PH the Hammett acidity function, "H", has been developed in connection with superacids. The concept of "unified pH scale" has been developed on the basis of the absolute chemical potential of the proton. This model uses the Lewis acid–base definition. This scale applies to liquids, gases and even solids. In 2010, a new "unified absolute pH scale" has been proposed that would allow various pH ranges across different solutions to use a common proton reference standard. Pure water is neutral. When an acid is dissolved in water, the pH will be less than 7 (25 °C). When a base, or alkali,
PH from aqueous pH values, because activities relate to different standard states. Hydrogen ion activity, "a", can be defined as: where "μ" is the chemical potential of the hydrogen ion, formula_9 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 due to different solvated proton ions such as lyonium ions, requiring an intersolvent scale which involves the transfer activity coefficient of hydronium/lyonium ion. pH is an example of an acidity function. Other acidity functions can be defined. For example,