Cationic resins are divided into two; Strong acid and weak acid resins.
Strong acid resins
Strong acid ones exchange cations and can operate at any pH.
It is mainly used in water softening applications as the first stage of deionization in softeners or multimedia filters. It removes cations from water and requires a large amount of regenerant. (Commonly hydrochloric acid [HCl]).
Weak acid resins
On the other hand, weak acid cation resins have a lower exchange capacity, they are not functional at low pH, they swell and contract constantly so they increase load losses or cause breakage in the tanks when they do not have enough space inside. However, it is a very efficient resin that needs less acid for its regeneration and it is common to regenerate them with the waste acid coming from the strong acid resins.