The chemical potential of oxygen (μO2) defined by the reactions  
has been determined using an electrochemical method with calcia-stabilized zirconia solid electrolytes. The Fe + Fe3O4, Fe + “FeO,” and Cu + Cu2O equilibria were measured using air as the reference, and these results are thus absolute determinations of μO2. Cu + Cu2O and Fe + “FeO” were then used as reference electrodes for other measurements reported both here and elsewhere.

The results for the μO2 values are [in J · mol−1, T in kelvins, with a reference pressure for O2 of 1 bar (105 Pa)]

forCu+Cu2O(±62),347705+246.096T12.9053T In T    (750<T<1330),
forFe+FeO(±100),605812+1366.718T182.7955T In T+0.10359T2    (833<T<1042),519357+59.427T+8.9276T In T    (1042<T<1184),551159+269.404T16.9484T In T    (1184<T<1450),
for“FeO”+Fe3O4(±306),581927-65.618T+38.7410T In T    (833<T<1270),
forFe+Fe3O4(±107),607673+1060.994T-132.3909T In T+0.06657T2    (750<T<833),
and forCu2O+CuO(±117),292245+377.012T-23.1976T In T    (800<T<1300).

The standard enthalpy of formation of Fe3O4 is −1115.4 ± 0.2 kj mol−1, in excellent agreement with that deduced from the quartz-fayalite-iron and quartz-fayalite-magnetite equilibria at higher temperatures.

The results for the Fe3O4 + Fe2O3 equilibrium are less certain and have therefore not been presented in the form of a simple equation. For the other equilibria, the results represent a significant improvement in accuracy over existing measurements.

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