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do you adjust pH for pure orginc mobile phase B?
Posted: Fri Jul 24, 2009 8:17 pm
by moonchips
We are using two mobile phases:
A, pH 9 aqueoues phase in water
B, pH 8 100% methanol with ammonium acetate+ammonium hydroxide.
Is it fair to measure pH in Methanol? should we just leave mobile phase B as 100% methanol instead of adding salt in organic phase?
thanks
Posted: Sat Jul 25, 2009 1:22 pm
by Uwe Neue
The procedure and the reference for measuring "pH" in methanol is complicated. My suggestion is to make your pH 9 solution at 100-fold concentration in water, and dilute 99:1 with methanol.
Posted: Sat Jul 25, 2009 4:36 pm
by HW Mueller
Besides, a pH = x in MeOH is not the same as pH = x in water, regarding reactions and thus equilibrium constants . . . . But unless one is doing some theoretical work it doesn´t matter as HPLC is "trial and error" to a large extent.
Posted: Sat Jul 25, 2009 10:51 pm
by tom jupille
This is a perennial!
For example:
viewtopic.php?t=5413
Posted: Sun Jul 26, 2009 9:19 am
by HW Mueller
Yes, somewhat boring, but indicating that the knowledge about pH is shaky. For instance, in
viewtopic.php?t=10983
I was reminded that a given pH is not even the same in different aqueous solutions (regarding reactivity).
Posted: Sun Jul 26, 2009 10:19 am
by Uwe Neue
Adjusting the pH is not that terribly complicated, unless you are doing theoretical stuff.
Rule number 1: measure the pH in water!
Rule number 2: use a proper buffer!
Definition of a buffer: a buffer is a buffer if it buffers the pH. If it doesn't, it isn't.
You can look up the pKs of buffers in tables in textbooks. A proper buffer has a pH close to the pKa of the buffer.
Posted: Mon Jul 27, 2009 2:23 am
by Bruce Hamilton
If you are developing a method, then the obvious solutions are to either not add salts to MeOH ( but the separation may be unacceptable ), or add the buffer using Uwe's method of dilution of the aqueous buffer with 99 parts of MeOH.
However, if you are following a proscribed procedure, then you should follow that procedure in detail. If the procedure doesn't provide details ( such as the quantities or concentrations of the CH3CO2NH4 and NH4OH ), then you should ask for them.
When using pH buffers, an important part of the mobile phase documentation is describing, in detail, how to prepare the buffered mobile phase components. It doesn't really matter if the procedure is not perfect, but it must be sufficiently detailed for others to replicate it.
Please keep having fun,
Bruce Hamilton
Posted: Mon Jul 27, 2009 12:32 pm
by lmh
Totally agreed! Please can we attach journal editors who encourage over-shortened methods sections to a nice solid object and throw rotten fruit at them?