An ion exchange resin is an insoluble matrix (or support structure) normally in the form of small (1-2mm diameter) beads, fabricated from an organic polymer substrate on the surface of which are sites with easily trapped and released ions in a process called ion exchange. There are multiple different types of ion exchange resin which are fabricated to selectively prefer one or several different types of ions.

This resin is used extensively for water softening during water purification. They were developed as a more flexible alternative to the use of natural or artificial zeolites

There are four main types differing in their functional groups:

• strongly acidic (sulfonic acid groups)
• strongly basic, (trimethylammonium groups)
• weakly acidic (carboxylic acid groups)
• weakly basic (amino groups)

There are also specialised types:

• chelating resin

Uses

Water Softening

In this application, ion-exchange 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 sodium ions in solution than was started with.

The resin can be recharged by washing it with a solution containing no magnesium and calcium ions, but 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 ion exchange resin which itself is used to soften the water so that limescale deposits are not left on the cooking and eating utensils being washed.

Water Purification

In this application, ion exchange resins are used to remove poisonous (e.g. copper) and heavy metal (e.g. lead) ions from solution, replacing them with more innocuous ions, such as sodium and potassium. Note that this is why it is not recommended to prepare artificial milk (infant formula) with softened water, as the increased levels of sodium and potassium in the diet can be deleterious to nursing infants.

Ion exchange resins do not remove chlorine or organic contaminants from water - this is usually done by using an activated charcoal filter mixed in with the resin. Domestic water purification is not usually recharged - the resin is discarded when it should no longer be used.

Sources

• http://www.remco.com/ix.htm
• http://www.rohmhaas.com/ionexchange/IP/faq_general.htm