Equilibrium of Nitrous Acid-Catalyzed Oxidation of Neptunium in Nitric Acid-TBP Extraction System

The redox equilibrium of neptunium with nitric and nitrous acids, NpO₂⁺+(3/2)H⁺+(1/2)NO₃^-=NpO₂²⁺+(1/2)HNO₂+(1/2)H₂O was studied in 3 to 6M nitric acid solutions by 30 vol% TBP-dodecane extraction. The equilibrium distribution ratio of neptunium was shown to be expressed by D=D₆R/(1+R), where R is [Np(VI)]/[Np(V)] and D₆ the distribution ratio of pure Np(VI). To examine the dependence of the apparent concentration constant for the reaction, K_app=[NP(VI)][HNO₂]^1/2/[Np(V)][HNO₃]_t² on nitric acid concentration, the redox potentials of HNO₃/HNO₂ solutions were measured which revealed that the activity correction on H⁺, NO₃^-, HNO₂ and H₂O was effective enough to give an ideal Nernst relationship for the potential of HNO₃/HNO₂. With using the activity coefficient quotient, Y=y_S²/(y_ua_w)^1/2 (y_S: stoichiometric activity coefficient of nitric acid, y_u: activity coefficient of HNO₂, aw: rational activity of water), the apparent concentration constant Kapp was shown to be expressed by K_app=K_mixY, where K_mix=[Np(VI)]{HNO₂}^1/2{H₂O}^1/2/[NP(V)]{H⁺}^3/2{NO₃^-}^1/2 and log K_mix=-2.77 and log Y=-0.13, 0.02 and 0.28 for [HNO₃]_t=3, 4 and 6M, respectively.