Spectrophotometric determination of microamounts of quercetin based on its complexation with copper(II)

The complexation process of the transition metal Cu(II) with quercetin was studied. The investigation was conducted spectrophotometrically in ethanol at the maximum absorption wavelength of 458.5 nm. Cu( II)-quercetin complex composition (1 : 1) was determined using the Job, Harvey Manning, and mole ratio methods. Complex stability constant was calculated by the Job and mole ratio methods and the respective logarithm values were 7.53 +/- 0.25 and 7.44 +/- 0.03. A new method for quantitative determination of the quercetin content in solution was developed in this work. At the optimal conditions quercetin was determined in concentrations ranging from 0.202 to 1.006 mu g cm(-3) with relative standard error of 2.5 % to 5.5 %. The lower detection limit was 0.067 mu g cm(-3). The method was found very accurate, reproducible, and sensitive, capable to determine microamounts of quercetin in pharmaceutical preparations.