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: CCP4BB@JISCMAIL.AC.UK
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: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of William Scott
Sent: 07 August 2008 23:27
To: Juergen Bosch
Cc: CCP4BB@JISCMAIL.AC.UK
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, [EMAIL PROTECTED] 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" <[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)
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