Oxidation of U(IV) in HNO3 solutions containing urea and Tc(VII)

Article abstract of DOI:10.1007/s11137-005-0049-y

The kinetics of U(IV) oxidation with nitric acid in aqueous solutions containing urea, catalyzed with technetium ions, were studied by sampling with subsequent colorimetric determination of the U(IV) concentration. At the constant ionic strength of the solution μ = 2 in the range of the initial concentrations of U(IV) from 2 × 10^-3 to 1.28 × 10 ^-2, Tc(VII) from 5 × 10^-5 to 1 × 10 ^-3, urea from 0.01 to 0.1, and hydrogen ions from 0.4 to 1.96 M, the reaction rate is described by the equation -d[U(IV)]/dt = k ₁[U(IV)] [Tc]^0.5[CO(NH₂)₂] × {[H⁺] ² + β₁[H⁺] + β₂} ^-1 - k ₂[U(IV)]₂[H⁺] ^0.4[CO(NH₂)₂]^1.6{ [H ⁺]² + β₁[H⁺]+ β₂}^-2, where k ₁ = 172 ± 10 mol^0.5 l^-0.5 min^-1 and k ₂ = (9.4±1.2)×10² mol l^-1 min^-1 at 25°C, β₁ and β₂ are the hydrolysis constants of U⁴⁺ ions. The activation energy is 63±2 kJ mol ^-1. A reaction mechanism is proposed, in which in the slow stages the complex ion U(OH) ₂ ²⁺ ?CO(NH₂) ₂ reacts with TcO²⁺ and TcO²⁺ ? CO(NH ₂)₂ ions. 2005 Pleiades Publishing, Inc.

Full text of DOI:10.1007/s11137-005-0049-y

Radiochemistry, Vol. 47, No. 1, 2005, pp. 63 68. Translated from Radiokhimiya, Vol. 47, No. 1, 2005, pp. 61 66.
Original Russian Text Copyright 2005 by Dvoeglazov, Marchenko, Koltunov.
Oxidation of U(IV) in HNO Solutions
3
Containing Urea and Tc(VII)
K. N. Dvoeglazov, V. I. Marchenko, and V. S. Koltunov
Bochvar Russian Research Institute of Inorganic Materials, State Scientific Center of the Russian Federation,
Moscow, Russia
Received February 10, 2004
Abstract The kinetics of U(IV) oxidation with nitric acid in aqueous solutions containing urea, catalyzed
with technetium ions, were studied by sampling with subsequent colorimetric determination of the U(IV) con-
centration. At the constant ionic strength of the solution = 2 in the range of the initial concentrations of
3
2
5
3
U(IV) from 2 10 to 1.28 10 , Tc(VII) from 5 10 to 1 10 , urea from 0.01 to 0.1, and hydrogen ions
0
.5
from 0.4 to 1.96 M, the reaction rate is described by the equation d[U(IV)]/dt = k [U(IV)][Tc] [CO(NH2)2]
1
+
2
+
1
2
+ 0.4
1.6
+ 2
+
2
{
1
[H ]
+
[H ] + 2}
k [U(IV)] [H ] [CO(NH ) ] {[H ] +
[H ] +
} , where k = 172
2 1
1
2
2 2
1
and
0
.5 0.5
1
2
1
1
0 mol
l
min and k = (9.4 1.2) 10 mol l min at 25 C,
of U ions. The activation energy is 63 2 kJ mol . A reaction mechanism is proposed, in which in the slow
stages the complex ion U(OH) CO(NH2)2 reacts with TcO and TcO CO(NH2)2 ions.
are the hydrolysis constants
2
1
2
4
+
1
2
+
2+
2+
2
Uranium(IV) is unstable in nitric acid solutions
because of formation and accumulation of HNO2,
which is kinetically more effective oxidizing agent
The concentration of U(IV) was determined colori-
metrically with Arsenazo I. The optical measurements
were carried out on a KFK-2-UKhL4.2 photocolorim-
eter with a red filter (wavelength 590 nm). In pre-
liminary experiments it was found that, at pH 1.2,
than HNO [1]. To prevent U(IV) oxidation, a com-
3
pound rapidly reacting with HNO , e.g., hydrazine, is
2
1
U(VI) (less than 10 g l ), urea, and Tc(VII) do not
added to the solution [2]. However, when solutions
contain technetium ions, which catalyze the reaction
interfere with determination of U(IV) by this method.
between U(IV) and HNO (nitrate ions), U(IV) is
3
oxidized even in the presence of hydrazine [3]. Simi-
lar trends are also observed with another stabilizing
agent for U(IV), hydroxylamine [4]. In this connec-
tion, it is of interest to study the catalytic effect of
technetium ions with respect to U(IV) in solutions
containing urea, which not only rapidly reacts with
RESULTS AND DISCUSSION
The kinetic curves of U(IV) oxidation at the initial
concentrations of Tc(VII) from 5 10 5 to 1 10 3
M
are exponential for the most part. The kinetic curves
are linearized in the coordinates ln[U(IV)] time
(
Fig. 1), which corresponds to the first order with
respect to U(IV). The apparent first-order rate constant
k ) evaluated from the slope of the straight lines
HNO [5] but also forms a complex with U(IV).
2
This work is aimed to study the kinetics and mech-
anism of U(IV) oxidation in aqueous nitric acid solu-
tions containing urea and technetium ions.
(
a
+
at [CO(NH ) ] = 0.05, [H ] = 1.5 M, 25 C, and ionic
2
2
EXPERIMENTAL
The procedure of kinetic experiments was as fol-
lows. Into a temperature-controlled 15-ml test tube,
aliquots of stock solutions of all the components
excluding U(IV) were added; the test tube was kept
for 10 min to warm up to the required temperature.
Then, a small aliquot of the stock solution of U(IV)
Fig. 1. Dependence of [U(IV)] on time at various initial
(from 0.1 to 0.6 ml) was added, and aliquots of the
+
concentrations of Tc(VII). [H ] = 1.5, [CO(NH ) ] =
2
2
working solution were withdrawn intermittently to
determine the U(IV) concentration.
5
4
0
.05 M, 25 C; [Tc(VII)] , M: (1) 5 10 , (2) 1 10 ,
0
4
4
(3) 2.5 10 , and (4) 5 10
.
1
066-3622/05/4701-0063 2005 Pleiades Publishing, Inc.

64
DVOEGLAZOV et al.
Table 1. Dependence of k on the urea concentration at
Thus, at a constant concentration of hydrogen
ions, the rate of U(IV) oxidation is described by the
equation
a
4
3
+
[
Tc(VII)] = 1 10 , [U(IV)] = 4.25 10 , [H ] = 1 M,
5 C, and = 2
2
ka, min 1
k2,* l1.5 mol 1.5 min 1
0.5
[
CO(NH ) ], M
d[U(IV)]/dt = (k0
k [U(IV)] )[U(IV)][Tc(VII)]
2
2
1
0
0
0
0
0
0
.010
.025
.050
.075
.100
0.016
0.04
0.08
0.13
0.15
157
159
155
169
148
[CO(NH ) ],
(2)
=
2
2
where k0
=
193 l1.5 mol 1.5 min 1, k1
7670 l2 mol min at [H ] = 1 M, 25 C, and
.5
2.5
1
+
=
2.
Average
158
8
The influence of the concentration of hydrogen
ions was studied in the range from 0.4 to 1.96 M at
a constant ionic strength of the solution ( = 2)
and 25 C. As seen from the data presented below
0
.5
*
k
= k /{[CO(NH ) )][Tc(VII)] }.
2 a 2 2
strength of the solution = 2 increases in proportion
to the square root of the Tc(VII) concentration:
(
[Tc(VII)] = 1 10 4 and [CO(NH ) ] = 0.05 M), the
2 2
rate of U(IV) oxidation decreases with increasing con-
centration of H ions; in this case, the reaction order
+
Tc(VII)] 104,
[Tc(VII)] 104,
[
1
k , min 1
ka, min
a
with respect to hydrogen ions decreases from 0.5 to
M
M
+
2
with increasing [H ] from 0.4 to 1.96 M (Fig. 2):
0
1
2
.5
0.025
0.039
0.061
5
7.5
10
0.091
0.104
0.127
+
k , min 1
0
+
k0, min 1
[
H ], M
[H ], M
.5
0
0
0
1
.4
.6
.8
0.17
0.13
0.1
1.2
1.5
1.8
0.06
0.039
0.025
0.022
The k value decreases with increasing initial con-
a
3
centration of U(IV) in the range from 2.1 10 to
0.095
1.96
1
.28 10 2 M according to Eq. (1)
The influence of temperature on the reaction
rate was studied in the range from 14.5 to 35.5 C
1
ka = k"0
k"[U(IV)] , min ,
1
0
(1)
3
4
at [U(IV)] = 4.2 10 , [Tc(VII)] = 1 10 ,
where k" = 0.096 min 1 and k" = 3.84 l mol 1 min 1
+
0
1
[CO(NH ) ] = 0.05, [H ] = 1 M, and = 2. From
2
2
at 25 C and
= 2.
the values obtained (see below), the activation energy
was determined to be 63 2 kJ mol .
1
The dependence of the reaction rate on the urea
concentration was studied in the range of [CO(NH ) ]
from 0.01 to 0.1 M at [Tc(VII)] = 1 10 , [U(IV)] =
.2 10 , [H ] = 1 M, 25 C, and = 2. As seen from
Table 1, k values increase in direct proportion to
2
2
k , min 1
0
1
T,
C
T,
30.5
35.5
C
k0, min
4
3
+
1
2
2
4.5
0
0.03
0.05
0.08
0.12
0.18
4
a
5
CO(NH2)2 concentration and, hence, the reaction
order with respect to urea is 1.
Based on published data [6 10] on the reactions
involving U(IV) and Tc(VII) in nitric acid solutions,
the following scheme of U(IV) oxidation can be
suggested:
3
U4+ + 2TcO4 = 3UO 22 + + 2TcO2+,
(3)
TcO4 + 2TcO2+ + 6H+
3TcO3+ + 3H O, (4)
2
TcO3+ + 2HNO + 5H O = 3TcO + 2NO + 10H , (5)
+
3
3
2
4
2
NO + HNO3 + H O
3HNO2.
(6)
2
+
Fig. 2. Dependence of lnk on ln[H ] at [U(IV)] = 4.25
2
3
4
10
, [Tc(VII)] = 1 10 , [CO(NH ) ] = 0.05 M, 25 C,
It is readily seen that summation of these stages
results in the stoichiometry of the overall reaction
2
2
and
= 2.
RADIOCHEMISTRY Vol. 47 No. 1 2005

OXIDATION OF U(IV) IN HNO3 SOLUTIONS
65
describing oxidation of U(IV) with HNO3:
where
k = {k + k [CO(NH ) ]}{1 + [CO(NH ) ]} . (15)
1
U4+ + HNO3 + H O = UO + HNO2 + 2H . (7)
2+
+
1
2
2 2
2 2
2
2
To determine the concentrations of various valence
forms of technetium, it is necessary to take into ac-
count that the rate of reaction (5) significantly exceeds
the rate of reaction (4) and the equilibrium between
two valence forms of technetium, Tc(VII) and Tc(IV),
is established. Then, applying the steady state prin-
ciple to Tc(IV) concentration, we can write
In the suggested scheme, reactions (5) and (6) are
considerably faster than reactions (3) and (4), which
are rate-determining stages. In the presence of urea,
nitrous acid formed is scavenged by the fast reaction
+
+
CO(NH ) + H + HNO = NH + N + H O + CO . (8)
2
2
2
4
2
2
2
A high rate of this reaction is confirmed by the
k [U(IV)][Tc(VII)] = k [Tc(IV)][Tc(VII)].
(16)
3
absence of peaks typical for HNO in the absorption
2
spectra of the working solutions.
Taking into account the mass balance with respect
to technetium, we obtain
It should also be noted that U(IV) can enter into
reaction (3) not only in the form of U4+ aqua ion, but
also in the form of a complex with urea
[Tc(VII)] = [Tc]
k [U(IV)]/k3,
(17)
(18)
3
U4+ CO(NH2)2 + 2Tc(VII) = 3U(VI) + 2Tc(IV)
3CO(NH ) . (9)
[Tc(IV)] = k [U(IV)]/k3,
+
where [Tc] is the total concentration of technetium in
the solution. After substitution of Eqs. (17) and (18)
2
2
Thus, considering the rate-determining stages (3),
4), and (9), the expression for the rate of U(IV) oxi-
in Eq. (14), we obtain (at constant concentrations of
+
(
H
ions and urea)
dation can be written in the form
2
2
d[U(IV)]/dt = 2k [Tc][U(IV)]
2(k ) [U(IV)] /k . (19)
3
4
+
d[U(IV)]/dt = k [U ][Tc(VII)]
1
The results of the experimental study show that the
reaction rate is described by the second-order equation
with respect to uranium concentration:
+
k [U4+ CO(NH ) ][Tc(VII)] + k [Tc(IV)][Tc(VII)], (10)
2
2 2
3
where k and k are the rate constants of oxidation of
1
2
4
+
4+
U
aqua ion and complex ion U CO(NH ) ac-
d[U(IV)]/dt = ka[U(IV)].
(20)
2
2
cording to Eqs. (3) and (9), respectively, and k is the
3
Equating the right sides of Eqs. (19) and (20), we
obtain the expression for the apparent first-order rate
constant
rate constant of reaction (4). The concentrations of
4
+
4+
U
and U CO(NH2)2 can be found from the
equation of mass balance for U(IV). Considering the
constant of complex formation of U(IV) with urea
2
ka = 2k [Tc]
2(k ) [U(IV)]/k ,
(21)
3
=
[U4+ CO(NH ) ][U ] [CO(NH ) ] ,
4+
1
1
(11)
2
2
2 2
according to which k is a linear function of U(IV)
a
concentration. This conclusion is in good agreement
with dependence (1) established experimentally at a
constant acidity and constant Tc concentration. Equa-
tion (21) is also in good agreement with the fractional
reaction order with respect to Tc, since in this equation
the reaction rate by one of the pathways is independent
of the Tc concentration, and that by the other pathway
is in direct proportion to the Tc concentration.
these concentrations are, respectively
4
+
1
[
U ] = [U(IV)]{1 + [CO(NH ) ]} ,
(12)
(13)
2
2
[
U(IV)][CO(NH ) ]
2 2
U4+ CO(NH ) ] =
[
.
2
2
1
+
[CO(NH ) ]
2 2
After substitution of Eqs. (12) and (13) in Eq. (10),
we obtain
Let us denote the initial concentration of tetravalent
uranium, [U(IV)] , by a and the change in the U(IV)
0
concentration, by x. Then, Eq. (19) takes the form
d[U(IV)]/dt = k [Tc(VII)][U(IV)] + k [Tc(IV)][Tc(VII)],
3
2
2
(
14)
d(a
x)/dt = 2k (a x)[Tc]
2(k ) (a x) /k . (22)
3
RADIOCHEMISTRY Vol. 47 No. 1 2005

66
DVOEGLAZOV et al.
Table 2. Dependence of rate constants k and k on concentration of U(IV), the amount of HNO formed
3
2
4
+
[
0
U(IV)] at [Tc(VII)] = 1 10 , [H ] = 1, [CO(NH ) ] =
increases; this HNO reacts with urea by reaction (8)
2
2
2
.05 M, 25 C, and
= 2
and decreases its concentration, and thus the urea con-
centration does not remain constant in the course of
the reaction.
U(IV)] 103,
[CO(NH2)2]
k
k3 10 6
[
M
l mol 1 min 1
[
U(IV)]
The rate constants k and k evaluated by Eq. (23)
3
using the experimental data at various CO(NH ) con-
2
2
2
4
4
6
8
.13
.18
.25
.38
.52
23.4
12
11.7
7.83
5.86
450
500
0.135
centrations are listed in Table 3.
Based on data listed in Table 3, we plotted k vs.
445
0.132
0.082
0.084
430
410
[CO(NH ) ]. This plot is a straight line. It can be
2
2
readily shown that Eq. (15) obtained with the sug-
gested mechanism transforms into a linear dependence
on the urea concentration only in the case when the
denominator is 1. Hence, should be less than 1, and
Eq. (15) thus takes the form
Average
447 41
0.108 0.03
After simple transformations of Eq. (22) and its
subsequent integration, we obtain the equation
a
x
a
k
k1 + k2 [CO(NH2)2].
(24)
ln
= 2k [Tc]t + ln
, (23)
[
Tc] k (a x)/k3
[Tc]
k a/k3
Since the straight line in the coordinates k
CO(NH ) ] passes practically through the origin, k
1
is close to 0 and, hence, the contribution of the reac-
[
2
2
presenting the linear function of its left side on time.
The numerical values of the rate constants k and k3
were found by selection of their ratio so that the cor-
relation coefficient of the linear equation was no less
than 0.9995. In the most cases, the influence of k on
linearization of the experimental data is insignificant
4+
tion pathway involving U aqua ions to the total
reaction rate can be neglected. Based on the above, the
slow stage of the first reaction pathway is described
by Eq. (9) and the expression for the reaction rate of
this pathway is
3
and, therefore, the error in determination of k is sig-
3
nificantly greater than that in determination of k. From
the slope of the straight line, we found k ; and then k3
d[U(IV)]/dt = k"[U(IV)][Tc][CO(NH ) ],
(25)
2
2
2
2
+
was evaluated from the ratio k /k . The rate constants
where k" = k = 9130 400 l mol min 1 at [H ] =
3
2
k and k found by the above procedure at various ini-
1 M, = 2, and 25 C. Since the complex formation
3
tial concentrations of U(IV) are listed in Table 2.
constant
is unknown, k in Eq. (24) cannot be
2
evaluated, and in further discussion of the reaction
Table 2 shows that k and k remain constant at a
3
mechanism we will use k".
fairly high (more than approximately 6) ratio of the
initial concentrations of urea and U(IV). At a lower
The reaction rate of the second slow stage, repro-
value of this ratio, k decreases and k increases. This
portionation of Tc(V), having the rate constant k , in-
3
3
is caused by the fact that, in solving Eq. (10), we as-
sumed that the urea concentration remains constant in
oxidation of U(IV). However, with increasing initial
creases with urea concentration (Table 3). The reac-
tion order with respect to urea, determined from data
listed in Table 3, is 0.4, and, hence, the expression for
4
+
Table 3. Dependence of k and k3 on urea concentration at [Tc(VII)] = 1 10 , [H ] = 1 M,
= 2, and 25 C
k
k3 104
5
k3 10 ,**
[
CO(NH ) ], M
k",* l mol 2 min 1
2
2
2
l1.4 mol 1.4 min 1
l mol 1 min 1
0
0
0
0
.0095
.025
.05
.075
.1
83
230
447
750
950
2.3
3.5
4.5
5.4
6.0
8700
9200
8940
9330
9500
1.48
1.53
1.49
1.52
1.51
0
Average
9130 400
1.51 0.03
0
.4
*
k" = k /[CO(NH ) ]. ** k = k /[CO(NH ) ]
.
2
2
3
3
2 2
RADIOCHEMISTRY Vol. 47 No. 1 2005

OXIDATION OF U(IV) IN HNO3 SOLUTIONS
67
+
4
Table 4. Dependence of k and k on the concentration of H ions at [Tc(VII)] = 1 10 , [CO(NH ) ] = 0.05 M, = 2,
3
2 2
and 25 C
4
5
5
k ,
k ,
k3 10 ,
k3 10 ,
k ** 10 ,
+
k ,* min 1
6
7
[
H ], M
l mol 1 min 1
l2 mol 2 min 1
l mol 1 min 1
l1.4 mol
1.4
min 1
l mol min
1
1
0
0
0
1
1
1
1
1
.4
.6
.8
1125
900
625
450
300
185
135
100
22500
18000
12500
9130
6000
3700
(3880)
(6820)
8310
9280
8860
8500
8900
7800
6.66
5.78
5.58
4.54
2.21
1.91
1.85
1.51
1.53
1.56
1.69
1.51
.2
.5
.8
.96
2700
2000
Average 8610 520
Average
1.57 0.08
+
2
+
+ 0.4
*
k
= k"([H ]
+
[H ] +
). ** k = k [H ]
.
6
1
2
7
3
the reaction rate by pathway (4) can be written in the
form
Equating the right side of Eq. (31) to the second
term of Eq. (14), we obtain the equation
d[U(IV)]/dt = k [Tc[IV)][Tc(VII)][CO(NH ) ]0.4, (26)
3
2 2
1
k = {k + k [CO(NH ) ]}{1 + [CO(NH ) ]} . (32)
3
4
5
3
2 2
3
2 2
where k = (1.51 0.03) 10 l mol 1.4 min 1 at
5
1.4
3
+
Using data listed in Table 3 and Eq. (32), we esti-
mated graphically the rate constants k and k in
[H ] = 1 M,
= 2, and 25 C.
4
5
The dependence of k on the urea concentration can
3
Eq. (29) and constants of complex formation ( )
3
be understood assuming that Tc(IV) enters into the
reaction with Tc(VII) along Eq. (4) not only in the
1
1
of urea with Tc(IV) to be 16200 l mol min ,
.1 10 l mol min , and 9 l mol , respectively,
5
2
2
1
1
1
2
+
form of the TcO ion but also in the form of a com-
plex ion containing urea:
at 25 C.
To determine the dependence of the rate constants
TcO2+ CO(NH ) + TcO
3TcO3+ + 2CO(NH ) . (27)
2 2
k and k on the concentration of hydrogen ions, the
3
2
2
2
4
experimental data were processed by Eq. (21). The
+
results are listed in Table 4 (the k values at [H ] >
1
3
The constant of formation of this complex
3
M are not given because of poor accuracy in their
TcO2+ + CO(NH2)
TcO2+ CO(NH2)2
determination). The dependence plotted in the coor-
dinates lnk" ln[H ] using these data (Fig. 3) is linear
(28)
2
+
in the range of concentration of hydrogen ions from
is equal to
+
0
.8 to 1.96 M with the slope equal to 2 (at [H ] <
[TcO2+ CO(NH ) ][TcO ] [CO(NH ) ] . (29)
2+
1
1
0.8 M, the dependence deviates from linearity).
=
3
2 2 3 2
The resulting dependence can be rationalized by
assuming that U(IV) enters into reaction (9) with
In this case, the equation for the rate of U(IV) con-
sumption along the second pathway can be written in
the form
Tc(VII) in the form of hydrolyzed ion U(OH)2
+
2
CO(NH ) , as suggested in [10]. Then, the expression
2
2
2
+
d[U(IV)]/dt = k [TcO ][Tc(VII)]
4
+
k5[TcO2+ CO(NH ) ][Tc(VII)].
(30)
2 2
From the equation of the mass balance for Tc(IV),
2+
2+
we can find the concentrations of TcO and TcO
CO(NH ) ions. Substituting these concentrations in
Eq. (30), we obtain
2
2
d[U(IV)] k4 + k5 3[CO(NH2)2]
+
4
Fig. 3. Dependence of lnk on ln[H ]. [Tc(VII)] = 1 10
CO(NH ) ] = 0.05 M, = 2, and 25 C.
,
=
[Tc(IV)][Tc(VII)]. (31)
[
dt
1 + 3[CO(NH2)2]
2 2
RADIOCHEMISTRY Vol. 47 No. 1 2005

68
DVOEGLAZOV et al.
for the rate of this reaction pathway can by presented
by the equation
Thus, the total kinetic equation for oxidation of
U(IV) obtained using the suggested reaction mech-
anism is
2
+
d[U(IV)]/dt = [U(OH) CO(NH ) ][Tc(VII)]. (33)
2
2 2
d[U(IV)]/dt = k [U(IV)][Tc]0.5[CO(NH2)2]
8
{[H+]2
+
[H ] +
+
} 1
Considering the mass balance with respect to
U(IV),
1
2
+ 2
k [U(IV)] [H+]0.4[CO(NH2)2]1.6{[H ] + [H ] + } ,
2
+
2
9
1
2
4
+
3+
2+
(39)
min 1
[
U(IV)] = [U ] + [UOH ] + [U(OH) ]
2
0
.5
0.5 0.5
where k = 2k /[Tc] = 172 10 mol
l
2
+
8
6
+
[U(OH)
CO(NH2)2],
(34)
2
and k = 2(k ) /k = (9.4 1.2) 10 mol l min 1 at
2
2
1
9
6
7
2
5 C and ionic strength equal to 2.
the expression for the concentration of this ion can be
written in the form
Comparison of Eq. (39) with kinetic equation (18)
established experimentally shows that the apparent
2
+
[U(OH)
CO(NH2)2]
2
first-order constant k is
a
[
U(IV)]
[CO(NH2)2]
0.5
+ 2
+
} 1
2
2
2
ka = k [Tc] [CO(NH ) ]{[H ]
+
[H ] +
8
2 2
1
=
,
(35)
+
2
+
[
H ]
+
[H ] +
+
2[CO(NH2)2]
2
k9[U(IV)][H+]0.4[CO(NH2)2]1.6{[H ] + [H ] + } .
+ 2
+
2
1
2
1
2
(
40)
where
and
are the constants of the first and
2
1
the second stages of U(IV) hydrolysis, and
is the
The k evaluated from this equation agrees within
12% with the values found experimentally under all
the studied conditions except the runs at low con-
2
a
equilibrium constant of formation of the complex
U(OH)2 CO(NH ) .
+
2
2 2
+
centrations of H ions (less than 0.4 M), where the
The term
[CO(NH ) ] in the denominator of
2
2
2 2
discrepancy exceeds 50%.
Eq. (35) can be neglected because of its small value in
comparison with the other terms, and then, after sub-
stitution of Eq. (35) in Eq. (33), we obtain
REFERENCES
1
. Koltunov, V.S., Kinetika reaktsii aktinoidov (Kinetics
of Actinide Reactions), Moscow: Atomizdat, 1974,
p. 58.
d[U(IV)]
[U(IV)]
[CO(NH ) ][Tc(VII)]
2
2
2 2
=
. (36)
[H+]2
+
[H ] +
+
dt
1
2
2
3
4
5
. Koltunov, V.S. and Marchenko, V.I., Kinet. Katal.,
1
966, vol. 7, no. 2, p. 224.
Comparison of Eqs. (34) and (23) shows that
. Koltunov, V.S. and Gomonova, T.V., Radiokhimiya,
1991, vol. 33, no. 5, p. 107.
. Taylor, R.J., Koltunov, V.S., Marchenko, V.I., et al.,
J. Nucl. Sci. Technol., 2002, suppl. 3, p. 355.
. Dvoeglazov, K.N. and Marchenko, V.I., Radiokhimiya,
+
2
+
1
k
= k {[H ]
+
[H ] +
} ,
(37)
. Expression (37) adequately de-
2 2
6
1
2
where k =
6
scribes the dependence of k" on the concentration of
2
005, vol. 47, no. 4, pp. 57 60.
6. Koltunov, V.S. and Gomonova, T.V., Radiokhimiya,
988, vol. 30, no. 6, pp. 756 760.
. Koltunov, V.S. and Gomonova, T.V., Radiokhimiya,
985, vol. 27, no. 4, pp. 394 399.
. Koltunov, V.S. and Gomonova, T.V., Radiokhimiya,
984, vol. 26, no. 3, pp. 328 332.
hydrogen ions in the range from 0.8 to 2 M at the hy-
drolysis constants
4
= 0.031 and
= 1 10 M [11].
1
2
1
Considering Eq. (37), the rate of the second reaction
pathway [Eq. (9)] can be described by the expression
7
8
1
d[U(IV)]
k [U(IV)][Tc][CO(NH ) ]
[H+]2
6
2 2
1
=
,
(38)
+
dt
+
[H ] +
1
2
9. Koltunov, V.S., Khimiya transuranovykh i oskoloch-
nykh elementov (Chemistry of Transuranium and Fis-
sion Elements), Moscow: Nauka, 1967, pp. 8 14.
1
+
in which k = 8610 520 min at [H ] = 0.8 2.0,
6
4
[
Tc(VII)] = 1 10 M,
= 2, and 25 C.
1
0. Koltunov, V.S. and Gomonova, T.V., Radiokhimiya,
The dependence of the rate constant k on the con-
centration of hydrogen ions is presented in Table 4.
These data show that the reaction order with respect to
1990, vol. 32, no. 5, pp. 70 72.
3
1
1. Nazarenko, V.A., Antonovich, V.P., and Nev-
skaya, E.M., Gidroliz ionov metallov v razbavlennykh
rastvorakh (Hydrolysis of Metal Ions in Dilute Solu-
tions), Moscow: Atomizdat, 1979.
+
H ions for the second pathway is 0.4 in the range of
+
[
H ] from 0.4 to 1 M.
RADIOCHEMISTRY Vol. 47 No. 1 2005