EPA Method 365.2

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EPA Method 365.2:
Phosphorus by Colorimetry. Official Name: Phosphorus, All Forms (Colorimetric, Ascorbic Acid, Single Reagent)

The sample is pretreated to select the phosphorus forms of interest; the forms are then converted to orthophosphate (which is amenable to the color chemistry of the method). A sample is filtered to select dissolved forms. Polyphosphates are converted to orthophosphate by sulfuric acid hydrolysis. Organic phosphorus is converted to orthophosphate using persulfate digestion.Ammonium molybdate and antimony potassium tartrate react in an acid medium with dilute solutions of phosphorus to form an antimony-phospho-molybdate complex, which is reduced with ascorbic acid to form an intense blue-colored complex. The absorbance of the complex is measured, and is proportional to the orthophosphate concentration.

This method can determine dissolved and total orthophosphate, hydrolyzable phosphate, and phosphorus (all forms) in drinking, surface and saline waters; domestic and industrial wastes.

Methods for the Chemical Analysis of Water and Wastes (MCAWW) (EPA/600/4-79/020)

(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.

QC Requirements:

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



0.01 to 0.5 mg P/L.

Sample Prep:

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


Revision Number:
Issued 1971