for the free energy in terms of potentials, the energy balance is given by a Nernst equation: For the iron and water example, considering the boundary line between the ferrous ion Fe2+ and hematite Fe2O3, the reaction equation is: Fe2O3(s) + 6 H+(aq) + 2 e^- <=> 2 Fe^{2+}(aq) + 3 H2O(l) with . The equation of the boundary line, expressed in base-10 logarithms is: As, the activities, or the concentrations, of the solid phases and water are always taken equal to unity by convention in the definition of the equilibrium constant : [Fe2O3] = [H2O] = 1. The Nernst equation thus limited to the dissolved species and is written as: For, [Fe2+] = 10−6 M, this yields: Note the negative slope (-0.1775) of this line in a Eh–pH diagram. The stability region of water In many cases, the possible conditions in a system are limited by the stability region of water. In the Pourbaix diagram for uranium presented here above, the limits of stability of water are marked by the two dashed green lines, and the stability region for water falls between these two lines. It is also depicted here beside by the two dashed red lines in the simplified Pourbaix diagram restricted to the water stability region only. Under highly reducing conditions (low EH), water is reduced to hydrogen according to: 2 H+ + 2e^- -> H2(g) (at low pH) and, 2 H2O + 2e^- -> H2(g) + 2 OH^- (at high pH) Using the Nernst equation, setting E0 = 0 V as defined by convention for the standard hydrogen electrode (SHE, serving as reference in the reduction potentials series) and the hydrogen gas fugacity (corresponding to chemical activity for a gas) at 1, the equation for the lower stability line of water in the Pourbaix diagram at standard temperature and pressure is: Below this line, water is reduced to hydrogen, and it will usually not be possible to pass beyond this line as long as there is still water present in the system to be reduced. Correspondingly, under highly oxidizing conditions (high EH) water is oxidized into oxygen gas according to: 2 H2O -> 4 H+ + O2(g) + 4e^- (at low pH) and, 4 OH^- -> O2(g) + 2 H_2O + 4e^- (at high pH) Using the Nernst equation as above, but with E0 = −ΔG0H2O/2F = 1.229 V for water oxidation, gives an upper stability