Anodic synthesis of Np(VII) compounds from acetate solutions

Article abstract of DOI:10.1134/S106636221202004X

A procedure was suggested for the synthesis of Np(VII) compounds by electrochemical oxidation in acetate solutions. The conditions for preparing compounds of type MNpO₄·nH₂O, where M is a singlecharged cation of alkali metals, ammonium, silver, guanidinium, or tetraalkylammonium, and of Np(VII) compounds with double-charged cations of alkaline-earth metals, Cu, Cd, and Zn were studied in detail. The compounds were characterized by chemical analysis and by IR and electronic spectroscopy. Pleiades Publishing, Inc., 2012.

Full text of DOI:10.1134/S106636221202004X

ISSN 1066-3622, Radiochemistry, 2012, Vol. 54, No. 2, pp. 128–131. © Pleiades Publishing, Inc., 2012.
Original Russian Text © A.M. Fedoseev, N.A. Budantseva, A.A. Bessonov, V.P. Shilov, 2012, published in Radiokhimiya, 2012, Vol. 54, No. 2, pp. 118–121.
Anodic Synthesis of Np(VII) Compounds from Acetate Solutions
A. M. Fedoseev*, N. A. Budantseva, A. A. Bessonov, and V. P. Shilov
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
Leninskii pr. 31, block 4, Moscow, 119071 Russia; * e-mail: a.fedosseev@gmail.com
Received April 18, 2011
Abstract—A procedure was suggested for the synthesis of Np(VII) compounds by electrochemical oxidation
in acetate solutions. The conditions for preparing compounds of type МNpO4·nH2O, where М is a single-
charged cation of alkali metals, ammonium, silver, guanidinium, or tetraalkylammonium, and of Np(VII) com-
pounds with double-charged cations of alkaline-earth metals, Cu, Cd, and Zn were studied in detail. The com-
pounds were characterized by chemical analysis and by IR and electronic spectroscopy.
Keywords: neptunates(VII), electrochemical synthesis, IR spectra, electronic absorption spectra
DOI: 10.1134/S106636221202004X
oxalate followed by its dissolution in concentrated
HNO3 on heating. Neptunium(VI) was prepared either
in the form of a Np(VI) perchlorate solution by evapo-
ration of a nitric acid solution with concentrated
HClO4, or as Np(VI) acetate by precipitation with
aqueous ammonia from a solution of Np(VI) perchlo-
rate or nitrate, followed by dissolution of the residue
(after washing with water) in excess 1–3 M aqueous
acetic acid. Solid Np(VI) acetate was prepared by
evaporation of the resulting Np(VI) acetate solution in
an air stream without heating. Neptunium(V) solutions
were prepared by reduction of aqueous solutions of
Np(VI) nitrate or perchlorate with sodium nitrite, fol-
lowed by precipitation of Np(V) hydroxide with an
aqueous ammonia solution, washing with water, and
dissolution in acetic or perchloric acid solutions. To
obtain neodymium acetate, the hydroxide precipitate
prepared by precipitation from a nitrate solution with
ammonia was washed with water and dissolved in ex-
cess 1–3 M acetic acid solution, and the solution was
slowly evaporated in an air stream without heating.
Barium, guanidinium, tetraethylammonium, and tetra-
butylammonium acetates were prepared by adding ex-
cess 1–3 M acetic acid solution to a definite amount of
guanidinium carbonate, barium hydroxide, or tetraal-
kylammonium hydroxide. The resulting solutions were
evaporated in an air stream on heating to no more than
40°C.
Preparation of Np(VII) compounds by electrochemi-
cal oxidation of Np(VI) in bicarbonate solutions was
studied in sufficient detail [1, 2]. A solution of sodium
or lithium bicarbonate [1] or of ammonium carbonate
[2] with “excess” bicarbonate/carbonate concentration
of 0.035–0.08 M was used as electrolyte. The current
density at the platinum anode was 1.5–20 mA cm–2,
which corresponded to the potentials of 1.5–1.9 V (vs.
standard hydrogen electrode). In so doing, the previ-
ously synthesized and studied compounds NaNpO4·
2.5H2O and LiNpO4·2.5H2O and the previously unknown
Np(VII) compound with ammonium ion, NH4NpO4·
nH2O, were isolated and identified. Relatively narrow
range of the Np(VII) compounds synthesized in carbon-
ate and bicarbonate solutions is restricted, on the one
hand, by relatively complex procedure for electrolyte
pretreatment and narrow interval of carbonate/bicar-
bonate concentrations and, on the other hand, by low
solubility of carbonates of various cations that would be
desirable to use in electrochemical synthesis of Np(VII)
compounds.
Previously we suggested a procedure for preparing
Np(VII) compounds by electrochemical oxidation of
Np(VI) and Np(V) in acetate solutions [3, 4], which con-
siderably extended the series of compounds prepared by
electrochemical oxidation. In particular, the compounds
with double-charged and organic cations were prepared.
Here we describe in more detail the synthesis of Np(VII)
compounds from acetate solutions.
Oxidation was performed in an Н-type electro-
chemical cell with the compartments separated by a
porous glass filter. The working electrode was a plati-
num plate of area ~1.8 cm2, and two pieces of platinum
wire served as auxiliary electrodes. The catholyte was
EXPERIMENTAL
In our study we used 237Np purified by anion ex-
change and additionally by precipitation of Np(IV)
128

ANODIC SYNTHESIS OF Np(VII) COMPOUNDS FROM ACETATE SOLUTIONS
129
a 0.01 M СН3СООН solution. In our study we used a
PI-50-1 potentiostat. The temperature was 20–22°С. The
electrolyte was stirring by argon bubbling. The concen-
trations of Np and Nd and the content of Np valence
forms were monitored spectrophotometrically in a cell
(l = 1 cm) with a Shimadzu UV 3101 PC device (Japan)
in the range 380–1300 nm. The IR spectra were recorded
with a Specord-80 device.
cations, the deposit is formed within 1–2 min, practi-
cally irrespective of the acetate concentration varied in
the interval 0.05–0.9 M, except tetrabutylammonium
acetate with which the deposit formed on the anode
only after 10–15 min of controlled potential electroly-
sis (CPE).
To analyze the products obtained, the deposit was
mechanically removed from the electrode, washed
with water, and dissolved in 1 M LiOH or dried in an
air stream. The dried samples were used for X-ray
phase analysis (XPA) and measurement of IR spectra.
The deposits obtained with acetates of alkali metals (Li,
Na, K, Rb, Cs) and ammonium are finely crystalline.
The compounds with Na+, Li+, and NH4+, according to
XPA, are isostructural to the corresponding com-
pounds obtained by electrochemical oxidation in bicar-
bonate and carbonate solutions [1, 2], i.e., NaNpO4·
2.5H2O, LiNpO4·2.5H2O, and NH4NpO4·nH2O. Pre-
sumably, compounds of the general formula
MNpO4·nH2O are formed, where M = Li, Na, K, Rb,
Cs, NH4, and Ag, with certain variation of n in this
series. It should be noted that the degree of crystallin-
ity is appreciably lower than in the synthesis from bi-
carbonate and carbonate solutions, which may be due
to lower pH value at which the compounds are depos-
ited. Under these conditions, the solubility of the com-
pounds being synthesized is expected to be lower,
which affects the crystallization process. With calcium,
strontium, barium, cadmium, copper, and zinc acetates,
the oxidation mainly occurs similarly to the systems
with acetates of single-charged cations. The deposits
obtained are fairly dense, and accumulation of large
amounts of the compounds is complicated. All the iso-
lated compounds are X-ray amorphous, which is ap-
parently caused by still lower solubility of the deposits
compared to the compounds with the single-charged
cations. It is difficult to prepare more than ~1 mg of
the products under the experimental conditions used,
but the presence of alkaline-earth elements, copper,
zinc, and cadmium was confirmed qualitatively.
RESULTS AND DISCUSSION
In solutions of acetates of alkali and alkaline-earth
metals, ammonium, silver, guanidinium, tetramethyl-,
tetraethyl-, and tetrabutylammonium, containing (5–10) ×
10–3 M perchlorate or acetate of Np(VI) or Np(V), a
black deposit is formed on the platinum electrode in
the potential interval 1.5–1.9 V along with the evolu-
tion of a small amount of a gaseous product; in the
process, in controlled potential electrolysis (CPE), the
current appreciably decreases (by a factor of 1.5–2).
At these potentials, under stirring, Np(V) is very
readily oxidized to Np(VI), but the deposit is formed
on the anode before the oxidation of Np(V) in solu-
tions of alkali metal acetates is complete. At the ace-
tate ion concentration lower than 0.05 M, the solid
phase on the electrode is not formed, but in the course
of CPE at these concentrations the solution color
changes, and the solution spectrum becomes similar to
that of Np(VII) in nitrate–acetate solutions [5]. This
may be due to low buffer capacity of solutions of such
composition and to fast acidification, in particular, in
the near-electrode layer, which prevents formation of
the solid phase on the electrode surface. With an in-
crease in the concentration of acetate ions in the elec-
trolyte, the minimal potential at which a solid Np(VII)
compound is deposited correspondingly increases, and
the acetate ion concentration of ~0.9 M is an upper
limit at which the formation of solid Np(VII) com-
pounds is possible. This fact is apparently associated
with an increase in the degree of the Np(VI) complexa-
tion owing to interaction with acetate ions and with the
corresponding increase in the potential of the Np(VII)/
Np(VI) couple and in the buffer capacity of the solu-
tion. An increase in the anode potential over 1.9 V
does not lead to the Np(VI) oxidation under all the ex-
amined experimental conditions, which is caused by
electrochemical decomposition of acetate ions, as it
takes place in electrochemical oxidation of Am(III) in
acetate solutions [6], with the decomposition products
rapidly reducing Np(VII).
Noticeable difference in the electrochemical oxida-
tion of Np(V) or Np(VI) is observed when using solu-
tions of uranyl acetate and neodymium acetate, or
when using solutions of Np(V) or Np(VI) acetate with-
out additional cations. For example, the rate of forma-
tion of the solid phase on the electrode in these sys-
tems at the same concentration of acetate ions and
same potential is considerably lower than in the pres-
ence of the examined single- and double-charged
cations, and in the case of neodymium acetate the char-
With the above-listed acetates of single-charged
RADIOCHEMISTRY Vol. 54 No. 2 2012

130
FEDOSEEV et al.
Characteristics of the IR spectra of solid Np(VII) compounds
Compound 1
Compound 2
position of maximum, cm–1
assignment
ν(HOH); ν(NH)
ν(CN3); δ(H2O)
δ(NH2)
position of maximum, cm–1
3530, 3420, 3280, 3184, 3082, 3042
1656, 1618, 1568
assignment
ν(HOH)
3536, 3472, 3456, 3262
2968, 2908, 2812
1624, 1532
νas(CH); νs(CH)
ν(NC4); δ(H2O)
δ(CH3)
1418, 1376, 1328
ρ(NH2)
1152, 1118
1472, 1420
ν(Np–O)
δ(CN3)
916, 908, 892
ρ(CH3)
1220, 1088
585, 540
ν(Np–O)
944, 912
δ(NC4)
488, 504
acter of the deposition also differs essentially. Whereas
in all the above-described systems a thin dense layer of
the deposit is formed on the anode fairly rapidly and
remains unchanged in the course of the subsequent
CPE, in solutions of uranyl, neodymium, and neptu-
nium acetates deposition of a black substance on the
walls of the anodic compartment is observed in the
course of prolonged CPE, and even visual observation
shows that the density of the electrode deposit is lower.
In preparation of Np(VII) compounds in the presence
of uranyl and neodymium acetates and in solutions of
Np(V) and Np(VI) acetates without other metals,
X-ray amorphous substances are formed. In solutions
prepared by dissolving the compounds in dilute HNO3,
uranyl and neodymium are not detected spectropho-
tometrically. Apparently, under these conditions the
compound NpO3OH·nH2O is formed.
The valence forms of Np present in the compounds
were determined spectrophotometrically after dissolv-
ing the washed deposit directly from the electrode in
1 M LiOH to determine Np(VII) from the absorption
maximum at 625 nm, with the subsequent acidification
of the solution with concentrated HNO3 to [H+] ≥
0.1 M to determine Np(V) from the absorption at
980 nm. The spectra of the solutions of the deposits in
1 M LiOH fully coincide with the published data [7].
Taking into account the molar extinction coefficients
(l mol–1 cm–1) of 400 [7] for Np(VII) at 625 nm and of
390 for Np(V) at 980 nm, we found that the Np(VII)
fraction in the compound prepared from an acetate so-
lution of Np(VI) is no less than 90% of the total
amount of Np determined in the compound.
Incorporation of single-charged organic cations into
the structure of the synthesized Np(VII) compounds is
confirmed by the spectroscopic data. The figure shows
the IR spectra of compounds of the presumed composi-
tion [C(NH2)3]NpO4·xH2O (1) and [(Met)4N]NpO4·
xH2O (2). Samples for measurements were prepared by
pelletizing with melted NaCl the deposits mechanically
separated from the electrode, washed with water, and
dried. It should be noted that the color and transpar-
ence of the samples did not change in the course of
pelletization and during the subsequent day, which
indicates that the compounds were stable under these
conditions.
The positions of the maxima of the main absorption
bands and their possible assignment taking into ac-
count published data [8–11] are given in the table.
The spectra of both compounds in the high-
frequency region, 3600–3100 cm–1, are characterized
by considerable overlap of the vibrations of water
molecules and characteristic vibrations of organic
cations. As a result, broad strong absorption bands
with numerous inflections corresponding to individual
vibration bands are observed. In the IR spectrum of 1,
the absorption at 3530–3280 cm–1 corresponds to the
vibrations of water molecules present in the com-
pound, whereas the group of low-frequency bands at
3184–3048 cm–1 is assigned to ν(NH) vibrations of the
guanidinium cation.
The IR spectrum of 2 is characterized by larger
half-width and more complex shape of the high-
frequency band. Presumably, the number of water
IR spectra of solid Np(VII) compounds with (1) guanidin-
ium and (2) tetramethylammonium cations.
RADIOCHEMISTRY Vol. 54 No. 2 2012

ANODIC SYNTHESIS OF Np(VII) COMPOUNDS FROM ACETATE SOLUTIONS
131
molecules in 2 is larger than in 1, and these molecules
have different status. The characteristic stretching vi-
brations ν(CH) of the methyl groups of the tetrame-
thylammonium cation are manifested in the range
2960–2800 cm–1 as three weak bands.
In the range 1800–1500 cm–1 in the spectra of both
compounds, there are bending vibration bands of water
molecules δ(H2O), noticeably overlapping with the
ν(CN) vibrations specific for organic cations. The
shape of the overall bands in this range is different,
probably because of different numbers of water mole-
cules in the compounds and specific features of the
structure of the cations. The possible formation of H
bonds by the guanidinium cation leads to a high-
frequency shift of the overall band.
acetate solutions considerably expands the range of
new Np(VII) compounds, especially with organic
cations, and allows preparation of NpO3OH·nH2O
without impurities of foreign cations, which is essen-
tial, because the structure and the very existence of this
compound are called in question.
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vibrations in the anionic group are observed in a rela-
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UV range, typical of Np(VII), and very weak bands in
the visible range. The positions of the absorption
maxima in the spectra of 1 and 2 appreciably differ:
429, 546, 627 nm for 1 and 420, 532, 632 nm for 2.
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Thus, the suggested procedure for preparing
Np(VII) compounds by electrochemical oxidation in
RADIOCHEMISTRY Vol. 54 No. 2 2012