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,
Acid dissociation constant the added complication that the neutral molecule is subject to an internal acid–base equilibrium in which the basic amino group attracts and binds the proton from the acidic carboxyl group, forming a zwitterion. At pH less than about 5 both the carboxylate group and the amino group are protonated. As pH increases the acid dissociates according to At high pH a second dissociation may take place. Thus the zwitterion, , is amphoteric because it may either be protonated or deprotonated. The equilibrium constant "K" for a base is usually defined as the "association" constant for protonation of the base, B,