![[logo] US EPA](http://www.epa.gov/epafiles/images/logo_epaseal.gif){kind=logo}
R9 Laboratory SOP 596
ANALYSIS OF TOTAL PHOSPHORUS IN WATER
Summary
This method is for the determination of total phosphorus in drinking and surface waters, domestic and industrial wastes, which are digested by the Kjeldahl digestion method. This SOP is based on EPA Method 365.4, issued 1974.
The colorimetric method is based on reactions that are specific for the orthophosphate ion.
The digestion converts the phosphorus in the sample to the orthophosphate ion.
The application of these procedures to samples with ferric iron concentrations in excess of 50 mg/L is not recommended due to a negative interference. The applicable range is 0.3 to 10 mg/L total phosphorus. The range can be extended for high level samples through dilution either before or after the digestion procedure. The quantitation limit for total phosphorus is 0.3 mg/L.
During the digestion procedure, the sample is heated in the presence of sulfuric acid, K2SO4, and HgSO4 for two and a half hours. With mercuric oxide as a catalyst, the phosphorus in the sample is converted to the orthophosphate ion. Potassium sulfate is also added to raise the boiling temperature of the digestion and speed the conversion to orthophosphate. The orthophosphate ion (PO4 3-) in the digested sample reacts with ammonium molybdate and antimony potassium tartrate to form an antimony-phosphomolybdate complex. This complex is reduced with ascorbic acid to form an intense blue color, which is read at 880 nm. The absorbance is proportional to the concentration of orthophosphate in the sample.