pH selection

[nitin]

hi dear all
my analyte pKa is 6.0 than what pH mobile phase is suitable for my analyte in RP.

[Uwe Neue]

Any pH based on a good buffer is suitable. However, there will be a larger variability with a buffer at pH 6, and you would need to make this buffer with a higher precision than any other pH.

You can use a phosphate buffer, pKa 7.2, best buffering and thus best pH around 7 to 7.5. You can use an acetate buffer, pKa 4.75, best buffering and best pH arounf 4.5 to 5. Or you can use formic acid or a formic acid buffer. Or you can use phosphoric acid or phosphate buffers around 2, or you can use an ammonium or even better an ammonium bicarbonate buffer around pH 10. The choice depnds on other conditions, such as compatibility with the intended detection scheme etc.

[zokitano]

There is a previous topic for pH and pH selection in RP chromatography:

http://www.sepsci.com/chromforum/viewto ... ght=ph+pka

Best regards

[yangz00g]

carbonate ( pK1 6.4 ) may be a good buffer for you, but the pH may change over the time since air has CO2.

[Uwe Neue]

It is the other way round: carbonate does not work at this pH because it looses the CO2 gas that is formed when you try to prepare such a buffer.