36
3 Technetium Coordinated by Inorganic Ligands in Aqueous …
overall process includes a set of consecutive reactions (3.10)–(3.12) with the initial
TcO 3 formation considered as the rate-determining step:
TcO
−
4 + 2H
+
+ e
− rds
→ TcO 3 + H 2 O
(3.10)
TcO 3 + e
−
TcO
−
3
(3.11)
TcO
−
3 + 3H
+
+ 2e
−
TcO(OH) + H 2 O
(3.12)
Cobble and coworkers reported the estimated value of the standard potential of
TcO
−
4 /TcO 3 redox couple in aqueous solutions equal to 0.7 V (Cobble et al. 1953).
They noted that TcO 3 is unstable in water and decomposes to pertechnetic acid and
technetium dioxide.
It is well established that Tc(VI) and Tc(V) are unstable in aqueous acidic solutions
and undergo disproportionation. For instance, Courson et al. (1999) suggested the
following reactions of the Tc(IV) and Tc(V) disintegration (3.13)–(3.14):
2TcO
2−
4 → TcO
3−
4 + TcO
−
4
(3.13)
2TcO
3−
4
H
+
→ TcO
2−
4 + TcO
2+
(3.14)
TcO
2+ ions may undergo a hydrolysis with the formation of TcO 2 (see: Table 3.3).
Recent EXAFS studies have shown that Tc(V) forms generated in a strongly acidic
environment (13 M H 2 SO 4 ) have TcO
3+ core (Poineau et al. 2013). Lawson and
coworkers (Lawson et al. 1984) reported that at pH 1-2 pertechnetates are reduced to
Tc(IV) species adsorbed at the surface of a carbon electrode. They pointed out that
also some Tc(V) forms are adsorbed at the electrode surface in formate solutions
with pH of 3.
Chotkowski and Czerwi´ nski (2012) investigated the electrochemical properties of
technetium species in aqueous solutions with a wide range of concentration of sulfuric
acid. Figure 3.1 presents typical CVs recorded in solutions containing from 1 to 4 M
of H 2 SO 4 with addition of 0.5 mM TcO 4
− . The voltammetry curves reveal currents
due to redox processes of technetium at potentials lower than ca. 1.1 V versus SHE
as well as currents due to oxidation and reduction of gold substrate surface which
are located above ca. 1 V. An analysis shows that an increase in the solution acidity
enhances currents of the reduction peak 1, which is attributed to the generation of
Tc(VI) and Tc(V) on the electrode surface (Fig. 3.1). The next reduction signal, a very
wide peak at ca. 0.5 V, is associated with formation of Tc(III) and Tc(IV) species.
Anodic section of the voltammetric curves recorded after changing direction of the
potential scan reveals three partially overlapping current peaks located below ca.
3 Technetium Coordinated by Inorganic Ligands in Aqueous …
overall process includes a set of consecutive reactions (3.10)–(3.12) with the initial
TcO 3 formation considered as the rate-determining step:
TcO
−
4 + 2H
+
+ e
− rds
→ TcO 3 + H 2 O
(3.10)
TcO 3 + e
−
TcO
−
3
(3.11)
TcO
−
3 + 3H
+
+ 2e
−
TcO(OH) + H 2 O
(3.12)
Cobble and coworkers reported the estimated value of the standard potential of
TcO
−
4 /TcO 3 redox couple in aqueous solutions equal to 0.7 V (Cobble et al. 1953).
They noted that TcO 3 is unstable in water and decomposes to pertechnetic acid and
technetium dioxide.
It is well established that Tc(VI) and Tc(V) are unstable in aqueous acidic solutions
and undergo disproportionation. For instance, Courson et al. (1999) suggested the
following reactions of the Tc(IV) and Tc(V) disintegration (3.13)–(3.14):
2TcO
2−
4 → TcO
3−
4 + TcO
−
4
(3.13)
2TcO
3−
4
H
+
→ TcO
2−
4 + TcO
2+
(3.14)
TcO
2+ ions may undergo a hydrolysis with the formation of TcO 2 (see: Table 3.3).
Recent EXAFS studies have shown that Tc(V) forms generated in a strongly acidic
environment (13 M H 2 SO 4 ) have TcO
3+ core (Poineau et al. 2013). Lawson and
coworkers (Lawson et al. 1984) reported that at pH 1-2 pertechnetates are reduced to
Tc(IV) species adsorbed at the surface of a carbon electrode. They pointed out that
also some Tc(V) forms are adsorbed at the electrode surface in formate solutions
with pH of 3.
Chotkowski and Czerwi´ nski (2012) investigated the electrochemical properties of
technetium species in aqueous solutions with a wide range of concentration of sulfuric
acid. Figure 3.1 presents typical CVs recorded in solutions containing from 1 to 4 M
of H 2 SO 4 with addition of 0.5 mM TcO 4
− . The voltammetry curves reveal currents
due to redox processes of technetium at potentials lower than ca. 1.1 V versus SHE
as well as currents due to oxidation and reduction of gold substrate surface which
are located above ca. 1 V. An analysis shows that an increase in the solution acidity
enhances currents of the reduction peak 1, which is attributed to the generation of
Tc(VI) and Tc(V) on the electrode surface (Fig. 3.1). The next reduction signal, a very
wide peak at ca. 0.5 V, is associated with formation of Tc(III) and Tc(IV) species.
Anodic section of the voltammetric curves recorded after changing direction of the
potential scan reveals three partially overlapping current peaks located below ca.