What's the difference between distilled and deionised water?

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What's the difference between distilled and deionised water?
"Pure" water is used in all types of laboratories for a range of uses - from making up calibration standards to cleaning and even in the rinse cycle in some laboratory glasswashers for a pure clean. There are different types of purified water available - but what is the difference? [caption id="attachment_6355" align="aligncenter" width="300"]Distilled or deionised water? What's the difference? Distilled or deionised water? What's the difference?[/caption]

Distilled water

Distilled water is simply made by heating water to evaporation, then re-condensing the vapour into a separate, clean vessel. Many salts dissolved in the water (such as calcium and magnesium salts which cause water hardness and many metals) are not carried over because they have a much higher boiling point than water. Impurities which have a low boiling point (such as some alcohols) could remain in water after is has been distilled if they were present in the source water - though if your source is tap water it is unlikely to contain any high concentrations of such species. Many laboratories produce distilled water in house using a water still. They are simple to operate and offer an easy way for labs that need small, regular amounts of distilled water to self manage rather than purchase it in. Double distillation models are also available to ensure extra purity.

Deionised water

Deionised water has been passed through an exchange resin which exchange charged ions in the water and create H+ and OH- ions, which then recombine to give the H2O.  This method removes ions such as chloride, calcium, potassium, nitrates and sulphates. Any species which are not ionic in the water could remain, though many industrial processes then pass the water through an activated charcoal filter to remove these species also. Laboratories which only need small volumes of deionised water usually buy this by the bottle. Labs which use a large amount of deionised water may use an in house deioniser to produce it on demand - these can range from small units for irregular use up to larger units with storage vessels for bigger sites. Both options offer the flexibility of in house generation with just the standard mains water supply as a feed. If you have any questions about purified water or producing your own distilled or deionised water in house, just contact us
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