In the CPC-based calcium dye-binding assay, which additive prevents magnesium interference?

• A. 8-hydroxyquinoline
• B. EDTA
• C. Arsenazo III
• D. Murexide

Answer: (A)

In this CPC-based calcium dye-binding assay, magnesium can interfere by reacting with the dye system and altering the color readout, which would skew the measured calcium concentration. The additive that prevents this interference must remove Mg2+ from participating without stealing Ca2+. 8-hydroxyquinoline does just that: it forms a stable complex with Mg2+, effectively sequestering magnesium away from the dye reaction, while calcium remains free to form the Ca-CPC complex and produce the correct color change.

EDTA would chelate both calcium and magnesium, disturbing the target Ca measurement. Arsenazo III and Murexide are dyes used for calcium detection themselves, not additives to suppress magnesium interference, so they don’t serve the purpose of masking Mg2+ in this assay.