USGS Test Method I-2601-90

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Method Name:
Phosphorus, Orthophosphate in Water by Colorimetry. Official Name: Phosphorus, Orthophosphate, Colorimetry, Phosphomolybdate, Automated-Segmented Flow

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).

This method is used to analyze most samples of water, wastewater, and brines containing from 0.01 to 1.0 mg/L of orthophosphate-phosphorus. Concentrations greater than 1.0 mg/L must be diluted. This modified method was implemented in the National Water Quality Laboratory in March 1988.

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

Barium, lead, and silver interfere by forming a phosphate precipitate but the effect is negligible in natural water. The interference from silica, which forms a pale-blue complex, is slight and also negligible. Nitrite interferes but can be oxidized to nitrate with hydrogen peroxide before analysis. Residual chlorine needs to be removed by boiling the sample. Arsenic as arsenate ion (AsO4 with minus three charge) produces a color similar to phosphate (Murphy and Riley, 1962) and might cause a positive interference. Arsenic concentrations as much as 100 ug/L do not interfere.

QC 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.

Maximum Holding Time:
30 days from sampling



0.01 - 1.0 (undiluted)

Single operator precision for dissolved orthophosphate-phosphorus, as determined for natural-water, is as follows: 147, 193, 252, 240 and 180 determinations were performed with mean recoveries of 0.06, 0.18, 0.48, 0.72 and 0.95 mg/L and 0.001, 0.001, 0.006, 0.013 and 0.008 mg/L standard deviation with 1.7, 0.56, 1.2, 1.8 and 0.84 %-RSD, respectively.

Method detection limits (MDLs) were determined for this method using the procedures of the U.S. Environmental Protection Agency (1997).

Revision Number:

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