Ion exchangers which can be employed in the invention include the commercially available types formed from synthetic resins, such as polystyrene or polyacrylate resins (for example styrene/divinylbenzene copolymers or acrylate/divinylbenzene copolymers) having the following conventional fixed ions: --SO3 - ions for strongly acidic cation exchangers, --COO- ions for weakly acidic cation exchangers,