Determination of quercetin in pharmaceutical formations via its reaction with potassium titanyloxalate. Determination of the stability constants of the quercetin titanyloxalato complex

Abstract

A simple, rapid and accurate procedure for the quantitative determination of quercetin in its pure form and in formulations has been developed. The method is based on the spectrophotometric determination of a complex formed between quercetin and potassium titanyloxalate in 50 % ethanolic solutions. To characterize the quercetin titanyloxalato complex, the stability constants of the complex were determinated potentiometrically and spectrophotometrically at different temperatures (T= 26.0°C, 34°C and 39.0°C), as well as at different ionic strengths (I= 5.0 × 10-4 mol dm-3, 3.0 × 10-2 mol dm-3 and 6.0 × 10-2 mol dm-3) and the thermodynamic parameters were calculated. As quercetin is usually conjugated to vitamin C in pharmaceutical formulations, two procedures for the quantitative determination of quercetin by this complexing reaction were tested - both in the absence and presence of ascorbic acid. In both procedures, the Beer law was obeyed over the same concentration range of quercetin, i.e., 0.85 μgm L-1 - 16.9 μg mL-1. In the first procedure in the absence of ascobic acid the molar absorptivity coefficient of the quercetin-titanyloxalate complex is a = 2.49 × 10 4 mol-1 dm3 cm-1, Sandells sensitivity of the method is S = 1.35 × 10-2 μg cm -2 and the detection limit is d = 0.67 μg mL-1. Whereas, in the presence of ascorbic acid (second procedure) a = 3.04 × 104 mol-1 dm3 cm-1, S = 1.11 × 10-2 μg mL-1. The proposed method was verified for the determination of quercetin in pharmaceutical dosage forms.