Pourbaix Diagram Stable Entries - where does the hydrogen go?

pymatgen
,
1

Hi,

I am confused about the stable entries I am getting using .get_stable_entry() for several pourbaix diagrams I have made. Specifically, why is it possible to have stable entries where hydrogen is not included in any of the entries? If the chemical system of interest interacts with H2O and forms some decomposition products, where does the hydrogen go if not included as one of the stable entries i.e. decomposition products? For example, in the documentation for the Pourbaix Diagram module, figure 2 shows a diagram in which multiple regions, with stable entries labelled, do not include any hydrogen. Thank you for the help!

2

Hi @mdresser , what we mean by stable entries is stable solids. So In the reducing region of most Pourbaix diagrams, the stable solid is either a solid metal like Fe(s) as shown in the example or a hydride phase (which you will see in diagrams for some of the alkali metals, for example).

If hydrogen does not appear in any of the stable entries, that doesn’t mean that H2 gas isn’t evolving; it’s just not being incorporated into the relevant solids. Does that make sense? (and does that answer your question? - do tell me if I’ve misunderstood)

3

Hi @rkingsbury thank you for this clarification, that makes sense.