Dioxouranium(VI)-Carboxylate Complexes. Speciation of UO22+-1,2,3-Propanetricarboxylate System in NaClaq at Different Ionic Strengths and at T = 25 °C

Abstract

The formation constants of dioxouranium(VI) - 1,2,3-propanetricarboxylate [tricarballylate (3-), TCA] complexes were determined in NaCl aqueous solutions at 0 ≤ I / mol L^-1 ≤ 1.0 and t = 25 °C, by potentiometry, ISE-[H⁺] glass electrode. The speciation model obtained at each ionic strength includes the following species: ML-, MLH⁰, ML₂^4- and ML₂H^3- (M = UO₂²⁺ and L = TCA). The dependence on ionic strength of protonation constants of 1,2,3-propanetricarboxylate and of the metal-ligand complexes was modeled by the SIT (Specific ion Interaction Theory) approach and by the Pitzer equations. The formation constants at infinite dilution are [for the generic equilibrium p UO₂²⁺ + q (L^3-) + r H⁺ = (UO₂²⁺)_p(L)_qH_r^(2p−3q+r); β_pqr]: log β₁₁₀ = 6.222±0.030, log β₁₁₁ = 11.251±0.009, log β₁₂₁ = 7.75±0.02, log β₁₂₁ = 14.33±0.06. The sequestering ability of 1,2,3-propanetricarboxylate towards UOUO₂²⁺ was quantified by using a sigmoid Boltzman type equation.