Cation exchange resins (R-H ): These resins are mainly styrene divinyl benzene copolymers, which on sulphonation or carboxylation, become capable to exchange their hydrogen ions with the cations in the water. Such resins have acidic functional groups like SO3H, - COOH, - OH (phenolic) capable of exchanging the cationic portion of minerals by their hydrogen ions and hence they are termed as cation exchangers. Ex: Amberlite IR-120,Dowex 50