Pharmaceutics: Dosage Form Design, Unit Operations and Stability
What the domain tests
Pharmaceutics in the ESIC pharmacist paper covers the physical and formulation science behind every marketed dosage form - preformulation, unit operations, conventional and novel delivery systems, and official standards. Questions are usually numerical or classification-based rather than descriptive, so equations, excipient roles and sterilisation limits carry the marks.
Core rules to hold
- Dissolution: Noyes–Whitney rate is proportional to surface area and the concentration gradient and inversely proportional to boundary-layer thickness; micronisation, salt formation and amorphous forms raise dissolution rate but can hurt stability.
- Tablets: diluents (lactose, dicalcium phosphate), binders (PVP, starch paste), disintegrants (croscarmellose sodium, sodium starch glycolate), glidants (colloidal silicon dioxide) and lubricants (magnesium stearate - hydrophobic, so kept at 0.5–2%).
- Sterilisation: moist heat 121 °C at 15 psi for 15 minutes for aqueous solutions; dry heat 160 °C for 2 hours for glassware and oils; 0.22 µm membrane for heat-labile solutions; ethylene oxide for plastic devices.
- Emulsions: the HLB system; Bancroft's rule - the phase in which the emulsifier is more soluble becomes the continuous phase.
- Stability: Arrhenius extrapolation of degradation rate constants to predict shelf life.
Worked example
An o/w emulsion needs a blend HLB of 12.0. Span 60 (HLB 4.7) and Tween 60 (HLB 14.9) are available. Let x be the Tween fraction: 14.9x + 4.7(1 − x) = 12.0, so 10.2x = 7.3 and x = 0.716. The blend is therefore 71.6% Tween 60 with 28.4% Span 60 by weight.
Traps that cost marks
Remembering HLB above about 10 as o/w and below as w/o but reversing it under exam pressure; writing autoclave conditions as 121 °C at 15 psi for 30 minutes; labelling magnesium stearate a glidant; applying Stokes' sedimentation law to dissolution problems.