What is the pKa? Why was pKa introduced?

  • The pH at which 50% of the drug molecules are ionized and 50% un-ionized (50% dissociated)
  • It is calculated as the negative logarithm to the base 10 of the dissociation constant (Ka) of an acid

 

pK_{a}=-\log K_{a}

  • Using the Ka can be inconvenient particularly for weak acids
    • Weak acids have a Ka which is a very small number (between 10-3 to 10-6)
  • pKa introduced as a simpler measure of the strength of an acid
    • The lower the pKa , the stronger the acid
    • Particularly useful for characterising weak acids
  • Like pH, pKa is a logarithmic scale. Therefore, a small reduction in pKarepresents a much larger increase in acidity
Acid
Ka
pKa
Lactic Acid
1.38 x 10-4
0.000138
3.9
Acetic Acid
1.76 x 10-5
0.0000176
4.8
Carbonic Acid
4.3 x 10-7
0.00000043
6.37
Ascorbic Acid
1.6 x 10-12
0.0000000000016
11.80