Buffer Solution for Mobile Phase

[shadow]

Can a buffer be prepared by adding a weak base and an acid that is not its conjugate acid?, would there still be some kind of buffering capacity? i.e dissolve ammonium formate and adjust pH with acetic acid. Just wondering the pros and cons of doing this. Thanks

[thiazole]

I've seen preps that do this, but you would use the same acid partner as your salt, ie, for ammonium formate, use formic acid to adjust the pH down. The only reason I can think to do this would be if your column isn't stable at the pH of the ammonium formate solution.

[HPLCaddict]

I've seen preps that do this, but you would use the same acid partner as your salt, ie, for ammonium formate, use formic acid to adjust the pH down. The only reason I can think to do this would be if your column isn't stable at the pH of the ammonium formate solution.

Erm, well, I bet you've seen procedures like that as it's probably the most common way to prepare a buffer (please don't start a thread concerning the "right" way) and the reason for adjusting the pH is not column stability but getting the right pH for the separation (remember that separations of ionizable compounds are pH-dependend). Just dissolving a salt in water is no sure way to get a true buffer, as the resulting pH might be out of the buffering range of that salt.

Concerning the origininal question, first of all why would you do that? I'd suppose that the buffering capacity of such a mixture depends heavily on the relative acid/base strengths of the species involved. Concerning your example, I'd say that using acetic acid with formate might actually work as acetic and formic acid are not so different concerning acid strength and the buffering range of acetate and formate buffers are quite similar. If you'd use e.g. conc. HCl to adjust your formate buffer, this might look different, though...but my chemistry lessons are long ago.