Fusion of a test portion with sodium carbonate and separation of the fluorine by distillation with sulphuric acid. Titration of the distillate with thorium nitrate solution using either sodium alizarinsulphonate with or without methylene blue as masking agents, or methylthymol blue as indicator.Cross References:BS 410BS 1792BS 1848BS 2586BS 3145BS 3422BS 3978