Applicable to products having contents up to 0.1 % (m/m). The method specified is not applicable in the presence of vanadium or high silica content. Molybdenum and chromium also interfere. The analysis bases on fusion of a test portion with sodium corbonate, extraction of the cooled melt with suphuric acid and the addition of hydrogen peroxide. Photometric measurement of the yellow coloured complex formed at a wavelength of about 420 nm.