Editing Acid dissociation constant (section)

== Definitions ==
According to [[Svante Arrhenius|Arrhenius]]'s [[Acid–base reaction#Arrhenius definition|original molecular definition]], an acid is a substance that [[Dissociation (chemistry)|dissociates]] in aqueous solution, releasing the hydrogen ion {{chem2|H+}} (a proton):<ref name=Miessler>{{cite book
| title = Inorganic Chemistry
| last1 = Miessler
| first1 = Gary L.
| last2 = Tarr
| first2 = Donald A.
| year = 1991
| publisher = Prentice Hall
| isbn = 0-13-465659-8
| edition = 2nd
}} Chapter 6: Acid–Base and Donor–Acceptor Chemistry</ref>
: <chem>HA <=> A- + H+</chem>

The equilibrium constant for this dissociation reaction is known as a [[dissociation constant]]. The liberated proton combines with a water molecule to give a [[hydronium ion|hydronium (or oxonium) ion]] {{chem2|H3O+}} (naked protons do not exist in solution), and so Arrhenius later proposed that the dissociation should be written as an [[acid–base reaction]]:
:<chem>HA + H2O <=> A- + H3O+</chem>

[[File:Acetic-acid-dissociation-3D-balls.png|thumb|upright=1.5|alt=Acetic acid, {{chem2|CH3COOH}}, is composed of a methyl group, {{chem2|CH3}}, bound chemically to a carboxylate group, COOH. The carboxylate group can lose a proton and donate it to a water molecule, {{chem2|H2O}}, leaving behind an acetate anion {{chem2|CH3COO-}} and creating a hydronium cation {{chem2|H3O}}. This is an equilibrium reaction, so the reverse process can also take place.|[[Acetic acid]], a [[weak acid]], donates a proton (hydrogen ion, highlighted in green) to water in an equilibrium reaction to give the [[acetate]] ion and the [[hydronium]] ion. Red: oxygen, black: carbon, white: hydrogen.]]

[[Brønsted–Lowry acid–base theory|Brønsted and Lowry]] generalised this further to a proton exchange reaction:<ref name = Bell>{{cite book
 |last = Bell
 |first = R.P.
 |title = The Proton in Chemistry
 |publisher = Chapman & Hall
 |location = London
 |year = 1973
 |edition = 2nd
 |isbn = 0-8014-0803-2
 |url-access = registration
 |url = https://archive.org/details/protoninchemistr0000bell
}} Includes discussion of many organic Brønsted acids.</ref><ref name=SA>
{{cite book
| last = Shriver
| first = D.F
| author2 = Atkins, P.W.
| title = Inorganic Chemistry
| edition = 3rd
| year = 1999
| publisher = Oxford University Press
| location = Oxford
| isbn = 0-19-850331-8
}} Chapter 5: Acids and Bases</ref><ref>{{Housecroft3rd}} Chapter 6: Acids, Bases and Ions in Aqueous Solution</ref>
: <math chem>\text{acid} + \text{base } \ce{<=>} \text{ conjugate base} + \text{conjugate acid}</math>

The acid loses a proton, leaving a conjugate base; the proton is transferred to the base, creating a conjugate acid. For aqueous solutions of an acid HA, the base is water; the conjugate base is {{chem2|A-}} and the conjugate acid is the hydronium ion. The Brønsted–Lowry definition applies to other solvents, such as [[dimethyl sulfoxide]]: the solvent S acts as a base, accepting a proton and forming the conjugate acid {{chem2|SH+}}.
:<chem>HA + S <=> A- + SH+</chem>

In solution chemistry, it is common to use {{chem2|H+}} as an abbreviation for the solvated hydrogen ion, regardless of the solvent. In aqueous solution {{chem2|H+}} denotes a [[hydronium#Solvation|solvated hydronium ion]] rather than a proton.<ref name=Headrick>
{{cite journal
| last = Headrick
| first = J.M.
| author2 = Diken, E.G.
| author3 = Walters, R. S.
| author4 = Hammer, N. I.
| author5 = Christie, R.A.
| author6 = Cui, J.
| author7 = Myshakin, E.M.
| author8 = Duncan, M.A.
| author9 = Johnson, M.A.|author10= Jordan, K.D.
| year = 2005
| title = Spectral Signatures of Hydrated Proton Vibrations in Water Clusters
| journal = Science
| volume = 308
| pages = 1765–69
| doi = 10.1126/science.1113094
| pmid = 15961665
| issue = 5729
| bibcode = 2005Sci...308.1765H
| s2cid = 40852810
}}</ref><ref name=Smiechowski>
{{cite journal
| last = Smiechowski
| first = M.
| author2 = Stangret, J.
| year = 2006
| title = Proton hydration in aqueous solution: Fourier transform infrared studies of HDO spectra
| journal = J. Chem. Phys.
| volume = 125
| pages = 204508–204522
| doi = 10.1063/1.2374891
| pmid = 17144716
| issue = 20
| bibcode = 2006JChPh.125t4508S
}}</ref>

The designation of an acid or base as "conjugate" depends on the context. The conjugate acid {{chem2|BH+}} of a base B dissociates according to
:<chem>BH+ + OH- <=> B + H2O</chem>

which is the reverse of the equilibrium
:<math chem>\ce{H2O}\text{ (acid)} + \ce{B}\text{ (base) } \ce{<=> OH-}\text{ (conjugate base)} + \ce{BH+}\text{ (conjugate acid)}</math>

The [[hydroxide ion]] {{chem2|OH-}}, a well known base, is here acting as the conjugate base of the acid water. Acids and bases are thus regarded simply as donors and acceptors of protons respectively.

A broader definition of acid dissociation includes [[hydrolysis]], in which protons are produced by the splitting of water molecules. For example, [[boric acid]] ({{chem2|B(OH)3}}) produces {{chem2|H3O+}} as if it were a proton donor,<ref name="Goldmine" /> but it has been confirmed by [[Raman spectroscopy]] that this is due to the hydrolysis equilibrium:<ref>{{cite book
| title = Modern Inorganic Chemistry
| last = Jolly
| first = William L.
| year = 1984
| pages = [https://archive.org/details/trent_0116300649799/page/198 198]
| publisher = McGraw-Hill
| isbn = 978-0-07-032760-3
| url = https://archive.org/details/trent_0116300649799/page/198
}}</ref>
:<chem>B(OH)3 + 2 H2O <=> B(OH)4- + H3O+</chem>

Similarly, [[hydrolysis#Hydrolysis of metal aqua ions|metal ion hydrolysis]] causes ions such as {{chem2|[Al(H2O)6](3+)}} to behave as weak acids:<ref name=Burgess>
{{cite book
| title = Metal Ions in Solution
| last = Burgess
| first = J.
| year = 1978
| publisher = Ellis Horwood
| isbn = 0-85312-027-7
}} Section 9.1 "Acidity of Solvated Cations" lists many p''K''<sub>a</sub> values.</ref>
: <chem>[Al(H2O)6]^3+ + H2O <=> [Al(H2O)5(OH)]^2+ + H3O+</chem>

According to [[Acid–base reaction#Lewis definition|Lewis]]'s original definition, an acid is a substance that accepts an [[electron pair]] to form a [[coordinate covalent bond]].<ref name=Petrucci>
{{cite book
| last = Petrucci
| first = R.H.
| author2 = Harwood, R.S.
| author3 = Herring, F.G.
| title = General Chemistry
| url = https://archive.org/details/generalchemistry00hill
| url-access = registration
| edition = 8th
| year = 2002
| publisher = Prentice Hall
| isbn = 0-13-014329-4
}} p.698</ref>