What pH to run in a multiple pKa compound

[Chrom user]

What pH should I maintain in my mobile phase when my compound is having 3 pKa values at 2,7 and 9?
I know that in reverse phase separation, a neutral compound will enhance retentivity in the column than an ioinised form.

[Bill Tindall]

I have successfully done trifunctional aromatic acids that are similar to your description with 0.15% phosphoric acid as water component.

[Uwe Neue]

The first question should be, in which pH range will you get adequate resolution of your compounds. Having said that, the very acidic pH range suggested by Bill is one possibility. Another very good possibility is the middle between the first 2 pK values, at pH 4.5, if an acetate buffer is suitable for detection. You should get stable chromatography there, with few issues. In the alkaline pH range, the two closely related pK values of your compound will result in less stable chromatography, meaning a high sensitivity of retention to small changes in pH. If you go to pH 10 to 11, you will get stable chromatography again. We have used pyrrolidine for this range.