Cation Resin is used for the removal of calcium and magnesium ions which are the hardness(Scale) forming constituents of water. When water is passed over an ion exchange bed in a softening process, Ca2+ and Mg2+ ions are exchanged for the equivalent Na+ ions. The water obtained contains only sodium salts, which do not produce boiler scales since they are readily soluble in water. This process is used for scale prevention in Low and Medium Pressure boilers. After every service cycle the resin is back washed with water followed by regeneration with 5-15% of NaCl solution.

In this application, cation resins are used to replace the magnesium and calcium ions found in hard water with sodium ions. When the resin is fresh, it contains sodium ions at its active sites. When in contact with a solution containing magnesium and calcium ions (but a low concentration of sodium ions), the magnesium and calcium ions preferentially migrate out of solution to the active sites on the resin, being replaced in solution by sodium ions. This process reaches equilibrium with a much lower concentration of magnesium and calcium ions in solution than was started with.

The cation exchange resin can be recharged by washing it with a solution containing a high concentration of sodium ions(e.g. it has large amounts of common salt (NaCl) dissolved in it). The calcium and magnesium ions migrate off the resin, being replaced by sodium ions from the solution until a new equilibrium is reached.

This is the method of operation used in dishwashers that require the use of 'dishwasher salt'. The salt is used to recharge an resin cation which itself is used to soften the water so that limescale deposits are not left on the cooking and eating utensils being washed.