The potential––pH diagram for rhenium

The aqueous chemistry of rhenium is of importance for the hydrometallurgical processes involving rhenium alloys, for the leaching of rhenium from its ores and alloys, and for the electrochemical deposition of thin rhenium films. This study aims to present the chemical and electrochemical equilibria in the Re – H₂O system in the form of the Pourbaix diagram. The existing variants of the Pourbaix diagrams for rhenium available in the literature were reviewed; it was shown that they significantly differ from each other in the composition and the values of the thermodynamic activities of rhenium species. The thermodynamic properties of aqueous rhenium species, including the standard Gibbs energies of formation and the standard electrode potentials of half-cell reactions involving it, were collected and analysed. The compounds of rhenium (+7) and rhenium (+6) not isolated in the free aqueous state, and rhenium hydride compounds were discussed. The self-consisted set of thermodynamic data for further calculations was obtained. Using these data, the basic chemical and electrochemical equilibria involving aqueous rhenium species at the temperature of 298 K and the air pressure of 1 bar were calculated. For the first time the potential – pH diagram for the Re – H₂O system at the temperature of 298 K, the air pressure of 1 bar and the different activities of rhenium species in the solution were constructed. The influence of thermodynamic activities of aqueous rhenium compounds on the stability of rhenium oxides was discussed. It was shown that rhenium trioxide ReO₃ is thermodynamically stable only in concentrated solutions (at the activities of rhenium species not less than 10^–6 М), rhenium dioxide ReO₂ loses the stability at the activities of rhenium species less than 10^–8 М, and rhenium sesquioxide Re₂O₃ does so at the activities of rhenium species not less than 10^–14 М. The necessity of the measurement of the stability constants of ortho- and mesorhenate anions, as well as nonahydridorhenates for a deeper understanding of the aqueous rhenium chemistry was discussed.