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 H /2 e for H 2 and 4 H /4 e with O 2 respectively), so it simplifies when solving the Nernst equation expressed as a function of pH.