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The pH evolution and corresponding changes in the UV−Vis−NIR absorption spectra of oxygenated neptunium (NpO 2 + and NpO 2 2+) and uranyl ions (UO 2 2+ ) in nitric acid are investigated during titration with an aqueous NH 3 solution. The speciation and precipitation regimes between acidic (pH 1.5) and alkaline (pH 10) conditions at room temperature are discussed to assess the suitability of Np(V) or Np(VI) in sol−gel conversion processes for fuel target fabrication. Under the applied experimental conditions, Np(V) hydrolyzes and precipitates into the insoluble hydroxide NpO 2 OH only above pH values 7.5 and an increase up to pH 10.0 is required to precipitate quantitatively. Np(VI) displays changes in the coordination environment of NpO 2 2+ ions in the pH interval 1.6− 4.0, similar to what is observed for U(VI). Precipitation into NpO 3 •H 2 O or other hydroxide compounds takes place between pH 4.0 and 5.9, which overlaps largely with precipitation of ammonium diuranate species from the U(VI) solution. The use of concentrated NH 3 aqueous solution, as commonly used in the external gelation process, will allow to quantitatively precipitate both Np(V) and Np(VI) species. Internal gelation process conditions, on the other hand, seem incompatible with the high pH required to precipitate Np(V) completely. For fabricating mixed-oxide (U,Np) targets using sol−gel conversion, a feed broth containing Np(VI) and U(VI) will be required to achieve homogeneous gelation.

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