pH buffer 5.2 which can be evaporated

[AndreB]

I would like to desalt peptides from its buffer (100ml scale or so). I would prefer to use a buffer that can be evaporated, maybe ammonium acetate. However, it nedds to buffer at around pH 5.2 - I think ammonium acetate buffers at pH7.

[juddc]

Ammonium acetate should be just fine at pH 5.2 as acetate's buffer range is approximately 3.6-5.6 if memory serves.

Good luck!

[AndreB]

Thanks,
I think pH 3.6-5.6 or so is for acetate/ acetic acid. NH4 CH3COO is a salt of a weak acid and a waek base, the buffering should good at around pH 7 or so.
Andre

[HW Mueller]

The pKa of HOAc is ~4.6, that of NH4+ is ~9.2, so the other "useful" buffering range of NH4OAc is ~8.2 - 10.2, not 7.

[Bryan Evans]

You could try a 2:8 v/v mixture of AcOH:NH4AcOH - just make sure
the acid and salt are the same ionic strength (i.e., 2:8 20mM AcOH:20mM NH4AcOH).

If you need more buffering capacity - increase the concentration of the acid and salt.

NH4AcOH has weak ionic strength - but it's worth a shot if you need something volatile at that pH.

[HW Mueller]

Bryan, what do you mean with "ionic strength" here?

[Bryan Evans]

Hi Hans -

I just wanted to say that NH4AcOH is a weak buffer.
Thank you.

[HW Mueller]

Bryan, OK, think I can follow you now: If you dissolve a "pure" NH4OAC in good H2O you get a pH ~7, where there is little buffering as mentioned above.
The source of my trouble stems from labeling mixtures of NH4+ and AcO- at pH of, lets say, 4.6 or 9.2 as NH4OAc buffers, and good ones at that.