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.
Jay
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Joshua Silverstein
Sent: Wednesday, January 04, 2012 4:08 PM
To: DK
Cc: [email protected]
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 <[email protected]> wrote:
> In article <[email protected]>, "Jayakumar,
> R" <[email protected]> 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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>
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