Equilibrium of Nitrous AcidCatalyzed Oxidation of Neptunium in Nitric AcidTBP Extraction System
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Abstract
The redox equilibrium of neptunium with nitric and nitrous acids, <BR>NpO<SUB>2</SUB><SUP>+</SUP>+(3/2)H<SUP>+</SUP>+(1/2)NO<SUB>3</SUB><SUP></SUP>=NpO<SUB>2</SUB><SUP>2+</SUP>+(1/2)HNO<SUB>2</SUB>+(1/2)H<SUB>2</SUB>O<BR>was studied in 3 to 6M nitric acid solutions by 30vol% TBPdodecane extraction. The equilibrium distribution ratio of neptunium was shown to be expressed by D=D<SUB>6</SUB>R/(1+R), where R is [Np(VI)]/[Np(V)] and D<SUB>6</SUB> the distribution ratio of pure Np(VI). To examine the dependence of the apparent concentration constant for the reaction, <BR>K<SUB>app</SUB>=[NP(VI)][HNO<SUB>2</SUB>]<SUP>1/2</SUP>/[Np(V)][HNO<SUB>3</SUB>]<SUB>t</SUB><SUP>2</SUP><BR>on nitric acid concentration, the redox potentials of HNO<SUB>3</SUB>/HNO<SUB>2</SUB> solutions were measured which revealed that the activity correction on H<SUP>+</SUP>, NO<SUB>3</SUB><SUP></SUP>, HNO<SUB>2</SUB> and H<SUB>2</SUB>O was effective enough to give an ideal Nernst relationship for the potential of HNO<SUB>3</SUB>/HNO<SUB>2</SUB>. With using the activity coefficient quotient, Y=y<SUB>S</SUB><SUP>2</SUP>/(y<SUB>u</SUB>a<SUB>w</SUB>)<SUP>1/2</SUP>(y<SUB>S</SUB>: stoichiometric activity coefficient of nitric acid, y<SUB>u</SUB>: activity coefficient of HNO<SUB>2</SUB>, a<SUB>w</SUB>: rational activity of water), the apparent concentration constant K<SUB>app</SUB> was shown to be expressed by K<SUB>app</SUB>=K<SUB>mix</SUB>Y, where <BR>K<SUB>mix</SUB>=[Np(VI)]{HNO<SUB>2</SUB>}<SUP>1/2</SUP>{H<SUB>2</SUB>O}<SUP>1/2</SUP>/[NP(V)]{H<SUP>+</SUP>}<SUP>3/2</SUP>{NO<SUB>3</SUB><SUP></SUP>}<SUP>1/2</SUP><BR>and log K<SUB>mix</SUB>=2.77 and log Y=0.13, 0.02 and 0.28 for [HNO<SUB>3</SUB>]<SUB>t</SUB>=3, 4 and 6M, respectively.
Journal

 Journal of Nuclear Science and Technology

Journal of Nuclear Science and Technology 32(1), 5059, 19950125
Atomic Energy Society of Japan