Organic solvent pH / pKa shifts

[ablue]

Hi,

I've searched the forum archives, and this topic has been covered, but I still don't completely understand, so I'm hoping someone is willing to help me!

I understand that adding alcohols to a buffer system changes the dielectric constant and therefore shifts the pH of the buffer. I have a few questions about this. I'm interested in fluoronated alcohols in particular, e.g. HFIP (hexafluoroisopropanol) and TFE (trifluoroethanol.)

1. Is there a way to measure the pH of the combined system (aqueous buffer + organic solvent)? Can a pH meter be used? Or, is this question irrelevant?

2. If you can know the pH of the organic solvent+buffer solution, and you dissolve a protein or other compound in the solution, do the pKa's of the solute also shift? In other words, will you get different ionization states for the solute in the organic+buffer solution vs. the buffer alone, even if the pH is constant?

I'm looking for references about this topic, or any help anyone can give me. Thanks a lot!

[tom jupille]

1. Is there a way to measure the pH of the combined system (aqueous buffer + organic solvent)? Can a pH meter be used? Or, is this question irrelevant?

It's *very* relevant, but not amenable to a simple reply. Read these three articles:
http://chromatographyonline.findanalyti ... rticle.pdf
http://chromatographyonline.findanalyti ... rticle.pdf
http://chromatographyonline.findanalyti ... rticle.pdf

2. If you can know the pH of the organic solvent+buffer solution, and you dissolve a protein or other compound in the solution, do the pKa's of the solute also shift?

Yes. To oversimplify, when you add organic, the equilibrium shifts in the direction of the less-ionized form.

[ablue]

Thanks so much for the articles. This is exactly what I was looking for!!