The Eh-pH Stability Field of Water

The theoretical stability field of water in Eh and pH terms (see Fig. 11.2) bounds all theoretical redox reactions taking place in water. The upper boundary of the water stability field is defined by Eh and pH values for which liquid water is in equilibrium with 02(g) at 1 bar pressure. The lower boundary is defined by Eh and pH values for which liquid water is in equilibrium with H2(g) at 1 bar pressure. 

The equation of the lower boundary can be derived from Eq. (11.17) for the hydrogen electrode. At the lower stability field of water, / n,(g) = 1 bar, and this equation reduces to 

The pQ,(g) pressure at the lower stability limit can be computed from the reaction 

Under atmospheric conditions where Pq, = 0.2 bar, this equation is practically unchanged and Eh = 

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