The Solubilities of Sodium and Potassium Dioxoneptunium(V) Carbonate Hydrates at 30, 50 and 75°C

To determine how the formation constants of dioxoneptunium(V) carbonate complexes change with temperature, we have measured the equilibrium concentration of 2 3 7 Np in aqueous solution over hydrated N a N p 0 2 C 0 3 and Na 3 Np0 2 (C0 3 ) 2 at 30, 50 and 75 °C. Solutions having a wide range of carbonate concentrations, a constant sodium ion concentration of 1.0 mol · d m 3 , and approximately constant ionic strength (NaC104) were contacted with the solids for 2—4 weeks. In some cases mixtures of N a N p 0 2 C 0 3 and Na3Np02(CC>3)2 were also used. At 30°C the observed solubility behaviour of N a N p 0 2 C 0 3 was similar to that reported in the literature for 25°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 K N p 0 2 C 0 3 with K 2 C0 3 /KC1 aqueous solutions. Powder X-ray diffraction data indicate that, at the higher temperatures, hydrated N a N p 0 2 C 0 3 is gradually converted to solids with higher ratios of sodium and carbonate to neptunium, such as hydrated Na 3 Np0 2 (C0 3 ) 2 . Conversely, if the concentration of added carbonate is low, hydrated N a 3 N p 0 2 ( C 0 3 ) 2 readily loses N a 2 C 0 3 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 N p 0 2 with carbonate. Interconversion of the solids is slow in both the sodium and potassium dioxoneptunium(V) carbonate systems.