Measuring pH in 95% DMSO solutions?
David F. Spencer
DSpencer at Dal.CA
Tue Dec 10 17:44:11 EST 2002
In article <uCxF9.36$N4.7401 at news.uchicago.edu>, "EK"
<khatipovNO-SPAM at NO-SPAMuchicago.edu> wrote:
> 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
> phenol is not miscible with water): you always assume that the inorganic
Phenol and water are mutually soluble to about 6 to 10%, depending on
the salt content of the aqueous component and which is dissolving in
which. If phenol and water were immiscible it would make no sense to
water saturate and buffer equilibrate phenol. That is why it would be
foolish to try to water saturate or buffer equilibrate chloroform.
Liquified phenol has about 8% water in it. This is also a poor analogy
because phenol, carbolic acid, is creating the acidity that you are
trying to correct and so the organic "solvent" in this case is not
> solvent does not contribute to pH value. Thus, mixing say 100 parts of
> phenol with 1 part of Tris buffer pH 8.0 is considered as preparation of
> phenol pH 8.0. Same for e.g. 80% acetone in phosphate buffer pH 7.0,
Mixing 100 parts phenol with 1 part Tris buffer pH 8 would not be
considered buffer equilibration of phenol and would have imperceptable
affect on pH.
What I attempted to convey in my initial response to the first query was
that the organic solvent component can in fact have, at high proportions
because of the effect of dielectric constants, a significant impact on
the pKas of Bronsted acids and bases and thus the observed or effective
pH of an aqueous/organic solution mixture. Adding aqueous pH 7 phosphate
buffer to acetone to a final 80% acetone would not give a final pH of 7.
It could very well be a standard protocol that serves its purpose well
but claiming the final pH is 7 would be wrong.
David F. Spencer, PhD
Dept. of Biochemistry and Molecular Biology
Halifax, Nova Scotia, Canada
DSpencer at Dal.CA
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