Total phosphorus is measured by first converting organic and inorganic-bound phosphorus compounds to soluble phosphorus through digestion. The AQUA Lab uses an alkaline persulfate digestion, which becomes acidic during autoclaving to facilitate the conversion of bound phosphorus compounds to soluble phosphorus.
After digestion, an antimony-phospho-molybdate complex is formed when acidic molybdate and antimony potassium tartrate react in the presence of orthophosphate. Ascorbic acid reduction of this complex yields phosphomolybdenum blue; the phosphorus concentration is proportional to the color, measured with a spectrophotometer at 880 nm.
Related references:
Ameel, J.J., R.P. Axler, C.J. Owen. 1993. Persulfate digestion for determination of total nitrogen and phosphorus in low nutrient water. American Environmental Laboratory. 5: 2-11.
Methods for the Determination of Inorganic Substances in Environmental Samples, USEPA 600/R 93/100, August 1993: Method 365.1, Rev. 2.0.
Standard Methods for the Examination of Water and Wastewater, APHA/AWWA/WEF, method 4500-P-F (1999 forward).
Valderrama, J.C., 1981. The Simultaneous Analysis of Total Nitrogen and Total Phosphorus in Natural Waters: Marine Chemistry, v. 21, p. 109–122.