Equilibrium of nitrous acid-catalyzed oxidation of neptunium in nitric acid-tbp extraction system

The redox equilibrium of neptunium with nitric and nitrous acids, [[inline]] was studied in 3 to 6 M nitric acid solutions by 30 vol% TBP-dodecane extraction. The equilibrium distribution ratio of neptunium was shown to be expressed by D=D₆R/(l+R), where R is [Np(VI)]/[Np(V)] and D₆the distribution ratio of pure Np(Vl). To examine the dependence of the apparent concentration constant for the reaction, on nitric acid concentration. the redox potentials of HNO₃HNO₂solutions were measured which revealed that the activity correction on H⁺, NO₃-, HNO₂and H₂O was effective enough to give an ideal Nernst relationship for the potential of HNO₃HNO₂ 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, a_w: rational activity of water), the apparent concentration constant K_appwas shown to be expressed by K_app=K_mixY, where [[inline]] and log K_mix=-2.77 and log Y=-0.13, 0.02 and 0.28 for [HNO₃]_t=3, 4 and 6 M, respectively.