Phosphorus is present in water in different forms: orthophosphates (H₂PO₄^- / HPO₄^2- / PO₄^3- depending on the pH of the water) and polyphosphates. The determination of total phosphorus therefore requires polyphosphate mineralization.

We will be interested here only in the determination of orthophosphates soluble in water. These will be H₂PO₄^- / HPO₄^2- / PO₄^3- depending on the pH of the water.