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++(3/2H++(1/2)NO3-=NpO22++(1/2)HNO2+(1/2)H2O 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=D(6)R/(1+R), where R is [Np(VI)]/[Np(V)] and D-6 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](2)(t) 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(s)(2)/(y(u)a(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-app was shown to be expressed by K-app=KmixY, 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 6 ar, respectively.