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How to understand the “potential-pH diagram” (i.e., Pourbaix diagram)? 1) Generally speaking, the Pourbaix diagram is actually a equilibrium diagram; it is no different from the temperature-pressure phase diagrams of pure substances, nor is there any substantial difference from the phase equilibrium diagrams of M-O-S systems. They are all the magnitudes of the Gibbs function for reactions (including phase transitions). The Pourbaix diagram simply provides the Gibbs function as a function of concentration at known temperatures and pressures ； The temperature-pressure phase diagram of a pure substance does not indicate the value of the Gibbs function, but it shows the lines at which the Gibbs function is 0 when the independent variables (temperature and pressure) change ； The phase equilibrium diagram of the M-O-S system represents the lines at which a reaction is in equilibrium at a specified temperature and total pressure, that is, the partial pressures of oxygen and sulfur at which the Gibbs free energy is 0 ； 2) Since the Pourbaix diagram is essentially a equilibrium diagram, it is also possible to use the Gibbs free energy as the vertical axis (in fact, potential is used), because we know that delta G = -nFE. Why switch to potential? Because potential is the connection point that links all reactions, all reactions can be communicated through potential; using the Gibbs function becomes inconvenient in such cases. 3) When using potential to represent the Gibbs function, certain methods are employed. It enables the voltage of direct current to be directly equivalent to the potential in the Pourbaix diagram. For example, the stable region of water lies between the aforementioned equilibrium lines. The above equilibrium lines are often represented by dashed lines when drawing other potential-pH diagrams, indicating the stable region of the solvent used. Species above the oxidation line or below the reduction line are unstable in water. 4) The potentials in the Pourbaix diagram are often very low; therefore, electrochemical instruments typically require low-voltage direct current ； 5) To provoke further discussion.