# Potentials in Pourbaix Diagrams

by Silvius
Tags: diagrams, potentials, pourbaix
 P: 13 Hi guys, I'm having a bit of difficulty understanding Pourboix diagrams. The biggest problem at the moment is that I don't clearly understand what exactly the E° values on the y-axis are of. All the resources I've consulted haven't been especially clear about this. Sometimes they make it sound as though they are some sort of "environment" potential - but what exactly this means, I'm not too sure. In other places they're made to sound like E° values of the reduction of the relevant species compared to SHE, but a reduction to what exactly, I'm not sure either. In a related problem, I don't understand whether we discern E° and what the most stable species present is for a given pH, or whether we only discern what the most stable species present is from a combination of pH and mystical E° information. Would anyone be able to help? Any assistance would be greatly appreciated! Hopefully my questions are clear enough... Thanks!
 Admin P: 21,706 Disclaimer: haven't used them in ages. Potential axis refers to the potential "as observed in the solution" - that is, if you would put an inert electrode into the solution (together with everything else that is needed for a measurement) that's the potential you would measure. It doesn't care about the source of the potential. Basically diagrams tells you what to expect in the solution of a given E and pH. Plus whatever you are able to discern about possible changes when modifying E/pH in any way.
 P: 13 I see, thank you very much, Borek! How does this relate to the idea that the potential at which a horizontal line divides two oxidation states of an element is equal to the potential of the reduction half equation which relates those two species? (For example where we had Fe$^{3+}$ (aq) and Fe$^{2+}$ (aq) divided by a horizontal line with the former on the top).
Yes but why do the potentials in this case represent the potentials for the reduction of Fe$^{3+}$ to Fe$^{2+}$ when there is other stuff which might be contributing to the "solution" potential? Is it simply because under these conditions, those two species are the dominant species?