Conversion of iron contained in a test sample to sulphate by treatment with hot sulphuric acid. Oxidation of the iron present in the solution by hydrogen peroxide and subsequently reduction of the iron(III) by hydroxylammonium chloride. Formation of the coloured complex iron(II)-2,2'bipyridyl in a buffered medium. Photometric measurement of the coloured complex at a wavelength of about 510 nm. - ISO 1391, Parts 1 to 4, replace ISO Recommendation R 1391-1970.