Calculation of Oxygen Fugacity — pH Diagrams

The method of calculating log fo -pH boundaries is illustrated with three examples. All other boundaries are derived the same way. Our examples include the boundaries for water stability and for coexisting minerals, and the aqueous solubility contours of a mineral. Notice that half-cell reactions are not involved in these calculations. 

For the water stability boundaries, the dissociation reaction of water is 

The upper boundary occurs at 1 atm 02(g) pressure or log /o = 0. The lower boundary is at 1 atm H2(g) pressure from the equilibrium constant (18.53) this corresponds to log /02 = —83.1. As noted before, water can exist outside of these boundaries, but only if the pressure of oxygen or hydrogen is greater than 1 bar. 

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