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 (original) (raw)

Abstract

The chemical potential of oxygen defined by the equilibrium: \operatorname{Re} (solid) + O_2 (gas) = \operatorname{Re} O_2 (solid)hasbeenmeasuredbetween850and1250Kviaanelectrochemicalmethodusingcalcia−stabilizedzirconiaelectrolytesandeitherNi+NiOorCu+Cu2Oasthereferenceelectrode.Theresultsare: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:hasbeenmeasuredbetween850and1250KviaanelectrochemicalmethodusingcalciastabilizedzirconiaelectrolytesandeitherNi+NiOorCu+Cu2Oasthereferenceelectrode.Theresultsare:\begin{array}{*{20}c} {\mu O_2 ^{\operatorname{Re} + \operatorname{Re} O_2 } ( \pm 400) = - 451020 + 297.595 T} \\ { - 14.6585 T In T(850< T< 1250)} \\ \end{array}

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-\(f_{O_2 }\)may be obtained from the above expression by dividing by R_T_ln10, 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.

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Authors and Affiliations

  1. CSIRO Division of Mineral Products, PO Box 124, 3207, Port Melbourne, Victoria, Australia
    Mark I. Pownceby
  2. Bayerisches Geoinstitut, Universität Bayreuth, Postfach 101251, D-95440, Bayreuth, Germany
    Hugh St. C. O'Neill

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  1. Mark I. Pownceby
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  2. Hugh St. C. O'Neill
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Pownceby, M.I., O'Neill, H.S.C. 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.Contr. Mineral. and Petrol. 118, 130–137 (1994). https://doi.org/10.1007/BF01052864

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