ISO 622:1981

For taking the phosphorus into solution the following two methods are specified: The carbonaceous material is removed by ashing in amuffle furnace under specified conditions and the phosphorus is extracted by treatment with hydrofluoric and sulphuric acids. The carbonaceous material is removed by repeated oxidation with nitric acid in the presence of sulphuric acid. The determination is carried out by adding ammonium molybdate and ascorbic acid solution and measuring the absorbance of the resulting blue solution by a siutable optical instrument.