exotic nuclei structure and reaction noyaux exotiques ... - IPN - IN2P3

(D 0
/D)-1
ln
complexes of Pa(V). In the DTPA system, the stability
constant of the Pa(DTPA) was deduced from
the slope of the linear variations of ((D 0 /D)-1). An
adjustment is illustrated on Fig.3 and the apparent
formation constants determined at 25°C are listed
in Table 1.
7
6
5
4
3
2
Same analysis of several extraction data sets were
performed at other temperatures allowing the use
of Van’t Hoff equation for the determination of thermodynamic
parameters (enthalpy, entropy, heat
capacity). The analysis of the variations of ln as
function of the inverse of temperature are illustrated
in Fig.4 for the system Pa/DTPA. In both
investigated systems, complexation reactions are
characterized by a strong positive entropic contribution.
In the complexation process, the desolvation
of both ligands and protactinium cation obviously
contributes to an increase of disorder in the
water structure around species due the liberation
of water molecules that occurs simultaneously to
the formation of ligand-Pa bond.
1
0
-1
0,000 0,005 0,010 0,015 0,020 0,025
C(DTPA)tot
Fig.3: Adjustment of extraction data relative to the
system Pa(V)/TTA/Toluene/H 2 0/NaClO 4 /HClO 4 /
H 5 DTPA at 10°C and µ=3M.
Equilibrium
log
PaO(OH) 2+ +C 2 O 4 2- +H + ↔PaO(C 2 O 4 ) + +H 2 O 7.4 ± 0.3
PaO(OH) 2+ +2C 2 O 4 2- +H + ↔PaO(C 2 O 4 ) 2 - +H 2 O 14.5 ± 0.8
PaO(OH) 2+ +3C 2 O 4 2- +H + ↔PaO(C 2 O 4 ) 3
3-
+H 2 O
19.1 ± 0.7
PaO(OH) 2+ +3H + +DTPA 5- ↔Pa(DTPA)+2H 2 O 28.6 ± 0.3
Table 1: Formation constants of oxalate and DTPA
complexes of Pa(V) at 25°C and µ=3M
The stability constants of oxalate complexes of Pa
(V) are of the same magnitude of order as those of
U(VI), but much higher than that of Np(V). The formation
constant of the complex Pa(DTPA) is similar
to that of actinides(IV) and once again much
higher than that of Np(V).
70
69
68
67
66
65
64
63
62
61
60
0,0030 0,0031 0,0032 0,0033 0,0034 0,0035 0,0036
1/T
Fig.4: Variation of formation constant of the complex
Pa(DTPA) as function of temperature
132