Speciation of the uranyl nitrate system via spectrophotometric titrations

The speciation of the uranyl nitrate system has been studied previously with limited success producing a wide range of stability constant values. The current literature has values for the mononitrate species, with scattered data for higher nitrate species. Furthermore, the reported values vary with experimental method. The work presented here examines the stability of the uranyl/nitrate/perchlorate/water system via spectrophotometric titrations with a focus on the predominance of the uranyl dinitrate species at low nitrate concentrations. Stability constants were determined at ionic strengths ranging from 1 to 6 molal followed by refinement with the specific ion interaction theory. The zero ionic strength stability constant of the uranyl dinitrate species was refined as log β (2,1)º = 3.37 ± 0.02 when including the stability constant for the uranyl mononitrate from Ahrland and 2.66 ± 0.02 without. These values are considerably higher than previous studies, which is attributed to the alternate speciation model used. The values generated in this work produce speciation diagrams that are consistent with published solvent extraction data of the uranyl nitrate system.