The specified method is applicable to the detection of Fe contents less than 0,0004 % (m/m). The principle consists in conversion of any iron present into the sulphate by evaporation to dryness in the presence of sulphuric acid, and reduction of iron (III) by means of hydroxylammonium chloride. Formation of a iron (II) 2,2'-bipyridyl complex. Photometric measurement of the coloured complex at a wavelength of about 522nm. As an alternative to measurement of absorbance the test solution prepared may be compared visually the Nessler cylinders with standard matching solution or with standard colorimetric discs.