The principle is based on evaporation of the test portion, taking up by means of hydrochloric acid and reduction of iron(III) by means of hydroxylammonium chloride. Formation of an iron(II) 2,2'-bipyridyl complex in a buffered medium at a pH value between 4.5 and 6. Spectrophotometric measurement of the coloured complex at a wavelength of about 522 nm. The method is applicable to the determination of iron content greater than 1 p.p.m.