Redox potentials Eh-pH diagrams

For nonstandard conditions, the redox potential of the 02(g)-H20(I) system is, from Eq. (6.26) 

redox potentials in natural environments will generally be less that the value of 1, given by Eq. (6.28). 

In the limit considered above, water acted as a reductant to limit the oxidation that can occur in nature. Water also acts as an oxidant to limit the reduction that can occur in nature. In this case, the halfreaction involving water is 

pH= -log[H (aq)], and the p utial pressure of hydrogen gas near the Earth s surface cannot exceed 1 atm, the minimum value of cell for a system in the presence of water is 

the redox potential in natural environments near the Earth s surface should not fall below the value given by Eq. (6.31). 

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