Measuring pH in 95% DMSO solutions?

D.K. dk at
Thu Nov 28 19:40:43 EST 2002

In article <uCxF9.36$N4.7401 at>, "EK" <khatipovNO-SPAM at> 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.


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