Hydrofluoric acid extraction by TBP and by amines. II. Mechanism, species formed and unique properties at high acid to amine molar ratios

Analysis of data concerning HF extraction by amines reveals a change in extraction mechanism when HF/amine molar ratio in organic phase (Z) reaches 2. Up to this point extraction is by ion-pair formation and after it by H-bonding. As a result, the two HF molecules initially extracted differ in nature from subsequent ones. The change in mechanism explains why seemingly unrelated properties such as water co-extraction dilution and diluent effects, selectivity and viscosity - all change when amine bifluoride composition is attained. Similar change in extraction of other mono-basic acids occurs at Z=1. The system with Z>2 for HF is equivalent to that with Z>1 for other mono-basic acids and to that of extraction by TBP. It therefore provides a model for extraction from concentrated aqueous solutions.