Applicable to products having F contents of more than 0.003 % (m/m). Dissolution of a test portion by attack by sulphuric acid under pressure. Separation of the fluorine by distillation at a temperature of (150 1) °C by sulphuric acid. The distillation is carried out by the method described in ISO 1693. Formation of a blue coloured complex between fluorine and alizarin complexone/lanthanum chloride reagent at a pH between 4.5 and 4.6. Addition of acetone to increase the stability and sensitivity of the fluorine complex. Spectrophotometric measurement of the complex at a wavelength of about 620 nm. Has also been approved by the IUPAC.