy. The alloy is treated with hydrochloric acid, after which ytterbium is extracted into the solution. The metal is precipitated as oxalate from solution. 

Recently, several kinds of layered compounds have been proposed for use as catalysts these include silicates, graphite, and acid salts of tetravalent metals. These materials can be expected to provide new applications such as shape selective ion-exchangers, adsorbents, and catalysts [1]. 

A synthetic ion-selective (ion-exchange) membrane is a dense, nonporous, mechanically stable polymer film about 0.01 —0.04 cm thick. By nonporosity we mean the absence of pores (possibly very tortuous transmembrane channels) with a typical radius above 5 — KM (10-8 cm). Structurally the membrane material is a cross-linked polyelectrolyte. This latter is a polymer containing chemical groups that while in contact with an aqueous solvent are capable of dissociation into charges which remain fixed to the polymer core and counterions which are free to move in the solution. 

IMPRINTED METAL-SELECTIVE ION EXCHANGER Masahiro Goto... 

ENTROPY-DRIVEN SELECTIVE ION EXCHANGE FOR HYDROPHOBIC IONIZABLE ORGANIC COMPOUNDS (HIOCs)... 

Electrodialysis (ED) is a unit operation for the separation or concentration of ions in solutions based on their selective electromigration through semi-permeable membranes under the influence of a potential gradient (Lacey and Loeb, 1972 Strathmann, 1992). Owing to their selectivity, ion-exchange membranes (IEM) allow transport of only cations (cation-exchange membranes) or anions (anion-exchange membranes) and thus can be used to concentrate, remove, or separate electrolytes. 

Fogg, A. M., Green, Y. M., Harvey, H. G. and O Hare, D. (1999). New separation science using shape-selective ion exchange intercalation chemistry. Adv. Mater. 11, 1466. 

Fogg, A. M., Dunn, J. S., Shyu, S. G., Cary, D. R. and O Hare, D. (1998). Selective ion-exchange intercalation of isomeric dicarboxylate anions into the layered double hydroxide [LiAl2(OH)6)]Cl, H20. Chem. Mater. 10, 351. 

Chabazite has found a wide variety of occasional uses as a cheap desiccant and molecular sieve and as a selective ion exchanger in the nuclear industry. Current interest is in its use as a selective sorbent (in Li form for instance) in PSA and related applications. 

RECOVERY OF MINERALS FROM SEAWATER Table 5 Uranium Selective Ion-Exchange Materials... 

The purpose of this review is to discuss IE kinetics in selected systems using the kinetic model developed in this author s research [26-27,45-50]. It is an attempt to demonstrate, in particular, that phenomenological regularities and criteria describing intraparticle diffusion kinetics for conventional ion exchange are not applicable for the selective ion-exchange systems. 

For all variants of the intraparticle IE process in selective ion exchangers the shape of the concentration distribution pattern in the exchanger bead depends on values of both selectivity (Kr /Krb 1) and diffiisivity (D /Dg > 1) factors (Figs. 3 and 4). The same conclusions with respect to the simultaneous qualitative influence of diffusional transport and the selectivity process in the cation exchange resin bead were reached later through the use of Fick s law and the same approach with respect to the assignment of two ( free and bound ) states of the exchanging of ions in the resin bead [65]. 

Another consequence of the rather limited number of molecular sieves that are used, is a high emphasis is being placed on post synthesis modification of molecular sieves. Especially, the introduction of a secondary pore structure (such as achieved for mordenites [263]), the deactivation of outer crystal surface and the adjustment of the acid strength by selected ion exchange procedures are examples for that approach. 

Here, the species to be separated are ions in solution (mobile phase) and the sorbent or stationary phase is a column of appropriately selected ion exchange resin. Hence ion exchange chromatography is a technique by which mixtures of ions in solution may be separated. Samuelson has presented a thorough survey of practical examples covering a host of elements. 

It is desirable to use the same solution for HPLC mobile phase and for the elution solvent from the cleanup cartridges to assure good reproducibility of the HPLC determination of the penicillins. After the investigation of the selected ion-exchange cartridges using 2 mL of the above-described mobile phase as the elution solvent, Sep-Pak Accell Plus QMA (QMA) produced the best results with all of the penicillins being completely eluted. 

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