Chromatographic behaviour of ionizable compounds in liquid chromatography. Part 1. pH scale, pKa and pHS values for standard buffers in tetrahydrofuran–water Original Research Article

The solvent characteristics of tetrahydrofuran (THF)–water mixtures that have an influence on the chromatographic behaviour of ionizable compounds in liquid chromatography (LC) were investigated (Debye–Hückel parameters, pH scale, LC-useful range, solvatochromic parameters). Solute properties, such as the pKa values of the acids used to prepare reference standard buffer solutions used in LC, depend on the composition of the mixture. The pKa values in several THF–water mixtures were subjected to factor analysis in order to determine how many factors affect the variation in the data sets, and then to target factor analysis to identify these factors. The influence of solvatochromic parameters on the interactions between acidic groups and the solvents studied was identified and quantified. Standard pH values, pHS, for the seven primary reference buffer solutions of the NIST scale and for acetate buffer in THF–water mixtures containing 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70% and 100% (w/w) THF were determined using IUPAC standardization rules. The quasi-lattice–quasi-chemical (QLQC) theory of preferential solvation was applied to obtain the degree of preferential solvation of hydrogen ions in these media in order to explain the variation in pHS values obtained over the range of mixtures studied. The relationships obtained allow calculation of the pHS values of the standard buffers in THF–water mixtures. Thus pH measurements in these media can be performed as in water. The chromatographic behaviour of the solute can then be related to the real pH value of the mobile phase used.