Ion-Exchange Chromatography of Inorganic Anions

A variety of inorganic anions may be separated using the stationary phases described in Section 3.3.1. They comprise  

Large ions, for example, iodide, exhibit a very strong affinity toward the stationary phase of an anion exchanger. In these large ions, the hydration enthalpy is partly counterbalanced by the cavity formation energy. Such ions are termed polarizable anions in the following their chromatography is discussed separately. 

In addition to the ionic radius in the hydrated state, the valency of an ion is another solute-specific property that affects retention. In general, retention is shifted forward with increasing valency. Thus, the monovalent nitrate elutes prior to the divalent sulfate. Exceptions are multivalent ions such as orthophosphate, where the retention depends on the eluent pH due to different dissociation equilibria. However, the size of an ion often influences the retention more strongly than the valency. Hence, the divalent sulfate elutes prior to the monovalent, but strongly polarizable, thiocyanate. 

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