Phosphorus is essential to the growth of algae and other biological organisms. The amount of phosphorus compounds present in wastewater discharge has to be controlled in order to avoid noxious algal blooms occurred in surface water. The usual forms of phosphorus found in aqueous solutions include the orthophosphate, polyphosphate, and organic phosphate. Three groups of phosphorus compounds have to be distinguished in phosphorus analysis of aqueous samples. The ortho-phosphate anion PO₄^3-, poly- and metaphosphates (examples see figure 8) which can be hydrolyzed to form ortho-phosphate, and phosphorus compounds which will not yield ortho-phosphate by Hydrolysis but by oxidative treatment. The latter group is mainly represented by organic phosphorus compounds. The sum of all three phosphorus species is designated as total phosphorus.

The ortho-phosphate anion is again determined by photometry after it has been transformed by addition of ammonium molybdate, potassium antimonyl tartrate and ascorbic acid to yield the intensely blue compound "molybdenum blue" (Greenberg et al. 1985) which is quantified by means of a photometer at 880 nm using phosphate calibration solutions of known phosphate concentrations.

Figure 8: Two examples of acid-hydrolyzable phosphates: di­phos­phate, P2O74-, and metaphosphate, P3O93-

The sum of ortho-phosphate and acid-hydrolyzable phosphorus is determined nearly in the same way except a Hydrolysis step prior to quantification of original and Hydrolysis-generated ortho-phosphate. The Hydrolysis is performed by gentle boiling of the wastewater sample after addition of a mixture of concentrated H₂SO₄ and concentrated HNO₃ (Greenberg et al. 1985). After cooling and neutralization with NaOH solution, the ortho-phosphate can be analyzed following the procedure given above.

Determination of total phosphorus requires oxidation as well as Hydrolysis prior to ortho-phosphate analysis. This is realized by boiling the wastewater sample after addition of concentrated HNO₃, evaporation on a steam bath, addition of 70 % perchloric acid and concentrated HNO₃, boiling until the mixture clears. After cooling the mixture, NaOH solution is added and the ortho-phosphate is determined as given above (Greenberg et al. 1985).