What is the product formed in the phosphate determination using the Fiske Subbarow method?

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

Answer: (A)

In the Fiske-Subbarow phosphate determination, phosphate reacts with ammonium molybdate in strongly acidic medium to form a phosphomolydate complex. This species is formed as an ammonium salt, known as ammonium phosphomolydate. It is the product of that initial reaction, which is then reduced (by agents like ascorbic acid) to give the blue molybdenum blue color used for measurement. So the product formed at this stage is ammonium phosphomolydate. The blue color you see later comes from its reduced form, molybdenum blue, not the immediate reaction product.