In the recent development of redox flow batteries (RFB), water-soluble organic molecules are at the forefront of attention due to their affordability, safety, and structural diversity. In water, pH will impact the protonated/deprotonated form of the reduced or oxidized molecules participating in the redox reaction. When the proton concentration is lowered, i.e., the pH is raised, even strong bases may not be protonated. Hence, pH-potential diagrams of the electrolyte species, known as Pourbaix diagrams, should be assessed early in the battery design process. The Pourbaix diagram can be constructed using the redox potential (E red. ) and pK a of the involved species.