Deionized water
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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.
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[edit] Properties
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 speciallized 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.
[edit] pH values
In theory, deionised water doesn't have a pH value, but in practice, it is usually considered by convention to be pH 7.0. pH is a logarithmic measurement of relative ion presence. Since there are no ions, there is nothing to measure. In practice, both chemical pH measuring systems and electronic pH meters will indicate a pH value. 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 indicate a random value since there is no conductance path to the electrode, but they should not be immersed in deionised water as the lack of any ions 'sucks' them out of the electrode degrading its performance.
Deionized water will quickly acquire a pH while in storage. Carbon dioxide, present in the atmosphere, will dissolve into 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 absence of a pH value.
[edit] Ultrapure deionized water
The uses of ultrapure deionized water are many and varied, often having application 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.
[edit] Uses
DI water is 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.
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 deionised water is in window cleaning, where window cleaners use pumped systems to brush and rinse windows with deionised water again without leaving any spots.
Deionized water is used in freshwater aquariums. Since it does not contain inpurities such as copper and chlorine, it keeps fishes 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 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 (5000 p.s.i.) 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. 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.
[edit] 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 micron 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.
[edit] See also
[edit] External links
- Deionized Water (Physics Van QA Forum)
