Ion Exchange In Mixed Aqueous-Organic Media:

Up to this point, you may be carrying an impression that ion exchange takes only when the aqueous solution is brought in contact with the solid exchanger. There must be ions in both the solution and the solid. The ions must be free to move and the exchange takes place. With water as a solvent and along with solids which qualify as an ion exchangers, these conditions are usually met. Water, because of its high dielectric constant, is an excellent solvent for most inorganics and quite a number of organic acids, bases and salts. This is what justifies the choice of water as a phase for ion exchange. Water, however, is by no means the only solvent which allows ion exchange to take place. There are other solvents along with high dielectric constant (ε) in which electrolytes can dissolve and dissociate and in which most of the ion exchangers are stable. These are ethylene glycol (ε = 41), methanol (ε =32), ethanol (ε = 26) and acetone (ε = 27). A last three solvents, within particular, have importance in ion exchange. They can be used with or without addition of water.

This particular section is devoted to ion exchange in mixed aqueous organic media. It has been observed that some irregular trends in the distribution of metal ions between mixed aqueous- organic media and the ion exchanger are exhibited when the percentage of the organic content is varied. The organic solvents used are water miscible oxygenated compounds like tetrahydrofuran, methanol, ethanol and acetone. The behaviour of the metal ions with varying organic content has been usefully exploited for achieving some difficult separations. Korkisch pioneered this technique and named it as combined ion exchange solvent extraction and coined the abbreviation CIESE for it. In sequence to explain the behaviour, in chromatography using these solutions two processes are operative: ion exchange and liquid-liquid extraction.