Questions
2–4 questions in B.Pharm semester exams
Difficulty
Medium-Hard
Importance
High yield for GPAT and university professional exams
Overview
This topic covers the foundational physical chemistry principles governing drug dissolution, physiological pH control, and surface tension dynamics. Understanding these concepts is essential for pharmaceutical formulation, pharmacokinetics, and clinical drug delivery mechanisms, making it a high-yield area for competitive exams.
Solubility and pH Partition Theory
Solubility describes the maximum concentration of a solute in a solvent at a specific temperature, while pH Partition Theory explains how non-ionized drug molecules permeate lipid membranes. This theory is crucial for predicting drug absorption based on pKa values and local pH environments.
- Henderson-Hasselbalch Equation for acids: pH = pKa + log([Ionized]/[Non-ionized])
- Henderson-Hasselbalch Equation for bases: pH = pKa + log([Non-ionized]/[Ionized])
- Solubility product constant (Ksp) measures the saturation of sparingly soluble salts
- Dielectric constant of solvents influences solute-solvent interactions
- Passive diffusion is dependent on the lipophilicity of the non-ionized form
Buffer Systems
Buffers are solutions that resist changes in pH upon the addition of small amounts of strong acid or base. In pharmacology, buffer systems maintain physiological pH and are vital for formulating stable parenteral and ophthalmic dosage forms.
- Buffer capacity (beta) is defined by the Van Slyke equation
- Common systems include acetate, phosphate, and citrate buffers
- Maximum buffer capacity occurs when pH equals pKa
- Buffer systems are essential for maintaining the stability of therapeutic proteins
- Physiological buffers like bicarbonate maintain blood pH at 7.4
Surface and Interfacial Phenomena
Surface phenomena deal with the behavior of molecules at the interface between two phases, such as liquid-gas or liquid-liquid. Understanding interfacial tension and adsorption is necessary for creating stable emulsions, suspensions, and effective drug absorption enhancers.
- Young-Laplace equation governs pressure difference across curved interfaces
- Gibbs Adsorption Isotherm relates surface tension to concentration
- Surfactants decrease interfacial tension by forming oriented monolayers
- HLB system (Hydrophile-Lipophile Balance) categorizes surfactants
- Contact angle measurements determine the wettability of solid surfaces
Formula Sheet
Henderson-Hasselbalch: pH = pKa + log([A-]/[HA])
Van Slyke: beta = dC/dpH
Young-Laplace: deltaP = 2gamma/r
Gibbs Adsorption: Gamma = -(1/RT) * (dgamma/dlnC)
Exam Tip
Always verify if the drug in your calculation is a weak acid or a weak base before applying the Henderson-Hasselbalch equation, as the log term orientation is critical.
Common Mistakes
- Confusing the Henderson-Hasselbalch signs for acidic vs. basic drugs during calculations.
- Neglecting the impact of temperature changes on solubility product constant (Ksp) values.
- Misinterpreting surface tension reduction as directly proportional to surfactant concentration at all ranges (ignoring the CMC).
More Revision Notes
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