In the phosphate colorimetric assay, the reduced phosphomolybdate yields which color?

• A. Blue
• B. Colorless
• C. Red
• D. Green

Answer: (A)

In this assay, phosphate reacts with molybdate under acidic conditions to form phosphomolybdic acid, which, when reduced, becomes molybdenum blue. That reduced complex is blue, and its intensity corresponds to the amount of phosphate present. So the color you observe after reduction is blue. The other colors don’t come from this reduced phosphomolydate reaction in the standard method.