USGS-NWQL: I-6600-88: Phosphorus, Total-in-Bottom Material, by Colorimetry
|
Official Method Name
|
Phosphorus, Colorimetry, Phosphomolybdate, Automated-Segmented Flow |
|---|---|
|
Current Revision
| 1993 |
|
Media
|
SOILS/SEDIMENT |
|
Instrumentation
|
Spectroscopy (Colorimetry; Photometry) |
|
Method Subcategory
|
Inorganic |
|
Method Source
|
|
|
Citation
|
M.J. Fishman, 1993, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory--Determination of inorganic and organic constituents in water and fluvial sediments: U.S. Geological Survey Open-File Report 93-125 |
|
Brief Method Summary
|
Bottom material samples are sieved through a 2.0-mm plastic mesh sieve, then ground and sieved through a 500-um mesh sieve. A 0.1g portion of mixed sample is run in duplicate and digested prior to colorimetric analysis. 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 reduction 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). |
|
Scope and Application
|
This method is used to analyze samples of bottom material containing from 40 to 4,000 mg/kg of phosphorus. This modified method was implemented in the National Water Quality Laboratory in March 1988. |
|
Applicable Concentration Range
|
40 - 4000 (undiluted) |
|
Interferences
|
The color of the molybdate blue complex is strongly affected by pH. Barium, lead, and silver interfere by forming a phosphorus precipitate but the effect is negligible. The interference from silica, which forms a pale-blue complex, is negligible. Residual chlorine needs to be removed by boiling the sample. Arsenic as arsenate (AsO4-3) 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. Greater concentrations were not investigated. |
|
Quality Control Requirements
|
Quality-control samples area analyzed at a minimum of one in every ten samples. These QC samples include at least one of each of the following: blanks, quality control samples, third party check solutions, replicates, and spikes. Correlation coefficients for calibration curves must be at least 0.99. QC samples must fall within 1.5 standard deviations of the mean value. If all of the data-acceptance criteria in the SOPs are met, then the analytical data are acceptable. |
|
Sample Handling
|
Description: 500 mL Polypropylene bottle, wide-mouth. |
|
Maximum Holding Time
|
180 days from sampling |
|
Relative Cost
|
$51 to $200 |
|
Sample Preparation Methods
|
P-0520-85, TWRI Book 5, Ch. A1, p. 45 |
