[Mysticallyenchanting]

I'm trying to work out what (aqueous) dilution of TFA is required to produce a pH2.5 mobile phase. I know I could just keep diluting in practice but I need the theory behind it for my write up.

pKa of TFA = 0.3
[H+] = 1exp-2.5 = 3.16exp-3

I get a little stuck after here.
I think I need to use this equation:
pH = pKa - log[HA]/[H+]

but not sure exactly how.
Any help would be greatly appreciated.

Will

[Borek]

Henderson-Hasselbalch equation is of no use here.

You have solution of a weak (albeit relatively strong) acid. Thus you have to find out at what acid concentration pH will reach 2.5.

http://www.chembuddy.com/?left=pH-calculation&right=pH-weak-acid-base

Discussion above reflects calculations other way around - pH from concentration, but should give you a hint on how to proceed, you may try to solve simplified equations 8.10 and 8.13 for concentration of acid.

What you will find is that with pKa = 0.3 acid is so strong, you may safely assume it is dissociated 100%. Thus all you need to find out its concentration is pH definition.