Water Softner Resin
In this application, Water Softner Resin 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 Water Softner 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 Water Softner Resin which itself is used to soften the water so that limescale deposits are not left on the cooking and eating utensils being washed.