Which calcium-dye yields a violet color in the dye-binding approach?

• A. CPC
• B. Arsenazo III
• C. Murexide
• D. Eriochrome Black T

Answer: (B)

In dye-binding calcium determinations, the indicator dye forms a colored complex with Ca2+, and the color reflects the bound state of the dye. Arsenazo III forms a Ca2+-Arsenazo III complex that has a violet hue, a distinct color that is easy to detect spectrophotometrically. This violet color arises from the dye’s chromophore behaving differently when coordinated to calcium, shifting its light absorption into the violet region. Other calcium indicators produce different colors when bound (for example, Eriochrome Black T gives a wine-red complex), so they do not yield violet. Therefore, Arsenazo III is the dye that provides the violet color in the dye-binding approach.