There are 3 pKa's for citric acid:
3.15, 4.77, and 5.19
so it is going to be a lousy buffer at pH 6.7 to 7.25. Also, you need
to know what concentration your buffer is, and whether that
concentration is with respect to the citrate ion or what. It won't be
tribasic with respect to ammonium ion near neutral pH.
So if for example you need 1M of this buffer, the simplest thing to do
is make up a 1M stock solution of tribasic ammonium citrate and a 1M
stock solution of of citric acid and then mix the two together. You
can calculate the ratio using the Henderson-Hasselbalch equation or
just titrate one into the other while stirring in the presence of a pH
electrode.
(You'll have to use the pH electrode approach if you took introductory
chemistry from anyone other than me at UCSC since my colleagues think
that polyprotic acids are too stressful for our undergrads.)
William G. Scott
Contact info:
http://chemistry.ucsc.edu/~wgscott/
On Aug 7, 2008, at 12:42 PM, E rajakumar wrote:
Dear All
Sorry for non crystallographic query.
Can any body mail me how to prepare Ammonium citrate
tribasic (citric acid triammonium salt) buffer pH 6.7
to 7.25 and also what is the pKa value.
Thanking you in advance
Rajakumara
E. Rajakumara
Postdoctoral Fellow
Strcutural Biology Program
Memorial Sloan-Kettering Cancer Center
New York-10021
NY
001 212 639 7986 (Lab)
001 917 674 6266 (Mobile)
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