Unified ph measurements of ethanol, methanol, and acetonitrile, and their mixtures with water

Abstract

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 (spH, solvent‐specific scales). The recent introduction H of the unified pH (pHabs) concept opens up the possibility of measuring pH, expressed as pH2o abs, 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 H pH2o abs values in mixtures of methanol, ethanol, and acetonitrile, with water, but without the presence of buffers or other additives. The aim was to assigned pH2oH abs values to solvent–water mixtures using differential potentiometry and the ‘pHabs‐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 H comparability between laboratories. Ranges of pH2o abs values for selected compositions of solvent– water mixtures are presented.