Ionic forms of resin:

It should be borne in mind that the selectivity of an ion exchange resin changes, usually decreases, as the resin is converted to that particular ionic form. In most cases, this effect is slight. The selectivity coefficient holds for one specific composition of the resin. The ionic form is usually expressed as the mole fraction of the resin which in the A forms at equilibrium and is designated as X_A. In general, the greater is the value of (K) A, the greater is its change from X_A.