Measuring pH in 95% DMSO solutions?

Wolfgang Schechinger hubahopp at
Fri Nov 29 10:33:01 EST 2002

Dear Dima,


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?


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

Since there's no law on how to prepare buffers, is there some common sense
at least?



At 12:40 AM 2002/11/29 GMT, D.K. wrote:
>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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