Which reagent forms a colored complex with calcium in the calcium colorimetric assay?

Which reagent forms a colored complex with calcium in the calcium colorimetric assay?

• A. Cresolphthalein complexone
• B. Lanthanum
• C. Malachite green
• D. Amino-naphthol-sulfonic acid

Answer: (A)

In calcium colorimetric assays, the dye chosen must form a colored complex specifically with Ca2+ to allow quantification by absorbance. Cresolphthalein complexone does exactly this: in an alkaline buffer it binds Ca2+ to produce a magenta-colored complex, and the intensity of this color (measured around 570–575 nm) increases with calcium concentration. This gives a direct readout of calcium levels in the sample. The other reagents aren’t used to form the Ca2+-dye complex: lanthanum is used to mask interfering phosphates, malachite green relates to phosphate assays, and amino-naphthol-sulfonic acid serves in different chromogenic chemistries.

In calcium colorimetric assays, the dye chosen must form a colored complex specifically with Ca2+ to allow quantification by absorbance. Cresolphthalein complexone does exactly this: in an alkaline buffer it binds Ca2+ to produce a magenta-colored complex, and the intensity of this color (measured around 570–575 nm) increases with calcium concentration. This gives a direct readout of calcium levels in the sample. The other reagents aren’t used to form the Ca2+-dye complex: lanthanum is used to mask interfering phosphates, malachite green relates to phosphate assays, and amino-naphthol-sulfonic acid serves in different chromogenic chemistries.