EPA Method 365.1

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EPA Method 365.1:
Phosphorus (all forms) by Semi-Automated Colorimetry. Official Name: Phosphorus, All Forms (Colorimetric, Automated, Ascorbic Acid)

A sample is appropriately treated to convert all phosphorus of interest to reactive orthophosphate. Ammonium molybdate and antimony potassium tartrate are added to the treated sample reacting with orthophosphate in an acidic medium to form an antimony-phospho-molybdate complex. This complex is reduced to an intensely blue-colored complex by ascorbic acid. The concentration of the orthophosphate is measured by detecting the absorbance of the complex with a spectrophotometer.Note: In Section 7.7 of the method (preparation of 11 N sulfuric acid solution), "600 mL reagent water" should read "300 mL reagent water."

This method determines total phosphorus, hydrolyzable phosphorus, and orthophosphate in drinking, ground, and surface waters; and domestic and industrial wastes.

Methods for the Determination of Inorganic Substances in Environmental Samples (EPA/600/R-93/100)

(A) Metals and silica: Copper, iron, and silica do not interfere at the levels reported in sea water, but excessively high concentrations of iron can cause precipitation and loss of phosphorus.(B) Salt error: Salt error for 5%-20% salt samples was less than 1%.(C) Arsenate: Arsenate can cause a positive interference, but is often at low concentrations.(D)Turbidity: Remove turbidity via filtration.

QC Requirements:
The minimum recommended quality control requirements include an initial demonstration of laboratory capability (determining the linear calibration range and method detection limit, and analyzing a quality control sample; LCR and MDL, and QCS), and the periodic analysis of laboratory reagent blanks.

Maximum Holding Time:
48 hours (orthophosphate, dissolved)28 days (Hydrolyzable and Total)24 hours (Total Dissolved).(MCAWW, Table 1).



0.01 - 1.0 mg/L

Precision and accuracy values were calculated using interlaboratory data from EPA-managed Water Pollution (WP) performance evaluation studies.

MDL procedure is equivalent to the procedure at 40 CFR part 136, Appendix B.

Revision Number:
Revision 2.0, August 1993

Test Description:
Orthophosphate (as P

Report Number:

Instrument used for this test: