The redox equilibrium of neptunium with nitric and nitrous acids,
NpO
2++(3/2)H
++(1/2)NO
3-=NpO
22++(1/2)HNO
2+(1/2)H
2O
was studied in 3 to 6M nitric acid solutions by 30vol% TBP-dodecane extraction. The equilibrium distribution ratio of neptunium was shown to be expressed by D=D
6R/(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]
t2on 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
2O 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
S2/(y
ua
w)
1/2(y
S: stoichiometric activity coefficient of nitric acid, y
u: activity coefficient of
HNO2
,
aw
: rational activity of water), the apparent concentration constant K
app was shown to be expressed by K
app=K
mixY, where
K
mix=[Np(VI)]{HNO
2}
1/2{H
2O}
1/2/[NP(V)]{H
+}
3/2{NO
3-}
1/2and log K
mix=-2.77 and log Y=-0.13, 0.02 and 0.28 for [HNO
3]
t=3, 4 and 6M, respectively.
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