Pharmaceutical Chemistry: Identity, Purity, and Assay
Pharmaceutical Chemistry in the Philippine Pharmacist Licensure Examination tests how drug identity, purity, stability, and strength are established through inorganic, organic, and analytical principles. Expect compendial reasoning, functional-group behavior, acid-base equilibria, and instrumental methods.
Core rules
- Identity vs purity vs assay: identity tests confirm structure; limit tests cap impurities; assay quantifies the active ingredient.
- Acid-base behavior: pH, buffers, pKa, and salt forms matter. Weak bases are often titrated nonaqueously with perchloric acid.
- Spectroscopy: UV-Vis uses Beer-Lambert (A = εbc); IR identifies functional groups; NMR and MS support structure.
- Chromatography: TLC Rf values, HPLC retention and system suitability, and GC volatility separate and quantify components.
- Titrimetry: acid-base, redox, complexometric (EDTA), and precipitation methods; distinguish endpoint from equivalence point.
Worked example
A 0.500 g monoprotic acidic drug (MW 180.16) is assayed by titration and consumes 24.50 mL of 0.1000 M NaOH. Moles NaOH = 0.02450 L × 0.1000 = 0.002450 mol. Because the reaction is 1:1, mass of drug = 0.002450 × 180.16 = 0.4414 g. Percent purity = 0.4414/0.500 × 100 = 88.28%. If the substance is a hydrate or salt, use the correct formula weight and stoichiometric factor.
Traps
Using the free-base molecular weight for a salt; ignoring reaction stoichiometry; treating a pH endpoint as the equivalence point; exceeding the Beer-Lambert linear range; assuming one HPLC peak proves purity; losing marks on unit conversions and significant figures.