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

Abstract

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⁺ + HSSA²⁻ → Eu(HSSA)⁺, and the deprotonation constant of the chelating reaction, Eu(HSSA)⁺ → Eu(SSA) + H⁺, were calculated at log β_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)8H₂O]²⁺ with the explicit involvement of the coordinated waters.