3.1 Acidic solutions
41
process:
TcO 2 + 2H
+
+ e
−
TcO
+
+ H 2 O
(3.21)
The equilibrium red–ox potential that corresponds to this reaction is given by
Eq. (3.22):
E = 0.319 − 0.1182 · pH − 0.0591 · log
TcO
+
(3.22)
These authors also stated that the oxidation of Tc(III) to Tc(IV) is faster than
subsequent oxidation of the Tc(IV) to the pertechnetates. Taking into account, the
hydrolysis constant of Tc(IV) (Table 3.3) one may expect the existence of more than
one hydrolyzed form of Tc(IV) in the aqueous solutions. It should be stressed that
one electron Nernstian slope predicted by Eq. (3.21) is expected also for Tc
3+ , which
is suggested by other authors as the Tc(IV) reduction product. The cationic structure
of the Tc(III) species is in line with the results reported by other researchers (e.g.
Salaria et al. 1963a; Rulfs et al. 1967). Oxidation of the Tc(III) may lead to formation
of TcO
2+ . Salaria with coworkers proposed Tc
3+ as the product of polarographic
electroreduction of the pertechnetates at pH < 4, Eq. (3.23):
TcO
−
4 + 8H
+
+ 4e
−
Tc
3+
+ 4H 2 O
(3.23)
These authors assumed this process as an irreversible one with the half-wave
potential given by Eq. (3.24):
E 1/2 = 0.12 − 0.11 3 · pH (0.25M Na 2 SO 4 + H 2 SO 4 )
(3.24)
Grassi reported that the oxidation of Tc(III) to Tc(IV) is accompanied by generation of some electro-inactive Tc(IV) species. Spitsyn proposed its structure as
a [Tc(OH) 2 (SO 4 ) 2 ]
2− complex. Today it is known that the reduced Tc(III/IV)
species can exist in aqueous solutions also as dimeric (or polymeric?) forms with
[Tc(μ−O) 2 Tc]
4+or 3+ core structure (Mausolf et al. 2011). Recent research carried
out in strongly acidic solutions indicates that reduced Tc species can also exist as
polymers with [Tc(μ−O)Tc]
6+ core structure (Poineau et al. 2018).
Chotkowski and Czerwi´ nski have shown that Tc(IV) polymeric species are electrooxidized at potentials higher than 0.7 V (Chotkowski and Czerwinski 2014a,
b). These authors investigated electroreduction of the pertechnetates and subsequent
oxidation of such obtained reduced technetium species in strongly acidic media using
UV–Vis spectroelectrochemistry, especially voltabsorbommetry (VAB).
The simultaneous use of both spectroscopic and electrochemical methods in examination of organic or inorganic compounds has been applied by many authors (see
e.g., Heineman et al. 1984; Kim et al. 1995; Kulesza et al. 1998; Astuti et al. 2004;
Duluard et al. 2010; Wang and He 2012). The spectroelectrochemical techniques
are particularly useful for determining the mechanism of electrochemical processes
with participation of species dissolved in solution. The VABs are calculated as a time
41
process:
TcO 2 + 2H
+
+ e
−
TcO
+
+ H 2 O
(3.21)
The equilibrium red–ox potential that corresponds to this reaction is given by
Eq. (3.22):
E = 0.319 − 0.1182 · pH − 0.0591 · log
TcO
+
(3.22)
These authors also stated that the oxidation of Tc(III) to Tc(IV) is faster than
subsequent oxidation of the Tc(IV) to the pertechnetates. Taking into account, the
hydrolysis constant of Tc(IV) (Table 3.3) one may expect the existence of more than
one hydrolyzed form of Tc(IV) in the aqueous solutions. It should be stressed that
one electron Nernstian slope predicted by Eq. (3.21) is expected also for Tc
3+ , which
is suggested by other authors as the Tc(IV) reduction product. The cationic structure
of the Tc(III) species is in line with the results reported by other researchers (e.g.
Salaria et al. 1963a; Rulfs et al. 1967). Oxidation of the Tc(III) may lead to formation
of TcO
2+ . Salaria with coworkers proposed Tc
3+ as the product of polarographic
electroreduction of the pertechnetates at pH < 4, Eq. (3.23):
TcO
−
4 + 8H
+
+ 4e
−
Tc
3+
+ 4H 2 O
(3.23)
These authors assumed this process as an irreversible one with the half-wave
potential given by Eq. (3.24):
E 1/2 = 0.12 − 0.11 3 · pH (0.25M Na 2 SO 4 + H 2 SO 4 )
(3.24)
Grassi reported that the oxidation of Tc(III) to Tc(IV) is accompanied by generation of some electro-inactive Tc(IV) species. Spitsyn proposed its structure as
a [Tc(OH) 2 (SO 4 ) 2 ]
2− complex. Today it is known that the reduced Tc(III/IV)
species can exist in aqueous solutions also as dimeric (or polymeric?) forms with
[Tc(μ−O) 2 Tc]
4+or 3+ core structure (Mausolf et al. 2011). Recent research carried
out in strongly acidic solutions indicates that reduced Tc species can also exist as
polymers with [Tc(μ−O)Tc]
6+ core structure (Poineau et al. 2018).
Chotkowski and Czerwi´ nski have shown that Tc(IV) polymeric species are electrooxidized at potentials higher than 0.7 V (Chotkowski and Czerwinski 2014a,
b). These authors investigated electroreduction of the pertechnetates and subsequent
oxidation of such obtained reduced technetium species in strongly acidic media using
UV–Vis spectroelectrochemistry, especially voltabsorbommetry (VAB).
The simultaneous use of both spectroscopic and electrochemical methods in examination of organic or inorganic compounds has been applied by many authors (see
e.g., Heineman et al. 1984; Kim et al. 1995; Kulesza et al. 1998; Astuti et al. 2004;
Duluard et al. 2010; Wang and He 2012). The spectroelectrochemical techniques
are particularly useful for determining the mechanism of electrochemical processes
with participation of species dissolved in solution. The VABs are calculated as a time