Good points! Not only citrate is inappropriate for buffers in the pH 6.5-7.5 range, but also (as pointed out by your colleagues at UCSC, but probabbly elsewhere, polyprotic acids are hard to handle, as their pKa change in a significant fashion depending on their concentration.
In this case, the Henderson-Hasselbalch equation must be written in terms of activities rather than concentrations, otherwise it is totally useless. Nadir Mrabet Selon "William G. Scott" <[EMAIL PROTECTED]>: > 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) > > > > > > > > Get your new Email address! > > Grab the Email name you've always wanted before someone else does! > > http://mail.promotions.yahoo.com/newdomains/aa/ > Pr. Nadir T. Mrabet Cellular & Molecular Biochemistry INSERM U-724 UHP - Nancy 1, School of Medicine 54505 Vandoeuvre-les-Nancy Cedex France Tel : +33 (0)3.83.68.32.73 Fax : +33 (0)3.83.68.32.79 E-mail : [EMAIL PROTECTED]
