Gargi - 19-3-2008 at 00:43
Hi everybody I am a new entrant. I feel thrilled to see this chemistry platform
Can anyone help me to elaborate on a standard method to find pH of a non - aqeous media, say I wish to check the pH of a acidic methanolic solution
containing reaction mass.
How much of the aliquot to be dissolved in water to read using pH meter??? are there any specific norms
Edit by Nicodem: Coincidentally, two threads on the same topic were contemporaneously open by two members. They were merged to avoid
confusion and ease searching.
[Edited on 21/3/2008 by Nicodem]
testing the PH of organic solutions
ssdd - 21-3-2008 at 09:52
I am wondering how one would go about testing the PH of an orgainic solution in an organic solvent.
Working with a product that has to be neutralized before extraction will work. So I am wondering how one goes about testing the PH of a solution (DMF
is the solvent, so it eats indicator strips, and the solution is colored which is not helping.)
How would you test the PH of this other than make a water - organic layer then test the PH of the water? Or is that the only way? I may have missed
something in my searches sorry if it is obvious.
-ssdd
*edit- spelling
[Edited on 21-3-2008 by ssdd]
vulture - 21-3-2008 at 10:49
pH is by definition in water, so yes, you'll have to resort to a water/organic mixture.
Nicodem - 21-3-2008 at 11:39
This can also simply be done by moistening the litmus paper and touching with a drop on a glass stick. But as far as I know pH papers (at least the
litmus ones) do show up color change from plain DMF solutions, so I'm unsure what problems you actually encountered (but of course, like Vulture said,
you can not measure the exact pH without having water present, just check if something is acidic or basic). The multicolor bands pH papers are of
course inappropriate for such things.
microcosmicus - 21-3-2008 at 14:25
One can extend the notion of pH to non-aqueous situations by
using a more general definition of acid-base such as the
Bronstein-Lowry or Lewis definitions and consider the
concentration (more accurately activity) of dissociated
molecules or their negative logarithm. The simplest example
would be with a protic solvent such as methanol. There, we
have the dissociation
2 MeOH <--> MeO- + MeOH2+
with equilibrium constant 2E-17. If we add appropriate solutes,
we can shift this equilibrium around, much as we can
shift the equilibrium
2 HOH <---> HO- + HOH2+
around by dissolving stuff in water.
When generalizing this notion, note that the neutral pH may
no longer be 7; for instance in pure methanol, we have
[MeOH2+] = 4.5E-9, so 8.3 would be the neutral pH value
for solutions in methanol.
Another thing to keep in mind is that what acts as a base
or what acts as an acid may depend on the choice of
solvent. For instance, Pauling gives H3PO4 as an example.
when dissolve in H20, it increases [H30+] at the expense of
[OH-] (equivalently, decreases p[H30+] below 7) hence acts as
an acid, but when dissolved in H2SO4, it instead decreases
[H3SO4+] at the expense of [HSO4-] (i.e. increases p[H3SO4+]
above 1.8) hence acts as a base. Thus, adding water and measuring
the pH may not give a result which reflects the acid-base chemistry
of the non-aqueous solution.
@ssdd:
In your case, DMF is an aprotic solvent, so it is not immediately
obvious that there is any acid-base chemistry going on. You
should need to first look at your system (solvent + solute) to
see what rxn's might be going on.
However, I suspect that, since you talked about extractions, I
guess that maybe something different is going on. Perhaps
the substance you are trying to extract is some organic acid
or base, but you are supposed to extract a salt of it instead.
In that case, you need not bother with what I said above, but
instead check that you have reacted all of your compound
into salt, so doing something like dissolving in water and
using indicator paper sounds like it should work.
[Edited on 21-3-2008 by microcosmicus]