Thermochemistry of the Extraction of Bismuth(III) with Bis(2‐ethylhexyl) Phosphoric and 2‐Ethyhexyl‐phenylphosphonic Acids

Abstract The extraction of bismuth(III) from perchloric acid medium by bis(2‐ethylhexyl) phosphoric (HDEHP) and 2‐ethylhexyl‐phenylphosphonic (HEH[Ph]P) acids dissolved in o‐xylene was studied in the temperature range 25–60°C. The extractions were investigated as a function of the aqueous perchloric acid concentration and organic phase extractant concentration. The slopes of the average extractant dependencies and acid dependencies were 2.8 and −2.9, respectively, for both extraction systems, and the extraction stoichiometries did not change with temperature. Enthalpy, entropy and free energy changes of the extraction process were derived from the measured extraction coefficients across this temperature range. The enthalpy change for the extraction with HDEHP was also studied directly by calorimetric measurement at 25°C. The extractability of bismuth by both extractants is enthalpy controlled. The enthalpy change for extraction from 1 M (Na/H)ClO4 calculated from calorimetric data at low metal loading, −1...