Which reagents produce the blue molybdenum complex used in the colorimetric determination of orthophosphate?

• A. Ammonium molybdate and ascorbic acid
• B. Potassium permanganate and sulfuric acid
• C. Nitric acid and sodium borohydride
• D. Phosphotungstic acid and lead nitrate

Answer: (A)

The blue molybdenum complex used to measure orthophosphate is formed when orthophosphate reacts with molybdate under acidic conditions to make a phosphomolybdate complex, which is then reduced to the blue molybdenum blue species. The essential reagents are ammonium molybdate, which provides the molybdate ion that forms the phosphomolybdate, and a reducing agent like ascorbic acid, which converts that complex into the blue form whose color intensity is proportional to the phosphate concentration. The other reagent combinations don’t generate this specific blue phosphomolybdenum complex for phosphate determination: they either don’t form the same molybdate-based complex, or involve different chemistries not used for this colorimetric method.