USDA LTAR Common Experiment measurement: Total phosphorus (TP) and total dissolved phosphorus (TDP) concentration v1

Total dissolved P (TDP) includes organic P (e.g., sugars and phospholipids) and orthophosphate. Dissolved orthophosphate (also termed soluble reactive P, dissolved reactive P, dissolved inorganic P, or filterable reactive P) is the form of P most readily available to algae and aquatic plants; however, numerous studies have shown that hydrolysis of other forms of P to the orthophosphate form is possible in natural waters. Therefore, when assessing the long-term potential for accelerated eutrophication of surface water due to P loading, many researchers and watershed managers want to know the total P concentration (regardless of P form) in water samples. Total dissolved P determination involves digesting filtered water samples to convert organic P species to orthophosphate, followed by colorimetric analysis or analysis by inductively coupled plasma with optical emission spectroscopy (ICP-OES). Determining TP involves the same digestion procedure applied to an unfiltered sample, followed by colorimetric or ICP-OES analysis. The recommended P digestion method is the persulfate oxidation technique, performed using an autoclave or microwave. The colorimetric determination of TDP and TP follows the ascorbic acid “blue” method (USEPA, 1978), whereby an intensely colored blue complex forms from the reaction between P compounds and ascorbic acid. The procedure involves the reaction of ammonium molybdate and potassium antimonyl tartrate with orthophosphate to form an antimony-phospho-molybdate complex. This complex is then reduced to an intensely blue-colored complex by addition of ascorbic acid. The intensity of the blue complex is proportional to the orthophosphate concentration in the sample.