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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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