Editing Hydronium (section)

==Acids and acidity==
The hydrated proton is very acidic: at 25&nbsp;°C, its [[Acid dissociation constant|p''K''<sub>a</sub>]] is approximately 0.<ref>{{cite journal |last1=Meister |first1=Erich |last2=Willeke |first2=Martin |last3=Angst |first3=Werner |last4=Togni |first4=Antonio |last5=Walde |first5=Peter |date=2014 |title=Confusing Quantitative Descriptions of Brønsted-Lowry Acid-Base Equilibria in Chemistry Textbooks – A Critical Review and Clarifications for Chemical Educators |journal=[[Helv. Chim. Acta]] |volume=97 |issue=1 |pages=1–31 |doi=10.1002/hlca.201300321}}</ref> The values commonly given for p''K''<sub>a</sub><sup>aq</sup>(H<sub>3</sub>O<sup>+</sup>) are 0 or −1.74. The former uses the convention that the activity of the solvent in a dilute solution (in this case, water) is 1, while the latter uses the value of the concentration of water in the pure liquid of 55.5 M. Silverstein has shown that the latter value is thermodynamically unsupportable.<ref>{{cite journal |last1=Silverstein |first1=T.P. |last2=Heller |first2=S.T.|title=pKa Values in the Undergraduate Curriculum: What Is the Real pKa of Water? |journal=J. Chem. Educ. |date=2017 |volume=94 |issue=6 |pages=690–695|doi=10.1021/acs.jchemed.6b00623 |bibcode=2017JChEd..94..690S}}</ref> The disagreement comes from the ambiguity that to define p''K''<sub>a</sub> of H<sub>3</sub>O<sup>+</sup> in water, H<sub>2</sub>O has to act simultaneously as a solute and the solvent. The IUPAC has not given an official definition of p''K''<sub>a</sub> that would resolve this ambiguity. Burgot has argued that H<sub>3</sub>O<sup>+</sup>(aq) + H<sub>2</sub>O (l) ⇄ H<sub>2</sub>O (aq) + H<sub>3</sub>O<sup>+</sup> (aq) is simply not a thermodynamically well-defined process. For an estimate of p''K''<sub>a</sub><sup>aq</sup>(H<sub>3</sub>O<sup>+</sup>), Burgot suggests taking the measured value p''K''<sub>a</sub><sup>EtOH</sup>(H<sub>3</sub>O<sup>+</sup>) = 0.3, the p''K''<sub>a</sub> of H<sub>3</sub>O<sup>+</sup> in ethanol, and applying the correlation equation p''K''<sub>a</sub><sup>aq</sup> = p''K''<sub>a</sub><sup>EtOH</sup> − 1.0 (± 0.3) to convert the ethanol p''K''<sub>a</sub> to an aqueous value, to give a value of p''K''<sub>a</sub><sup>aq</sup>(H<sub>3</sub>O<sup>+</sup>) = −0.7 (± 0.3).<ref>{{Cite journal|last=Burgot|first=Jean-Louis|date=1998|title=PerspectiveNew point of view on the meaning and on the values of Ka○(H<sub>3</sub>O<sup>+</sup>, H<sub>2</sub>O) and Kb○(H<sub>2</sub>O, OH<sup>−</sup>) pairs in water|url=http://xlink.rsc.org/?DOI=a705491b|journal=The Analyst|volume=123|issue=2|pages=409–410|doi=10.1039/a705491b|doi-access=free|bibcode=1998Ana...123..409B }}</ref> On the other hand, Silverstein has shown that Ballinger and Long's experimental results <ref name="Ballinger1960">{{cite journal |last1=Ballinger |first1=P. |last2=Long |first2=F.A. |title=Acid Ionization Constants of Alcohols. II. Acidities of Some Substituted Methanols and Related Compounds |journal=J. Am. Chem. Soc. |date=1960 |volume=82 |issue=4 |pages=795–798|doi=10.1021/ja01489a008}}</ref> support a p''K''<sub>a</sub> of 0.0 for the aqueous proton.<ref>{{cite journal |last1=Silverstein |first1=T.P. |title=The aqueous proton is hydrated by more than one water molecule: Is the hydronium ion a useful conceit? |journal=J. Chem. Educ. |date=2014 |volume=91 |issue=4 |pages=608–610|doi=10.1021/ed400559t |bibcode=2014JChEd..91..608S}}</ref> Neils and Schaertel provide added arguments for a p''K''<sub>a</sub> of 0.0 <ref>{{cite web| title = What is the pKa of Water| url = http://chemwiki.ucdavis.edu/Core/Organic_Chemistry/Fundamentals/What_is_the_pKa_of_water%3F| publisher = [[University of California, Davis]]| date = 2015-08-09| access-date = 2022-04-03| archive-date = 2016-02-14| archive-url = https://web.archive.org/web/20160214222524/http://chemwiki.ucdavis.edu/Core/Organic_Chemistry/Fundamentals/What_is_the_pKa_of_water%3F| url-status = dead}}</ref>

The aqueous proton is [[Leveling effect|the most acidic species that can exist in water]] (assuming sufficient water for dissolution): any stronger acid will ionize and yield a hydrated proton. The acidity of {{chem2|H+}}(aq) is the implicit standard used to judge the strength of an acid in water: [[strong acid]]s must be better proton donors than {{chem2|H+}}(aq), as otherwise a significant portion of acid will exist in a non-ionized state (i.e.: a weak acid). Unlike {{chem2|H+}}(aq) in neutral solutions that result from water's autodissociation, in acidic solutions, {{chem2|H+}}(aq) is long-lasting and concentrated, in proportion to the strength of the dissolved acid.

pH was originally conceived to be a measure of the [[hydrogen ion]] concentration of aqueous solution.<ref>{{cite journal |last1=Sorensen |first1=S. P. L. |date=1909 |title=Ueber die Messung und die Bedeutung der Wasserstoffionenkonzentration bei enzymatischen Prozessen |journal=[[Biochemische Zeitschrift]] |language=de |volume=21 |pages=131–304}}</ref> Virtually all such free protons are quickly hydrated; acidity of an aqueous solution is therefore more accurately characterized by its concentration of {{chem2|H+}}(aq). In organic syntheses, such as acid catalyzed reactions, the hydronium ion ({{H3O+}}) is used interchangeably with the {{chem2|H+}} ion; choosing one over the other has no significant effect on the mechanism of reaction.