EPA Method 365.5

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EPA Method 365.5:
Orthophosphate in Estuarine and Coastal Waters by Colorimetry. Official Name: Determination of Orthophosphate in Estuarine and Coastal Waters by Automated Colorimetric Analysis

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.

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

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

(A) Hydrogen sulfide: Dilute samples with high sulfide contents to minimize interferences.(B) Turbidity: Remove turbidity using filtration.(C) Metals: High iron concentrations can precipitate phosphate.

QC 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.

Maximum Holding Time:
24 hours; two months if stored at -20oC



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

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.

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 Number:
Revision 1.4, September 1997