3.2 Alkaline solutions
53
It is worth mentioning here that the knowledge about the structure of the Tc(VI)
species that are present in an aqueous environment is incomplete and ideas of various
researchers are often inconsistent.
Kissel and Feldberg (1969) investigated the pertechnenates reduction in 1 M
NaOH. They found that disproportionation) of Tc(VI) is very fast and competes with
generation of Tc(V). This process is presented by Eq. (3.43):
2TcO
2−
4 → TcO
−
4 + TcO
3−
4
(3.43)
The respective rate constant determined by these authors is equal to 1.5 × 10
5
dm
3 ·mol
−l ·s
−1 . The Tc(VI) disproportionate with a much faster rate than the Tc(V)
whose rate constant is equal to 2.4 × 10
3 dm
3 ·mol
−l ·s
−1 (see: Lukens et al. 2001).
Kissel and Feldberg (1969) concluded that the overall reduction of the pertechnetates
in alkaline media is a two electron process.
Nowadays, the recommended value of the standard redox potential for the
Tc(VII)/Tc(V) couple, Eq. (3.44):
Tc(VII) + 2e
−
Tc(V)
(3.44)
is equal to −0.60 ± 0.05 V (Rard et al. 1999) being only slightly higher than the
redox potential for Tc(VII)/Tc(VI) couple, Eq. (3.45), which is equal to E
⦵
= −
0.64 ± 0.03 V:
Tc(VII) + e
−
Tc(VI)
(3.45)
Kryuchkov et al. (1979) determined the protonation constants of the following
reactions of the technetates(VI) (Eqs. (3.46) and (3.47)):
TcO
2−
4 + H
+
HTcO
−
4
(3.46)
HTcO
−
4 + H
+
H 2 TcO 4
(3.47)
log 10 K 1 of reaction (3.46) is equal to 8.7 ± 0.5 while for the process (3.47) log 10 K 1
≤ 1. Accuracy of determination of these values is hard to evaluate due to instability
of the discussed Tc species.
The values of the protonation constant of the pertechnetates (reaction (3.48)) given
by various authors are inconsistent and reported log 10 K varies from −0.39 to 0.60
(see: Rard et al. 1999, pp. 101–102), Eq. (3.48):
TcO
−
4 + H
+
HTcO 4
(3.48)
Regardless on these discrepancies, one may assume that these values are high
enough as to assume that the HTcO 4 is fully dissociated in aqueous solutions.
53
It is worth mentioning here that the knowledge about the structure of the Tc(VI)
species that are present in an aqueous environment is incomplete and ideas of various
researchers are often inconsistent.
Kissel and Feldberg (1969) investigated the pertechnenates reduction in 1 M
NaOH. They found that disproportionation) of Tc(VI) is very fast and competes with
generation of Tc(V). This process is presented by Eq. (3.43):
2TcO
2−
4 → TcO
−
4 + TcO
3−
4
(3.43)
The respective rate constant determined by these authors is equal to 1.5 × 10
5
dm
3 ·mol
−l ·s
−1 . The Tc(VI) disproportionate with a much faster rate than the Tc(V)
whose rate constant is equal to 2.4 × 10
3 dm
3 ·mol
−l ·s
−1 (see: Lukens et al. 2001).
Kissel and Feldberg (1969) concluded that the overall reduction of the pertechnetates
in alkaline media is a two electron process.
Nowadays, the recommended value of the standard redox potential for the
Tc(VII)/Tc(V) couple, Eq. (3.44):
Tc(VII) + 2e
−
Tc(V)
(3.44)
is equal to −0.60 ± 0.05 V (Rard et al. 1999) being only slightly higher than the
redox potential for Tc(VII)/Tc(VI) couple, Eq. (3.45), which is equal to E
⦵
= −
0.64 ± 0.03 V:
Tc(VII) + e
−
Tc(VI)
(3.45)
Kryuchkov et al. (1979) determined the protonation constants of the following
reactions of the technetates(VI) (Eqs. (3.46) and (3.47)):
TcO
2−
4 + H
+
HTcO
−
4
(3.46)
HTcO
−
4 + H
+
H 2 TcO 4
(3.47)
log 10 K 1 of reaction (3.46) is equal to 8.7 ± 0.5 while for the process (3.47) log 10 K 1
≤ 1. Accuracy of determination of these values is hard to evaluate due to instability
of the discussed Tc species.
The values of the protonation constant of the pertechnetates (reaction (3.48)) given
by various authors are inconsistent and reported log 10 K varies from −0.39 to 0.60
(see: Rard et al. 1999, pp. 101–102), Eq. (3.48):
TcO
−
4 + H
+
HTcO 4
(3.48)
Regardless on these discrepancies, one may assume that these values are high
enough as to assume that the HTcO 4 is fully dissociated in aqueous solutions.