AbstractsChemistry

Quercetin (3, 3', 4', 5, 7-pentahydroxyflavone) reacts with hexavalent molybdenum, tungsten and chromium to form chelates soluble in an ethanol-water medium. This color reaction has been investigated with a view to its possible application in the spectrophotometric determination of trace amounts of these metals. In the development of the method, several variables affecting the metal-quercetin color system have been studied in order to establish optimum conditions for color development. These include wavelength, pH, ethanol concentration, quercetin concentration and time. The molybdenum-quercetin chelate obeys Beer's law from 0.1 to 1.8 ppm at 420 mμ where maximum absorbance occurs. The molar absorptivity at this wavelength is about 34,000 cm⁻¹mole⁻¹1. The optimum concentration for maximum precision at 420 mμ corresponds to 0.6 to 1.2 ppm of molybdenum. By operating at 450 mμ where the molar absorptivity is only about 15,000 cm⁻¹mole⁻¹1 one can extend the workable range to 15 ppm of molybdenum. The tungsten and chromium chelates with quercetin both show deviations from Beer's law, although a working curve can still be established. The maximum absorbances for the tungsten and chromium complexes occur at 420 mμ and 435 mμ, respectively. The optimum concentration in both cases is 3 to 5 ppm of metal ion. The molar absorptivities are 18,900 cm⁻¹mole⁻¹1 for tungsten and 4600 cm⁻¹mole⁻¹1 for chromium. Studies have been made of the composition of the complexes in solution using the slope-ratio method and the "gerade" method of Asmus. Both methods indicated a 1:1 mole ratio of metal to ligand for all three chelates, although there is some evidence that additional species may be present in the case of tungsten and chromium. Approximate values for the instability constants of the chelates have been obtained using a spectrophotometric method. These were found to be about 3.9 x 10⁻⁵ for molybdenum, 1.4 x 10⁻⁵ for tungsten and approximately 10⁻⁵ for chromium. A study of the effect of foreign ions indicates a general lack of specificity of the reagent and points to the probable need for a prior separation of the metal ion before its determination. An attempt was made to run a potentiometric titration of quercetin, both alone and in the presence of the metal ions, but the results were inconclusive.