A potentiometric study of Eu3+ complexation with acetate ligand from 25 to 170°C at Psat

Abstract Thermodynamic properties for europium acetate (EuCH 3 COO 2+ and Eu(CH 3 COO) 2 + ) complexes were studied by potentiometry at temperatures from 25 to 170°C at the saturation pressure of water. The thermodynamic association constants (K 1 and K 2 ) for the reaction Eu 3+ + n Acite − = EuAc n 3−n . were determined by two different experimental approaches at 25 to 75°C and 75 to 170°C. Logarithms of measured association constants increase with increasing temperature, showing that Eu-Ac complexing increases as temperature rises. Log K 1 for EuCH 3 COO 2+ increases from 2.91 at 25°C to 4.25 at 170°C, whereas log K 2 for Eu(CH 3 COO) 2 + increases from 4.83 at 25°C to 7.39 at 170°C. Species distribution calculated in this study using the experimentally determined association constants suggests that in acetate-bearing solutions (≥0.05 m ) Eu-acetate complexes dominate over the free ion above pH of 4 to temperatures of at least 200°C. At equal total acetate and carbonate concentrations and at 25°C, the relative stability of carbonate and acetate complexes is close. However, as the temperature rises the relative stability of carbonate versus acetate complexes is model dependent.