Chromatography Forum

Hi

One of the method I copied used formic acid to adjust the pH of ammonium acetate buffer.

I did the same and adjusted pH of 15 mM ammonium acetate buffer to 5.0 with formic acid. My boss is not happy with me and says that its not buffer anymore?

Is it true?
The amount of formic acid needed was not much. Why the buffer strength will change much.

Please give me your feed back

Thanks

The pKa of acetic acid / acetate is 4.8; you are at pH 5.0. That is well within the buffering range of acetate. Personally, I would have preferred acetic acid for the adjustment, just to keep thing simple, but I don't see any fatal problem with formic. In any case, if you are following a validated procedure, you must follow it *exactly* -- even if it is obviously incorrect. Correctness is nice, but consistency is critical!

You might want to (diplomatically) ask your boss to explain why the use of formic acid for the adjustment would be unacceptable (for your education, of course!).

I'm in agreement with Tom J. here.

Logically, acetic acid would be the correct acid to make a buffered system with ammonium acetate.

However, if that way with formic acid works, then keep using that until you can (and want) to investigate and document formally.

We do have a procedure here where we make a buffer with perchlorate and then add phosphoric acid, came from a published application, and no one here really wanted to get perchloric acid in.

booby12

the important point is to bring in H+. The counter ion is of lower interest. Therefore you did no mistake.

bobby12

the important point is to bring in H+. The counter ion is of lower interest. Therefore you did no mistake.

"No mistake/difference" from a technical standpoint and "no mistake/difference" from a cGMP/GLP standpoint can be completely different.

One should document and differences from validated procedures that use different chemical/reagent.

My boss is still mad and saying it will make a big difference to the buffering strength of ammonium acetate which I dont understand why?

Can somebody explain me? I really appreciate it.

The explanation is simple: your boss is (mostly) wrong!

To give him the benefit of the doubt: if the pH had been adjusted with acetic acid, that would have effectively increased the acetate/acetic acid concentration a bit above 15 mM, which might result in a very slight increase in buffer capacity.

My boss is still mad and saying it will make a big difference to the buffering strength of ammonium acetate which I dont understand why?

Can somebody explain me? I really appreciate it.

I would have just re-made the buffer with HOAc by now

Yeah but the method has already been transfered to another lab:)

Hi

May you kindly share the document or article you copied the method from?

Regards

Mufaro

There are essentially two buffers (formic acid/formate and acetic acid/acetate) when adjusting the pH of ammonium acetate with formic acid. The pH of 5 is outside the buffer range of formic acid/formate and within the buffer range of acetic acid/acetate.Since formic acid is a stronger acid than acetic acid, it would require adding more acetic acid than formic acid to adjust the pH to 5. Therefore the buffer capacity of your solution is lower when compared to adjusting pH with acetic acid.