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 of the unified pH (pHabs) concept opens up the possibility of measuring pH, expressed as pHabsH2O, 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 pHabsH2O values in mixtures of methanol, ethanol, and acetonitrile, with water, but without the presence of buffers or other additives. The aim was to assigned pHabsH2O 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 comparability between laboratories. Ranges of pHabsH2O values for selected compositions of solvent-water mixtures are presented.

Czech Metrology Institute Okružní 31 63800 Brno Czech Republic

DFM A S Kogle Allé 5 2970 Hørsholm Denmark

Faculty of Mechanical Engineering Slovak University of Technology Namestie Slobody 17 SK 81231 Bratislava 1 Slovakia

Government Office of the Capital City Budapest Németvölgyi út 37 39 1124 Budapest Hungary

Institute of Chemistry University of Tartu Ravila Street 14a 50411 Tartu Estonia

Instituto Português da Qualidade R António Gião 2 2828 513 Caparica Portugal

Laboratoire National de Metrologie et d'Essais 1 rue Gaston Boissier 75015 Paris France

Physikalisch Technische Bundesanstalt Bundesallee 100 38116 Braunschweig Germany

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