Thermodynamic studies of actinide complexes. 1. A reappraisal of the solution equilibria between plutonium(4) and ethylene-diamine-tetraacetic acid (EDTAH4) in nitric media

A detailed reevaluation of the complexation equilibria of plutonium(IV) with the poly-amino-carboxylic sequestering agent EDTA4- has been performed in 1 M (H,K)NO3 media at 298 K by means of visible absorption spectrophotometry and glass-electrode potentiometry at millimolar concentration levels. The high binding affinity even under strongly acidic conditions supports the exclusive formation of the neutral Pu(EDTA) complex over the range 0.01≤ [H+] ≤ 0.9 M with an apparent formation constant of log β110 = 25.8(1) at 0.9 M HNO3. Extrapolation to zero ionic strength using the SIT approach provides the first ever-reported NEA-TDB compliant estimate of β0110 (logβ0110=32.2(3)). A global modeling procedure of the potentiometric data collected for three different metal over ligand ratios (1:1, 1:1.5, and 1:2) ascertained the formation of four additional species for pH values comprised between 2 and 6.5, namely [Pu(EDTA)(OH)]-, [Pu(EDTA)2H2]2-, [Pu(EDTA)2H]3-, and [Pu(EDTA)2]4-. The visible absorption spectra for the first three complexes have also been determined. Data recorded above pH 3.5 (for a 1:1 molar ratio) or 6.5 (for a 1:2 molar ratio) were finally excluded from the global fitting procedure as speciation calculations suggested the precipitation of amorphous Pu(OH)4(am) according to the latest critical value quoted in the literature for its solubility product. (authors)