This International Standard specifies a flameless atomic absorption spectrometric method for the determination of the mercury content of sodium hydroxide for industrial use. The method is applicable to liquid or solid products having mercury (Hg) contents greater than 0,02 mg/kg. NOTE - The alternative dithizone photometric method for the determination of mercury content, specified in ISO 5992, is intended for use in laboratories where flameless atomic absorption equipment is not availa ble.