1 pKa is the negative of the logarithm of the dissociation constant for the -COOH group. 2 pKb is the negative of the logarithm of the dissociation constant for the -NH3 group. 3 pKx is the negative of the logarithm of the dissociation constant for any other group in the molecule. 4 pl is the pH at the isoelectric point. Reference: D.R. Lide, Handbook of Chemistry and Physics, 72nd Edition ...