Editing PH (section)

=== Non-aqueous solutions ===
pH values can be measured in non-aqueous solutions, but they are based on a different scale from aqueous pH values because the [[Standard state|standard states]] used for calculating hydrogen ion concentrations ([[Activity (chemistry)|activities]]) are different. The hydrogen ion activity, ''a''<sub>H<sup>+</sup></sub>, is defined<ref name="GoldBook2">{{GoldBookRef|title=activity (relative activity), ''a''|file=A00115}}</ref><ref name="GreenBook2">{{GreenBookRef2nd|pages=49–50}}</ref> as:
: <math chem="">a_\ce{H+} = \exp\left (\frac{\mu_\ce{H+} - \mu^{\ominus}_\ce{H+}}{RT}\right )</math>
where ''μ''<sub>H<sup>+</sup></sub> is the [[chemical potential]] of the hydrogen cation, <math chem="">\mu^{\ominus}_\ce{H+}</math> is its chemical potential in the chosen standard state, ''R'' is the [[molar gas constant]] and ''T'' is the [[thermodynamic temperature]]. Therefore, pH values on the different scales cannot be compared directly because of differences in the solvated proton ions, such as lyonium ions, which require an insolvent scale that involves the transfer activity coefficient of [[Lyonium ion|hydronium/lyonium ion]].

pH is an example of an [[acidity function]], but others can be defined. For example, the [[Hammett acidity function]], ''H''<sub>0</sub>, has been developed in connection with [[Superacid]]s.