ISO 2216:1972

The principle of the method consists in determination of the Na2O content by treatment of an aqueous extract with an excess of hydrochloric acid solution, followed by back-titration with standard volumetric sodium hydroxide solution in the presence of methyl red as indicator. Subsequent titration of the boric oxide with standard volumetric sodium hydroxide solution in the presence of mannitol or sorbitol (the solution turns red; on titration with sodium hydroxide solution the colour of the solution changes to yellow and turns pink at the end point) and phenolphthalein as indicator to a distinct pink colour.