Ionic equilibria in aqueous organic solvent mixtures. Speciation of hydrofluoric acid in several ethanol/water solutions

Abstract

The presence of different species derived from hydrofluoric acid, together with the corresponding formation equilibria, has been investigated in 70% (w/w) ethanol/water solutions at analytical concentrations up to 5 mol L−1. In the same way, hydrofluoric acid solutions in 50% (w/w) (up to 2 mol L−1) and in 30% (w/w) ethanol/water (at 0.003–0.005 mol L−1) have been examined too. Conductometric and potentiometric (glass electrode and fluoride-ion-selective electrode) measurements have been used to demonstrate that HF and HF2− predominate in dilute solutions regardless the solvent composition, whereas HF and polyhomoconjugated species prevail in the concentrated ones.Thermodynamic dissociation and homoconjugation constants have been determined in hydrofluoric acid dilute solutions. The more ethanol is contained in the solvent, the higher is the formation constant for the homoconjugated species (between 1.1 and 1.3 as logKhomo). In concentrated HF solutions at 70% and 50% (w/w) ethanol/water medium, the polyhomoconjugated species H2F3− (in 70% and 50% ethanol/water) and H3F4− (in 70% ethanol/water) are formed, with successive formation constants between about 4.1 and 6.9 logarithmic units, being the values obtained for the first homoconjugated species (about 1.2 in logarithmic units), very close to those obtained in dilute HF solutions.