Applies for the determination of natural and synthetic materials having a molar ratio sodium fluoride/aluminium trifluoride between 3 and 1,7 approximately. The method is applicable to products having an iron content equal to or greater than 0,020 % (m/m). The principle consists in dissolution of a test portion by either alkaline or acid fusion. Reduction of iron(III) with hydroxylammonium chloride, formation of the complex iron(II)-1, 10-phenathroline in a buffered medium at pH value between 3,5 and 4,2. Photometric measurement of the coloured complex at a wavelength of about 510 nm. Applies ISO Recommendation R 1694-1970.