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

Osamu Tochiyama, Yoshinobu Nakamura, Yasushi Katayama, Yasushi Inoue

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3 Citations (Scopus)


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, Kapp=[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=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 Kapp was shown to be expressed by Kapp=KmixY, where Kmix=[Np(VI)]{HNO2}1/2{H2O}1/2/[NP(V)]{H+}3/2{NO3-}1/2 and log Kmix=-2.77 and log Y=-0.13, 0.02 and 0.28 for [HNO3]t=3, 4 and 6M, respectively.

Original languageEnglish
Pages (from-to)50-59
Number of pages10
JournalJournal of Nuclear Science and Technology
Issue number1
Publication statusPublished - 1995 Jan 1
Externally publishedYes


  • Purex process
  • TBP
  • activity coefficient
  • neptunium
  • nitric acid
  • nitrous acid
  • pH value
  • redox potential
  • redox reaction
  • solvent extraction

ASJC Scopus subject areas

  • Nuclear and High Energy Physics
  • Nuclear Energy and Engineering


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