We humans are not the only ones who are picky: When an ion exchanger preferentially absorbs certain ions, we speak of SELECTIVITY. This would be the case with a cation exchanger, for example, since it only exchanges cations and not anions. But it's even more demanding: A cation exchanger prefers polyvalent ions to monovalent ones, which means that it binds ions that have multiple positive charges (for example calcium/Ca²+) more firmly than monovalent ions (potassium/K⁺).