The passivation potential of iron is a function of pH. At one time it was proposed to identify iron's passive film by calculating the pH dependency of the equilibrium potential of various iron oxides and comparing them to the measured variation of the passivation potential with pH. The oxide whose equilibrium potential varied with pH in the same way as the passivation potential would be identified as the passive film. To check on the viability of the suggestion, calculate the pH dependency of the equilibrium potential of the following oxides, hydroxides and oxyhydroxides of iron: FeO, Fe(OH)2, Fe:O4, Fe2O3, Fe(OH)3, and FeOOH.