Ultrapure Water Process: Detailed Explanation of Anion Exchange Groups

Anion Exchange Groups

Anion exchange resins contain primary, secondary, tertiary, and quaternary ammonium functional groups linked to stereopolymers. These groups dissociate in electrolyte solutions to release OH⁻ ions, which then exchange with anions. Primary ammonium anion resins can dissociate and exchange ions across almost any pH range and are called strongly basic anion resins. Anion resins containing trimethyl quaternary ammonium groups belong to Type I; those containing dimethylethanolamine quaternary ammonium groups belong to Type II. Strongly basic anion resins can completely adsorb small amounts of acids such as carbonic acid and silicic acid, and can release bases from salts independently, as shown in the following reaction:

R＝N⁻OH + NaCl ------ R==N⁻Cl + NaOH

Type I strongly basic anion resins are more basic than Type II resins, and their heat resistance, oxidation resistance, lifespan, and strength are all superior to Type II; however, Type II resins have better regeneration efficiency and antifouling ability than Type I resins, requiring less regenerant. Strongly basic anion exchange resins are not very sensitive to hydrolysis; the anions originally adsorbed in the resin will not be displaced by pure or ultrapure water. Commercially available strongly basic anion exchange resins are generally in the chloride form (R=N-Cl), requiring pretreatment with NaOH solution before use to convert them to R=N-OH, allowing them to exchange with other anions in the solution as OH-.

Primary, secondary, and tertiary amine anion exchange resins, due to their lower basicity, are called slightly basic anion exchange resins. They cannot adsorb very weak acids such as carbonic acid. Weakly basic anion exchange resins are more sensitive to hydrolysis; the anions originally adsorbed can be displaced by pure or ultrapure water:

R==NH₂Cl + H₂O ---- R=NH₂OH + HCl

Weakly basic anion exchange resins cannot undergo ion exchange with neutral salts of strong acids and strong bases, but they can undergo neutralization and double displacement reactions with salts of strong acids or strong acids and weak bases:

R==NH₂OH + HCl --- R=NH₂Cl + H₂O