THE SOLUBILITIES OF SODIUM AND POTASSIUM DIOXONEPTUNIUM(V) CARBONATE HYDRATES AT 30-DEGREES-C, 50-DEGREES-C AND 75-DEGREES-C

To determine how the formation constants of dioxoneptunium(V) carbonate complexes change with temperature, we have measured the equilibrium concentration of Np-237 in aqueous solution over hydrated NaNpO2CO3 and Na3NpO2(CO3)2 at 30, 50 and 75-degrees-C. Solutions having a wide range of carbonate concentrations, a constant sodium ion concentration of 1.0 mol . dm-3, and approximately constant ionic strength (NaClO4) were contacted with the solids for 2-4 weeks. In some cases mixtures of NaNpO2CO3 and Na3NpO2(CO3)2 were also used. At 30-degrees-C the observed solubility behaviour of NaNpO2CO3 was similar to that reported in the literature for 25-degrees-C, whereas, at higher temperatures, the solubility minimum is less pronounced and appears to be shifted to higher carbonate concentrations. A limited number of similar measurements were done by equilibrating hydrated KNpO2CO3 with K2CO3/KCl aqueous solutions. Powder X-ray diffraction data indicate that, at the higher temperatures, hydrated NaNpO2CO3 is gradually converted to solids with higher ratios of sodium and carbonate to neptunium, such as hydrated Na3NpO2(CO3)2. Conversely, if the concentration of added carbonate is low, hydrated Na3NpO2(CO3)2 readily loses Na2CO3 to the solution. The data are interpreted in terms of the relative stabilities of the sodium dioxoneptunium(V) carbonate solids and the temperature dependence of the stability constants of complexes of NpO2+ with carbonate. Interconversion of the solids is slow in both the sodium and potassium dioxoneptunium(V) carbonate systems.