3.1 Acidic solutions
51
2. The authors stated that this redox couple is electrochemically reversible in strongly
acidic solutions. At pH between 2 and 5, the slope in the plot of pH versus E 1/2 was
equal to 35 mV per pH unit.
3.2 Alkaline solutions
Early works on reduction of the pertechnetates in alkaline solutions reported a multistep process although the identity of the initial stage of the process is disputable. For
example, Salaria et al. (1963b) observed a wave at a half-wave potential of −0.81 V
versus SCE in a solution with pH of 13, which was attributed to an irreversible threeelectron process. Further reduction of such generated technetium species to Tc(III)
occurred at E 1/2 = −1.02 V and was identified as an adsorption-controlled process.
Münze (1968), on the other hand, reported a four-electron process composed of three
stages. It begins with a transfer of two electrons (Tc(VII) → Tc(V)) at E 1/2 = −
0.8 V which is followed by a one electron reaction (Tc(V) → Tc(IV)) at E 1/2 = −
0.9 V and ends with another one electron step (Tc(IV) → Tc(III)) at E 1/2 = −1.1 V
versus SCE. In contrast, Colton et al. (1960) concluded that the pertechnetates are
reduced in 0.1 M KOH to Tc(IV) and not to Tc(III). This process includes two- (E 1/2
= −0.85 V versus SCE) and one-electron reduction steps (E 1/2 = −1.15 V).
The electroreduction of the pertechnetates in alkaline solutions is assumed nowadays as a multistage process with Tc(V) as the product generated through Tc(VI)
intermediates. The Tc(V) species can be further reduced to Tc(IV). It cannot be ruled
out that electrolysis of a solution containing Tc(IV) at strongly cathodic potentials
can lead to the formation of Tc(III) species and, under specific conditions, even
metallic Tc can be generated.
The hexavalent technetium species and its stability were examined using UV–
Vis by Deutsch et al. (1978). These researchers conducted pulse radiolysis of 0.01 ÷
0.1 mM TcO
−
4 in 0.1 M KOH solutions. They observed that both aqueous electron and
TcO
−
4 disappear according to a first-order kinetics. This process was accompanied
by generation of a new form of Tc, which was described the researchers as possibly
TcO
2−
4 . They found the rate constant of this process equals to k 2 = (2.48 ± 0.05)·10
10
dm
3 ·mol
−l ·s
−l at 25 °C. The Tc(VI) form was characterized by two bands: the first
one with a maximum at approximately 335 nm and the second one, with a broad, flat
peak in the range of 500 ÷ 530 nm. Deutsch et al. stated that Tc(VI) is apparently
stable for 10 ms but later on its reactions accelerate and all the species eventually
disappears within 50 ms. In addition, cyclic voltammetry experiments (Fig. 3.10)
revealed that a reduction wave at a potential of about −0.8 V versus SCE appears
only for very high scan rates of the working electrode (Fig. 3.10b, c). The researchers
attributed this wave to the Tc(VII)/Tc(VI) couple. Application of Randles–Sevcik
equation in analysis of the voltammetric results allows to calculate number of the
electrons exchanged in the discussed process, which was equal to n = 1. The standard
reduction potential of the Tc(VII)/Tc(VI) system was determined at a level of −
0.61 V versus NHE.
51
2. The authors stated that this redox couple is electrochemically reversible in strongly
acidic solutions. At pH between 2 and 5, the slope in the plot of pH versus E 1/2 was
equal to 35 mV per pH unit.
3.2 Alkaline solutions
Early works on reduction of the pertechnetates in alkaline solutions reported a multistep process although the identity of the initial stage of the process is disputable. For
example, Salaria et al. (1963b) observed a wave at a half-wave potential of −0.81 V
versus SCE in a solution with pH of 13, which was attributed to an irreversible threeelectron process. Further reduction of such generated technetium species to Tc(III)
occurred at E 1/2 = −1.02 V and was identified as an adsorption-controlled process.
Münze (1968), on the other hand, reported a four-electron process composed of three
stages. It begins with a transfer of two electrons (Tc(VII) → Tc(V)) at E 1/2 = −
0.8 V which is followed by a one electron reaction (Tc(V) → Tc(IV)) at E 1/2 = −
0.9 V and ends with another one electron step (Tc(IV) → Tc(III)) at E 1/2 = −1.1 V
versus SCE. In contrast, Colton et al. (1960) concluded that the pertechnetates are
reduced in 0.1 M KOH to Tc(IV) and not to Tc(III). This process includes two- (E 1/2
= −0.85 V versus SCE) and one-electron reduction steps (E 1/2 = −1.15 V).
The electroreduction of the pertechnetates in alkaline solutions is assumed nowadays as a multistage process with Tc(V) as the product generated through Tc(VI)
intermediates. The Tc(V) species can be further reduced to Tc(IV). It cannot be ruled
out that electrolysis of a solution containing Tc(IV) at strongly cathodic potentials
can lead to the formation of Tc(III) species and, under specific conditions, even
metallic Tc can be generated.
The hexavalent technetium species and its stability were examined using UV–
Vis by Deutsch et al. (1978). These researchers conducted pulse radiolysis of 0.01 ÷
0.1 mM TcO
−
4 in 0.1 M KOH solutions. They observed that both aqueous electron and
TcO
−
4 disappear according to a first-order kinetics. This process was accompanied
by generation of a new form of Tc, which was described the researchers as possibly
TcO
2−
4 . They found the rate constant of this process equals to k 2 = (2.48 ± 0.05)·10
10
dm
3 ·mol
−l ·s
−l at 25 °C. The Tc(VI) form was characterized by two bands: the first
one with a maximum at approximately 335 nm and the second one, with a broad, flat
peak in the range of 500 ÷ 530 nm. Deutsch et al. stated that Tc(VI) is apparently
stable for 10 ms but later on its reactions accelerate and all the species eventually
disappears within 50 ms. In addition, cyclic voltammetry experiments (Fig. 3.10)
revealed that a reduction wave at a potential of about −0.8 V versus SCE appears
only for very high scan rates of the working electrode (Fig. 3.10b, c). The researchers
attributed this wave to the Tc(VII)/Tc(VI) couple. Application of Randles–Sevcik
equation in analysis of the voltammetric results allows to calculate number of the
electrons exchanged in the discussed process, which was equal to n = 1. The standard
reduction potential of the Tc(VII)/Tc(VI) system was determined at a level of −
0.61 V versus NHE.