JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Pharmacy II Year PHYSICAL PHARMACEUTICS-I Previous Question Paper (September-2021 - R17)

The dissociation constant (Ka) of a weak acid or base is determined primarily through potentiometric titration or spectrophotometric methods. In potentiometric titration, a solution of the weak acid is titrated with a strong base using a pH meter. The pH is recorded as a function of the volume of base added. According to the Henderson-Hasselbalch equation, when the acid is half-neutralized, the pH equals the pKa of the acid. Thus, by identifying the equivalence point and determining the pH at the half-neutralization point, the pKa is calculated.

Alternatively, the spectrophotometric method is used for substances that exhibit different absorption spectra in their ionized and unionized forms. By measuring the absorbance of the substance at various pH levels, the ratio of ionized to unionized species is determined, allowing for the calculation of the dissociation constant using the modified Beer-Lambert law.

The dissociation constant is a critical parameter in pharmaceutical sciences with several key applications:

• Solubility prediction: The solubility of weakly acidic or basic drugs is highly pH-dependent. Knowledge of the pKa allows for the calculation of the pH at which a drug remains in solution or precipitates, which is vital for formulation development.
• Bioavailability and Absorption: According to the pH-partition hypothesis, only the unionized form of a drug can readily permeate biological membranes. The pKa determines the fraction of the drug that exists in the unionized state at physiological pH levels (e.g., pH 1.2 in the stomach or pH 7.4 in the blood), directly influencing drug absorption.
• Drug Stability: The rate of chemical degradation, such as hydrolysis, is often influenced by the ionization state of the molecule. Understanding the pKa helps in optimizing the pH of liquid formulations to ensure maximum stability.
• Formulation Compatibility: The pKa is essential for selecting appropriate buffer systems to maintain the drug in its most stable and soluble form during shelf life.
• Protein Binding: The extent of drug binding to plasma proteins is often influenced by the ionization state, which is governed by the pKa of the drug molecule.