EPA-NERL: 365.1: Phosphorus (all forms) by Semi-Automated Colorimetry

Summary
Analytes
Revisions
Data and Sites

Official Method Name

Phosphorus, All Forms (Colorimetric, Automated, Ascorbic Acid)

Current Revision

Revision 2.0, August 1993

Media

WATER

Instrumentation

Automated Spectrophotometer

Method Subcategory

Inorganic

Method Source

EPA-NERL

Citation

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

Brief Method Summary

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."

Scope and Application

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

Applicable Concentration Range

0.01 - 1.0 mg/L

Interferences

(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.

Quality Control 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.

Sample Handling

Samples should be collected in throughly clean plastic or glass bottles and should be of sufficient volume to ensure a representative sample.

Samples must be preserved with H₂SO₄ to a pH < 2 and cooled to 4^oC at the time of collection.

Maximum Holding Time

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

Relative Cost

Less than $50

Sample Preparation Methods

This method has 2 analytes associated with it.

Analyte	Detection Level	Bias	Precision	Pct False Positive	Pct False Negative	Spiking Level
Phosphate(14265-44-2) Orthophosphate Orthophosphate, total Phosphate Phosphorus (as phosphate) Phosphorus, phosphate, ortho Reactive phosphorus Total orthophosphate Total phosphorus	0.010 mg/L	102% Rec (ML)	9.00 % RSD (ML)
Phosphorus(7723-14-0) Black phosphorus P Phosphorous, Red Phosphorus Phosphorus (yellow or white) Red phosphorus Violet phosphorus White phosphorus Yellow phosphorus	0.010 mg/L	99% Rec (ML)	8.00 % RSD (ML)

Precision Descriptor Notes:	Precision and accuracy values were calculated using interlaboratory data from EPA-managed Water Pollution (WP) performance evaluation studies.
Detection Level Note:	MDL procedure is equivalent to the procedure at 40 CFR part 136, Appendix B.

Revision	PDF/Link
Revision 2.0, August 1993
Issued 1971; Editorial Revision 1974 and 1978

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EPA-NERL: 365.1: Phosphorus (all forms) by Semi-Automated Colorimetry

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