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Specialty Chemistry Forums => Biochemistry and Chemical Biology Forum => Topic started by: Seks on March 16, 2008, 02:07:52 PM

Title: titrating an amino acid
Post by: Seks on March 16, 2008, 02:07:52 PM: 300 ml of 0.2 M of one of the 20 standard amino acids.
Titrate with 1.0M NaOH.
I am given a titration curve.

How do I determine which amino acid got titrated?

.3*0.2 = .06 mol of amino acid

Assuming using .3L of NaOH,

.3*1 = .3 mol of NaOH

I'm not sure where to go from here? ???

For some reason I always have trouble with titration problems :(
Title: Re: titrating an amino acid
Post by: Borek on March 16, 2008, 02:50:11 PM: It's a shape of the curve that holds necessary information.
Title: Re: titrating an amino acid
Post by: enahs on March 16, 2008, 04:42:23 PM: Here are the pKa's for the 20 amino acids and their acidic groups.

http://www.bmrb.wisc.edu/referenc/aapka.html

Also notice and memorize the general pKa's for the Carboxylic group and the amine group.
Title: Re: titrating an amino acid
Post by: Seks on March 16, 2008, 10:58:03 PM: On the curve,

0.5 OH- added, it's at pH 2
1.5 OH- added, it's at pH 8
2.5 OH- added, it's at pH 10ish

(pH for 1.0, 2.0, 3.0 OH- added are indicated but I dont' think those values are relevant??)

I use that pH = pKa + log [A-]/[HA] equation, where A- = .3 and HA = .06.

But for pH 2 and 8, I get pKa 1.3 and 7.3 respectively. And those values don't jive with any pKa values on my table of amino acids.

I must be missing something???? or a step somewhere...
Title: Re: titrating an amino acid
Post by: Yggdrasil on March 16, 2008, 11:35:41 PM: Exactly between equivalence points of a titration curve, [A-] = [HA]. What does this tell you about the pKa values for the various functional groups on the amino acid?

[edit: thanks for the correction Borek]
Title: Re: titrating an amino acid
Post by: Borek on March 17, 2008, 03:58:01 AM: Quote from: Yggdrasil on March 16, 2008, 11:35:41 PM
At the equivalence point of a titration curve (where pH changes sharply with added OH), [A-] = [HA].

Sounds like one of these "before coffee" answers ;)

What Ygg is trying to point out is that there are points on the curve where pH=pKa without any further corrections.
Title: Re: titrating an amino acid
Post by: Seks on March 17, 2008, 11:35:00 PM: Well, when 0.5 OH- is added, pKa = pH, so which means that's pKa 2.
1.5 OH-, pKa = 8
so on

But using 1 mol/L of OH- for 0.3L of 0.2 mol/L amino acid, how will those amounts/quantities affect what I'm trying to find? :(
Title: Re: titrating an amino acid
Post by: Borek on March 18, 2008, 03:57:53 AM: Quote from: Seks on March 17, 2008, 11:35:00 PM
But using 1 mol/L of OH- for 0.3L of 0.2 mol/L amino acid, how will those amounts/quantities affect what I'm trying to find? :(

Assuming 0.5 OH^- means just molar ratio of base added to your aminoacid, concentrations don't matter (much, they do a little, but the effect can be neglected). It is their ratio that counts - and ratio is what you use as input.