Fluoroacidity evaluation in molten salts

The fluoroacidity of several alkaline fluoride media was studied by monitoring the concentration of electroactive species which is decreasing vs. time due to a gas species release, such as silicon fluorides, as indicated by the reaction: SiF_{4 + x}^x− = SiF_4(g) + xF⁻. This article relates the Si(IV) reaction study to define a relative fluoroacidity scale by studying the silicon ions stability in different melts. Electrochemical techniques allow the measurement of SiF_{4 + x}^x− concentration evolution and thus the reaction rate constant to be calculated at different temperatures and for several fluoride media. The article shows that the free F⁻ content depends on the fluoride mixture and that the rate values are correlated with the fluoroacidity allowing a qualitative estimation. Then a fluoride solvents fluoroacidity scale was proposed, scaling the different eutectic melts from basic melt to acidic one: NaF–KF < LiF–KF < NaF–MgF₂ < NaF–CaF₂ < LiF–NaF < LiF < LiF–CaF₂.