Chemistry Forums for Students > Undergraduate General Chemistry Forum

pH, is this possible?

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Demotivator:
Conventional wisdom has ph ranging from 0 to 14, but

pH + pOH = 14
Is it possible to have ph > 14 if pOH is negative?
ie. > 1 M OH solution .

movies:
Sure, it's possible.  You just have to find something soluble enough in water.

Remember that pH is defined as -log [H+] (or for pOH, -log [OH-] )

So you would need an [H+] greater than 1 M.  That's not hard to do with HCl or H2SO4.  Concentrated HCl is about 12 M for a pH of about -1.1 (pOH of ~15.1).

Demotivator:
Ok, thanks. But although theoretically possible it may or may not be practically.
Molar concentrations are quick and dirty alternatives to activities. At high concentrations, the activity coefficients may reduce appreciably, no?
I just thought of that!

movies:
You wouldn't be departing far from the accuracy of the concentration approximation if you had a 2 M solution of HCl, and that's enough to get you a negative pH.  They sell pH paper that can measure in the negative pH range.

I'm not even sure that the activity necessarily has an effect here, since the pH is defined by the concentration.  I guess it could have an effect if you had so little water that the HCl wasn't actually dissociating.  That's probably hard to do though.  Concentrated HCl is only 37% HCl (remainder water), so I doubt you're even approaching that situation with 12 M conc. HCl.

budullewraagh:

--- Quote --- Concentrated HCl is only 37% HCl (remainder water), so I doubt you're even approaching that situation with 12 M conc. HCl.
--- End quote ---
unless it's supersaturated like mine:)

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