The Development of Organic Ion Exchange Resins

The previous chapter dealt with the discovery and properties of aluminosilicate ion exchangers whereby it is immediately apparent that three fundamental requirements have to be met to confer ion exchange properties upon a material  

An inert host structure which allows diffusion of hydrated ions, 

The host structure must carry a fixed ionic charge, termed the fixed ion. 

Electrical neutrality of the structure must be established by the presence of a mobile ion of opposite charge to that of the fixed ion, called the counter-ion. 

With the above listed essential requirements in mind the ideal characteristics of an ion exchange material listed at the outset of Chapter 1 are reviewed and restated as follows  

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It is appropriate to refer here to the development of non-suppressed ion chromatography. A simple chromatographic system for anions which uses a conductivity detector but requires no suppressor column has been described by Fritz and co-workers.28 The anions are separated on a column of macroporous anion exchange resin which has a very low capacity, so that only a very dilute solution (ca 10 4M) of an aromatic organic acid salt (e.g. sodium phthalate) is required as the eluant. The low conductance of the eluant eliminates the need for a suppressor column and the separated anions can be detected by electrical conductance. In general, however, non-suppressed ion chromatography is an order of magnitude less sensitive than the suppressed mode. 

Selective ion electrodes (SIE). Selective ion electrodes are essentially variants of the well-known pH meter. They are membrane indicator types of electrodes in which a potential is developed across a membrane in the presence of the ion the size of the potential is related to the concentration and hence can be used to quantitatively detect and measure the species. However, instead of a glass membrane, as in the pH meter, the membranes consist of organics that are immersible in water. For example, anion-sensitive electrodes use a solution of an anion exchange resin in an organic solvent the liquid can be held in the form of a gel, for example, in polyvinyl chloride. The ion reacts with the organic membrane, setting up an equilibrium between the free ion in solution and the ion bound to the membrane, generating a potential difference, which is measured. 

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