Measuring pH in 95% DMSO solutions?
David F. Spencer
DSpencer at Dal.CA
Thu Nov 28 12:48:35 EST 2002
In article <20021030163433.3406.qmail at ww02.hostica.com>, jsp at bio.auc.dk
> Hi there,
> I'm trying to make a 95% DMSO, 4.5% H2O, 0.5% CH2ClCOOH solution at pH 5,
> by adjusting pH with HCl.
> Im using a standard pH-meter (Hamilton minitrode), but I can't seem to
> get stable readings due to the low water activity. If I clean the
> electrode and re-introduce it into the DMSO solution I get a completly
> different reading. Does anyone have a good suggestion how to accurately
> measure the pH?
There are many layers to the problem presented here.
First, chloroacetic acid is a fairly strong acid (pKa 2.85) so the fact
that you have been adding HCl to lower the "pH" to 5 should be the first
sign that this is not a simple problem. The normal strategy here would
be to make up a 20X stock of chloroacetic acid (because you have room
for only 5ml per 100ml) with the pH adjusted to 5 (with say NaOH) and
add that to the DMSO. However, that would not give a pH of 5 when added
to the DMSO, assuming that you could measure the pH of this solution
with a conventional pH meter, and also assuming that the whole concept
of pH is applicable to what is essentially a non aqueous solution. Note
that even in aqueous solution adjusting chloroacetic acid to a pH of 5
would be tricky because that is more than 2 pH units above the pKa so
less than 1% of the chloroacetic would be free acid.
The pK of acids (and bases) is a function of several factors the simple
Henderson-Hasselbach equation does not account for, such as ionic
strength and the dielectric constant of the solution. The dielectric
constant for DMSO (45) is considerably less than that of water (78.5 at
25C) and that factor shifts the observed pKa of a carboxylic acid up
(i.e., towards 7). There are equations to calculate this more properly
but they are not trivial to work with. The ionic strength factor is
another can of worms.
You would predict that in 95% DMSO choroacetic acid would be poorly
ionized and so trying to adjust the pH down to 5 with acid would not be
useful. The analogy would be trying to adjust the pH of say a NaCl
solution to 5 or 7, whatever. You can only attain a stable pH if one of
the solutes in a solution has a pK in the general vicinity of the
desired end point. With the chloroacetic acid in DMSO example, if the
observed pKa for the acid has risen to say 6 to 7, there is little
buffering capacity at pH 5 (if that even means anything in this
situation). If the chloroacetic acid is essentially unionized in DMSO
the problem is even more fundamental and so buffering is not possible.
You did not explain what you were trying to do or why but I assume this
can't be an established protocol. You might get better feedback here if
you elaborated more on the specific goals. I assume that you know that
it is advisable (before dispensing) to purge the DMSO stock bottle (pure
liquid) with a gas such as nitrogen to clear out the dimethyl sulphide,
a breakdown product of DMSO.
David F. Spencer, PhD
Dept. of Biochemistry and Molecular Biology
Halifax, Nova Scotia, Canada
DSpencer at Dal.CA
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