The method is applicable to products having a silica content between 0.005 % (m/m) and 0.025 % (m/m) provided that the content of phosphorus pentoxide plus vanadium pentoxide does not exceed 0.02 % (m/m). The principle of the method is based on the formation of oxidized molybdosilicate (yellow) under well-defined conditions of acidity, concentration of reagents, temperature and time. Selective reduction of the complex in a highly acidic medium of sulphuric acid and in the presence of tartaric acid. Spectrophotometric measurement of the reduced coloured complex at a wavelength of about 815 nm. - Replaces ISO Recommendation R 1232-1970.