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Some properties of heptavalent plutonium in molten KOH-NaOH eutectic
INORG. NUCL. CHEM. LETTERS
Vol. 8,
pp. 861-867, 1972. Pergamon Press. Printed
in
Great
Britain.
SOME PROPERTIES OF HEPTAVALENT PLUTONIUM IN MOLTEN KOH-NaOH EUTECTIC L. Martinot* and G. Duyckaerts Laboratory of Nuclear Chemistry University of Liege rue A. St@vart 2 B - 4000 LIEGE (Belgium) (Received 12 May 1972)
ABSTRACT We have investigated
the electrochemical
reduction of Pu
in
molten KOH-NaOH at 240°C. This reduction proceeds in two steps whose last one is rate determining PuO~- + e-
, >
PuO~ + + 2e-
PuO~ + + 302>
PuO 2
-'7INTRODUCTION In study
a previous
of
the
paper
reduction
(1), of
we h a v e
heptavalent
reported
a chronopotentiometric
Neptunium
in
molten
KOH-NaOH
eutectic. A similar
work has
now been
carried
out
with
Plutonium.
of Pu(VII) has allowed only to approach the qualitative
The short
aspect of the
question.
* Chercheur
i
l'Institut
Interuniversitaire
861
des
Sciences
life
Nucl6aires.
:
862
H E P T A V A L E N T PLUTONIUM
Vol. 8, No. 10
EXPERIMENTAL The melt preparation,
the chronopotentiometric
electrodes have been described earlier
circuit and the
(1). The indicator electrode
was a graphite single crystal with cleavage plane exposed to the melt
(S = O.16 cm2). All attempts to oxidize PuO 2 dissolved in the bath by continuous
flowing of 02 were unsuccessful The Pu(VII)
: only Pu(VI) was obtained.
solutions were easily prepared by dissolving weighted
amounts of Na3PuO 5 in the melt whose color turned immediately
to dark
green. This compound was obtained according to the method proposed by Keller and Seifert
(2). A typical sample was obtaimed by mixing
together and heating 54.2 mg of PuO 2 and 46.6 mg of Na202 for 26 hours, at 420°C, in a platinum erucible and in a pure oxygen flow. To check the efficiency of the reaction,
a ~ 10 mg sample is dissolved in
3M NaOH and the absorption spectrum is taken : Pu(VII) peaks appear at 400 - 410 n.m. and at 620 - 630 n.m. peaks at 820 - 830 n.m. and at 1010 - 1020 n.m.
characteristic
(2,3). Pu(VI)
(3) never had appeared.
The composition of the melt under investigation was also checked by comparing the visible spectrum of a "frozen" part of the melt, dissolved in water, with the literature data (2,3)
RESULTS
Preliminary
experiments have shown that when Na3PuO 5 is introduced
into the melt at 240°C, Pu(VI) the Pu(VII)
appeared within 10 - 15 minutes. At 290°C,
life time was even shorter and no experiment was ~udertaken.
Vol. 8, No. 10
H E P T A V A L E N T PLUTONIUM
A typical is
shown in
Fig.
steps reduction
1.
chronopotentiogram The s h a p e o f
but no overshoot
the
of
the
863
reduction
w a v e seems t o
fit
of
Pu(~I)
with
a two
for the first transition
appears
clearly. We use the classical Sand's io~ I
I/2
formula
- where i o is the current
time - for the first reaction. calculations
and further
(I) to calculate
the product
density and ~I the first transition
Large discrepancies
estimations
arise in these
of the diffusion
coefficient
would be meaningless.
FIG.
1
A typical chronopotentiogram for the reduction
of PuO53-
~Pu053-~ = 1.285.10-3 i~ = 0.55 mA
m.f.
864
HEPTAVALENT
Pu(VII~
x 10 3
PLUTONIUM
Vol.
i ° I 11/2(m A sec 1/2)
Number of
at 240 ° C
determinations
0.635
O. 18 + 0.09
5
1.285
0.39 + 0.25
8
1.490
0.43 + 0.2
6
1.510
0.38 + 0.2
4
m.f.
8,
The ratio between the second transition time 1 2 and the first one, is about 11 but this value increases markedly with the elapsed time. c) Tafel lines Tafel lines were drawn for the last two solutions according to the galvanostatic method as already reported by us (2,4). The experimemtal slopes ~ E / ~log i
o
- E beimg the potential off,he
indicator electrode - for these experiments equal - 0.048 and - 0.055
(Fi~. 2) E (volt -0.20
o
Oo
O
-0.2@
® L
i
,
0.1
FIG. 2
•
,
,
1 l
,
,
Tafel lines for the reduction of PuO 3-
[ uo 1 o
•
O.5 L0 ImAl.--~
No.
I0
Vol. 80No. 10
HEPTAVALENT PLUTONIUM
865
DISCUSSION As a preliminary hypothesis, and Pu(VII)
we shall consider that both Np(VII)
exist in the melt as anions
: PuO~-, NpO~-. The same situ-
ation holds true in concentrated KOH or NaOH aqueous solutions. Furthermore,
the chronopotentiometric
reduction proceeds in two steps,
the last one involving PuO 2 precipitation
on the indicator electrode
as it is the rule in uranyl and neptunyl ions reduction According to Delahay's theory
(1).
(5) on chronopotentiometry,
two consecutive reactions occur, the transition time ~ 2
when
of the second
one is given by :
~nl/
in1 / J
n I : number of electrons of the first reaction n 2 : number of electrons for the second reaction For
these
two consecutive
Pu(VII) Pu(VI)
reactions
:
+ e-
>
Pu(VI)
nI = 1
+ 2e-
>
Pu(IV)
n2 = 2
the
ratio
%2/~1
the
best
one to
will
be 8.
account
This
for
As the experimental ratio of of Pu(VI) in the melt
the
combination high
~2/'~1
of
partial
experimental
value
reductions obtained
is :
increases with spontaneous formation
- checked by visible spectra - we may explai~ that
the theoretical value ~2/~[I = 8 is not observed, ning of each experiment,
even at the very begin-
because a small amount of Pu(VI) is immediately
formed when Na3PuO 5 is poured into the bath.
866
H E P T A V A L E N T PLUTONIUM
Vol. 8, No. 10
The preparation of a pure Pu(Vll) solution is therefore impossible and this would lead to a greater value of the ratio T2/~I. A similar phenomenon is also observed for the reduction UO2(VI) + e-
~
UO2(V) | UO2(V) + e-
--~
UO 2 in LiC1-KC1 (6)
where the theoretical ratio ~ 2/~q = 3 is only observed with pure UO2(VI) solutions. If UO2(V) appears in the melt, the ratio 1 2 / ~ 1 increases rapidly. Thus the most probable reaction scheme for the electrochemical reduction of Pu(VII) will be : PuO~- + e-
~
PuO~ + + 2e-
PuO~+ + 302~> PuO 2
When Na3PuO 5 is added to the bath at 240°C, a chemical reduction by the OH a)
ions or an internal oxido-reduction occurs :
2PuO
+ 2OH-
~
2PuO + + 602- + 2H + + 02
which is followed by 2H + + 2OHb)
2PuO~-
~
~
2H20 ; 2H20 + 202-
~
40H-
22PuO~ + + 402- + 02
This electrochemical two steps mechanism is well supported by the analysis of the slope~ E / ~ l o g
i ° of the Tafel lines.
When an electrochemical reaction proceeds in several steps whose jt~ is the slow determining step, Mauser (7) uses the formula E/~log
i° =
F2.3Aj RT
for the calculation of the Tafel slope values.
Aj is a complex term depending on the number of electrons nj transferred in the slow step, the electrochemical transfer coefficient a and the number of electrons _ ~ n i in all the steps preceding the jth one.
AJ = m nj + ~ j-lq
ni
Vol. 8, No. I 0
HEPTAVALENT PLUTONIUM
From the above quoted values f o r ~ E / ~ l o g
867
io, it comes that A J C 2
with 2"3F RT = 0.102 at 240°C. If we assume that PuO~ + + 2eis the rate determining step, nj = 2 and
~ 1 j-1
>
PuO 2
ni
Aj = 2 fits well the experimental data for = = 0.5. This value of the electrochemical transfer coefficient has already been pointed out for the reduction of NpO~" (I) in molten KOH - NaOH
CONCLUSION As it is observed in aqueous alkaline solutions, the stability of heptavalent Plutonium is inferior to that of heptavalent Neptunium in a KOH-NaOH melt at 240°0. Spontaneous reduction of Pu(VII) gives only Pu(VI) as a reaction product. The electrochemical reduction proceeds in two steps : PuO;- + ePuO~ + + 2e-
>
PuO~+ + 302>
PuO 2 (rate determining step)
ACKNOWLEDGMENTS We thank the I.I.S.N. for the financial and general support of this work.
REFERENCES
(1) L. MARTINOT and (].. D~CKAERTS, I . N . C . L . , (2) e . KELLER and H. SEIFERT, I . N . 0 . L . ,
6 , 546 (1970)
~ , 51 (1969)
(3) V.I. SPITSYN et al., J.I.N.C., 31, 2733 (1969) E. HENRICH, Thesis from Ruprecht - Karl - UniversitMt, Heidelberg
1971 (4) L. MARTINOT and G. DUYCKAERTS, Anal. Letters, 4, 1 (1971)
(5) P. DELAHAY, New I n s t r u m e n t a l
Methods i n E l e c t r o c h e m i s t r y ,
Interscienoe Publishers, New-York 1964, P. 179 (6) F. CALIGARA, L. MARTINOT and G. DUYCKAERTS, I.N.C.L., 4, 169 (1960) (7) F. MAUSF~, Zeit. Elektrochemie, 62, 419 (195S).
Vol. 8,
pp. 861-867, 1972. Pergamon Press. Printed
in
Great
Britain.
SOME PROPERTIES OF HEPTAVALENT PLUTONIUM IN MOLTEN KOH-NaOH EUTECTIC L. Martinot* and G. Duyckaerts Laboratory of Nuclear Chemistry University of Liege rue A. St@vart 2 B - 4000 LIEGE (Belgium) (Received 12 May 1972)
ABSTRACT We have investigated
the electrochemical
reduction of Pu
in
molten KOH-NaOH at 240°C. This reduction proceeds in two steps whose last one is rate determining PuO~- + e-
, >
PuO~ + + 2e-
PuO~ + + 302>
PuO 2
-'7INTRODUCTION In study
a previous
of
the
paper
reduction
(1), of
we h a v e
heptavalent
reported
a chronopotentiometric
Neptunium
in
molten
KOH-NaOH
eutectic. A similar
work has
now been
carried
out
with
Plutonium.
of Pu(VII) has allowed only to approach the qualitative
The short
aspect of the
question.
* Chercheur
i
l'Institut
Interuniversitaire
861
des
Sciences
life
Nucl6aires.
:
862
H E P T A V A L E N T PLUTONIUM
Vol. 8, No. 10
EXPERIMENTAL The melt preparation,
the chronopotentiometric
electrodes have been described earlier
circuit and the
(1). The indicator electrode
was a graphite single crystal with cleavage plane exposed to the melt
(S = O.16 cm2). All attempts to oxidize PuO 2 dissolved in the bath by continuous
flowing of 02 were unsuccessful The Pu(VII)
: only Pu(VI) was obtained.
solutions were easily prepared by dissolving weighted
amounts of Na3PuO 5 in the melt whose color turned immediately
to dark
green. This compound was obtained according to the method proposed by Keller and Seifert
(2). A typical sample was obtaimed by mixing
together and heating 54.2 mg of PuO 2 and 46.6 mg of Na202 for 26 hours, at 420°C, in a platinum erucible and in a pure oxygen flow. To check the efficiency of the reaction,
a ~ 10 mg sample is dissolved in
3M NaOH and the absorption spectrum is taken : Pu(VII) peaks appear at 400 - 410 n.m. and at 620 - 630 n.m. peaks at 820 - 830 n.m. and at 1010 - 1020 n.m.
characteristic
(2,3). Pu(VI)
(3) never had appeared.
The composition of the melt under investigation was also checked by comparing the visible spectrum of a "frozen" part of the melt, dissolved in water, with the literature data (2,3)
RESULTS
Preliminary
experiments have shown that when Na3PuO 5 is introduced
into the melt at 240°C, Pu(VI) the Pu(VII)
appeared within 10 - 15 minutes. At 290°C,
life time was even shorter and no experiment was ~udertaken.
Vol. 8, No. 10
H E P T A V A L E N T PLUTONIUM
A typical is
shown in
Fig.
steps reduction
1.
chronopotentiogram The s h a p e o f
but no overshoot
the
of
the
863
reduction
w a v e seems t o
fit
of
Pu(~I)
with
a two
for the first transition
appears
clearly. We use the classical Sand's io~ I
I/2
formula
- where i o is the current
time - for the first reaction. calculations
and further
(I) to calculate
the product
density and ~I the first transition
Large discrepancies
estimations
arise in these
of the diffusion
coefficient
would be meaningless.
FIG.
1
A typical chronopotentiogram for the reduction
of PuO53-
~Pu053-~ = 1.285.10-3 i~ = 0.55 mA
m.f.
864
HEPTAVALENT
Pu(VII~
x 10 3
PLUTONIUM
Vol.
i ° I 11/2(m A sec 1/2)
Number of
at 240 ° C
determinations
0.635
O. 18 + 0.09
5
1.285
0.39 + 0.25
8
1.490
0.43 + 0.2
6
1.510
0.38 + 0.2
4
m.f.
8,
The ratio between the second transition time 1 2 and the first one, is about 11 but this value increases markedly with the elapsed time. c) Tafel lines Tafel lines were drawn for the last two solutions according to the galvanostatic method as already reported by us (2,4). The experimemtal slopes ~ E / ~log i
o
- E beimg the potential off,he
indicator electrode - for these experiments equal - 0.048 and - 0.055
(Fi~. 2) E (volt -0.20
o
Oo
O
-0.2@
® L
i
,
0.1
FIG. 2
•
,
,
1 l
,
,
Tafel lines for the reduction of PuO 3-
[ uo 1 o
•
O.5 L0 ImAl.--~
No.
I0
Vol. 80No. 10
HEPTAVALENT PLUTONIUM
865
DISCUSSION As a preliminary hypothesis, and Pu(VII)
we shall consider that both Np(VII)
exist in the melt as anions
: PuO~-, NpO~-. The same situ-
ation holds true in concentrated KOH or NaOH aqueous solutions. Furthermore,
the chronopotentiometric
reduction proceeds in two steps,
the last one involving PuO 2 precipitation
on the indicator electrode
as it is the rule in uranyl and neptunyl ions reduction According to Delahay's theory
(1).
(5) on chronopotentiometry,
two consecutive reactions occur, the transition time ~ 2
when
of the second
one is given by :
~nl/
in1 / J
n I : number of electrons of the first reaction n 2 : number of electrons for the second reaction For
these
two consecutive
Pu(VII) Pu(VI)
reactions
:
+ e-
>
Pu(VI)
nI = 1
+ 2e-
>
Pu(IV)
n2 = 2
the
ratio
%2/~1
the
best
one to
will
be 8.
account
This
for
As the experimental ratio of of Pu(VI) in the melt
the
combination high
~2/'~1
of
partial
experimental
value
reductions obtained
is :
increases with spontaneous formation
- checked by visible spectra - we may explai~ that
the theoretical value ~2/~[I = 8 is not observed, ning of each experiment,
even at the very begin-
because a small amount of Pu(VI) is immediately
formed when Na3PuO 5 is poured into the bath.
866
H E P T A V A L E N T PLUTONIUM
Vol. 8, No. 10
The preparation of a pure Pu(Vll) solution is therefore impossible and this would lead to a greater value of the ratio T2/~I. A similar phenomenon is also observed for the reduction UO2(VI) + e-
~
UO2(V) | UO2(V) + e-
--~
UO 2 in LiC1-KC1 (6)
where the theoretical ratio ~ 2/~q = 3 is only observed with pure UO2(VI) solutions. If UO2(V) appears in the melt, the ratio 1 2 / ~ 1 increases rapidly. Thus the most probable reaction scheme for the electrochemical reduction of Pu(VII) will be : PuO~- + e-
~
PuO~ + + 2e-
PuO~+ + 302~> PuO 2
When Na3PuO 5 is added to the bath at 240°C, a chemical reduction by the OH a)
ions or an internal oxido-reduction occurs :
2PuO
+ 2OH-
~
2PuO + + 602- + 2H + + 02
which is followed by 2H + + 2OHb)
2PuO~-
~
~
2H20 ; 2H20 + 202-
~
40H-
22PuO~ + + 402- + 02
This electrochemical two steps mechanism is well supported by the analysis of the slope~ E / ~ l o g
i ° of the Tafel lines.
When an electrochemical reaction proceeds in several steps whose jt~ is the slow determining step, Mauser (7) uses the formula E/~log
i° =
F2.3Aj RT
for the calculation of the Tafel slope values.
Aj is a complex term depending on the number of electrons nj transferred in the slow step, the electrochemical transfer coefficient a and the number of electrons _ ~ n i in all the steps preceding the jth one.
AJ = m nj + ~ j-lq
ni
Vol. 8, No. I 0
HEPTAVALENT PLUTONIUM
From the above quoted values f o r ~ E / ~ l o g
867
io, it comes that A J C 2
with 2"3F RT = 0.102 at 240°C. If we assume that PuO~ + + 2eis the rate determining step, nj = 2 and
~ 1 j-1
>
PuO 2
ni
Aj = 2 fits well the experimental data for = = 0.5. This value of the electrochemical transfer coefficient has already been pointed out for the reduction of NpO~" (I) in molten KOH - NaOH
CONCLUSION As it is observed in aqueous alkaline solutions, the stability of heptavalent Plutonium is inferior to that of heptavalent Neptunium in a KOH-NaOH melt at 240°0. Spontaneous reduction of Pu(VII) gives only Pu(VI) as a reaction product. The electrochemical reduction proceeds in two steps : PuO;- + ePuO~ + + 2e-
>
PuO~+ + 302>
PuO 2 (rate determining step)
ACKNOWLEDGMENTS We thank the I.I.S.N. for the financial and general support of this work.
REFERENCES
(1) L. MARTINOT and (].. D~CKAERTS, I . N . C . L . , (2) e . KELLER and H. SEIFERT, I . N . 0 . L . ,
6 , 546 (1970)
~ , 51 (1969)
(3) V.I. SPITSYN et al., J.I.N.C., 31, 2733 (1969) E. HENRICH, Thesis from Ruprecht - Karl - UniversitMt, Heidelberg
1971 (4) L. MARTINOT and G. DUYCKAERTS, Anal. Letters, 4, 1 (1971)
(5) P. DELAHAY, New I n s t r u m e n t a l
Methods i n E l e c t r o c h e m i s t r y ,
Interscienoe Publishers, New-York 1964, P. 179 (6) F. CALIGARA, L. MARTINOT and G. DUYCKAERTS, I.N.C.L., 4, 169 (1960) (7) F. MAUSF~, Zeit. Elektrochemie, 62, 419 (195S).