pKa
pKa measurement by spectroscopy is indicated when
solubility is too low, or when the pKa is particularly
low (11) for determination by potentiometry.
The method depends on determining the
ratio of molecular species (neutral molecular at pH
< pKa - 2 (acid), pH > pKa + 2 (base)) to the ionized
species in a series of seven buffers (pH = ±1 of pKJ.
An analytical wavelength (A) is chosen where the
greatest difference in absorbance exists between the
molecular species and the pure ionized moiety at two
pH units from the pKa.