pKa same as your pH of mobile phase?

[ldcarthy]

why would you use a pH the same as your pKa for anything other than a zwitterion?

[Juan2011]

Do you mean, Ph=PKA? Never use that, at that point your compound is 50% ionised if ionic. Use + or - 2 of Pka

[ldcarthy]

Hi
sorry I wasnt clear I know what is standard practice in chromatography.

my problem is I ve come across a method that pKa =pH and I ve never come across this before and was wondering if anyone could of any reason why anyone would ever do this?

[tom jupille]

If you plot reversed-phase retention as a function of pH, you get an S-shaped plot which is flat at both high and low pH and steepest at pH = pKa. It's *usually* a good idea to begin method development with pH on one of the flat portions because that provides the most robust methods.

As with any statement that includes the word "usually", there *are* exceptions; in some cases an intermediate pH is required to get appropriate selectivity, and the robustness with respect to pH may not be so poor as to cause a problem.

[HW Mueller]

The use of buffers reduces pH changes, thus imparting robustness even for situations as mentioned here.