Abstract
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.
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