Scope and Application

This method covers the determination of the orthophosphate ion in drinking, surface and saline waters, domestic and industrial wastes using APHA Standard Method 4500-P F. Depending on the prescribed pretreatment of the sample, the various forms of phosphorus in Figure 1 may be determined.


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. Insoluble forms of phosphorus are determined by calculation.

Summary of Method

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. Only orthophosphate forms a blue color in this test. Polyphosphates (and some organic phosphorus compounds) may be converted to the orthophosphate form by manual sulfuric acid hydrolysis. Organic phosphorus compounds may be converted to the orthophosphate form by manual persulfate digestion. The developed color is measured automatically on the AutoAnalyzer.

 Sample Handling and Preservation

If benthic deposits are present in the area being sampled, great care should be taken not to include these deposits. Sample containers may be of plastic material; such as cubitainers, or of Pyrex glass. If the analysis cannot be performed the same day of collection, the sample should be preserved by the addition of 2 ml conc. H2SO4 per liter and refrigeration at 4 degrees C.


Arsenate is determined similarly to phosphorus and should be considered when present in concentrations higher than phosphorus. However, at concentrations found in sea water, it does not interfere. Sample turbidity must be removed by filtration prior to analysis for orthophosphate. Samples for total or total hydrolyzable phosphorus should be filtered only after digestion. Sample color that absorbs in the photometric range used for analysis will also interfere.