Cation position in ion-exchanged

Analysis of Cation Position in Ion-Exchanged Y Zeolites by 23Na NMR... 

The sites adopted by cations in these structures are complex and will not be considered here but, broadly, an increase in Si/Al (decrease in framework charge) depopulates Sites I and T in the hydrated zeolites in favor of positions in the large cage. Removal of water causes an increase in population of the Sites I and T regardless of framework charge. Details of individual cation sitings in ion-exchanged forms of X and Y are available, with water molecule positions, where known. 

The first selection in ion-exchange methods development is to choose either cation or anion exchange, which is decided by the charge on the analyte (Fig. 6.8). First, check the pAT of the solute and determine whether it can be a positive, negative, or a neutral species. Examples of several different compounds are shown in Figure 6.8. Compounds such as paraquat or diquat (herbicides) are always protonated thus, they will always bind by cation exchange. On the other hand, the triazine, prometryn, has a pAT of 4.8 thus, the molecule will be 50% protonated at pH 4.8, and totally protonated at pH 2.8. At pH 6.8 it is a neutral species. Thus, it is possible to isolate this compound effectively by cation exchange only when the pH is acidic. 

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