Jayakumar, R via methods%40net.bio.net (by R.Jayakumar from roswellpark.org)
Thu Jan 5 10:22:47 EST 2012

DK and others,
   This is getting sillier.  I cannot believe this.  I dilute buffers nearly everyday for the past 20 years.  I also check the pH before and after dilution because I am scrupulous.   There is a reason why buffers do not change pH upon dilution within its buffering range.  I just checked it out again for your sake (check it out yourselves, use a 3 point calibrated pH stick, rarely does anyone calibrate their pH meters before every use, but you should).

High school experiment1:  Took 5 ml of 1 M Tris (pH before dilution: 7.81) and diluted it 10 fold (pH after dilution: 7.83).  Just to be sure that the "ions" are not misbehaving, I took 10X TBS (I did not have PBS stock presently in the lab), (before dilution pH 7.84) and diluted 10 fold to 50 ml (pH after dilution: 7.85).  I used milliQue water of 17.7 Mohm-Cm for the experiment.  

    I feel that there is no necessity to teach about pH to this group  of scientists.  But to know why deionised water cannot change the pH of buffers significantly (within a certain buffer range for a particular buffer), and to refurbish you memories, please READ UP ON Lehninger's Principles of biochemistry chapter on Buffers and pKa (very important concept this) and also on the importance of conjugate bases and acids (explains why we use citric acid to adjust citrate buffers and not HCL).  Check out the buffer change curves and you will realize why water is different from buffers and the importance of pKa range which is the buffering range for any buffer(THAT IS WHY IT IS CALLED A pH "BUFFER").  MOST IMPORTANT TERM IS pKa. Check it out before replying.
   If diluting buffers with deionised water changes pH, then a lot of companies will be out of business selling stock buffers.
Best of luck. 


-----Original Message-----
From: methods-bounces from oat.bio.indiana.edu [mailto:methods-bounces from oat.bio.indiana.edu] On Behalf Of Joshua Silverstein
Sent: Wednesday, January 04, 2012 4:08 PM
To: DK
Cc: methods from magpie.bio.indiana.edu
Subject: Re: 10X PBS

You don't need physical chemistry, just general.

If you take 10X PBS and dilute it with deionized water, it will certainly
change the pH.  For a 10-fold dilution, it should change the pH by
approximately 1 (log scale).

On Wed, Jan 4, 2012 at 3:38 PM, DK <dk from noemail.thankstospam.net> wrote:

> In article <mailman.75.1325703460.3721.methods from net.bio.net>, "Jayakumar,
> R" <R.Jayakumar from roswellpark.org> wrote:
> >Ideally the pH should not change much between 10X and 1X if deionised
> water was
> > used for dilution.  Presnce of NaCl has nothing to do with pH changes
> since it
> > contributes neither hydroxyl nor hydrogen ions
> You need to refresh your physical chemistry.
> (Hint: cations compete with protons for binding to anions)
> DK
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