Chemistry of Technetium

Precise and reliable thermodynamic constants for the oxidation/reduction relationships of the technetium ions in various environments are necessary to model the transport behaviour of 99 Tc from the nuclear waste repositories to the bio and geosphere. The current knowledge of the Tc chemistry indicates that the most important oxidation states of Tc in natural environment would be VII in oxidant conditions and IV, in an-oxidant media. However, even the value of the formal oxidation potential of the Tc O4-/Tc (IV) couple in simple media is questionable, because it relies only upon one kind of data: the values of the potential of the half electrochemical cell Tc O4-/Tc O2. These potentials are reversible, but sparingly soluble Tc O2 is not in fast equilibrium with other Tc (IV) complexes. The measured potentials are surely Tc O4-/Tc O2 potentials, but not the Tc O4-/Tc(IV) potentials if Tc O2 is not the thermodynamically stable form of Tc (IV). Quantitative data for the Tc O2 equilibria with Tc (IV) complexes are difficult to obtain because of the slowness of the ligand exchange reactions and the general lack of knowledge of the chemistry of the VIIb group tetravalent ions. The present investigation deals with a new approach for the determination of the formal oxidation potential of the Tc O4-/Tc (IV) couple. Tc was used at trace level, in order to avoid the kinetic and thermodynamic sinks made by insoluble Tc O2. Oxidation and reduction of Tc were governed by controlling the ion concentration of a fast electron exchanger couple of which the formal potential has been measured carefully, presently Fe (III)/Fe (II). Distribution ratios of Tc between aqueous and immiscible organic solution of tetra-phenyl arsenic chloride (TPA.Cl) into chloroform served as the probe for the advancement of oxidation and reduction reaction. In these solvent, only Tc (VII) as Tc O4- anions is extracted. (authors)