Applicable to products having contents of more than 0.0001 % (m/m). The procedure described is suitable for contents between 0.0001 % (m/m) and 0.0005 % (m/m). It bases on distillation, with entrainment by steam, of the ammonia in the presence of an excess of sodium hydroxide and collection of the distillate in an excess of acid solution. Neutralization of the dexcess of acid and formation of the coloured complex by treatment with sodium phenate and sodium hypochlorite in the presence of acetone. Spectrophotometric measurement of the indophenol obtained at a wavelength of about 630 nm. Has also been approved by the IUPAC.