3.1 Acidic solutions
43
active under the experimentel conditions applied. This process manifests itself by
formation of a positive spectroscopic wave (for 440 nm) at potentials below 0.2 V.
The subsequent oxidation of such generated TcO
−
4 electroreduction products leads to
formation of some Tc(IV) species. Electrochemical oxidation of polymeric Tc(IV)
forms starts at potentials higher than ca. 0.8 V as follows from an analysis of the
wave at 502 nm. Chotkowski et al. (2018a) proposed the schemes of formation of
these species as the products of mutual interactions between reduced Tc species,
Eqs. (3.25)–(3.26):
TcO
+
+ TcO
2+
→ [Tc(μ−O) 2 Tc]
3+
(3.25)
TcO
2+
+ TcO
2+
→ [Tc(μ−O) 2 Tc]
4+
(3.26)
or synproportionation of Tc(III) with Tc(VII) species, Eq. 3.27:
3TcO
+
+ TcO
−
4 + 6H
+
→ 2[Tc(μ−O) 2 Tc]
4+
+ 3H 2 O
(3.27)
Vichot (2001) discussed possible reversible reactions inside the polymeric
technetium(III/IV) structures, Eq. (3.28):
Tc
IV
− Tc
IV
+ e
−
Tc
III
− Tc
IV
+ e
−
Tc
III
− Tc
III
(3.28)
This author observed that reduction of Tc(VII) with a concentration of μM carried
out at −0.4 V versus Ag, AgCl(KCl(sat.)) in a solution with pH = 1.75 and containing
[SO
2−
4 ] = 0.01 M leads to formation of a UV–Vis band at 497 nm, which is characteristic of polymeric Tc(IV) form. Longer than 1.3 h polarization at the same potential,
up to 3.1 h, resulted in the disappearance of this band and formation of another one
at 570 nm. This spectroscopic signal has been attributed to Tc
III
− Tc
IV forms with
3.5 as the formal charge of the Tc atom.
Other papers devoted to the technetium systems containing polymeric Tc(IV)
species include works of Vongsouthi (2009). The reported experiments were carried
out in 1 ÷ 9 M HTFMS. This author stated that the Tc(IV) polymer is stabilized in
strongly acidic solutions. The standard redox potential of this newly described polymeric Tc(IV)/Tc
3+ couple was reported to be equal to 0.469 V versus NHE. When
concentration of the pertechnetates is low (below ~ 0.5 mM), their electroreduction
leads to the formation of Tc(III). When the technetium concentration increases above
c.a. 0.2 mM a process of the Tc polymerization is observed. An increase in the acid
concentration results also in stabilization of monomeric species containing Tc(III)
and their reoxidation products, i.e., Tc(IV). The last step of the process comprises
the formation of the Tc(III) as follows from the presence of an UV–Vis absorption
band around 460 ÷ 480 nm. Some of these Tc(III) may exist in polymeric forms.
Chotkowski and Czerwi´ nski (2014a) studied also reduction of dimeric reduced
technetium species. They investigated this process in strongly acidic media (4 M
H 2 SO 4 ) using UV–Vis spectroelectrochemistry with RVC-OTTLE electrodes. They
43
active under the experimentel conditions applied. This process manifests itself by
formation of a positive spectroscopic wave (for 440 nm) at potentials below 0.2 V.
The subsequent oxidation of such generated TcO
−
4 electroreduction products leads to
formation of some Tc(IV) species. Electrochemical oxidation of polymeric Tc(IV)
forms starts at potentials higher than ca. 0.8 V as follows from an analysis of the
wave at 502 nm. Chotkowski et al. (2018a) proposed the schemes of formation of
these species as the products of mutual interactions between reduced Tc species,
Eqs. (3.25)–(3.26):
TcO
+
+ TcO
2+
→ [Tc(μ−O) 2 Tc]
3+
(3.25)
TcO
2+
+ TcO
2+
→ [Tc(μ−O) 2 Tc]
4+
(3.26)
or synproportionation of Tc(III) with Tc(VII) species, Eq. 3.27:
3TcO
+
+ TcO
−
4 + 6H
+
→ 2[Tc(μ−O) 2 Tc]
4+
+ 3H 2 O
(3.27)
Vichot (2001) discussed possible reversible reactions inside the polymeric
technetium(III/IV) structures, Eq. (3.28):
Tc
IV
− Tc
IV
+ e
−
Tc
III
− Tc
IV
+ e
−
Tc
III
− Tc
III
(3.28)
This author observed that reduction of Tc(VII) with a concentration of μM carried
out at −0.4 V versus Ag, AgCl(KCl(sat.)) in a solution with pH = 1.75 and containing
[SO
2−
4 ] = 0.01 M leads to formation of a UV–Vis band at 497 nm, which is characteristic of polymeric Tc(IV) form. Longer than 1.3 h polarization at the same potential,
up to 3.1 h, resulted in the disappearance of this band and formation of another one
at 570 nm. This spectroscopic signal has been attributed to Tc
III
− Tc
IV forms with
3.5 as the formal charge of the Tc atom.
Other papers devoted to the technetium systems containing polymeric Tc(IV)
species include works of Vongsouthi (2009). The reported experiments were carried
out in 1 ÷ 9 M HTFMS. This author stated that the Tc(IV) polymer is stabilized in
strongly acidic solutions. The standard redox potential of this newly described polymeric Tc(IV)/Tc
3+ couple was reported to be equal to 0.469 V versus NHE. When
concentration of the pertechnetates is low (below ~ 0.5 mM), their electroreduction
leads to the formation of Tc(III). When the technetium concentration increases above
c.a. 0.2 mM a process of the Tc polymerization is observed. An increase in the acid
concentration results also in stabilization of monomeric species containing Tc(III)
and their reoxidation products, i.e., Tc(IV). The last step of the process comprises
the formation of the Tc(III) as follows from the presence of an UV–Vis absorption
band around 460 ÷ 480 nm. Some of these Tc(III) may exist in polymeric forms.
Chotkowski and Czerwi´ nski (2014a) studied also reduction of dimeric reduced
technetium species. They investigated this process in strongly acidic media (4 M
H 2 SO 4 ) using UV–Vis spectroelectrochemistry with RVC-OTTLE electrodes. They