该方法适用于含量等于或大于0.1 mg/kg的产品。该原理适用于用氯化羟胺还原铁（III）。在缓冲溶液中形成铁（II）-1,10-菲二氮杂菲络合物。在约510 nm波长下对有色络合物进行光度测量。

The method is applicable to products having a content equal to or greater than 0,1 mg/kg. The principle concists in reduction of the iron(III) by hydroxylammoniumchloride. Formation of a iron(II)-1,10-phennanthroline complex in a buffered solution. Photometric measurement of the coloured complex at a wavelength of about 510 nm.