ISO 352:1981

The sample is burnt in a stream of oxygen, in a tube furnace at a temperature of 1250 or 1350 °C, and the acid gases (chlorine and oxides of sulphur) formed are absorbed in hydrogen peroxide. The acid solution is neutralized with sodium tetraborate and the sodium chlorine formed is converted by reaction with mercury(II) oxycyanide to sodium hydroxide which is determined volumetrically.