limit of water as a function of the pH value: at standard temperature and pressure. Above this line, water is oxidized to form oxygen gas, 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 oxidized. The two upper and lower stability lines having the same negative slope (−59 mV/pH unit), they are parallel in a Pourbaix diagram and the reduction potential decreases with pH. Applications Pourbaix diagrams have many applications in different fields dealing with e.g., corrosion problems, geochemistry, and environmental sciences. Using the Pourbaix diagram correctly will help shedding light not only on the nature of the species present in aqueous solution, or in the solid phases, but may also help to understand the reaction mechanism. Concept of in environmental chemistry Pourbaix diagrams are widely used to describe the behaviour of chemical species in the hydrosphere. In this context, reduction potential is often used instead of . The main advantage is to directly work with a logarithm scale. is a dimensionless number and can easily be related to by the equation: Where, is the thermal voltage, with , the gas constant (), , the absolute temperature in Kelvin (298.15 K = 25 °C = 77 °F), and , the Faraday constant (96 485 coulomb/mol of ). Lambda, λ = ln(10) ≈ 2.3026. Moreover, , an expression with a similar form to that of pH. values in environmental chemistry ranges from −12 to +25, since at low or high potentials water will be respectively reduced or oxidized. In environmental applications, the concentration of dissolved species is usually set to a value between 10−2 M and 10−5 M for the determination of the equilibrium lines. Gallery See also Nernst equation Dependency of reduction potential on pH Ellingham diagram Latimer diagram Frost diagram Ionic partition diagram Bjerrum plot Notes References External links Marcel Pourbaix — Corrosion Doctors DoITPoMS Teaching and Learning Package- "The Nernst Equation and Pourbaix Diagrams" Software ChemEQL Free software for calculation of chemical equilibria from Eawag. FactSage Commercial thermodynamic databank software, also available in a free web application. The Geochemist's Workbench Commercial geochemical modeling software from Aqueous Solutions LLC. GWB Community Edition Free download of the popular geochemical modeling software package. HYDRA/MEDUSA Free software for creating chemical equilibrium diagrams from the KTH Department of Chemistry. HSC Chemistry Commercial thermochemical calculation software from Outotec