Stability Range of water

[San]

Hi

I have a question regarding the stability range of water in Pourbaix diagram. In the attached file (Pourbaix diagram of Zn in an aqueous solution containing NaCl) the fact that Zn is just stable outside the stability range of water means that we can not have "Zn" in the aqueous condition containing NaCl? Could you please clarify how you interpret it?

Can Zn be in equilibrium with Zn++ in this system at all? Dose it need that all water in the system be reduced first?

Thank you

Zn in NaCl.ac2

[Brian Farrell]

Hi, 

In a traditional diagram like this, with Zn++ as the main species, adding Na+ to the “in the presence of” section does nothing. Neither does swapping NaOH in for the Na+. I don’t understand why you’ve swapped HCl(aq) in for the Cl-, either. If it’s just a salt solution you’re dealing with, just add the Cl- directly. HCl(aq) is much less stable than Cl- under the conditions of the diagram, so assuming HCl(aq) is present at the activity you specified is like assuming Cl- is present at a much, much larger activity. 

Whether you have Cl- in solution or not, metallic zinc is not thermodynamically stable within the area denoted by the water stability limits.

Hope this helps,

Brian Farrell
Aqueous Solutions LLC