Thermodynamic data from redox reactions at high temperatures. II. The MnO-0Mn₃O₄ oxygen buffer, and implications for the thermodynamic properties of MnO and Mn₃O₄

Abstract

The$\mu _{O_2 }$ defined by the reaction 6 MnO+O₂ =2 Mn₃O₄ has been determined from 917 to 1,433 K using electrochemical cells (with calcia-stabilized zirconta, CSZ) of the type:

Steady emfs were achieved rapidly at all temperatures on both increasing and decreasing temperature, indicating that the MnO-Mn₃O₄ oxygen buffer equilibrates relatively easily. It therefore makes a useful alternative choice in experimental petrology to Fe₂O₃-Fe₃O₄ for buffering oxygen potentials at oxidized values. The results are (in J/mol, temperature in K, reference pressure 1 bar);$\mu _{O_2 }$ (±200)=-563,241+1,761.758T-220.490T inT+0.101819T ² with an uncertainty of ±200 J/mol. Third law analysis of these data, including a correction for the deviations in stoichiometry of MnO, impliesS _298.15 for Mn₃O₄ of 166.6 J/K · mol, which is 2.5 J/K · mol higher than the calorimetric determination of Robie and Hemingway (1985). The low value of the calorimetric entropy may be due to incomplete ordering of the magnetic spins. The third law value of Δ _r H ⁰_298.15 is-450.09 kJ/mol, which is significantly different from the calorimetric value of-457.5±3.4 kJ/mol, calculated from Δ _f H ⁰_298.15 of MnO and Mn₃O₄, implying a small error in one or both of these latter.