pK_a Determination

Synonyms

Acid dissociation constant; Acid dissociation constant determination; Acidic dissociation constant; Acidity constant; Acidity constant determination; Ionization constant; Ionization constant determination

Definition

The acid dissociation constant (K_a) (also known as ionization constant, as acidic dissociation constant, or sometimes as acidity constant) is among the most relevant physicochemical parameters from a (bio)pharmaceutical perspective. For a weak acid AH, K_a is provided by the following equilibrium and expression:

$$ \mathrm{AH}\rightleftharpoons {\mathrm{A}}^{-}+{\mathrm{H}}^{+}\, {K}_a=\frac{\left[{\mathrm{A}}^{-}\right]\left[{\mathrm{H}}^{+}\right]}{\left[\mathrm{AH}\right]} $$

(1)

whereas for a weak base B:

$$ {\mathrm{BH}}^{+}\rightleftharpoons \mathrm{B}+{\mathrm{H}}^{+}\quad {K}_a=\frac{\left[\mathrm{B}\right]\left[{\mathrm{H}}^{+}\right]}{\left[{\mathrm{BH}}^{+}\right]}\, $$

(2)

Generalizing, K_acan be thus obtained from the concentration ratio of the...