Reconciling the pHe measurements of bioethanol: pHabs measurements of buffered 50-50 wt% water-ethanol mixtures

Water-ethanol mixtures intended for specific purposes, such as bioethanol fuel, can be subject to national quality standards, including the measurement of pHe – a solvent-specific quantification of acidity. This work discusses the shortcomings of the use of pHe in these quality standards, including the lack of metrological traceability of pHe measurements made using combination pH electrodes calibrated using aqueous pH buffers. The feasibility of measuring the acidity of 50-50 wt% water-ethanol mixtures on a non-solvent-specific, unified pH scale, which is traceable to the conventional aqueous pH scale (${\text{pH}}_{\text{abs}}^{{\text{H}}_2\text{O}}$) is demonstrated. ${\text{pH}}_{\text{abs}}^{{\text{H}}_2\text{O}}$ measurements of buffered and un-buffered water-ethanol mixtures using two cell configurations, including the use of an ionic liquid salt bridge (ILSB), show good agreement. The cell configuration, consisting of a commercial glass (half-cell) electrode and a reference electrode incorporating an ILSB, can be readily adopted by measurement laboratories.