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-Hasselbach-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
|