How to Calculate a Pourbaix Diagram for Nickel

Trace the x- and y-axis on a piece of paper. The y-axis represents the equilibrium potential, abbreviated as ̶0;Ee̶1;. When calculating a Pourbaix diagram Ee is calculated in Volts relative to the standard hydrogen electrode --or SHE -- also known as a reference electrode. The x-axis represents the pH. The origin of the axes coincides with the pH value of zero and a value of -1.8 on the y-axis. Mark the vertical axis every 0.2 units until reaching the level of +2. The horizontal axis should contain values starting from zero to 16.

Trace two lines on the diagram representing the number of possible cathodic reactions for water.

The reactions for water are as follows:
2H++ 2e- ͛6; H2
H2O͛6; ½ O2 +2H++ 2e-

Obtain the values for the equilibrium potential for each of these reactions according to the Nerst equation. For the first equation Ee= -0.059pH, while for the second, Ee=1.223-0.059pH. Trace the sloping lines representing these reactions on the diagram. Draw a descending line, originating from point zero on the y-axis and another line descending from the 1.223 level on the y-axis. These lines are drawn in the same manner in all Pourbaix diagrams for all metals and their species.

Find out the number of oxidizing species for nickel, including Ni2+ ,Ni (OH)2 and NiO. Calculate the equilibrium potential for each of these species with the Nerst equation using the formula:

Ee = E0 -2.303 RT/zF x

log(reduced)/(oxidized)

"E0" represents the standard potential. Use 8.3145 J K-1 mol-1,
which is the value of ̶0;R̶1; --the universal gas constant, and 96 485 C per mole of electrons for ̶0;F,̶1; or the Faraday constant. In this equation, ̶0;T̶1; is the absolute temperature and ̶0;z̶1; is the number of electron moles that take part in the reaction. Calculate the logarithm using a calculator and use the number of reduced and oxidized species in the reaction. For instance in the reaction:

Ni2+ +2H2O͛6; Ni (OH)2 +2H+

calculate log Ni (OH)2(H+)2 /(Ni2+)(H2O)2

Considering that the continuous phases of Ni(OH)2 and H2O have activities of one, calculate log (H+)2 /(Ni2+).

Mark all the results in your diagram after calculating the equilibrium potential for each of the nickel oxidizing species. The oxidizing species Ni2+ is a result of a pure acid-base reaction and it is not dependent on potential, so trace it as a vertical line in your diagram. Interpret the diagram after tracing all lines. Nickel is immune to corrosion when it is in a liquid with conditions under any of the lines you see in your diagram.