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The hydrolysis, colloid formation and solubility of Np(IV) are investigated in aqueous HClO 4 -NaClO 4  solutions (log [H + ] = 0 to -2.5) by absorption spectroscopy in the wavelength range of 680-1000 nm. Applying Laser induced photoacoustic spectroscopy (LPAS) in the range of 680-760 nm, the study is extended to low Np(IV) concentrations of 10 -6  mol/l in  DClO 4 -NaClO 4 -D 2 O solutions up to log [D + ]=-3.3. Laser induced breakdown detection (LIBD) demonstrates the formation of Np(IV) colloids when the Np(IV) concentration exceeds the solubility of Np(OH) 4 (am) at given pH. The simultaneous decrease of the Np(IV) absorption bands at 723 and 960 nm cannot be ascribed to the formation of the mononuclear complex Np(OH) 3+  as assumed in the literature. It is found to be caused by polynucleation. In undersaturated Np(IV) solutions below 10 -4  mol/l, the position and intensity of the absorption maximum at 723 nm are practically insensitive to the pH change. In oversaturated solutions the absorption band decreases significantly. The spectroscopically determined pH-dependent equilibrium concentration of mononuclear Np(IV) species above freshly formed solid or colloidal Np(IV) particles indicates that Np(OH) 2  2+  is the predominant species in the pH range of 1.5-3. This finding is in agreement with the Np(IV) hydrolysis constants reported in the literature from a solvent extraction study with  239 Np(IV) trace concentrations. The solubility product of freshly formed Np(OH) 4 (am) particles is determined to be log  K ' sp  = -54.4±0.4 in 0.1 M HClO 4 -NaClO 4  and log  K ° sp =-56.5±0.4  (converted to  I =0 by applying the SIT).

A spectroscopic study of the hydrolysis, colloid formation and solubility of Np(IV)