Editing Acid dissociation constant (section)

=== Water self-ionization ===
{{Main|Self-ionization of water}}
The water molecule may either gain or lose a proton. It is said to be [[amphiprotic]]. The ionization equilibrium can be written
: <chem>H2O <=> OH- + H+</chem>

where in aqueous solution {{chem2|H+}} denotes a [[solvation|solvated]] proton. Often this is written as the [[hydronium]] ion {{chem2|H3O+}}, but this formula is not exact because in fact there is solvation by more than one water molecule and species such as {{chem2|H5O2+}}, {{chem2|H7O3+}}, and {{chem2|H9O4+}} are also present.<ref>{{Housecroft2nd|page=163}}</ref>

The equilibrium constant is given by
:<math alt="The acidity constant K A for water equals the concentration of H + times that of O H minus divided by the concentration of water, H 2 O." >
 K_\text{a} = \mathrm{\frac{[H^+] [OH^-]}{[H_2O]}}
</math>

With solutions in which the solute concentrations are not very high, the concentration {{chem2|[H2O]}} can be assumed to be constant, regardless of solute(s); this expression may then be replaced by
:<math alt="The ionization constant of water K w equals the concentration of H + times the concentration of O H minus." >
 K_\text{w} = [\mathrm{H}^+] [\mathrm{OH}^-]\,
</math>

The [[self-ionization of water|self-ionization]] constant of water, ''K''<sub>w</sub>, is thus just a special case of an acid dissociation constant. A logarithmic form analogous to p''K''<sub>a</sub> may also be defined
:<math>\mathrm{p}K_\text{w} = - \log_{10}\left(K_\text{w}\right)</math>

{| class="wikitable" style="text-align:center"
|+ p''K''<sub>w</sub> values for pure water at various temperatures<ref>{{cite book
| last1 = Harned
| first1 = H.S.
| last2 = Owen
| first2 = B.B
| title = The Physical Chemistry of Electrolytic Solutions
| url = https://archive.org/details/physicalchemistr0003harn
| url-access = registration
| year = 1958
| publisher = Reinhold Publishing Corp.
| location = New York
| pages = [https://archive.org/details/physicalchemistr0003harn/page/634 634]–649, 752–754
}}</ref>
|-
! scope="row" | ''T'' (°C)
| 0 || 5 || 10 || 15 || 20 || 25 || 30 || 35 || 40 || 45 || 50
|-
! scope="row" | p''K''<sub>w</sub>
| 14.943 || 14.734 || 14.535 || 14.346 || 14.167 || 13.997 || 13.830 || 13.680 || 13.535 || 13.396 || 13.262
|}

These data can be modelled by a [[parabola]] with
: <math>\mathrm p K_\mathrm w = 14.94 - 0.04209\ T + 0.0001718\ T^2</math>

From this equation, p''K''<sub>w</sub>&nbsp;=&nbsp;14 at 24.87&nbsp;°C. At that temperature both hydrogen and hydroxide ions have a concentration of 10<sup>−7</sup>&nbsp;M.