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 of the unified pH (pHabs) concept opens up the possibility of measuring pH, expressed as , 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 values in mixtures of methanol, ethanol, and acetonitrile, with water, but without the presence of buffers or other additives. The aim was to assigned 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 values for selected compositions of solvent–water mixtures are presented.
Highlights
solid-contact glass electrodes (SCGE) have previously been proposed as the preferred electrodes for the differential potentiometry measurements enabling calculation of pHH
A small, but noteworthy, consideration remains the details of the differential potentiometry method, including the need to fill both measurement chambers with identical masses of sample, and timing the immersion of the pair of electrodes, such that equal forces are experienced on the ionic liquid salt bridge (ILSB) by both chambers
The data presented in the paper provide means of understanding the sensitivities and challenges of the pHH
Summary
Measurements of acid-base properties in non-aqueous solvents, and solvent–water mixtures, can be realized using the same definition as aqueous solutions: pH = ́log aH , where aH is the activity of protons in a given solvent or solvent–water mixture [3,4]. In this case, it is critical to point out that each non-aqueous solvent, including different ratios of solvent–water mixtures, has its own, solvent-specific, pH scale, termed s pH, where the s superscript denotes the solvent (or solvent–water mixture). The s pH window is fixed by the autoprotolysis constant of a given solvent
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