16
2 Comprehensive Electrochemistry of Tc …
Dioxides of technetium and its neighbors from the periodic table are well known
and their chemistry is well recognized. Among them only manganese dioxide is
a strongly oxidizing agent. The standard redox potential of MnO 2 /Mn
2+ couple
in acidic solutions is high and equals 1.23 V. TcO 2 or ReO 2 is not considered as
oxidants.
An analysis of the values of the standard redox potentials of MO
−
4 /M(IV) couples
of the manganese group elements reveals that the values of these parameters decrease
in the order from manganese through technetium to rhenium. In contrast to pertechnetates and perrhenates, the permanganates are strong oxidizing agents in acidified aqueous solutions. The values of the respective standard redox potentials for
TcO
−
4 /TcO 2 or ReO
−
4 /ReO 2 systems are relatively low and equal to 0.746 V and
0.51 V, respectively. Similar periodic changes of the redox potentials are observed
also for MO
−
4 /M(VI) couples. The electroreduction of MnO
−
4 to stable in alkaline
solution MnO
2−
4 proceeds easily and quickly and its completion does not cause significant experimental problems (Freeman and Mamantov 1976; Norwell and Mamantov
1977). In the case of technetium, on the other hand, this process is more complicated
due to instability of Tc(VI). The latter is generated in the first step of the pertechnetates electroreduction and rapidly disporoportionates to Tc(VII) and Tc(IV) via
another one unstable intermediate, i.e. Tc(V) (Rard et al. 1999). It should be noted,
however, that the latest report indicates possibility of extending the life time of Tc(VI)
in alkaline environment with high ionic strength (Chatterjee et al. 2018).
What distinguishes technetium (and rhenium) from manganese is the lack of ionic
form with +2 oxidation state that are stable in aqueous solutions. Mn
2+ is especially
stable in an aqueous environment and can be oxidized on solid electrodes to Mn
3+ and
further to MnO 2 (Chotkowski et al. 2011; Rogulski et al. 2006). Lee and coworkers
performed electrochemical studies on properties of manganese dioxide (Lee et al.
1977, 1980). Both Mn
3+ and Tc
3+ ions are sensitive to the presence of oxygen,
Tc(III) is quickly converted to Tc(IV) in a reaction with oxygen. On the basis of
spectroelectrochemical results, Córdoba de Torresi and Gorenstein (1992) concluded
that the first step of the MnO 2 reduction leads to the generation of MnO(OH). This
reversible stage involves a one electron reduction of Mn(IV) accompanied by addition
of a proton to MnO 2 . The proton originates from decomposition of a water molecule,
Eq. (2.1).
MnO 2 + H 2 O + e
−
MnO(OH) + OH
−
(2.1)
MnO(OH) can be further reduced to a mixture of hydrated manganese oxides (II
and III), MnO 2 Mn 3 O 4 that can be further reduced to Mn(OH) 2 . Early papers devoted
to electrochemical properties of Tc assume the existence of its mixed oxides, e.g.,
Tc 3 O 4 , and hydroxides, e.g., Tc(OH) 2 (Cartledge 1971; Mazzocchin et al. 1974).
Contemporary reports, however, are not conclusive as to their existence and structure
(Rard et al. 1999). Moreover, in contrast to technetium and rhenium, manganese is
the only manganese group member have found broad applications in galvanic cells,
including classical Leclanché batteries, their modifications and other power sources
2 Comprehensive Electrochemistry of Tc …
Dioxides of technetium and its neighbors from the periodic table are well known
and their chemistry is well recognized. Among them only manganese dioxide is
a strongly oxidizing agent. The standard redox potential of MnO 2 /Mn
2+ couple
in acidic solutions is high and equals 1.23 V. TcO 2 or ReO 2 is not considered as
oxidants.
An analysis of the values of the standard redox potentials of MO
−
4 /M(IV) couples
of the manganese group elements reveals that the values of these parameters decrease
in the order from manganese through technetium to rhenium. In contrast to pertechnetates and perrhenates, the permanganates are strong oxidizing agents in acidified aqueous solutions. The values of the respective standard redox potentials for
TcO
−
4 /TcO 2 or ReO
−
4 /ReO 2 systems are relatively low and equal to 0.746 V and
0.51 V, respectively. Similar periodic changes of the redox potentials are observed
also for MO
−
4 /M(VI) couples. The electroreduction of MnO
−
4 to stable in alkaline
solution MnO
2−
4 proceeds easily and quickly and its completion does not cause significant experimental problems (Freeman and Mamantov 1976; Norwell and Mamantov
1977). In the case of technetium, on the other hand, this process is more complicated
due to instability of Tc(VI). The latter is generated in the first step of the pertechnetates electroreduction and rapidly disporoportionates to Tc(VII) and Tc(IV) via
another one unstable intermediate, i.e. Tc(V) (Rard et al. 1999). It should be noted,
however, that the latest report indicates possibility of extending the life time of Tc(VI)
in alkaline environment with high ionic strength (Chatterjee et al. 2018).
What distinguishes technetium (and rhenium) from manganese is the lack of ionic
form with +2 oxidation state that are stable in aqueous solutions. Mn
2+ is especially
stable in an aqueous environment and can be oxidized on solid electrodes to Mn
3+ and
further to MnO 2 (Chotkowski et al. 2011; Rogulski et al. 2006). Lee and coworkers
performed electrochemical studies on properties of manganese dioxide (Lee et al.
1977, 1980). Both Mn
3+ and Tc
3+ ions are sensitive to the presence of oxygen,
Tc(III) is quickly converted to Tc(IV) in a reaction with oxygen. On the basis of
spectroelectrochemical results, Córdoba de Torresi and Gorenstein (1992) concluded
that the first step of the MnO 2 reduction leads to the generation of MnO(OH). This
reversible stage involves a one electron reduction of Mn(IV) accompanied by addition
of a proton to MnO 2 . The proton originates from decomposition of a water molecule,
Eq. (2.1).
MnO 2 + H 2 O + e
−
MnO(OH) + OH
−
(2.1)
MnO(OH) can be further reduced to a mixture of hydrated manganese oxides (II
and III), MnO 2 Mn 3 O 4 that can be further reduced to Mn(OH) 2 . Early papers devoted
to electrochemical properties of Tc assume the existence of its mixed oxides, e.g.,
Tc 3 O 4 , and hydroxides, e.g., Tc(OH) 2 (Cartledge 1971; Mazzocchin et al. 1974).
Contemporary reports, however, are not conclusive as to their existence and structure
(Rard et al. 1999). Moreover, in contrast to technetium and rhenium, manganese is
the only manganese group member have found broad applications in galvanic cells,
including classical Leclanché batteries, their modifications and other power sources