Thermodynamics of the Extraction of Lead, Copper, Cadmium, Cobalt, and Nickel with Mono‐2‐ethylhexyl Phosphoric Acid

The extraction of Pb2+, Cu2+, Cd2+, Co2+, and Ni2+ from aqueous nitric acid medium by mono‐2‐ethylhexyl phosphoric acid (H2MEHP) dissolved in o‐xylene and aggregation of the extractant was studied in the temperature range 25–60°C. The extractions were investigated as a function of aqueous nitric acid concentration and extractant organic phase concentration. The extractant aggregation and extraction stoichiometries for the metal ions did not change with temperature. The thermodynamics of the extraction process was studied by measuring extraction coefficients in this temperature range. The slopes of the extractant dependencies are unity and acid dependencies have values equal to negative values of the formal charge on the metal ions; except for Pb2+ where extraction is favored by both enthalpy and entropy, extraction of the other metal ions is strongly opposed by entropy.