Measuring pH in 95% DMSO solutions?

D.K. dk at no.email.thankstospam.net
Fri Nov 29 10:54:31 EST 2002


In article <3.0.6.32.20021129163851.007c4b00 at pop.gmx.net>, hubahopp at gmx.de (Wolfgang Schechinger) wrote:
>Dear Dima,
>
><sarcastic_mode>
>
>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?
>
></sarcastic_mode>
>
>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
>results?

I don't know. In many cases published pH turns out to be pH of the 20X
stock, and when one attempts to do things right, the results are not 
reproducible. In other cases it does not really matter. IMHO, it only 
makes sense to report final pH and - unless temperature probe is
installed on a pH meter - measure pH at room temperature only. 

DK



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