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Hi Pete: The pKa (NH4OH) is about 9, so it shouldn't be relevant to the buffer calculation. The ammonia gas equilibrium would be a reason not to use this as a buffer, however. Bill William G. Scott Contact info: http://chemistry.ucsc.edu/~wgscott/ On Aug 9, 2008, at 2:54 PM, Dunten, Pete W. wrote: > Bill, You still lose a point for missing > the fourth pKa in ammonium citrate, the > ammonium ion. > > Kind regards, Pete > > > -----Original Message----- > From: CCP4 bulletin board on behalf of William G. Scott > Sent: Sat 8/9/2008 12:09 PM > To: [log in to unmask] > Subject: Re: [ccp4bb] Ammonium citrate tribasic buffer > > Well, as long as I was right. That is the important thing. ;) > > > William G. Scott > > Contact info: > http://chemistry.ucsc.edu/~wgscott/ > > > On Aug 9, 2008, at 11:11 AM, Ian Tickle wrote: > >> >> I think the problem is that Britannica, or even Wikipedia, is and >> has never been the final arbiter of truth, I would put my faith in a >> specialist publication such as: >> >> http://books.google.co.uk/books?id=bwbCVCFPNI4C&pg=PA175&lpg=PA175&dq=%22karl+hasselbalch%22&source=web&ots=yHzTUiNE1g&sig=7JmNU-OfHIkmXxR5ivGTrpS7_Us&hl=en&sa=X&oi=book_result&resnum=5&ct=result >> >> -- Ian >> >>> -----Original Message----- >>> From: [log in to unmask] >>> [mailto:[log in to unmask]] On Behalf Of William Scott >>> Sent: 07 August 2008 23:27 >>> To: Juergen Bosch >>> Cc: [log in to unmask] >>> Subject: Re: [ccp4bb] Ammonium citrate tribasic buffer >>> >>> Wikipedia claims Hasselbach is a misspelling: >>> >>> http://en.wikipedia.org/wiki/Henderson-Hasselbalch_equation >>> >>> Since Wikipedia has now replaced the social consensus as the final >>> arbiter of truth ... >>> >>> >>> On Aug 7, 2008, at 3:22 PM, Juergen Bosch wrote: >>> >>>> Just on a side note, can someone clarify why Hasselbach is not >>>> Hasselbalch or vice versa ? Or is that the same guy just somewhere >>>> sometime misspelled and for ever in the records ? >>>> >>>> See here: >>>> >>> http://www.britannica.com/EBchecked/topic/261188/Henderson-Has >> selbach-equation >>>> >>>> Jürgen >>>> >>>> >>>> On 7 Aug 2008, at 15:12, [log in to unmask] wrote: >>>> >>>>> 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" <[log in to unmask]>: >>>>> >>>>>> 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 : [log in to unmask] >>>> >>>> - >>>> Jürgen Bosch >>>> University of Washington >>>> Dept. of Biochemistry, K-426 >>>> 1705 NE Pacific Street >>>> Seattle, WA 98195 >>>> Box 357742 >>>> Phone: +1-206-616-4510 >>>> FAX: +1-206-685-7002 >>>> Web: http://faculty.washington.edu/jbosch >>> >>> >> >> >> Disclaimer >> This communication is confidential and may contain privileged >> information intended solely for the named addressee(s). It may not >> be used or disclosed except for the purpose for which it has been >> sent. If you are not the intended recipient you must not review, >> use, disclose, copy, distribute or take any action in reliance upon >> it. 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