Editing Hydronium (section)

==Relation to pH==
The [[molar concentration]] of hydronium or {{chem2|H+}} ions determines a solution's [[pH]] according to

:pH = −log([{{chem2|H3O+}}]/M)

where M = mol/L. The concentration of [[hydroxide]] ions analogously determines a solution's [[pH#pOH|pOH]]. The molecules in pure water [[self-ionization of water|auto-dissociate]] into aqueous protons and hydroxide ions in the following equilibrium:

:{{chem2| H2O <-> OH-(aq) + H+(aq)}}

In pure water, there is an equal number of hydroxide and {{chem2|H+}} ions, so it is a neutral solution. At {{convert|25|C|F}}, pure water has a pH of 7 and a pOH of 7 (this varies when the temperature changes: see [[self-ionization of water]]). A pH value less than 7 indicates an acidic solution, and a pH value more than 7 indicates a basic solution.<ref name="usgs">{{cite web|url=https://www.usgs.gov/special-topic/water-science-school/science/ph-and-water?qt-science_center_objects=0#qt-science_center_objects|title=pH and Water|author=<!--No listed author-->|publisher=United States Geological Survey|access-date=9 November 2021}}</ref>