Direct determination of stability constants of lanthanide ion chelates by laser-excited europium(III) luminescence spectroscopy: application to cyclic and acyclic aminocarboxylate complexes

Abstract

Excitation spectroscopy of the ⁷ F ₀ → ⁵ D ₀ transition of Eu ³⁺ in the visible has been used to quantitate the complexation of this ion to various chelating carboxylate ligands including 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate (dota). Two methods were employed. The first involves the measurement of conditional stability constants at constant low pH values where hydrogen ions compete with Eu ³⁺ for the ligand. The second method involves measurement of the concentration of the Eu ³⁺ –ligand complex as a function of pH in the low pH region. The data are then fitted by an equation which accounts for the multiple equilibria involved (ligand and complex protonation) with the stability constant being a parameter to be fit. For [Eu(dota)] ^- a log K value of 26.21 ± 0.10 was obtained which falls among a wide range of reported values. The values for the remaining ligands are in excellent accord with literature values. The log K values for complexes of dota were obtained for the entire Ln ³⁺ ion series by a metal ion competition method. This ligand discriminates effectively against the larger members of this series.