The Determination of Fluoride-Ion Activities in Moderately Concentrated Aqueous Hydrogen-Fluoride

Abstract

A lead-lead chlorofluoride electrode has been used to determine fluoride ion activities, aF-, in 0-6 M aqueous HF. Our data show that aF- increases, passes through a maximum and then decreases with increasing HF concentration. The conductivities of concentrated HF- water mixtures have also been measured. Both the conductometric and potentiometric data are used in the development of a quantitative model of the equilibria existing between fluoride ions and HF molecules in the solutions studied. The data support the view that the ions (HF)2F-, (HF)3F- and (HF)4F- are important species in moderately concentrated aqueous HF.

Ab Initio Molecular Dynamics Simulation of a Water–Hydrogen Fluoride Equimolar Mixture

“…[11] The other polyfluoride ions F(HF) n À (n = 2) either have been revealed by potentiometry and electrical conductivity experiments around 10 % molar or are supposed to be formed at higher concentrations for n = 3, 4. [12,13] But, as for bifluoride, infrared and Raman experiments do not support these results. [10,11] Some more recent spectroscopic investigations even support the description of the liquid as hydronium fluoride.…”

Solubility and dissolution rate of silica in acid fluoride solutions

“…None of these species are important in HF solutions containing less than 1 m HF (Farrer and Rossotti, 1964). They form by the reactions (McTigue et al, 1985) 2HF Gennick et al (1977) reported the existence of all the above polymeric species in the solid state and X-ray diffraction studies. Coyle et al (1970), Forrester et al (1963, and Ibers (1964) confirm the structures of all the polymers except H 3 F 4 À .…”

“…We used pK = 3.16 (IUPAC, 1979;Kresge and Chiang, 1973;Perrin, 1982) for all our calculations. Other significant studies (Knotter, 2000;McTigue et al, 1985) also used this value. Hefter (1984) performed a critical study of all reported acidity constants for HF and suggested a pK value of 3.18 ± 0.02, which is similar to the value used in this study within error.…”

“…1 shows the distribution of the fluoride species as a function of pH and log a F À . McTigue et al (1985) suggested that the apparent acidity of HF-H 2 O mixtures increases with increasing HF concentration such that, "from being a weak acid in dilute aqueous solutions, pure liquid HF is a super-acid". They attribute this to the formation of polymeric HF aqueous species (H n F nþ1 À ) resulting from the solvation of F À by molecular HF by the reaction…”

Electric field assisted hydrogen fluoride etching of silica

“…Infrared spectroscopic data seem to indicate that molecular HF is practically absent and instead ion pairs H 3 O + F − are formed (Giguere & Turrell 1980). But conductivity and potentiometric data indicate that a large amount of HF does exist (McTigue et al 1985). Ion concentrations and activity coefficients at high concentrations of that acid are unknown.…”