Chromatographic behaviour of ionizable compounds in liquid chromatography.Part 1. pH scale, pK(a) and pH(s) values for standard buffers in tetrahydrofuran-water

The solvent characteristics of tetrahydrofuran (THF)-water mixtures that ha ve an influence on the chromatographic behaviour of ionizable compounds in liquid chromatography (LC) were investigated (Debye-Huckel parameters, pH s cale, LC-useful range, solvatochromic parameters). Solute properties, such as the pK(a) values of the acids used to prepare reference standard buffer solutions used in LC, depend on the composition of the mixture. The pK(a) v alues in several THF-water mixtures were subjected to factor analysis in or der to determine how many factors affect the variation in the data sets, an d then to target factor analysis to identify these factors. The influence o f solvatochromic parameters on the interactions between acidic groups and t he 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 solva tion was applied to obtain the degree of preferential solvation of hydrogen ions in these media in order to explain the variation in pHs values obtain ed over the range of mixtures studied. The relationships obtained allow cal culation of the pHs values of the standard buffers in THF-water mixtures. T hus pH measurements in these media can be performed as in water. The chroma tographic behaviour of the solute can then be related to the real pH value of the mobile phase used. (C) 1999 Elsevier Science B.V, All rights reserve d.