USGS-NWQL: I-4607: Phosphorus, total, colorimetric, ASF
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Official Method Name
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Phosphorus, low ionic-strength water, colorimetry, phosphomolybdate, automated-segmented flow |
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Current Revision
| Mar-90 |
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Media
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WATER |
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Instrumentation
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Automated Spectrophotometer |
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Method Subcategory
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Inorganic |
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Method Source
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Citation
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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 |
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Brief Method Summary
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All forms of phosphorus, including organic phosphorus compounds, are converted to orthophosphate by an acid-persulfate digestion. Orthophosphate ion reacts with ammonium molybdate in acidic solution to form phosphomolybdic acid, which upon reaction with ascorbic acid produces an intensely blue complex. Antimony potassium tartrate is added to increase the rate of reduction (Murphy and Riley, 1962; Gales and others, 1966; Pai and others, 1990). |
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Scope and Application
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This method is used to analyze samples of precipitation or natural water with a specific conductance of less than 100 uS/cm containing from 0.001 to 0.20 mg/L of phosphorus. Concentrations greater than 0.20 mg/L must be diluted. This method was implemented in the National Water Quality Laboratory in March 1990. |
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Applicable Concentration Range
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0.001 to 0.20 mg/L |
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Interferences
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Barium, lead, and silver interfere by forming a phosphorus precipitate but the effect is negligible in natural water. The interference from silica, which forms a pale blue complex, is small and may be considered negligible. Nitrite interferes but can be oxidized to nitrate with hydrogen peroxide before analysis. Residual chloride needs to be removed by boiling the sample. Arsenic as arsenate produces a similar color as phosphate (Murphy and Riley, 1962) and might cause a positive interference. Arsenic concentrations as much as 100 ug/L do not interfere. |
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Quality Control Requirements
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Calibrate instrument using calibration standards (CAL); quality control samples (QCS); and laboratory blanks (LB) analyzed at a minimum of 1 for every 10 samples. |
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Sample Handling
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Container Description: 125 mL brown polyethylene bottle. Treatment and Handling: Use unfiltered sample to rinse container; preservation with mercuric chloride; chilled to 4 degrees C and ship immediately. |
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Maximum Holding Time
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30 days |
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Relative Cost
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Less than $50 |
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Sample Preparation Methods
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