3.1 Acidic solutions
45
Chemistry of halides containing reduced technetium is one of the most extensively explored areas of the technetium chemistry. This is due to the fact that these
compounds are important for various fields of the nuclear industry, including production of the nuclear fuel (e.g. volatile TcF 6 ) and management of radioactive waste (e.g.
immobilization of poorly reactive compounds in solid forms). Technetium halides
also found application as precursors of various radiopharmaceuticals.
Trop (1979) examined electrochemistry of halogen complexes of technetium.
He analyzed [n–Bu 4 N] 2 [TcCl 6 ] irreversible reduction in CH 3 CN with 0.1 M
[n–Bu 4 N][ClO 4 ] used as supporting electrolyte and determined respective half-wave
potential value, E 1/2 , as equal to −0.34 V versus SCE. Oxidation of this Tc compound
was also irreversible with E 1/2 = 1.88 V. The respective E 1/2 of bromide complexes
of Tc, TcBr
2−
6 , are lower than the above values and are equal to −0.27 V and 1.70 V,
respectively. This behavior suggests that TcX
2−
6 (X = Cl or Br) are kinetically labile.
Rajec i Macášek (1981) dealt with studies on electoreduction process of Tc(V)
chlorocomplexes in 4 M HCl. The investigated Tc species, TcOCl
2−
5 , were obtained
as product of a reaction of TcO 4
− with 11.8 M HCl. The electroreduction of Tc(VII)
at −0.2 V and −0.4 V versus Ag, AgCl leads to the formation of TcCl 5 (H 2 O)
−
and TcCl
2−
6 . On the basis of UV–Vis spectroscopy measurements in 286–357 nm
region, these authors determined the concentration ratio of selected Tc(V) forms,
such as, e.g., TcOCl
2−
5 with λ max = 238, 325 nm. The equilibrium constant (K) for
the reaction given by Eq. (3.30):
TcCl
−
5 + Cl
−
TcCl
2−
6
(3.30)
was found to be equal to 0.80±0.04 M
−1 (T ~ 25 °C).
The speciation of Tc(IV) in chloride solutions was of interest for Liu et al. (2005).
Electrophoretic experiments allowed them determining mobility of selected Tc(IV)
forms. In 1 M HCl/NaCl electrolyte with pH of 1 and at 25 °C, these values are equal to
5.47·10
−4 cm
2 ·V
−1 ·S
−1 and 2.13·10
−4 cm
2 ·V
−1 ·S
−1 for TcCl
2−
6 and [TcCl 5 (H 2 O)]
−
respectively.
The work of the Deutsch group broadened significantly knowledge about the electrochemical properties of technetium halide complexes (e.g. Huber et al. 1987, 1981).
These authors examined chloro- and bromotechnetium compounds in concentrated
HX/X
− (X: Cl, Br) and nonaqueous solutions. Selection of type of the solution was
driven by the fact that all TcX
2−
6 , except TcF
2−
6 , undergo a fast hydrolysis in neutral
and weakly acidic aqueous solutions with formation of insoluble TcO 2 .
Huber et al. (1987) also analyzed the process of the TcX
2−
6 (X : Cl, Br) electroreduction in 2 ÷ 4 M HX/NaX(2 ÷ 5.1 M for Cl or 2 ÷ 4 M for Br). The overall
reaction of reduction of Tc(IV) to Tc(III) was given by Eq. (3.31):
Tc(IV) + e
−
Tc(III) + n h H
+
+ n X X
−
(3.31)
while the corresponding Nernst equation is given by (3.32):
45
Chemistry of halides containing reduced technetium is one of the most extensively explored areas of the technetium chemistry. This is due to the fact that these
compounds are important for various fields of the nuclear industry, including production of the nuclear fuel (e.g. volatile TcF 6 ) and management of radioactive waste (e.g.
immobilization of poorly reactive compounds in solid forms). Technetium halides
also found application as precursors of various radiopharmaceuticals.
Trop (1979) examined electrochemistry of halogen complexes of technetium.
He analyzed [n–Bu 4 N] 2 [TcCl 6 ] irreversible reduction in CH 3 CN with 0.1 M
[n–Bu 4 N][ClO 4 ] used as supporting electrolyte and determined respective half-wave
potential value, E 1/2 , as equal to −0.34 V versus SCE. Oxidation of this Tc compound
was also irreversible with E 1/2 = 1.88 V. The respective E 1/2 of bromide complexes
of Tc, TcBr
2−
6 , are lower than the above values and are equal to −0.27 V and 1.70 V,
respectively. This behavior suggests that TcX
2−
6 (X = Cl or Br) are kinetically labile.
Rajec i Macášek (1981) dealt with studies on electoreduction process of Tc(V)
chlorocomplexes in 4 M HCl. The investigated Tc species, TcOCl
2−
5 , were obtained
as product of a reaction of TcO 4
− with 11.8 M HCl. The electroreduction of Tc(VII)
at −0.2 V and −0.4 V versus Ag, AgCl leads to the formation of TcCl 5 (H 2 O)
−
and TcCl
2−
6 . On the basis of UV–Vis spectroscopy measurements in 286–357 nm
region, these authors determined the concentration ratio of selected Tc(V) forms,
such as, e.g., TcOCl
2−
5 with λ max = 238, 325 nm. The equilibrium constant (K) for
the reaction given by Eq. (3.30):
TcCl
−
5 + Cl
−
TcCl
2−
6
(3.30)
was found to be equal to 0.80±0.04 M
−1 (T ~ 25 °C).
The speciation of Tc(IV) in chloride solutions was of interest for Liu et al. (2005).
Electrophoretic experiments allowed them determining mobility of selected Tc(IV)
forms. In 1 M HCl/NaCl electrolyte with pH of 1 and at 25 °C, these values are equal to
5.47·10
−4 cm
2 ·V
−1 ·S
−1 and 2.13·10
−4 cm
2 ·V
−1 ·S
−1 for TcCl
2−
6 and [TcCl 5 (H 2 O)]
−
respectively.
The work of the Deutsch group broadened significantly knowledge about the electrochemical properties of technetium halide complexes (e.g. Huber et al. 1987, 1981).
These authors examined chloro- and bromotechnetium compounds in concentrated
HX/X
− (X: Cl, Br) and nonaqueous solutions. Selection of type of the solution was
driven by the fact that all TcX
2−
6 , except TcF
2−
6 , undergo a fast hydrolysis in neutral
and weakly acidic aqueous solutions with formation of insoluble TcO 2 .
Huber et al. (1987) also analyzed the process of the TcX
2−
6 (X : Cl, Br) electroreduction in 2 ÷ 4 M HX/NaX(2 ÷ 5.1 M for Cl or 2 ÷ 4 M for Br). The overall
reaction of reduction of Tc(IV) to Tc(III) was given by Eq. (3.31):
Tc(IV) + e
−
Tc(III) + n h H
+
+ n X X
−
(3.31)
while the corresponding Nernst equation is given by (3.32):