Polystyrene sulfonates are polymers derived from polystyrene but containing sulfonic acid or sulfonate functional groups. The linear polymer is white when very pure and is water-soluble. The crosslinked material (called a resin) does not dissolve in water and typically appears amber in color. These polymers are classified as  polysalts and ionomers. They are widely used to remove ions from a solution in technical or medical applications.