Purified water

Bottle for Distilled water in the Real Farmacia in Madrid

Purified water is water from any source that is physically processed to remove impurities. Distilled water and deionized (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. 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. Purified water in colloquial English can also refer to water which has been treated ("rendered potable") to neutralize, but not necessarily remove contaminants considered harmful to humans and/or animals.

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 than 10 mg/litre.^[1] Distillation involves boiling the water and then condensing the vapour into a clean container, leaving solid contaminants behind. Distillation produces very pure water but also leaves behind a 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.

Purification methods

Double distillation

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.

Deionization

Deionized water, also known as demineralized water^[2] (DI water, DIW or de-ionized water), 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 and therefore are impossible to remove.

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. Processes rendering water potable but not necessarily closer to being pure H₂O / hydroxide + hydronium ions include use of dilute sodium hypochlorite, mixed-oxidants (electro-catalyzed H₂O + NaCl), and iodine; See discussion regarding potable water treatments under "Health effects" below.

Uses

Purified water is suitable for many applications, including autoclaves, hand-pieces, laboratory testing, laser cutting, and automotive use.^[3] Purification removes contaminants which may interfere with processes, or leave residues upon evaporation. Although water is generally considered to be a good electrical conductor—for example domestic electrical systems are considered particularly hazardous to people if they may be in contact with wet surfaces—pure water is a poor conductor. Conductivity of sea-water is typically 5 S/m,^[4] drinking water is typically in the range of 5-50 mS/m, while highly purified water can be as low as 5.5 μS/m, a ratio of about 1,000,000:1,000:1.

Laboratory use

Technical standards on water quality have been established by a number of professional organizations, including the American Chemical Society (ACS), ASTM International, the U.S. 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.

Maximum Contaminant Levels in Highly Purified Water^[5]

		ISO 3696 (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 (20°C)	USP
Ions	Resistivity at 25 °C/MΩ·cm	10	1	0.2	18	1.0	0.25	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	4.0	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.(μg/l)	-	-	-	50	50	200	-	<50	<200	<1000	<500	<500
Total Solids	mg/kg	-	1	2	-	-	-	-	0.1	1	5	-	-
Colloids	Silicia/μg/mL	-	-	-	<3	<3	<500	-	<0.05	<0.1	<1	-	-
Bacteria	CFU/mL	-	-	-	-	-	-	-	<10	<1000	-	<100	<100

Criticism

A member of the ASTM D19 (Water) Committee, Erich L. Gibbs, criticized ASTM Standard D1193, by saying "Type I water could be almost anything – water that meets some or all of the limits, part or all of the time, at the same or different points in the production process."^[6]

Electrical Conductivity

Electrical conductivity of ultra-pure water is 5.5 × 10⁻⁶ S·m⁻¹ (18 MΩ cm) and is due only to H⁺ and OH^- ions produced in the water dissociation equilibrium.^[7][8] This low conductivity is only achieved, however, in the presence of dissolved monoatomic gases. Completely de-gassed ultra-pure water has conductivity of 1.2 × 10⁻⁴ S·m⁻¹, whereas upon equilibration to the atmosphere it is 7.5  × 10⁻⁵ S·m⁻¹ due to dissolved CO₂ in it.^[7] 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.

Examples of laboratory quality sources include Milli-Q, Elga Ultra, or Purite Neptune.

Non-laboratory uses

Distilled or deionized water is commonly used to top up lead-acid batteries used in cars and trucks and for other applications. The presence of foreign ions commonly found in tap water will drastically shorten the lifespan of a lead-acid battery.

Distilled or deionized water is preferable to tap water for use in automotive cooling systems.^[9] 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.^[10][11] Distilled or deionized water is especially important in automotive hybrid system component cooling systems, mixed with hybrid system coolant, to prevent corrosion or electrolysis of hybrid components.^[12]

Using deionised or distilled water in appliances which evaporate water, such as steam irons and humidifiers, can reduce the build-up of mineral scale, which shortens appliance life. Some appliance manufacturers say that deionised water is no longer necessary.^[13][14]

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

Water (sometimes mixed with methanol) has been used to extend the performance of aircraft engines. In piston engines it acts to delay the onset of engine knocking. In turbine engines it allows more fuel flow for a given turbine temperature limit, and increases mass flow. As an example, it was used on early Boeing 707 models.^[15] Advanced materials and engineering have since rendered such systems obsolete for new designs, however spray-cooling of incoming air-charge is still used to a limited extent with off-road turbo-charged engines (road-race track cars).

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.

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 the Sandia National Laboratories Z Machine.

Distilled water can be used in PC watercooling systems and Laser Marking 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 reduces risk of electrical damage in the event of a leak.

When used as a rinse after washing cars, windows, and similar applications, purified water dries without leaving spots caused by dissolved solutes.

Deionized water is used in water-fog fire-extinguishing systems used in sensitive environments, such as where high-voltage electrical and sensitive electronic equipment is used. The 'sprinkler' nozzles use much finer spray jets than other systems, 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 fine droplets of water are nonconducting (when deionized) and are less likely to damage sensitive equipment. 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 the water. It is not generally considered acceptable to spray water on electrical circuits that are powered, and it is generally considered undesirable to use water in electrical contexts.^{[16][17][18][19]}

Health effects of drinking purified water

Distillation removes all minerals from water, and the membrane methods of reverse osmosis and nanofiltration remove most, or virtually all, minerals. This results in demineralized water which is not an ideal replacement for 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 nutrients in water can help to protect against nutritional deficiency. Demineralized water may also increase the risk from toxic metals because it more readily absorbs them, and because the presence of calcium and magnesium in water can 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 2–4 mmol/L. At water hardness above 5 mmol/L, higher incidence of gallstones, kidney stones, urinary stones, arthrosis, and arthropathies have been observed. For fluoride the concentration recommended for dental health is 0.5–1.0 mg/L, with a maximum guideline value of 1.5 mg/L to avoid dental fluorosis.^[20]

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^[21][22][23] are not removed through conventional filtration; however, distillation and reverse osmosis eliminate nearly all of these impurities.

The drinking of purified water as a replacement of drinking water has been both advocated and discouraged for health reasons. Purified water lacks minerals and ions such as calcium that play key roles in biological functions such as in nervous system homeostasis, and are normally found in potable water. The lack of naturally-occurring minerals in distilled water has raised some concerns. The Journal of General Internal Medicine^[24] 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 people 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.^[25] Since distilled water is free of minerals, it will not have these potential benefits.

Where a public water supply contains significant fluoride, naturally or added by fluoridation, people who prefer to limit fluoride intake can install filters appropriate to reduce fluoride content, although this may harm dental health.^[26]

See also

Water portal

• Hydrogen production
• Ionized water
• Water ionizer
• Electrodeionization
• Atmospheric water generator
• Heavy water
• Water softening
• Milli-Q water

References

1. ^ "Frequently asked questions about bottled water". Health Canada. http://www.hc-sc.gc.ca/fn-an/securit/facts-faits/faqs_bottle_water-eau_embouteillee-eng.php. Retrieved 2009-05-24. 
2. ^ http://www.image2output.com/user_resources/TextFiles/pdfs/Deionised_Water.pdf
3. ^ http://www.pureklenz.com
4. ^ http://www.lenntech.com/water-conductivity.htm
5. ^ "The Importance of Water Quality is Critical". http://www.elgalabwater.com/water-quality-en.html. 
6. ^ A Critique of ASTM Standard D1193. http://www.high-q.com/pdf/astm_d1193_critique.pdf. 
7. ^ ^{a b} Pashley, R. M.; Rzechowicz, M.; Pashley, L. R.; Francis, M. J. (2005). "De-Gassed Water Is a Better Cleaning Agent". J. Phys. Chem. B 109 (3): 1231–1238. doi:10.1021/jp045975a. PMID 16851085.  See in particular page 1235. Note that values in this paper are given in S/cm, not S/m, which differs by a factor of 100.
8. ^ [1], Resistivity / Conductivity Measurement of Purified Water, Lab Manager Magazine
9. ^ http://www.peakantifreeze.com/faq.html#B
10. ^ http://distilledwater.net.au/index2.php?option=com_content&do_pdf=1&id=14
11. ^ http://www.processregister.com/Distilled_Water/Suppliers/pid12830.htm
12. ^ http://www.eetcorp.com/antifreeze/antifreeze-faq.htm
13. ^ http://www.consumersearch.com/steam-irons/important-features
14. ^ http://www.homeinstitute.com/steam-iron-buying-guide.htm
15. ^ SP-4221 The Space Shuttle Decision Retrieved 25 April 2008
16. ^ http://www.lytron.com/tools-technical/notes/deionized-water.aspx
17. ^ http://www.tmasc.com/cleaning_chemicals.htm
18. ^ http://www.hghouston.com/Technical%20FAQs/TFAQ021.html
19. ^ http://www.safetymgmt.com/AIGRiskTools/Knowledge_Center/General_Industry/ELECTRICAL_SAFETY.pdf
20. ^ Kozisek F (2005). "Health risks from drinking demineralised water" (PDF). Nutrients in Drinking Water. World Health Organization. pp. 148–63. ISBN 92-4-159398-9. http://www.who.int/water_sanitation_health/dwq/nutrientschap12.pdf. 
21. ^ http://watersystems.walton.com/
22. ^ http://www.drinkmorewater.com/technology/technology/
23. ^ http://www.homeenvironmentcenter.com/10stage.htm
24. ^ Azoulay A, Garzon P, Eisenberg MJ (2001). "Comparison of the mineral content of tap water and bottled waters". J Gen Intern Med 16 (3): 168–75. doi:10.1111/j.1525-1497.2001.04189.x. PMC 1495189. PMID 11318912. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1495189. 
25. ^ Voors, A. W. (April 1, 1971). "Mineral in the municipal water and atherosclerotic heart death". American Journal of Epidemiology 93 (4): pp. 259–266. PMID 5550342. http://aje.oxfordjournals.org/cgi/content/abstract/93/4/259 
26. ^ Hobson WL, Knochel ML, Byington CL, Young PC, Hoff CJ, Buchi KF (2007). "Bottled, filtered, and tap water use in Latino and non-Latino children". Arch Pediatr Adolesc Med 161 (5): 457–61. doi:10.1001/archpedi.161.5.457. PMID 17485621. http://archpedi.ama-assn.org/cgi/content/full/161/5/457.