|Abstract or Summary
- 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
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.