pH shift when adding MeOH to buffered solution

[LCFlo]

Dear all,

I have a mobile phase consisting of 300 mL of an aqueous sodium octanesulfonate solution (2 g/L, pH = 3.5 with CH3COOH) and 700 mL MeOH.

During method troubleshooting of the method using the mentioned mobile phase, the pH came up as a possible pitfall, because the solution contains no buffer. Thus, I added 20 mM Ammonium acetate to the aqueous part before adjusting the pH. I compared the final pH values of the two mixtures:

A. 700 mL MeOH + 300 mL non-buffered aqueous solution
B. 700 mL MeOH + 300 mL buffered aqueous solution

The pH of B was higher than the pH of A, even B contains a buffer.

What is wrong here?

Thank you for every hint.

Florian

[danko]

Ammonium acetate at pH 3.5 is not a buffer!
There are other ussues with regards to pH in organic solvents, but the topic has been discussed exessivly, so you might like to search the forum for more info on the subject.

Best Regards

[LCFlo]

Danko,

thanks for your prompt reply. I used an acetate due to the fact that the method is derived from an EP method using CH3COOH.

But even considering that I increased the ionic strength only without establishing a real buffer effect: why is the pH shift after adding MeOH bigger than the shift that is obeserved using an aqueous solution without ammonium acetate?

Florian

[Uwe Neue]

If you compare the pH of an acetic acid solution in water to the pH of an acetate buffer, you will find that the solution of the acid has a lower pH than the buffer. If you create an acetate buffer by mixing acetic acid with an acetate solution equal molarity, you will get an aqueous buffer of pH 4.75.

If you do the same comparison after having added 70% methanol, you will measure a higher pH for both solutions than in water. However, the relationship should remain the same: the acid solution still should have a lower pH than the buffer solution. I think that this is what you have observed, right? The reason is that the pH scale is changing when you add the methanol. However, the relationships between buffer and acid solution remain the same.

[LCFlo]

Thank you, Uwe.

For me, a consequence might be:

during the development of a method when I don't use a buffer, I have a pH X of the aqueous part of the mobile phase. The final "pH" of the mobile phase is Y (Y > X).
After implementation of a buffer, I might have to change X to get an equal separation using the resulting mobile phase.

[Uwe Neue]

If you change the pH, and your method is sensitive to changes in the pH, you will get a different separation.

This might sound trivial, but:
If you have such a method, and you want to keep the same results as you had before, you should not change the pH.

[HW Mueller]

What is it now? Did you do what Uwe guessed or did you start out with pH = 3.5 for both the AcOH and the NH4Ac? The MeOH was then added to both and the resulting pH was higher for both, but highest for the NH4AC/MeOH?
If the latter is correct it could have something to do with the change in ionization, maybe NH4OAC is more favored than AcOH? To be honest I would have expected it the other way around based on what I expect in equilibrium shifts.
Also, Danko already pointed it out: You did not "implement" a buffer.

[Uwe Neue]

To be honest I would have expected it the other way around based on what I expect in equilibrium shifts

Why? An acid is always more acidic than the buffer of this acid, no matter what you do...

[tom jupille]

These three articles by Bill Tindall really cover the subject thoroughly:

http://chromatographyonline.findanalyti ... rticle.pdf

http://chromatographyonline.findanalyti ... rticle.pdf

http://chromatographyonline.findanalyti ... rticle.pdf

[LCFlo]

Thanks a lot for you inputs.

HW: You asked:
Did start out with pH = 3.5 for both the AcOH and the NH4Ac? The MeOH was then added to both and the resulting pH was higher for both, but highest for the NH4AC/MeOH?

My answer: your description is correct


Tom: thank you for the links; I will start reading now

[HW Mueller]

At a given pH the acidity of an acid and the salt of this acid must neccessarily be the same.
As a first approximation the dissociation changes in going from an aqu. solution to H2O/MeOH would be dominant. This is also cited in the first article by Bill Tindall in Tom´s links: Acetic acid would be expected to be less dissociated if MEOH is present, so the acidity (H+) would be down. The NH4+ would tend toward NH3 with MeOH, so the acidity would go up (that is what I would predict, the acetic doing the same as in the HOAc counterpart). But, if MeOH caused a shift toward NH4OAC the acidity would be expected to go down.
Now I wonder how the measurements were made, whether the system was at eqilibrium, also, what were the values, what were the standard deviations?

[LCFlo]

HW,

I prepared the solution with and without NH4 acetate twice:

The pH values I set were:

without NH4 acetate: 3.52 / 3.54
with NH4 acetate: 3.49 / 3.49

after addition of MeOH:

without NH4 acetate: 4.70 / 4.71
with NH4 acetate: 4.91 / 4.92

Florian

[HW Mueller]

Did you recheck the calibration after these measurements? In any case it shows that prediction is difficult. Have you read the linked articles? Do you see an explanation there?