Chemistry Reference
In-Depth Information
As the complex formation is preceded by the acid dissociation of the beta-diketone, its
p
K
a
value has an inverse dependence on the metal extraction efficiency (
23
), the lower
the p
K
a
value of the beta-diketone becomes, the higher its efficiency as an extractan-
tis. This inverse linear correlation of the p
K
a
of the beta-diketone with its extraction
constant was reported by Batzar et al. using acyclic ligands, HTTA (Figure 2.3a),
HDBM, acac, etc., for U(VI) extraction (
23
). However, these beta-diketones are capa-
ble of extracting tetravalent actinides from moderate acidic solutions and hexavalent
actinides from weakly acidic solutions. They extract trivalent actinides poorly. Beta-
diketones containing a heterocyclic moiety such as HPAI (3-phenyl-4-acetyl-5-isox-
azolone, p
K
a
: 1.31; Figure 2.3b), HPBI (3-phenyl-4-benzoyl-5-isoxazolone, p
K
a
: 1.12;
Figure 2.3b), and HPMBP (1-phenyl-3-methyl-4-benzoyl-5-pyrazolone, p
K
a
: 4.10;
Figure 2.3c) have been employed for the extraction of actinides from acidic/complex-
ing media. The tetravalent actinide ions are usually extracted more efficiently than
hexavalent uranium (Table 2.1). The inverse linear correlation of extraction constant
(log
K
ex
) with respect to the p
K
a
of the extractant (observed distinctly in the case
of U(VI)-acyclic beta-diketones) was not observed for Pu(IV) with beta-diketones
like HPMBP and HPBI, in which the heterocyclic ring contained one of the ketonic
groups. It was ascribed to the increased stereochemical constraints (
24
). However,
very large extraction constants of Pu(IV) with HPMBP led to the development of
Ph
Me
N
R
CF
3
N
R
O
N
Ph
Ph
O
O
O
O
O
O
R = Thenoyl; HTTA
R = Ph; BTFA
R = Ph; HPBI
R = Me; HPAI
HPMBP
FIGURe 2.3
Structures of some beta-diketones used for the extraction of actinides.
tAbLe 2.1
two-phase extraction Constants for Actinides with
several beta-Diketones
Log
K
ex
Metal Ion
HttA
HPMbP
HPbI
p
K
a
6.24
4.10
1.12
U(VI)
-2.44 (
28
)
0.63 (
27
)
1.40 (
28
)
Th(IV)
2.25 (
29
)
6.96 (
29
)
8.26 (
30
)
U(IV)
5.42 (
32
)
-
-
Np(IV)
5.68 (
32
)
-
10.11 (
34
)
Pu(IV)
7.31 (
32
)
13.75 (
33
)
10.76 (
24
)
