effect of DMSO on pH

Wolfgang Schechinger Wolfgang.Schechinger at med.uni-tuebingen.de
Sat Mar 18 16:24:39 EST 2000


since I wouldn't expect DMSO to dissociate in a noteworthy amount, it
shouldn't affect pH in a measurable way. You may check the pK value of
DMSO for reference. 

Another point of view: in my experience, very strong alkali (e.g.
natrium metal) is required for abstracting one proton and producing
the salt (dimsylate) wich is a very nice nucleophil. Though it is not
stable in water (will yield DMSO and OH-). The equlibrium will be
totally on the side of DMSO. Water will not be able to deprotonate

Thus DMSO shouldn't affect pH when added in tiny amounts to buffers.
If you suspect any influence to the pH *measurement*, simply measure
pH before adding DMSO. (There could be some influence on the
electrode's electrical signal due to the redox properties of DMSO. The
CRC Handbook of Chemistry and Physics maybe will elucidate this

Finally some theoretical brain twisting: When you have say 50mol%
(half of all molecules) DMSO in water, what will happen to the pH?

A suggestion:

Kw = ([H+]x[OH-])/[H2O] is the equilibrium constant for water

If you change [H2O] (as you do when you substitute half of water
molecules by DMSO) then [H+]x[OH-] also will have to be half. Since
both factors are equal, [H+] and [OH-] will change absolutely by 
1/(square root of two) (makes .707something).  Since [H+] and [OH-]
still are equal (DMSO will not add or remove any, I assume it not
being dissociated at all), the solution still will be neutral and the
pH will not change by -log(.707sth) as one could suspect at a first

Would you agree?

(I have no idea if there is a bug in the argumentation)


> From:          Bill_A_Nussbaumer at ms.bd.com
> Subject:       effect of DMSO on pH
> Date:          17 Mar 2000 21:45:33 -0000
> Organization:  BIOSCI/MRC Human Genome Mapping Project Resource
> Centre X-To:          methods at hgmp.mrc.ac.uk To:           
> methods at hgmp.mrc.ac.uk

> Bill A Nussbaumer at BDX
> 03/17/2000 04:44 PM
> Hi all,
> Does anyone have any information or experience regarding the effect
> on pH vs the effect on a pH meter when DMSO is added to a buffer?
> Actual data, references, theories, and postulations all welcome.
> TIA,
> Bill Nussbaumer
> ---
This message is encrypted. Use your brain to decode it.
Dr. Wolfgang Schechinger, Dept. of Pathobiochemistry
University of Tuebingen, Germany
email: wolfgang.schechinger at med.uni-tuebingen.de 
wwWait: http://www.medizin.uni-tuebingen.de/~wgschech/start.htm
usual disclaimers apply 

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