124
5 Metallic Technetium, Corrosion, Technetium Alloys …
containing 0.1 or 1 g dm
−3 TcO
−
4 using Au, GC and Pt electrodes under hydrodynamic conditions (RDE). The authors stated that the currents due to the pertechnetate
reactions were indistinguishable from the TcO 4
− free background.
The experiments carried out by Bebko (2011) showed that even small addition of
HNO 3 to a solution containing the pertechnetates significantly affects the currents
due to TcO 4
− reactions. A large, distorted reduction wave was observed in 4 M
H 2 SO 4 solutions containing 50 mM HNO 3 and 0.32 mM TcO
−
4 at potentials below
0.2 V (vs. Hg, Hg 2 SO 4 , 0.5 M H 2 SO 4 ). This signal contains charge associated with
the electrochemical reduction of TcO
-
4 but the main contribution comes from electrochemical degradation of HNO 3 . Additional spectroelectrochemical measurements
confirmed the formation of HNO 2 and NO x .
Before starting more precise characterization of the catalytic properties of Tc in
HNO 3 solutions, it is necessary to discuss possible redox couples containing NO x ,
which may exist in the nitric acid solutions. HNO 3 –HNO 2 –NO x systems exhibit
relatively high values of the standard redox potentials (Compton and Sanders 1996)
(5.6–5.8):
NO
−
3 + 4H
+
+ 3e
−
HNO 2 + H 2 O E
= 0.94 V
(5.6)
NO
−
3 + 4H
+
+ 3e
−
NO + 2H 2 O E
= 0.96 V
(5.7)
2NO
−
3 + 4H
+
+ 2e
−
N 2 O 4 + 2H 2 O E
= 0.81 V
(5.8)
The above standard redox potential values are higher than those for the reduced
technetium species (Rard et al. 1999) which means that the latter may be involved
in redox reactions with the nitrates. A further complication in the analysis of the
HNO 3 –Tc systems comes from the decomposition of HNO 3 accelerated by the
accumulation of HNO 2 (5.9):
NO
−
3 + HNO 2 + H
+
2NO 2 + H 2 O
(5.9)
Unstable HNO 2 shows strong oxidizing properties (5.10):
HNO 2 + H
+
+ e
−
NO + H 2 O E
= 0.99 V
(5.10)
and is able to oxidize the reduced Tc.
Chotkowski and Czerwi´ nski (2016) investigated the stability of technetium
species with intermediate oxidation states of in the presence of nitric acid. The solutions containing initially electrochemically generated polymeric Tc(III/IV) species
were acidified by addition of HNO 3 . Oxidation of these Tc forms was accompanied
by generation of a new band at 465 nm (Fig. 5.9). The maximum of this wave is close
to the value of 480 nm assigned to the Tc(V) ions by Rotmanov (2015). This wave is
broad and poorly shaped, which suggests that apart from Tc(V) also other Tc species
5 Metallic Technetium, Corrosion, Technetium Alloys …
containing 0.1 or 1 g dm
−3 TcO
−
4 using Au, GC and Pt electrodes under hydrodynamic conditions (RDE). The authors stated that the currents due to the pertechnetate
reactions were indistinguishable from the TcO 4
− free background.
The experiments carried out by Bebko (2011) showed that even small addition of
HNO 3 to a solution containing the pertechnetates significantly affects the currents
due to TcO 4
− reactions. A large, distorted reduction wave was observed in 4 M
H 2 SO 4 solutions containing 50 mM HNO 3 and 0.32 mM TcO
−
4 at potentials below
0.2 V (vs. Hg, Hg 2 SO 4 , 0.5 M H 2 SO 4 ). This signal contains charge associated with
the electrochemical reduction of TcO
-
4 but the main contribution comes from electrochemical degradation of HNO 3 . Additional spectroelectrochemical measurements
confirmed the formation of HNO 2 and NO x .
Before starting more precise characterization of the catalytic properties of Tc in
HNO 3 solutions, it is necessary to discuss possible redox couples containing NO x ,
which may exist in the nitric acid solutions. HNO 3 –HNO 2 –NO x systems exhibit
relatively high values of the standard redox potentials (Compton and Sanders 1996)
(5.6–5.8):
NO
−
3 + 4H
+
+ 3e
−
HNO 2 + H 2 O E
= 0.94 V
(5.6)
NO
−
3 + 4H
+
+ 3e
−
NO + 2H 2 O E
= 0.96 V
(5.7)
2NO
−
3 + 4H
+
+ 2e
−
N 2 O 4 + 2H 2 O E
= 0.81 V
(5.8)
The above standard redox potential values are higher than those for the reduced
technetium species (Rard et al. 1999) which means that the latter may be involved
in redox reactions with the nitrates. A further complication in the analysis of the
HNO 3 –Tc systems comes from the decomposition of HNO 3 accelerated by the
accumulation of HNO 2 (5.9):
NO
−
3 + HNO 2 + H
+
2NO 2 + H 2 O
(5.9)
Unstable HNO 2 shows strong oxidizing properties (5.10):
HNO 2 + H
+
+ e
−
NO + H 2 O E
= 0.99 V
(5.10)
and is able to oxidize the reduced Tc.
Chotkowski and Czerwi´ nski (2016) investigated the stability of technetium
species with intermediate oxidation states of in the presence of nitric acid. The solutions containing initially electrochemically generated polymeric Tc(III/IV) species
were acidified by addition of HNO 3 . Oxidation of these Tc forms was accompanied
by generation of a new band at 465 nm (Fig. 5.9). The maximum of this wave is close
to the value of 480 nm assigned to the Tc(V) ions by Rotmanov (2015). This wave is
broad and poorly shaped, which suggests that apart from Tc(V) also other Tc species