Stability constants for europium(III) complexes with substituted methane diphosphonic acids in acid solutions

The stability constants of europium complexes with methane diphosphonic (MDPA), ethane-1,1- diphosphonic (EDPA), 1-hydroxyethane-1,1-diphosphonic (HEDPA), vinylidene-1,1-diphosphonic (VDPA) and 1,2-dihydroxyethane-1,1-diphosphonic (DHEDPA) acids were determined by solvent extraction distribution experiments in 2.0 M Na/HNO3 media at 25 °C. The protonation constants for these tetrabasic acids were also determined by potentiometric titration. The latter two diphosphonic acids have not been previously reported in the open literature. Stability constants for a variety of protonated complexes with M:L stoichiometries varying from 1:1 to 1:3 are indicated in the least-squares analysis of the distribution data. Calculations based on the thermodynamic data indicate that diphosphonic acids are superior complexing agents for europium compared to carboxylic acids and aminopolycarboxylic acids in solutions of acidity greater than 0.01 M. The implication of these complexes for possible applications in separation processes is discussed.