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Meaning of pKa
Name: Natalie H.
Status: student
Age: 19
Location: N/A
Country: N/A
Date: 7/26/2004
Question:
i was wondering if you could tell me what exactly pKa
means, what it measures, anything about it, and how it is related to the
pH. also, how does an acidic and a basic solution affect amino acids
and nucleobases if they were to be mixed together?
Replies:
As you know the definition of pH is: pH = - log[H+] where the log is base
10 and [H+] = hydrogen ion concentration. The reason for using a "log" scale
is that [H+] is a small number (usually) and can vary over
about +/- 7 factors of ten. The value of ionization constants of partially
ionized acids is also frequently a small number, and like pH, can vary over
several factors of 10. So by analogy to pH, the pKa = - log (Ka) where Ka is
the ionization constant for the partially ionized acid, and the log is also
base 10.
Vince Calder
Natalie,
I can give you a brief discussion on the acid dissociation constant,
Ka. Many acids are weak electrolytes, and they ionize to an extent that
is much less than 100%. We say that they are weak acids, but they are not
equally so. Just how weak varies from acid to acid, so we could clearly
use a method for comparing their relative strengths as acids. This is Ka,
the acid ionization constant.
Ka = the Hydrogen ion concentration times the conjugate base
concentration divided by the weak Bronsted acid concentration.
I hope that this helps. If you want more information, I recommend that
you refer to a good freshman College Chemistry book of even a good High
School Chemistry book.
Sincerely,
Bob Trach
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Update: June 2012
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