USGS-NWQL: I-2598-85:  Phosphorus, orthoposphate in water by automated discrete colorimetry, phosphomolybdate

  • Summary
  • Analytes
  • Revision
  • Data and Sites
Official Method Name
Phosphorus, orthoposphate, colorimetric, phosphomolybdate, automated-discrete
Current Revision
Method Subcategory
Method Source
Skougstad, M.W., Fishman, M.J., Friedman, L.C., Erdmann, D.E., and Duncan, S.S., eds., 1979, Methods for determination of inorganic substances in water and fluvial sediments: U.S. Geological Survey Techniques of Water-Resources Investigations, book 5, chap. A1, 626 p.
Brief Method Summary
This method is no longer available through the USGS National Water Quality Laboratory. If using a local laboratory, refer to method report for more information.
Scope and Application
This method may be used to analyze water, wastewater, brines, and water-suspended sediment containing from 0.01 to 2.0 mg/L orthophosphate-phosphorus. Samples containing greater concentrations need to be diluted. To determine total orthophosphate phosphorus, the suspended sediment in an unfiltered, unacidified sample is allowed to settle in the sample bottle and a portion of the clear supernatant solution is decanted for analysis.
Applicable Concentration Range
Because the phosphorus is easily adsorbed on sediment, the orthophosphate recovered from the supernatant solution above a water-suspended sediment after some time has elapsed may be less than the orthophosphate that would have been determined in the filtrate from a sample filtered at the time of collection. The amount recovered may also depend on the type of sediment. Barium, lead, and silver interfere by forming a precipitate. The interference from silica is slight and can be considered neglegible. Nitrite interferes but can be oxidized to nitrate with hydrogen peroxide before analysis. 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. The blue color produced will coat the flow cell, causing a small but significant high bias. A 0.10 mg/L P spike is dispensed into the four leading wash tubes with the color reagent. The resulting blue color then coats the flow cell prior to the introduction of the working standards. By this action, the high bias is effectively aliminated.
Quality Control Requirements
Sample Handling
Maximum Holding Time
Relative Cost
Sample Preparation Methods