USGS Test Method I-2602


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Source:
USGS

Method Name:
Phosphorus, orthophosphate plus hydrolyzable, dissolved, colorimetric, ASF. Official Name: Phosphorus, orthophosphate plus hydrolyzable, colorimetric, phosphomolybdate, automated-segmented flow

Description:
Polyphosphates (P2O7)-4, (P3O10)-5, etc., and a few organic phosphorus compounds are converted to orthophosphate by an acid hydrolysis.Orthophosphate ion reacts with ammonium molybdate in acidic solution to form phosphomolybdic acid, which, upon reduction with ascorbic acid, produces an intensely colored blue complex. Antimony potassium tartrate is added to increase the rate of reduction (Murphy and Riley, 1962; Gales and others, 1966). Mercuric chloride-preserved samples are fortified with a minimum of 85 mg/L NaCl to overcome the interference from mercury in the analysis.

Scope:
This method may be used to analyze most water and brines containing between 0.01 and 1.0 mg/L combined acid hydrolyzable and orthophosphate phosphorus. Samples containing greater concentrations need to be diluted.

Citation:
Methods for the Determination of Inorganic Substances in Water and Fluvial Sediments, Techniques of Water-Resources Investigations of the United States Geological Survey, Book 5, Chapter A1 Edited by Marvin J. Fishman and Linda C. Friedman

Interferences:
Barium, lead, and silver interfere by forming a precipitate. Silica gives a pale-blue color that is additive to the phosphate color and may require correction. The affect of silica is somewhat dependent on the reagents; therefore, an appropriate silica correction should be determined for each batch of reagents. Nitrite interferes, but can be oxidized to nitrate with hydrogen peroxide before analysis. Residual chlorine must be removed by boiling the sample.Mercuric chloride interferes when the chloride concentration is less than 50 mg/L. Mercuric chloride-preserved samples are fortified with a minimum of 85 mg/L NaCl to overcome this interference.Arsenic as arsenate produces a color similar to that of phosphate and may cause a positive interference. Arsenic concentrations as much as 100 ug/L do not interfere. Greater concentrations were not investigated.

QC Requirements:
Calibrate instrument using calibration standards (CAL); quality control samples (QCS); and laboratory blanks (LB) analyzed at a minimum of 1 for every 10 samples.

Maximum Holding Time:
30 days

Media:
WATER

Subcategory:
Inorganic

Concentration:
0.01 to 1.0 mg/L

Precision:
It is estimated that the percent relative standard deviation for dissolved orthophosphate plus hydrolyzable phosphorus will be equal to that reported for dissolved phosphorus by the automated phosphomolybdate method.

Detection:
Reporting Level defined as smallest measured concentration of a constituent that may be reliably reported using a given analytical method.

Revision Number:
1973

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