Detailed Explanation of the Functional Group Characteristics of Ultrapure Water Cation Exchange Resins

Functional Groups

Ion exchange resins contain numerous charged fixed ions and counterions bonded to the fixed ions via ionic bonds on their macromolecular carbon chains. These counterions dissociate into free ions in the electrolyte solution, allowing them to exchange with other similar ions. For example, hydrogen cation exchange resins (R-H) can electrolyze to produce hydrogen ions, which can exchange with various surrounding metal cations. Hydroxyl cation exchange resins (R-OH) can electrolyze to produce OH⁻, which can exchange with various surrounding anions. Based on the different functional groups, ion exchange resins can be classified into cation exchange resins, anion exchange resins, and other types of exchange resins.

Cation Exchange Groups

Cation exchange resins have acidic groups such as sulfonic acid groups, carboxylic acid groups, or benzoic acid groups attached to the stereopolymer. These acidic groups can ionize to produce hydrogen ions in the electrolyte solution. Using "R" to represent the carbon chain portion of the resin polymer, cation exchange resins can be represented as R-SO₃H, R-COOH, and R-OH. Their ionization process in electrolyte solutions can be represented as:

R－SO3H-----R－SO3－+H+ R-COOH――R－COO－+H+

Resins containing sulfonic acid groups have acidity equivalent to sulfuric acid and hydrochloric acid, and are called strong acid resins, similar to solidified sulfuric acid. These resins are highly hygroscopic and are broad-spectrum cation exchange resins, capable of exchanging cations in alkaline and acidic solutions. Commercially available strong acid cation exchange resins are mostly sodium salt type, requiring pretreatment with hydrochloric acid or sulfuric acid before use to convert them to the hydrogen form, allowing them to exchange with metal cations or other cations in water as H+.

Resins with functional groups such as carboxyl and phosphate groups are not easily ionized, and their acidity is equivalent to organic acids such as formic acid and acetic acid; these are called colic acid cation exchange resins. Some acid resins can only dissociate in neutral or alkaline solutions. In water treatment desalination processes, they can adsorb cations corresponding to bicarbonate, such as CA2+, Mg2+, and Na+, while simultaneously releasing carbonic acid.