EPA Method 365.3

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EPA Method 365.3:
Phosphorus by Colorimetry. Official Name: Phosphorus, All Forms (Colorimetric, Ascorbic Acid, Two Reagent)

Summary:
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.

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

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

Interferences:
(A) Arsenate: Arsenate is determined similarly to phosphorus, and should be eliminated by reducing arsenic acid to arseniuos acid with sodium bisulfite.(B)Iron: Iron can make a low recovery because it will react with the reducing reagent. The bisulfite treatment will eliminate this interference as well.

QC Requirements:
None.

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

Media:
WATER

Subcategory:
Inorganic

Concentration:
0.01 to 1.2 mg P/L.

Sample Prep:
None.

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

Detection:
None.

Revision Number:
Issued 1978

Test Description:
Phosphorus - colorim

Report Number:
600/4-79-020

Instrument used for this test:
UV-VIS