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 formationequilibria, has been investigated in 70% (w/w) ethanol/water solutions at analytical concentrations up to5molL−1. In the same way, hydrofluoric acid solutions in 50% (w/w) (up to 2 mol L−1) and in 30% (w/w) etha-nol/water (at 0.003-0.005 mol L−1) have been examined too. Conductometric and potentiometric (glass elec-trode andfluoride-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 spe-cies prevail in the concentrated ones.Thermodynamic dissociation and homoconjugation constants have been determined in hydrofluoric acid dilute so-lutions. The more ethanol is contained in the solvent, the higher is the formation constant for the homoconjugatedspecies (between 1.1 and 1.3 as logKhomo). In concentrated HF solutions at 70% and 50% (w/w) ethanol/water me-dium, 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 valuesobtained for thefirst homoconjugated species (about 1.2 in logarithmic units), very close to those obtained in diluteHF solutions.