buffer: NaOAc, pH6.8 ?

[kiknos]

hi all,

i just read an application on Taxanes, in which the conditions are:
solvent A= 50mM NaOAc, pH6.8
solvent B= acetonitrile

for sure there are some gradient and the separation is nicely done on the total 15 taxane compounds.

my question is: is NaOAc good buffer?(we more often find NH4OAc is used) and even it is, does that work well(i mean the buffer capacity) on the pH value? if the answer is no and no, it should be expected to be some method not "stable" enough.

your comments? thanks!

best regards,
kiknos in shanghai,China

[syx]

OAc (pKa = 4. is effective in pH range 3.8 - 5.8
The most effective buffer in pH 6.8 is phosphate (pKa2 = 7.2)
That 's what I know.

[HW Mueller]

Ammonium acetate can also not be considered a buffer at pH 6.8. Some things work far from ideal, maybe most of the time, but not all the time (modified American saying).

[Bill Tindall]

acetate will not have much buffering capacity at this pH, no matter if it is sodium or ammonium acetate. The buffer capacity will be about 25 times less than the idea case of pH 4.8. However, even though it is a poor buffer it will be much more buffered than a case where just water is used with no buffer. Therefore, it may work just fine.

[Victor]

In my experience, the viability of Ammonium acetate around neutral pH depends on the pKa of the ionisable groups on the solute. For example, if you have a basic solute with pKa 10 then some change in the pH of the mobile phase (say +/- 1 in pH ) will not affect its state of ionisation-it will be mostly protonated whatever happens. If the pKa of the compound is close to pH 6.8, you may have trouble, because then any mobile phase pH changes will cause big changes in the state of solute protonation.

I don't know anything about taxanes. Another thing about a buffer is that it keeps the state of ionisation of any column groups constant. If that is all it is there for then the requirements on the buffer are not very stringent.

[kiknos]

Thank you all for your kind comments!

I think actually two things are mentioned here: 1) the buffer capacity around the pH which is beyond its most suitable range; 2) and whether and how the pH varience would affect the analysis(by Victor).