A 20 mL aliquot of a water sample is digested sequentially with a sodium hydroxide/potassium persulfate reagent, then with a sulfuric acid/potassium persulfate reagent in a sealed Folin-Wu tube. The digestion steps take place on an electrically heated block at 150°C. The digested sample is analyzed colorimetrically for total phosphorus as orthophosphate.
(See Method Reference No. 41)