Influence of Di- n -butyl Phosphoric Acid on Cerium Redox and Speciation in Tri- n -butyl Phosphate.

The effects of simulated radiolytic degradation of tri- n- butyl phosphate (TBP) on the chemical speciation of cerium were studied by spectrophotometry and electrochemistry of TBP solutions containing increasing amounts of di- n -butyl phosphoric acid (HDBP), a common degradation product of TBP. Tetravalent cerium was found to exchange coordinated nitrate for the dibutyl phosphate anion, forming dinuclear complexes of the formula (CeOCe)(NO 3 ) (6- d ) (DBP) d ·3TBP ( d = 0-3). Compared to Ce(IV), Ce(III) was complexed less strongly by HDBP in TBP, but HDBP displaced both nitrate and TBP to form the series of mononuclear complexes Ce(NO 3 ) (3- d ) (HDBP·DBP) d ·(3- d )TBP ( d = 0-3). Dibutyl phosphate coordination caused large negative shifts in the Ce(IV/III) reduction potential in TBP, indicating a strong stabilization of the tetravalent state. Electrochemical investigation of the reduction of Ce(IV) in TBP revealed it to be a two-electron process in accordance with the dinuclear nature of the organic-phase Ce(IV) complexes. The diffusion coefficients of the d = 0 dinuclear Ce(IV)-nitrate-TBP complex and mononuclear Ce(III)-nitrate-TBP complex in TBP equilibrated with 7 M HNO 3 were determined to be (1.16 ± 0.06) × 10 -7 cm 2 /s and (1.9 ± 0.4) × 10 -7 cm 2 /s, respectively, which also is consistent with the larger molecular volume of the dinuclear Ce(IV) complexes.