Dioxouranium(VI) – Carboxylate Complexes. Interaction with Dicarboxylic Acids in Alqueous Solution: Speciation and Structure

Abstract

In this paper we report the results of an investigation performed by potentiometric (H+-glass electrode) and visible spectrophotometric measurements on the interaction of UO2(2+) ion towards some carboxylic ligands (acetate, malonate, succinate, azelate). The measurements were carried out at T= 25 degrees C in different ionic media (KNO3 and NaCl) at different ionic strengths (0.1 < or = I/mol L(-1) < or = 1.0, NaCl; I/mol L(-1) = 0.1, KNO3). The dependence on ionic strength of formation constants was taken into account by using both a simple Debye-Hückel type equation and the SIT (Specific ion Interaction Theory) approach. Different speciation models (depending on concentration of reagents, ionic strength, pH-range) both for different carboxylates and different ionic media have been obtained. Linear combinations between formation constants, stoichiometric coefficients and length of alkyl chain of dicarboxylates have been observed and predicted formation constants at I= 0 mol L(-1) are reported for the interaction of UO2(2+) with HOOC-(CH2)n-COOH with 1 < or = n < or = 7. Finally, a visible absorption spectrum for each complex reaching a significant percentage of formation in solution (KNO3 medium) has been calculated to characterise the compounds found by pH-metric refinement.

Assessing the formation of weak sodium complexes with negatively charged ligands

“…8 Recently, the detection of sodium ion complexes and relative stability constants were performed at constant values of ionic strength, by employing a sodium ion-selective electrode (ISE), and it was confirmed unequivocally the formation of sodium complexes, even with bidentate carboxylic ligands. 9 Furthermore, the values of stability constants of the sodium complexes, determined by ISE, fall in agreement with those measured with pH-metric technique. Daniele et al suggested that the chemical bond in these compounds has a predominately ionic nature, favouring ligand with negatively charged oxygen donor groups, 9 which is also supported by theoretical calculations (see discussion).…”

“…9 Furthermore, the values of stability constants of the sodium complexes, determined by ISE, fall in agreement with those measured with pH-metric technique. Daniele et al suggested that the chemical bond in these compounds has a predominately ionic nature, favouring ligand with negatively charged oxygen donor groups, 9 which is also supported by theoretical calculations (see discussion). However, our preliminary observations suggested 9 that the stability of the Na complexes is highly affected by nitrogen donor groups of the ligands.…”

“…In order to support the experimental evidence of the complex formation, an estimation of pNa-combined uncertainty is proposed. The uncertainty budgets were evaluated following the procedure described elsewere 9,11 for a potentiometric apparatus calibrated according to the concentration scale. The procedure follows the metrological literature recommendations.…”

“…9 The expanded Debye-Hückel formalism of Eq. (1) is applied to compute the formation constants for various ionic strengths: (1) where K eq (I') is the experimental formation constant at ionic strength I', K eq (I) is the formation constant obtained at ionic strength I, C and D are calculated from Eq.…”

“…react -Σ (stoichiometric coef.) prod and z* = Σ (charge) The main contribution to the uncertainty of the formation constants (u(log NaL )) is the uncertainty of E 0 (u(E 0 )), therefore, in order to estimate u(log NaL ) the u(E 0 ) 9 was applied in the elaboration of titration data with the BSTAC software. The difference between the log NaL obtained using…”