Reaction of Np(VI) with cyclohexanediaminetetraacetic acid in HClO4 solutions

Article abstract of DOI:10.1134/S1066362216020041

The stoichiometry of the reaction of Np(VI) with cis-cyclohexanediaminetetraacetic acid (CHDTA, H4chdta) in 0.05 M HClO₄ solution was studied by spectrophotometry. With Np(VI) in excess, 1 mol of the complexone converts 4 mol of Np(VI) into Np(V). In 0.115–0.98 M HClO₄ solutions (the ionic strength of 1.0 was supported with LiClO₄) containing 3–29 mM CHDTA at 20–45°С, Np(VI) at a concentration of 0.2–3.3 mM is consumed in accordance with the first-order rate law until less than 40% of Np(VI) remains. After that, the reaction decelerates. The reaction rate has first order with respect to [CHDTA] and the order of–1.2 with respect to [H⁺]. The activated complex is formed with the loss of one and two Н⁺ ions. The activation energy is 82.3 ± 3.8 kJ mol^–1.

Full text of DOI:10.1134/S1066362216020041

ISSN 1066-3622, Radiochemistry, 2016, Vol. 58, No. 2, pp. 128–130. © Pleiades Publishing, Inc., 2016.
Original Russian Text © V.P. Shilov, A.M. Fedoseev, 2016, published in Radiokhimiya, 2016, Vol. 58, No. 2, pp. 114–116.
Reaction of Np(VI) with Cyclohexanediaminetetraacetic Acid
in HClO4 Solutions
V. P. Shilov* and A. M. Fedoseev
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
Leninskii pr. 31, korp. 4, Moscow, 119071 Russia
* e-mail: ShilovV@ipc.rssi.ru
Received September 7, 2015
Abstract—The stoichiometry of the reaction of Np(VI) with cis-cyclohexanediaminetetraacetic acid (CHDTA,
H4chdta) in 0.05 M HClO4 solution was studied by spectrophotometry. With Np(VI) in excess, 1 mol of the
complexone converts 4 mol of Np(VI) into Np(V). In 0.115–0.98 M HClO4 solutions (the ionic strength of 1.0
was supported with LiClO4) containing 3–29 mM CHDTA at 20–45°С, Np(VI) at a concentration of 0.2–
3.3 mM is consumed in accordance with the first-order rate law until less than 40% of Np(VI) remains. After
that, the reaction decelerates. The reaction rate has first order with respect to [CHDTA] and the order of –1.2
with respect to [H+]. The activated complex is formed with the loss of one and two Н+ ions. The activation en-
ergy is 82.3 ± 3.8 kJ mol–1.
Keywords: neptunium(VI), cyclohexanediaminetetraacetic acid, reduction, kinetics
DOI: 10.1134/S1066362216020041
Replacement of the ethylene group in the EDTA
molecule by the cyclohexane ring leads to different,
compared to EDTA, electron density distribution on
nitrogen atoms and on terminal carboxy groups. As a
result, the dissociation constants and the solubility in
water and acid solutions change. As for the reducing
power of CHDTA, it is difficult to predict the changes
relative to EDTA a priori, and experimental studies are
required.
prepared in double-distilled water. The solutions were
standardized by common procedures.
In the experiments on studying the reaction
stoichiometry, into a quartz cell (l = 1 cm) with a solu-
tion of Np(VI) in HClO4 we added an aliquot of a
Na2H2chdta solution. The optical density at 981 nm,
corresponding to the Np(V) absorption, was measured
at regular intervals with a Shimadzu PC 3100 (Japan)
or SF-46 (Leningrad Optical and Mechanical Associa-
tion, Russia) spectrophotometer. In kinetic studies, we
added an aliquot of the Np(VI) stock solution into a
temperature-controlled cell with an HClO4 + H4chdta
solution and monitored an increase in the Np(V) con-
tent. Each experiment was repeated 2–3 times.
This study deals with the reaction of CHDTA with
Np(VI) in perchloric acid solutions.
EXPERIMENTAL
Our experiments were performed with 237Np. Stock
solution of Np(VI) was prepared and standardized by
the known procedure [1]. 1,2-Cyclohexanediamine-
tetraacetic acid (pure grade) was recrystallized from
water and dried in air. A weighed portion of CHDTA
was mixed with a NaOH solution in 1 : 2 molar ratio.
The Na2H2chdta solution was quantitatively transferred
into a volumetric flask. Chemically pure grade HClO4,
ultrapure grade NaOH, and pure grade LiClO4 were
used. Lithium perchlorate was additionally purified by
recrystallization and dried in air. All solutions were
RESULTS AND DISCUSSION
The stoichiometry of the Np(VI) + H4chdta reaction
was studied in 0.05 M HClO4 at 25°С. The initial con-
centrations of Np(VI) and H4chdta were 3.6 and
0.5 mM, respectively. The Np(V) concentration was
calculated using the Np(V) molar extinction coefficient
ε981 = 403 ± 3 L mol–1 cm–1. From three experiments,
we obtained Δ[Np(V)]/Δ[H4chdta] = 4. The results of
one of the experiments are given below.
128

REACTION OF Np(VI) WITH CYCLOHEXANEDIAMINETETRAACETIC ACID
129
Time, [Np(V)], Time, [Np(V)], Time, [Np(V)],
min
1
2
3
mM
0.82
1.342
1.515
1.652
min
15
42.2
68
mM
1.80
1.95
2.02
2.09
min
150.4
187
mM
2.12
2.14
2.16
236
5
105.4
The reaction between Np(VI) and H4chdta is com-
plete in 50–55 min. Further accumulation of Np(V) is
caused by slow reaction of Np(VI) with CHDTA
decomposition products. As can be concluded from the
data obtained, the following reactions occur in HClO4
solutions containing Np(VI) and H4chdta:
Fig. 1. Kinetic curves of the Np(VI) reduction with
CHDTA molecules. [HClO4] = 0.115 M, [LiClO4] =
0.875 M, [NaClO4] = 0.010 M, [H4chdta] = 5 mM; 24°С.
[Np(VI)], mM: (1) 3.3, (2) 1.0, and (3) 0.23.
Np(VI) + H4chdta → Np(V) + R1 (slow),
Np(VI) + R1 → Np(V) + R2 (fast),
(1)
(2)
(3)
(4)
the reaction course insignificantly.
Np(VI) + R2 → Np(V) + R3 (fast),
The linearity of the kinetic curves in semilog coor-
dinates and equal slope of these curve portions in the
case of 1.0 and 3.3 mM Np(VI) indicate that the reac-
tion is first-order with respect to Np(VI). The reaction
rate is described by the equation
Np(VI) + R3 → Np(V) + low-active products (fast),
Np(VI) + low-active products → Np(V)
+ inactive products (slow).
(5)
–d[Np(VI)]/dt = k'[Np(VI)] = k'([Np(V)]∞ – [Np(V)]),
The kinetics of the Np(VI) + H4chdta reaction was
studied in 0.115–0.98 M HClO4 solutions at an ionic
strength I = 1.0, supported by adding LiClO4. Figure 1
shows the kinetic curves in the coordinates time–
log(D∞ – D) for 0.115 M HClO4 + 0.875 M LiClO4 +
0.010 M NaClO4 solutions containing 5 mM H4chdta
and 3.3, 1.0, and 0.23 mM Np(VI). For the first two
Np solutions, the kinetic dependences are linear up to
80% conversion, after which the reaction decelerates.
In the case of the third solution, the linear portion of
the kinetic curve has a smaller slope than the slope of
the kinetic curves of the first and second solutions.
Deviation from the linearity starts at the moment corre-
sponding to 60% Np(VI) conversion. Similar character
of kinetic curves in [1] was attributed to the fact that
the radicals arising in reactions (1)–(3) react with dis-
solved О2. We performed an experiment in which ar-
gon was passed for 20 min through a 0.48 M HClO4 +
0.5 M LiClO4 + 0.02 M NaClO4 + 10 mM H4chdta
solution at 25°С, after which Np(VI) was added to
a concentration of 1.2 mM. The hole in the cell lid was
closed with a Teflon stopper, and the Np(V) accumula-
tion was monitored. Similar experiment was repeated
with saturation of the solution with О2. We found that
the kinetic curves obtained in solutions saturated with
argon and О2 did not differ from those for the aerated
solutions. Hence it follows that dissolved О2 influences
(6)
where k' is the first-order rate constant. In the integral
form, after the replacement of [Np(V)] by the quantity
D proportional to the concentration, Eq. (6) transforms
into Eq. (7):
2.3log(D∞ – D) = –k't + const.
(7)
The first-order rate constant increases with increas-
ing H4chdta concentration. This can be illustrated by
the example of 1 M HClO4 solutions containing 1 mM
Np(VI) and 3–29 mM H4chdta at 22°С.
[H4chdta], mM
k' × 103, s–1
3
5
10
20
29
0.253 0.406 0.777 1.46 2.20
0.082 0.081 0.078 0.073 0.076
k, L mol–1 s–1
The bimolecular rate constant k = k'/[H4chdta] in
the examined range of H4chdta concentrations remains
essentially constant, which suggests the first order of
the reaction with respect to the reductant. The k values
obtained under different conditions are given in the
table.
As seen from the table, as the perchloric acid con-
centration is increased, the Np(VI) reduction rate ap-
preciably decreases. Figure 2 shows the log–log plot of
RADIOCHEMISTRY Vol. 58 No. 2 2016

130
SHILOV, FEDOSEEV
k vs. [H+]. At 25°С, the experimental points are fitted
by a straight line with a slope of –1.2. Hence it follows
that the reduction occurs via formation of an activated
complex with the elimination of one and two protons.
For cis- and trans-cyclohexanediaminetetraacetic acid
at an ionic strength of 1.0 and 20°С, рK1 = 2.4 [2];
therefore, in a dilute perchloric acid solution the com-
plexone exists in the form of the H3chdta– ion and
H4chdta molecule. The activated complex is formed by
the reactions
Influence of conditions on the rate constant of the reaction
Np(VI) + H4chdta. I = 1.0 M, [H4chdta] = 5–10 mM,
[Np(VI)] = 1 mM
T, °C
25
25
[HClO4], M
0.115
0.24
k, L mol–1 s–1
1.64
0.648
0.266
0.122
0.064
0.216
0.355
0.59
25
0.48
25
0.98
20
0.98
30
0.98
NpO22+ + H3chdta = NpO2H2chdta + H+,
NpO22+ + H4chdta = NpO2H2chdta + 2H+.
35
0.98
40
0.98
45
0.98
0.98
The charge transfer leads to the formation of Np(V)
and radical R.
Figure 3 shows the temperature dependence of k in
the Arrhenius coordinates. The activation energy found
from this dependence is 82.3 ± 3.8 kJ mol–1.
Variation of the ionic strength only slightly influ-
ences the rate of the Np(VI) + H4chdta reaction. In a
0.115 M HClO4 solution at 25°С, k = 1.99 L mol–1 s–1,
and in a 0.115 M HClO4 + 0.875 M LiClO4 + 0.02 M
NaClO4 solution, k = 1.64 L mol–1 s–1. A decrease in
the reaction rate with increasing ionic strength is typi-
cal of reactions between oppositely charged species,
but low extent of this decrease suggests participation
of a neutral species also. All these facts indicate that
the activated complex is formed with the participation
of H3chdta– and H4chdta.
Fig. 2. Influence of the concentration of Н+ ions on the rate
constant of the Np(VI) reduction with ions and molecules
of H4chdta in a solution with an ionic strength of 1.0 at
25°С.
Comparison of the kinetic relationships for the re-
actions of Np(VI) with H4edta and with H4chdta shows
that, under equal conditions, the rate constants are
close and the mechanisms are similar.
REFERENCES
1. Shilov, V.P. and Fedoseev, A.M., Radiochemistry,
2015, vol. 57, no. 3, pp. 255–258.
2. Martell, A.E. and Smith, R.M., Critical Stability Con-
Fig. 3. Logarithm of the rate constant of the Np(VI)
reduction with ions and molecules of H4chdta as a function
of reciprocal temperature. [HClO4] = 0.98 M, [H4chdta] =
10 mM.
stants, vol. 1: Amino Acids, New York: Plenum, 1974.
Translated by G. Sidorenko
RADIOCHEMISTRY Vol. 58 No. 2 2016