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Deionized water

Deionized water (DI water or de-ionized water; also spelled deionised water, see spelling differences) is water that lacks ions, such as cations from sodium, calcium, iron, copper and anions such as chloride and bromide. This means it has been purified from all other ions except H3O+ and OH, but it may still contain other non-ionic types of impurities such as organic compounds. This type of water is produced using an ion exchange process. Deionized water is similar to distilled water, in that it is useful for scientific experiments where the presence of impurities may be undesirable.



The lack of ions causes the water's resistivity to increase. Ultra-pure deionized water can have a theoretical maximum resistivity up to 18.31 MΩ·cm, compared to around 15 kΩ·cm for common tap water. Deionized water's high resistivity allows it, in some very highly specialized instances, to be used as a coolant in direct contact with high-voltage electrical equipment. Because of its high relative dielectric constant (~80), it is also used (for short durations) as a high voltage dielectric in many pulsed power applications, such as Sandia's Z Machine.

pH values

The pH is a logarithmic measurement of proton presence; the true pH of deionized water is 7.0, because the ionization constant of water (Kw) ~ 10-14, so p[Kw] = 14, and pH + pOH = p[Kw]

In practice, the indication from chemical indicators can give a value of usually between pH 5.0 and pH 9.0 depending on the indicator used (the indication being the ions introduced by the indicator itself, its solvent and its impurities). Electronic pH meters will output an unpredictable value since the absence of ions in the liquid means that the two parts of the electrode are insulated from each other and thus would generate no EMF. In practice since absolutely pure water is an unattainable goal, the liquid will contain a very small amount of ions, but the current this would allow the probe to generate will be far smaller than that required to operate the metering circuit.

Electrodes of a pH meter should not be immersed in deionised water for prolonged periods as the lack of any ions 'sucks' them out of the electrode degrading its performance. Deionised water should be used for cleaning only rarely as the effect is cumulative. Electrodes should be cleaned using proper cleaning solution (usually very acidic), and rinsed between samples; ideally it should be rinsed using an extract from the next sample to be tested, but failing that, a pH neutral liquid such as tap water or pH 7.0 buffer solution is suitable.

Deionized water will quickly acquire a pH when exposed to air. Carbon dioxide, present in the atmosphere, will dissolve in the water, introducing ions and giving an acidic pH of around 5.0. The limited buffering capacity of DI water will not inhibit the formation of carbonic acid H2CO3. Boiling the water will remove the carbon dioxide to restore the pH to 7.0.

Ultrapure deionized water

The uses of ultrapure deionized water are many and varied, often having applications in scientific experimentation such as when very pure chemical reagent solutions are needed in a chemical reaction or when a biological growth medium needs to be sterile and very pure. Laboratory grade ultra pure water cannot be stored in glass or plastic containers because such materials leach contaminants at very low concentrations into the water. Storage vessels made of silica are used for less demanding applications but for highest purity uses, containers made from ultra pure Tin are used.


Process utilizing specially-manufactured ion exchange resins which remove ionized salts from water can theoretically remove 100% of salts. Deionization typically does not remove organics, virus or bacteria, except through "accidental" trapping in the resin and specially made strong base anion resins which will remove gram-negative bacteria.


DI water is necessary in clinical and medical laboratories, where its addition to various reagents and solvents enable accurate measurements, often on automated machines which use very little specimen (i.e. plasma, serum, etc.)

DI water is also used extensively in the semiconductor industry to process and clean silicon wafers and sometimes in the optics industry when very highly clean optical surfaces are required for coating. DI water is also often used as a final rinse when washing scientific glassware.

Deionized water is very often used as an "ingredient" in many cosmetics and pharmaceuticals where it is sometimes referred to as "aqua" on product ingredient labels. This use again owes to its lack of potential for causing undesired chemical reactions due to impurities.

It can also be used in PC watercooling systems. The lack of impurity in the water means that the system stays clean and prevents a build up of bacteria and algae. This enables the machine to work at optimal efficiency even after extensive periods of time without water exchange.

A recent use of DI water is that of a final rinse in some car washes where, because it contains no dissolved solutes, the car dries without leaving any spots. Another use of deionized water is in window cleaning, where window cleaners use pumped systems to brush and rinse windows with deionized water again without leaving any spots.

Deionized water is used in freshwater aquariums. Since it does not contain impurities such as copper and chlorine, it keeps fish free from diseases, as well as avoiding the build-up of algae on aquarium plants, due to its lack of phosphate and silicate. Deionized water should be re-mineralized before used in aquaria, since it also lacks many macro and micro-nutrients needed by both plants and fish.

Deionized water is also commonly used in Marine (salt water) aquariums. A deionization stage is almost exclusively used as the final stage in a combination reverse osmosis - deionization unit. Use of deionized water helps avoid harmful contaminants and unwanted algae growth.

Deionized water has also recently found a use in an up to date version of water fog fire extinguishing systems. Such systems can be used in sensitive environments such as where high voltage electrical and sensitive electronic equipment is used. The 'sprinkler' nozzles use much finer spray jets and operate at up 350 bar (35 MPa, 5000 psi) of pressure. The extremely fine mist produced takes the heat out of a fire rapidly and the deionized water coupled with the fine droplets is non conducting and does not damage sensitive equipment, not already damaged by fire[citation needed]. The system is perfectly safe to discharge when personnel are present. Apart from getting a little damp, there are no other hazards associated with the system.

Small scale water deionizing for hydrogen production

For small scale production of hydrogen, water purifiers are installed to prevent formation of minerals on the surface of the electrodes and to remove organics and chlorine from utility water. First the water passes through a 20 micrometre interference (mesh or screen filter) filter to remove sand and dust particles, second, a charcoal filter (activated carbon) to remove organics and chlorine, third stage, a de-ionizing filter to remove metallic ions. A test can be done before and after the filter for proper functioning on barium, calcium, potassium, magnesium, sodium and silicon.

Another used method is reverse osmosis. Another method still is using a combination of ion exchange resins with ordinary water.

See also

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Deionized_water". A list of authors is available in Wikipedia.
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