When H and OH ions are not involved in the reaction, the boundary line is horizontal and independent of pH. <br />The reaction equation is thus written:
As, the standard Gibbs free energy :
Using the definition of the electrode potential ∆G = -zFE, where F is the Faraday constant, this may be rewritten as a Nernst equation:
or, using base-10 logarithms:
For the equilibrium /, taken as example here, considering the boundary line between Fe and Fe, the half-reaction equation is:
Since H  ions are not involved in this redox reaction, it is independent of pH. 
E = 0.771 V with only one electron involved in the redox reaction.
The potential E is a function of temperature via the thermal voltage  and directly depends on the ratio of the concentrations of the  and  ions:
For both ionic species at the same concentration (e.g., ) at STP, log 1 = 0, so, , and the boundary will be a horizontal line at E = 0.771 volts. The potential will vary with temperature.