Method 365.3

Phosphorous, All Forms (Colorimetric, Ascorbic Acid, Two Reagent)

This method is for the determination of specified forms of phosphorus in drinking, surface and saline waters, domestic and industrial wastes. The method is based on reactions that are specific for the orthophosphate ion and depending on the prescribed pretreatment of the sample, the various forms may be determined.  Ammonium molybdate and antimony potassium tartrate react in an acid medium with dilute solutions of phosphorus to form an antimony-phospho-molybdate complex. This complex is reduced to an intensely blue-colored complex by ascorbic acid. The color is proportional to the phosphorus concentration and only orthophosphate forms a blue color in this test. Polyphosphates (and some organic phosphorus compounds) may be converted to the orthophosphate form by sulfuric acid hydrolysis. Organic phosphorus compounds may be converted to the orthophosphate form by persulfate digestion.

Except for in-depth and detailed studies, the most commonly measured forms are phosphorus and dissolved phosphorus, and orthophosphate and dissolved orthophosphate. Hydrolyzable phosphorus is normally found only in sewage-type samples and insoluble forms of phosphorus are determined by calculation.

The range of the method is in the 0.01 to 1.2 mg P/L range. 

(EPA: Office of Water)


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Method Data

Hold Times, Preservatives, Preps, Collection, Analytical & Documentation
Holding Time:   Holding time from sample collection to analysis are: 24 hours for total dissolved phosphorus, 48 hours for dissolved orthophosphate and 28 days for hydrolyzable and total phosphorus.
Preservatives:   Preserve with 2 mL concentrated sulfuric acid per liter and refrigerate sample 4°C.
Required Preps:   100 mL HDPE or amber glass bottle
Collection Method:   Grab sampling
Analytical Methodology:   Spectrophotometer or filter photometer
Documentation:   365.3

Analyte List*

Analyte Formula CAS Number Detection Limit

* The analytes and detection limits listed for each method represent the typical detection limits and analytes reported for that particular method. Keep in mind that analyte lists may vary from laboratory to laboratory. Detection limits may also vary from lab to lab and are dependent upon the sample size, matrix, and any interferences that may be present in the sample.