Weak acid cation exchangers are mostly polyacrylic resins, generally with carboxyl groups as functional groups. They have a largely selective effect on cations that can be assigned to the anion of carbonic acid, i.e. are strongly dissociated in the water. The main field of application is therefore also decarbonisation, i.e. the removal of hydrogen carbonates from water with the release of carbon dioxide and retention of the hardness ions calcium and magnesium at the exchanger. These exchangers can also be used for the neutralisation of regeneration wastewater.

Strong acid cation exchangers are polystyrene resins with sulfonic acid groups as functional groups. In the Na form, they are used to soften water. The hardness ions are absorbed by the exchanger and exchanged for sodium ions. In the H form, strongly acidic cation exchangers are used for the decationisation of water. They absorb the cations from the electrolyte and release hydrogen ions, so that mineral acids (HCl, H2SO4 and partly HNO3) are freely present in the effluent.

Weakly basic anion exchangers are polyacrylamide or polystyrene resins with a macroporous backbone and tertiary amino groups as functional groups. As a rule, ion exchange does not really take place on weakly basic exchangers, but they accumulate free acids, primarily the strong mineral acids, with protinisation of the functional groups. For this reason, it is common to connect a weakly basic anion exchanger behind a strongly acidic cation exchanger in order to be able to remove the free mineral acids from the water.

Strong base anion exchangers are also polystyrene or polyacrylamide resins with quaternary ammonium groups as functional groups. They can absorb the anions of both weak and strong acids, even from salts, in exchange for hydroxide ions. In addition, there are macroporous resins in the Cl form here, which are also used as adsorbers upstream of the cation stage to remove orgnic substances from water (so-called scavengers).