Pharmaceutical Chemistry and Analysis: quality control calculations
What chemistry and analysis tests
This domain tests medicinal chemistry of common drug classes and the analytical methods used in quality control: titrimetry, spectrophotometry, chromatography, pH, limit tests and pharmacopoeial standards. D.Pharm questions often ask for a calculation, a functional-group reaction or the reason a method is chosen.
Core rules
- Quality control confirms identity, purity, potency and uniformity; pharmacopoeial monographs specify tests and limits.
- In acid-base titrimetry, normality and molarity are not interchangeable when reaction stoichiometry is not 1:1. Always write the balanced equation.
- Spectrophotometry depends on the Beer-Lambert law: absorbance is proportional to concentration and path length; linearity must be validated.
- Chromatography separates by differential partitioning or affinity; TLC is a limit or identity test, while HPLC is common for assay.
- Common functional-group tests: phenols with ferric chloride, aldehydes with 2,4-DNP, carboxylic acids by bicarbonate effervescence.
Worked example
A 0.400 g sodium bicarbonate sample is assayed by titration with 0.1000 M HCl; 42.0 mL is required to endpoint. NaHCO3 + HCl -> NaCl + CO2 + H2O, so 1 mol HCl reacts with 1 mol NaHCO3. 1.00 mL of 0.1000 M HCl = 0.1000 mmol = 0.1000 x 84.01 mg = 8.401 mg NaHCO3. Mass found = 42.0 x 8.401 = 352.8 mg = 0.3528 g. Percentage purity = (0.3528/0.400) x 100 = 88.2%. This is below a typical 99.0-100.5% monograph limit, so the batch fails.
Traps
Using 84.01 g/mol as grams instead of mg; forgetting the blank; reading the meniscus wrongly; and reporting purity without checking the pharmacopoeial limit.