Antioxidant study of quercetin and their metal complex and determination of stability constant by spectrophotometry method

Quercetin found chelate cadmium ions, scavenge free radicals produced by cadmium. Hence new complex, quercetin with cadmium was synthesised, and the synthesised complex structures were determined by UV–vis spectrophotometry, infrared spectroscopy, thermogravimetry and differential thermal analysis techniques (UV–vis, IR, TGA and DTA). The equilibrium stability constants of quercetin–cadmium complex were determined by Job’s method. The determined stability constant value of quercetin–cadminum complex at pH 4.4 is 2.27 × 106 and at pH 7.4 is 7.80 × 106. It was found that the quercetin and cadmium ion form 1:1 complex in both pH 4.4 and pH 7.4. The structure of the compounds was elucidated on the basis of obtained results. Furthermore, the antioxidant activity of the free quercetin and quercetin–cadmium complexes were determined by DPPH and ABTS assays.