On March 10, 2023, Ivo Leito presented “Unified pH – what, why and how?” at the Chemistry and Chemical Technology 2023 Vilnius (Lithuania).

Acidity is one of the most critical characteristics of solutions. Its measurement is crucial to understanding and controlling essential processes in fundamental chemistry, industry and living organisms, such as catalysis, extraction, chromatography, processes in micelles/bilayers, etc.

Acidity refers to the activity of the solvated proton and is typically expressed as pH. However, the conventional pH scale is well established only in dilute aqueous solutions at medium pH values. It has severe limitations at extreme values, in other solvents or more complex media where most real-life chemistry occurs. Most importantly, comparing the conventional pH values between different media is impossible because every solvent has its pH scale.

Given the above, a decade ago, the concept of a “unified pH scale” was put forward, defining unified pH via the absolute chemical potential of the solvated proton. The merits of this approach are a strict thermodynamic foundation and direct comparability of values between any media.

In recent years, to a large part thanks to the European Union UnipHied (17FUN09) project, the measurement possibilities have been developed and the concept has now been published as an IUPAC technical report.

The first part of the presentation gave theoretical background and explained the need for unified pH. The second part described the experimental method, and the last part gave an overview of the work done and future perspectives.