It is not frequent that a new quantitative measure of some physicochemical characteristic is introduced. Recently our group managed to do exactly that by introducing a novel approach – termed as biphasic pKa values – of measuring the acidity/basicity of lipophilic compounds embedded in water-immiscible media (membranes or solutions) at equilibrium with aqueous solution.
In this approach, the H+ ion activity is determined in the aqueous phase, while the concentration ratio of protonated and deprotonated forms of acid/base is measured in the lipophilic phase with any suitable technique. The approach has been demonstrated on the example of biphasic pKa values of some lipophilic indicator bases in the octanol: water system, measured using UV-Vis spectrophotometry. The counterion identity and concentration, as well as the overall ionic strength in the aqueous phase, are included in the standard state definition.
This work has been published S. Selberg, S. Tshepelevitsh, I. Leito Croat. Chem. Acta 2018, 91, (4) and will form a part of the PhD thesis of Sigrid Selberg.
Biphasic pKa values are envisaged to be useful for characterizing lipophilic indicators, synthetic receptors, phase transfer catalysts and other implements that are used in hydrophobic media (e.g. in sensor membranes) in contact with water.
(On photo: Sigrid Selberg)