Complexation of Eu(III) by citrate in aqueous solution

Full text of publication follows: Citric acid, a natural hydroxy-carboxylic acid, forms different complexes with actinide and lanthanide ions with bidentate, ter-dentate, binuclear and polymeric conformation. Therefore, the stability of the citrate complex is fairly high providing the useful information on modeling the complexation of actinides and lanthanides with large molecules. The speciation of Ln(III)-citrate system is not fully understood especially under conditions where citrate is present in large excess since trivalent Ln ions have more degree of freedom for the coordination geometries than actinyl ions which have oxygen in axial parts. Actinide ions are known to form polynuclear complex with citrate. On the other hand, lanthanide aquo ions seldom form polynuclear complexes in aqueous solution. The complexation of lanthanides depends on pH and the metal-hydroxo ternary complex can be formed around neutral or alkaline condition. This may suggest the formation of polynuclear complexes of lanthanides, but only a few studies focused on the coordination structure of lanthanide-citrate complex in aqueous solution have been performed. The formation of polynuclear complexes has the relationship with the hydrolysis reactions. Therefore, the hydrolysis behavior in Eu(III)-citrate system is essential to understand the formation of polynuclear complexes. In this work, the understanding of stoichiometry for complexation of Eu(III) by citrate was investigated. Laser spectroscopy and potentiometric titration are useful techniques to determine the composition of the species present in the aquatic solutions and hydrolysis reaction can be determined via these methods. The hydration state, q, of Eu(III)-citric acid complexes can be evaluated by observing luminescence lifetime of Eu(III). In the present study, we observed the luminescence lifetime of Eu(III) in H2O and D2O solution in the presence and in the absence of citric acids (total concentration of Eu(III) 2.5, 5.0, 10.0 mM; total concentration of citrate = 167, 250, 450, 900 mM) at [Na+] = 3.0 M and pH = 8.5. According to the empirical analysis proposed by Beeby et al [1], the hydration states q was estimated at 2.0-3.0. All of the q values decreased as Ccit increased. Other techniques (e.g. potentiometric titration) will be performed to identify the stoichiometry of the Ln(III)-citrate system. [1] A. Beeby, et al., J. Chem. Soc., Perkin Trans.2, 1999, (3), 493-504. (authors)