Britannica Sources used in this answer

How do fixed negative charges in a resin exchange positive ions?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

chemical structure of cation exchanger — ion-exchange reaction Figure 1: Chemical structure of cation exchanger. The exchangeable ions are marked ⁺. The whole structure is permeated by solvent molecules, usually water (not shown).

In an ion-exchange resin, fixed negative charges facilitate the exchange of positive ions through a process known as cation exchange.1

The mechanism relies on two primary requirements:

Fixed Ionic Charges: The resin consists of a porous, three-dimensional polymeric network containing fixed, negatively charged groups—such as sulfonic or carboxylic acid salts—attached to its molecular framework.21
Permeability: The resin must be permeable to the surrounding solution. When the resin is immersed in a solution, it absorbs the liquid and swells, allowing small, mobile positive ions (cations) to move into the interstices of the lattice structure.21

Because the fixed negative charges are part of the resin's solid framework, they attract and hold mobile positive ions to maintain electrical neutrality. When the resin is in contact with a solution, these mobile positive ions can change places with other positive ions present in the liquid, effectively swapping them based on the chemical equilibrium of the system.1