ISO 5442:1980

Applicable to products having contents, expressed as sulphate2- , between 0.03 % (m/m) and 0.5 % (m/m). Reduction of the compounds to hydrogen sulphide by heating with a mixture of hydriodic and hypophosphorus acids in the presence of hydrochloric acid. Entrainment of the hydrogen sulphide in a flow of nitrogen and absorption in an excess of ammoniacal cadmium acetate solution. Liberation of the absorbed hydrogen sulphide in the presence of an excess of standard volumetric iodate/iodide solution. Back-titration of the excess iodine with standard volumetric sodium thiosulphate solution using starch as the indicator. An apparatus for reduction, entrainment and absorption is shown in a figure. Has also been approved by the IUPAC.