EQUILIBRIA IN MODERATELY CONCENTRATED AQUEOUS HYDROGEN-FLUORIDE SOLUTIONS

Hydrogen and fluoride ion molalities have been estimated potentiometrically in HF/H2O mixtures up to 10 mol% HF (c. 6 mol kg-1 HF) at 298 K using the galvanic cells: Ag/AgCl/KCl(m1), HF(m2)\PbClF/Pb (two-phase amalgam) (1) Pt/H-2\KCl(m1), HF(m2)\AgCl/Ag (2) The data show progressive solvation of F- by HF to form first HF2- and then H2F3-; this association is directly responsible for the rapid rise in acidity as the analytical HF concentration rises above 1 molal. No evidence was found for any significant further association of HF with F- at molalities below 6.