A 20 mL aliquot of a water sample is digested sequentially in a sealed Folin-Wu tube, first with a sodium hydroxide/potassium persulfate reagent, then with a sulfuric acid/potassium persulfate reagent. The digestion steps take place on a digestion block heated to 150°C. The digested sample is analyzed colorimetrically for total phosphorus as orthophosphate 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 and measured on a Lachat 8500 FIA at 880 nm.) Acidified samples must be neutralized to a pH between 5 and 9.
(See Method Reference No. 41)
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