Ion Exchange
What is Ion Exchange?
Ion Exchange (IX) in laboratory water purification is a process of deionisation in which the water is passed through one or more beds of ion-exchange beads where impurity ions in water are taken up on the beads and replaced by hydrogen and hydroxyl ions, purifying the water.
What does Ion Exchange remove from water?
IX removes impurity ions from water. It is highly effective and can reduce ionic levels in the product water to sub-ppt levels. However, the beds have a finite capacity and when they are nearly fully used they start to release the weakly held ions. IX beds are also used after exposure of water to short-wavelength UV light to remove the charged organic molecules produced and reduce Total Organic Carbon (TOC) levels to low ppb concentrations.
How does Ion Exchange work?
IX resins used in water purification are sub-mm porous beads of highly cross-linked insoluble polymers with large numbers of strongly ionic exchange sites. As water is passed through the resin bed ions in solution migrate into the beads where they compete for exchange sites. Deionisation beads are either cationic or anionic and exchange, respectively, either hydrogen ions for cations or hydroxyl ions for anions. The resultant ions combine to produce water. To achieve the lowest levels of ionic impurities the anion and cation beads are mixed together. When most of the ion-exchange capacity of the resins has been used up the resins have to be replaced (or, in large systems, regenerated in-situ).
What are the benefits of Ion Exchange?
Mixed-bed IX is by far the most effective way of removing the last traces of ionic contamination from water including charged species allowing the highest levels of purity (and water resistivity) to be achieved. Modern high purity resins rinse-up rapidly in use to achieve low background levels of organic contaminants.
How does ELGA LabWater use Ion Exchange?
ELGA uses the highest purity IX resins. In ELGA lab systems, typically, IX forms part of a 'polishing' loop including UV and filtration, through which water is repeatedly circulated to maintain quality. UV light is used to photo-oxidise organic impurities, enabling their removal by IX. Prior removal of most ions and large organic molecules by reverse osmosis is used to optimize the effectiveness of IX. To optimise IX performance and maximise resin bed life ELGA uses the PureSure system in which the IX bed is split into two packs and the resistivity after the first pack is monitored. This ensures that the ionic content of the product water is maintained. With this approach a PURELAB Chorus 1 can produce water with a resistivity of 18.2 MΩ.cm, total ionic impurity levels <1ppb and a TOC of <3ppbC.
