Measuring pH in 95% DMSO solutions?

EK khatipovNO-SPAM at NO-SPAMuchicago.edu
Sat Nov 30 00:58:59 EST 2002


Yeah! You are right, Wo. The important thing is does "it" work when the
buffer is prepared this way and not the other. I always try to post here
solutions rather than to show my erudition (which is in fact quite limited,
exp. in chemistry :-)). I think in practice the _only_ way to pH 95% DMSO is
to mix DMSO with the pre-adjusted water-based buffer. If that results in
problems with the experiment, mix with the buffer at different pH and so on
until the experiment works, i.e. take the empirical approach. After all, the
important thing is to describe your method so that others would be able to
repeat it after you, right? I don't think I would like to describe in the
Methods section of my paper how I calculated "real" pH of 95% DMSO and what
supercomplicated equations I used - most probably it is not the point of the
paper.
-Emir


"Wolfgang Schechinger" <hubahopp at gmx.de> wrote in message
news:3.0.6.32.20021129163851.007c4b00 at pop.gmx.net...
> Dear Dima,
>
> <sarcastic_mode>
>
> isn't the real problem that usually nobody describes *how* the buffer
> really was prepared? By statistics, there are many possilities:
>
> May one assume that first, the aqueous buffer was prepared and adjusted
and
> then it was mixed with 95% DMSO? Were 95ml of DMSO mixed with 5ml of
buffer
> or was DMSO added q.s to 5ml of buffer or was buffer added q.s. to 95ml of
> DMSO? Or was pH adjusted after mixing all compounds (i.e. was a pH
> electrode inserted into the solution and acid or base was added until the
> digits on the screen were congruent with the researcher's desire)? Is the
> published pH the pH measured corrected for activities?
>
> </sarcastic_mode>
>
> Anyway, back to the bench, how relevant is the garbage I wrote above? I
> mean, though there are many ways to prepare  buffers for physicochemist's
> delight, if we use them for biological experiments, will there be
different
> results?
>
> Since there's no law on how to prepare buffers, is there some common sense
> at least?
>
> Regards,
>
> Wo
>
> At 12:40 AM 2002/11/29 GMT, D.K. wrote:
> >In article <uCxF9.36$N4.7401 at news.uchicago.edu>, "EK"
> <khatipovNO-SPAM at NO-SPAMuchicago.edu> wrote:
> >>DMSO does not dissociate to form H+ or OH-, thus it does not contribute
to
> >>pH (after all, a measure of concentration of H+)
> >
> >pH is a function of activity. In non-polar solvents, dielectic constant
> >is so different from one in water than activity ~ concentration
> >no longer holds truth.
> >
> >>I would suggest just making a 10% CH2ClCOOH in water (4.5 parts H2O, 0.5
> >>parts  CH2ClCOOH ), adjust pH to 5 and mix it with 95 parts of DMSO.
Same
> >>concept applies to, e.g., preparation of Tris-buffered phenol (even
though
> >>phenol is not miscible with water): you always assume that the inorganic
> >>solvent does not contribute to pH value.
> >
> >pKa for phenol is 9.9. It is soluble in water at low concentrations (~ 50
mM
> >as found in many over the counter cold "medicines"). Its alternative
names
> >are carbolic acid and phenyl hydroxide.
> >
> >In my opinion, there is no truly accurate way of measuring pH in
> >95% DMSO.
> >
> >DK
> >
> >
>
> ---





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