ION EXCHANGE RESIN 101

Ion exchange is a reversible chemical reaction wherein an ion (an atom or molecule that has lost or gained an electron and thus acquired an electrical charge) from solution is exchanged for a similarly charged ion attached to an immobile solid particle. These solid ion exchange particles are either naturally occurring inorganic zeolites or synthetically produced organic resins. The synthetic organic resins are the predominant type used today because their characteristics can be tailored to specific applications.

An organic ion exchange resin is composed of high-molecular-weight polyelectrolytes that can exchange their mobile ions for ions of similar charge from the surrounding medium. Each resin has a distinct number of mobile ion sites that set the maximum quantity of exchanges per unit of resin.

Ion exchange is a simple process where an organic molecule has been substituted with acid or base exchange sites. As a solution with exchangeable ions passes through the resin, the ions are captured and exchanged for the ions on the resin. For example, some weak acid ion exchange resins can use the carboxylic acid radical, in the sodium form, R-COO-Na as the exchange site. As solutions of metal ions such as Magnesium (Mg), Calcium (Ca), or Copper (Cu) pass through the resin, they are exchanged for the Na (Sodium) at that site. The molecule would now appear like this R-COO-Ca, for a Calcium exchange.

R-COO-Na + Ca+2 = R-COO-Ca + 2 Na+

These resins are used in water softeners where the alkaline earth metals such as Calcium and Magnesium are exchanged for Sodium. In a water softener Sodium Chloride (salt, NaCl), is used to "regenerate" the resin. The salt solution is passed through the resin in very high concentrations and the sodium displaces the metals attached to the active sites. When the concentration of sodium is low, the sites prefer the alkaline metals again and the resin can be used to "soften" water again.