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How to Calculate a Pourbaix Diagram

The Pourbaix diagrams, also known as potential pH diagrams, play an important role in electrochemistry and corrosion science. The Russian scientist Marcel Pourbaix came up with the idea for these diagrams in 1938, and they indicate potential stable phases of aqueous electrochemical systems. Knowing and predicting the corrosion rates of various metals in contact with liquids with variable pH values can improve safety and lifetime of ships and offshore platforms. You can create a diagram for metals and their oxidized species to find which species are thermodynamically stable at various values of electrode potential and pH.

Things You'll Need

Pencil and paper
Ruler
Scientific calculator

Instructions

- 1
  Draw two axes, a vertical and a horizontal one. Tag the vertical axis or y-axis with "Ee," which stands for the equilibrium potential, also known as the reversible potential and is the equilibrium state between two oxidation states of the same chemical element. The equilibrium potential is a variable and changes with temperature, concentration and pressure variations. The horizontal axis stands for the pH. The origin of the axes is the level zero for pH and -2 or -1.8 for the Ee. Mark the y-axis every 0.2 units. Mark the pH axis with levels starting from 0 to 16. Ee is measured in volts, while the pH is measured in units.
- 2
  Find the number of possible cathodic reactions for water. There are two cathodic reactions, so draw two lines representing these reactions. One line starts from zero on the y-axis and decreases as the pH increases. The second line starts at the level of 1.2 on the y-axis and descends as the pH grows. These lines are the same on all Pourbaix diagrams, regardless of the metal that is being studied.
- 3
  Establish the number of oxidized species for the metal you are studying. Use the Nerst equation for each oxidized species to calculate the equilibrium potential.
  The Nerst equation establishes the value of the equilibrium potential Ee:
  
  Ee = E0 -2.303 RT/zF x log(reduced)/(oxidized)
  
  In this equation "E0" is the standard potential. "R" stands for the universal gas constant and equals 8.3145 J K-1 mol-1. "T" is the absolute temperature, "z" is the number of moles of electrons that participate in the reaction, while "F" is the Faraday constant equaling 96.485 C per mole of electrons. The notation ̶0;reduced̶1; stands for all the species that are reduced in the electrode reaction, while "(oxidized)" represents the oxidized species.
- 4
  Obtain the values of Ee for each species of the metal you study. The equilibrium potential will depend on the pH. Calculate the Ee for different pH levels and trace your results in the diagram. The metal will be immune to corrosion when in conditions that are under any lines you draw in the Pourbaix diagram.

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