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Runoff from mines or mine tailings can produce some extremely low pH values.<ref>{{cite journal |last1=Nordstrom |first1=D. Kirk |last2=Alpers |first2=Charles N. |date=March 1999 |title=Negative pH, efflorescent mineralogy, and consequences for environmental restoration at the Iron Mountain Superfund site, California |url=http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1495&context=usgsstaffpub |url-status=live |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=96 |issue=7 |pages=3455–62 |bibcode=1999PNAS...96.3455N |doi=10.1073/pnas.96.7.3455 |pmc=34288 |pmid=10097057 |archive-url=https://web.archive.org/web/20170923012227/http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1495&context=usgsstaffpub |archive-date=23 September 2017 |access-date=4 November 2018 |doi-access=free}}</ref>
== Applications ==▼
▲==Applications==
The pH scale ranges from 0 to 14, with 7 being neutral. Pure water has a pH of 7 at 25°C, meaning it is neutral. When an [[acid]] is dissolved in water, the pH will be less than 7, while a [[Base (chemistry)|base]], or [[alkali]], will have a pH greater than 7. A strong acid, such as [[hydrochloric acid]], at concentration 1 mol dm<sup>−3</sup> has a pH of 0, while a strong alkali like [[sodium hydroxide]], at the same concentration, has a pH of 14. Since pH is a logarithmic scale, a difference of one in pH is equivalent to a tenfold difference in hydrogen ion concentration.
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: {{chem|CO|2|+ H|2|O {{eqm}} HCO|3|-|+ H|+}}
=== pH in
{{See also|Soil pH}}
[[File:Soil_pH_effect_on_nutrient_availability.svg|thumb|Nutritional elements availability within soil varies with pH. Light blue color represents the ideal range for most plants.]]
The United States Department of Agriculture [[Natural Resources Conservation Service]], formerly Soil Conservation Service classifies [[soil pH]] ranges as follows:<ref>{{cite web |author=Soil Survey Division Staff |title=Soil survey manual.1993. Chapter 3, selected chemical properties. |url=http://soils.usda.gov/technical/manual/contents/chapter3.html |url-status=dead |archive-url=https://web.archive.org/web/20110514151830/http://soils.usda.gov/technical/manual/contents/chapter3.html |archive-date=14 May 2011 |access-date=2011-03-12 |publisher=Soil Conservation Service. U.S. Department of Agriculture Handbook 18}}</ref>
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In Europe, topsoil pH is influenced by soil parent material, erosional effects, climate and vegetation. A recent map<ref>{{Cite journal |last1=Ballabio |first1=Cristiano |last2=Lugato |first2=Emanuele |last3=Fernández-Ugalde |first3=Oihane |last4=Orgiazzi |first4=Alberto |last5=Jones |first5=Arwyn |last6=Borrelli |first6=Pasquale |last7=Montanarella |first7=Luca |last8=Panagos |first8=Panos |date=2019 |title=Mapping LUCAS topsoil chemical properties at European scale using Gaussian process regression |journal=Geoderma |language=en |volume=355 |pages=113912 |bibcode=2019Geode.355k3912B |doi=10.1016/j.geoderma.2019.113912 |pmc=6743211 |pmid=31798185 |doi-access=free}}</ref> of topsoil pH in Europe shows the alkaline soils in Mediterranean, Hungary, East Romania, North France. Scandinavian countries, Portugal, Poland and North Germany have more acid soils.
===pH in
[[File:Lemon_-_whole_and_split.jpg|thumb|[[Lemon juice]] tastes sour because it contains 5% to 6% [[citric acid]] and has a pH of 2.2 (high acidity).]]
Plants contain pH-dependent [[Plant pigment|pigments]] that can be used as [[PH indicator|pH indicators]], such as those found in [[hibiscus]], [[red cabbage]] ([[anthocyanin]]), and grapes ([[red wine]]). [[Citrus]] fruits have acidic juice primarily due to the presence of [[citric acid]], while other [[Carboxylic acid|carboxylic acids]] can be found in various living systems. For instance, [[muscle]] activity results in the production of [[lactic acid]]. The [[protonation]] state of [[phosphate]] derivatives, including [[Adenosine triphosphate|ATP]], is pH-dependent. [[Hemoglobin]], an oxygen-transport enzyme, is also affected by pH in a phenomenon known as the [[Root effect]].
=== pH in
{{See also|Seawater#pH value|Ocean#pH and alkalinity}}
{{See also|Ocean acidification}}The pH of [[seawater]] is typically limited to a range between 7.4 and 8.5.<ref name="Chester Marine Geochem2">{{cite book |last=Chester, Jickells |first=Roy, Tim |title=Marine Geochemistry |date=2012 |publisher=Blackwell Publishing |isbn=978-1-118-34907-6}}</ref> It plays an important role in the ocean's [[Carbon cycle#Ocean|carbon cycle]], and there is evidence of ongoing [[ocean acidification]] caused by [[Carbon dioxide emission#Greenhouse gas emissions|carbon dioxide emissions]].<ref name="raven052">{{cite book |author=Royal Society |url=http://dge.stanford.edu/labs/caldeiralab/Caldeira%20downloads/RoyalSociety_OceanAcidification.pdf |title=Ocean acidification due to increasing atmospheric carbon dioxide |year=2005 |isbn=978-0-85403-617-2 |archive-url=https://web.archive.org/web/20100716000207/http://dge.stanford.edu/labs/caldeiralab/Caldeira%20downloads/RoyalSociety_OceanAcidification.pdf |archive-date=16 July 2010 |url-status=live}}</ref> However, pH measurement is complicated by the [[Chemical property|chemical properties]] of seawater, and several distinct pH scales exist in [[chemical oceanography]].<ref name="zeebe2">Zeebe, R. E. and Wolf-Gladrow, D. (2001) ''CO<sub>2</sub> in seawater: equilibrium, kinetics, isotopes'', Elsevier Science B.V., Amsterdam, Netherlands {{ISBN|0-444-50946-1}}</ref>▼
▲
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In practical terms, the three seawater pH scales differ in their pH values up to 0.10, differences that are much larger than the accuracy of pH measurements typically required, in particular, in relation to the ocean's [[Total inorganic carbon|carbonate system]].<ref name="zeebe2" /> Since it omits consideration of sulfate and fluoride ions, the free scale is significantly different from both the total and seawater scales. Because of the relative unimportance of the fluoride ion, the total and seawater scales differ only very slightly.
===
:{| class="wikitable"
|+pH in living systems<ref name=Boron2012>{{cite book |last1=Boron |first1=Walter, F. |url= https://books.google.com/books?id=54mxMgO5H_YC&dq=pH%20in%20living%20systems&pg=PA652 |title=Medical Physiology: A Cellular And Molecular Approach |last2=Boulpaep |first2=Emile L. |date=13 January 2012 |publisher=[[Elsevier Health Sciences]], Saunders |isbn=9781455711819 |pages=652–671 |oclc=1017876653 |access-date=8 May 2022 |archive-date=8 May 2022 |archive-url=https://web.archive.org/web/20220508051939/https://www.google.co.in/books/edition/_/54mxMgO5H_YC?hl=en&gbpv=1&dq=pH+in+living+systems&pg=PA652 |url-status=live |edition=2nd }}</ref>
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Blood pH is usually slightly basic, with a pH of 7.365, referred to as physiological pH in biology and medicine. [[Dental plaque|Plaque]] formation in teeth can create a local acidic environment that results in [[tooth decay]] through demineralization. [[Enzyme|Enzymes]] and other [[Protein|proteins]] have an optimal pH range for function and can become inactivated or [[Denaturation (biochemistry)|denatured]] outside this range.
== pH
When calculating the pH of a solution containing acids and/or bases, a [[Determination of equilibrium constants#Speciation calculations|chemical speciation calculation]] is used to determine the concentration of all chemical species present in the solution. The complexity of the procedure depends on the nature of the solution. Strong acids and bases are compounds that are almost completely dissociated in water, which simplifies the calculation. However, for weak acids, a [[quadratic equation]] must be solved, and for weak bases, a cubic equation is required. In general, a set of [[non-linear]] [[Simultaneous equation|simultaneous equations]] must be solved.
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