What is the product formed in the Fiske Subbarow phosphate assay?

• A. Ammonium phosphomolybdate
• B. Phosphomolybdic acid
• C. Molybdenum blue
• D. Phosphoric acid

Answer: (A)

In this assay the amount of phosphate is determined by the blue color produced after a reduction step. Phosphate reacts with ammonium molybdate in acidic medium to form a phosphomolydate complex. This complex is then reduced by a suitable agent to form molybdenum blue, whose intensity is proportional to the phosphate concentration. So the detectable product that is measured is the reduced molybdenum blue. The intermediate phosphomolydate complex (often described as ammonium phosphomolydate or phosphomolybdic acid) forms first, but the final colored species used for quantification is molybdenum blue.