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The equilibrium acidity scale (pKa scale) in acetonitrile has been supplemented by numerous new compounds and new ΔpKa measurements. It now contains altogether 231 acids – over twice more than published previously – linked by 569 ΔpKa measurements and spans between the pKa values of hydrogen iodide (2.8) and indole (32.57), covering close to 30 orders of magnitude. Measurement results acquired over the last 15 years were added to the scale and new least‐squares treatment was carried out. The treatment yielded revised pKa values for the compounds published previously, with the root mean square difference between revised and previous values 0.04, demonstrating very good stability of the scale. Correlation equations were developed for estimating pKa values for the studied types of compounds in water, DMSO, DMF, and 1,2‐dichloroethane on the basis of pKa values in acetonitrile. These equations enable predicting pKa values with an average error around or less than 1 pKa unit, which is a sufficient accuracy for many applications. The scale is expected to be a useful tool for the widest possible research areas in organic chemistry, electrochemical power sources, catalysis, etc.

Rationale The first comprehensive quantitative scale of the efficiency of electrospray ionization (ESI) in the positive mode by monoprotonation, containing 62 compounds, was published in 2010. Several trends were found between the compound structure and ionization efficiency (IE) but, possibly because of the limited diversity of the compounds, some questions remained. This work undertakes to align the new data with the originally published IE scale and carry out statistical analysis of the resulting more extensive and diverse data set to derive more grounded relationships and offer a possibility of predicting logIE values. Methods Recently, several new IE studies with numerous compounds have been conducted. In several of them, more detailed investigations of the influence of compound structure, solvent properties, or instrument settings have been conducted. IE data from these studies and results from this work were combined, and the multilinear regression method was applied to relate IE to various compound parameters. Results The most comprehensive IE scale available, containing 334 compounds of highly diverse chemical nature and spanning 6 orders of magnitude of IE, has been compiled. Several useful trends were revealed. Conclusions The ESI ionization efficiency of a compound by protonation is mainly affected by three factors: basicity (expressed by pKaH in water), molecular size (expressed by molar volume or surface area), and hydrophobicity of the ion (expressed by charge delocalization in the ion or its partition coefficient between a water–acetonitrile mixture and hexane). The presented models can be used for tentative prediction of logIE of new compounds (under the used conditions) from parameters that can be computed using commercially available software. The root mean square error of prediction is in the range of 0.7–0.8 log units.

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This work explores the 7-amino-4-(trifluoromethyl)coumarin backbone for designing lipophilic fluorescent pH indicators. Four novel fluorescent phosphazene bases with an advantageous combination of properties-high lipophilicity of both neutral and charged forms, lack of localized charges in the cations, and prominent spectral changes upon protonation/deprotonation-were synthesized using the Staudinger reaction. The molecular structures of the synthesized compounds were confirmed using attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FT-IR), nuclear magnetic resonance (NMR), and Fourier-transform ion-cyclotron resonance (FT-ICR)high-resolution mass spectrometry. The compounds were further characterized by ultraviolet-visible (UV-Vis) absorption and fluorescence emission spectra, lipophilicities (logP values) and pK a values in water and acetonitrile, as well as "biphasic" pK a values in the water/1-octanol solvent system. The bases could potentially be useful as working agents in optical pH sensors and acid-base indicators for lipophilic membranes.

A novel approach-termed here as biphasic pKa values-is presented for quantifying the acidity/basicity of lipophilic compounds embedded in water-immiscible media (membranes or solutions) at equilibrium with aqueous solution. In this approach the hydrogen ion activity is determined in the aqueous phase, while the concentration ratio of protonated and deprotonated forms of acid/base is measured in the lipophilic phase with any suitable technique. The approach has been demonstrated on the example of biphasic pKa values of some lipophilic indicator bases in the octanol : water system, measured using UV-Vis spectrophotometry. The measurement is straightforward if the counterion identity and concentration and overall ionic strength in the aqueous phase are included in the standard state definition. Biphasic pKa values are envisaged to be useful for characterizing lipophilic indicators, synthetic receptors, phase transfer catalysts and other implements that are used in hydrophobic media (e.g. in sensor membranes) in contact with water.

Brønsted acidities of 19 fluorocompounds in the pKa range from 3.5 to 10 have been measured in aqueous solution using an approach involving absolute and relative pKa measurements with 19F NMR. 19F NMR has not been frequently used for determining pKa values, but it has a number of advantages, especially when relative values are considered: accurate concentrations not needed, tolerant to impurities, several compounds can be measured simultaneously, several uncertainty sources cancel out.

Monoaminoacridines (1-, 2-, 3-, 4-, and 9-aminoacridine) were studied for suitability as matrices in the negative ion mode matrix-assisted laser desorption/ionization mass spectrometry (MALDI(−)-MS) analysis of various samples. This is the first study to examine 1-, 2-, and 4-aminoacridine as potential matrix material candidates for MALDI(−)-MS. In addition, spectral (UV−Vis absorption and fluorescence), proton transfer-related (basicity and autoprotolysis), and crystallization properties of these compounds were characterized experimentally and/or computationally. For testing the capabilities of these aminoacridines as matrix materials, four samples related to cultural heritage materialsstearic acid, colophony resin, dyer's madder dye, and a resinous case-study sample from a shipwreckwere analyzed with MALDI(−)-MS. A novel algorithm (implemented as an executable Python script) for MS data analysis was developed to compare the five matrix materials and to help mass spectrometrists rapidly identify peaks originating from the sample and matrix material. It was determined that all five of the studied aminoacridines can successfully be used as matrix materials in MALDI(−)-MS analysis. As an interesting finding, in several cases, the best mass spectra were obtained by using a relatively small amount of matrix material mixed with an excess amount of sample. 3-and 4-aminoacridine outperformed the other aminoacridines in the ease of obtaining acceptable spectra, average number of ions identified in the mass spectra, and low dependence of the sample-to-matrix mass ratio on experimental results.