based). Examples with a Pourbaix diagram To illustrate the dependency of the reduction potential on pH, one can simply consider the two oxido-reduction equilibria determining the water stability domain in a Pourbaix diagram . When water is submitted to electrolysis by applying a sufficient difference of electrical potential between two electrodes immersed in water, hydrogen is produced at the cathode (reduction of water protons) while oxygen is formed at the anode (oxidation of water oxygen atoms). The same may occur if a reductant stronger than hydrogen (e.g., metallic Na) or an oxidant stronger than oxygen (e.g., F2) enters in contact with water and reacts with it. In the here beside (the simplest possible version of a Pourbaix diagram), the water stability domain (grey surface) is delimited in term of redox potential by two inclined red dashed lines: Lower stability line with hydrogen gas evolution due to the proton reduction at very low Eh: (cathode: reduction) Higher stability line with oxygen gas evolution due to water oxygen oxidation at very high Eh: (anode: oxidation) When solving the Nernst equation for each corresponding reduction reaction (need to revert the water oxidation reaction producing oxygen), both equations have a similar form because the number of protons and the number of electrons involved within a reaction are the same and their ratio is one (2/2 for H2 and 4/4 with respectively), so it simplifies when solving the Nernst equation expressed as a function of pH. The result can be numerically expressed as follows: Note that the slopes of the two water stability domain upper and lower lines are the same (-59.16 mV/pH unit), so they are parallel on a Pourbaix diagram. As the slopes are negative, at high pH, both hydrogen and oxygen evolution requires a much lower reduction potential than at low pH. For the reduction of H+ into H2 the here above mentioned relationship becomes: because by convention = 0 V for the standard hydrogen electrode (SHE: pH = 1). So, at pH = 7, = -0.414 V for the reduction of protons. For the reduction of O2 into 2 H2O the here above mentioned relationship becomes: because = +1.229 V with respect to the standard hydrogen electrode (SHE: pH = 1). So, at pH = 7, = +0.815 V for the reduction of oxygen. The offset of -414 mV in is the same for both reduction reactions because they share the