Chromatography Forum

Some recent publications in Analytical Chemistry describe 'one run' pKa estimation using mobile phase with a constant % organic and pH gradient.



t=0 50% MeOH 50% pH 9 buffer 0% pH 3 Buffer

final 50% MeOH 0% pH 9 buffer 50% pH 3 Buffer

Are there many with experience with this type of method? If so, what variety of compounds?

I'm not sure to what extent this method is useful for estimating multiple pKa's for poly protic analytes, however, for single-pKa compounds, the method appears to be a rational and quick method for pKa estimation..

Any experiences?

Because pKa and pH, as determened by a glass electrode, and hydrogen ion activity(the value needed to make thermodynamic pKa calculations), all are dependent on solvent composition, the concept seemes flawed to me. I don't see any way of producing an easily predictable hydrogen ion acitivity gradient(as opposed to a pH gradient which has no thermodynamic relevance). It is certain that the hydrogen ion activity is not constant from the description you provided. If you provide a reference I will look it up and check the details. We may want to pursue this issue off line.