6.1 Acidic and Neutral Solutions
145
An increase in the Cl
− ions concentration in the solution decreases intensity of the
peak B although its potential remains approximately unchanged. In contrast to the
peak B, the potential of the peak A is shifted toward lower values when the NaCl
concentration increases while its intensity remains relatively unchanged. Such influence of the chlorides on the electrochemical behavior of the technetium species can
be understood when the discussed process leads to formation of technetium-chloro
or technetium-oxochloro species rather than pertechnetates. Lewis et al. determined
technetium in a buffer solution without chlorides by means of chronocoulometric
measurements. They reported the detection limit equal to (1.00 ± 0.01) · 10
−7 mol
dm
−3 . The procedure required deposition of Tc at −0.90 V for 10 min followed
by shifting the potential to −0.10 V and finally to 0.60 V. The measurements were
carried out at −0.10 V rather than at −0.90 V in order to reduce contributions from
the double layer charging on the blank chronocoulogram. Lewis et al. concluded that
the proposed method of the Tc determination is affected by several factors, including
the pH value and the presence of the chlorides which influence height of the current
peaks. Changes in the electrode surface due to aging processes, on the other hand,
affect reproducibility of the results.
A combination of a high pressure liquid chromatography (HPLC) with electrochemical detection of technetium was applied by Lewis et al. (1983) for analysis of
solutions eluted from a
99 Mo/
99m Tc generator. The surface of the stationary phase of
the HPLC contained –NH 2 functional groups. The tests were carried out in acetates
solutions or in phosphate buffers (pH = 5/6) using carbon electrodes or static mercury
drop electrode (SMDE). The Tc detection limits obtained for the carbon and mercury
electrodes were equal to 8.5 · 10
−9 and 2.1 · 10
−8 mol dm
−3 , respectively. It is worth
mentioning that the technetium solubility in the liquid mercury is low. Kozin estimated its value as equal to 1.1 · 10
−9 at% at 25 °C (Guminski and Galus 1986). Therefore, application of the Hg as a liquid electrode is intended to obtain a reproducible
surface rather than Hg(Tc) amalgam.
Using adsorption stripping voltammetry at HMDE Friedrich and Ruf (1986) determined the technetium concentration at the level of approx. 10
−11 g ml
−1 (~10
−10 mol
dm
−3 ). The system was preconcentrated for 120 s at the potential of −0.4 V versus Ag,
AgCl. The measurements were performed out using the differential cyclic voltammetry technique with a modulation amplitude of −40 mV, an electrode polarization
rate of 15 mV s
−1 and a potential ranging from −0.8 V to −1.5 V. The influence of the
pulse amplitude (from −40 to −300 mV) on the high of the stripping voltammetric
peak of technetium turned out to be relatively small. The measurements were carried
out in solutions containing H 2 SO 4 with a concentration of about 5 mM and various
concentrations of SCN
− . An analysis of the currents due to technetium reaction at −
1.27 V revealed that the highest signal is observed for the thiocyanates concentration
of 0.66 mM (Fig. 6.2).
Friedrich and Ruf stated that application of solutions with high acid concentration
is disadvantageous due to a possible reaction between the Tc(VII) ions with the
metallic mercury electrode. Therefore, the experiments were carried out in solutions
with a pH above 2. The presence of NaCl and Na 2 SO 4 significantly influenced the
height of the recorded current signals. Thus, when the concentration of these salts
145
An increase in the Cl
− ions concentration in the solution decreases intensity of the
peak B although its potential remains approximately unchanged. In contrast to the
peak B, the potential of the peak A is shifted toward lower values when the NaCl
concentration increases while its intensity remains relatively unchanged. Such influence of the chlorides on the electrochemical behavior of the technetium species can
be understood when the discussed process leads to formation of technetium-chloro
or technetium-oxochloro species rather than pertechnetates. Lewis et al. determined
technetium in a buffer solution without chlorides by means of chronocoulometric
measurements. They reported the detection limit equal to (1.00 ± 0.01) · 10
−7 mol
dm
−3 . The procedure required deposition of Tc at −0.90 V for 10 min followed
by shifting the potential to −0.10 V and finally to 0.60 V. The measurements were
carried out at −0.10 V rather than at −0.90 V in order to reduce contributions from
the double layer charging on the blank chronocoulogram. Lewis et al. concluded that
the proposed method of the Tc determination is affected by several factors, including
the pH value and the presence of the chlorides which influence height of the current
peaks. Changes in the electrode surface due to aging processes, on the other hand,
affect reproducibility of the results.
A combination of a high pressure liquid chromatography (HPLC) with electrochemical detection of technetium was applied by Lewis et al. (1983) for analysis of
solutions eluted from a
99 Mo/
99m Tc generator. The surface of the stationary phase of
the HPLC contained –NH 2 functional groups. The tests were carried out in acetates
solutions or in phosphate buffers (pH = 5/6) using carbon electrodes or static mercury
drop electrode (SMDE). The Tc detection limits obtained for the carbon and mercury
electrodes were equal to 8.5 · 10
−9 and 2.1 · 10
−8 mol dm
−3 , respectively. It is worth
mentioning that the technetium solubility in the liquid mercury is low. Kozin estimated its value as equal to 1.1 · 10
−9 at% at 25 °C (Guminski and Galus 1986). Therefore, application of the Hg as a liquid electrode is intended to obtain a reproducible
surface rather than Hg(Tc) amalgam.
Using adsorption stripping voltammetry at HMDE Friedrich and Ruf (1986) determined the technetium concentration at the level of approx. 10
−11 g ml
−1 (~10
−10 mol
dm
−3 ). The system was preconcentrated for 120 s at the potential of −0.4 V versus Ag,
AgCl. The measurements were performed out using the differential cyclic voltammetry technique with a modulation amplitude of −40 mV, an electrode polarization
rate of 15 mV s
−1 and a potential ranging from −0.8 V to −1.5 V. The influence of the
pulse amplitude (from −40 to −300 mV) on the high of the stripping voltammetric
peak of technetium turned out to be relatively small. The measurements were carried
out in solutions containing H 2 SO 4 with a concentration of about 5 mM and various
concentrations of SCN
− . An analysis of the currents due to technetium reaction at −
1.27 V revealed that the highest signal is observed for the thiocyanates concentration
of 0.66 mM (Fig. 6.2).
Friedrich and Ruf stated that application of solutions with high acid concentration
is disadvantageous due to a possible reaction between the Tc(VII) ions with the
metallic mercury electrode. Therefore, the experiments were carried out in solutions
with a pH above 2. The presence of NaCl and Na 2 SO 4 significantly influenced the
height of the recorded current signals. Thus, when the concentration of these salts