EPA-NERL: 365.3: Phosphorus by Colorimetry

Summary
Analytes
Revision
Data and Sites

Official Method Name

Phosphorus, All Forms (Colorimetric, Ascorbic Acid, Two Reagent)

Current Revision

Issued 1978

Media

WATER

Instrumentation

Spectroscopy (Colorimetry; Photometry)

Method Subcategory

Inorganic

Method Source

EPA-NERL

Citation

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

Brief Method 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 and Application

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

Applicable Concentration Range

0.01 to 1.2 mg P/L.

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.

Quality Control Requirements

None.

Sample Handling

Samples should be collected in plastic or Pyrex glass containers. Samples should be preserved with 2 mL of concentrated sulfuric acid and refrigerated at 4^oC. If samples are collected near benthic deposits, take care not to include materials from the deposits.

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

None.

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	N/A	99% Rec (ML)	10.60 % RSD (ML)			0.10 mg/L
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	100% Rec (ML)	8.80 % RSD (ML)			0.50 mg/L

Precision Descriptor Notes:	Precision and accuracy values were calculated using interlaboratory data from EPA-managed Water Pollution (WP) performance evaluation studies.
Detection Level Note:	None.

Revision	PDF/Link
Issued 1978

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EPA-NERL: 365.3: Phosphorus by Colorimetry

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