Standard Methods: 4500-P E:  Phosphorus by Ascorbic Acid

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Official Method Name
4500-P B. Ascorbic Acid Method
Current Revision
Standard Methods Online
Spectroscopy (Colorimetry; Photometry)
Method Subcategory
Method Source
  Standard Methods
  Standard Methods Online - Standard Methods for the Examination of Water and Wastewater
Brief Method Summary
Ammonium molybdate and potassium antimonyl tartrate react in acid medium with orthophosphate to form a heteropoly acid-phosphomolybdic acid-that is reduced to intensely colored molybdenum blue by ascorbic acid.
Scope and Application
This method is most suited for measuring phosphorus in water.
Applicable Concentration Range
0.01 to 6 mg-P/L
Arsenates react with the molybdate reagent to produce a blue color similar to that formed with phosphate. Concentrations as low as 0.1 mg As/L interfere with the phosphate determination. Hexavalent chromium and NO2- interfere to give results about 3% low at concentrations of 1 mg/L and 10 to 15% low at 10 mg/L. Sulfide (Na2S) and silicate do not interfere at concentrations of 1.0 and 10 mg/L.
Quality Control Requirements
See Section 4020 Quality Assurance/Quality Control.
Sample Handling
If dissolved phosphorus forms are to be differentiated, filter sample immediately after collection. Preserve by freezing at or below -10 degrees C. Add 40 mg HgCl2/L to the samples, especially when they are to be stored for long periods. Do not add either acid or CHCl3 as a preservative when phosphorus forms are to be determined. If total phosphorus alone is to be determined, add 1 mL conc HCl/L or freeze without any additions. Do not store samples containing low concentrations of phosphorus in plastic bottles unless kept in a frozen state because phosphates may be adsorbed onto the walls of plastic bottles. Rinse all glass containers with hot dilute HCl, then rinse several times in distilled water. Never use commercial detergents containing phosphate for cleaning glassware used in phosphate analysis.
Maximum Holding Time
48 hours (See Section 1060)
Relative Cost
Less than $50
Sample Preparation Methods
See 4500-P B