Purified water
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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, including in science and engineering laboratories and industries, and is produced in a range of purities.
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[edit] Methods of water purification
[edit] Distillation
Distilled water has virtually all of its impurities removed through distillation. Distillation involves boiling the water and then condensing the steam into a clean cup, leaving nearly all of the solid contaminants behind. Distillation produces very pure water but also leaves behind a leftover white or yellowish mineral scale, which requires that the distillation apparatus be frequently cleaned.
For many applications, cheaper alternatives such as deionized water are preferred over distilled water.
[edit] Double-distillation
Double distilled water (abbreviated "ddH2O" or "Bidest. water") 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.
[edit] Deionization
Deionized water which is also known as demineralized water (DI water or de-ionized water; also spelled deionised water, see spelling differences) is water that has had its minerals 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 quickly and without scale buildup. However, deionization does not significantly remove uncharged organic molecules, viruses or bacteria, except through "accidental" trapping by the resin. Specially made strong base anion resins can remove Gram-negative bacteria. Deionization can be done continuously using Electrodeionization.
It should be noted that removal of all ions from water is next to impossible, since water self-ionizes quickly to reach equilibrium.
[edit] 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. Generally, each process is well suited to removing particular impurities while not being as good with other impurities.
[edit] Parameters of purified water
[edit] Resistivity and conductivity
Removal of ions causes water's resistivity to increase, providing a convenient measurement for the exact extent of deionization. Ultrapure deionized water has a theoretical maximum resistivity of 18.31 MΩ·cm and a theoretical minimum conductivity of .0545 microsiemens/cm, compared to around 15 kΩ·cm and 70 microsiemens/cm for 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.
[edit] pH values
The theoretical pH of highly purifed water is 7.0. In practice, however, most purified water will have a pH that is slightly acidic (less than 7.0) due to the presence of dissolved carbon dioxide (CO2) from the atmosphere. Dissolved carbon dioxide reacts slowly with water to give the bicarbonate and hydronium ions.
- CO2 (g) + 2H2O(l) ⇋ HCO3- + H3O+
Note that carbonic acid, H2CO3, is only formed in strongly acid solutions. Distillation temporarily removes dissolved CO2 from the water. However, during condensation the water reabsorbs CO2 again resulting in a pH that is slightly less than 7.0.
[edit] Uses
[edit] Laboratory use
Water quality standards 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 Types I-III depending upon the level of purity. While there is some general consensus across these standards, there are still some differences.
Most ultrapure laboratory water is currently produced by a combination of the physical purification processes described above.
Regardless of which water quality standard 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 may need to be DNase or RNase-free, whereas water for microbiology experiments needs to be completely sterile. Water used to analyze trace metals needs an even higher level of impurity removal.
| Contaminant | Parameter | Type I | Type II | Type III |
|---|---|---|---|---|
| Ions | Resistivity at 25°C (megaohms-cm) | >18.0 | >1.0 | >0.05 |
| Conductivity at 25°C (microsiemens/cm) | <0.056 | <1.0 | <20 | |
| Organics | TOC (ppb) | <10 | <50 | <200 |
| Pyrogens | Eu/mL | <0.03 | NA | NA |
| Particulates | size | <0.2 µm | NA | NA |
| Colloids | Silicia (ppb) | <10 | <100 | <1000 |
| Bacteria | CFU/mL | <1 | <100 | <1000 |
The highest grades of ultrapure water should not be stored in glass or plastic containers because such materials leach contaminants at very low concentrations. Storage vessels made of silica are used for less demanding applications and of ultrapure tin for the highest purity use.
[edit] Other 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.[citation needed] Distilled or deionized water is especially important in automotive hybrid system component cooling systems, mixed with hybrid system coolant, to prevent corrosion and/or electrolisys of hybrid components.[citation needed]
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.[citation needed]
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.
Another application is to cool off airplane engines before takeoff, was used on the early Boeing 707. This is not as common today due to cost.[2]
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.
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. 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 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 <http://www.hi-fog.com/en/land-applications/index_land-applications_EN.shtml>. 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] Drinking purified water
Many beverage manufacturers use distilled water to ensure a drink's purity and taste. Bottled distilled water is sold as well, and can usually be found in supermarkets. Water purification, such as distillation, is especially important in regions where water resources or tap water is not suitable for ingesting without boiling or chemical treatment.
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[citation needed] 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. The lack of naturally-occurring minerals in distilled water has raised some concerns. The Journal of General Internal Medicine[3] published a study on the mineral contents of different waters available in the US. The study concluded, "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," and further 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.
It is often observed that consumption of "hard" water, or water that has some 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.[4] Since distilled water is free of minerals, it will not have these potential benefits.
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 due without this element. However, the benefit of fluoride supplementation for any purpose should be cautioned due to potential negative effects to IQ and motor functions[5] and a lack of strong evidence that fluoridation of drinking water is effective at cavity prevention,[6] especially when compared to other ways of using fluorides.
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.[citation needed]
[edit] See also
- Hydrogen production
- Ionized water
- Water ionizer
- Electrodeionization
- Atmospheric water generator (Make distilled water from air)
- Heavy water
- Water softening
[edit] References
- ^ Laboratory Water Types.
- ^ SP-4221 The Space Shuttle Decision Retrieved 25 Apr 2008
- ^ Azoulay, Arik; Garzon, Philippe & Eisenberg, Mark (2001), “Comparison of the Mineral Content of Tap Water and Bottled Waters”, Journal of General Internal Medicine 16 (3): 168-175, <http://www.blackwell-synergy.com/links/doi/10.1111/j.1525-1497.2001.04189.x/enhancedabs/>
- ^ Voors, A. W. (1971), “Mineral in the municipal water and atherosclerotic heart death”, American Journal of Epidemiology 93 (4): 259-266, <http://aje.oxfordjournals.org/cgi/content/abstract/93/4/259>
- ^ Statement from Dr. Phyllis Mullenix on the Neurotoxicity of Fluoride at Fluoride Action Network
- ^ York Review, Executive Summary http://www.york.ac.uk/inst/crd/pdf/summary.pdf
[edit] External links
- Deionized Water (Physics Van QA Forum)
- [1] (Res-Kem Corp. Conductivity/Resistivity Conversion Chart)
