Which calcium measurement method is categorized as colorimetric?

• A. AAS
• B. ISE
• C. Dye binding
• D. Precipitation/Redox techniques (2)

Answer: (C)

Colorimetric calcium measurements rely on a dye that binds calcium to form a colored complex, and the color intensity is read with a spectrophotometer to determine concentration. This dye-binding approach is the classic colorimetric method because the signal comes from a visible color change directly proportional to calcium levels, often using dyes like o-cresolphthalein complexone or Arsenazo dyes.

AAS measures the amount of light absorbed by free calcium atoms in a flame or graphite furnace, which is an atomic technique, not a color change. ISE detects calcium by a electrode potential, reflecting ion activity rather than a color change. Precipitation or redox methods rely on turbidity, precipitate formation, or redox reactions to infer concentration, not on a color that can be measured spectrophotometrically.