Standard Methods: 4500-P E: Phosphorus by Ascorbic Acid

Summary
Analytes
Revisions
Data and Sites

Official Method Name

4500-P B. Ascorbic Acid Method

Current Revision

Standard Methods Online

Media

WATER

Instrumentation

Spectroscopy (Colorimetry; Photometry)

Method Subcategory

Inorganic

Method Source

Standard Methods

Citation

Standard Methods Online - Standard Methods for the Examination of Water and Wastewater

Brief Method Summary

Ammonium molybdate and potassium antimonyl tartrate react in acid medium with orthophosphate to form a heteropoly acid-phosphomolybdic acid-that is reduced to intensely colored molybdenum blue by ascorbic acid.

Scope and Application

This method is most suited for measuring phosphorus in water.

Applicable Concentration Range

0.01 to 6 mg-P/L

Interferences

Arsenates react with the molybdate reagent to produce a blue color similar to that formed with phosphate. Concentrations as low as 0.1 mg As/L interfere with the phosphate determination. Hexavalent chromium and NO2- interfere to give results about 3% low at concentrations of 1 mg/L and 10 to 15% low at 10 mg/L. Sulfide (Na2S) and silicate do not interfere at concentrations of 1.0 and 10 mg/L.

Quality Control Requirements

See Section 4020 Quality Assurance/Quality Control.

Sample Handling

If dissolved phosphorus forms are to be differentiated, filter sample immediately after collection. Preserve by freezing at or below -10 degrees C. Add 40 mg HgCl2/L to the samples, especially when they are to be stored for long periods. Do not add either acid or CHCl3 as a preservative when phosphorus forms are to be determined. If total phosphorus alone is to be determined, add 1 mL conc HCl/L or freeze without any additions. Do not store samples containing low concentrations of phosphorus in plastic bottles unless kept in a frozen state because phosphates may be adsorbed onto the walls of plastic bottles. Rinse all glass containers with hot dilute HCl, then rinse several times in distilled water. Never use commercial detergents containing phosphate for cleaning glassware used in phosphate analysis.

Maximum Holding Time

48 hours (See Section 1060)

Relative Cost

Less than $50

Sample Preparation Methods

See 4500-P B

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	10.000 ug/L	10Rel. Error (ML)	9.10 % RSD (ML)			100.00 ug/L
Phosphorus(7723-14-0) Black phosphorus P Phosphorous, Red Phosphorus Phosphorus (yellow or white) Red phosphorus Violet phosphorus White phosphorus Yellow phosphorus	10.000 ug/L	10Rel. Error (ML)	9.10 % RSD (ML)			100.00 ug/L

Precision Descriptor Notes:	Precision and bias values are given in a table in the method; data presented were generated using three laboratories. They are for a single-solution procedure from the 13th edition. The current procedure differs in reagent-to-sample ratios, no addition of solvent, and acidity conditions. It is superior in precision and bias to the previous technique in the analysis of both distilled water and river water at the 228-ug P/L level. Note: Data for total phosphorus may vary from that provided in NEMI due to the sample preparation procedure used.
Detection Level Note:	A 10 ug-P/L minimum detectable concentration can be acheived using a 5.0 cm light path. Note: The acheivable detection limit for total phosphorus may vary from that provided in NEMI due to the sample preparation procedure used.

Precision Descriptor Notes:

Precision and bias values are given in a table in the method; data presented were generated using three laboratories. They are for a single-solution procedure from the 13th edition. The current procedure differs in reagent-to-sample ratios, no addition of solvent, and acidity conditions. It is superior in precision and bias to the previous technique in the analysis of both distilled water and river water at the 228-ug P/L level.

Note: Data for total phosphorus may vary from that provided in NEMI due to the sample preparation procedure used.

Detection Level Note:

A 10 ug-P/L minimum detectable concentration can be acheived using a 5.0 cm light path.

Note: The acheivable detection limit for total phosphorus may vary from that provided in NEMI due to the sample preparation procedure used.

Revision	PDF/Link
Standard Methods Online
Standard Methods 18th Edition (1992)
Standard Methods 19th Edition (1995)
Standard Methods 18th, 19th, 20th ed.

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Standard Methods: 4500-P E: Phosphorus by Ascorbic Acid

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