Oxygen Fugacity

Definition

Oxygen fugacity is an equivalent of the partial pressure of oxygen in a particular environment (atmosphere, rocks, etc.) corrected for the nonideal character of the gas.

Overview

There are multiple ways of characterizing how reducing or how oxidizing an environment is. An example of a redox reaction in an ideal gas mixture of carbon dioxide, methane, water, and oxygen is:

$$ {\mathrm{CO}}_2+2{\mathrm{H}}_2\mathrm{O}\iff {\mathrm{O}}_2+{\mathrm{CH}}_4 $$

(1)

The status of this reaction can be evaluated by writing the equilibrium equation:

$$ \frac{P_{{\mathrm{O}}_2}{P}_{{\mathrm{CH}}_4}}{P_{{\mathrm{CO}}_2}{P}_{{\mathrm{H}}_2\mathrm{O}}^2}={K}_1(T) $$

(2)

where P _i stands for the partial pressure of gas i and K ₁(T) is the reaction coefficient at temperature T. For real gases, pressure P _i should be replaced by fugacities f _i . Fugacity and pressure converge toward each other when the total pressure diminishes.

Likewise, the reduction of magnetite, which contains one Fe²⁺...