Applicable to products whether or not they are homogeneous and whether or not they contain polyphosphoric acids. Specifies preliminary hydrolysis of the polyphosphoric acids by boiling in the presence of hydrochloric acid. The precipitation of the phosphoric acid is carried out in the form of quinoline phosphomolybdate in the presence of acetone. Filtration by a filter crucible, washing, drying and weighing of the precipitate.