The principle in dissolution of a test portion and separation of the insoluble matter. Precipitation of sulphate as barium sulphate in dilute hydrochloric acid medium. Separation of the precipitate obtained and ignition at 800 °C +- 25 °C and weighing. The method is applicable to products having a sodium sulphate content equal to or greater than 0,01 % (m/m). Replaces ISO Recommendation R 743-1968.