Table 16.1 shows the displacement series for ion exchange materials. When an ion species high in the table is in solution, it can displace ion species in the insoluble material below it in the table and, thus, be removed from solution. As noted in this table, to remove any cation in solution, the displaceable cation must be the proton H+; and to remove any anion, the displaceable anion must be the hydroxyl ion OH-.
Originally, natural and synthetic alumino silicates, called zeolites, were the only ones used as exchange materials. Presently, they have been largely replaced by synthetic resins. Synthetic resins are insoluble polymers to which are added, by certain chemical reactions, acidic and basic groups called functional groups. These groups are capable of performing reversible exchange reactions with ions in solution. The total number of these groups determines the exchange capacity of the exchange material, while the type of functional group determines ion selectivity. When the exchange capacity of the exchange material is exhausted, the exchanger may be regenerated by the reverse reactions above. The principles of regeneration are discussed in the section on "Sodium, Hydrogen Cycle, and Regeneration."
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