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United States Department of Agriculture

Agricultural Research Service

Title: Analyzing for Dissolved Reactive P in Water Samples

Authors
item Pote, Daniel
item Daniel, Tommy - UNIV OF ARKANSAS

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: March 14, 2000
Publication Date: June 1, 2000
Citation: POTE, D.H., DANIEL, T.C. 2000. ANALYZING FOR DISSOLVED REACTIVE P IN WATER SAMPLES. BOOK CHAPTER. IN METHODS OF PHOSPHORUS ANALYSIS FOR SOILS, SEDIMENTS, RESIDUALS, AND WATERS. 396:91-93.

Technical Abstract: Dissolved reactive P (DRP) refers to the phosphorus fraction that passes through a membrane filter of 0.45-micron pore diameter and responds to the molybdate colorimetric test with no preliminary hydrolysis or oxidative digestion of the water sample. Because DRP is largely a measure of dissolved orthophosphate, the P form most readily available to aquatic plants, it is often considered the most critical P fraction contributing to accelerated eutrophication of surface water. Filtering out all particles larger than 0.45 micron does not completely separate dissolved and suspended P forms, but the method can be easily replicated and provides a convenient technique for clearly defining the analytical separation of dissolved and suspended P fractions. Development of the molybdate colorimetric test for ortho-P in water samples was based on the observation that ammonium molybdate and potassium antimonyl tartrate react with dilute ortho-P solutions in an acid medium to form an antimony-phospho-molybdate complex. Reduction of this complex by ascorbic acid gives it an intense blue color that is proportional to the ortho-P concentration. Prototypes of this colorimetric method were used as early as 1936, but the combination of time and temperature requirements for full color development allowed some hydrolysis of organic P compounds to ortho-P. The addition of antimony to the reagent (first reported in 1962) provided full color development in 10 minutes at room temperature, and allowed accurate measurement of DRP concentrations as low as 10-micron/L. The technique has since been modified for use on autoanalyzers. A complete description and discussion of the procedure, including interferences, are included in this chapter.

Last Modified: 9/21/2014