MadSci Network: Biochemistry

Re: Concentration of amino acid ionic species

Date: Mon Sep 19 10:49:06 2005
Posted By: Karin Crowhurst, Post-doc/Fellow, Biochemistry
Area of science: Biochemistry
ID: 1127011192.Bc

Hi Patty,

If I understand your question correctly, you are having trouble using all 
the information you have been given with the Henderson-Hasselbach 
equation.  So, as you mentioned, you know the pH and pKa values for the 
equation, but cannot convert the [A-]/[HA] ratio into actual 
concentrations.  That's where a second equation comes into play.  So, in 
your example, you know that

[A-] + [HA] = 0.1M, or 0.1 - [A-] = [HA].  

If you insert this information into the H.H. equation, you can solve for 
one of the concentrations.  So, in the case of Arginine, the first pKa is 
1.82 (for COOH).  The H.H. equation then becomes

4 = 1.82 + log ([A-]/([0.1 - [A-])),
or 2.18 = log ([A-]/([0.1 - [A-])).  

This rearranges to 

10^(2.18) = ([A-])/(0.1 - [A-]),

which you can rearrange further to get [A-].  Once you know [A-], you 
substitute this value back into [A-] + [HA] = 0.1M to get the value of 
[HA].  And so on for all your calculations.  Hope this helps!  

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