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

Thermodynamic properties for europium acetate (EuCH3COO2+ and Eu(CH3COO)(2)(+)) complexes were studied by potentiometry at temperatures from 25 to 170degreesC at the saturation pressure of water. The thermodynamic association constants (K-1 and K-2) for the reaction Eu3+ + n Ac- = EuAcn3-n, were determined by two different experimental approaches at 25 to 75degreesC and 75 to 170degreesC. Logarithms of measured association constants increase with increasing temperature. showing that Eu-Ac Complexing increases as temperature rises. Log K-1 for EuCH3COO2+ increases from 2.91 at 25degreesC to 4.25 at 170degreesC, whereas log K-2 for Eu(CH3COO)(2)(+) increases front 4.83 at 25degreesC to 7.39 at 170degreesC. Species distribution calculated in this study using the experimentally determined association constants suggests that in acetate-bearing solutions (greater than or equal to0.05 m) Eu-acetate complexes dominate over the free ion above pH of 4 to temperatures of at least 200degreesC. At equal total acetate and carbonate concentrations and at 25degreesC, 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, Copyright (C) 2002 Elsevier Science Ltd.