pKa
The negative logarithm of the acid dissociation constant, giving a convenient scale of acid strength.
Formula / equation
pKa = −log Ka
Unit
Dimensionless
In depth
A lower pKa means a stronger acid, and each unit represents a tenfold difference in Ka. The pKa also equals the pH at which an acid is exactly half ionised, which makes it the single most useful number for predicting the charge state of a molecule at a given pH.
Examples in the real world
Ethanoic acid has pKa 4.76, phenol 10.0, and ethanol 16 — a range of eleven orders of magnitude.