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Purified water is water from any source that is physically processed to remove impurities. Distilled water and deionized water have been the most common forms of purified water, but water can also be purified by other processes including reverse osmosis, carbon filtration, microporous filtration, ultrafiltration, ultraviolet oxidation, or electrodialysis. In recent decades, a combination of the above processes have come into use to produce water of such high purity that its trace contaminants are measured in parts per billion (ppb) or parts per trillion (ppt). Purified water has many uses, largely in science and engineering laboratories and industries, and is produced in a range of purities.
 Methods of purifying water
Distilled water is often defined as bottled water that has been produced by a process of distillation and has an electrical conductivity of not more than 10 µS/cm and total dissolved solids of less that 10mg/L. Distillation involves boiling the water and then condensing the steam into a clean container, leaving most solid contaminants behind. Distillation produces very pure water but also leaves behind a leftover white or yellowish mineral scale on the distillation apparatus, which requires that the apparatus be frequently cleaned. Distillation does not guarantee the absence of bacteria in drinking water; unless the reservoir and/or bottle are sterilized before being filled, and once the bottle has been opened, there is a risk of presence of bacteria
For many applications, cheaper alternatives such as deionized water are used in place of distilled water.
Double-distilled water (abbreviated “ddH2O”, “Bidest. water” or “DDW”) is prepared by double distillation of water. Historically, it was the de facto standard for highly purified laboratory water for biochemistry and trace analysis until combination methods of purification became widespread.
Deionized water which is also known as demineralized water (DI water or de-ionized water; can also be spelled deionised water, see spelling differences) is water that has had its mineral ions removed, such as cations from sodium, calcium, iron, copper and anions such as chloride and bromide. Deionization is a physical process which uses specially-manufactured ion exchange resins which bind to and filter out the mineral salts from water. Because the majority of water impurities are dissolved salts, deionization produces a high purity water that is generally similar to distilled water, and this process is quick and without scale buildup. However, deionization 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. Deionization can be done continuously and inexpensively using electrodeionization.
Deionization does not remove the hydroxide or hydronium ions from water. These are the products of the self-ionization of water to equilibrium, so removing them would lead to the removal of the water itself.
 Other processes
Other processes are also used to purify water, including reverse osmosis, carbon filtration, microporous filtration, ultrafiltration, ultraviolet oxidation, or electrodialysis. These are used in place of, or in addition to the processes listed above.
 Laboratory use
Water quality “norms” for purified water have been established by a number of professional organizations, including the American Chemical Society (ACS), the American Society for Testing and Materials (ASTM), the National Committee for Clinical Laboratory Standards (NCCLS) which is now CLSI, and the U.S. Pharmacopeia (USP). The ASTM, NCCLS, and ISO 3696 classify purified water into Grade 1-3 or Types I-IV depending upon the level of purity. These organizations have similar, although not identical parameters for highly purified water.
Regardless of which organization’s water quality norm is used, even Type I water may require further purification depending upon the specific laboratory application. For example, water that is being used for molecular biology experiments needs to be DNase or RNase-free, which requires special additional treatment or functional testing. Water for microbiology experiments needs to be completely sterile, which is usually accomplished by autoclaving. Water used to analyze trace metals may require elimination of trace metals to a standard beyond that of the Type I water norm.
|ISO 3698 (1987)||ASTM (D1193-91)||NCCLS (1988)||Pharmacopoeia|
|Contaminant||Parameter||Grade 1||Grade 2||Grade 3||Type I*||Type II**||Type III***||Type IV||Type I||Type II||Type III||EP||USP|
|Ions||Resistivity at 25 °C/MΩ•cm||10||1||0.2||18.0||1.0||4.0||0.2||>10||>1||>0.1||>0.23||>0.77|
|Conductivity at 25 °C/μS•cm-1||0.1||1.0||5.0||0.056||1.0||0.25||5.0||<0.1||<1||<10||<4.3||<1.3|
|Acidity/Alkalinity||pH at 25 °C||–||–||5.0-7.5||–||–||–||5.0-8.0||–||–||5.0-8.0||–||–|
|Organics||Total Organic Content/p.p.b.||–||–||–||100||50||200||–||<50||<200||<1000||<500||<500|
The highest grades of ultrapure water should not be stored in glass or plastic containers because these container materials leach (release) contaminants at very low concentrations. Storage vessels made of silica are used for less demanding applications and vessels of ultrapure tin are used for the highest purity applications.
 Non-laboratory uses
Distilled or deionized water are commonly used to top up lead acid batteries used in cars and trucks. The presence of foreign ions commonly found in tap water will cause a drastic reduction in an automobile’s battery lifespan.
Distilled or deionized water is preferable to tap water for use in automotive cooling systems. The minerals and ions typically found in tap water can be corrosive to internal engine components, and can cause a more rapid depletion of the anti-corrosion additives found in most antifreeze formulations. Distilled or deionized water is especially important in automotive hybrid system component cooling systems, mixed with hybrid system coolant, to prevent corrosion and/or electrolysis of hybrid components.
Using distilled water in steam irons for pressing clothes, as well as other appliances such as humidifiers and cigar humidors which boil water, can reduce mineral scale build-up and help the appliance last longer. However, many iron manufacturers say that distilled water is no longer necessary in their irons.
Purified water is used in freshwater and marine 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 very often used as an “ingredient” in many cosmetics and pharmaceuticals where it is sometimes referred to as “aqua” on product ingredient labels; see International Nomenclature of Cosmetic Ingredients. This use again owes to its lack of potential for causing undesired chemical reactions due to impurities.
Because of its high relative dielectric constant (~80), deionized water is also used (for short durations) as a high voltage dielectric in many pulsed power applications, such as Sandia National Laboratories‘ Z Machine.
Purified water 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. Also, the low conductance leads to less risk of electrical damage in the event of a leak or spillage. This enables the machine to work at optimal efficiency even after extensive periods of time without water exchange.
A recent use of purified 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 has also recently found a use in an up to date version of water fog fire extinguishing systems. Such systems have been 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 35 MPa (350 bar; 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 (when deionized) and may not damage sensitive equipment, not already damaged by fire. Deionized water, however, is inherently acidic and contaminants such as copper, dust, stainless and carbon steel and many other common materials rapidly supply ions thus re-ionizing deionized water. The very lack of ions make deionized water unusually corrosive and one of the most aggressive solvents known. It is not generally considered acceptable to spray water on electronic circuits that are in use or with power applied. It is wise to avoid mixing water and electricity. 
 Health effects
Distillation removes all minerals from water, and the membrane methods of reverse osmosis and nanofiltration remove most to all minerals. This results in demineralized water which is not considered ideal drinking water. The World Health Organization investigated the health effects of demineralized water in 1980, and its experiments in humans found that demineralized water increased diuresis and the elimination of electrolytes, with decreased serum potassium concentration. Magnesium, calcium, and other minerals in water can help to protect against nutritional deficiency. Demineralized water may also increase the risk from toxic metals because it more readily absorbs materials and minerals such as calcium and magnesium can help prevent absorption of lead and cadmium. Recommendations for magnesium have been put at a minimum of 10 mg/L with 20-30 mg/L optimum; for calcium a 20 mg/L minimum and a 40-80 mg/L optimum, and a total water hardness (adding magnesium and calcium) of 2 to 4 mmol/L. At water hardness above 5 mmol/L, higher incidence of gallstones, kidney stones, urinary stones, arthrosis, and arthropathies have been observed.
Water filtration devices are becoming increasingly common in households. Most of these devices do not distill water, though there continues to be an increase in consumer-oriented water distillers and reverse osmosis machines being sold and used. Municipal water supplies often add or have trace impurities at levels which are regulated to be safe for consumption. Much of these additional impurities, such as volatile organic compounds, fluoride, and an estimated 75,000+ other chemical compounds are not removed through conventional filtration; however, distillation and reverse osmosis eliminate nearly all of these impurities.
The drinking of purified water has been both advocated and discouraged for health reasons. Purified water lacks minerals and ions, such as calcium, which are normally found in potable (drinking) water, and which have important biological functions such as in nervous system homeostasis. Some percentage of our daily consumption of these minerals and ions come from our drinking water, but most of them come from the food we eat, making DI water perfectly fine to drink if one has food in his or her system. The lack of naturally-occurring minerals in distilled water has raised some concerns. The Journal of General Internal Medicine published a study on the mineral contents of different waters available in the US. The study found that “drinking water sources available to North Americans may contain high levels of calcium, magnesium, and sodium and may provide clinically important portions of the recommended dietary intake of these minerals”. It encouraged individuals to “check the mineral content of their drinking water, whether tap or bottled, and choose water most appropriate for their needs”. Since distilled water is devoid of minerals, supplemental mineral intake through diet is needed to maintain proper health.
The consumption of “hard” water (water with minerals) is associated with beneficial cardiovascular effects. As noted in the American Journal of Epidemiology, consumption of hard drinking water is negatively correlated with atherosclerotic heart disease. Since distilled water is free of minerals, it will not have these potential benefits.
Purified water may have little to no fluoride, as pitcher or faucet-mounted filters do not alter fluoride; the more-expensive reverse osmosis filters remove 65%–95% of fluoride, and distillation filters remove all fluoride.
It has been suggested that, because distilled water lacks fluoride ions that are added by a minority of governments (e.g., municipalities in the United States) at water treatment plants using sodium hexafluorosilicate or hexafluorosilicic acid for their effect on the inhibition of cavity formation, the drinking of distilled water may increase the risk of tooth decay. However, many medical authorities argue that the effects of fluoridation on teeth are topical (brushed on) rather than systemic (swallowed).  Also the benefit of fluoride supplementation, for any purpose, should be cautioned due to potential negative effects to IQ and motor functions.
The costs associated with water distillation have generally been prohibitive. However, distilling water with solar water distillers is becoming increasingly popular around the world; they can be relatively simple to design and build.
 See also
- Hydrogen production
- Ionized water
- Water ionizer
- Atmospheric water generator (Make distilled water from air)
- Heavy water
- Water softening
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