Stoichiometry, stability constants and coordination geometry of Eu(III) 5-sulfosalicylate complex in aqueous system - A TRLIFS study

The complex formation between Eu(III) and 5-sulfosalicylate, (HSSA)(2-), has been investigated by means of TRLIFS (time resolved laser induced fluorescence spectroscopy). The concentration of free ligand in the solution was determined from the fluorescence emission of 5-sulfosalicylate by subtracting the dynamic quenching effect from the observed quenching of fluorescence emission by means of lifetime analysis, and the stoichiometry and the corresponding formation constants were obtained. A carboxylate coordinated complex, Eu(HSSA)(+), and also a chelate complex, Eu(SSA), were identified, and the formation constants of the complex Eu(HSSA)(2+) for the reaction, Eu3+ + HSSA(2-) --> Eu(HSSA)(+), and the deprotonation constant of the chelating reaction, Eu(HSSA)(+) --> Eu(SSA) + H+, were calculated at log beta(1,1) = 1.79 and log K' = -5.78, respectively. TRLIFS using the fluorescence emission from Eu(III) was performed in order to determine the number of coordination waters of the complex Eu(HSSA)(2+). The quenching of the Eu(III) fluorescence caused by (SSA)(3-) disturbed the lifetime analysis of the 'intrinsic lifetime' of Eu(III) in Eu(HSSA)(2+), however the problem was successfully solved by the analysis of emission intensity and lifetime, and the formulation of the complex was determined as [Eu(HSSA)8H(2)O](2+) with the explicit involvement of the coordinated waters.