You are correct about certain buffers as interferents. Certain buffer
species will coordinate with or precipitate silver or mercurous ions
that are present in the reference electrode compartment of the
combination pH electrodes. Tris is notorious for clogging the little
porous frit on the reference electrode, and this, along with reference
solution cation ion depletion, will drive the electrode crazy until the
frit is cleared and/or the reference electrode filling solution is
replenished. Making 1M stock Tris solutions is enough to knock out a pH
electrode for several hours if you overexpose it to the solution.
Cheers,
_______________________________________
Roger S. Rowlett
Gordon & Dorothy Kline Professor
Department of Chemistry
Colgate University
13 Oak Drive
Hamilton, NY 13346
tel: (315)-228-7245
ofc: (315)-228-7395
fax: (315)-228-7935
email: [email protected]
On 1/30/2014 12:23 PM, Shane Caldwell wrote:
Hi Sreetama,
For most buffers, I use Katherine's method, but in the case of citrate
I'd recommend just titrating citric acid with NaOH. I've made a pH
series from citric acid and Na3Cit before, and it's a huge pain. It's
very difficult to calculate how much of each you'll need, because
citrate is triprotic and the pKas overlap. When I made a pH series
this way, I ended up using much more stock than I anticipated, and
just had an overall unpleasant afternoon.
One other point - high citrate concentrations can cause your pH meter
to drift, so don't leave the probe in the citrate solution any longer
than needed, and calibrate it frequently. I'm told this is because
citrate chelates metals and that throws off the electrode, though
admittedly I don't know the mechanism and can't find any references to
back me up - it's just been lab folklore for me. Either way it might
be worth testing the stability of your electrode over time to get an
idea.
Shane Caldwell
McGill University
On Thu, Jan 30, 2014 at 10:40 AM, Katherine Sippel
<[email protected] <mailto:[email protected]>> wrote:
Alternatively you could make a stock solution of citric acid (say
1 M for example) and stock solution of sodium citrate (also 1 M).
Mix them in the appropriate ratio to ballpark the right pH and
just adjust up or down with the stock solution. The concentration
of citrate will be the same no matter the final volume. You can
then dilute that down to whatever your final concentration of
citrate needs to be.
If you are looking for the actual method to do the calculations I
would suggest finding a chemistry textbook and looking at the
chapter on buffering and the Henderson-Hasselbalch equation.
Cheers,
Katherine
On Thu, Jan 30, 2014 at 9:31 AM, Daniel Picot
<[email protected] <mailto:[email protected]>> wrote:
But you have to be aware that pH depends on the concentration
of the buffer. This is especially the case for phosphate and
citrate buffer.
Daniel
Le 30/01/2014 15:51, Schnicker, Nicholas J a écrit :
It's a pain but I usually just make each pH of whatever
buffer I'm using (if you make it concentrated then you'll
only have to do it once). Also, if you haven't already found
it, Hampton has a nice link to calculate volume of components
while designing a tray as long as you tell it the concentrations.
http://hamptonresearch.com/make_tray.aspx
Nick
From: Roger Rowlett <[email protected]
<mailto:[email protected]>>
Reply-To: Roger Rowlett <[email protected]
<mailto:[email protected]>>
Date: Thursday, January 30, 2014 at 7:23 AM
To: "[email protected] <mailto:[email protected]>"
<[email protected] <mailto:[email protected]>>
Subject: Re: [ccp4bb] preparation of citrate buffer pH3-6.5
The easiest way to produce repeatable conditions is to
titrate a stock solution (say 1M) of citric acid with NaOH to
the desired pH and use that to mix your screen. That's what
Hampton does anyway.
If fine sampling pH, you can mix various ratios of pH 3 and
6.5 buffers. The pH won't be linear with mixing ratio, but
will be easily repeatable. The actual pH of the final, magic
solution can be directly measured if desired. Calculations
will never be exactly right; pKa values are ionic strength
dependent. Better to measure.
Roger Rowlett
On Jan 30, 2014 2:37 AM, "sreetama das"
<[email protected] <mailto:[email protected]>> wrote:
Dear All,
We have obtained many tiny protein crystals in
a condition containing 0.1M citric acid pH 3.5, 2M
ammonium sulfate. The crystals are too small for mounting
in loops.
We intend to vary the salt concentration & pH
to obtain larger crystals.
Could anyone direct us to some links, or
provide us with a method (with calculations) to calculate
the amounts of citric acid & trisodium citrate required
to obtain buffers in a range of pH 3 - 6.5?
I have come across online buffer calculators
and links where the amounts of the components required
are mentioned in grams, but none explaining how those
values were arrived at.
Thanks & regards,
sreetama
--
"Nil illegitimo carborundum"/- /Didactylos
