A problem in the spectrophotometric determination of dissolved total phosphorus in brackish anoxic waters

When dissolved total phosphorus in anoxic brackish waters was determined spectrophotometrically by the heteropoly molybdenum blue method after oxidation with potassium peroxodisulfate, negative dissolved organic phosphorus contents were obtained from the differences between the total phosphorus and inorganic phosphorus contents. A major cause of this inconsistency is coprecipitation of phosphate with the colloidal hydrated iron(III) oxide produced fro miron in these waters, or the formation of iron(III) phosphate during the oxidation process. The problem can be avoided by oxidation with nitric and perchloric acids.