Liquid ion-exchange agents

Type 2 (Fig. 19.3-lc). These carry the difliising species across the membrane by incorporating carrier or chelating compounds in the membrane. This kind of canier-mediated transport can be illustrated by the separation of various metal ions, such as cadmium, chromium, copper, and mercuty, from their aqueous solutions by the use of oil-type liquid membranes containing oil-soluble liquid ion-exchange agents. " " These mechanisms have been described in detail elsewhere and are not repeated here. 

LIX is the General Mills trademark for various oil-soluble, liquid ion-exchange agents. "As complex anions. 

Figure 1.6 Schematic examples of carrier facilitated transport of gas and ions. The gas transport example shows the transport of oxygen across a membrane using hemoglobin as the carrier agent. The ion transport example shows the transport of copper ions across a membrane using a liquid ion-exchange reagent as the carrier agent...

A number of organic liquids are known, however, that exhibit extreme selectivi-ties between chemically related species. Perhaps the best known examples are the crown ethers, which form stable complexes with potassium ion but not with sodium ion. Also important are the so-called liquid ion exchangers which can exchange metal ions for hydrogen ions or vice versa, depending on pH. Chemically, these agents are typically oximes, quinolines, or amines. They are frequently highly hydrophobic. 

In commercial electrodes, the liquid ion-exchanger is in a form in which the chelating agent is immobilized in a hydrophobic polymer membrane like poly(vinylchloride) (Pig-ure 2.4.4). Electrodes based on this design (called polymer or plastic membrane ISEs) are more rugged and generally offer superior performance. 

Research should continue on traditional separation methods. For example, there is a continuing need for more selective extraction agents for liquid-liquid and ion-exchange extractions. High-temperature processes that use liquid metals or molten salts as extraction agents should have potential in nuclear fuel reprocessing and... 

A variety of preconcentration procedures has been used, including solvent extraction of metal chelates, coprecipitation, chelating ion exchange, adsorption onto other solids such as silica-bonded organic complexing agents, and liquid-liquid extraction. 

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