Cation Exchange in Aqueous Solution

The cation exchange properties of zeolites give rise to their largest single application in terms of volume, as detergent additives. They are used for the softening of water i.e. the replacement of calcium ions by sodium ions) in domestic 

In discussing the equilibrium between solution and zeolite, the exchange of cation P in the solution for in the zeolite can be described by the equation  

Plotting these values of Z against the analogous equivalent fractions of ions in solution, Sp and Sq, enables the selectivities of binary cation exchange experiments to be displayed, whereas a thermodynamic treatment of Ka can be used to obtain the values of AG°, AH° and AS° for the exchange. 

One additional effect is observed when zeolites are suspended in concentrated salt solutions ( 0.5 M). Under these conditions it becomes thermodynamically favourable for additional inclusion of both cationic and anionic species, in addition to cationic species required for charge balance. This is known as salt imbibition or occlusion. 

Different types of cation exchange behaviour observed in zeolites. Isotherms represent equilibrium plots of the fractional concentration of extra-framework cations in the zeolite compared with the fractional concentration of cations in the solution in contact with the zeolite (see text for discussion). In case (a) cation M is taken in preference over a competing cation type for the entire relative concentration range, whereas the preference is inverted in case (c). The situation where there is no preference is represented by (b). Type (d) isotherms occur when only a certain fraction of the cations may be exchanged (experimentally, kinetic barriers may also result in this behaviour). Finally, isotherms of type (e) indicate that the selectivity changes as the relative concentration of the cations in solution changes. 

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