Unified pH Measurements of Ethanol, Methanol, and Acetonitrile, and Their Mixtures with Water

Measurement of pH in aqueous-organic mixtures with different compositions is of high importance in science and technology, but it is, at the same time, challenging both from a conceptual and practical standpoint. A big part of the difficulty comes from the fundamental incomparability of conventional pH values between solvents ((s)pH, solvent-specific scales). The recent introduction of the unified pH (pH(abs)) concept opens up the possibility of measuring pH, expressed as [Formula: see text] , in a way that is comparable between solvent, and, thereby, removing the conceptual problem. However, practical issues remain. This work presents the experience of the authors with measuring [Formula: see text] values in mixtures of methanol, ethanol, and acetonitrile, with water, but without the presence of buffers or other additives. The aim was to assigned [Formula: see text] values to solvent–water mixtures using differential potentiometry and the ‘pH(abs)-ladder’ method. Measurements were made of the potential difference between glass electrodes immersed in different solutions, separated by an ionic liquid salt bridge. Data were acquired for a series of solutions of varying solvent content. This work includes experiences related to: a selection of commercial electrodes, purity of starting material, and comparability between laboratories. Ranges of [Formula: see text] values for selected compositions of solvent–water mixtures are presented.