The measurement of oxygen fugacities in flowing gas mixtures at temperatures below 1200 degrees C

We measured oxygen fugacities in H-2-CO2 and CO-CO2 gas mixtures in the temperature interval 700-1350 degrees C using an yttria-stabilized zirconia (YSZ) oxygen sensor. At high temperatures in excess of 1200 degrees C, measured emfs are consistent with expectations based on the gas composition. At lower temperatures in H-2-CO2 gas mixtures, the oxygen fugacity (f(O2)) obtained assuming Nernstian behavior of the oxygen sensor is as much as two log units more reducing (similar to 900 degrees C) to one log unit more oxidizing (similar to 700 degrees C) than expected by assuming equilibrium speciation. The deviations in H-2-CO2 gas mixtures arise from two sources: (1) poor contact between the electrode and the zirconia electrolyte, leading to apparent f(o2) values that are higher than expected and (2) disequilibrium in the vapor, leading to lower than expected f(o2) values in the temperature range similar to 700-1200 degrees C (for experiments near the iron-wustite (TW) buffer) and higher than expected Sets at lower temperatures. The first problem can be alleviated by spring-loading and lightly sintering a Pt mesh internal electrode against the electrolyte and the second by forcing the entire gas stream to equilibrate by passing it through a Pt catalyst. With these measures, experiments employing H-2-CO2 gas mixtures can be conducted routinely in the temperature range 700-1200 degrees C and the f(o2) determined with an accuracy comparable to that obtained at higher temperatures (2 sigma < +/-0.1 log units). Above similar to 770 degrees C, apparent oxygen fugacities measured using an oxygen sensor in CO-CO2 gas mixtures near IW are consistent with equilibration in the vapor regardless of whether or not a Pt catalyst is present. At lower temperatures, however, the measured values are more oxidizing than the expected equilibrium values. Under more reducing conditions, the deviations begin to occur at even higher temperatures, similar to 930 degrees C for IW-3. The anomalously high f(o2) values are probably related to the condensation of graphite, which removes C from the gas and generates a lower temperature limit for practical gas mixing experiments using CO-CO2 gas mixtures. Copyright (C) 1997 Elsevier Science Ltd.