Most of the official drugs containing calcium and zinc are assayed by:

• A. Gravimetry
• B. Precipitimetry
• C. Acidimetry
• D. Complexometry

Answer: (D)

Complexometric titration uses a chelating agent to bind metal ions and determine their concentration. In assays for calcium and zinc, a strong, water-soluble chelator like EDTA is added to the dissolved drug sample. EDTA forms very stable complexes with Ca2+ and Zn2+, and the amount of EDTA required to bind all the free metal ions corresponds to the metal content in the sample. The endpoint is typically detected with an indicator that changes color when metal ions are fully complexed, often under a buffered pH that favors EDTA binding to these ions. This approach is highly specific for divalent metal ions and works well even in the presence of other substances found in pharmaceutical formulations, making it efficient for routine official assays. Other methods, such as gravimetry, precipitation-based titrimetry, or acid-base titration, are less suitable here because they either require isolating a solid, are less selective, or do not directly quantify the metal ions in solution.