The principle of the method specified is heating the material under test in a stream of dry air, absorbing the gases in 0,1 M sodium hydroxide solution, determining the amount of halogen acid by acidifying the solution with nitric acid adding a measured volume of 0,1 M silver nitrate solution and back titrating the excess with 0,1 M ammonium thiocyanate using ferric ammonium sulphate as the indicator, or by any other equivalent analytical method having at least the same accuracy. For reasons of accuracy this method is not recommended for use where the amount of halogen acid evolved is less than 5 mg/g of the sample taken.