EPA-NERL: 365.3:  Phosphorus by Colorimetry

  • Summary
  • Analytes
  • Revision
  • Data and Sites
Official Method Name
Phosphorus, All Forms (Colorimetric, Ascorbic Acid, Two Reagent)
Current Revision
Issued 1978
Spectroscopy (Colorimetry; Photometry)
Method Subcategory
Method Source
  Methods for the Chemical Analysis of Water and Wastes (MCAWW) (EPA/600/4-79/020)
Brief Method Summary
The sample is pretreated to select the phosphorus forms of interest; the forms are then converted to orthophosphate (which is amenable to the color chemistry of the method). A sample is filtered to select dissolved forms. Polyphosphates are converted to orthophosphate by sulfuric acid hydrolysis. Organic phosphorus is converted to orthophosphate using persulfate digestion.
Ammonium molybdate and antimony potassium tartrate react in an acid medium with dilute solutions of phosphorus to form an antimony-phospho-molybdate complex, which is reduced with ascorbic acid to form an intense blue-colored complex. The absorbance of the complex is measured, and is proportional to the orthophosphate concentration.
Scope and Application
This method can determine dissolved and total orthophosphate, hydrolyzable phosphate, and phosphorus (all forms) in drinking, surface and saline waters; domestic and industrial wastes.
Applicable Concentration Range
0.01 to 1.2 mg P/L.
(A) Arsenate: Arsenate is determined similarly to phosphorus, and should be eliminated by reducing arsenic acid to arseniuos acid with sodium bisulfite.
(B)Iron: Iron can make a low recovery because it will react with the reducing reagent. The bisulfite treatment will eliminate this interference as well.
Quality Control Requirements
Sample Handling
Samples should be collected in plastic or Pyrex glass containers. Samples should be preserved with 2 mL of concentrated sulfuric acid and refrigerated at 4oC. If samples are collected near benthic deposits, take care not to include materials from the deposits.
Maximum Holding Time
48 hours (orthophosphate, dissolved)
28 days (Hydrolyzable and Total)
24 hours (Total Dissolved).
(MCAWW, Table 1).
Relative Cost
Less than $50
Sample Preparation Methods