Potentials in Pourbaix Diagrams

[Silvius]

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!

 

[Borek]

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.

 

[Silvius]

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).

 

[Borek]

Isn't it obvious? When the potential goes up, system is dominated by the more oxidized form, when the potential goes down, system is dominated by the less oxidized form. That's exactly kind of conclusion you would expect from the simple analysis of the redox potential.

 

[Silvius]

Isn't it obvious? When the potential goes up, system is dominated by the more oxidized form, when the potential goes down, system is dominated by the less oxidized form. That's exactly kind of conclusion you would expect from the simple analysis of the redox potential.

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?

 

[Borek]

You treat other species as if they were responsible for forcing the solution potential. But - for the Fe(II)/Fe(III) system - it doesn't matter what the other system is, nor how it reacts. What matters is the observed potential.

It is not much different from pH. You have a solution that is buffered at pH=8.0, you add some acetic acid and ask "what dominates the solution at this pH, HAcetate or Acetate^-?". And you can easily calculate it knowing just what the pH (and pKa of acetic acid) is, you don't care about what the buffering system is. The only important thing is that the other system keeps pH at 4.5.

Same with potential - you don't care about other systems present, how they react and what the are. The only thing that is important is that the potential measured in the solution is E. And you ask "what dominates the solution, Fe²⁺ or Fe³⁺ at this potential" - and you read the answer from the diagram (you can also calculate it from the Nernst equation, knowing standard potential for Fe²⁺/Fe³⁺ system).

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