Distilled or Deionised Water? What's the difference?

Many laboratory staff ask for purified water and use the terms distilled and deionised interchangeably.  However the actual products are different and are produced differently.

Most commonly now deionised water is supplied when people ask for purified water.

Purified water is water that is mechanically filtered or processed to be cleaned for consumption. Distilled water and deionised (DI) water have been the most common forms of purified water, but water can also be purified by other processes including Reverse osmosis, carbon filtration, microfiltration, ultrafiltration, ultraviolet oxidation, or electrodialysis.

Distilled water is produced by a process of distillation and has an electrical conductivity of not more than 11 µS/cm and total dissolved solids of less than 10 mg/litre.  Distillation involves boiling the water and then condensing the vapour into a clean container, leaving solid contaminants behind. Distillation produces very pure water. A white or yellowish mineral scale is left in the distillation apparatus, which requires regular cleaning. Distillation alone does not guarantee the absence of bacteria in drinking water unless containers are also sterilized. For many procedures more economical alternatives are available such as deionised water and, is used in place of distilled water.

Double distillation - Double-distilled water is prepared by double distillation of water. Historically, it was the de facto standard for highly purified laboratory water for biochemistry and, by the method of trace analysis until combination methods of purification became widespread.

A water still (Stuart Merit W4000)

Deionisation - Deionised water, also known as demineralised water, is water that has had its mineral ions removed, such as cations like sodium, calcium, iron, and copper, and anions such as chloride and sulfate. Deionisation is a chemical process that uses specially manufactured ion-exchange resins which exchange hydrogen ion and hydroxide ion for dissolved minerals, which then recombine to form water. Because the majority of water impurities are dissolved salts, deionisation produces a high purity water that is generally similar to distilled water, and this process is quick and without scale buildup. However, deionisation does not significantly remove uncharged organic molecules, viruses or bacteria, except by incidental trapping in the resin. Specially made strong base anion resins can remove Gram-negative bacteria. Deionisation can be done continuously and inexpensively using electrodeionisation.

Purite Labwater Deioniser

Outside of the laboratory deionised or distilled water is used to top us lead-acid car batteries although many units are now sealed.  Purified water is also used in freshwater and marine aquariums. As it doesn't contain impurities such as copper and chlorine, it helps to keep fish free from diseases and avoids the build-up of algae on aquarium plants due to its lack of phosphate and silicate.

Deionised water is available from P&R Labpak in small through to large containers!  We can also supply equipment if you need to make your own.

To read more on water:-
https://en.wikipedia.org/wiki/Deionised_water
http://www.prlabs.co.uk/lab-supplies.php?N=ANALYST-40-WITH-BOOST-PUMP&Id=40933
http://www.prlabs.co.uk/lab-supplies.php?N=STILL-MERIT-MODEL-W4000&Id=45943