Thermodynamic data from redox reactions at high temperatures. IV. Calibration of the Re-ReO₂ oxygen buffer from EMF and NiO+Ni-Pd redox sensor measurements

The chemical potential of oxygen defined by the equilibrium: {Mathematical expression} has been measured between 850 and 1250 K via an electrochemical method using calcia-stabilized zirconia electrolytes and either Ni+NiO or Cu+Cu₂O as the reference electrode. The results are: {Mathematical expression} where μO₂ is in J·mol^-1, T in kelvins, and the reference pressure for O₂ is 1 bar (10⁵ Pa). (Values in terms of log- {Mathematical expression}may be obtained from the above expression by dividing by R Tln10, where R=8.31441 J.K^-1·mol^-1). The standard enthalpy of formation of ReO₂ is-444.350±0.400 (1σ) kJ·mol^-1, requiring a significant modification to previously published estimates. These results were checked in hydrothermal experiments using the double capsule method with NiO+Ni-Pd alloy as an oxygen sensor. Reversals at P=1 kbar over the T range 823 to 1073 K are in good agreement with the electrochemical measurements. These latter results also serve to demonstrate: (1) the usefulness of the "redox sensor" method; (2) the viability of using Re+ReO₂ as a buffer in hydrothermal experiments. Re+ReO₂ lies nearly midway between the Ni+NiO and Fe₃O₄+Fe₂O₃ buffers in μO₂-T space, and thus fills a petrologically important gap in the range of μO₂s which can be covered by accurately calibrated oxygen buffers.