Concept of absolute acidity
Recently, we constructed a unified pH scale for all phases which allows comparison of Bronsted acidities in different phases. In our opinion, Bronsted acidity is thermodynamically "chemical potential of the proton". In our absolute acidity scale, we use the gaseous proton as universal reference acid and set its absolute chemical standard potential µabs(H+) to zero. In real bulk phases, the chemical potential of the proton is reduced by interaction with its environment which leads to negative µabs(H+) values.