Colorimetric analysis by the molybdate-blue/ascorbic acid method. Orthophosphate reacts with ammonium molybdate and potassium antimonyl tartrate to form phosphomolybdic acid. This complex is reduced by ascorbic acid to intensely colored molybdenum blue measured colorimetrically on an Lachat 8500 FIA at 880 nm. Acidified samples must be neutralized to a pH between 5 and 9.
(See Method References No. 40 and No. 51)