Applicable to products having Fe contents between 0.0004 % (m/m) and 0.1 % (m/m). The residue from the determination of the loss of mass according to ISO 5444 is used to prepare the test sample. Decomposition of the hexafluorosilicate and elimination of silica by evaporation with hydrofluoric and sulphuric acids. Dissolution of the residue in hydrochloric acid and reduction of the iron(III) with hydroxylammonium chloride. Formation of a complex between iron(II) and 1,10-phenanthroline in a buffered medium, spectrophotometric measurement of the coloured complex at a wavelength of about 510 nm. Has also been approved by the IUPAC.