Disproportionation of Pu(V) in K10P2W 17O61 solutions

Article abstract of DOI:10.1134/S106636220701002X

In solutions of unsaturated heteropolytungstate K₁₀P ₂W₁₇O₆₁, Pu(V) disproportionates in a wide pH range; it is a first-order reaction with respect to Pu(V), and its rate only slightly changes in the pH range from 0.7 to 4.0. The activation energy E _a of Pu(V) disproportionation was determined as 78.6±2.0 and 64.2±3 kJ mol^-1 at pH 2.0±0.1 and 4.0±0.2, respectively. The thermodynamic parameters of activation ΔH^≠ and ΔS^≠ were evaluated. Published data on disproportionation of Np(V) and Am(V) in K₁₀P₂W ₁₇O₆₁ solutions were analyzed.

Full text of DOI:10.1134/S106636220701002X

ISSN 1066-3622, Radiochemistry, 2007, Vol. 49, No. 1, pp. 14 19. Pleiades Publishing, Inc., 2007.
Original Russian Text A.B. Yusov, V.P. Shilov, A.M. Fedoseev, L.N. Astafurova, 2007, published in Radiokhimiya, 2007, Vol. 49, No. 1, pp. 15 19.
Disproportionation of Pu(V) in K10P2W17O61 Solutions
A. B. Yusov, V. P. Shilov, A. M. Fedoseev, and L. N. Astafurova
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, Russia
Received October 10, 2005
Abstract In solutions of unsaturated heteropolytungstate K10P2W17O61, Pu(V) disproportionates in a wide
pH range; it is a first-order reaction with respect to Pu(V), and its rate only slightly changes in the pH range
from 0.7 to 4.0. The activation energy Ea of Pu(V) disproportionation was determined as 78.6 2.0 and 64.2
1
3 kJ mol at pH 2.0 0.1 and 4.0 0.2, respectively. The thermodynamic parameters of activation H and
S were evaluated. Published data on disproportionation of Np(V) and Am(V) in K10P2W17O61 solutions
were analyzed.
PACS numbers: 82.20.Pm, 82.30.Hk
DOI: 10.1134/S106636220701002X
In solutions of unsaturated heteropolytungstates
of the 11th and 17th series, the redox properties of
f elements change substantially. In particular, Np(V)
and Am(V) ions, which are relatively stable in the
absence of strong ligands in a wide range of hy-
drogen ion concentrations, rapidly disproportionate in
required volume of water, we obtained a Pu(VI) solu-
tion with a concentration of 0.005 M and pH about
3.5 4.0. The Pu(VI) concentration was determined
spectrophotometrically at the wavelength 830.4 nm
1
1
using the extinction coefficient 550 l mol cm [7].
To prepare Pu(V), the solution of Pu(VI) was treated
with H O (taken in 15 20% excess to stoichiometry)
K P W O solutions [1 5], and this reaction is
10 2 17 61
2 2
first-order with respect to the actinide ion. This is
and the resulting solution was heated on a water bath
to 80 85 C for several minutes. Within this time,
Pu(VI) was nearly quantitatively ( 95%) reduced to
Pu(V).
+
+
only possible if the reaction between NpO or AmO
2
2
10
and the P W O
ligand is the limiting stage of
2
17 61
disproportionation. However, only tentative assump-
tions were made on the mechanism and structure of
the intermediate complexes [2, 4, 5]. Thus, the sys-
Solutions of K P W O nH O ( isomer)
2
10 2 17 61
2
were prepared by the procedure given elsewhere [5].
Chemically pure grade acids, KOH, and KCl were
used without additional purification.
tems containing actinides in the oxidation state 5+ and
10
P W O
anions are of particular interest and the
2
17 61
processes involved are rather unusual. A systematic
study of the behavior of An(V) would give insight
into these processes. Here we report on the first study
The spectrophotometric measurements were carried
out on a UVPC-3100 automatic device (Shimadzu,
+
Japan); the concentration of hydrogen ions H was
of the behavior of Pu(V) in K P W O solutions.
10 2 17 61
measured using an HI-8314 pH meter (Hanna Instru-
ments, Portugal).
It is known [6] that dissolution of K P W O in
10 2 17 61
(x = 6 7) anions; herein-
10 x
water yields K P W O
x 2 17 61
The experiments were performed as follows. A
solution (3 ml) with a required concentration of
after, for simplicity these heteropolyanions are desig-
nated as P W without indicating the charge.
2
17
K P W O and known acidity was prepared. Then
10 2 17 61
a cell with this solution was placed in a temperature-
controlled spectrophotometric compartment, and, after
attainment of the required temperature, an aliquot of
the 0.005 M solution of Pu(V) heated to the same
temperature was quickly added; this aliquot did not
significantly affect the pH of solution. Then the kinet-
ic curve of the optical density at 841 nm was re-
corded. After the reaction completion, the spectrum of
the solution in the 400 1300 nm range was recorded
and the pH of solution was measured.
EXPERIMENTAL
Plutonium-239 was purified by anion-exchange
chromatography; Pu(IV) oxalate was precipitated and
then dissolved in HNO . An aliquot of the Pu(IV)
3
nitrate solution was added to a portion of HClO , and
4
then the aqueous phase was carefully evaporated to
the wet residue and almost complete disappearance of
the white vapor. After dissolution of the residue in a
14

DISPROPORTIONATION OF Pu(V) IN K10P2W17O61 SOLUTIONS
RESULTS AND DISCUSSION
15
Main Features of Disproportionation of Pu(V)
in K P W O Solutions
10 2 17 61
The spectrum of a weakly acidic solution of Pu(V)
exhibits no strong absorption bands (Fig. 1, curve 1).
Upon addition of Pu(V) up to a concentration of 5
4
10 M into 0.001 0.01 M solution of K P W O ,
10 2 17 61
at the initial moment the absorption spectrum resem-
bles the spectrum of the PuO aqua ion, although low
+
2
absorption intensity does not allow unambiguous con-
clusions on its identity. We assumed that under these
experimental conditions complexation of P W with
Fig. 1. (1) Spectrum of a Pu(V) solution of concentration
2
17
3
Pu(V) is incomplete and the spectrum of the resulting
5
10 M, pH 4, and (2) spectrum of solution after ter-
+
mination of the reaction in the system of the following
complex is close to that of PuO aqua ion. Immedi-
2
4
composition: [Pu(V)] 4.8 10 M, [K P W
O ] 3.0
ately after mixing the reactants, spectral changes are
observed and the reaction is often completed in sever-
al tens of minutes. A typical spectrum of the solution
after the reaction completion is shown in Fig. 1
(curve 2). As seen, the final solution contains only
10 2 17 61
3
10 M, 20 C, pH 3.9. Asterisks and crosses denote the ab-
sorption bands belonging to heteropolytungstate complexes
of Pu(VI) and Pu(IV), respectively.
Pu(VI) and Pu(IV), i.e., in a K P W O solution
10 2 17 61
Pu(V) almost completely disproportionates.
As shown in [8], in solutions of mineral acids dis-
proportionation of Pu(V) yields Pu(III), Pu(IV), and
Pu(VI) ions. Pu(III) is formed by the following reac-
tions:
2PuO+2 + 4H+
Pu4+ + PuO+2
Pu4+ + PuO22+ + 2H2O,
Pu3+ + PuO22+.
(1)
(2)
IV
Fig. 2. Absorption spectrum of Pu (P W ) .
2
17 2
tively low, and they can be used only to estimate the
content of Pu(IV) after the reaction completion.
The absence of Pu(III) in K P W O solutions
10 2 17 61
is probably due to the fact that the potential of the
Pu(IV)/Pu(III) couple [similarly to the couples of the
other f elements M(IV)/M(III)] decreases in this medi-
The strongest band of the complex of Pu(VI) with
K P W O at 840 nm (Fig. 1, curve 2) is the most
10 2 17 61
um by nearly 1 V as compared to 1 M HClO solu-
4
suitable to monitor disproportionation. The results of
the spectrophotometric study of interaction of Pu(VI)
tions, and reaction (2) becomes impossible. In dispro-
portionation of Pu(V) in oxalate solutions, reaction (2)
is also impossible [9, 10] owing to the formation of
with K P W O will be presented in a separate
10 2 17 61
paper. Here we only mention that the position and
intensity of the main absorption band of the complex
2
very strong Pu(IV) complexes with C O anions.
2
4
VI
Pu O (P W ) strongly depends on the concentration
To calculate the concentrations of the final prod-
ucts using the spectrophotometric data, the molar
extinction coefficients of Pu(IV) and Pu(VI) in solu-
2
2
17
of hydrogen ions and the other single-charged cations.
We used the following molar extinction coefficients of
the complex Pu O (P W ) determined in the ab-
VI
tions of K P W O are required. As shown in [11],
2
2
17
10 2 17 61
IV
sorption maximum at the given pH:
the complexes Pu (P W ) are formed at the molar
2
17 2
ratio [P W ] : [Pu(IV)] 2 : 1 even in 2 M HNO .
Their absorption spectra in nitrate [11] and neutral [12]
solutions are similar. The spectrum of Pu (P W )
(9.4 10 M) similar to that given in [11] is shown
in Fig. 2 using the scale of the extinction coefficients.
The intensities of the strongest bands at 650 and
2
17
3
pH
1
2
3
4 5
max, nm
840.3
840.7
841.1
841.7 841.8
IV
2
17 2
,
445 10 430 10 415 10 405 10
4
l mol 1 cm
1
After the reaction completion, the absorption spec-
trum of the solution was recorded and the concen-
1
1
1050 nm [ 52 3 and 72 4 l mol cm ] are rela-
RADIOCHEMISTRY Vol. 49 No. 1 2007

16
YUSOV et al.
verted to the curves of Pu(V) disproportionation:
[Pu(V)] = [Pu(V)]0 2 D841 (3)
is the increase in the optical density
/
,
841
where
at
D
841
841 nm in the course of the reaction.
In reaction (1) of Pu disproportionation, hydrogen
+
ions H are consumed. At initial pH 3.5 4, the
+
concentration of H strongly changes in the course
of the reaction; at pH > 4, the buffer capacity of
K P W O is insufficient to maintain pH with an
10 2 17 61
accuracy of 0.1, and each arising Pu(IV) ion binds
two P W anions. To decrease possible distortion
2
17
in the kinetics owing to changes in the concentrations
+
of H and free P W anions in the course of the reac-
2
17
tion, the majority of experiments were carried out at
pH 3 and at least fourfold excess of P W with
2
17
Fig. 3. Kinetics of Pu(V) consumption in the course of dis-
respect to the total concentration of Pu. The kinetic
curves recorded at various concentrations of Pu(V) are
presented as straight lines with similar slopes in the
t log[Pu(V)] coordinates (Fig. 3). The treatment of
these curves using TableCurve 2D program (Scientific
Software JANDEL) gave the correlation coefficient
close to 1 (using the sum of two parallel first-order
reactions). The correlation coefficients calculated for a
single first-order reaction were about 0.992 0.995.
The results of mathematical treatment assuming sec-
ond-order reactions were inadequate. Thus, with suf-
ficient approximation we can consider that, similarly
to Np(V) [2, 3, 5] and Am(V) [4], disproportionation
of Pu(V) in the given medium predominantly proceeds
as a first-order reaction with respect to Pu(V). Under
the other experimental conditions, the kinetic curves
can also be described by a first-order equation with
respect to Pu:
3
proportionation. [K P W
O
] 1.9 10
M, pH 2.9
10 , M: (1) 0.98, y = 0.0461x
3.0444; (2) 0.57, y = 0.0442x 3.2969; and (3) 0.24, y =
0.0443x 3.6625.
10 2 17 61
3
0.2, 28 0.5 C. [Pu(V)]
in
1
Fig. 4. First-order rate constant k (min ) of Pu(V) dispro-
portionation as a function of concentration of K P W O
10 2 17 61
4
[P W ]. [Pu(V)] = (4.35 4.95) 10 M, [KCl] 0.05 M
2
17
in
(in the test with [P W ] 0.0095 M, [KCl] 0.03 M), pH
3.1 0.2, 301.0 0.5 K. The data for Np(V) were taken
2
17
v = d[Pu(V)]/dt = k [Pu(V)].
(4)
from [5], Tables 3, 4. (1) Np, pH 2.7, y = 0.5972x 0.1846;
(2) Np, 1 M HClO , y = 1.0157x 0.6623, and (3) Pu,
pH 3.1, y = 0.8448x + 0.9984.
The dependence of the rate constant of dispropor-
4
tionation k on the concentration of P W anions at
2
17
pH 3.1 is close to direct proportionality. Under the
0.84
trations of Pu(VI) and Pu(IV) were determined. At
pH < 1.5 the presence of Pu(VI) aqua ion was taken
into account. In all the experiments the concentrations
of Pu(VI) and Pu(IV) ions were equal, [Pu(IV)] =
[Pu(VI)], within the experimental error of the concen-
tration measurements ( 10%). No other oxidation
states of plutonium were observed. In the course of
reaction, the sum of the concentrations [Pu(IV)] +
[Pu(VI)] was equal to the decrease in [Pu(V)]; no
complexes of Pu(V) [similar to those of Np(V) with
the absorption band at 1048 nm] were formed in
detectable amounts. Taking into account these results,
the kinetic curves of Pu(VI) accumulation were con-
conditions given in Fig. 4, k = k[P W ]
and the
(5)
2
17
rate equation can be presented as follows:
v = k[Pu(V)][P2W17]n (n = 0.84 0.1).
Similar results were obtained previously for Np(V)
[5]; in Fig. 4 these data are given for comparison.
+
The effect of [H ] on the rate of Pu(V) dispropor-
tionation was studied under the following conditions:
4
[Pu(V)] (5.0 0.3) 10 M, [K P W O ] 2.5
10 2 17 61
3
10 M, 301 K, pH 0.70, 1.00, 2.25, 2.90, and 4.00.
In the latter three solutions, 0.05 M KCl was added to
RADIOCHEMISTRY Vol. 49 No. 1 2007

DISPROPORTIONATION OF Pu(V) IN K10P2W17O61 SOLUTIONS
reduce the effect of the change in the ionic strength of
17
the solution on passing to more acidic solution. As
seen from Fig. 5, k only slightly depends on pH, but
at pH 0.7 and 4.0 the deviations from the first-order
law increase. We calculated the rate constant from the
average slope of the straight line in the coordinates
t log[Pu(V)] throughout the reaction period. At
+
[H ] = 0.2 M, common disproportionation of Pu(V)
can occur at a significant rate [8]; however, under the
given experimental conditions its contribution to the
total reaction rate is negligible. In Fig. 5, published
data on Np(V) [2, 5] and Am(V) [4] are given for
comparison. All the dependences pass through a maxi-
mum, which is most pronounced for Am(V) and less
pronounced for Pu(V). At high pH, the reaction ac-
Fig. 5. First-order rate constant k of Pu(V) disproportiona-
4
tion as a function of pH {[Pu(V)] 5 10 M, [P W
]
2
17
3
2.0 10 M} and related data for Np(V) [2, 5] and Am(V)
[4]. (1) Np, variable I [2], (2) Np, I = 1 [5], (3) Pu, and
(4) Am.
+
celerates with increasing [H ]; for Np(V) at a constant
ionic strength of 1 [5], the dependence is steeper as
compared to the case when the ionic strength was not
maintained constant [2].
of Np(V) complex with P W (absorption at 988 nm)
2
17
(Tables 3, 4 [5]). The dependences of k on [P W ]
2
17
are probably due to the different stability of the result-
ing Np(V), Pu(V), and Am(V) complexes with P W .
Mechanism of Disproportionation of Pu(V ), Np(V),
2
17
and Am(V) in K P W O Solutions
10 2 17 61
The pH dependences of the rate of Np(V), Pu(V),
and Am(V) disproportionation are also different
(Fig. 5). The causes of the influence of pH on the rate
of Np(V) disproportionation were discussed previ-
ously [5]. The maximum appears under the effect of
factors similar to those in the oxalate solutions, where
The most important common features of dis-
proportionation of Pu(V), Np(V), and Am(V) in
K P W O solutions is that the reaction is first-
10 2 17 61
order with respect to actinide cation. This fact sug-
gests that in all the cases the main stages of reaction
proceed by similar mechanism; in the case of Np,
along with disproportionation, a Np(V) complex
species appears with the absorption band at 1048 nm.
+
at high H concentrations the rate of Np(V) [13] and
Pu(V) [9, 10] disproportionation also decreases.
Depending on the conditions, the highest reaction rate
is observed at different pH [9, 10]. It should be noted
that for Pu(V) the effect of pH on the disproportiona-
tion rate in P W solutions is the weakest. The same
The dependences of the reaction rates on pH and
[K P W O ] are different for the actinides ana-
10 2 17 61
2
17
lyzed. This fact does not contradict our conclusion
that the disproportionation mechanism is similar,
because both dependences are governed by a number
of factors, and the final result depends on the contri-
bution of each factor.
is true for common disproportionation of Pu(V): its
+
rate equation involves the concentration of H in the
first power, and that of Np(V) disproportionation,
in the second power [8]. Probably some stages of
reactions in the presence of P W proceed by the
2
17
way similar to disproportionation in mineral acids. It
is assumed that, similarly to perchlorate solutions,
the reactions in oxalate solutions occur through addi-
tion of the proton to the yl oxygen atom of M(V)
ion bound in the complex. Then, this ion reacts with
The rate constant of Am disproportionation k at
pH 3 and [P W ] > [Am(V)] is independent of
2
17
[P W ] [4]. In [4] it was considered that internal
2
17
+
rearrangement of the complex of AmO and P W is
2
2
17
the limiting stage of disproportionation, because its
rate should be independent of the concentration of free
P W . However, in disproportionation of Pu(V)
+
MO and the reaction proceeds by the common sec-
2
ond-order mechanism with respect to M(V). Thus, the
oxalate anions in the primary event only weaken the
2
17
(pH 3.1), k changes proportionally to [P W ]. The
2
17
+
Np O or Pu O bonds, so that protonation of MO
direct proportional dependence was observed for
2
cations becomes significant even at high pH. Since the
stabilities of the oxalate complexes of M(V) [9] and
complexes of Np(V) with P W (with adsorption
at 988 nm) [5] are similar, in our case the acidity of
the yl oxygen atoms also decreases, and, in turn,
the probability of their interaction with the proton
Np(V) in 1 M HClO , with increasing pH to 2.7 the
4
dependence slope decreases (Fig. 4). At pH 6.5, the
effect of the increase in the P W concentration on
2
17
2
17
the reaction kinetics is more complex: it accelerates
+
consumption of free NpO (absorption at 980 nm),
2
but does not noticeably affect the rate of consumption
RADIOCHEMISTRY Vol. 49 No. 1 2007

18
YUSOV et al.
at a low ionic strength [5]; they slightly increase with
decreasing pH.
Negative values of the activation entropy indicate
that, with formation of the activated complex, the
number of degrees of freedom decreases. Our experi-
mental data do not allow detailed description of dis-
proportionation, but suggest that disproportionation of
Np(V) and Pu(V) in solutions of K P W O oc-
10 2 17 61
curs by the similar mechanism. Disproportionation of
Np(V) is characterized by the parallel formation of
a complex of unknown structure (characterized by the
absorption band at 1048 nm). This complex is not
absolutely stable and gradually disproportionates with
time. Formation of a similar Pu(V) complex was not
observed. We suggest that this is due to the lowest
stability of the Pu(V) complex: as seen from the sys-
tem of the oxidation potentials, plutonium dispropor-
tionates in a wider range of experimental conditions as
compared to Np(V). Such a complex, if formed, dis-
proportionates so rapidly that its equilibrium concen-
tration in solutions remains too low to be detected.
This assumption can be indirectly confirmed by the
Fig. 6. Rate constant k of Pu(V) disproportionation as
4
a function of temperature. [Pu(V)] (4.8 0.2) 10
M,
3
[P W ] (2.0 0.2) 10 M; average ratio [P W ]/[Pu(V)]
2
17
2
17
6.0. (1) pH 4, y = 3353.8x 8.7418 and (2) pH 2, y =
4106.3 11.105.
increases. Protonation of the yl ion bound in the
complex with P W and weakening of the Np O and
2
17
Pu O bond are probably the important stages of dis-
proportionation.
The temperature dependence of the rate constant k
1
(s ) of Pu(V) disproportionation is shown in Fig. 6.
Using these data, we evaluated the activation energy
E and, in accordance with the theory of absolute
a
reaction rates, calculated the activation entropy
using the following expression:
S
+
following facts. Addition of NpO excess to the solu-
2
tion of the metastable complex (absorption maximum
at 1048 nm) in solution containing no excess P W
2
17
S
= R[2.303lgA
2.303lg(kT/h)
1].
does not provide rapid disproportionation. At the
+
same time, addition of PuO to the solution of such
2
The calculated thermodynamic functions of activa-
tion of Pu(V) disproportionation are listed in the
1
2+
Np(V) complex causes rapid formation of PuO and
2
+
complex of Np(IV) with P W . This suggests that
2
17
table. The activation energy of PuO dispropor-
2
the oxidation potential of the metastable complex of
tionation in 1 M HClO is somewhat higher: E
4
a
+
1
Np(V) is sufficiently high to oxidize PuO . The
more so, PuO should be oxidized by the related
2
82.3 kJ mol [13], whereas in oxalate solutions it is
+
1
2
lower: E 57.5 kJ mol
at pH 3.1 3.7 [9] and
a
1
Pu(V) complex, because the standard potential of the
Pu(V)/Pu(IV) couple is 0.332 V, which is higher than
that of the Np(V)/Np(IV) couple [14]. The complex of
Pu(V) with P W , similar to the metastable complex
42.0 kJ mol at pH 2 [10].
The activation energies evaluated are close to those
calculated previously for disproportionation of Np(V)
2
17
+
of Np(V), when formed, rapidly reacts with PuO
2
Thermodynamic functions of activation of reactions
aqua ions and, hence, its formation cannot be detected.
between Np(V) and Pu(V)* in K10P2W17O61 solutions.
4
Experimental conditions: [Np(V)] (5.7 0.2) 10
M,
4
ACKNOWLEDGMENTS
[Pu(V)] (4.8 0.2) 10
M, and [P2W17] (2.9 0.2)
3
10
M
This study was financially supported by the Rus-
sian Foundation for Basic Research (project no. 03-
03-32203).
Reac-
tion
Ea,
kJ mol
H ,
kJ mol
S ,
logA
1
1
1
J K 1 mol
1
10
1
2
3
8.7
11.1
11.5
64.2
78.6
77.6
61.7
76.1
75.1
86.0
40.7
33.5
The reaction was performed with P W
O
deficit, i.e., the
2
17 61
reaction mixture contained metastable Np(V) complex and
NpO and PuO aqua ions; NpO ion does not react with
+
+
+
2
2
2
+
PuO ions and the absence of free P W excludes its com-
2
2
17
+
* Reactions: (1) disproportionation of Pu(V), pH 4; (2) the
same, pH 2; and (3) consumption of Np(V) (overall reaction),
pH 4.6 [5].
plexation with PuO . Hence, this reaction cannot be explained
2
2+
+
by decrease in the potential of the PuO /PuO couple in the
2
2
complex with P W
.
2
17
RADIOCHEMISTRY Vol. 49 No. 1 2007

DISPROPORTIONATION OF Pu(V) IN K10P2W17O61 SOLUTIONS
19
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