Thermodynamic data from redox reactions at high temperatures. III. Activity-composition relations in Ni-Pd alloys from EMF measurements at 850?1250 K, and calibration of the NiO+Ni-Pd assemblage as a redox sensor

Abstract

The assemblage NiO+Ni-Pd alloy has been calibrated as a precise oxygen fugacity sensor in the temperature range 850–1250 K at 1 bar, using an electrochemical technique with oxygen-specific CSZ electrolytes, and Ni+NiO and Cu+Cu2O as the reference electrodes. Nine compositions were studied, ranging from 0.12 to 0.83 X Ni alloy . Steady EMFs, implying equilibrium, were rapidly achieved in all cells, and were found to be reversible on increasing and decreasing temperature with a precision approaching 0.1 mV. The estimated accuracy of the measurements on each cell is ±0.2 mV (1σ, corresponding to ±0.003 log-bar units in fo2 at 1273 K). Compositions of the Ni-Pd alloys were measured after each run by electron microprobe, and these compositions were then checked for internal consistency by measuring the lattice parameter by X-ray diffraction. Nickel-rich alloys show positive deviations from ideality and endothermic enthalpies of mixing, but palladium-rich compositions have exothermic enthalpies of mixing and strong negative deviations from ideality. The excess entropies of mixing are positive for all compositions, and correlate approximately with the excess volumes of mixing. The highly asymmetrical deviations from ideality are well described by a polynomial expression of the Redlich-Kister form, with three terms for the enthalpies, and two for the excess entropies and volumes of mixing. The experimental data from this study have been used to re-formulate the Ni-Pd oxygen fugacity sensor to give an expression; μ O2 ss = μ O2 NNO − 2RT ln X Ni alloy − [2 · (1 − X Ni alloy )2 · [(−2165−7.958 · T) + (9409 − 0.888 · T) · (4 X Ni alloy − 1) + 2089 · (6 X Ni alloy − 1) · (2 X Ni alloy − 1)]](850<T<1300) where μ O2 ss is in J mol-1, T is in kelvins, and the expression for μ O2 NNO is that given by O'Neill and Pownceby (1993). Values in terms of log fo2 may be obtained from the above by dividing by RT ln 10. The estimated standard error in μ O2 ss is on the order of ±200 J mol-1, which is approximately ±0.01 log-bar units in fo2 at 1273 K.