Clark-Collip oxalic acid precipitation is used to precipitate calcium with which reagent?

• A. Oxalic acid
• B. Sodium citrate
• C. Ammonium oxalate
• D. Citric acid

Answer: (A)

Calcium can be removed from solution by forming an insoluble salt with oxalate ions. In the Clark‑Collip approach, oxalic acid is added to the sample, supplying C2O4^2− that reacts with Ca^2+ to form calcium oxalate, CaC2O4, which precipitates out. This precipitation relies on the very low solubility of calcium oxalate, effectively pulling calcium out of the solution so it no longer interferes with other measurements. Among the options, oxalic acid provides the oxalate needed to form the precipitate directly. Citrate or citric acid would chelate calcium rather than form the insoluble salt, and ammonium oxalate would also precipitate calcium but is not the reagent specified in this method.