Describes a method applicable to iron contents greater than or equal to 0.00005 % (m/m). The principle is based on the ignition of the material at 800 °C and fusion of the residue with anhydrous potassium hydrogen sulphate. Solution of the cooled melt in hydrochloric acid and reduction of iron(III) by means of hydroxylammonium chloride. Formation of an iron(II)-2,2'bipyridyl complex in the presence of a buffered solution (pH value between 4.5 and 6). Photometric measurement of the coloured complex at a wavelength of about 522 nm.