REDOX BEHAVIOUR OF THE Tc(VII)/Tc(IV) COUPLE IN DILUTED NaCl SOLUTION IN VARIOUS REDUCING SYSTEMS

The redox behaviour of the Tc(VII)/Tc(IV) couple over a wide range of pH conditions in 0.1 M NaCl/NaOH solution was investigated in various homogenous and heterogenous reducing systems. Stock solution of Tc(VII) was added to each reducing system, and after given periods, the Tc concentration was measured and compared to the initial Tc(VII) concentration (1.10(-5) mol/dm(3)). The results can be systematized according to Eh-pH conditions in solution and a general borderline for the reduction of Tc(VII) to Tc(IV) independent of the reducing systems is obtained. The experimental borderline is slightly lower than the calculated equilibrium line between TcO4- and TcO2(s)center dot xH(2)O(s). This may suggest that more soluble solid phase such as small Tc(IV) oxyhydroxide particles are formed under the given conditions. Reaction kinetics are also discussed and correlated to the measured redox potentials and the reduction borderline.