3 Technetium Coordinated by Inorganic Ligands in Aqueous …
33
Table 3.2 Pitzer parameters
in Na + /Cs + /TcO
−
4 /H 2 O
system at 25 °C (Könnecke
et al. 1997)
Binary parameters
i/j
β (0)
β (1)
C φ
Na + /TcO
−
4
0.01111
0.1595
0.00236
Cs + /TcO
−
4
−0.1884
−0.1588
0
Mixing parameters
i/j/k
φ ijk
ψ ijk
TcO
−
4 /Cl − /Na +
0.067
−0.0085
TcO
−
4 /Cl − /Cs +
0.067
−0.0011
applied here, Eq. (3.4):
Λ = Λ
0
−
0.2300 · Λ
0
+ 60.64
· I
1/2
c
(3.4)
The recommended value of the tracer diffusion coefficient of the pertechnetates
in water at 25 °C is equal to (1.48 ± 0.01)·10
−5 cm
2 ·s
−1 . According to Astheimer
and Schwochau (1976), the latter parameter decreases in electrolyte solutions and
in 1 mM solutions of TcO
−
4 in 1 M NaOH and 1 M LiCl has the values of (1.27 ±
0.01)·10
−5 and (1.24 ± 0.01)·10
−5 cm
2 ·s
−1 , respectively.
Tc(IV) may be contained in various ionic and nonionic species and this leads to
significant discrepancies between the standard potential values reported by various
authors for the TcO
−
4 /TcO 2 xH 2 O system (Rard et al. 1999). The net reaction of the
pertechnetates reduction to the technetium dioxide is presented by Eq. (3.5):
TcO
−
4 + 4H
+
+ 3e
−
TcO 2 · xH 2 O (s) + (2 − x)H 2 O
(3.5)
Unfortunately, the stoichiometry of the hydrated technetium dioxide which is
generated in this reaction is still unclear. An arbitrary value of x equal to 1.6 was
assumed in the review by Rard et al. (1999), but it is known that the degree of
the TcO 2 hydration changes very slowly with time in aqueous solutions. Yalçınta¸ s
studied Tc(IV) speciation in the NaCl environment assuming x equal to 0.6 (e.g.
Yalçintas et al. 2016). Thus, it is possible that under equilibrium conditions x may
reach a value equal to 0 (see: Grambow et al. 2020).
The experimentally determined value of the standard redox potential, E
⦵ , of the
system given by Eq. (3.1) with TcO 2 (am, hyd, fresh) as the reaction product is
equal E
⦵
= 0.746 ± 0.012 V at 25 °C (Rard et al. 1999). On the other hand, the
standard potential for TcO
−
4 /TcO 2 redox system calculated on the basis of solubility
of TcO 2 (am, hyd, aged) is slightly higher than the above value and is equal to
E
⦵
= 0.757 ± 0.006 V (see: Grambow et al. 2020). The experimental emf of this
system determined in low ionic strength solutions (I m ≤ 10 mmol·kg
−1 ) at 25 °C is
represented by Eq. (3.6) (E, in V):
E = E
0
− 0.0789 · pH + 0.0197 · log 10
a TcO
−
4
(3.6)
33
Table 3.2 Pitzer parameters
in Na + /Cs + /TcO
−
4 /H 2 O
system at 25 °C (Könnecke
et al. 1997)
Binary parameters
i/j
β (0)
β (1)
C φ
Na + /TcO
−
4
0.01111
0.1595
0.00236
Cs + /TcO
−
4
−0.1884
−0.1588
0
Mixing parameters
i/j/k
φ ijk
ψ ijk
TcO
−
4 /Cl − /Na +
0.067
−0.0085
TcO
−
4 /Cl − /Cs +
0.067
−0.0011
applied here, Eq. (3.4):
Λ = Λ
0
−
0.2300 · Λ
0
+ 60.64
· I
1/2
c
(3.4)
The recommended value of the tracer diffusion coefficient of the pertechnetates
in water at 25 °C is equal to (1.48 ± 0.01)·10
−5 cm
2 ·s
−1 . According to Astheimer
and Schwochau (1976), the latter parameter decreases in electrolyte solutions and
in 1 mM solutions of TcO
−
4 in 1 M NaOH and 1 M LiCl has the values of (1.27 ±
0.01)·10
−5 and (1.24 ± 0.01)·10
−5 cm
2 ·s
−1 , respectively.
Tc(IV) may be contained in various ionic and nonionic species and this leads to
significant discrepancies between the standard potential values reported by various
authors for the TcO
−
4 /TcO 2 xH 2 O system (Rard et al. 1999). The net reaction of the
pertechnetates reduction to the technetium dioxide is presented by Eq. (3.5):
TcO
−
4 + 4H
+
+ 3e
−
TcO 2 · xH 2 O (s) + (2 − x)H 2 O
(3.5)
Unfortunately, the stoichiometry of the hydrated technetium dioxide which is
generated in this reaction is still unclear. An arbitrary value of x equal to 1.6 was
assumed in the review by Rard et al. (1999), but it is known that the degree of
the TcO 2 hydration changes very slowly with time in aqueous solutions. Yalçınta¸ s
studied Tc(IV) speciation in the NaCl environment assuming x equal to 0.6 (e.g.
Yalçintas et al. 2016). Thus, it is possible that under equilibrium conditions x may
reach a value equal to 0 (see: Grambow et al. 2020).
The experimentally determined value of the standard redox potential, E
⦵ , of the
system given by Eq. (3.1) with TcO 2 (am, hyd, fresh) as the reaction product is
equal E
⦵
= 0.746 ± 0.012 V at 25 °C (Rard et al. 1999). On the other hand, the
standard potential for TcO
−
4 /TcO 2 redox system calculated on the basis of solubility
of TcO 2 (am, hyd, aged) is slightly higher than the above value and is equal to
E
⦵
= 0.757 ± 0.006 V (see: Grambow et al. 2020). The experimental emf of this
system determined in low ionic strength solutions (I m ≤ 10 mmol·kg
−1 ) at 25 °C is
represented by Eq. (3.6) (E, in V):
E = E
0
− 0.0789 · pH + 0.0197 · log 10
a TcO
−
4
(3.6)