Fluoroacidity evaluation in molten salts

Abstract

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: SiF4+xx−=SiF4(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 SiF4+xx− 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.