The method is applicable to sodium contents equal to or greater than 0.05 % (m/m). The principle of the procedure consists in the fusion, at a controlled temperature, of a test portion with a mixture oflithium carbonate and boron trioxide or a mixture of lithium carbonate and lithium tetraborate. Dissolution of the fused mass in hydrochloric acid. Aspiration of the solution into a flame and spectrophotometric measurement of the intensity of the light emitted at a wavelength of 589 nm. - Replaces ISO Recommendation R 1617-1970.