Nature of non-exchanging ions:
After getting an idea about exchanging ions, let us now consider the nature of non- exchanging ions also. A nature of non-exchanging or co-ions has little effect on selectivity unless they tend to form complexes or slightly ionized molecules with the exchanging ion. A significant example in this respect is that of ferric ion in concentrated solution of chloride ions. In this case, ferric ion has a tendency to exist as FeCl4- (a complex ion) which is strongly held by a quaternary ammonium anion exchange resin. The iron can be removed from the resin by rinsing with water or dilute acid since the complex breaks down when chloride ions are not present in high concentration. Such examples are numerous in the literature and are successfully exploited for various metal ion separations.