EPA-NERL: 365.5: Orthophosphate in Estuarine and Coastal Waters by Colorimetry

Summary
Analytes
Revision
Data and Sites

Official Method Name

Determination of Orthophosphate in Estuarine and Coastal Waters by Automated Colorimetric Analysis

Current Revision

Revision 1.4, September 1997

Media

WATER

Instrumentation

Automated Spectrophotometer

Method Subcategory

Inorganic

Method Source

EPA-NERL

Citation

Methods for Determination of Chemical Substances in Marine and Estuarine Matrices - 2nd Edition (EPA/600/R-97/072)

Brief Method Summary

This method is based on the method of Murphy and Riley. A sample is introduced into an automated segmented flow analyzer. Orthophosphate reacts with ammonium molybdate and ammonium potassium tartrate in an acidic medium to form an antimony-phoso-molybdate complex. The complex is reduced using ascorbic acid, and the concentration of orthophosphate is determined by measuring the absorbance of the reduced complex with an on-line colorimeter.

Scope and Application

This method determines low-level orthophosphate concentrations in estuarine and coastal waters.

Applicable Concentration Range

The method is linear up to 0.39 mg-P/L

Interferences

(A) Hydrogen sulfide: Dilute samples with high sulfide contents to minimize interferences.

(B) Turbidity: Remove turbidity using filtration.

Quality Control Requirements

Initial demonstration of performance, continued analysis of Laboratory Reagent Blanks (LRB), laboratory duplicates, and Laboratory Fortified Blanks (LFB) with each set of samples as a continuing check on performance.

Sample Handling

Samples collected using hydrocast or submersible pump systems. Turbid samples must be filtered through a 0.45 um membrane or glass fiber filter immediately after collection. Samples to be analyzed within 24 hours should be stored at 4^oC until analysis. If samples cannot be analyzed within 24 hours, then freezing at -20^oC for a maximum period of two months is acceptable.

Maximum Holding Time

24 hours; two months if stored at -20^oC

Relative Cost

Less than $50

Sample Preparation Methods

This method has 1 analyte 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	0.001 mg/L	97% Rec (ML)	8.50 % RSD (ML)			0.01 mg/L

Precision Descriptor Notes:	Precision and accuracy data determined using regression equations in method for reagent water; the regression equation was generated using data from nine laboratories (see Table 1 of the method). Additional data, including data on method performance at vaious salinities, are provided in the method.
Detection Level Note:	The MDL was determined in an analyte-fortified saline water (3 parts per thousand) according to the MDL procedure at 40 CFR part 136, Appendix B. The procedure also is detailed in Section 9.2.2 of the method.

Revision	PDF/Link
Revision 1.4, September 1997

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EPA-NERL: 365.5: Orthophosphate in Estuarine and Coastal Waters by Colorimetry

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