Describes a method of analysis of anhydrous sodium carbonate and sodium carbonate decahydrate. The principle of analysis consists in heating of thin layers of a test portion to constant mass at 250 °C. The residue constitutes the non-volatile matter at 250 °C. The loss of mass represents water in its various forms plus half the carbon dioxide content of the hydrogen carbonate present. Replaces ISO Recommendation R 745-1968.