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:

<SNIP>

> 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

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 
irrelevant.

> 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.

Dave

-- 
David F. Spencer, PhD
Dept. of Biochemistry and Molecular Biology
Dalhousie University
Halifax, Nova Scotia, Canada

DSpencer at Dal.CA



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