• 2025-07-09

Active groups (ion exchange functional groups)

Some articles state that "lively metals in the lower left corner" (such as Na/K) are unrelated to the functional groups of the resin. In practical applications:

Active groups (ion exchange functional groups)

Active groups refer to the non-movable functional groups immobilized in ion exchange resins through covalent bonds, with the following characteristics:

Structural features

Cationic resins: Strong acid type sulfonic acid group (—SO₃⁻), weak acid type carboxyl group (—COO⁻)

Weakly basic anion exchange resin: quaternary ammonium type (—N⁺(CH₃)₃), weakly basic amine type (—NH₂)

Electrochemical behavior

Bound to exchangeable counter-ions (such as H⁺/Na⁺/Cl⁻) through electrostatic attraction

Maintains charge neutrality in accordance with the Donnan equilibrium principle

Ion exchange selectivity rules

Resin type Active group pKa value Cation selectivity order Anion selectivity order

Strong acid cation resin <1.0 Fe³⁺＞Al³⁺＞Ca²⁺＞Mg²⁺＞K⁺＞Na⁺ -

Strong base anion resin >13.0 - SO₄²⁻＞NO₃⁻＞Cl⁻＞HCO₃⁻

Technical Key Points

■ Correction of Element Reactivity Misconceptions

Exchange potential of sodium-type strong acid resin (—SO₃Na): Ca²⁺ (selection coefficient 5.2) > Na⁺ (1.0)

Hydrogen-type weak acid resin (—COOH) ionizes only when pH > 6

■ Industrial resin parameters

Exchange capacity: ≥4.2 mmol/g (strong acid resin GB/T 13659-2017)

Water content: 45-55% (crosslinking degree 7% DVB)

Particle size range: 0.3-1.2mm (uniformity coefficient ≤1.6)