PH
Non-aqueous solutions

where μ is the chemical potential of the hydrogen ion, μ H + ⊖ {\displaystyle \mu _{{\ce {H+}}}^{\ominus }} is its chemical potential in the chosen standard state, R is the 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 intersolvent scale which involves the transfer activity coefficient of hydronium/lyonium ion.
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