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=ys2(yuaw)1/2(ys: stoichiometric activity coefficient of nitric acid, yu: activity coefficient of HNO2, aw: rational activity of water), the apparent concentration constant Kappwas shown to be expressed by Kapp=KmixY, where [[inline]] and log Kmix=-2.77 and log Y=-0.13, 0.02 and 0.28 for [HNO3]t=3, 4 and 6 M, respectively.
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