Which method is used for phosphate determination as described?

• A. Fiske Subbarow
• B. Bradford
• C. Biuret
• D. Lowry

Answer: (A)

Phosphate is measured with a colorimetric assay that forms a phosphomolybdate complex and then reduces it to a blue color whose intensity reflects the amount of phosphate present. In the Fiske-Subbarow method, phosphate in the sample reacts with ammonium molybdate in acidic solution to produce phosphomolybdate. A reducing agent (such as ascorbic acid or another reducer) converts this to a blue molybdenum complex. The resulting color is measured spectrophotometrically, and its intensity is proportional to inorganic phosphate concentration. This technique is a classic choice for phosphate determination in biological samples because it is sensitive and straightforward. The other listed methods are protein assays and are not used to quantify phosphate, which is why this method is the appropriate one.