The pK
a value is the threshold
pH (i.e.
hydrogen concentration) at which a potential hydrogen acceptor/donator site will accept/donate a hydrogen. To try and make that simpler here's an example:
Glycine is a very simple
amino acid (H
2NCH
2COOH) with the structure:
NH2
|
HCH
|
COOH
This molecule has two potential hydrogen acceptor/donator sites:
1) the NH
2 (
amine) site can be further
hydrogenated to give NH
3+
2) the COOH (
carboxyl) site could be
dehydrogenated to give COO
-
These two sites have different threshold pK
a values though. The amine site will become hydrogenated below pH 2.34 and the carboxyl site below 9.60.
Thus as the pH increases (
basic to
acidic) glycine can be found in three different states:
pH <2.34 2.34-9.60 >9.60
NH2 NH3+ NH3+
State | | |
HCH HCH HCH
| | |
COO- COO- COOH
This has obvious effects on it's net
charge and further effects on
solubility, amongst other things.
The pKa value is the -log10 of the Ka (just as pH is the -log10 of the hydrogen ion concentration). Ka is the association constant that links the concentrations of the free ions and the undissociated parent molecule - the larger the Ka the greater the tendency to dissociate (e.g. very high for molecules that freely dissociate in solution, like NaCl, and very low for things that don't, like sucrose).