Thermodynamics on the complexation of uranium (VI) with N-methylethylenediamine-N,N′,N'-triacetic acid in aqueous Solution: Potentiometry and microcalorimetry

Stability constants and enthalpies of N-methylethylenediamine-N,N′,N'-triacetic acid (MEDTA, denoted as H₃L) complexes with uranyl were measured using potentiometry and microcalorimetry, I = 1.0 mol L⁻¹ NaClO₄ solution at 25 °C. Thermodynamic analyses revealed three U(VI)/MEDTA complexes: UO₂(HL), UO₂L⁻, and UO₂(OH)L²⁻. The results showed that the complex formation of UO₂L⁻ (UO₂²⁺ ​+ ​L³⁻ = UO₂L⁻) is endothermic and driven solely by entropy. MEDTA in the UO₂L⁻ complex is a tetradentate and coordinates with U(VI) along the ethylenediamine backbone. Stability constant of mononuclear uranyl complex with MEDTA (UO₂L⁻) is lower than UO₂²⁺/EDDA complex but higher than UO₂²⁺/HEDTA complex, indicating that dangling acetate arm of MEDTA or hydroxyethyl group of HEDTA would lower the stability.