Investigation of fluoro-acidity in molten salts

This thesis deals with the study of one physico-chemical property of molten fluorides media, fluoro-acidity (pF which is equivalent to the pH for aprotic solvent), and its impact on the process. The fluoro-acidity (pF = -log(a_F-)) of molten fluorides is presently unknown and the purpose of this work is to build a relative scale of fluoro-acidity. This study was conducted in the first part using two approaches: - a kinetic study of stability of B(III) and Si(IV) ions, where the reaction leading to gaseous species (BF_3(g) and SiF_4(g)) is affected by the fluoro-acidity. By comparing the rate constants of gas release, the different media were sorted and a relative scale of fluoro-acidity was proposed. - the influence of fluoro-acidity on the transport in solution, determined by measuring the diffusion coefficient and by calculating the non-dimensional Schmidt number (Sc=ν/D) was investigated. Relationships between bridged fluorines, viscosity and fluoro-acidity was evidenced, and the proposed fluoro-acidity scale validated. Then, a stable reference electrode in molten fluorides was developed. Electromotive forces measurements of concentration cells were performed and the fluoro-acidity impact on the potential of a redox couple was demonstrated. Thanks to this reference electrode, free F activities ratios in different solvents were determined. (author)