The basicities of simple organic bases – aliphatic and aromatic amines, amidines, phosphazenes, as well as saturated and unsaturated nitrogen heterocycles – are examined in acetonitrile, dimethyl sulfoxide, tetrahydrofuran, water and the gas phase. The basicities (pKaH values) of conjugate acids of a large variety of bases in these media are presented and discussed. Equations employing easily usable structural descriptors have been derived for approximately converting basicities from acetonitrile to other solvents. Recommendations are given on their practical use and a number of pKaH values that are experimentally unavailable are estimated from these relationships. An important part of the minireview is a large compilation of pKaH and GB values of the compounds in solvents and the gas phase, respectively, as well as the revised basicity scale in acetonitrile, now containing more than 270 pKaH values.

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In this work we explored the relationship between the structure and solvent effects on the basicity of a large selection of conjugated N‐heterocyclic nitrogen bases in different media: the polar aprotic solvent acetonitrile, the polar protic solvent water and the gas phase. Altogether, 58 previously unpublished basicity values in different media for 39 compounds are presented, including 30 experimentally determined pKa values in acetonitrile. We present the pKa and gas‐phase basicity values for quino[7,8‐h]quinoline, which is one of the most basic conjugated nitrogen heterocyclic compounds without basicity‐enhancing substituents. The trends in basicity are rationalized by comparing the basicity data of related compounds in different solvents, as well as by using isodesmic reactions. The gas‐phase basicity is predominantly determined by the ability of a molecule to disperse the excess positive charge over a large number of atoms. In solution the situation is less clear and smaller systems with localized charge often lead to higher basicities because of solvent effects. In particular, it was found that the fusion of an additional benzene ring does not always lead to an increase in basicity in solution: its effect can be either basicity‐increasing or ‐decreasing, depending on the ring size, number and position of nitrogen atoms and medium. A correlation between the measured pKa values in MeCN and in water suggests that these two different solvents exert a similar effect on the basicity of the studied heterocycles.

2 The abbreviations used are: LPMO, lytic polysaccharide monooxygenase; AscA, ascorbic acid; GA, gallic acid; CNW, chitin nanowhisker; MHQ, methyl hydroquinone, NaAc, sodium acetate; NAG eq , N-acetylglucosamine equiv-alents; SmLPMO10A, chitin active lytic polysaccharide monooxygenase from Serratia marcescens; R, reductant.

A unified pH scale of absolute values (pHabs scale) enables the comparison of acidities in different solvents. To date, very few different experimental setups have been used for the measurement of values on this scale. The article describes the design and performance of the different symmetric cells used for unified pH measurement by several institutions. Well-established and reliable standard aqueous buffer solutions are the first step of method validation necessary to achieve a robust metrological level for more complex media. The pH of aqueous standard buffers was measured by differential potentiometry, where the potential between two glass electrodes is measured directly. All the tested electrochemical cells prove to be suitable for unified pH measurements. This validation highlights that the method is, to a large extent, independent of the used equipment, including the cell geometry. The inherent symmetry of the cell design helps to reduce the experimental workload and improve the accuracy of obtained results.

Ionization efficiency and mechanism in ESI is strongly affected by the properties of mobile phase. The use of mobile-phase properties to accurately describe droplets in ESI source is convenient but may be inadequate as the composition of the droplets is changing in the plume due to electrochemical reactions occurring in the needle tip as well as continuous drying and fission of droplets. Presently, there is paucity of research on the effect of the polarity of the ESI mode on mobile phase composition in the droplets. In this paper, the change in the organic solvent content, pH, and droplet size are studied in the ESI plume in both ESI+ and ESI- ionization mode. We introduce a rigorous way - the absolute pH (pH) - to describe pH change in the plume that takes into account organic solvent content in the mobile phase. pH enables comparing acidities of ESI droplets with different organic solvent contents. The results are surprisingly similar for both ionization modes, indicating that the dynamics of the change of mobile-phase properties is independent from the ESI mode used. This allows us to conclude that the evolution of ESI droplets first of all proceeds via the evaporation of the organic modifier and to a lesser extent via fission of smaller droplets from parent droplets. Secondly, our study shows that qualitative findings related to the ESI process obtained on the ESI+ mode can almost directly be applied also in the ESI- mode. Graphical Abstract ᅟ.

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.

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.