effect of DMSO on pH
Bill_A_N...@ms.bd.com
Bill A Nussbaumer@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
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Bernard Murray, PhD
Bill_A_N...@ms.bd.com wrote:
If handwaving is permitted, then.....
DMSO is an aprotic solvent. Therefore pure DMSO shouldn't
change the pH at all (but watch those contaminants!).
How did I do?
Bernard
--
Bernard P. Murray, PhD
bpmurray at cgl . ucsf . edu
Department of Cellular & Molecular Pharmacology, UCSF
J. Martinez-Irujo
What about the effect of DMSO on dielectric constant? It was also reported that DMSO
forms stable coordination complexes with metals.
--
Juan Jose Martinez Irujo
Departamento de Bioquimica
Universidad de Navarra
Spain
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Wolfgang Schechinger
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
DMSO.
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
aspect).
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
glance.
Would you agree?
(I have no idea if there is a bug in the argumentation)
Wolfgang
> From: Bill_A_N...@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: met...@hgmp.mrc.ac.uk To:
> met...@hgmp.mrc.ac.uk
>
>
>
>
> Bill A Nussbaumer@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.
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Dr. Wolfgang Schechinger, Dept. of Pathobiochemistry
University of Tuebingen, Germany
email: wolfgang.s...@med.uni-tuebingen.de
wwWait: http://www.medizin.uni-tuebingen.de/~wgschech/start.htm
-----
usual disclaimers apply
-----
---
J. Martinez-Irujo
>
> ...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
> glance.
>
> Would you agree?
>
> (I have no idea if there is a bug in the argumentation)
>
> Wolfgang
In my opinion there is two concepts mixed in your argumentation. First: a
neutral solution has [H+] = [OH-] but this does not mean that neutral
solutions always have pH=7. In fact, ionization of water is an endothermic
process and ionization constant increases with temperature. Neutral water
solutions at 0, 24 and 50 ºC have pH equal to 7.473, 7.000 and 6.631,
respectively. So a pH =7 solution is neutral at 24 degrees but acid at 0
degrees and basic at 50 degrees. Second: dielectric constant of water is
87.74 while for DMSO is 45. I would expect that H+ and OH- to interact
more in DMSO that in water. Equilibrium dissociation constant must
decrease, and thus a relatively concentrated DMSO solution should have a
slightly alkaline pH... (only playing).
May be Bill can tell us what actually hapens in the pHmeter....
--
Juan J. Martinez Irujo
Departamento de Bioquimica
UNIVERSIDAD DE NAVARRA
Pamplona, Spain
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