Editing PH (section)

{{Short description|Measure of the level of acidity or basicity of an aqueous solution}}
{{Other uses|PH (disambiguation)}}
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{{Lowercase title}}
{{Use dmy dates|date=February 2021}}
[[File:PH_scale_3.jpg|upright=1|thumb|right|Test tubes containing solutions of pH 1–10 colored with [[Universal indicator|an indicator]]]]
{{Acids and bases}}In [[chemistry]], '''pH''' ({{IPAc-en|p|iː|ˈ|eɪ|tʃ}} {{respelling|pee|AYCH}}), also referred to as ''acidity'' or ''basicity'', historically denotes "[[Chemical potential|potential]] of [[hydrogen]]" (or "power of hydrogen").<ref>{{cite journal |last1=Jensen |first1=William B. |author1-link=William B. Jensen |date=2004 |title=The Symbol for pH |url=http://www.che.uc.edu/jensen/W.%20B.%20Jensen/Reprints/102.%20pH.pdf |url-status=live |journal=Journal of Chemical Education |volume=81 |issue=1 |pages=21 |bibcode=2004JChEd..81...21J |doi=10.1021/ed081p21 |archive-url=https://web.archive.org/web/20191214110759/http://www.che.uc.edu/jensen/w.%20b.%20jensen/reprints/102.%20ph.pdf |archive-date=14 December 2019 |access-date=15 July 2020}}</ref> It is a [[logarithmic scale]] used to specify the [[Acid|acidity]] or [[Base (chemistry)|basicity]] of [[aqueous solution]]s. Acidic solutions (solutions with higher concentrations of hydrogen ([[Hydrogen ion#Cation (positively charged)|{{chem2|H+}}]]) [[Cation|cations]]) are measured to have lower pH values than basic or [[alkali]]ne solutions.

The pH scale is logarithmic and inversely indicates the [[Thermodynamic activity|activity]] of [[Hydronium|hydrogen cations]] in the solution
: <math chem="">\ce{pH} = - \log_{10}(a_\ce{H+}) \thickapprox -\log_{10}([\ce{H+}]/\text{M})</math>
where [H<sup>+</sup>] is the [[Equilibrium chemistry|equilibrium]] [[molar concentration]] of H<sup>+</sup> (in M = [[Mole (unit)|mol]]/[[Litre|L]]) in the solution. At 25&nbsp;[[Celsius|°C]] (77&nbsp;[[Fahrenheit|°F]]), solutions of which the pH is less than 7 are acidic, and solutions of which the pH is greater than 7 are basic. Solutions with a pH of 7 at 25&nbsp;°C are neutral (i.e. have the same concentration of H<sup>+</sup> ions as OH<sup>−</sup> ions, i.e. the same as [[pure water]]). The neutral value of the pH depends on the temperature and is lower than 7 if the temperature increases above 25&nbsp;°C. The pH range is commonly given as zero to 14, but a pH value can be less than 0 for very concentrated [[Acid strength|strong acids]] or greater than 14 for very concentrated [[Base (chemistry)#Strong bases|strong bases]].<ref>{{cite journal |last1=Lim |first1=Kieran F. |year=2006 |title=Negative pH Does Exist |journal=Journal of Chemical Education |volume=83 |issue=10 |pages=1465 |bibcode=2006JChEd..83.1465L |doi=10.1021/ed083p1465 |doi-access=}}</ref>

The pH scale is [[Measurement traceability|traceable]] to a set of standard solutions whose pH is established by international agreement.<ref name="covington3">{{cite journal |last1=Covington |first1=A. K. |last2=Bates |first2=R. G. |last3=Durst |first3=R. A. |year=1985 |title=Definitions of pH scales, standard reference values, measurement of pH, and related terminology |url=http://www.iupac.org/publications/pac/1985/pdf/5703x0531.pdf |url-status=live |journal=Pure Appl. Chem. |volume=57 |issue=3 |pages=531–542 |doi=10.1351/pac198557030531 |s2cid=14182410 |archive-url=https://web.archive.org/web/20070924235637/http://www.iupac.org/publications/pac/1985/pdf/5703x0531.pdf |archive-date=24 September 2007}}</ref> Primary pH standard values are determined using a [[Galvanic cell|concentration cell with transference]] by measuring the potential difference between a [[hydrogen electrode]] and a [[Standard electrode potential (data page)|standard electrode]] such as the [[silver chloride electrode]]. The pH of aqueous solutions can be measured with a [[glass electrode]] and a [[pH meter]] or a color-changing [[PH indicator|indicator]]. Measurements of pH are important in [[chemistry]], [[agronomy]], medicine, water treatment, and many other applications.