pKa or Dissociation Constants

Among the physicochemical characteristics of interest is the extent of dissociation or ionization of drug substances. This is important because the extent of ionization has an important effect on the formulation and pharmacokinetic parameters of the drug.

The extent of dissociation or ionization in many cases is highly dependent on the pH of the medium containing the drug. In formulation, often the vehicle is adjusted to a certain pH to obtain a certain level of ionization of the drug for solubility and stability.

In the pharmacokinetic area, the extent of ionization of a drug has a strong effect on its extent of absorption, distribution, and elimination. The dissociation constant, or pKa, is usually determined by potentiometric titration.

Simply, pKa is a number that shows how weak or strong an acid is. A strong acid will have a pKa of less than zero, more precisely a negative value.

The dissociation of a weak acid in water is given by this expression:

HA ↔ (H+) + (A-)

K1 [HA] ↔ K2 [H+][A-]

At equilibrium, the reaction rate constants K1 and K2 are equal.

For the dissociation of a weak base that does not contain a hydroxyl group, the following relationship can be used:

BH+ ↔ (H+) + (B)

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