Glass electrode half-cells for measuring unified pH in ethanol–water mixtures

Abstract

Abstract. The acidities of any given solvent or mixtures thereof can be compared by pH measurements on a unified scale, so-called pHabsH2O measurements. The method is quite new and has not been characterized with respect to metrological criteria to date. Metal solid-contact glass electrode half-cells, three commercial, conventional glass electrode half-cells with inner liquid filling and one pair of combined electrodes were used to investigate the stability of the measurement and the reproducibility of pHabsH2O results of ethanol mixtures with water. All electrodes are suitable for unified acidity measurements in standard aqueous buffers. In ethanol mixtures, the combined electrodes were found to be unsuitable. The half-cell electrodes can be reasonably used only in buffered solutions.

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

“…However, the SCGE used here is not commercially available on a large scale. Similar advantages may be seen in other commercial half-cell electrodes for measurements [ 20 ]. However, differences between commercial electrodes have been noted when measuring solvent-specific pHe in anhydrous and hydrous bio-ethanol according to ASTM D6423-14 [ 26 , 27 ], as well as buffered methanol–water and ethanol–water solutions [ 28 ].…”

“…SCGE (electrode A) differential potentiometry have previously been shown to allow the determination of values for buffered aqueous solutions described in the [ 25 ] and solvent–water [ 19 , 20 , 49 , 50 ] mixtures, as well as p K a values in acetonitrile [ 51 ]. The use of these electrodes in Cell I has been agreed to be the ‘reference’ method for value determination [ 36 ].…”

“…( b ) water–ethanol mixtures: potential difference measured using combination (F, ×), half-cell (G, *; H, + and −; I, ◦; J, ◊), and SCGE (A, ●). Values for electrodes H, I, and J are taken from [ 20 ]. ( c ) water–acetonitrile mixtures: potential difference measured using combination (F, ×), half-cell (G, * and H, +), and SCGE (A, ●).…”

“…Suu et al [ 19 ] demonstrated the practical realization of measurements via differential potentiometry. In the most recent version of this method [ 20 ] GEs are immersed in two solutions of differing composition separated by an ionic liquid salt bridge (ILSB), triethylamylammonium bis((trifluoromethyl)sulfonyl)imide [N 2225 ][NTf 2 ]. It has been demonstrated [ 21 , 22 , 23 ] that this ILSB has several advantages, including: the elimination of the LJP between the two solutions, slower mixing kinetics between IL and organic solvent compared to the more common 3 M KCl (aq) salt bridge, and allowing the assignment of the contribution to uncertainty of the LJP without extra-thermodynamic assumptions.…”

“…These electrodes consist of a solid-contact between the glass membrane and the internal sensor [ 19 , 24 ], and are referred to as solid-contact glass electrodes (SCGE). The use of half-cells eliminates the possibility of interactions between the inner filling solution and solution under test [ 20 , 25 ]. However, the SCGE used here is not commercially available on a large scale.…”