2 Comprehensive Electrochemistry of Tc …
21
both aqueous and nonaqueous environment. This conclusion is in line with the results
reported by Astheimer and Schwochau (1976). The relative difference in the respective values calculated for solutions of both acids does not exceed 3% for concentrations of up to 1.0 mol kg
−1 (Boyd 1978). Thus, one may deduce transport parameters
for Tc on the basis of respective data available for Re. Such an approach is justified and does not generate large errors as long as rhenium solutions with millimolar
concentrations are considered.
Ironically, what makes the electrochemical properties of technetium and rhenium
similar in aqueous solutions is their incomplete description and understanding.
The electrochemical properties of rhenium have been the subject of intense studies
for about half of a XX century. The studies carried out in 1960s and 1970s focused
on analysis of the electroreduction of perrhenates using polarographic techniques
(Shropshire 1968; Letcher et al. 1970, 1971). Based on the results of experiments
with a dropping mercury electrode, Shropshire (1968) suggested that in an acidic
environment this process proceeds with formation of rhenium compounds with +VI
oxidation state. Such generated Re(VI) undergoes a slow disproportionation reaction,
which results in the formation of Re(IV) and Re(VII), according to Eq. (2.2).
3Re(VI) 2Re(VII) + Re(IV)
(2.2)
Letcher et al. (1970) reported an additional evidence that indicates that the reduction of perrhenate ion in 4M HClO 4 proceeds via an intermediate Re(V) state. Such
behavior makes rhenium similar to technetium because also for the latter species
with +V oxidation state (Tc(V)) are observed as the products of an electrode process
in strongly acidic solutions (Chotkowski and Czerwi´ nski 2012).
Till now, however, the products of the ReO
−
4 electroreduction in an acidic environment have not been unequivocally identified. A general reduction scheme discussed
by e.g., Horányi and Bakos (1993, 1994) include two main stages: formation of an
oxide layer (1st stage) followed by generation of metallic rhenium (2nd stage). Both
composition and structure of the oxide layer strongly depend on the experimental
conditions, Eq. (2.3):
ReO
−
4 → Re (oxides layer) → Re (metallic)
(2.3)
The above-described reactions scheme only partially reflects current state of
knowledge on the reduction of pertechnetates.
Schrebler et al. (2001) analyzed the electroreduction of ReO
−
4 on gold electrode
in slightly acidic solutions (0.1 M Na 2 SO 4 , pH 2). At potentials lower than −0.75 V
versus SCE, two parallel processes were observed: the formation of metallic rhenium
and the hydrogen evolution. These researchers pointed out that generated hydrogen
can facilitate reduction of perrhenate ions adsorbed on the gold surface. Such behavior
differentiates rhenium from technetium since the electroreduction of TcO
−
4 ions starts
at potentials higher than the hydrogen evolution onset.
The formation of rhenium layers on a gold surface through the nucleation and
growth mechanism during electroreduction of ReO
−
4 in acidic solutions (0.1 M
21
both aqueous and nonaqueous environment. This conclusion is in line with the results
reported by Astheimer and Schwochau (1976). The relative difference in the respective values calculated for solutions of both acids does not exceed 3% for concentrations of up to 1.0 mol kg
−1 (Boyd 1978). Thus, one may deduce transport parameters
for Tc on the basis of respective data available for Re. Such an approach is justified and does not generate large errors as long as rhenium solutions with millimolar
concentrations are considered.
Ironically, what makes the electrochemical properties of technetium and rhenium
similar in aqueous solutions is their incomplete description and understanding.
The electrochemical properties of rhenium have been the subject of intense studies
for about half of a XX century. The studies carried out in 1960s and 1970s focused
on analysis of the electroreduction of perrhenates using polarographic techniques
(Shropshire 1968; Letcher et al. 1970, 1971). Based on the results of experiments
with a dropping mercury electrode, Shropshire (1968) suggested that in an acidic
environment this process proceeds with formation of rhenium compounds with +VI
oxidation state. Such generated Re(VI) undergoes a slow disproportionation reaction,
which results in the formation of Re(IV) and Re(VII), according to Eq. (2.2).
3Re(VI) 2Re(VII) + Re(IV)
(2.2)
Letcher et al. (1970) reported an additional evidence that indicates that the reduction of perrhenate ion in 4M HClO 4 proceeds via an intermediate Re(V) state. Such
behavior makes rhenium similar to technetium because also for the latter species
with +V oxidation state (Tc(V)) are observed as the products of an electrode process
in strongly acidic solutions (Chotkowski and Czerwi´ nski 2012).
Till now, however, the products of the ReO
−
4 electroreduction in an acidic environment have not been unequivocally identified. A general reduction scheme discussed
by e.g., Horányi and Bakos (1993, 1994) include two main stages: formation of an
oxide layer (1st stage) followed by generation of metallic rhenium (2nd stage). Both
composition and structure of the oxide layer strongly depend on the experimental
conditions, Eq. (2.3):
ReO
−
4 → Re (oxides layer) → Re (metallic)
(2.3)
The above-described reactions scheme only partially reflects current state of
knowledge on the reduction of pertechnetates.
Schrebler et al. (2001) analyzed the electroreduction of ReO
−
4 on gold electrode
in slightly acidic solutions (0.1 M Na 2 SO 4 , pH 2). At potentials lower than −0.75 V
versus SCE, two parallel processes were observed: the formation of metallic rhenium
and the hydrogen evolution. These researchers pointed out that generated hydrogen
can facilitate reduction of perrhenate ions adsorbed on the gold surface. Such behavior
differentiates rhenium from technetium since the electroreduction of TcO
−
4 ions starts
at potentials higher than the hydrogen evolution onset.
The formation of rhenium layers on a gold surface through the nucleation and
growth mechanism during electroreduction of ReO
−
4 in acidic solutions (0.1 M