Carbonate Buffer

[tnlambert]

Hey all -

I have a question about carbonate buffers. I am trying to use a 25mM Sodium Carbonate buffer in a seperation I am developing. We made the choice to try a Carbonate buffer because it has a reported pH buffering range of 5.1-7.1 and the seperation needs to be performed at or around pH=5.7.

I seem to be having trouble with pH drift after pH adjustment. The initial pH of the Sodium Carbonate solution was around 11. I adjusted the buffer to 5.7 with Phosphoric Acid. Some bubbles were evolved, which I expected. I waited a few minutes and monitored the pH to ensure that equilibration had been reached. Then, I brought the buffer to volume with dI Water and filtered it through a 0.45 uM membrane. I rechecked the pH of the buffer after filtration and it appeared to have drifted up to 6.0.

I readjusted the pH to 5.7 with Phosphoric Acid and monitored the pH for about 5 minutes. This time it drifted up 0.02 pH units.

What might be causing this? Did I not wait long enough for equilibration? Should I have used a different acid for my pH adjustment? Is there a different buffer in this range that I should consider? I know that acetate has a buffering range of 3.8-5.8, but acetate buffer was already tried and dismissed as a possibility.

Any input anyone might have would be great!

Thanks,

Tom

[Uwe Neue]

We have also attempted to use carbonate buffers around pH 7, and we ran into similar difficulties. The pH shifted into the alkaline range after some period of time. The carbonate is not stable and converts to CO2. The CO2 slowly escapes out of the system. Thus it does not appear to be possible to create stable carbonate buffers in the neutral to slightly acidic pH range.

[Bill Tindall]

Phthalic acid is an excellent buffer for the pH range you need. Bicarbonate works well at a higher pH than you are needing but as you discovered it isn't useful at that low a pH. phosphate may be ok if you don't need a lot of buffering capacity.

[tnlambert]

Thanks to everyone for the advice. I appreciate it!