## Abstract

The redox equilibrium of neptunium with nitric and nitrous acids, NpO_{2}^{+}+(3/2)H^{+}+(1/2)NO_{3}^{-}=NpO_{2}^{2+}+(1/2)HNO_{2}+(1/2)H_{2}O 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=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)][HNO_{2}]^{1/2}/[Np(V)][HNO_{3}]_{t}^{2} on nitric acid concentration, the redox potentials of HNO_{3}/HNO_{2} solutions were measured which revealed that the activity correction on H^{+}, NO_{3}^{-}, HNO_{2} and H_{2}O was effective enough to give an ideal Nernst relationship for the potential of HNO_{3}/HNO_{2}. 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 HNO_{2}, aw: rational activity of water), the apparent concentration constant Kapp was shown to be expressed by K_{app}=K_{mix}Y, where K_{mix}=[Np(VI)]{HNO_{2}}^{1/2}{H_{2}O}^{1/2}/[NP(V)]{H^{+}}^{3/2}{NO_{3}^{-}}^{1/2} and log K_{mix}=-2.77 and log Y=-0.13, 0.02 and 0.28 for [HNO_{3}]_{t}=3, 4 and 6M, respectively.

Original language | English |
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Pages (from-to) | 50-59 |

Number of pages | 10 |

Journal | Journal of Nuclear Science and Technology |

Volume | 32 |

Issue number | 1 |

DOIs | |

Publication status | Published - 1995 Jan 1 |

Externally published | Yes |

## Keywords

- 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