Calcium exchange resin
Water softening calcium exchange resin
In this application, ion-exchange resins are used to replace the magnesium and calcium ions found in hard water with sodium ions. When the calcium exchange 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. calcium exchange resin could reduce the content of calcium ion in water.
The 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 from the resin, being replaced by sodium ions from the solution until a new equilibrium is reached. The salt is used to recharge an ion-exchange resin, calcium exchange resin itself is used to soften the water.