Anion Exchange Chromatography of Inorganic Anions

A variety of inorganic anions may be separated using the stationary phases described in Section 3.4. These are as follows  

and IO3 Oxygen-containing phosphorus compounds  

When classifying these anions, it is important to remember that water, unlike other liquids, exhibits good solvent properties for salts because of its specific structure and the special interaction mechanism between the ion and the water molecule. When an ion is solvated by water, hydrogen bonds are broken (cavity effect) and the water structure is destroyed. The larger the ion, the higher the energy required for the formation of a cavity with molecular dimension. On the other hand, electrostatic ion-dipole interactions occur that lead to the formation of a new structure. Thus, the smaller the ionic radius and the higher the 

Large ions such as iodide exhibit a very strong affinity toward the stationary phase of an anion exchanger. In these ions, the hydration enthalpy is partly counterbalanced by the cavity formation energy. These anions are called polarizable their chromatography is discussed separately. 

The polarizability of an ion is directly related to the ionic radius in the hydrated state it is one of the solute-specific properties that determines the affinity of an ion toward the stationary phase. In general, the retention time increases with increasing ionic radius in the hydrated state and thus, with stronger polarizabihty. Accordingly, halide ions elute in the orden fluoride chloride bromide iodide. The retention time difference between bromide and iodide is already so large that the set of halide ions can be analyzed only in a single run by using special eluents or stationary phases. 

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 increases 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 eluant pH (the pH influences the 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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Anion-Exchange Chromatography of Inorganic Anions Il85... 

Anion-Exchange Chromatography of Inorganic Anions Table 3 3 Comparison of theoretical and measured values for the quotient y/x. 

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