The affinity of an ion for a charged site on the resin depends on the hydrated radius of the ion. As this radius is smaller for ions with greater charge, the relative affinities depend on the charge or valence as shown:

single-charged ion < ion with 2 charges < ion with 3 charges < multi-charged ion

This is ideal for most practical situations because we can prepare a resin in the form of a monovalent ion such as Na⁺ or H⁺ and exchange for an ion such as Ca⁺⁺, Mg⁺⁺, or Fe⁺⁺⁺ and have the relative affinity in our favor.

Although we might expect that an ion exchange resin that has a great affinity for a particular ion should be the best choice for a process, this is probably not true. Very strong binding means that removal of that ion for regeneration of the resin will be difficult. The concentration required for the regenerating ion and the cost will be less if a resin is selected that is adequate for the task but does not hold the ion too tightly.

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