The characterization of Np2O5(c) and its dissolution in CO2-free aqueous solutions at pH 6 to 13 and 25 C

A literature method was adapted to successfully prepare pure crystalline Np2O5 in this work. Detailed and quantitative analysis of the as-prepared and post-run solid materials by XPS, XRD and SEM suggest that there was some recrystallization but no phase transformation of Np2O5(c) during the dissolution experiments, and that Np2O5(c) was stable under our experimental conditions. The solubility results show that the dissolution of Np2O5(c) in aqueous solutions reaches equilibrium in 200 d. Only the first hydrolyzed species (NpO2OH(aq)) and the free neptunyl ion (NpO2+(aq)) are necessary and sufficient to interpret all the experimental results over the entire pH range investigated (6-13); hydrolysis constants for the second- and higher-order hydrolyzed species cannot be derived from our data. The solubility product of Np2O5(c) (log KSP) and the first hydrolysis constant (log β1) for NpO2+(aq) are determined to be, respectively, -10.1 ± 0.2 and 5.38 ± 0.07. (orig.)